Bandpass 4 orders for 12 drawing. Subwoofers are bandpass type. Subwoofer Enclosure Types
Subwoofer bandpass
The bandpass subwoofer is a device that is increasingly used by modern drivers in cars. The bandpass subwoofer has a number of specific advantages over other models, which makes it indispensable for many car owners. Many companies well known in the car audio world produce subwoofers of this design.
What is a subwoofer
The subwoofer is one of the most necessary and important elements. After all, it is he who is responsible for reproducing low frequencies that are not amenable to conventional speakers, which is why it got its name (low frequencies).
For some reason, it is the bass that interests almost all car owners. In their opinion, the louder they are, the better.
Note: if you like music without low frequencies, then you can do without using a subwoofer.
Where are low frequencies used?
The transmission of low frequencies is especially important when watching modern films where scenes with special effects are used, when listening to electronic music. Low frequencies are also common in video games. In fact, this is one of the most important advantages of a bandpass subwoofer: it can be used not only in a car speaker system, but also in the house.
Note: but for this you will need to purchase additional cables and adapters.
Types of subwoofers
Today there are 2 types of subwoofers:
- Active
- Passive
Each of these types has its own distinctive characteristics. a built-in power amplifier and an active crossover are used, which filters all high frequencies and coordinates the work with the rest of the speaker system.
Very often has many functions for adjustment. In addition, it is equipped with a huge number of connectivity options (pass-through, etc.).
A passive subwoofer, on the contrary, is distinguished by the absence of a power amplifier and is connected in two ways: in parallel or separately, to another channel. Due to the fact that it does not have any means of adjustment and, due to its design features, is very picky about placement in the room, you need to find a suitable place where there is “good bass”.
Materials used in production
Currently, several materials are used to make speakers, each of which has its own specific advantages:
- Multilayer plywood. It has a low cost, but is easy to process;
- Chipboard consists of natural materials, due to which its properties are significantly increased;
- Fiberboard has a fairly dense structure, so it is highly resistant to moisture.
Subwoofer Enclosure Types
There are several types of subwoofer enclosures, the production of which is practiced by modern companies:
- Sealed housing. Today this is the most common and at the same time the simplest type of case. It is a completely insulated, closed box that creates space inside itself to control the speaker membrane, which allows it to operate at such powers;
- Phase inverted housing. It is used mainly in home systems and, in particular, in car audio systems.
At lower power, the bass becomes louder than in a sealed space. It should be noted that this type of case has its disadvantages. When you try to reproduce frequencies that are lower than the “factory” ones, distortion appears.
Note: problems may also arise due to high humidity during storage.
- Isobaric design. From a commercial point of view, one of the most profitable types of subwoofer. In a sealed box there are two completely identical speakers, which must work in unison with each other, which creates a certain effect. The disadvantages include the fact that there is actually one speaker working, not 2.
- Bandpass. This type of subwoofer design combines both a sealed and phase-inverted enclosure, which are located in two different chambers. This type shows excellent performance in delivering low frequencies along with limiting resonance. Of course, this is one of the best types of subwoofer designs, which we will explain in more detail. Bandpass categories
Bandpass is divided into 2 categories, which differ in design and structure:
- Bandpass 4. This type of design is standard and was described above;
- Bandpass 6. This type combines 2 bass reflexes at once. Thanks to this structure, the sound is very high quality with deep and rich bass.
Review of the Mystery MBP-2500 model
For example, let's take the first BandPass subwoofer we came across - the Mystery MBP-2500.
Note. Typically, numbers like “speakers.subwoofers” are used for designation. For example, in a multi-channel system with five speakers and one subwoofer, the designation will look like this - “5.1”.
Problems with subwoofers
Most problems can be solved by simply adjusting the frequency and phase. If such manipulations are not provided initially or you use, you will have to try to place it in the room in order to find that very “point” of correct location.
If you need high-quality sound from your speakers, then BandPass is certainly the best choice. This is one of the most effective subwoofers with various design variations and combinations of different types of designs.
In addition, you can install such a device without any problems with your own hands. This can be done even at home. However, in the process of work it is necessary to view photos on this topic.
Detailed instructions recorded on video may also be needed. The price of the device depends mainly on which company manufactured it. However, you need to look not only at the cost, but also at the technical data.
Introduction
A subwoofer is a low-frequency loudspeaker that reproduces the lowest bass frequencies that are not included in the frequency range of full-range speaker systems working in conjunction with it.
The reason for writing this article was the rather small number of detailed articles on the Internet on ready-made structures. So I decided to make my contribution. For some reason, most of the articles concerned car subs - it’s simply amazing how many people we have who are ready to give up half the trunk in order to drive with a roar and jump rhythmically :). In addition, noise at speed will still not give pleasure from the sound. My opinion is that you just need to buy good acoustics for your car, that’s usually enough. But at home you can listen to music in a quiet environment and watch a movie with surround sound. The article was intended as a brief summary of the theory with its reflection in practice, so I tried to summarize the theory from other articles - not to invent it myself.
