964 Spectral Analysis
09-30-2006, 10:04 PM
#1
Racer
Thread Starter
964 Spectral Analysis
I made some measurements of the engine sound in my C2, and thought maybe others might be interested. I used an Ivie Technologies IE35 handheld spectrum analyzer to record several wave files at idle, 1000, 2000, 3000, 4000 and 5000 rpm. This analyzer has an excellent Type II mic, so the recordings are good fidelity.
In the driver's seat, windows closed, the SPL was as follows:
Idle: 89.7 dB
1000: 90.0 dB
2000: 96.7 dB
3000: 97.0 dB
4000: 97.5 dB
5000: 98.3 dB
It's interesting to see the jump in loudness between idle and 2000 rpm: it really picks up. And then above 4000rpm it picks up again. At 5000rpm 98dB SPL is pretty darned loud! (My engine compartment is missing the foam insulation, and I wonder what effect that would have.)
I also analysed the frequency distribution at the various rpms. The spectra are shown HERE
If you look at those, you can see how there is a build up of sound level across the spectrum as the revs increase, and there are also noticeable peaks in the lower frequencies which probably are fundamentals or harmonics of the engine speed.
E.g. at 5000 rpm (which is 83Hz) you can see the biggest peak on the spectrum is at ~80Hz.
In the driver's seat, windows closed, the SPL was as follows:
Idle: 89.7 dB
1000: 90.0 dB
2000: 96.7 dB
3000: 97.0 dB
4000: 97.5 dB
5000: 98.3 dB
It's interesting to see the jump in loudness between idle and 2000 rpm: it really picks up. And then above 4000rpm it picks up again. At 5000rpm 98dB SPL is pretty darned loud! (My engine compartment is missing the foam insulation, and I wonder what effect that would have.)
I also analysed the frequency distribution at the various rpms. The spectra are shown HERE
If you look at those, you can see how there is a build up of sound level across the spectrum as the revs increase, and there are also noticeable peaks in the lower frequencies which probably are fundamentals or harmonics of the engine speed.
E.g. at 5000 rpm (which is 83Hz) you can see the biggest peak on the spectrum is at ~80Hz.
10-01-2006, 01:26 AM
#2
Interesting... I wonder what the sound attenuation would be with the insulation. My guess is pretty good. I don't think mine is nearly that loud, and mine has the insulation.
10-01-2006, 09:41 AM
#3
Addict
Rennlist Member
Rennlist Member
Are readings weighted? Normally for human perception, sound levels are reported as dBA, where the A weighting reflects the frequency response of human hearing. If these are dBA readings, they seem really high - will into the "hearing damage" range.
Can your hear the radio? What do you have for mufflers? Windows and sunroof open or closed? What exhaust are you running? My C2 is bolt stock. I have not measured the SPL, but the cabin is quiet enough to enjoy the stereo if I like.
Can your hear the radio? What do you have for mufflers? Windows and sunroof open or closed? What exhaust are you running? My C2 is bolt stock. I have not measured the SPL, but the cabin is quiet enough to enjoy the stereo if I like.
10-01-2006, 01:59 PM
#4
Racer
Thread Starter
Originally Posted by springer3
Are readings weighted? Normally for human perception, sound levels are reported as dBA, where the A weighting reflects the frequency response of human hearing. If these are dBA readings, they seem really high - will into the "hearing damage" range.
Can your hear the radio? What do you have for mufflers? Windows and sunroof open or closed? What exhaust are you running? My C2 is bolt stock. I have not measured the SPL, but the cabin is quiet enough to enjoy the stereo if I like.
Can your hear the radio? What do you have for mufflers? Windows and sunroof open or closed? What exhaust are you running? My C2 is bolt stock. I have not measured the SPL, but the cabin is quiet enough to enjoy the stereo if I like.
The car is completely stock. The windows and sunroof were shut. I don't often listen to the radio/CD but I can do so quite comfortably. But it's by no means a quiet car (thank goodness!).