# Are helmholtz resonators and diffusers all we need?



## omholt

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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## bpape

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.

Bryan


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## omholt

bpape said:


> 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.



bpape said:


> 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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## bpape

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.

Bryan


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## omholt

bpape said:


> 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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## bpape

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. 

Bryan


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## omholt

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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## bpape

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.

Bryan


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## fredk

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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## omholt

fredk said:


> 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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## atledreier

So, I have a huge room node at 33,4Hz (measured). How will a helmholzt work vs EQ? I have basstraps (Rockwool absorbers) that do a great job down to 60-70Hz, but nothing below that. 

The timedomain issues with helmholzt is news to me. What are the issues vs EQ?

sorry ro hijack the thread, Omholt, I feel it's relevant, though.


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## bpape

A Helmholz could be a good solution for you. 

Just to be clear, there's no time domain problem specific to a Helmholz. My point was that if you only treat a small, narrow band, then that's the only place where you're getting some decay time reduction and you're leaving the rest unaddressed. If you already have some broadband in the room, that shouldn't be an issue.

Bryan


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## atledreier

So how do one know how much/many H-boxes to build? Does it go by surface area liek broadband absorbers or by volume or what? In my case it's obviously a front/backwall resonance, and I have lots of room against my back wall. Are Hellmholzt more effective on the velocity or pressure zones? Do I go with one big one or several smaller ones?


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## bpape

You'd need several of them. Size is going to be determined by the frequency it needs to be tuned to.


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## omholt

Bryan,

I think you guys at GIK really should start producing Helmholtz resonators at different sizes that can be tuned. That would really complete your product line. Right now, you don't have anything that treats the lowest bass.


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## bpape

The Monsters and Tri Traps do pretty well down into the 50's. Below that, it's pretty tough to deal with. 

Doing Helmholz Resonators would be interesting for sure - It's just such a narrow tuning that they'd almost need to be custom built for each application which makes it a tough product to market. That's why nobody else has them either.

Bryan


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## omholt

This swedish company sells some:
http://www.diffusor.com/Basabsorbenter.htm

The first one can be tuned in the 60-100Hz area and the second between 25-60Hz. They also have one that goes even deeper, but it's not on the site. So, is possible to make Helmholtz' that are tunable in a certain range. Not 100% sure how it works, but I think the tuning works by simply how much you open the "door".


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## bpape

Those certainly look interesting. I'd agree on the slot size changing doing the tuning. I wish they had specs for those 2 products like they do for the others on the site. 

Bryan


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## atledreier

Should be real easy to make, though. It's basically a box, they have the measurements in the specs. Make a slot, make some sliding arrangements for the port, and Bob's your uncle. Do these things need any damping inside or will that throw the tuning off?

Also, do you have any info on placement? DO they go in the pressure zone or the velocity zone of the node?


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## MatrixDweller

omholt said:


> This swedish company sells some:
> http://www.diffusor.com/Basabsorbenter.htm


I'm sure that the pictures the company has on their site are to help market their products...but get a load of this room (below). Pretty futuristic looking. Their Golden Horn product looks neat too. I've always wondered how to make something like those. There are formulas for the Skyline type but I can't find any for the RPG Hemifusor type.


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## atledreier

All that stuff on the walls, and very little in the ceiling, and a rug on the floor... :scratchhead:


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## omholt

Here's some pictures from another room that has acoustic products from the same company. They have a very different approach then GIK and RealTraps. Helmholtz resonators for the bass and diffusorers for midrange and treble. I'm not sure if they use any fiberglass/rockwool behind their diffusers, but I'm quite sure they don't. Some say this is the best room they ever been in, but I've also heard other comments.


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## atledreier

Once again.. all that stuff, and a thin rug on the floor... 

I'd at least have SOMETHING in the 1.st reflection point after all that money spent..


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## avaserfi

Depending on the speakers polar response and resonances characteristics the something you will want in the first reflection locations will be a _reflective_. Of course, this is presuming the loudspeaker is placed such that this reflection would arrive to the listener in 4-10ms range. 

I am not saying that this is the ideal option for most speakers, it isn't as most are too resonant, but it is an example that shows there is no ideal treatment for one room. It is very dependant on loudspeaker-room interaction. This is a case where a problem can be minimized with proper room treatments while this same treatment would actually stifle realism in a higher quality system.


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## omholt

atledreier said:


> Once again.. all that stuff, and a thin rug on the floor...
> 
> I'd at least have SOMETHING in the 1.st reflection point after all that money spent..


Why, when it's measuring flat? They have another room where there's actually tiles on the floor. But together with all the acoustics treatment, it's not a problem. They way they have set up the room is by taking measures after every acoustic treatment. These guys know what they're doing. 

The discussion is really more if this approach is better then using broadband bass traps. I personally don't know, but wish to find out. It probably depends on the speakers. But if we stick to speakers that measures neutral, I think it's probably a matter of taste. People who sell their products say though that most people prefer a room that's treated this way over a room with rockwool/broadband panels when they get to listen to both.


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## bpape

Again, there's a lot more to it than just frequency response. Impulse response and decay times are also important.

As for reflective at the 1st reflection points, that can work if:

- You're within the proper timeframe
- The speaker has an almost perfect power response way off axis or they're extremely directional
- You don't have close proximity of the speaker to the side boundary that would require SBIR correction.

There are lots of different ways to attack room acoustics and lots of different preferences for how a person wants the room to sound.

The hard floors shown above wouldn't necessarily be bad in a purely 2 channel setup and as shown since the ceiling is pretty much completely covered with diffusion and tuned resonators.

Bryan


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## fredk

Thanks for the link omholt. The debth of the resonators is in the same ballpark as I was modeling using some online tools.

I have some space that I could use for resonators, but I have no idea if it would be enough to be effective.



atledreier said:


> So how do one know how much/many H-boxes to build?You'd need several of them.


After doing a lot of reading, that about sums up what I have found. I have yet to find anyone who has published information quantifying the effectiveness of helmholtz resonators.

I did find one book on Amazon that got quite deep into the math behind figuring out the tuning of resonators and difusors, but there was nothing to quantify effectiveness. It may well be that the nature and complexity of room acoustics does not lend itself to quantification.


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## bpape

They can vary quite a bit. I suspect that's why there are no specs on that site since they will vary greatly depending on how it's tuned. It would be nice to have a sample set though at say 2 different tunings so one could properly estimate what would be needed.

Bryan


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## giankam

How can it be that adjusting the 38Hz region it affects also 100/200Hz range?

Please anybody can explain or make any consideration? I really don't get it.

Thanks


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## bpape

4 x 38 = 152Hz. If there was a harmonic of the 38Hz that was interacting with something else, it's not there now and the other can be showing through.

Bryan


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