# Panel Absorbers



## Chriswil (Jun 14, 2010)

Hi

Has anyone had any success with panel bass absorbers? 

So far we have not been able to match what we build with what the various calculators predict.

Thanks

Chris


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## bpape (Sep 14, 2006)

What are you trying to do? Are you trying to build the membrane types that use wood, etc. for the front membrane?

Bryan


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## Chriswil (Jun 14, 2010)

Hello Bryan

Yes, Panel absorber with plywood membrane, sealed box and fiberglass damping inside. I have an equation from Feuerbacher but this does not appear to agree with actual measurements. Do you know of any reliable calculators as well as any practical examples?

rgds

Chris


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## bpape (Sep 14, 2006)

Actually, there's a calculator at the link below.

http://www.mh-audio.nl/Helmholtzabsorber.asp#PanelAbsorber

Here is the forumla - f=60/SQRT(md)

Remember that d is in mm and m is in kg per unit area

Just understand a few things.

- Front panel spec has nothing to do with thickness, it's all about mass
- Absorbing material should be close to the front plate but not touch it.
- Nothing can be touching the front plate except where it's attached to the surrounding frame
- The assembly must be completely air tight to function properly.

The first item is critical. Every sheet of wood is different so weigh what you're going to use to determine the actual mass. 

Also understand that as you change the dimension of the plates in terms of length and width, the center tuning frequency will change slightly.

Plywood is tough unless it's pretty much cabinet grade as the gaps, filler, etc. all change the mass and how the panel resonates. If you can get enough depth to get the tuning you want, you can use something like Lexan which is very even in terms of mass. 

Bryan


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## Chriswil (Jun 14, 2010)

Thank you for the link to mh audio calculator Bryan. The result I get from this calculator is around the same as mine. The problem with the difference between the actual and measured values we get may have to do with ours being wedge shapped in order to fit into the corner. We calculated using the average depth. Do you know if this is the correct method or should we calculate in a different way in this case?

I am also wondering about the way we measure it. Basically we give it a thump and see at which frequency it rings. Should still give a correct result if the initial thump is ignored, or do you have any other suggestions? One worry is if we use the speakers we just end up recording the room influences.

rgds

Chris


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## bpape (Sep 14, 2006)

The average depth should get you close. 

How far are you off? What is the design frequency vs what you're getting?

A contact mic is really the best way to check what you actually have when you're done.

Bryan


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## Chriswil (Jun 14, 2010)

Hello Bryan,

The design frequency is 60Hz and the measured is 15Hz. The panel is 4mm ply in order to reduce edge stiffness which was then loaded in the middle with weights, with the idea both to increase mass as well as try and keep it working as a piston and to try and avoid the first partial from forming (which in our opinion would create positive extension at one half of the board and negative at the other so basically just shuffeling the air from one side to the other and not compressing the air at all.) average mass is 5.8kg/sqm. Resonator panel dimension 116.4cm x 73.5cm.

I will try and get a picture of the measurement this evening.

Chris


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## bpape (Sep 14, 2006)

Try just removing the weights. Understand your thinking but IMO, you're trying to 'cheat' the physics by wanting the flexibility and the mass. Try using a standard material of the same density. I'd bet you a beer that it will tune differently.

Not sure how you're measuring but I seriously doubt that the tuning is actually 15Hz. What is the cavity depth? 

Bryan


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## Chriswil (Jun 14, 2010)

Hello Bryan

We have been trying to resolve the measurement problem. We bought a contact (piezo) mic and stuck it on the box. We measured two peaks. 35Hz and 80Hz. 

However 

The resonator is wedge shaped (to fit into the corner). Outside dimensions: Sides are 53.4cm deep, front is 75.5cm wide and the whole thing is 120cm high. Wall thickness of box is 19mm and the resonator is 4mm ply. We calculate a volume of 143l and Average depth is calculated as 17.6cm. Resonator panel "free" vibrating area= 116.2cm x 71.7cm. 

We attached weights on the 4mm ply to acheive a total panel weight of 4.91 kg, i.e. 5.42Kg/m2 This should result in a Resonant frequency of 61Hz.

The room response measurement also confirms the effect is closer to 80Hz. 

I don't see an issue with placing weights to increase the mass. Bass piano strings are made in this way, i.e. a thin core with copper wound on the outside leaving the part that runs over the bridge to be thin and flexible, and speakers are heavier in the middle and have a flexible surround. Having a stiff edge. 

However what we might have is too much weight localised in one place, i.e. its forcing a mode where the weight is localised. this might suggest that there is insufficient edge flexibility.

In anycase we intend to drag it out into the garden and test it there in case we are getting some room interference.

If we still have a problem then we will replace the membrane as you previously suggested and check again.

rgds

Chris


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## Chriswil (Jun 14, 2010)

Sorry for disappearing, I travel a lot for work and it isn't always easy to find time or INTERNET.

Anyway the latest is that the measurements made in the garden show the same response from the panel resonator. 

I begin to believe there isn't enough damping going on. That is to say, it resonates and puts the stuff back in the room. 

To have an effect it must either resonate out of phase with the frequency we want to damp, or we need to convert the sound energy into something else. It is a small box with only short distances for the air to travel within. I wonder if there is enough air velocity going through the fiberglass to cause any significant absorption. 

I don't think sealed panel resonators are the way to go. No one appears to have successfully made one work with any efficiency.

Has anyone here been successful with sealed panel resonators? Or how do you deal with modes below 80Hz?

Chris


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