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DIY Acoustic low-frequency conversion absorber measurements

ppataki

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I thought I would share my measurements about my first DIY conversion absorber
It is a plywood sheet attached to a timber frame and filled with damping material then attached to the wall
(the photo shows before totally filling it with damping material)

20210331_180807.jpg


It is tuned at 68Hz

Let's see the measurements at listening position

Frequency response:

1617362431488.png


The red is with my HOFA natural bass traps, green is without them and blue is the conversion absorber. Close to zero differences below 100Hz, some differences between 100 and 200Hz

T60M graphs:

1617362666608.png


We can see that the HOFA does a way better job with broadband absorption above 100Hz but below 100Hz the conversion absorber performs slightly better

Waterfall diagrams:

No absorber
1617362881767.png


With HOFA

1617362910249.png


HOFA decreased the ringing above 80Hz but strangely it increased the ringing at about 70Hz

With the conversion absorber

1617362940779.png


Ringing at tuning frequency is reduced (but only compared to HOFA and not when compared to having nothing in the room). Ringing above 100Hz is not as effectively reduced as with HOFA

I must say that I am totally NOT impressed by the performance of the conversion absorber
Maybe I screwed up the build, not sure - anyway I was expecting some more impact overall

Any builds/comments/advice would be appreciated
 

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Vict0r

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Thanks for that link! These seem very affordable to DIY. I still need some absorbers for my office, so I'm going to give this BBC design a try.

Schermafbeelding 2021-04-02 om 16.20.45.png
 

Geert

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My impression is you didn't use the right type of absorber. You're supposed to use mineral wool, which is a material that allows air through. If I'm not mistaken the material you used doesn't.
 
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P

ppataki

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I have concerns using mineral wool in a living room (health risks)
Are there any bio alternatives? Maybe combed wool would do?
Thank you
 

Geert

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That's a valid concern. There a different specialised acoustic materials like Akotherm D40.
 

JPA

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It's a little hard to tell from your picture, but it looks like the foam is in contact with the plywood membrane. You don't want that. The membrane has to have complete freedom of movement except where it's attached at the edges.
 

JPA

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@JPA How could I prevent the filling from contacting the membrane?

You need to pull it away from the membrane so there is at least a 1cm air gap. It looks like you're using two layers. Try only using one layer to make room for the gap. If it is glued in place you'll have to cut it out and re-glue it.

I recommend reading the section on diaphragmatic absorbers in "Master Handbook of Acoustics" by F. Alton Everest.
 
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