What inspired me to sculpt such a structure? A whole series of events led to this. The DVD generation is coming, everyone around is preoccupied with buying players and discs (fortunately, they now cost $4-6). It was already difficult to stay away, and my long-standing dream of transferring the family archive from a video camera to digital did not give me rest. I sat down on the Internet, studying articles on DVDs, from equipment to creating my own discs, fortunately I already knew how to digitize and process videos. It was decided to wait until the summer, when DVD writers would become a little cheaper, and buy all this newfangled equipment. To begin with, it was decided to assemble everything ourselves in order to get by at the lowest possible cost. The first thing I bought was a DVD player, which was purchased with the intention of testing my DVD film library on a home player. Moreover, I couldn’t wait until the summer :) I chose it based on price and capabilities, and also because I don’t have a receiver and probably won’t have one in the near future, so it had to have a decoder and an output for 6 channels at once. As a result, I bought a BBK-919PS (at that time it still had a Panasonic drive).
I had a pair of S-30 speakers connected to the computer through a homemade computer amplifier, so I used them for the fronts. There was nothing for the rear, so I bought 15AC-315 (small plastic speakers, but they sound very poor in the mid and high range) and the Vega-120 amplifier for a nominal fee. I decided to connect the central channel to the TV, since it can be separately adjusted from the remote control, and the sound there is sufficient to reproduce voices. And if you have a 29" TV, you can generally do without a central speaker, because there, as a rule, the sound is very decent. Then, of course, I began testing all the channels in films, since I only had a stereo amplifier. And then it turned out that that in other channels there is a lot of sound that is not in the front speakers :), and in the sub there are sounds that are not in any of the channels. Having read on the Internet about the recommendations of the best sub-cavods, as well as recommendations from friends who own a house. cinemas, I was obviously fired up.
And then a friend advised me to put this together myself, slipped me a couple of articles on 75GDN and recommended that I go to the market and look at the speaker. After much torment and thought, I bought a 75GDN-1-4 on the market for $23, as the cheapest and best described on the Internet, although I looked at different ones, including JBL from $80 to $250 :). I must say, the first thing I did before purchasing was to study the prices. The most affordable price for purchase turned out to be Sven-Audio, but it’s still a bit expensive and the sound quality is questionable. So, the simplest sub would cost $180, and a normal sub would cost about $300-350. All expenses for the production of the subwoofer were recorded, so I have calculated the cost of the design :), but first things first...
Theory and measurements of speaker parameters
To calculate the speaker, you need to know at least the basic parameters of the speakers. These parameters are in abundance on the Internet, but the trouble is that all as one, give implausible parameters for 75GDN-1-4 (I don’t know for others). On one of the sites I found a scanned passport for this speaker, and I trust these parameters more. There are probably reasons for this, one of them is that such heads were produced by a whole bunch of factories and for quite a long time by Soviet industry, so the parameters may have changed over time. But the fact is that they have changed, as it turned out, 2 times! And with the parameters from the reference books, when calculated in the program, the volume of the box turned out to be 5 liters, which alerted me. By the way, in almost all the articles I read there were recommendations to measure YOUR speaker parameters. The decision to take the parameters of my speaker was made after a week of fiddling with programs for calculating the subwoofer (despite the terrible laziness of doing this), in which it turned out that these parameters greatly affect the size of the box and the frequency response, respectively. In order to understand what parameters are needed and what they are used with, as well as how to get them without having any sensible measuring instruments at hand, this chapter will be discussed.
I will write briefly; anyone who wants can read in more detail in the attached literature from which the material was collected.
It may seem strange, but the speaker is mainly characterized by three parameters proposed by Till and Small:
Fs- this is the resonance frequency of the speaker without any acoustic design. This is how it is measured - the speaker is suspended in the air at the greatest possible distance from surrounding objects, so that now its resonance will depend only on its own characteristics - the mass of the moving system and the stiffness of the suspension.
Qts- the ratio of the transfer function of the speaker at frequency Fs to the transfer function at frequencies where the amplitude-frequency response (AFC) of the speaker is horizontal, i.e. at frequencies above Fs. In other words, Qts characterizes the efficiency of the speaker at the resonant frequency.
Vas– the volume of air that has flexibility (the inverse value of elasticity) is the same as the movable speaker system. When a speaker is placed in a closed box (CC), the flexibility of the air inside the box adds to the flexibility of the speaker's moving system and its resonant frequency changes. There is the following pattern: when a speaker is placed in a box of volume Vas, its resonant frequency Fs and quality factor Qts increase by 1.4 times.
Measuring these parameters at the first glance at the design is quite a hassle, but having done it once, all doubts disappear - everything turns out to be quite simple. First you need to prepare:
- download signal generator program for sound card download Marchand Function Generator
- You can also download Oscilloscope 2.51 - an oscilloscope for your sound card. By connecting the output to the input, you can see what the generator is doing
- find a 1 kOhm resistor
- take a stereo, at least a power amplifier, because you need to amplify first the signal itself, and then the measured signal
- It is advisable to take a digital voltmeter, so as not to recalculate or change the ranges. I took digital and pointer, and compared the results to check.
Next you need to assemble the following circuit:
- Take a pen and paper
- We launch the generator program, set the volume on the computer to the middle (otherwise the sine wave will be cut off), and the rest will be corrected with an amplifier.
- We connect a voltmeter to points A and C (i.e. to the output of the amplifier), and set the voltage to 10-20 V at a frequency of 500-1000 Hz, adjusting the volume on the amplifier.
- We connect the voltmeter to points B and C (i.e. to the speaker).
- We set the generator to ~5Hz and REMOVE the SPEAKER away from all objects and walls (you can hang it up if possible). Practice has shown that, far from objects and on the floor, a speaker lying on the floor still gives different readings, but they are insignificant, but will affect the purity of the experiment.
