Hello Eomsy, thanks for contributing to PSW R/E/P's "Acoustics in Motion"
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
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
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
This include helmholtz resonator which we measure and measurement microphone.
We only may move covers and do measurements... anytihng other in room should not
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...