| tda |
I have a big question.
How big can be the volume of the box for a given Vas of the driver? Nobody seems to treat this subject on the forum. Does it matters if the box is sealed or bass-reflex?
Are there any clear formulas or just an rule of thumb? |
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| tda |
My problem is with a ported box.
The Vas of the driver is 4,4 l and Sd=65 cm^2.The box volume is 27 l
In any simulation that I made it (WinISD, MJK MathCAD sheets), The Fb of the box should roll off around 55 Hz...But my FR plots show that actually it rolls off at 85 Hz (very close to the Fs of the driver). So I'm questioning myself if the problem is not the Volume Box of the driver, 6 times bigger than Vas. Why in any simulation that I made this problem is not appearing? |
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| SY |
Let's try to straighten out some terminology and a few realities:
Fb is a box tuning frequency. It is a single number, hence can't "roll off." You can determine it by looking at the impedance curve.
The box tuning frequency does NOT necessarily correspond to the box/driver's rolloff frequency except in the special case of a discrete alignment.
With a tiny Vas like 4 liters, actual boxes will not have the effective volume you think they have, especially once you mount a driver and a port into the box. Presumably, you'll want to add some damping materials on the walls so that driver reflections don't come back through the cone (a real problem in small boxes), and that will affect both the effective volume and the box Q.
Driver parameters are not generally the same as they are on spec sheets. Vas in particular can vary wildly, even with small changes in temperature.
Trying to get a driver with such a tiny Vas to produce bass much below 100Hz is going to be an exercise in frustration. |
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| tda |
SY,
You are right about last thing:
<Trying to get a driver with such a tiny Vas to produce bass much below 100Hz is going to be an exercise in frustration.
My common sense tells me the same think. I know the box is just too big and probably is impossible to reach 55 Hz with this small driver. BUT I want an answer from the mathematical and physically point of view. I want to know why in every simulation that I made it the port response looks very good, when actually should be almost negligible???
I want to know if is there any rule for Ported Box regarding VAS and volume of the enclosure. For example: “You cannot put a driver in a ported box with VAS smaller than half of the enclosure volume”
You see, this questions really frustrates me for the last week J |
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| SY |
"An analogy is not an equality."
Because simulations assume the correctness of the assumptions. Let's take an example: shape the cabinet so that it's really, really thin and really, really tall. The simulation will show no difference from a box with normal proportions. Measure something like that in reality and you'll find that the model is not predictive.
In this case, the cabinet is effectively infinite in size. The driver can't effectively drive the slug of air in the port- it's like trying to push a golf ball uphill using a soggy piece of spaghetti. The assumptions in the Thiel-Small model (note that last word!) are not met. |
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| kelticwizard |
Well, I cannot tell you precisely how big an enclosure has to be compared to Vas of the speaker before the rule book gets thrown out.
But I can tell you why nobody has bothered to investigate much, if that puts your mind at ease.
If you take a look at the Thiele-Small charts, you will see that there really are not any useful alignments which require much over Vas times 2. After that, distortion takes over.
The famous audio writer David Weems tried this out. He took a speaker with the Qts of 0.6, put it in a box over 2 times Vas, and pronounced the distortion too high to be useful. Thiele-Small were being generous when they put their last useful alignment at Qts=0.57.
You see, from a practical standpoint, nobody even wants to build a reflex box with a volume much over Vas times 2, in fact, most people don't even want to go that big in the first place.
The larger the box volume compared to Vas, the higher the distortion and the worse the transient response. So somewhere above Vas times 2, the Thiele-Small parameters mean progressively less and less until they don't matter at all. Few have bothered to investigate, or at least publish their findings, because if you want to build a good reflex, you aren't going anywhere near those enclosure sizes anyway.
Perhaps you can add blocks of wood and bricks to your enclosure to reduce volume and let us know where the Thiele-Small parameters kick in. It might be fun, and you already have built the test enclosure. :)
Below is a graph for the step responses for various Thiele-Small alignments. The sloppier the response, the worse the distortion.
