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      01-10-2019, 02:59 PM   #94
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evanevery's Avatar

Drives: i8 Roadster, M4, Tesla MX
Join Date: Feb 2012
Location: Wisconsin

iTrader: (0)

I moved my i8 from the house to the office garage to swap cars for a weekend trip. I decided to grab a quick sound sample along the way. Unfortunately, I forgot to turn-on auto-leveling for the recording level. I got enough to get a quick sample for analysis but not good enough to post as a sample for a fair evaluation.

This clearly needs to be done properly later but here are a couple of things I noted (I had the Maserati Sound Module in place):

- I mounted my Tascam DR-05 recorder to a blank region of my license plate using a suction cup RAM mount (with an improvised safety strap running into the trunk). I oriented the recording mikes downward below the license plate. I think this is a pretty good mounting position to get a good a high fidelity sample of the exhaust sound. (Too bad I didn't set up my recorder optimally...)

- It was a short trip on side roads. I worked to establish a major section of the drive steady at 4000 RPM and another steady at 2000 RPM (as displayed on the tach).

- I then loaded the captured wav file into an editor (Audacity) and isolated and extracted a solid region for both those RPM's.

- The sound is pretty unimpressive when you hear it from the outside. The idle is better than the sound when moving. (Remember, when you are driving you are mostly hearing the BMW interior sound). I'm hoping it sounds better when I get the recording level set properly...

- I loaded each of those individual regions into a Software Spectrum Analyzer (WaveSpectra) to see what they looked like.

- At 4000 RPM, the dominant frequency appears to be 226 Hz. If my prior assumptions for 4 cyl exhaust sound are correct (Cyl * RPM / 120 = Hz), this would equate to:
  • a 6.78 Cyl engine at exactly 4000 RPM
  • a 6 Cyl engine at 4520 RPM
  • a 8 Cyl engine at 3390 RPM

- At 2000 RPM, the dominant frequency appears to be 118 Hz. This one jumps around a bit more which may be due to the relatively low exhaust sound level mixing with more prominent ambient sounds (road noise). Again, if my assumptions are correct, this would equate to:
  • a 7.08 Cyl engine at exactly 2000 RPM
  • a 6 Cyl engine at 2360 RPM
  • a 8 Cyl engine at 1770 RPM

- So its not tracking the RPM as a steady multiple, but its close! Its also clear I need to get better samples (particularly for the relatively low volume of the 2K RPM test) but that will have to wait until I get back from my trip this weekend...