using Audacity as a simple sound meter

[wrmacint]

I am involved with teacher education. I want to know if Audacity can be used to measure the loudness - in decibels [the number]? We would like students in the classroom to monitor the volume of sound on their computers as they work. Can it be done in a simple way?

thanks

[kozikowski]

Yeeeees...... [he said with rising inflection].

You can launch Audacity and then click once inside the red record meters and they will wake up and meter what they perceive to be the incoming show. It's a diagnostic mode and you don't have to actually record anything to get it. The ballistics are peak digital and the meter only measures the positive portion of the waveform. This can lead to significant errors if your voice has a bias to it like most people have.

You were fuzzy on what exactly the goal is and what you're trying to measure.

Koz

[steve]

in decibels

That's a tricky bit.
Unlike cm or litres, dB are not an absolute measurement - they describe a ratio of loudness against some reference point.
The meters in Audacity are referenced against "full scale" - that is, a range of +1.0 to -1.0 (these values are shown on the vertical scale of the tracks, and relate to the actual numeric values of each digital sample).

How does this relate to "loudness"? It doesn't. If I turn up the recording level and record a quiet sound, then the level shown on the meters may be the same as if I turn the recording level down and record a loud sound. The level on the meter also depends on the sensitivity of the microphone, how far away it is from the sound source, how much gain is applied by the microphone pre-amp.....

The best you will get is a relative level of "the class room is more noisy now than it was 10 minutes ago". Unless you are able to calibrate the system you have no way of working out "how loud" in dB.

If you're interested in pursuing this further:
http://en.wikipedia.org/wiki/Decibel
http://en.wikipedia.org/wiki/Sound_pressure

[wrmacint]

Thanks Koz and Steventhefiddle

In the past, students were measuring the 'loudness' of homemade instruments in elementary/ middle school using a sound meter / that recorded a value in dB. As schools do not have many sound meters, preservice students were wanting a device that could record "straight" value in dB. We thought that Audacity would be able to give us a "value" when playing 4 different lengths of st

As noted that the dB value is relative so I am now thinking that we will need to use the sound meter and use the 'reading on the sound meter' to make a calibrated system for the audacity meter.

Sooo is there anyone who has done this? produce a simple calibration scale that would work with different sound instruments? Perhaps I should get some preservice students to conduct an inquiry into this! Many thanks for furthering my thinking about the use of Audacity.

Bill

[kozikowski]

That was in dBSPL. Sound Pressure Level. That one is an absolute and there are special meters that can measure it and tell you numbers...like this.

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

Work has the digital one and I have the analog one. They still sell both because you can't measure music with the fancy digital one.

Note that each instrument is an enclosed unit. Everything is matched inside and calibrated to always do the same thing the same way. You can't get a collection of different bits and pieces to do that. If you have one real sound meter, you can make one computer simulate a dBSPL meter by writing down where all the computer settings are, but then you can't ever change anything on the computer, any of the sound panels, any of the cables or any other software. If you do, you will have to compare it to the original instrument (calibrate it) all over again.

And that will only give you the "C" sound curve, not the more critical"A" curve.

Most people meet these instruments the first time when somebody complains that the workplace noises are a health hazard. They rarely are, but the laws are written in "A" curve.

Those Radio Shack meters are the cheapest way out I've ever seen. They're accurate enough that they get used in some of the more important theaters in Hollywood.

Koz

[kozikowski]

If you go the comparison route, all you really need is to subject the meter and the microphone to the same sound. Write down the difference between the two readings. The difference should be consistent. The computer is always 12 dB low. Or the meter is always 15dB higher. Something like that.

Since nobody will stop breathing if your numbers are off a bit, this should be OK, but again, you can't adjust any of the computer sound panels after you calibrate or compare them the first time. That will throw the calibration off.

Another way you can get killed is to drive the microphone into distortion or overloading by accident. This is easy to do on a computer sound card. That will kill the calibration, too. "How come the sound is going up but the meter isn't?" Because you're damaging the sound card with sound that's too loud. The real meter is calibrated to avoid that.

Koz