I took this chart from Small's article on vented boxes. He has charts that deal with K and B in his article, so I took it from there. All the colored printing is my addition. It was unclear to me what the Keibs and SC4 alignments were, so I added no additional info. However, four of the charts I added the Vas÷Vb ratio, and the F3÷Fs ratio.
a=(Vas÷Vb)
Note the response gets smoother and less distorted the smaller Vb is compared to Vas, and the higher F3 is compared to Fs. Of course, this is the precise opposite to the way you tried to build your box. |
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| roddyama |
It seems to me (being at work and not having any of my resources at hand) that as the box gets bigger (compared to VAS) you start to "grow" into an Infinite Baffle design, which will be akin to the open baffles that been all over the forum for the past months. The drivers response will start to resemble that of the driver in free air (fc=fs).
I'm not quite sure what distortion your speaking of KW, but I would think that the level of distortion will depend a lot on the quality of the driver. If it were strictly an alignment issue then Bozak and Linkwitz were mistaken in some of their efforts. |
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| kelticwizard |
Roddy:
The distortion that I am sure Weems was talking about was the "overshoot" and "ringing" which the charts indicate occurs the greater the box size is compared to Vas. |
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| sreten |
You also need to quote the Fs and Qt of the driver to allow analysis.
One of the major issue of commercial speakers is volume effiency.
What can you get out of a box of a given size.
Normally the Box size is much less than Vas and this dominates,
the real issue being the Qt at the open air resonant frequency.
Using extreme alignments the following is generally true :
going beyond Box volume = Vas is generally not worth it,
the driver suspension begins to dominate.
Once box volume = 2 Vas then there hardly any point going any
larger, a 2 Vas box will give uncontrollable reflex alignments.
E.G. for a Tb driver Vas=3 litre, Fs = 81 Hz and Qt =0.49,
reflex alignments above 6 litres are impossible without
the reflex peaking.
:) sreten. |
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| tda |
Ok,
The thinks are pretty clear to me. Yestarday I put some bricks inside the box, and the graphs start to look good when I reach Vb=2.5 * Vas. Of course the roll off start at 85 Hz in the simulations, just like in the FR measured graph. I was searching for a couple of hours on the Internet to find some subwoofers with the Vas smaller the ½ Vb, but I didn’t find any. Most of them have the Vas LARGER than Vb.
So, in conclusion, now I’m sure that is impossible to reach 55 Hz with such a small driver;
And second, EVERY TIME I START BUILDING A ENCLOSURE I HAVE TO CHECK IF VB IS MAXIMUM TWICE THE VAS OF THE DRIVER.
Thank you very much for your patience and for you very helpful replies. |
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| tda |
I researched a little bit more on the Internet and on Bob Speaker Stuff (http://www.geocities.com/rbrines1/) I found a project called Peerless Pipe. Well, I calculate the volume of the TL enclosure, and guess what: Vb=70 l, Vas of the Peerless Driver=15 l…. so Vb is 4 times bigger than Vas. This project is very well known in TL diy world, and is based on MKJ MathCAD sheets. Also the measured FR plots looks very good, just like in the simulations. So, where is the theory that says nobody wanted to build something bigger than twice of the Vas?
I think in most of the cases of TL enclosure, Vb is bigger than Vas…
Maybe the actual problem is the fs of the driver…maybe it’s impossible to achieve an fb much more lower than fs.
I’m in trouble again…J |
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| Ouroboros |
But the Peerless pipe is a quarter-wave transmission line, which is a very different beast to a reflex enclosure.
I use Winspeakerz to analyse sealed-box and Reflex speakers, but a transmission line is difficult to analyse. TL speakers seem to sound better than the simulations would suggest! |
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| SY |
| A TL enclosure is a totally different animal and lies outside of what Thiel-Small models can predict. T-S handles sealed and ported boxes. |
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| roddyama |
| quote: | Originally posted by tda
I researched a little bit more on the Internet and on Bob Speaker Stuff (http://www.geocities.com/rbrines1/) I found a project called Peerless Pipe. Well, I calculate the volume of the TL enclosure, and guess what: Vb=70 l, Vas of the Peerless Driver=15 l…. so Vb is 4 times bigger than Vas. This project is very well known in TL diy world, and is based on MKJ MathCAD sheets. Also the measured FR plots looks very good, just like in the simulations. So, where is the theory that says nobody wanted to build something bigger than twice of the Vas?
