Control of sound signal 1

[elogesh]

Hai,

There may be different standards for evaluating the sound reduction index of materials.
I want to do an experiment for measurement of sound reduction of structures.
What I want to do is two step procedure as mentioned below,

1) Initialy,I will place miniature size speaker driven by controlled signal.I will measure the sound pressure or intensity at 10 points on the hemisphere at a distance of 1m(Measurement points are according to ISO standard)only due to speaker.We have anechoic room for conducting the measurements.
2)Then I will take the structure to be evaluted.The structure is a dome like structure(Not spherical shape) may be less than 0.1m radius.I will place this structure over the speaker.Speaker and structure are rested on hard floor walls.The resting faces of the structure are exactly flat.
I will drive the speaker by the same signal mentioned in 1.
I will measure the sound pressure or intensity at the same 10 points on the hemisphere at a distance of 1m(The measure ment point will be according to ISO standards)

Then I will subtract the sound pressure or intensity values of 2(structure over the speaker) from 1(only speaker) to evalute the sound reduction characteristics.

In this regard I need few informations,

1)Whatever I am planning to do is appropriate one for simple evaluation of sound reduction characteristics?

2) How to control the frequency characteristics of the sound signal by controling the input signal to speaker?
We are having function generator at our company.
I am planning to apply constant amplitude signal vs frequency.
3) What type of speakers are recommended for this measurement, so that good signal to noise ratio will be maintained?

With Thanks and Regards,
Logesh.e

 

[GregLocock]

1) Yes, sort of, but do you realise that the loudspeaker will behave differently in an enclosure (ie your second test) than when you test it by itself. In fact, the answer is more nearly no.

2) Put a monitoring mic in the near field of the speaker and use that to drive the level of excitation in a feedback loop.

3)S/N is a big problem. Typical attenuations for enclosures are of the order of 20 dB, so you need to be generating 90-100 dB to get a decent signal at the mic, tricky with such a small test item.

Basically you have got the wrong approach, you need a sound source that delivers a constant acoustic power whatever the impedance of the surroundings. A transmission loss suite is such a beast.

However, I have thought of a neat alternative.

The principle of reciprocity says that you can replace each mic by a loudspeaker, and vice versa.

So you mount a mic at the centre of the hemisphere, then use an omnidirectonal sound source at each of the ten locations on the hemisphere. This solves your S/N problem since the speaker can be fairly large.

This should give the right answer. Bruel and Kjaer make omni directional sound sources, I'm sure other people do as well.

Your suggested approach is OK for a crude estimate but it has many flaws, I'm afraid.





Cheers

Greg Locock

 

[elogesh]

Thanks Greg Locock.

I will prefer the alternative approach mentioned by you.

If I get something positive and interesting after this experiment, I will get back to this thread once again.

With Thanks and Regards,
E.Logesh