- When changing the frequency of the generator, we look at the voltmeter readings - we are interested in the maximum and minimum voltage. Approximately near the resonant frequency, the voltage increases sharply, and then drops sharply. At maximum voltage, we look at the frequency - this is Fs.
From these records we get something like this graph:
Where, Fs is the resonant frequency, and Us is the corresponding voltage. Um is the minimum voltage, U12 is calculated in Excel after entering the data into the cells. Again we turn the frequency again and look for when the voltmeter readings coincide with the value of U12, remember the frequency. There should be two such values, as can be seen from the graph. These will be F1 and F2. We enter them into Excel. That's it - look at the value of Qts. I made two calculation methods to check the correctness of what I calculated :) in the end, the readings converged, and the small difference is explained by the calculation error. You don’t have to count Vas, but take it from the reference data, it is similar to the truth, and this parameter does not greatly affect the calculation of the box. In addition, to calculate it, you will have to build a plywood box that is sufficiently rigid and airtight, and then repeat the measurements. I set myself a goal to create something like a summary of the materials that I read, in order to discard the unnecessary, and for an in-depth study of the issue, I provide all the links at the end of the material. So we removed the speaker parameters. For 75GDN-1-4 this is what happened:
The most important parameter is Qts. How do you feel about the difference? TWICE! I must say that I downloaded about a dozen speaker reference books, other parameters in them fluctuated, but this one was surprisingly stable. So trust people after this...
Let's move on. Now you need to choose the type of box where you can put all this stuff. And this is where the exact speaker parameters come in handy.
There are only three most common types of acoustic design:
| Drawer type | Closed Box (ZY) Closed | PhaseInverter (FI) Vented | Bandpass (BP) 4th and 6th orders Bandpass |
| Selection criterion | Qts< 0.8-1.0 , оптимально 0,7 Fs/Qts=50 | Qts<0.6, оптимум - 0,39 Fs/Qts=85 | Fs/Qts=105 |
| Distinctive characteristics | This is the easiest type of speaker system to manufacture. Despite the simplicity of the design, it has many advantages, but the C.P.P. is the smallest compared to any other type of acoustic design - as a consequence, the need for considerable power and possible failure of the speaker (from excessive efforts:) | In its operating range, the bass reflex creates completely greenhouse conditions for the speaker, and exactly at the tuning frequency the oscillation amplitude is minimal, and most of the sound is emitted by the tunnel. The permissible input power is maximum here, and the distortion introduced by the speaker is, on the contrary, minimal. The bass reflex is much more capricious in the selection of parameters and settings, since three parameters are subject to selection for a specific speaker: box volume, cross-section and tunnel length. | Efficiency champion. By selecting the appropriate volumes and frequency tuning of the front chamber, it is possible to build a subwoofer with a wide bandwidth but limited output, that is, the bell will be low and wide, or one with a narrow bandwidth and very high efficiency. in this strip. The bandpass is a capricious thing to calculate and the most labor-intensive to manufacture. But the speaker is buried inside - there is less risk of damaging the speaker and there is practically no need for a bandpass filter (although in practice it turned out that it is still desirable) |
FI I didn’t want to do this for three reasons: 1) I need to make a super even hole and then cover it with something, otherwise my child will immediately condemn the speaker 2) I need to install a filter to cut off frequencies above 200Hz, otherwise the 75GDN then sounds not cool at all. 3) a normal low-end roll-off was obtained with a wild box size of 120-150 liters (another closet in the apartment), my wife would have kicked me out along with this subwoofer :) BUT! Fs/Qts=74, i.e. the speaker is most suitable for FI, and gives great lows, only it is SIZE 8(. Here we must take into account that almost the same speakers are suitable for single bandpasses as for bass reflexes.
Bandpass I liked it and liked it the most. There is no need to make a filter - the housing itself filters. The speaker is hidden inside so you can't puncture it. And calculations in the programs showed the best results with the appropriate size...
Calculation and design of the box
Calculations showed that the Bandpass had relatively good dimensions and a good drop-off at the bottom, however, the drop-off still depended heavily on the volume and we had to make a compromise by slightly reducing the box to 65 liters. I carried out the calculations in three programs at once in order to check the accuracy of what I had built. The results were almost identical. I used WinISD 0.44, WinISD Pro Aplha and download JBL SpeakerShop or download BassBox (called find 10 differences). I liked the first program the most, the second was terribly buggy (that’s why it’s Alpha), but in some respects useful, the third simply confirmed my calculations (it has a very inconvenient interface - it’s bad to change parameters on the fly, selecting values for the size of cameras and bass reflexes, and after each boot you need to switch to the metric system). So what happened - look at the graphs (you can take the project files later):
This can be seen in the comparison between BP and FI. With equal box volumes, the FI is significantly inferior to the Bandpass. And for large ones it’s the other way around. So if you want to have a new closet at home, then you should take a closer look at FI. About the hump in the middle, see below. And almost the same in JBL SS:
Here the volume of the FI is slightly smaller, but it is still 2 times larger than the given BP. For PSU, the volume is also critical; you can make it larger, thereby reducing the dip in the middle and improving the roll-off at the bottom.
The calculation comes down to selecting the displacement of the cameras and the frequencies to which they are tuned, and checking the frequency response. I think the other three graphs will not bother you :).
