mini/micro subwoofer thread

[gene_stl]

BL Product, is the product of B the magnetic strength of the motors' magnet , and the Length of voice coil in the magnetic gap.
It is related to the efficiency of the magnetic motor. All other things being equal more BL product is better than less and more magnetic motors (ie drivers) are better than fewer.

 

[Wolf]

^So the passive radiator is 5 1/4" I'm guessing by it's looks, the driver is also 5 1/4"


Cambridge Audio is yet another company that makes a couple of mini subs although their 6 1/2" Minx sub doesn't go any deeper than any of my much cheaper 6 1/2"-8 "subs but it can play louder

Yes, the active driver is the TB W5-1138, and the PRs (2x) are the Peerless 830880 with no added mass. All 5.25" drivers. I used the 25W plate amp available at the time, but a 40-50W plate amp would also get the job done well. This is a slick little sub and cost me about $100 to build at the time.

 

[617]

AFAIK the term room gain is usually used for the sound pressure increase with ideally 12 dB/octave under the lowest room mode, in German language it's called translated pressurisation chamber effect.

Yes, in English it is most commonly called cabin gain (since it is mostly observed in car cabins), which as you put it is a 12db/octave gain which occurs under the lowest room mode (sometimes called the volume mode). In car audio, people add foam and rubber gaskets to stiffen and seal their cars to take advantage of this gain, but it is not really a factor according to for example Earl Geddes, and I don't think it would be a factor in any normal space. It certainly shouldn't be an assumed effect for a subwoofer.

Also it is worth mentioning that dipoles do not produce pressurization gain, and that non-sealed subwoofers (such as bandpass, ported and passive radiator enclosures) are dipoles in their lower bandwidth, so the pressurization gain doesn't even apply to those subwoofers. Not sure about horns.

I read somewhere that the corner frequency for cabin bass boost is "figured by dividing 565 by the longest dimension of the car interior(in feet)". So if we imagine a small room of say 10' x 18' x 8', the longest diagonal is 22', giving a corner frequency of 25hz. For a 25x20x12 room, the corner frequency is 16hz. In a car it isn't hard to imagine your longest dimension being around 12', which puts the cabin gain at 47hz, so it's no wonder that you're more likely to excite this mode in a car than a room (with ducts, windows, doors, air conditioning, etc).

 

[KSTR]

It’s not just the small woofers, it’s also the air volume in those boxes limiting output, especially at the lowest end.

Well, the internal volume does not really matter as long as you can control the distortion it brings (which is why you need to have some form of motional feedback with ultra-small internal volumes). Displaced air volume, as seen from the outside, is what counts to yield a certain sound pressure at a certain frequency.
https://www.linkwitzlab.com/spl_max1.xls

 

[voodooless]

Well, the internal volume does not really matter as long as you can control the distortion it brings (which is why you need to have some form of motional feedback with ultra-small internal volumes). Displaced air volume, as seen from the outside, is what counts to yield a certain sound pressure at a certain frequency.
https://www.linkwitzlab.com/spl_max1.xls

Try doing a proper simulation with WinISD or Hornresp. Double the volume and compare maxSPL. You’ll see that it easily can make a difference of 5 to 8 dB at the very low end. That is quite significant! I’m mostly talking about closed boxes here, BR or PR is a bit more complex, but it mostly still holds.

 

[KSTR]

max acoustic output is strictly and solely governed by max displacement the driver is mechanically capable of, assuming you don't hit other barriers first (notably power dissipation).
Your are talking about voltage-based sensitivity which of course goes down as the driver is working against a stiffer spring (below resonance, which goes up with smaller volume). EQ it and throw more amp power on the driver, then.

 

[voodooless]

nope, I’m talking maxSPL.. just do the sim!

 

[richard12511]

When KEF shows 'room gain' they don't show all frequencies getting louder, they show the low bass getting (conveniently) extended, but in reality moving from a less to more restricted space boosts all frequencies (this is what a horn does, in essence.)

