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Q & A: Doppler on the Road - Where Sounds Come From

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Most recent answer: 10/22/2007
Q:
When I’m out walking on a country road, I can tell which direction a car is coming at me from ie. either from behind me or in front of me. I think that the sound from behind has a different tone than the sound from in front of me. Is this correct? If so, does this happen because I’m picking up the vibration/sound at a different point on the sound wave curve - you know the curve either below the line or above that depicts how sound travels? Or am I right off beat here?
- Veronicah Hampton (age 32)
Alliant International New Zealand Limited, New Zealand
A:
Veronica-

Knowing whether the car is in front of you or behind you has to do with how your brain interprets the signals that it gets from your ears. When you are walking down the street, you are probably turning your head very slightly from side to side as you go. If, for instance, a sound is coming from in front of you, and you turn your head to the right, then your left ear will hear the sound slightly sooner than your right one does. (Because the sound would have to travel all the way around your head to get to the right one.) If you turn your head the other way, then the other ear would hear it sooner.

Your brain takes all this information and puts it together to determine exactly where the sound is coming from - i.e. in front of you, to the side of you, or behind you. Owls have actually developed this ability so well that they can identify the direction that a sound comes from to within an angle of just 1 or 2 degrees.

It is also interesting to think about how we can determine how high up a sound comes from. This has to do with the shape of the inside of our ears. The shape of our ears allows sound waves from higher up to enter more easily than sounds from lower down, which lets our brain figure out a little bit about how high up a sound is (although we are actually not very good at this).

Again, owls are much better at this than people are. Owls' left and right ears are actually shaped differently. The right ear canal points slightly upwards and the left one slightly downwards. The feathers around the ears are also placed differently so that each ear gets slightly different information. By putting together both signals, the owl can judge exactly how far up or down the sound is coming from. This is what allows owls to hunt even when it's totally dark outside. If they hear a mouse moving around on the ground, they can tell exactly where it is just by paying attention to the sounds that it makes.

You are right that part of your sense of what's going on with the car does have to do with tone. The "Doppler effect" tells you if a sound is coming from an object that's moving towards you or away from you. The explanation of it is a little different from the one you guessed. Remember that the tone is the impression produced by the frequency of the sound- how many times per second the sound pressure cycles up or down. (You're sensitive to the whole pressure change, not just where it crosses some particular level.)

Let's say that the sound travels at 300 m/s, and that you're hearing a sound emitted from the car at 1000 Hz (1000 cycles per second). Let's also say the car is travelling toward you at 100 km/hr, or about 30 m/s. Between the time it emits one pressure crest and the time it emits the next, the first crest has moved toward you 300 m/s * 0.001 s = 0.3 m. The car itself has travelled 30 m/s *0.001 s = 0.03 m toward you. So the second crest lags the first by 0.27 m. The crests are still travelling at 300 m/s, so the time between their arrivals at your ears is only 0.27 m /(300 m/s) = 0.0009 s. The frequency is then 1111 Hz.

If the car is travelling away from you, the spacing between the crests becomes 0.33 m, so the frequency is only 909 Hz. That's why the pitch definitely is different when the car is going toward you or away from you.


Tamara & Mike W

(published on 10/22/2007)

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