Are helmholtz resonators and diffusers all we need? - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 30 Old 10-25-08, 08:59 AM Thread Starter
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Are helmholtz resonators and diffusers all we need?

I see that some claim that to avoid the room becoming too dead and boxed in, it's better to use diffusers for midrange and treble and then helmholtz' for the bass. Is this a better way to treat the room rather then using broadband bass traps?

It's said by those who support this that often it's enough to treat the lowest bass frequencies problem. Treating the lowest ones, also resolves the upper bass frequencies.

I've added two pictures that demonstrates this. The first one without any acoustic treatment, and then the second one where only the lowest one (around 32Hz) has been adressed with a helmholtz resonator.



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post #2 of 30 Old 10-25-08, 09:10 AM
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Re: Are helmholtz resonators and diffusers all we need?

If you only have 1 large bass problem, from a frequency response standpoint, that may well work. From a decay time standpoint, a certain amount of broadband bass control is beneficial. Helmholz resonators are very narrow in scope and will not address ringing and decay time anomolies across the bandwidth. In addition, Helmholz resonators when tuned very very low, can introduce their own anomolies in the time domain.

I'm not saying they're not useful, they certainly are in the right application and as a part of an overall control scheme. They're just not an end all solution.

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post #3 of 30 Old 10-25-08, 10:39 AM Thread Starter
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Re: Are helmholtz resonators and diffusers all we need?

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If you only have 1 large bass problem, from a frequency response standpoint, that may well work.
If you look at the first graph there's more then just one bass problem. There's a peak around 48Hz and a dip at 55Hz. These have been flattened out even though only the 32Hz problem has been treated. The second graph also shows improvement above 100Hz despite the fact that it hasn't directly been adressed.

So, if treating the lowest (or sometimes the two lowest bass problems) resolves most of the peak/dips in the bass area, I don't see why we should use a bunch of thick panel absorbers.

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From a decay time standpoint, a certain amount of broadband bass control is beneficial. Helmholz resonators are very narrow in scope and will not address ringing and decay time anomolies across the bandwidth. In addition, Helmholz resonators when tuned very very low, can introduce their own anomolies in the time domain.

I'm not saying they're not useful, they certainly are in the right application and as a part of an overall control scheme. They're just not an end all solution.

Bryan
Here you definetly have a point (ringing and decay time). So perhaps the best way to go is using a combination of Helmholtz' placed in corners and some absorbers in the first reflection points? A advantage using mainly Helmholtz reonators for the bass, is that it will take less panel absorbers in the room and it will look better. Who want's a room like Ethan's and perhaps it's not necessary either... (Sorry Ethan)

I think this Helmholtz resonator looks quite nice by the way. Can be used as table.
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post #4 of 30 Old 10-25-08, 12:58 PM
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Re: Are helmholtz resonators and diffusers all we need?

Understand that there was more than one problem. But, only one really bad NULL. Yes - you'll get some benefits from harmonics of the Helmholz. As I said though, do the same experiment and look at decay times before and after when you tune a Helmholz that low. It can actually get worse.

I'll agree that sometimes, a combination of broadband and tuned absorbers are the best solution. Personally, I prefer the panel/membrane type absorbers as they're a bit broader but still leave the high end pretty much alone.

Diffusion also requires that you have sufficient distance to allow the diffusion to work. Sometimes you do, sometimes you don't. They can be very effective though.

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post #5 of 30 Old 10-26-08, 06:15 AM Thread Starter
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Re: Are helmholtz resonators and diffusers all we need?

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Personally, I prefer the panel/membrane type absorbers as they're a bit broader but still leave the high end pretty much alone.

Diffusion also requires that you have sufficient distance to allow the diffusion to work. Sometimes you do, sometimes you don't. They can be very effective though.

Bryan
Are the GIK panels and corner bass traps made with membrane this way?

According to Ethan's video on diffusion it differs how much distance diffusers need. Some can be quite close to the ears, others need 6-12 feet.

And what about the uneven absorption coefficient broadband traps introduce? How are you suppose do get an even absorption in the bass by only using panels/traps?
If we take look at how 244s and Monster Bass Traps perform, they don't exactly absorb with linearity. Especially the Monster Bass Trap which has a coefficient as high as 3 at 80Hz, 1.85 at 165Hz and much lower then 3 under 75Hz. Isn't this a problem?
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post #6 of 30 Old 10-26-08, 09:12 AM
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Re: Are helmholtz resonators and diffusers all we need?

Correct. They don't absorb linearly - and they're not supposed to. The idea is not to absorb the same across the spectrum. The idea is to bring the decay time down into a predetermined curve based on room size and usage.

Generally, most rooms have a reasonable amount of upper mid and high frequency absorption by default (carpeting, furniture, curtains, air - yes, air) but very short on bass absorption. That's why our products are designed to perform the way they do. You can use enough of them to bring the bottom end under control without over-deadening the mids and highs. You'll find that Ethan's Real Traps products are designed to do the same thing and for the same reason.

As for diffusion, we'll just have to agree to disagree. QRD type diffusers operate not only in the physical domain but also the time domain. Potentially in the very very high frequencies, if the diffuser is done properly, you might be able to sit a bit closer and still have it be effective - but that would trade off a broader diffusive nature in the areas of the spectrum where most of the energy exists. I respect Ethan but we do disagree on diffusion as does Jason, the person who designed our diffusion products.

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post #7 of 30 Old 10-26-08, 10:59 AM Thread Starter
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Re: Are helmholtz resonators and diffusers all we need?

Bryan, I'm not really talking about the unlinear absorption across the bandwidth. I understand that less absorption in mid and high frequencies is an advantage because of the room's furnitures. But I'm asking about the uneven absorption in the bass area which is clearly seen on the measurements. This is not a problem with a Helmholtz resonator. Isn't linear absorption of the bass important?

Interesting to hear that you have a different view on diffusion then Ethan.
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post #8 of 30 Old 10-26-08, 11:45 AM
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Re: Are helmholtz resonators and diffusers all we need?

Correct. Helmholz's don't have that issue - because they're not broadband. They're about an octave at best. Rigid membranes are around 2 octaves or a bit more.

Most 'soft' membranes will show a spike at a specific frequency - just a function of the physics. Now, if you use a variety of different panels, things balance out nicely. Also, standard reflection type absorption will also fill in some in the areas where it's a bit lower.

Lastly, people and stuffed furniture also have some nice absorption characteristics in that 80-200Hz range.

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post #9 of 30 Old 10-26-08, 08:12 PM
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Re: Are helmholtz resonators and diffusers all we need?

Omholt. Where did those graphs come from. I have been looking for some information on the effectiveness of helmholtz and pannel resonators, but have not had much luck.

Fred
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post #10 of 30 Old 10-27-08, 03:11 AM Thread Starter
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Re: Are helmholtz resonators and diffusers all we need?

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Omholt. Where did those graphs come from. I have been looking for some information on the effectiveness of helmholtz and pannel resonators, but have not had much luck.

Fred
They came from this company:
http://www.engelholmaudio.com/eng_index2.htm
He sells tuned Helmholtz' from this company:
http://www.diffusor.com/index.htm
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