Isn't this the way it occurs in real rooms, though, or am I just getting incredibly lucky? I've got 4 rooms with subwoofers atm, and all of them show consistently increasing spl as the frequency drops, despite the natural response of the subwoofer being consistently decreasing.

As an example(stealing from @chucky7 here since I don't have measurements on this machine), compare this in room response to the natural anechoic response of the same subwoofer.

8Hz is up ~10dB in room over 100Hz, despite being ~10dB down anechoically. This is the same thing that I see in all my system measurements. The anechoic response is consistently decreasing, while the in room response is increasing up until the point it really starts to fall off.

I've always understood that the amount average 'room gain' increases, as the frequency decreases(below a certain frequency), but if I'm understanding you correctly, that understanding is not correct? Is the fact that so many in room responses increase below 30Hz(?) just a coincidence?

 

[617]

Isn't this the way it occurs in real rooms, though, or am I just getting incredibly lucky? I've got 4 rooms with subwoofers atm, and all of them show consistently increasing spl as the frequency drops, despite the natural response of the subwoofer being consistently decreasing.

As an example(stealing from @chucky7 here since I don't have measurements on this machine), compare this in room response to the natural anechoic response of the same subwoofer.

View attachment 127235
View attachment 127236

This is the same thing that I see in all my system measurements. The anechoic response is consistently decreasing, while the in room response is increasing up until the point it really starts to fall off.

I've always understood that the amount average 'room gain' increases, as the frequency decreases(below a certain frequency), but if I'm understanding you correctly, that understanding is not correct? Is the fact that so many in room responses increase below 30Hz(?) just a coincidence?

Are these measurements spatially averaged?

 

[KSTR]

nope, I’m talking maxSPL.. just do the sim!

Then the sim is either wrong (I never trusted WinISD for that matter) or was wrongly set up.
Trust physics, trust Mr. Linkwitz. SPL is directly proportional to displaced air volume, there are no other factors involved. Therefore, available excursion is what limits SPL and internal box volume is irrelevant for this limit.

 

[Wolf]

[QUOTE="617, post: 765695, member: 6871Also it is worth mentioning that dipoles do not produce pressurization gain, and that non-sealed subwoofers (such as bandpass, ported and passive radiator enclosures) are dipoles in their lower bandwidth, so the pressurization gain doesn't even apply to those subwoofers. Not sure about horns..[/QUOTE]

This is not exactly true. There is in fact more pressure generated within a vented box than a sealed box.

It's not dipolar response in vented boxes as there is not equal output to either side of the alignment. When the port takes over for the woofer, it has the output at max for a limited bandwidth, while the driver is minimally engaged. Vice versa when the driver is playing and way above tuning. There is no null to either side of the system either as in dipolar systems.

Bandpasses as systems can be variable in method, but 6th order being vented to both sides of the driver applies the same as standard vented boxes. It's not a dipolar response. If it's a 4th order bandpass, one cavity is sealed, and the port is the only output device. This is therefore not dipolar either.

As to gain in pressurization, if you have output, and it is not:
Ripole
Cardioid
Dipole

...which each represent a soundfield with nulls in it's polar response, then I see no reason why pressurization cannot be achieved.

I have felt my eardrums flap with both sealed and ported subs in the car system of my past, so this does not add up.

 

[richard12511]

Are these measurements spatially averaged?

I think so? They're not mine, so I don't know for sure.

I do know that my own measurements (that show the same trend) are MM measurements with 60+ points. Does that have something to do with it?

*Edit: I've always assumed that that room gain started at 100Hz and increased as the frequency decreased(as that's what my measurements always seem to show), but after doing some reading, that is very wrong. Apparently room gain doesn't start until about 29Hz for me. I have more questions now, though. I did some max spl testing back in December. Let me see if I can find those. From memory, it was a smooth up slope from 100Hz to 7Hz or so, with peaks above reference(115dB).

 

[voodooless]

Then the sim is either wrong (I never trusted WinISD for that matter) or was wrongly set up.
Trust physics, trust Mr. Linkwitz. SPL is directly proportional to displaced air volume, there are no other factors involved. Therefore, available excursion is what limits SPL and internal box volume is irrelevant for this limit.

