Help on Sound pressure Levels of sine waves

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Foufou

Joined: 22 Oct 2006
Posts: 173
Helped: 1

15 Mar 2008 16:28 Re: Help on Sound pressure Levels of sine waves

Hi, I am doing experience on measuring the SPL of sine waves generated from computer by a simple Sound Level meter using the A weighting parameter ,
The expression of the resulting signal is 1*sin(2*pi*f*t) and f varying from 125 Hz to 8 kHz
the results are :
125:: -> 54 dB
250 Hz:: -> 69.9 dB
500Hz:: -> 86.3 db
1khz::--> 101.6dB
2kHz:: --> 111.4 dB
4kHz:: --> 120.6 dB
8kHz:: --> 101.3 dB
According to http://www.beis.de/Elektronik/AudioMeasure/WeightingFilters.html the human ear act like the A weighting filter. The frequency response gived by the above link assume that the A filter for the specifics values is given by
‘’A’’ filter :
125:: -> -16.18dB
250 Hz:: -> -8.67 dB
500Hz:: -> -3.24 dB
1khz::--> 0dB
2kHz:: --> 1.2 dB
4kHz:: --> 0.96 dB
8kHz:: --> -1.14 dB
This response shows that for the frequencies low then 1 kHz the measurements values must be attenuated for each frequency.
For frequencies high then 1 kHz the measurements must be amplified according to the values given above.
The problem is that according to the A weighted filter the measured value for the frequency 2kHz must be greater than the one measured for 4 kHz but it is not the case
The second problem is that for all the frequencies, the attenuation and amplification is not exactly the values given by the A filter ??
I really need an explication

Thank before

Added after 1 hours 18 minutes:

any one can help me it is urgent §§§

FvM

Joined: 22 Jan 2008
Posts: 5156
Helped: 767
Location: Bochum, Germany

15 Mar 2008 16:45 Re: Help on Sound pressure Levels of sine waves

Quote:

measuring the SPL of sine waves generated from computer by a simple Sound Level meter

To my opinion the rating "simple sound level meter" implies that the "instrument" must not be necessarily very exact. You also didn't tell how you applied the test signal to the level meter. Has it a line input? How much is the deviation and what's the accuracy specification of the level meter? Also the sound card may have a frequency characteristic (although less likely). Did you measure the output voltage with an independant instrument?

P.S: Please don't tell that you used a speaker to apply the test signal to the level meter...

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Foufou

Joined: 22 Oct 2006
Posts: 173
Helped: 1

16 Mar 2008 9:25 Re: Help on Sound pressure Levels of sine waves

FvL

I have used Codiam Scientific Sound Level Meter and a TDH39P telephonics headphones. I simple put the head of the Sound Level Meter near to the headphone (left or right) and I hold the same position for all my mesurements using the A weighting Filter (specific to the Sound Level meter)

thanks

FvM

Joined: 22 Jan 2008
Posts: 5156
Helped: 767
Location: Bochum, Germany

16 Mar 2008 9:44 Re: Help on Sound pressure Levels of sine waves

O. K., in this case I think it's simply a problem of transducer frequency characteristics and probably additional coupling effects. Generally, level meter microphone have to be calibrated for frequency characteristic in free field. In contrast, the said audiometer headphones are most likely calibrated with an ear model and a small measurement microphone. Both, microphone and headphone have directional frequency characteristics, and it's additionally matters, if the opposite tranducer is near or far.

Foufou

Joined: 22 Oct 2006
Posts: 173
Helped: 1

17 Mar 2008 8:31 Re: Help on Sound pressure Levels of sine waves

how can I calibrate my Sound Level Meter,?

thanks

Foufou

Joined: 22 Oct 2006
Posts: 173
Helped: 1

17 Mar 2008 15:43 Re: Help on Sound pressure Levels of sine waves

any one can help me

FvM

Joined: 22 Jan 2008
Posts: 5156
Helped: 767
Location: Bochum, Germany

17 Mar 2008 15:53 Re: Help on Sound pressure Levels of sine waves

Usual calibration is possible with a dedicated calibration unit at 1 kHz only (to adjust the sensitivity). Calibration at different frequencies wouldn't be possible without special equipment (dead room, reference microphone). It must be assumed, that a sufficiant accuracy is achieved by design of the instrument.