71 dB
Headphoneus Supremus
We are splitting hairs here. Slowly dropping means incredibly slow (hardly noticeable by the observer), but it is a mathematical fact. When the car is x meters from the observer approaching, the frequencies emitted by the car get multiplied due to Doppler effect byIt doesn’t slowly drop, it stays constant (assuming a constant speed) and then drops quite quickly as it passes and then stays constant again. The pitch change actually occurs over a period typically of 0.5 - 1 second, although obviously depends on the speed of the vehicle.
c/(c-v*cos(𝜶)),
where 𝜶 is the angle from the car to the observer compared to the street the car is driving. This angle changes while the car drives by the observer causing the multiplier effect dynamically change. The angle 𝜶 is calculated as
tan(𝜶) = y/x ⇒ 𝜶 = arctan(y/x).
Or
cos(𝜶) = x/(√(x²+y²)) ⇒ 𝜶 = arccos (x/(√(x²+y²))).
Let's assume a car is driving by with a minimum distance of y=5 m at a speed of v=72 km/h (20 m/s) in conditions were the speed of sound was c=345 m/s and these was no wind. When the car is x=50 meters away approaching, the Doppler multiplier is
(345/(345-20*cos(arctan(5/50)))) = 1.061214363...
When the car is only x=40 meters away, the Doppler multiplier is
(345/(345-20*cos(arctan(5/40)))) = 1.06103425...
When the car is x=30 meters away, the Doppler multiplier is
(345/(345-20*cos(arctan(5/30)))) = 1.060650382...
When the car is x=20 meters away, the Doppler multiplier is
(345/(345-20*cos(arctan(5/20)))) = 1.059591584...
When the car is x=10 meters away, the Doppler multiplier is
(345/(345-20*cos(arctan(5/10)))) = 1.054686387...
Not a dramatic change for sure, but not "constant" either.
Yeah, good point! This is more relevant in movie sound for obvious reasons...It is sometimes but it’s quite rare in music production. It’s very common in film sound though, obviously there are often vehicle passbys, including aircraft. If fact there are entire sound libraries of passby but sometimes we need to create our own for various things, from artillery and other weapons or projectiles to flying insects. There are a number of plugins to help with the process and all sound designers/mixers have at least one in their collection.
G