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Measuring Helmholtz absorber resonant frequency in non-anechoic space.

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Greetings everyone!

I was asked by a friend of mine to post on this forum about  getting information of Helmholtz resonator resonant  frequency  from  a measurements taken in non-anechoic space ...

So, first of all let me tell u something about assembling square based H. absorber/resonator. Iíve taken 16mm water resistant chipboard for the box, did a whole air sealing with water resistant silicon on inside edges and  Iíve heavily implied rock wool on itís sides for a wider Q. So now I had two boxes in one with similar volumes, one 88 liter and the other one 88,7 liter and the desired resonant frequency was approx 100 Hz.

Here are some pictures of assembilng the box ...

I couldnít measure resonant frequency of the H. absorber outside the studio (Iím living in country where my backside of studio is looking on forest which is approx 30 meters from my house, so it would be convenient to measure outside), Ďcouse the temperature was under zero, so the only way was inside.
 Iíve done 4 holes with 11cm in diameter on the 88 liter one and put my measurement microphone on the center just outside of one hole, closed all 4 holes with screwing chipboard on it without touching mic and took first measurement. That was measurement of the room. Than Iíve released one hole (nearest and below to microphone), took measurement, than released two holes, and repeated the same process till I released whole holes Ö Then Iíve sent the measurements to my friend and he successfully extracted the measurement frequencies of each hole individually from a room (all holes closed) measurement.
He will explain in details what he did to accomplish that.

I hope that this will be useful for someone.

Hello Eomsy, thanks for contributing to PSW R/E/P's "Acoustics in Motion"

--- Quote from: Eomsy on March 20, 2011, 02:14:22 PM ---Greetings everyone!

I was asked by a friend of mine to post on this forum about  getting information of
Helmholtz resonator resonant  frequency  from  a measurements taken in non-anechoic
space ...

--- End quote ---
Yes, I asked Eomsy to post our expiriences here, beacause it can be useful for others,
 and... in old forum, we have thread with subject "how to measure bass trap"... or
 similar... then.... here is one method... anechoic space isn't needed, measuring
outdoors isn't needed too.

There aren't easy way to measure wideband absorbers or some complicated
 constructions (MyRoom for example) without special equipment and/or anechoic
 room (or outdoors)...  but simple helmhotz resonator we can measure even
 in non-anechoic space, because we know where they absorb sound pressure.

Helmholtz absorber sink air pressure at its port, and we can expect frequency region
of lowest pressure near to opening or port, at resonant frequency. Air/sound pressure
we can easily measure with measurement microphone, and with one loudspeaker we
can do measurements for tunning Helmholtz resonators in any available space.

Here it is. Some Helmholtz absorber with three holes.

First we must have covers for already opened holes in helmholtz absorber.
Fixing with screws is a good idea.
We need to position microphone near to center of one of holes and with distance of
about one height of cover, from (already installed) cover.

It's essential to not moving anything inside room where you do measurements, during
measurement process.
This include helmholtz resonator which we measure and measurement microphone.
We only may move covers and do measurements... anytihng other in room should not
be moved.
When all things are at place, we can do first measurements with all holes covered.
This measurement can be called "room response"

Second we must uncover hole below microphone (look at picture below)

And do another measurement without move anything in room other than cover1. This
measurement can be called "one hole" (for example...)

Second you may remove cover2 and do another measurement without touching microphone.
Microphone stay where was from beginning:

Then we do measurement and call it "two holes" (or whatever you like)

Third we may remove a last cover and do another measurement (without moving... )
as you can see on picture below:

Then we do last measurement and call it "three holes"

Now we have four measurements and we need  to export ascii (txt) response from
RoomEQ Wizard or ARTA (whatever you use for measurements) for all this

30-200Hz is ok for frequency band for export... more than this isn't practical to handling.

To be continued...

For results analyze we need any spreadsheet utility (Excel or similar)

We need to calculate, for every frequency point, difference
between magnitude response with room magnitude response with one or more holes opened
and room magnitude response with all holes closed, expressed in decibels SPL.

At graphs we can see how resonant frequency translate to higher frequencies
Because Helmholtz absorber works that way: more holes - higher resonant frequency.

All graphs display SPL (Sound Pressure Level) near the one of holes (in all measurements
this is a same hole!)
Numbers on the graphs in different colors, means numbers of uncovered holes in absorbers....
Here are some already measured results.:

-All this results are from measurements which do Eomsy with his own helmholtz absorbers in his room.
-I only give him directions how to measure
-I do all calculations in spreadsheet and graphs/illustrations artwork.
-I obtain results only for two, three or more opened holes from Eomsy.
-Anyhow, Helmholtz resonator isn't really useful with single hole (if hole isn't huge)
-More holes means more efficient absorber...
-If we need even more efficient absorber we need to open more holes (resonant frequency increases)
and insert pipes with same outside diameter as holes, then with length of pipe we can tune Helmholtz
absorber (decreasing resonant frequency)... described measurement method can be still usefull fot that purpose.
-All holes has 110mm diameter.
-all other (smaller) resonances you can see on graphs are measurement artifacts because room modes.
-when you use it in your room, best position for holes of helmholtz absorber is in area where
you get peak/maximum pressure at desired frequency.
-use measurement microphone to find SPL peaks at desired frequency.
-with all helmholtz absorbers we are tried to hit 100Hz resonant frequency... with more or less success.

Hope this can help someone...

Eomsy thank you very much! :)

Yep, don't mention it  :).

And thank U for U're help!

You could also do the subtraction from within REW, use the "All SPL" tab and in the controls set trace B as the room response then trace A as each of the other measurements in turn, select "A-B" as the operation and click "Generate" for each trace.


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