Finding the CORRECTED octave band sound pressure levels

[sandow]

Hi everyone,
I have got a curious question and am wondering if I can get some clarification from you experts out there.
I am given a typical calibration chart (V/Pa) for a free-field microphone.
On the chart I am given the free-field response and the pressure response for a typical free-field microphone, where the free field response graph is fairly straight at 0dB, while the pressure response starts droping down at about 1000Hz.

I am also given the following data,
At 2000Hz the (pressure level) Lp=92 re 2*10^-5 Pa
Also at 2000Hz (x-axis on the graph) extending a vertical line up to the response curve and then a horizontal line up to the y-axis, I obtain the following value:
-0.5dB.

My objective is to find the CORRECTED octave band sound pressure levels.

Would any be kind enough to direct me to the correct way to attack this problem??
Thank you in Advance.

 

[MikeyP]

Without knowing the details of the calibration chart. I would interpret it as follows:

The calibration chart shows the sensitivity of the microphone output voltage to an input pressure. A sensitivity of -0.5 dB at 2000 Hz on the chart means that it is LESS sensitive than it is at lower frequencies, ie it reads a smaller pressure than is actually present. The reading you get from the microphone needs to be INCREASED to compensate for this. So if the pressure measured by the microphone is 92 dB SPL @ 2kHz, when you are assuming that the sensitivity of the mic is that at low frequencies, then the actual SPL will be 92.5 dB SPL.

You may well find that a difference of 0.5 dB is small compared to the variance of a number of repeated measurements and is therefore not very significant. Also additional factors such as directivity may be more important at high frequencies.

Michael

 

[sandow]

Yep! that makes sense. I was getting confused on how to interpret the values that i obtained from the chart to the values that were given to me... but your post made it all clear..
I would like to thank you very much for this helpful post Michael, i really do appreciate it.
Thank you

 

[Rob45]

I think the actual interpretation of your calibration data might be just a little different.
The two values you are given are for use of the same mike as a "free field" mike or as a "pressure field" mike.
A "free field" mike is used for making sound level measurements in accordance with IEC Type 1, while a "pressure field" mike is used for ANSI Type 2 measurements.

Free-field-response microphones are used for measuring sound coming mainly from one direction. Their frequency-response curve is designed to compensate for the pressure build-up at the diaphragm caused by interference and diffraction effects. Measured sound-pressure levels are therefore equal to those that would exist in the sound field if the microphone were not present.
Pressure-response microphones do not compensate for the pressure build-up at the microphone diaphragm — they measure the actual sound- pressure level at the diaphragm. Uses include measuring sound- pressure levels at a surface (if the microphone is flush-mounted), or in a closed cavity (where the microphone is part of the cavity wall). Pressure-response microphones can be used as free- field microphones if they are oriented at right-angles to the direction of sound propagation — but their effective frequency range is then reduced.