Resonator Freq. and Absorption Measuring Method

Hi,

I have a question in order to know how to measure the Resonant Frequency of a HR with some open ports and how I can estimate the absorption coefficient using a Home Measurement Set-Up and REW software?....I found something very interesting discussion in another forum (http://repforums.prosoundweb.com/index.php?board=29.0) but the post is very old.

I will appreciate any help!
Opacheco,

opacheco said:

I have a question in order to know how to measure the Resonant Frequency of a HR with some open ports and how I can estimate the absorption coefficient using a Home Measurement Set-Up and REW software?...

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Finding the resonant frequency is not too difficult; a basic frequency response of the room with the mic close to any of the holes will show a dip at the resonant frequency, as long as the Q is not too wide. Now the absorption coefficient is not a direct measurement; it can only be calculated from room response measurements; since the coefficient is evaluated as its contribution to the whole surface, it is very difficult to evaluate in an actual room. Mesurement of industrial absorbers is done in a purpose-built room where the material to be tested is used on a large proportion of the surface; they are particularly difficult to do at low frequencies because vibrations in the structure and air leaks introduce a significant error.
Indeed, knowing the absorption coefficient with reasonable accuracy would be of value for predicting the effect of room treatment, but I don't think it's accessible to a DIYer. Rule-of-thumb would say the absorption coefficient is the ratio of open holes area vs. the frontal area of the resonator. That would be only at the resonant frequency. True only if lossless (very narrow Q). Absorption coefficient decreases as frequency moves from resonant frequency.

I found something very interesting discussion in another forum (http://repforums.prosoundweb.com/index.php?board=29.0) but the post is very old.

Click to expand...

I read this and wonder why they measured the resonator with only one, two or three holes open. Everybody knows it changes the resonant frequency and how. It seems they are making a mistery of data processing, and finally I don't see a definite conclusion. The important data are verification of the tuning frequency (because Helmoltz resonators are to be used for taming primary_mode resonances, that have a very precise resonant frequency) and comparison of the frequency response before and after.

One more question regarding their measurements.

Velocity at absorption frequency is high directly at the holes, isn't it?
So would sticking the mic in the box lead to a mirrored measurement?
Mics measure the pressure.

With some of my random build panel absorbers I simply measure the working freq by hand.
So I plug my sinus generator in the speakers and sweep while lightly touching the panel.
Maximum movement of the panel will be maximum absorption.

I'd stick a mice inside the HR and hit the thing.
The tone it makes would be the resonant frequency or is this naive?

DerEber said:

Velocity at absorption frequency is high directly at the holes, isn't it?

Click to expand...

Correct.

So would sticking the mic in the box lead to a mirrored measurement?

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Yes; pressure may not be uniform in the box, but at the nominal resonant frequency, you shouldn't see much difference.

Mics measure the pressure.

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Measurement mics (omni) do; a cardioid mic would not be accurate there.

With some of my random build panel absorbers I simply measure the working freq by hand.
So I plug my sinus generator in the speakers and sweep while lightly touching the panel.
Maximum movement of the panel will be maximum absorption.

Click to expand...

That's a correct method, although its sensitivity is mediocre.

I'd stick a mice inside the HR and hit the thing.
The tone it makes would be the resonant frequency or is this naive?

Click to expand...

That is correct. The stimulus must be applied to the air; I suggest you hit one of the holes with the flat of your hand. Some percussion instruments are played that way.

So I need more of those percussion Instruments to get my bass tight.  ;D

Thanks for confirming!
So it looks rather easy to measure the Helmholzresonator frequency.
Go ahead Opacheco!

The panel resonances and also HR resonance (if it's built in a box) can be measured with a laser doppler vibrometer  . But a piezo  transducer (from a buzzer) will do, IMO. The output signal should be conditioned with an active DI-box and then analyzed with a  software. 
@Opacheco
You should place your HR to the corner of your room where all mods are active when you are measuring it, IMO.

DerEber said:

So I need more of those percussion Instruments to get my bass tight.  ;D

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I guess you know the Greek put differently tuned amphors in their theatres. The first explorers that discovered them concluded the Greek drank a lot when going to the theatre.

abbey road d enfer said:

.......Indeed, knowing the absorption coefficient with reasonable accuracy would be of value for predicting the effect of room treatment, but I don't think it's accessible to a DIYer. Rule-of-thumb would say the absorption coefficient is the ratio of open holes area vs. the frontal area of the resonator. That would be only at the resonant frequency. True only if lossless (very narrow Q). Absorption coefficient decreases as frequency moves from resonant frequency.....

Click to expand...

abbey road d enfer,

Thanks for your time taken for do son clear comments like this!. Very interesting to do an coarse approximation by the area ratios....

May be a Theorical Analysis will be better than a home measurements; in ths way I found one study for that but the math is very high level. I will try to read and get some idea about....IF I CAN!

I will post the link soon!

abbey road d enfer said:

I read this and wonder why they measured the resonator with only one, two or three holes open. Everybody knows it changes the resonant frequency and how. It seems they are making a mistery of data processing, and finally I don't see a definite conclusion. The important data are verification of the tuning frequency (because Helmoltz resonators are to be used for taming primary_mode resonances, that have a very precise resonant frequency) and comparison of the frequency response before and after.

Click to expand...

Maybe they are doing some measurements in order to know the effect of each port added!....or for funtionality purpose! like more ports more absorption like a EQ for your rooms.

Thanks a lot again,
Opacheco

opacheco said:

Maybe they are doing some measurements in order to know the effect of each port added!....or for funtionality purpose! like more ports more absorption like a EQ for your rooms.

Click to expand...

the graphs show exactly that; the more open holes, the higher the frequency (standard Helmoltz formula) and the more absorption (because more absorptive area). The holes are almost 100% absorbent, the more of them, the more the absorption. I guess they tried to prove the validity of their method.

abbey road d enfer said:

the graphs show exactly that; the more open holes, the higher the frequency (standard Helmoltz formula) and the more absorption (because more absorptive area). The holes are almost 100% absorbent, the more of them, the more the absorption. I guess they tried to prove the validity of their method.

Click to expand...

yeah!, I believe that.

In other hand, this is the link that I find in order to "A theoretical Model to Predict the Low-Frequency Sound
Absorption of a Helmholtz Resonator Array" by my profesor Yang-Hann Kim and the other scientifics:

https://pdfs.semanticscholar.org/aac5/40d8a45fd0741859c908b5d17c3af66f30b6.pdf

Maybe someone can be able to comment about this prediction formulation for absorption coeficiente.

opacheco.

moamps said:

The panel resonances and also HR resonance (if it's built in a box) can be measured with a laser doppler vibrometer  . But a piezo  transducer (from a buzzer) will do, IMO. The output signal should be conditioned with an active DI-box and then analyzed with a  software. 
@Opacheco
You should place your HR to the corner of your room where all mods are active when you are measuring it, IMO.

Click to expand...

moamps, interesting notes!...I will need to study about these measurements options.

Thanks
opacheco.