The hole in the middle could not be removed at all - such is the speaker :). I didn’t want to increase the volume, I had to fit into the room, by the way, the box is quite big anyway. But I think such a failure can be neglected - after all, a 3dB dip is very small (the graph is simply stretched in height), and if you take into account the uneven frequency response of the speaker itself at 10dB, then you can simply forget about it. In addition, this is still an idealized frequency response, in life everything is much more complicated and confusing :) The dip can be made smaller by narrowing the frequency, but I wanted to extend the frequency response to 200Hz, which was not entirely possible, but it certainly got to 150Hz :). I’ll note right away that the middle is still audible in the subwoofer through a thick hole, so an active filter won’t hurt, which I then did in the amplifier.
The calculation of bass reflexes comes down to setting the internal diameter of the pipe in meters and checking the “Vent mach” value to green, when it turns red it’s bad – there’s too much air flow, i.e. it’s no longer a subwoofer, but a musical instrument of the “trumpet” type. Here you need to look at the length of the bass reflex so that it fits into the box, and preferably up to the middle. For a long time I could not fit the size, because by increasing the diameter, for normal air flow, the length of the phase immediately became immeasurable. The length depends on the diameter and frequency to which the phase is tuned - so you can also play with the frequency. In this case, the frequency response will change, keep this in mind.
At first, a sub was made with identical pipes, fortunately in the upper one, the program shows less air resistance, but after assembling the 100W amplifier and listening, it turned out that after ~50W the effect of air exhaust (popping) began just in the upper compartment (the smallest one, however) ). I had to disassemble everything and cut a large hole for a thick 105mm pipe inside, so I almost fit right into the height of the camera - 2cm remained. Considering that you also need to put a sound absorber there, this is a very small margin. I used plastic sewer pipes. Here I note that there are 70mm pipes, but they are not as big as 50mm and 105mm. As the upper phase increased, the lower phase immediately began to work better. I really don’t recommend using 50mm - for such a fool it’s very small. The result is this: for a sub, the dimensions of the elements are the most important.
Well, the volumes have been determined, it’s time to calculate the box and structure. Since I do 3D modeling as part of my job, I did just that - I took the SolidWorks program and created a 3D model there. If you noticed, 3D graphics are also involved in the site design :) The program itself calculated the volume for me. It’s difficult to do this yourself accurately, since all the connecting bars in the structure eat up a decent amount of space, and the design had to be invented and changed on the fly. Another problem was the material and its thickness, and the program immediately made it possible to see the dimensions of each element, taking into account, precisely, the thickness of the sheet and the connection with each other, i.e. I automatically got the dimensions of each element.
Separately about the material. Well, I didn’t even think about MDF, although of course the best option. The task was to find 20-22 mm chipboard, but it turned out to be practically impossible. The most common chipboard is 16mm or 18mm laminated Polish. 16mm is not enough, and laminated on both sides is probably bad for sound, and it also turns out to be expensive. Then I realized how right I was by not buying laminated chipboard. And it took about 1.5 weeks to find a regular 18mm one. Those that were in stores are impossible to take home, because the sheet is very large. I have already called all the offices and visited all the markets. My nerves were starting to give in - I almost regretted that I contacted the sub, because everything except the box material had already been purchased. I started thinking about how to glue 16mm chipboard and 4mm plywood, but I stubbornly didn’t want to glue it - this requires special glue and a strong press. And then a friend called and asked for help to bring cement to his home. So, while we were running around looking for cement and its owners throughout the industrial zone, we accidentally came across an office selling and sawing that very notorious laminated Polish chipboard. They used 22mm chipboard sheets as spacers in the racks. But the owner was not there and we had to wait... After looking again for the owners of the cement and not finding them :) We returned to the chipboard again. The owner did not give me the 22mm, citing the fact that it was difficult to pull out, and they seemed to have already sagged (in short, I was too lazy to pull out) and offered me 28mm. This is cool, I thought, and refused, looking at these thick sheets. And then he offered me 18mm, simple sanded chipboard - it turns out that it is used as transport sheets for laminated (top and bottom). So, all the good stuff, including sawing on an imported machine, cost me $5 (cutting $0.3/m). You can’t cut it like this in life - evenly, exactly to size. Draw your own conclusions...
Yes, what am I talking about? Oh yes - about the box. Let's see what happened:
As you guessed, this is a 3D model. The length of the bass reflexes is visible in the screenshots of the program above and is 19 cm at the top and 25 cm at the bottom, the internal diameters are 105 mm and 70 mm, respectively. There is a hole in the back wall for the connector socket. The legs were made of steel spikes at the factory and hardened. I chose the size according to my taste. In the articles I found, people made the spikes 2 times larger, but I didn’t want to put the sub too high so that the spikes wouldn’t be visible, because the design is not low anyway. You need 4 spikes, I tried it on 3 stands - a terribly unstable design. In the columns you can get by with three, since they are not deep and their center of gravity is in the front. I leveled the height of the spikes with a washer; only one was needed, then I checked it on a obviously flat surface (the back wall).
Next, let's look at the stages of assembling the structure...
Assembling the box
I will omit the cutout of the holes in the chipboard, we will assume that they are cut out. I cut everything out with a jigsaw. Without him, I probably would have had a pipe. They are also great at cutting timber, but one thing is bad - it buzzes incredibly.
