# sound problem

Discussion in 'Physics' started by recca02, Jun 25, 2007.

1. ### recca02 Thread Starter Senior Member

Apr 2, 2007
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i recently underwent a fortnight long training in a power plant(explains my brief absence).
while visiting the turbogenerator section me and my friend who were obsessed with efficiency and application of science found that the turbogenerator set was the noisiest (had to be). so we thought how much amount of energy was actually being lost by this.

so my question is this.
how can we measure the energy of sound?
is there some instrument for this?
can anyone tell the principal which can be used to measure this.
will the knowledge of dB be helpful? (i think it wont be sufficient though.)

2. ### n9352527 AAC Fanatic!

Oct 14, 2005
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Pretty easy if you could fit the turbo generator in an anechoic chamber This is similar with measuring a speaker efficiency (dB/W) with a dB meter. Otherwise, I think it'd be ballpark figure only due to the reflections etc.

Surely the energy is insignificant compared to the generator output?

3. ### recca02 Thread Starter Senior Member

Apr 2, 2007
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yeah it has to be insignificant(few kw) or my nervous system wud have suffered significantly.
it was a gas turbine which powered a abt 260 MW compressor and generated abt 144 MW power coupled with a generator there were 3 in all + one steam one producing 224.1 MW(all German make).

say, can u describe abt how actually is it measured a theoretical answer wud suffice.

i have never HEARD abt this method before (must be the due to the noise)

4. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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There are two types of sound measurements, I think The first one is the pressure (loudness) and the other one is power. If the reflections etc. can be controlled, as in a chamber, then the power per area at a certain distance can be measured for a given distance and area. The problem is for accurate measurement, the chamber has to be much larger than the source volume, like 100 times or so, which can be difficult for a big machine.

I'm no expert on this, the little that I know was from discussions with a colleague that researched this area. I think the actual power is distributed on wide spectrum of frequency, with peaks on several natural resonances of the machine.

Just curious, was it an SST combined-cycle steam plant?

5. ### recca02 Thread Starter Senior Member

Apr 2, 2007
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yes it was a combined cycle plant,
three gas turbines outlet were directed to the boilers.
will the measurement suffer much without the chamber?
what i m really intrested in knowing is what does the sensor sense (vibrations??)
and what sort of theory or formula or idea goes in converting that to sensed parameter to power?
i was wondering abt the frequency range too.
i concur that there may be a vast range of frequency present some even not audible.
thanks for the ideas thus far.

6. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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The parameters measured depend to some extent on the measurement methods. Have a look at this short glossary of methods.

This is a longer articles on sound power measurements, the methods are largely similar with the above.

7. ### recca02 Thread Starter Senior Member

Apr 2, 2007
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thanks,
those links really helped a lot, specially the 2nd one.
one small question.
what about the power rating of a speaker is it the sound power radiated at max volume,
does frequency component come into picture.

abt the two types of measurement you mentioned from what i learned from the links you posted i think they are inter convertible
right?

8. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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I think the obvious differences between the loudness and power measurements are the area and time. The loudness can be pretty much determined at one point and time. Power, OTOH, is measured over a given area, could be sampled over an area or points, and also there is a time factor. Except when the sound is perfectly homogenous with respect to area and time. So if we have enough pressure measurements spread over an area and time, I would think they are convertible.

The power rating of a speaker is just the maximum electrical power that the coil can handle. The loudness depends on the efficiency, which is given by dB/W at a given frequency at a certain distance (usually 1m) in front of the speaker. For example, a 50W speaker with eficiency of 89dB/W would output 89dB at 1W. To increase the loudness, for every 3dB, the input power had to be doubled, i.e. 92dB at 2W.

9. ### recca02 Thread Starter Senior Member

Apr 2, 2007
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thank u very much mr n9352527,
u r simply gr8.

10. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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Your welcome. And if you knew me better you would've never put great and my name in a same sentence

There is another point that stuck in my mind, calibration probably plays a big part in this kind of measurement. Sound power is an absolute quantity, and therefore the instrumentation chain has to be calibrated, from microphone sensor to output amplifier. Probably from a sound source with a known power? Otherwise the result would be meaningless.

11. ### recca02 Thread Starter Senior Member

Apr 2, 2007
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probably my vehicle it always makes a constant noise and its power to unnerve ppl is well known.