I think in most of the cases of TL enclosure, Vb is bigger than Vas…
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The VAS to box volume ratio limitation holds for closed and ported (and PR) designs. As I pointed out in my earlier posts, this ratio limitation doesn't hold for infinite baffle and open baffle designs. It also doesn't hold for TL designs, although it will effect the FR as the box gets smaller. TL designs depend more on the wavelength of the tuning frequency than on the volume of the air in the box.
You should provide some more information about your driver. |
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| tda |
Yes,
I know all this things very well, my problem started when I tried to simulate on Ported Box MathCad sheet an enclosure for my small driver. The result where disastrous so I'm beginning to ask myself if I know how to use this MathCAD sheet. I used Ported Box sheet because MJK told me that is the most advanced and complete sheet. So, the simulations was for a quarter wave system (I used MJK sheets) but the mesured results indicates a classic ported box system where the Vb/Vas problem ocurred. |
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| MJK |
Hello tda,
I have been thinking about the question of driver Vas and enclosure Vb. The driver Vas is a constant and known from measurements. If we look at the method used to evaluate Vas, the formaula Vas = Vb/(rho x c^2), then we see that an inherent assumption is made that the pressure is constant everywhere in the enclosure. If standing waves exist then the expression for enclosure impedance becomes very complicated and exhibits many peaks and nulls associated with the resonances. I don't think that there is any magic ratio of Vb/Vas that will tell you that your simulation is inaccurate. But if your internal dimensions are big enough to support standing waves, then I think that the simple expression for Vas is no longer applicable and the simulations will be inaccurate.
The Peerless TL referenced above has a large Vb compared to the Vas, and one very long dimension. The MathCad worksheet takes the long dimension into account and predicts and accounts for the standing waves in the system response. The response is accurately predicted and correlates well with the computer model. The MathCad worksheets can accurately handle one long dimension. But if several long dimensions exist in the design, the MathCad worksheet will not be accurate. The MathCad worksheets take you one step further then then the lumped parameter models like WINISD. A true 3D representation of the enclosure is the next level of calculation.
If you really want to see if your enclosure is behaving like the computer model predicts, then I recommend you remove all fiber stuffing and measure the impedance magnitude and phase, as a functions of frequency, for the driver in a baffle and the driver mounted in the enclosure. These curves will tell you how accurate the computer models are and show all system resonances. If there are unpredicted resonances they should be obvious. The sorce of the resoance can be determined and then corrective actions can be tried in the enclosure.
Hope that helps, |
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| tda |
Hello MJK,
If I understood you right, the MathCAD sheets are good as long as the So and Sl are small (close to the Sd)...so the enclosure is very tall and thin. If the enclosure starts to look like a cube for example, the results are not predictable. Am I right? My test enclosure is almost a cube, so all 3 dimensions are almost equal...Could be this the reason of my problem?
And another general question: You think that finally all the enclosure (ported, TL, closed, horns, etc) is just special cases and all of them should be theoretically simulated with just one set of formulas, no matters the enclosure shape, volume, etc? |
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| MJK |
Hi tda
If we look at a TL that is basically a square enclosure with the bottom end open then the length of the TL is equal to the width and depth of the cross-section. Along the length you get a quarter wavelength standing wave at frequency fo. From side to side and from front to back you will have half wavelength standing waves of frequency 2 x fo. These frequencies are probably too close for the enclosure to act as a TL and there will be destructive interference between the modes, this seems to be what your test data is telling us. If I have to guess about the minimum length to width or height ratio, I would probably use a factor of 5 as a minimum. If the classic TL is five times longer then the other two dimensions, then the difference between the frequencies of the fundamental standing waves would be a factor of 10. This is just a guess and should not be taken too seriously, but I hope it makes the point.