Stage 1
Stage 2
Stage 3
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We glue the bass reflexes to the front wall. I glued them with liquid metal - such a solid multi-component garbage, it smells like epoxy. She kneads it with her hands, it turns out like plasticine - soft and terribly sticky. It hardens almost instantly as soon as it cools down, and if it doesn’t cool down, it still hardens after 5-10 minutes, so I then removed it from my hands with pumice stone along with the skin :) I made a thickening along the entire radius at the base, for reliable fastening - they still stick out far. In addition, I personally was not able to perfectly accurately cut such a hole with a jigsaw, and these unevenness were miraculously sealed with liquid metal. I wrapped the bass reflexes with window paper tape in 5-7 layers, smoothing everything very tightly. Then I covered them with linoleum with insulation. The thickness of the phases turned out to be about 7 mm. |
Stage 5
We install the speaker on the sealant, fasten it with bolts (I don’t remember the size). We coat it well in a circle - you can see the white sealant in the photo. Solder the wires. I attached a block with connectors to the back wall.
Stage 6
Now all this needs to be covered with a sound absorber. There are many material options. I used foamed linoleum with insulation, and then, in the end, thick padding polyester ~2.5-4 cm, which, in addition to pasting, was also fluffed in a large chamber. The linoleum also needs to be glued to the top and back walls. Can be pasted over in several layers. Thick batting is a good thing, but I couldn’t get it.
Stage 7
We put the top cover and the back cover on the sealant, without glue - then you may have to remove them. I fastened them with furniture ties under an internal hexagon - an excellent thing, it fastens very firmly, the main thing is not to tear it off if you tighten it with a drill, like I do. I attached it at the end, here you have to be careful with the chipboard - it can delaminate. For ties, first drill one long hole, and then enlarge the screwed hole with a larger drill. Two ties are at the top in the front wall, the rest are on top of the lid, and the back one is just around the entire perimeter and in the center. You can see how it is attracted by the protruding sealant, but it is attracted well.
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In the photographs there is still the first option, with a small upper bass reflex and padding polyester not yet pasted over. Later I covered the bars with linoleum too. This was, so to speak, the first test assembly, in order to listen to whether it would sound or not. Sounded. But so far I only had a 25W amplifier, it was fine for now.... Then I gave up the whole thing and started assembling a 100W amplifier, I had to test the design to the maximum possible. And it was not for nothing that I was afraid of a more powerful amplifier - then it turned out that at maximum the upper phase squelched, but the case was sealed the first time - it was not for nothing that I thoroughly sealed all the seams and corners with sealant. I had to disassemble the entire structure, cut out a larger hole and install a thicker pipe. Here I have already completely “insulated” everything with padding polyester. Inside it became soft, white and warm like a snowdrift. What can I say, it sounded quite good - smooth, soft bass. At maximum volume of the amplifier, the speaker begins to jam, but we must take into account that its nominal value is 50W. It's enough for an apartment higher than the roof, especially since it shouldn't be heard in music, just when you cut it - as volume is added to the music and such a solid low sound. In short - good work...
I plan to cover the entire structure with wood-look film. But first, you need to putty and sand the edges - no matter how accurately I didn’t cut it on a machine, no matter how I tried to carefully assemble everything - there is still a total error of 0.5-1mm. That's why laminated chipboard is still bad, because you have to immediately glue the tape to the end - in short, it will be crooked.
From time to time, bandpass subwoofers come into our field of vision. Otherwise - bandpasses...
Most often, it should be noted, in the form of finished products. Much less often - as an element of a system built according to an individual project. The basics (the very basics) of the theory and practice of this type of design were published in our country a long time ago, about 15 years ago. There is an opinion (from the author) that it would be necessary to clarify something in this matter, or else, you see, and remind, not forgetting about the “newly discovered circumstances...”
1. Relatively narrow frequency range.
2. Increased complexity of manufacturing and configuration.
We see more advantages.
1. Availability of an internal low-pass filter.
2. Ability to reproduce low bass.
3. Acoustic amplification.
4. The speaker is protected from external influences.
The first minus and the first plus are two sides of the same coin, which confirm the well-known wisdom that every thing is good in its place. That is, the bandpass is of little use for the role of a broadband bass radiator (in a three-way acoustic system, say), while in the position of a subwoofer it would be very useful. The second drawback was a factor that significantly limited the spread of band-pass designs. Moreover, the problem is not even in manufacturing; after all, the difference between PP and FI is only in the presence of an additional inner wall. The difficulty is that during the fine-tuning process, one of the walls will probably have to be moved, changing the “front” volume. In a single production this is indeed inconvenient, but when preparing a series (even a small one) - what is the problem?
Now about the positives. As you can imagine, the presence of a natural low-pass filter was responsible for some of the popularity of bandpasses in the past, when passive filters were considered a natural way to separate frequency bands. A passive low-pass filter, tuned to a frequency typical for subwoofers, involves the use of large and expensive coils. As we moved towards dedicated bass amplifiers and active filters, this factor became irrelevant. However, considering that in our industry most truly musical amplifiers are equipped with second-order filters, the presence of an additional (acoustic) low-pass filter will be very useful.
