Power Mac Noise - Analyzing

After you have recorded the noise from your Power Mac, you can use Amadeus to analyze it. The goal is to identify where your Power Mac is making the most noise. This will make it easier to decide what you should do. It may even help in the discussions with Apple. A noisy case fan may be fixable with firmware, but a noise power supply requires physically replacing the power supply.

Bring up the spectrum analysis from your recording. You can should save this spectrum to a file for future reference:

By replacing individual fans in the Power Mac and comparing the spectrum before and after, I am fairly confident of the type of noise each fan generates. The table below explains what causes noise in each of the frequency ranges you are likely to see a peak or spike. The frequencies may vary slightly depending on the system, but that won't invalidate the results.

Frequency	Sound component
120Hz	Vibration from the two 60mm Delta power supply fans. Because of the power supply design, the fans are in direct contact with the case and vibration appears to show up exactly at 120hz. I am guessing this is the 60hz AC power multiplied by the two fans.
0-200Hz	Excluding the spike at 120Hz from the power supply, the rest of this appears to be air movement. Ideally, you want to see all noise in this range.
600-800Hz	The 120mm Delta case fan. This is the huge fan at the bottom of the case which cools the CPU and hard drives. This fan is variable speed and theoretically should remain quiet until it is needed. The so called "leafblower" noise is caused by this fan spinning at a high rate.
1300-1500Hz	The two 60mm Delta power supply fans. These fans are small, but spin at a fixed rate. For most people, this is the cause of the annoying whining noise.

Amadeus has a nice feature that allows you to see the exact frequency and decibel for any frequency. Just move your cursor over the spectrum so that it lines up veritically with the noise you are interested in. The frequency and decibel will be displayed in the top left hand corner. Here's the example with my cursor over the left most spike.

Now write down the information for each of the peaks and you will have something like this:

Frequency	Decibels	Relative Decibels
120Hz	-44dB	-4dB
660Hz	-40dB	0dB
1520Hz	-50dB	-10dB

The relative decibels column is obtained simply by setting the largest decibel value (-40dB at 660Hz) to 0dB and adding the same amount to the other frequency ranges. This makes it easier to see how the various noise components compare to each other. In this case, it looks like the case fan is hte noisiest. If I remember correctly, each 3dB is approximately approximately double the volume. So, the case fan is more than twice as loud as the power supply vibration and more than 8 times as loud as the power supply fan itself.

Now when you make any changes to your Power Mac, you can quantitatively measure the difference in noise. For example, the spectrum shown on this page is the Power Mac after I replaced the Delta 60mm power supply fans. After that, I made another change to put a piece of leather in between the power supply fans and the case. Here's the spectrum and analysis for that:

Frequency	Decibels	Relative Decibels
120Hz	-67dB	-19dB
660Hz	-48dB	0dB
1520Hz	-57dB	-11dB

The noise level at 660hz should have stayed the same, but for some reason, it dropped by 8dB. That might simply mean that the microphone placement made the second recording quieter. Nevertheless, the results are still useful by looking at the relative decibels. Note that the relative decibels has dropped 15dB at 120Hz and 1dB at 1520Hz. I had not noticed this result before I wrote this page and it surprised me. This indicates that the vibration transmitted by the power supply fans is actually at 120Hz and not at 1520Hz as I initially thought (I had to rewrite the frequency table at the top of the page). Since I had already replaced the power supply fans before I add the leather insert, it just means that the power supply fans themselves were already fairly quiet.

So in this example, the 120mm case fans are now the loudest noise in the Power Mac by far. It was at this point that I replaced the 120mm case fan and approached nirvana, but that's another web page altogether. :-)

Hopefully, this web page shows how you can:

Identify the noisiest component in your system.
Measure the impact of making a change to the system.
Rinse and repeat.

Please post your results to the Yahoo G4Noise group. If you would prefer not to join, then email them to me at (Chun Hsu) hsuc@centurytel.net. We need as much data as we can to confirm these theories.

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