DIY Helmholtz Plates

[mike w]

Has anyone built a Helmholtz absorber to deal with specific room mode? If so, how did it work? If I look at a modeling calculator, I would expect a very efficient absorber. The reason I'm interested in this technology is that I could get a wood panel thru the architectural committee as compared to 8" of foam glued to the walls.

As a practical matter, I suspect that these do not work nearly as well as the model says or there would be more of them commercially available.

http://www.acousticmodelling.com/helmholtz.php

Mike

 

[Matthew J Poes]

Has anyone built a Helmholtz absorber to deal with specific room mode? If so, how did it work? If I look at a modeling calculator, I would expect a very efficient absorber. The reason I'm interested in this technology is that I could get a wood panel thru the architectural committee as compared to 8" of foam glued to the walls.

As a practical matter, I suspect that these do not work nearly as well as the model says or there would be more of them commercially available.

http://www.acousticmodelling.com/helmholtz.php

Mike

They can work very efficiently, but over an extremly narrow bandwidth. You have to treat them like PEQ, meaning that you need to design the resonator to precisely hit the mode in both frequency and Q. That isn't so easy to do. Further, their overall effectiveness is largely a factor of their surface area. They are much more efficient than velocity absorbers for a given area, but see the effect of those results still requires a good number of them. To successfully treat a number of different modes, you need to apply many of them tuned to different frequencies. As the Q lowers, the efficiency goes down. A lot of people have had bad luck implementing them. I've built and used them, and I use a similar theory in my basic room design, but as a whole, I generally don't recomend them to people.

If you don't mind a little DIY fun and are ok with it not working out, give it a shot. You never know, might work wonders for you!

In general EQ is far more effective at achieving the same results, so if you are treating a stereo system or home theater rahter than a studio, I recomend using EQ. I know that people sometimes feel like the acoustic approach is better, but in this specific case, they are identical in end result.

 

[DanDan]

The efficiency can look very high, but you have to multiply that by Surface Area to get an idea of efficacy. Many people have searched and researched the 'best' bass trap.
The VPR seems at the top of the pile. It is sold by RPG as the Modex Plate. Diaphragm/panel designs do well also. Hard to beat the old BBC one with the simple hardboard front.
I concur in not recommending DIY unless you really have just one distinct mode. If you do go there, make it adjustable. I have found resonant devices in a room can change frequency with location.
Acoustic treatment is difficult at the lowest frequencies, in which case PEQ is really the only option. But treatment helps throughout the space, while Eq can make the response worse in parts of the room.
DD

 

[Matthew J Poes]

The efficiency can look very high, but you have to multiply that by Surface Area to get an idea of efficacy. Many people have searched and researched the 'best' bass trap.
The VPR seems at the top of the pile. It is sold by RPG as the Modex Plate. Diaphragm/panel designs do well also. Hard to beat the old BBC one with the simple hardboard front.
I concur in not recommending DIY unless you really have just one distinct mode. If you do go there, make it adjustable. I have found resonant devices in a room can change frequency with location.
Acoustic treatment is difficult at the lowest frequencies, in which case PEQ is really the only option. But treatment helps throughout the space, while Eq can make the response worse in parts of the room.
DD

This is more an aside but the Modex plate is very hard to get. After RPG’s financial problems and reorganization I have found it nearly impossible to get. I lost my contact within RPG and can no longer obtain the plates. Even dealers have tried to order them and called me back to cancel the order.

There are some copies being made but they can’t ship them. They are constructed on site and so the cost is high for end customers not near these manufacturers. I work with one such company but they are based in California. I need a big job to justify using their products.

 

[DanDan]

703/705 is impossible to get here, and in Europe in general afaik.
I think Renz, Germany, make the VPR also. Don_audio sell the Caruso Isobond which should make DIY a lot easier.
I believe he has made a shipping arrangement, bulk buy or such, for the USA.
Hardboard has been used in Studios for decades. Interesting to see what it can do in a real situation.
https://www.gearslutz.com/board/showpost.php?p=8992852&postcount=91


DD

 

[Matthew J Poes]

703/705 is impossible to get here, and in Europe in general afaik.
I think Renz, Germany, make the VPR also. Don_audio sell the Caruso Isobond which should make DIY a lot easier.
I believe he has made a shipping arrangement, bulk buy or such, for the USA.
Hardboard has been used in Studios for decades. Interesting to see what it can do in a real situation.
https://www.gearslutz.com/board/showpost.php?p=8992852&postcount=91


DD

I use it quite often. I have a stack of hardboard and pegboard in my garage as we speak.

I can’t always make my panels for clients. It depends on the project. Sometimes the liability of the success of the project dictates that I use tested products.

In general I still prefer composite walls as panel bass traps than adding them, but that isn’t always possible. A damped composite wall provides the largest possibel LF damping without taking up additional floor space. Having now designed and built a handful of rooms (including my own) I have a pretty good formula for how to maximize the LF damping of the wall itself. I, of course, also have modeling software and spreadsheets to aid. The biggest problem with this design approach is that it doesn’t function at 150hz. It’s maximum efficiency is down at more like 50hz. You can move the peak but it will always fall below 80hz. I figure that is ok because the lowest stuff is the hardest to deal with through passive acoustics anyway. My room should have a terrible room mode at 50hz or so but I have no evidence of it because 100% of the walls and ceilings are a damped composite wall mounted on resilient mounts (hat channel and isolated clips). Of course none of this is new, lots of research into the concept. Just not widely used for some reason.

 

[DanDan]

Yup, a good friend acoustician Andre Vare calls it 'Invisible Alpha' tm ™
DD

 

[mike w]

Thanks Matt and DanDan.

DanDan: I’ve followed many of your sensible comments on the GearSlutz forum. I guess I cured myself of this idea after reading “ My Experiment with a Metal Panel Absorber” thread. Many of the Pro Audio solutions are not practical for me in my mixed use listening environment.

Since hope springs eternal, my focus has been on reducing the Topt time. Toole says that if you can get the reverb time down, modal ringing below 100 Hz is not really audible. I interpret that comment to mean I can achieve a more articulate bass response without going crazy with bass absorption.

The problem is that I have a house with an open floor plan (few walls) with minimal furnishings so, the reverb time is high (700- 800mS). I’ve been adding furnishings plus another lean diffusor. (I get a kitchen pass with the lean diffusor because my wife likes the way they look.)

I’m not done with my changes but the truth is that I haven’t really move the needle much if at all. The slight difference in REW measurements I attribute to measurement variation. So, in the end, I may have to settle and be happy with things as they are.

 

[DanDan]

Visuals win! Fortunately a Win Win situation has evolved. http://www.sto.co.uk/media/documents/brochures/stosilent_brochure.pdf
DD