The ability to reproduce low bass is a good thing, but not for us. Let's say, a subwoofer for a home theater in a home theater can play bass from 50 Hz, in a design with FI - from 32, and in a PP design - from 25, and if you sacrifice the uniformity of the frequency response, you can reduce the frequency limit to 20 Hz. However, in our industry there seem to be no problems with low bass; the subwoofer in the ZYA plays (in theory) from 0 Hz. We have other problems - with the uniformity of the mid-bass, since the transition from free field to compression mode occurs at 55 - 70 Hz (-3 dB rise frequency). All subwoofers with FI provide a rise, starting from this frequency and downwards; a SF, tuned exactly to the transition frequency, will give a straight-line response, but, naturally, without any rise. With all other settings, the VZ will give either a rise in the mid-bass region of 70 - 50 Hz, or, even worse, a decline at the same frequencies.
So, a fourth-order bandpass (we will only talk about it below; we will avoid unnecessary complications) can give a rise exactly where it is needed, near the upper limit of its operating band, and in salon conditions maintain an attractive shape of the frequency response. I think this alone is enough to take a closer look at it. Well, the last of the above advantages is also worth something.
So, let's start taking a closer look. Let's take some unnamed speaker, we are not interested in its characteristics for the time being. For now, we are building the characteristics in a free field, it’s more clear. Let's see how the frequency response changes depending on the size of the “rear” (that is, closed) volume.
Rice. 1. Change in characteristics depending on the size of the closed volume
Rice. 2. Change in characteristics depending on the size of the front volume
Rice. 3. Change in characteristics depending on the air mass inside the port
Rice. 4. Variation of characteristics depending on the bandwidth
As you can see, even when the volume changes more than three times (by 10 dB), the bandwidth changes insignificantly. However, as the volume decreases, the rise in the characteristic shifts to the lower limit of the passband. But we don't need this. Therefore, the guideline will be simple, the first volume should correspond to a ground cell with a Butterworth quality factor or slightly lower, say, from 0.62 to 0.72 (green curve on the graph). To avoid bulky boxes, we must immediately focus on heads with a quality factor at least 25% lower than this indicator, that is, no higher than 0.47 - 0.5.
Now notice that all curves pass through some point. This is the resonance frequency of the port and the front volume, we will call it the center frequency. In this case, it is 38 Hz, this choice is typical for a home subwoofer; in our field, the center frequency must be chosen significantly higher.
The upper frequency limit most depends on the size of the second (front) volume.
By reducing this volume, we extend the frequency range upward, while simultaneously obtaining a boost in the characteristic. The shape of the high-frequency part of the characteristic is due to the resonance of the bass reflex with joint elasticity due to the rigidity of the suspension, the elasticity of the rear air and the elasticity of the front volume air. A 3 dB boost (orange curve) is obtained when the high resonance frequency is an octave higher than the center frequency. The air mass inside the port affects the lower frequency limit and at the same time the shape of the characteristic.
The performance in this area is due to the resonance of the combined air mass inside the port and diffuser with the joint elasticity of the suspension and rear volume. Luckily, we don’t need a lot of mass; it’s enough to move the lower resonance 1/3 octave away from the center frequency (brown curve). To understand the nature of the bandbass, it will be useful to see how the shape of the characteristic will change if we maintain the port setting, but at the same time change both the mass of air inside it and the volume of the front chamber.
The graphs are shown in an idealized form, which is obtained without taking into account leakage (as well as non-rigidity) of the housing. If this is taken into account, the central part of the graphs will never reach the 0 dB level. However, in the theory available today, losses are not taken into account.
Now let's move on to practice. Let's select two heads from test No. 2 for this year. Their parameters meet the Qts criterion<0,5. Как показало моделирование, чтобы избежать провала характеристики на суббасовых частотах, надо, чтобы частота резонанса головки в ЗЯ (заднем объёме) была не выше 50 Гц. Потому добавляем ещё один критерий: Fs <35 Гц. Вот что мы выбрали:
Head No. 1. Fs = 31.6 Hz; Vas = 29.3 l; Qts = 0.480.
Head No. 2. Fs = 33.4 Hz; Vas = 22.0 l; Qts = 0.454.
To calculate the PP of fourth-order design, you can use many of the ready-made programs: not all of them give the expected results, but the old JBL Speakershop (aka Bass Box) in this case gives quite correct ones. If, as already mentioned, we ignore possible housing leaks. But for meaningful calculations it is advisable to have some starting points. For example, for the first head, first calculate the volume of the cell in which the Butterworth (or slightly lower) quality factor is achieved. For Butterworth we get 24.7 liters. If, after installing the head in such a box, we measure the impedance characteristic, it will look something like in Fig. 5:
Rice. 5. Characteristics of the impedance of head No. 1 in the GZ
Now we need to determine the anterior volume. A good starting point would be 1/3 of the rear, in this case 8.2L. As a result of the simulation, the refined value of the front volume is 8.06 liters, which is almost the same. During the calculation, we obtain a free-field characteristic as shown in Fig. 6. The graph shows the approximate position of the port resonance frequency.
Rice. 6. Characteristics of the PP subwoofer in a free field
However, we are much more interested in the characteristics of the subwoofer in a car showroom environment. If we superimpose the standard “AutoSound” transfer function onto the previous graph (it was published in its entirety in No. 8/2000), we get the picture depicted in Fig. 7.
Rice. 7. Characteristics of the PP subwoofer in a typical cabin. Head No. 1, cutoff frequency 100 Hz
As you can see, the maximum amplitude spread over the entire operating range from infrasound to 95 Hz is less than 2 dB. A similar characteristic can be obtained using the SG, but on the bandpass side there is also a constant acoustic gain of 3 dB plus an effective roll-off slope of 18 dB/oct on top.