So what can be done to "save" your enclosure. Have you tried dividing the cross-sectional area internally with pieces of cardboard. If, looking down from the top, you put two pieces of cardboard inside the enclosure forming a plus sign then you would effectively divide the TL into four parallel TL's each with 1/4 of the cross-sectional area of the original enclosure. You would need to allow communication at the top and bottom of the line so that the boundary conditions at each end of the four TL's would be the same. The next step might be to divide the areas again forming more parallel TL's. If this were done then I think the system performance would start to converge to the simulation's predictions. Sounds like an easy thing to try.
Hope that helps, |
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| aaroncgi |
My Emerald Towers use the same driver as the Peerless Pipe (850122 - 7" CSX). Now, I didn't measure my the T/S on my drivers, but they are of a recent batch, so can only assume they probably have the newer specs, with a low Vas and high Fs. I did some quick and dirty Fs measurements, and they came out to the higher version, at 50Hz. Assuming Vas also jives with the new version, my enclosure is almost three times Vas. It's vented at the bottom. Here's where it gets interesting. My speakers are more or less pipe shaped, with the drivers up at the top, and the length of the pipe being just over five times the greatest width. I wasn't intentionally making a TL design...
What concerns me is all this talk of distortion. I certainly feel I should say this is not what I have experienced. The bass out of my speakers is among the most accurate I've ever heard, and has no problems maintaining control.
I don't have the answers, just wanted to throw out that such a large Vb/Vas ratio isn't always a recipe for disaster. :)
Aaron Gilbert |
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| kelticwizard |
Aaron:
What was the volume of your enclosure and what did you tune it to? Oh, and the vent dimensions?
One more thing. Did you break in your drivers before measuring the Fs? Breaking in drivers tends to lower Fs and increase Vas, I believe. |
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| aaroncgi |
The volume of the enclosure - that's a bit tricky. Do you know how to calculate the area of a D shape cross section? It is a 12" cylinder (11.25" internally) but with the 'front' flattened for a baffle approximately 8.5" wide. It's 62" long, but then subtract 3/4" for the top cap insert. So, if we were to assume it was just an 11" cylinder, that would come out to about 3.36 cubic feet or 95 liters. I figure subtract at least 0.3 cubic feet for the drivers, crossover, port, and extra wood behind the baffle, so let's call it 3 cubic feet or 85 liters. I suppose it could be a bit less, so if anyone knows how to calculate area of a D shape, we could get more exact. :)
In any case, this is the volume (85 liters) I used to tune the enclosure. I initially started with a 3" x 17" port (flared), which WinISD tells me was about 18Hz (I was still thinking the 850122 had an Fs of 37Hz). That sounded a bit too thin, so I cut the center section in half, giving me 11" of port length. This in theory is somewhere around 22Hz. I could probably go down to the minimum length of 5", and expect the sound might not change all that much because the Fb would still be way below Fs. F3 would go down a little and output above 30 Hz would be greater. But, that would push the group delay peak farther up into the audible range.
The drivers were played pretty hard for a couple weeks before I measured Fs, so they should have been broken in. The fact that they measured between 49-52Hz seems to point in that direction, as most notes I have seen put the new batch at around 55Hz Fs. I also noted that the Fs was a few Hz lower at higher signal levels (like 4-6V+).
Of course now, I don't use them full range, because I have my subwoofer, so all that bass is lost. Truth be told though, my new room doesn't have nearly as much gain as my old apartment did, which is bad for my ET's full range performance, but good for my giant subwoofer. :) |
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| Pjotr |
Hi,
Don’t understand the relation Vb needs to be somewhere below 2x Vas. With most speakers I end up with a much smaller BR box than Vas, to over 4 times smaller and rarely over 1x time Vas. Note that Qt also plays a big role here.
Be careful with Vas. If you want to measure Vas yourself then be sure the speaker is run-in fully. Vas reflects the compliance of the speaker and this can vary a lot after running in.
;) |
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| tda |
Hi MJK,
The constructed enclosure was just a test enclosure. I will build a new enclosure with a very large dimension comparing the other two dimensions, so the enclosure will act as a TL. In you reply you mention the word “Classic TL”. All your discussion is referring only to classic TL (TL offset), or also to all your sheets (Ported Box and ML TQWT)? Do you think that in a classic ported box(modeled with WinISD for example) the pressure is equal everywhere inside the box and in a ML TQWT the pressure is not equal? |
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| sreten |
| quote: | Originally posted by Pjotr
Hi,
Don’t understand the relation Vb needs to be somewhere below 2x Vas. With most speakers I end up with a much smaller BR box than Vas, to over 4 times smaller and rarely over 1x time Vas. Note that Qt also plays a big role here.
