Multiple Small Subs - Geddes Approach

[gedlee]

Originally Posted by Speedskater

I think that we have a typo here.

Yes, quite true, thanks. See how easy it is to confuse the two!!

It should read:

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

[gedlee]

Originally Posted by mattlong8

I think many will agree there are nuts in both the hifi and pro sound camps. IMO pro sound has a tougher job having a much larger listening area in which to keep the system aligned. I have a great deal of respect for both Earl and Art having learned much from both you. Can we not accept that these "different worlds" both contain the good, the bad, and the insane?

-Matt

Absolutely, but consider the title of this thread. Why would anyone come here and use a "Pro" perspective? It's clearly not about "Pro"!

I get annoyed when people don't consider the context and criticize my comments from a perspective that is clearly NOT what this thread is about.

[Soundtrackmixer]

Originally Posted by bentoronto

Frankly, my intuition is that sound for movies for home use are as badly mangled and exaggerated for "lowest common denominator" playback systems as is the norm for pop music recordings.

Actually you are incorrect. We mix for the highest attainable quality, and let the chips fall where they may in the field.

I see no reason to expect the producers of these sound tracks to show any more taste and good judgment than they show in developing their nonsense movie content for the consumption of teens.

The producers of soundtracks are sound editors, sound designers, and re-recording mixers. These are quite different people than those who do the production of the film.

[nigelwright7557]

I used to work for a mobile disco and the man who ran it built his own speakers.
He had one cabinet with 12 small subs in it and it sounded very good.

[mattlong8]

Originally Posted by gedlee

Absolutely, but consider the title of this thread. Why would anyone come here and use a "Pro" perspective? It's clearly not about "Pro"!
I get annoyed when people don't consider the context and criticize my comments from a perspective that is clearly NOT what this thread is about.

Understood, however I'm a pro engineer and was attracted by the thread title. I'm interested in the modularity of small subs for a few reasons: Scaleable arrays, multiple concurrent events, rentals etc. but also I find myself working in medium-small rooms where modes are very problematic. Using multiple subs at the nodes of the room modes has given me good results especially when the walls are pretty rigid.

That being said I do understand that thing on audio forums often get taken out of context.

-Matt

[Omholt]

Originally Posted by gedlee

I absolutely believe this is the case.

When current flows through a copper wire it heats up and its resistance changes (this change happens instantly). As the resistance changes the signals current through the wire gets modulated by this change. The larger the resistance change the more the "dynamics" get crushed.

There are two factors in the actual amount of resistance change that occurs - the amount of copper in the voice coil and the amount of current through it. The more copper there is the more thermal capacity it has and hence the less the temperature change for a given amount of heat. The more current there is the greater the heat.

The higher the efficiency the less current is required to generate a given SPL so the lower the heat generation, but high efficiency requires bigger voice coils so there is also more copper. Hence simply put, thermal modulation is proportional to the inverse square of the efficiency.

By my calculations the difference between a 1" dome tweeter and a 1" compression driver is somewhere in the thousands. I have measured this difference as well, being on the order of several dB - as much as 6 in some cases. It is NOT an insignificant effect. For woofers it is not as great, but it is still a factor. The real issue is that to have high efficiency at the high frequencies where it is a major effect and most audible you also have to have it at the lower frequencies as well.

Is efficiency here the same as sensitivity?

If higher sensitivity is important, do we know where the upper limit is for an improvement? Or does it simply get better and better?

Though 95 dB is high compared to commercial speakers, it's a good distance to horn speakers with sensitivity of 100, 105 or 110 dB.

[gedlee]

Originally Posted by Omholt

Is efficiency here the same as sensitivity?

If higher sensitivity is important, do we know where the upper limit is for an improvement? Or does it simply get better and better?

Though 95 dB is high compared to commercial speakers, it's a good distance to horn speakers with sensitivity of 100, 105 or 110 dB.

Yes, I used efficiency here as synonymous to sensitivity. "Better and better" is only relative to what is audible and we don't know what that is.

At first I doubted that this effect could be audible. But when I did some calculations and found that the difference between a 1" dome tweeter and a high sensitivity compression driver on a waveguide could be as high a three to four orders of magnitude I figured that audibility would surely lie in that range somewhere.

But I doubt that the situation could get "better and better" compared to a 105 dB per watt compression driver.

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.

[Patrick Bateman]

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.

Here's a funny story, to illustrate this:



This week I was at CES, and had a chance to hear Dynaudio's new flagship. It's something like $95,000. This is your basic audiophile dream speaker, with imaging to die for, and a soundstage that's wide and deep.

While listening to it, I can honestly say that it's one of the best sounding speakers I've ever heard.

But someone asked Dynaudio to 'crank it up', and I noticed something...

It was starting to 'harden' at surprisingly low SPLs. I'd estimate around 100dB it was starting to compress.

I have a pair of Summas, and I've never even heard a *hint* of this, and I've listened at levels that exceeded what Dynaudio was playing by easily 20dB.

Also, this 'hardening' of the sound wasn't coming with big bass hits, or drums, or anything like that. It was just plain ol' vocals. If I had to hazard a guess, I'd bet that the low order crossover on the Dynaudio midranges was causing them to get close to their xmax.


That really made me think, though. IE, it *did* sound like one of the best speakers I've ever heard, but that was mostly because of it's enormous soundstage. And in my experience, most recordings don't have much of an image. For every well-recorded album there are ten more that are practically mono. And can you imagine spending ninety five thousand dollars on a set of speakers that image like a champ, but can't exceed 95dB without getting strident?

[Russell Dawkins]

I have had a similar experience with a very highly regarded (within a small circle) two-way with a "waveguide on a 1" dome tweeter (not compression driver) and an 8" woofer known for low distortion.
At low levels it was gorgeous, but I could hear hardening starting at levels averaging 75dB, C-weighted. I sold them to someone who is still delighted with them after 2+ years but who, like most I guess, simply expect speakers to sound stressed when turned up. That is the common understanding of "loud" in the context of home hi-fi - the sound of stress.

I had always assumed it was the result of some form of non-linear behavior in one or more of the drivers - break-up - and am surprised to think that it may be a function of thermal modulation, but I am seeing this again and again. I had assumed that thermal modulation effects would occupy too long a time scale to affect short transients. I find it difficult to visualize, though, why diffraction effects would vary with loudness, if that I understand you correctly, Dr Geddes.

I remember an early ATC speaker being described as getting "bigger" sounding, more than "louder" as the volume control was advanced, and that description stayed with me.

[Russell Dawkins]

Originally Posted by Patrick Bateman

I find it strange that they are set up to fire straight down the room. I cannot think of any circumstance where this would be the preferred configuration - even if the speakers need to be heard off axis due to nasty beaming of mids or highs. This only works well when the listener is precisely centered; with any lateral movement the image moves to the near speaker, doesn't it?
Crossing axes in front of the listener reduces this, widens the listening area and also reduces the first side wall reflections, yet I see this all the time.

The only rationale for it can be the visual aspect (some kind of symmetry) or the speakers are way too hot in the HF and need to be listened to 30º off axis to be tolerable. I guess.