To complete the picture, you can look at Fig. 8 impedance characteristic. The port's tuning frequency is 64.3 Hz, the lower resonance is tuned an octave lower, the upper one about 2/3 of an octave higher.
Rice. 8. Characteristics of the impedance of the subwoofer PP. Head No. 1, cutoff frequency 100 Hz
What happens if we make a mistake with the volume of the front drawer during manufacturing? Say, by 10% (Fig. 9)?
Rice. 9. Front drawer volume detuning by ±10%
Nothing particularly scary. The upper frequency limit changes by 5%, but the shape of the frequency response does not change radically. But it is not recommended to make a big mistake in realizing the air mass inside the port; the consequences are shown in Fig. 10.
Rice. 10. Port mass detuning by ±10%
A deviation of the air mass inside the port by 10% in each direction (that is, a 5% change in the tuning frequency) leads to a deviation of the frequency response by 1 dB in one direction or the other.
With head No. 2, although not quite identical, a similar set of characteristics is achieved. In this case, we set the first volume to a quality factor of 0.66; for this you will need a box (part of a box) with a volume of 21 liters. The resonance frequency in such a cell would be equal to 48.8 Hz. As a result of modeling, the front volume was chosen to be 7.3 liters. The calculated frequency response is shown in Fig. eleven.
Rice. 11. Characteristics of the PP subwoofer in a typical cabin. Head No. 2, cutoff frequency 100 Hz
Unevenness over the entire range is no more than 2 dB, the upper limit frequency is about 95 Hz. The slope of the decline is the same 18 dB/oct. If we now turn to the impedance characteristic (Fig. 12a), it will be easy to see that the characteristic frequencies here are almost the same as in the case of head No. 1.
Rice. 12. Impedance characteristics of the PP subwoofer. Head No. 2: a) cut-off frequency 100 Hz; b) cut-off frequency 80 Hz
If we need to work in a narrower band, the settings will change in a predictable way (Fig. 12b). To get an upper frequency limit of 80 Hz instead of 100 Hz, we will need to lower the center frequency and the upper resonance frequency by about 10%. In this case, the frequency response takes the form as in Fig. 13.
Rice. 13. Characteristics of the PP subwoofer in a typical cabin. Head No. 2, cutoff frequency 80 Hz
The unevenness increased to 3 dB, although unlike FI, which tend to focus on the lower bass, in the PP design the decline begins around 30 Hz, where, in fact, the informative bass ends. True, in order to narrow the strip at the top, the front volume had to be increased to almost 10 liters. Paradoxically, in our conditions, bandpass design works better over a wider band.
There's just a little bit left to add. That the bandpass, for all its apparent “technocraticism,” emits like a closed box, and therefore its transient characteristics are more interesting than those of the FI. For the same reason, its bass (properly tuned) turns out to be more collected, which is important for SQ installations. Finally, in terms of total volume, the bandpass does not lose that much to phasics; we can say that the amount of loss is equal to the value of the front volume. This is the price for uniform acoustic amplification, in other words, for increased efficiency.
From the history of the bandpass
To avoid the impression that bandpasses were just invented, here are two illustrations with dates. The first is a US patent issued in the name of a man with the noble French surname d'Alton. He came up with a bandpass as a narrowband emitter, hoping to assemble broadband acoustic systems of great efficiency from many of them, tuned to different frequencies, in the manner of a church organ.
The idea is outlandish and, as history has shown, of little use, but the nobleman understood both the device and, as far as can be judged from the lonely graph, the characteristics, in principle, correctly.
Twenty years later, when the speaker (that is, the direct radiation electrodynamic loudspeaker) finally replaced other means of sound emission, another patent appeared. It already states in plain text that this is a way to expand the frequency band downward using a separate, relatively narrow-band acoustic unit. That is, exactly what we today call the word “subwoofer”.
Moreover, Henry Lang's explanations already include sound pressure curves very similar to those discussed in today's publication. “The process began” from that moment.
Subwoofer bandpass
On this page we will analyze such acoustic design as a bandpass or a bandpass loudspeaker. This is perhaps the most complex type of subwoofer design; it consists of two chambers, between which a speaker is mounted.
Peculiarities
The main feature of the bandpass is that it has a natural low-pass filter, which made it popular in the past when “natural” filters were the usual way to separate frequency bands. But after the advent and spread of specialized bass amplifiers with active filters, this factor became insignificant. In addition to their features, bandpasses also have bright advantages, which is why they are used in car audio, although not as often compared to other types of design.
Pros:
- High quality low bass
- Efficiency higher than that of ZYa and FI
- The subwoofer is protected from external influences
Minuses:
- Relatively narrow frequency range
- Complexity of calculation and manufacturing
- The required volume is greater than that of ZY and FI
Bandpasses are found in the 4th and 6th orders. The order is the slope of the filter.
n - filter order
A body with two cameras, one of which is essentially a closed box, and the second is a bass reflex. It is the 4th order bandpass that is most often used in audio systems. The calculation consists of selecting the volumes of the chambers, their ratio and determining the area and length of the port.
A 6th order bandpass is a box with two bass reflexes. Therefore, the definition of the second port, as well as the ratio of the settings of both bass reflexes, is added to the previous calculations. Such a box has 2 main types: type 1: the port goes out from both chambers; view-2 the port of one camera goes into another.