;) |
Vb does not need to be 2 x Vas at all. Its that there's not much
point going beyond 2 x Vas in trying to extend bass because
the speaker compliance begins to dominate.
At Vb = Vas the compliances are equal and I'd call this a big reflex
box, as you say Qt is usually arranged such that Vb is typically
1/2 Vas but this does depend on the Qt of the driver.
Vb >/= ~ 1.5xVas generally needs very low port tuning to
avoid bass peaking if it is possible at all with the driver.
I'd also say if Vb > Vas then its a rule of thumb an alternative
driver would give better performance with the box volume,
especially in bass SPL capability, as the implication is a bigger
driver could have been used.
;) sreten. |
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| Pjotr |
Ok, thanks Sreten,
That makes it clearer. For BR boxes Vas is not very useful as a selection criterion IMHO. Qt is more suitable. To high a Qt and you end up with a large box indeed and sometimes it is impossible to tune to a decent alignment at all. It is not for nothing Qt of units intended for BR’s is somewhere between 0.2 and 0.3.
For a TL, a Qt of around 0.5 is a more suitable choice.
Cheers ;) |
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| sreten |
| quote: | Originally posted by Pjotr
Ok, thanks Sreten,
That makes it clearer. For BR boxes Vas is not very useful as a selection criterion IMHO. Qt is more suitable. To high a Qt and you end up with a large box indeed and sometimes it is impossible to tune to a decent alignment at all. It is not for nothing Qt of units intended for BR’s is somewhere between 0.2 and 0.3.
For a TL, a Qt of around 0.5 is a more suitable choice.
Cheers ;) |
IMO you have to consider Qt, Vas and Fs because its
simple to make a low Qt and Fs driver by having a high Vas.
What you need is low Fs combined with moderate Vas
and a Qt around 0.3 to 0.35 for good extension.
For a 88dB/W 6.5" driver something like :
30Hz Fs, 25 litre Vas and Qt = 0.35.
I regard an innappropriate Vas as a rejection criteria.
:) sreten. |
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| Pjotr |
Hi Sreten,
Low Fs and a moderate/small Vas are contrary to each other in some way. If a speaker has both its efficiency is unavoidable low then.
Cheers ;) |
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| kelticwizard |
| quote: | Originally posted by Pjotr
Hi,
Don’t understand the relation Vb needs to be somewhere below 2x Vas. With most speakers I end up with a much smaller BR box than Vas, to over 4 times smaller and rarely over 1x time Vas. Note that Qt also plays a big role here.
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I am a little surprised to hear that. A speaker has to have a Qt well over .4 and be tuned below Fs for a box much larger than Vas to make sense.
Thomas W took a HE 15 and put it into an enclosure about twice it's Vas. It was the only way he could get it to have output below the Fs of 25 Hz. He used an equalizer to take care of the dip above the tuning frequency of 18 Hz. That was not the ideal situation, but it was what he was forced to rely on to use a driver with a 1.5" one way excursion to get low bass output out of it.
In most cases, there is a wide range of choices for drivers with Qts of .4 or below. Not that many take drivers with Qts over .5 to use for reflex encloures. As Small has shown here, the transient response gets worse the greater the Vb is compared to the Vas.
http://www.diyaudio.com/forums/show...3547#post303547
If anything, on a few threads here I found myself the only person arguing that a driver with a Qts of .5 can be put into an enclosure larger than it's Vas and be tuned to below it's Fs and produce a perfectly usable box, as long as Vb is not that much larger than Vas. Making a reflex box with the Vb four times the Vas is cetrtainly beyond normal practice-very few would be inclined to do so, considering that there is no Thiele-Small alignment which calls for it.:) |
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| kelticwizard |
| quote: | Originally posted by aaroncgi
... so let's call it 3 cubic feet or 85 liters. I suppose it could be a bit less, so if anyone knows how to calculate area of a D shape, we could get more exact. :)
In any case, this is the volume (85 liters) I used to tune the enclosure. This in theory is somewhere around 22Hz.
