Power Mac Noise - Recording

Overview

I put together this page to encourage other people to do an analysis of their noisy Power Mac computers.  My main interest right now is finding out, in a concrete fashion, whether or not there is a difference between the noisy and quiet Power Mac computers.  Right now, the definition of a noisy Power Mac is somewhat subjective.  Are people complaining because they are more sensitive?  Do all Power Mac's sound the same, or are the noisy ones truly different?  I think doing a spectrum analysis will help.  Of course, it's possible that it will turn out that all Power Mac's sound the same, but I would really like to find out.

Note that I am not trying to measure the volume of the noise, but the frequency characteristics.   The reason is that I don't have an accurate way of measuring volume which would be very sensitive to the microphone location and background noise.

Microphone

The Power Mac has an audio in minijack on the rear.  However, it looks like it requires a powered microphone which I do not have.  If you have a USB microphone, I assume that would work as well.  In my case, I had none of these things, so I used a minidisc player/recorder.  For a microphone, I improvised and used a Plantronics headset that I bought for one of my other computers.  This is not an ideal setup, but when I played it back, the recordings did sound reasonably accurate.

For the recording, I placed the microphone on top of the computer case with the end of the microphone approximately 2 inches in front of the computer.  I placed it there because I noticed that the sound from the rear of the Power Mac is more of the normal low frequency whooshing sound, where as the annoying droning comes out from the front.  This is probably due to the fact that the two 60mm power supply fans are located towards the front of the case.

Recording Software

In order to analyze the noise, the audio sample has to be recorded or transferred to a Mac.  I downloaded a 15 day trial version of Amadeus from HairerSoft.  I have never used the software, or for that matter, any other audio analysis software.  However, this seemed to fit the bill, and I will register the software if I end up doing much more analysis.

Recording

The steps for recording are pretty much the same whether you record directly to your Power Mac, or transfer the recording from something like a minidisc recorder.  First, startup Amadeus.  It should automatically open a new audio file.  Then select Sound->Record from the menu.




This will bring up the recording dialog.



Check the playthrough option.  This will automatically play whatever is coming through the line-in jack through your speakers.  Adjust the Input Gain until you can hearly hear the audio sample.  I had to max out the Input Gain in order to hear my recordings.  Either the microphone was not very sensitive, or else the noise simply wasn't that loud (certainly true after all of the modifications).  Push the record button to start, and stop when you are finished.  The main goal is to get a clean sample somewhere in the recording.  The entire sample doesn't need to be clean, because we are going to select the part that we want to analyze later.

Click OK once you are satisfied you have a reasonable audio sample.  I generally recorded 15-20 seconds of audio just in case.  Save the audio sample to a file for future reference.  Next, find a clean section of audio in your recording by using your mouse to select a portion of the audio sample.



Hit the space key to listen to the selected portion.  Keep adjusting your selection until you are satisfied that you have a reasonably clean audio sample.  Now you can run the spectrum analysis by selecting Analyze->Spectrum.



A dialog will pop up.  I just clicked the OK button without changing any options.  The spectrum analysis should appear.



Now you are ready to analyze your results.

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