Bandpass 6th order. View-1.
Bandpass 6th order. View-2.
Features of choice
In principle, the complexity of manufacturing is not as terrible as it seems at first glance; problems will be caused by many variables in construction. These are the volumes of the cameras separately, their ratio, port settings, the ratio of the settings of two ports. As you can see, there are a lot of them, add to this the fact that calculations do not always agree with practice and you will have to make adjustments to the body.
However, such acoustic design is often found at car audio competitions, and in the sound pressure format it is not at all limited to the box—the role of one of the cameras can be played by the car interior.
I would like to say right away that this is not the first subwoofer on this speaker. Before the BP (Bandpass) of the 4th order, there was a closed box and a FI (bass reflex). Looking ahead, I will say that a 4th order power supply on this speaker, with slightly larger dimensions, sounds much better than one designed in a ZYA (closed box) or FI, despite the speaker’s parameters being “not suitable” for this. Speaking of parameters, I measured them and for a specific speaker I got the following values:
Qts = 0.84
Vas = 34 l
Fs = 34 Hz
Re = 3.6 Ohm
Le = 1.32 H
Xmax = 7.5 mm
Z = 4 Ohm
Qms = 8.27
Qes = 0.93
SPL = 87 db
Pe = 150 W
BL = 7.6
Dia = 0.17 m
Sd = 242 m^2
As you can see from the parameters, the speaker is not at all suitable not only for BP, but also not even suitable for FI. In fact, everything is not so sad. By substituting the parameters into the program for calculating the subwoofer enclosure (I used WinISD), I selected the optimal acoustic design configuration for this speaker. When calculating, I tried to get the flatst possible frequency response, with minimal housing dimensions, as well as an acceptable frequency range. In the end I succeeded in all this. Estimated frequency response of the received registration:
Frequency range at -3dB level........ 30 - 90 Hz.
Which is more than enough for good music, considering that the lowest note on the bass has a frequency of 31.5 Hz.
After receiving the desired calculated values, I started designing the housing and received the following drawing:
Case dimensions - 407 x 460 x 407 mm
Front camera volume - 22.7 liters (~20 liters minus the speaker and FI pipe)
Rear camera volume - 30.6 liters (~ 26 liters excluding speaker)
FI port tuning frequency - 53 Hz
After all the calculations and design of the body, I started building the box itself.
The first thing I did was cut blanks for all the panels of the body.
For the body material, I used 21 mm chipboard from used furniture.
Assembly begins with gluing the future front and rear walls, and then the side wall, on which the terminal with input terminals will be located.
I didn’t have solid pieces of chipboard for the side and central walls, so I had to glue the panel together from two pieces. The halves are hermetically glued and the joints are generously coated with sealant.
I generously coated the joints between the panels with acrylic sealant.
I cut the corners from 30x30 timber, sawing it lengthwise at an angle of 45 degrees.
The next step was to mark and cut out a mounting hole for the speaker, bass fan port and input terminal.
The diameter of the hole for the speaker is 233 mm.
I check the accuracy of the resulting hole with a small piece of pipe similar to the one that will be the bass fan pipe in the future.
The outer diameter of the pipe is 88 mm, the inner diameter is 74 mm. Pipe length - 165 mm.
After cutting out the hole for the speaker, the panel is glued into the body. The corners are installed, the joints are coated with sealant.
Sound-absorbing material - batting - is glued to all internal flat surfaces.
When all surfaces are covered with batting, the speaker is installed and securely secured using self-tapping screws.
After installing the speaker, wires are pulled from the speaker itself to the future location of the input terminal. Wire - PVA cores 1mm2.
The next step was to glue in the bass fan pipe. The joint between the pipe and the body was generously coated with acrylic sealant. The outside of the pipe was also covered with batting.
After making sure that the speaker was securely fastened, I started gluing the second side wall.
I filled each of the chambers of the box halfway with padding polyester.
The box is ready to be covered with a lid.
I also glued batting to the inside of the lid.
Along the perimeter of the box, as well as on the end of the central panel (on which the speaker is attached), I liberally applied glue so that after pressing the lid, no gaps would form between it and the other panels. In addition to the glue, I secured the lid with long self-tapping screws.
All sides of the box were first sanded using a random orbital sander using a 40 grit abrasive wheel, then a 120 grit abrasive wheel.
After this, all sides were puttied. They sanded it again, putty it again, and so on. Grinding was carried out using abrasive wheels with grits of 120, 180, 220 and 400. With each new layer of putty, the abrasive grain size decreased. And so on until the sides of the box are perfectly smooth.
After the sanding was completed, I began painting the box.
The box was painted using rollers. Rollers with a diameter of 20 mm and a width of 100 mm. Paint for decorative coatings, red, manufacturer MAV.
5 layers of paint were applied to each of the walls of the building. Before the last coat, I sanded the previous layers of paint with a 400 grit wheel.
And in the end, this is the subwoofer that came out:
What happened in the end? The result is a beautiful and good subwoofer, which I am very pleased with. The sub sounds very pleasant, soft, without buzzing and at the same time quite assertive and powerful. It’s also nice that the program didn’t cheat with the calculations - by ear, the subwoofer’s AHF turned out exactly the way I saw it in the program. I recommend this program for everyone to use.