....The drivers were played pretty hard for a couple weeks before I measured Fs, so they should have been broken in. The fact that they measured between 49-52Hz seems to point in that direction, as most notes I have seen put the new batch at around 55Hz Fs.
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I did some figuring. Assuming similar Vas/Fs ratios, if your drivers Fs was 50 Hz, the Vas would shrink to about 17 liters. Which would make your enclosure about six times the Vas.
You tuned the enclosure to 22 Hz. Just as a quick experiment, have you considered running a tone of say, 25 Hz or so through your speakers, and see if there is any difference when you cover the port and when you leave it clear? There are freeware online tone generators, such as this one provided by the Taylor family on their home page:
http://www.satsignal.net "Audio Tools" |
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| sreten |
| quote: | Originally posted by Pjotr
Hi Sreten,
Low Fs and a moderate/small Vas are contrary to each other in some way. If a speaker has both its efficiency is unavoidable low then.
Cheers ;) |
Indeed they are - and as you say efficiency is implied to be low.
Once you've decided on your approximate box size and the
amount of bass extension you want you select the driver on
Fs, Qt and Vas parameters - but as you say your effectively
choosing the eficiency of the driver to suit your requirements.
The thing is you need to know the other two parameters to
judge the third, you cannot use any of them in isolation for
driver selection.
:) sreten. |
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| MJK |
Hi tda,
| quote: | | The constructed enclosure was just a test enclosure. I will build a new enclosure with a very large dimension comparing the other two dimensions, so the enclosure will act as a TL. In you reply you mention the word “Classic TL”. All your discussion is referring only to classic TL (TL offset), or also to all your sheets (Ported Box and ML TQWT)? Do you think that in a classic ported box(modeled with WinISD for example) the pressure is equal everywhere inside the box and in a ML TQWT the pressure is not equal? |
My discussion was simplified to a classic TL, not mass loaded by a port, so that the wavelengths of sound are directly related to the lengths. Once you mass load, add a port, then the long dimension becomes much shorter and closer to the width and depth dimensions. I wanted a simple explanation without this additional complication.
When you use a lumped parameter model, like WinISD, you are assuming that the pressure in the box is uniform. That is how the equations were formulated. In reality at some frequency this assumption breaks down and the simulation is very inaccurate. The frequency at which this occurs is related to the dimensions of the enclosure. There are always standing waves in enclosures! The program will not tell you when this happens. The user needs to be aware of the limitations of any computer model and double check that they are not asking for something outside the scope of the assumptions used to derive the equations. Garbage in, garbage out, and you don't get any warning.
My MathCad worksheets extend the lumped parameter model to include one long dimension and the associated standing waves. If your enclosure is dimensioned to be consistent with this type of geometry, the worksheets are accurate. If your dimensions are nearly equal then you have stepped outside the intent of the worksheets. Garbage in and garbage out again. This is true of all my MathCad worksheets.
But there is one situation that will still work, if your enclosure is small enough that you do not get standing waves in any of the three dimensions in the bass frequency range then the MathCad worksheets will simulate a constant pressure in the box and a lumped parameter solution.
Bottom line, whenever you use any computer model you need to have a real good understanding of the assumptions and limitations that are associated with the derivation of the equations used in the simulation. Just because you can enter data and produce a SPL response curve, there is no double check to make sure that the results will be accurate. These programs are only as smart as the user. |
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| kelticwizard |
A) What are the dimensions of the original box? Inside or outside will do, as long as you tell us which, and the thickness of your building material.
B) Tda is using a 4 inch woofer, which naturally has a Vas which is a small fraction of the 27 liter, (almost 1 Ft³), box. What if tda replaced that with a 6½" or even 8" woofer with a Vas nearly 27 liter or even larger? The standing wave situation should not change with the change of woofer size or Vas size, will it?
If tda can dig up a 6½" (160 mm), woofer or 8" (200mm), woofer, stick it in the same or similar sized box and finds that the output is more nearly that of the predicted response, then we have a pretty good indicator that the irregularity is due to the Vas being too small relative to box size rather than any standing wave situation.
