The Doppler effect

The Doppler effect is the perceived change in sound frequency caused by the movement of the observer and the sound source moving in relation to each other. When they approach each other, the pitch increases as the waves become denser. When the distance increases, the pitch decreases as the density of the waves decreases. The Doppler effect can also be illustrated visually.

The sound source may, for example, be a car driving on a motorway. As the car passes the observer, the pitch of the sound created by the car decreases. The speed of the car can be determined from the sound interval using the following table. If the falling interval observed when the car is passing by is a major third, the car is speeding (the speed is over 120 km/h).

Ratio	Vehicle speed	Difference in 1200^th parts of octave (cents)	Interval
s	*(1-2)1220/(s+1)**	*3986log s**
0,71	207	593	Tritone
0,72	199	569
0,73	190	545
0,74	182	521
0,75	174	498	Fourth
0,76	166	485
0,77	159	452
0,78	151	430
0,79	143	408	Major third
0.80	136	386	Natural major third
0.81	128	365
0.82	121	344
0.83	113	323
0.84	106	302	Minor third
0.85	99	281
0.86	92	261
0.87	85	241
0.88	78	221
0.89	71	202	Major second

Kaava Doppler-ilmiön laskemiseksi:


Distance decreases		Distance increases

The frequency of a car horn is 440 Hz. What is the frequency
a) when the car is approaching at 90 km/h (25 m/s) > 475 Hz
b) when the car is receding at 90 km/h > 410 Hz

= 343
= 25
= 440

When the car is passing by, what is perceived by the ear is the difference between the frequencies.
In this case, the difference is that between a whole tone and a minor third (255 cents).

For this, the table below gives the ratio of 410/475 = 0.863... -> approx. 250260 C.

Temperature also has a minor effect on the pitch change of the Doppler effect:

	0° C	13° C	20° C
Sound velocity km/h	1191	1220	1235
Vehicle speed required to maintain the same interval	96,6	98,9	100,1