Acoustic signal used in this section was collected under the setup shown below. The acoustic sensor is an microphone (RadioShack) and connected to a laptop computer. The measured acoustic signal is sampled at 11 kHz with 8 bit resolution.

Fig.1 Field test setup

The following field testes were done in Richmond Field Station (RFS) with a Mazada 626. First, figure 2 and 3 show the background acoustic signals without any vehicle, and its frequency domain respectively. Notice that the acoustic energy in the background is highly dependent on wind-age and the wind-age energy mainly concentrates between DC and 500Hz.

Fig.2 Background acoustic signal	Fig.3 Background acoustic signal in frequency domain

Next, the acoustic signals were measured from a stationary vehicle with its engine turned on. Figure 4 and 5 show the acoustic signals with the microphone placed close to the engine exhaust. While Figure 6 and 7 show that with the microphone placed under the front bumper. Compared to the background acoustic spectrum, the engine acoustic signal has harmonics above 500Hz, which could be coming from engine cranking.

Fig.4 Exhaust acoustic signal	Fig5 Exhaust acoustic signal in frequency domain
Fig.6 Engine acoustic signal	Fig.7 Engine acoustic signal in frequency domain

Finally, the acoustic signals were measured for slow and fast moving vehicles. Figure 8 and 9 show the acoustic signal from the Mazda 626 when it was running at about 5 mph. Notice that the time domain waveform is severely smeared by strong wind disturbance. Figure 10 and 11 show the acoustic signal when it is running at about 20 mph.

Fig.8 Acoustic signal of Mazda 626 running at 5mph	Fig.9 Acoustic signal in frequency domain of Mazda 626 running at 5mph
Fig.10 Acoustic signal of Mazda 626 running at 20mph	Fig.11 Acoustic signal in frequency domain of Mazda 626 running at 20mph
The links to some of the available acoustic data bank are shown on the right.