On the other hand, if the larger woofer exhibits the same situation as the original woofer and does not seem to follow the predicted output, then we know that it is due to the standing wave situation inside the box.
Remember, this is for a bass reflex box only. Transmission Lines are a whole different story, even though Thiele-Small parameters are used in the calculations.
Might be fun to try. :) |
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| MJK |
kelticwizard,
tda designed and optimized a box based on the properties of his small driver. Granted the size of the dirver and box made the system a very extreme alignment. If he starts switching drivers then he is not working to understand his own design. He is just randomly changing his system hoping for some improvement. Whatever results will not be an optimum combination. Nothing wrong with that but I would be more interested in undersatnding what went wrong in the first place.
I still recommend doing an impedance measurement for the unstuffed cabinet and seeing what standing waves can be seen in this plot. Then I would add the cardboard internal baffles to push the speaker design towards the assumptions in the simulation, only allow standing waves in the "long" direction and see if things improve. |
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| kelticwizard |
tda:
Okay, before you ruin the driver cutout by putting in a 6½" or 8" driver, do it Martin's way to trace the standing waves thru the impedance change. |
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| Pjotr |
Hmm,
You can better have a look at the phase change of the impedance, it is more sensitive. But with proper damping of a BR box, standing waves shouldn’t be a problem.
From Keele’s pocket calculator method to find the optimum box size for a BR:
Vb = 15 * Qt^2.87 * Vas
This will give an alignment without hump or dip.
Tda, if you have a chance get also this book by David Weems:
http://topics.practical.org/browse/Loudspeakers
It is much more structured than Dickason and has a good introduction.
Cheers ;) |
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| aaroncgi |
Kelticwizard,
That is a really awesome signal generator, Audio SweepGen! I have a standalone signal generator at work, but it's accuracy is in question, so I may switch over to this computer based one.
Anyhow, I did your little experiment, blocked off the port, and came up with some interesting and somewhat unexpected results:
- Both versions have strong output down to 30Hz. I find this amazing in a sealed box using a woofer with an Fs of 50Hz!
- The sealed box starts to noticeably lose output below about 40Hz, but very smoothly and gradually all the way down to 20Hz, where it's audible but barely.
- The vented box seems to have a dip from 27-30Hz, a broad peak from 21-26Hz, and still has significant output at 18Hz. It has much more output below 30Hz than the sealed box.
- Woofer excursion in the vented box appears lowest right at 20Hz, so that seems pretty close to expected.
- There is something rattling in my right speaker (suspect a loose crossover nut).
- My subwoofer tune is right where it's supposed to be, at 11-12Hz.
- I won't need an air conditioner in the summer; just set the frequency to 10Hz, crank the volume way up, and attach an elbow to direct air from my subwoofer ports. :cool:
Of course, putting either speaker in a corner seriously helps the low end, and above 30 Hz can either make up or totally skew the difference between them. This would explain why I had so darned much bass in my apartment with these speakers, as both L/R were in a corner. Now, the L speaker is about 10' from either side wall, and the R speaker about 2.5' from the side.
Next week I'll grab my SPL meter from work and bring it home, so I can put some actual numbers to each frequency.
Based on this, it would certainly seem there really is not a huge benefit to be gained from my port except below 30Hz. The sealed response seems a bit smoother, and extension down to 30 Hz should be just fine for most kinds of music. Since I use my subwoofer 99.9% of the time, low end extension is not relevant anyway. I can certainly experiment and make the port shorter, but I suspect what would happen is the peak at the bottom end would grow higher in amplitude and frequency.
I'm still amazed just how much low end there is without the port. Is this a sort of infinite baffle reponse, with a super shallow rolloff below F3 (which may be quite high)?
