Multiple Small Subs - Geddes Approach

[ßart West-VL.]

Originally Posted by Russell Dawkins

I find it strange that they are set up to fire straight down the room. ...

They're not, look at the baseplate and the feet of the enclosures, a little bit of too in.

[Omholt]

Originally Posted by weltersys

Thermal modulation is not always proportional to the inverse square of the efficiency.
In testing a pair of Lab 12" compared to a BC 18SW115-4 (18 inch) in a tapped horn, the Lab 12" were almost 3 dB more efficient, but the 18SW115 was able to dissipate the heat from the voice coil much better, so it had much better ultimate dynamic range.

At high power, the 3 dB advantage the Lab 12"s had succumbed to thermal compression, and the 18SW115 was able to put out more SPL, showing virtually no thermal change from one watt to 1500 watts of sine wave, double the AES rating.
The pair of Lab 12"s, each getting about 400 watts sine wave, were quite hot to the touch, while the 18SW115 barely rose over the ambient temperature with 1500 watts.

Had I not done the high power tests, I would have concluded the B&C was overpriced compared to the Lab 12".
Having done the tests, I consider the 18SW115 a bargain, at least it was before the price of Neodymium went through the roof. Fortunately, B&C has come up with the ceramic magnet 18TBW100, nearly the equivalent of the 18SW115, kind of like a giant big brother to the 12TBX100 you are using.

Art

A bit off-topic, but how does the JBL 2242H compare to the B&C 18SW115? The 2242H had higher sensitivity and require less power. But also needs a bigger cabinet to reach same lows. Anything else?

[weltersys]

Originally Posted by Omholt

A bit off-topic, but how does the JBL 2242H compare to the B&C 18SW115? The 2242H had higher sensitivity and require less power. But also needs a bigger cabinet to reach same lows. Anything else?

The JBL 2242H has only 7.87mm Xmax compared to the BC18SW115's 15mm Xmax, and 800 watt power handling compared to 1500.
That means you would need almost two 2242H to equal the output of one BC18SW115.

Of course, if you subscribe to the multiple location sub approach, and don't need 122-128 dB output per sub, both these drivers are overkill.

[mattlong8]

Originally Posted by weltersys

The JBL 2242H has only 7.87mm Xmax compared to the BC18SW115's 15mm Xmax, and 800 watt power handling compared to 1500.
That means you would need almost two 2242H to equal the output of one BC18SW115.

It's definitely true that the specs favor ,b&c, and I can personally say that the 12" and the 21" in the TBX line are beasts sounding great even at high spl so I'm sure the 18 neo version is too. However, I have worked with the 2241H(G) for about 9 years and I love it(read: slightly biased). It has been the most reliable subwoofer driver for me. I've only messed around with the 2242 but it has the same motor structure but with 9mm of xmax compared to the 7.6 of the 2241(though Art may be citing his direct measurement not the spec sheet.). My experience, and the numerous papers I like that JBL has put out, has led me to believe that JBL often understates its it's power handling.

Less than a year ago I drove some sr4719xs with dual 2241s(reconed) with over 2000w each to the point where I heard the vc former touching(euphemism) the back plate. Naturally practically toppled the entire crowd of people on my way to adjust, but held it at just a bit inside what we can call xlim. This was probably right around 2000w. Amazed at the new found power handling I kept this up for 3-4 nights a week for 3 months. The only failure I had was one broken lead wire that was easily repaired.

The unique venting that the 224x series has deserves a closer look I think.

@Bateman thanks for sharing that tidbit, it makes me feel all warm inside.

It would seem high spl testing is important, I guess us pro sound guy can be of some use after all
Send me your drivers and I will break them!

And just to tie us back to the topic: driving multiple subs at eigenmode mode node points is good, but how does one deal with problems in the height of a room?
-Matt

[weltersys]

Originally Posted by mattlong8

I have worked with the 2241H(G) for about 9 years and I love it(read: slightly biased). It has been the most reliable subwoofer driver for me. I've only messed around with the 2242 but it has the same motor structure but with 9mm of xmax compared to the 7.6 of the 2241(though Art may be citing his direct measurement not the spec sheet.).
And just to tie us back to the topic: driving multiple subs at eigenmode mode node points is good, but how does one deal with problems in the height of a room?

Matt,

Usually one deal with problems in the height of a room with a building contractor, but as this is a DIY forum, go ahead and get out the chainsaw and wrecking bar.

I have not measured the 2241 or 2242, just going from the JBL TS spec sheet, which shows 7.62mm Xmax for the 2241, 7.87mm for the 2242.

I have measured the BC18SW115-4, it definitely has 15mm Xmax with distortion under 10%. It also will not "bottom out" with 120 volts (about 3600 watts) of sine/square wave, as I found out three times in a row when I made a testing mistake.

Art

[Pallas]

Dr. Geddes, a few questions about your statements this ~week:

Originally Posted by gedlee

***I have given up on subs of any variety except closed boxes - as big as you can afford. Everything else (ported, bandpass, LF horns, etc.) is just fluff with little real impact on what matters. (I assume that one is using multiple subs and a controller of course, but in this thread that's a pretty reasonable assumption.)

Interesting. Now I feel less like a slacker and more like someone who was on the cutting edge for having never gotten around to building 4th order BP subs to compare with the closed boxes I've been using (set up by your methods, in different rooms) for years.

But - leaving aside the lower distortion of the 4th order BP (I never thought that would be a major concern if one uses basically good drive-units to start with), you're not longer that impressed with the efficiency gains of the bandpass? That is to say, with enough cones pumping and sufficient power, you're no longer convinced that the efficiency bump a 4th order BP provides in the bottom half (or so) of its passband is that useful.