Well, Linkwitz isn't wrong. But you'll have to consider the maximum power available as well. And since you need more power in a small box, that does become a real issue to content with. See here a large woofer with massive power handling (1500W) in an 30 and 100 liter box: it's already has a 4dB difference at 30 hz. Don't forget we're feeding it 1500W..

I have seem more extreme examples as well.

But it all depends on the specific setup and driver used. Something like an Dayton UM8 is practically unlimited in the low end because it has enough power handling. I cannot imagine those dinky 6.5" KEF wooflets having 1000W RMS power handling. It's probably realistically around 200~300W. Surely you must consider the crest factor in music, which will help a lot in pumping out some additional output down low. Still I would not expect miracles. In the other thread we've done the math (as you say by displacement), and it wasn't magic. Usually it's the woofers with the lower Qts that gain from bigger box most, which makes sense I guess. Then you say, but you should not use those in a closed box.. But wasn't it Linkwitz who said that it doesn't matter anymore?

So yes, there is no replacement for displacement, it just has some caviats to consider: You need enough power and power handling to actually reach the maximum excurtion. If power handling is ample, you can practically reduce box size and correct response in DSP. If not, then a bigger box might help squeeze out some additional dB's.

 

[nm4711]

@KSTR already made the exception for power handling.

max acoustic output is strictly and solely governed by max displacement the driver is mechanically capable of, assuming you don't hit other barriers first (notably power dissipation).

 

[jhaider]

Also it is worth mentioning that dipoles do not produce pressurization gain, and that non-sealed subwoofers (such as bandpass, ported and passive radiator enclosures) are dipoles in their lower bandwidth, so the pressurization gain doesn't even apply to those subwoofers. Not sure about horns.

A 4th order bandpass doesn’t go dipole. The back wave is isolated from the front wave by a closed chamber.

I consider “horn subwoofer” a marketing term for bandpass. They can be whatever order.

 

[voodooless]

@nm4711 Well yes, but assumptions are the mother of all f*ckups . That’s why you should always simulate. It often does have practical implications.

And the real point is actually a bit different: a bigger box gives you better power handling, meaning you need less power for the same excursion. If that means that you can stay below the max power of the driver, you’ll gain dB’s.

 

[sigbergaudio]

A couple of comments:

Room gain is indeed a real thing, and not only in cars but also in rooms. The effect varies with room size. We're generally more blessed here in Europe, since the effect is increased when the room is smaller. So if you are in a very large living room, the effect will be less than a smaller living room. In smaller rooms you'd typically see the effect start to kick in at 25hz and below, and it can indeed be quite significant.

When it comes to smaller volume subwoofers, it's also generally true that reduced subwoofer volume can be compensated by increasing power, within reasonable limits. We've done some testing, and have seen a minimal (practically insignificant) increase in distortion, but otherwise no negative effects with quite significant reduction in cabinet size (up to 50%). Doing this is more feasible these days with highly effective class D amplifiers. This doesnt' mean you can fool phsysics of course, so in the example of the 2x6.5" from Kef for instance, it can still only push so much air. Compensating this with ultra long throw drivers isn't ideal from a sound quality perspective (not implying anything about the sound quality of the Kef, just talking in general terms on this point).

 

[voodooless]

Compensating this with ultra long throw drivers isn't ideal from a sound quality perspective (not implying anything about the sound quality of the Kef, just talking in general terms on this point).

To be fair, that is not the only thing they did. There is some kind of motion feedback system present to reduce distortion, and I guess the opposing setup with single motor helps as well. Not sure about the two voice coil sizes though.

 

[polmuaddib]

Does having multiple subwoofers help with low end extension? I know multiple subs are cruical for even bass response, but do two or more subs play lower when combined then individually?

 

[KSTR]

Does having multiple subwoofers help with low end extension? I know multiple subs are cruical for even bass response, but do two or more subs play lower when combined then individually?

They do, under certain circumstances. A tight 2x2 or 3x3 stack is slightly more efficient that the same amount of speakers distributed because the acoustic loading is better.