Cheers,
Aaron Gilbert |
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| MJK |
Aaron,
| quote: | | Based on this, it would certainly seem there really is not a huge benefit to be gained from my port except below 30Hz. The sealed response seems a bit smoother, and extension down to 30 Hz should be just fine for most kinds of music. |
While the bass response of the closed and ported versions sounds like an interesting trade-off, take a look at the driver excursion when you sweep this frequency range. I think that you may find that for the ported box the driver's motion is significantly attenuated at the tuning frequency. This may give you some advantage when you really push the speakers to higher volume in terms of distortion and running out of displacement. On the other hand the ported box will unload the woofer below the tuning frequency but it sounds like this is low enough in frequency to not be a concern, unless you are battling the old turntable's rumble. |
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| aaroncgi |
MJK,
| quote: | | While the bass response of the closed and ported versions sounds like an interesting trade-off, take a look at the driver excursion when you sweep this frequency range. I think that you may find that for the ported box the driver's motion is significantly attenuated at the tuning frequency. This may give you some advantage when you really push the speakers to higher volume in terms of distortion and running out of displacement. On the other hand the ported box will unload the woofer below the tuning frequency but it sounds like this is low enough in frequency to not be a concern, unless you are battling the old turntable's rumble. |
Quite right you are. I did notice the much increased cone excursion with the port sealed. These 7" have a pretty healthy Xmax of 5.5mm though, so with two per speaker, that gives me slightly more cone area than a single 12", albeit still with a low 5.5mm Xmax. :) At the tuning frequency, the ported version's woofers aren't moving much at all, as you would expect, whereas with sealed, they are going crazy and not giving much output to boot.
For normal music it's true, the tuning is low enough that unloading is not a concern. I have some CDs and DVDs that have significant below 20Hz information though, and for that material, the subwoofer gets the nod, as it doesn't start to unload until 10Hz and below. :bigeyes:
All that said, I deal with commercial speakers every day at my job, and four Peerless 850122's vented kick the **** out of a LOT of subwoofers. :) But it's not very often that I see a commercial subwoofer use six cubic feet of enclosure. In fact, I don't know that I have ever seen one of those - yet.
Aaron Gilbert |
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| kelticwizard |
Tda:
Any progress on that woofer in the big box? |
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| tda |
| For the moment I left the big box and I built another enclosure with a 6’’ driver. This time I follow MJK advises, the enclosure 1 m long, the top and bottom area are just 260 cm^2. The result is excellent. The response is almost exactly as the MathCAD sheets predicts it. The volume of the enclosure is around 26 l and the Vas of the driver is 9 l…so is 3 times smaller, but in this kind of enclosure it seems that the rule of VAS for ported and sealed enclosure is not applying. MJK has the answer for the behavior of quarter wave’s designs, and most probably he’s right. |
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| tda |
I just finished all the nearfield and farfiled measurements for my new system. On nearfield measurements everything is corresponding to MathCAD sheets, the results are smooth between 45 Hz and 2000 Hz. In farfield measurements, the baffle step is almost exactly like the Excel Baffles Step Simulator predicts it, so I’m very happy with the results…with only one exception….the maximum power applied to the system is around 8-10 W. If I increase the power, the speaker excursion became large, over XMAS specifications, and the sound at low frequency is distortional.
Of course, I can build a Subwoofer and make a crossover at 100 Hz, but then what is the sense of the Transmission line? I can put my 6’’ driver in closed box instead of a TL large enclosure…I’m curious if anybody faced the same problem. I read a MJK reply on this forum and he doesn’t seem to be concerned about cone excursion. He used in one of his project 6’’ ½ drivers… I wonder if he faced the cone excursion problem. |
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| MJK |
| quote: | | I read a MJK reply on this forum and he doesn’t seem to be concerned about cone excursion. He used in one of his project 6’’ ½ drivers… I wonder if he faced the cone excursion problem. |
Hi tda,
I have not really had any cone excursion problems. This might be a function of the music I like and the volume level it is played at. I listen to acoustic music, almost exclusively old mainstream jazz. I have pushed some of my speakers very hard on occasion but have not noticed any real degradation of the sound. If I watch the driver I do not see large excursions.
Somewhere in my CD collection I have the Telarc 1812 disc which I have used on occasion when a visitor wants a real thrill. I used to have a big Focal 3 way speaker and did manage to bottom the ported 10" woofers very hard a couple of times. I put that disc away and don't play it through my Lowther or Fostex systems.
Maybe your reproduction needs are more severe then mine, |
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