How to compute SPL from sound inputed from a microphone.

[lance]

Dear all
I am new to audacity.
I want to develop an ap with function of measure sound level. I have not enough knowlegde about that. May I have your suggestion? Is there some code in audacity I can reference?
My programme language is C# and MFC.
Any suggestion will be greatly appreciated.

Email:lanxiaolin007@sina.com
Xiaolin.lan

[Irish]

It is not as simple as that.

To measure sound level, you have to know the following:

With [this microphone], a sound pressure level (SPL) of [this number of decibels] at [this frequency] at a distance of 1 metre from the mic gives an output of [this number of millivolts].
With [this sound card] on [this computer] using [this operating system], and with [these input settings in Control Panel] and [these input settings in Audacity], an input of [this number of millivolts] gives a level of [this number of decibels] on the recording level meters in Audacity.

That's eight variables between the sound and the display you will get in Audacity. Change any one of them and your answer will be wrong.

Now, having completely discouraged you, there is one way you could get an appoximate value for SPL using Audacity;
Beg, borrow or rent a Sound Pressure Level meter. Put its microphone and your microphone (connected to Audacity) side by side and record a range of sounds.
Note the reading on the SPL meter and Audacity's level meter for each sound.

Now you know that with that microphone, that computer and that setup, a level of "x" dB on the Audacity meter means that the sound level is "y" dB.

That's about as close as you will get.

PO'L

[lance]

Dear PO'L
Thanks for your reply.
I will make the solution according your suggestion.
Could you tell me which file the code is used to compute sound level locate at?
I have read the code for a day, but I am so dizzy because of too many source code files.

[Irish]

I don't think the source code will tell you a lot.

Audacity doesn't calculate SPL values; it just converts the analogue input to digital and assigns 0dB on the recording level meters to a certain input level, which corresponds to the maximum value of the digital signal. I'm not familiar with the source code, but I don't think you will find a part of it that you can take out and use as a separate application.

Maybe someone else can point you to the part of the code that generates the meter display.

PO'L

[lance]

OK!
I should try to look up some other solution.
Thanks!
Lance

[kozikowski]

<<<That's about as close as you will get.>>>

Real SPL meters also have curves. Workplace rules for dangerous noises are written in "A" curve which gives less importance to very high and very low pitch sounds. The "C" curve isn't a curve at all, but gives equal importance to all pitches of sound. So even if you got the code to work, you'd still only have one of the two meters.

dBSPL meters are not all that big a deal to walk out and buy one.

http://www.radioshack.com/search/index. ... meter&sr=1

I have an analog one and the company owns a digital one. They still make both because you can't measure music with the digital one.

Koz