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Alfa Romeo V6 engine exhaust noise
(Automobile noise analysis 3)

The exhaust sound at the time of idling of ALFA ROMEO 164 was measured for comparing with that of Ferrari Mondial t.

Date 2002/01/29 9:00 a.m.
Place Tokyo, JAPAN
Weather Fair weather (Air temperature 10 degree Celsius, Humidity 44%)
Measurement device Personal computer: SONY VAIO PCG-Z505GR/K
Video camera: SONY DCR-TRV300
Stereo microphone: Audio Technica AT9450 
Software Realtime Analyzer and Sound Analyzer of DSSF3
Measurement object 1994 ALFA ROMEO 164 Super 24V
3 liter water-cooled V 6-cylinder DOHC 24 valve
WAVE file alfaromeo164.wav (44.1kHz / Stereo / 10sec / 1.68MB)


I could hear a low-pitched "booow" sounds from the muffler. The external stereo microphone was attached to the video camera and placed to the position of the above left photograph. Sound data of the digital video was transferred to the PC, reproduced by the Windows Media Player, and then recorded by the Realtime Analyzer.

The running ACF for 10 seconds of sound was measured. Sound was recorded on this window, and saved as a .wav format. Once the wav file is saved on the hard disk, it can be loaded directly. 

Then, the recorded sound was analyzed by the Sound Analyzer. 

A calculation condition setup by the sound analyzer. Sound is analyzed in the interval of 0.1 seconds. Integration time of 2 sec means that the 2 s portion of sound is analyzed.

This graph shows the time course of F(0) (relative sound level). The sound level we hear is almost constant (range is 0.5 dB).

te of alpha was 18 - 27ms. It is longer than te of 6 - 8 ms of Ferrari. It means that this sound is more reverberant. It seems the character of Alpha sound like "booow".

The t1 (representative frequency) of alpha was about 2.4 ms (417 Hz). It is a little higher pitch than 2.8 ms (357 Hz) of Ferrari.

The f1 (pitch strength) has some variations. 

IACC is about 0.8. This parameter measures a degree of correlation between stereo inputs, and expresses the spatial impression of sound, originally used in the concert hall acoustics (see more information). We hear a diffuse sound (i.e. spatially extended sound) when the IACC value is low. On the contrary, we hear a very sharp localization of sound when the IACC is high.

It is the example of the ACF of sound from Alpha V6. Secrets of the sound quality may be hidden in this waveform.

The spectrum for the same data portion. We can see the harmonic components at 50 Hz, 75 Hz, 100 Hz, and so on. Though the amplitude is small, the fundamental frequency can be seen at 25 Hz. It is corresponded to the ACF waveform. The maximum peak at 40 ms in the ACF is 1000 / 40 = 25 Hz.

This time, I reported the measurement of the exhaust sound at the time of the idling of ALFA ROMEO 164 by the running ACF analysis. Some of the acoustic parameters were different from Ferrari Mondial, reflecting the hearing impression to some extent. 

January 2002 by Masatsugu Sakurai