Does that change (assuming a suitable PR is available - I had no idea PE discontinued their "shallow" subs and PRs) if one takes a "2-way" sub approach, i.e. in your terminology separate "BroadBand" fairly high-tuned boxes to play in the modal region, and a single closed box for the ULF?

Originally Posted by gedlee

What specifications are relavent for a sub?
Resonance? - doesn't matter, it will be EQ'd.
Power handling? - look that up on the B&C website - 12TBX100
Efficiency? - also on the B&C website.

When mentioning efficiency as relevant, do you mean primarily for headroom in the modal region, or overall?

After all, isn't the "efficiency" of a subwoofer at the bottom of its range determined substantially by the size of the box, regardless of the driver's native efficiency?

Originally Posted by gedlee

***I have always been interested in why speaker sound quality falls apart as the SPL goes higher and why some speakers do so at much lower SPLs than others. I used to think that it was nonlinear distortion and so I studied this quite a bit (and I mean quite a bit!) As you know I, and almost everyone else who has looked at this, have concluded that nonlinear distortion is simply not the answer. So what is it? There is clearly a difference in the way speakers handle dynamics, but its not what we thought it was for a long time. It has to be something else. I now believe that it is diffraction and thermal modulation. Eliminate these two things and the speakers will play at insane levels without even the slightest loss of quality. I've done those tests.

Just to be clear, that means for subwoofers one needs to look at thermal modulation. That is to say, diffraction isn't so much an issue for bass drivers, though it's of high importance for mains.

[weltersys]

Originally Posted by Pallas

Just to be clear, that means for subwoofers one needs to look at thermal modulation.

Just to be clear, Earl wrote In post 1510:

"As expected people are confusing thermal compression with thermal modulation. they are related but distinctly different effects.

"Thermal compression" is well understood, documented and sometimes even measured. It is a long term effect that happens after a long exposure to a signal and the entire motor structure heats and eventually reaches a thermal equilibrium. In this effect the heat dissipation is critical as it determines the long term temperature."

And in 1541:

"Thermal modulation is also, for the most part, independent of the current voice coil temperature."

The thermal modulation Earl believes happens in ultralight voice coil dome tweeters would not happen in the large, heavy voice coils used in a good subwoofer, and thermal compression is not a problem unless high average levels are maintained.

As he wrote in post #1517:

"The time constants tend to be too long to be a factor for low frequencies, but not for tweeters. A burst of HF material and the tweeter heats and a burst about 1 ms later will be lower in level with a change in frequency response.

This is dominately a HF effect, which is why its kind of out of place here."

Art

[Omholt]

What studies show that lower sensitivity drivers, for let's say frequencies below 200 Hz, leads to inferiour sound quality?
Do we know this for sure?
Isn't it possible to avoid power compression with multiple high quality smaller drivers and suited amplifier power?

[Moonfly]

Cabinet design affects a drivers output more than sensitivity, but I dont think there is a direct relationship between sensitivity resultant sound quality. You can avoid power compression by using a high power handling driver in a cabinet that results in it only using half that power, and there are driver out there like that. You just need enough headroom in your system so your not pushing it too hard and you will avoid things like power compression whether your using one big sub or multiple subs that are large or small.

[gedlee]

Originally Posted by Pallas

Dr. Geddes, a few questions about your statements this ~week:



you're not longer that impressed with the efficiency gains of the bandpass? That is to say, with enough cones pumping and sufficient power, you're no longer convinced that the efficiency bump a 4th order BP provides in the bottom half (or so) of its passband is that useful.

Does that change (assuming a suitable PR is available - I had no idea PE discontinued their "shallow" subs and PRs) if one takes a "2-way" sub approach, i.e. in your terminology separate "BroadBand" fairly high-tuned boxes to play in the modal region, and a single closed box for the ULF?

The efficiency gain and bass extension of the bandpass is just not a good "a value", not that it doesn't exist. I cease to see any situation in audio where "value" is not a real criteria (except arguments on forums like this). There is always a cost involved and it is virtually always a factor. A closed box is simply the best value when one is going to use DSP and multiple subs. Adding a bandpass port increase the size substantially and a little to the cost (people tend to not like either of those) and using a PR adds a lot o the cost and a little to the size (same issues). Closed box with ample power handling is cost effective and as small as practicable.

When mentioning efficiency as relevant, do you mean primarily for headroom in the modal region, or overall?

After all, isn't the "efficiency" of a subwoofer at the bottom of its range determined substantially by the size of the box, regardless of the driver's native efficiency?

Just to be clear, that means for subwoofers one needs to look at thermal modulation. That is to say, diffraction isn't so much an issue for bass drivers, though it's of high importance for mains.

Efficiency is simply a means of looking at headroom. When I did tests of "powered subs" in almost every case the subs "crashed" from amp clipping even when it appeared that sufficient amp power was available. the higher the efficiency the lower likelihood of this happening. Yes the native efficiency and the "bottom end" efficiency are different, but they are completely correlated, so if one is "high" then so it the other" Box design has an effect, but its not huge, more of a secondary thing.

Subs have no issue with "thermal modulation", its a mid to high frequency problem. And diffraction at sub frequencies is not an issue either. For subs its all about how much SPL can be achieved without massive overloading, i.e. distortion in the 10's and 20% as occurs when something is being used beyond its design limitation. As I said, I find that this is usually an amp that is too low power, but sometimes a driver that goes beyond its magnetic gap limits. The magnetic gap thing however can be very low order and hence inaudible, while the amp clipping is very high order and highly audible. So, to me, X-max is less of an issue in a good sub driver than power handling and efficiency.