I have a basic understanding of the Doppler Effect and how it impacts sound and light waves.
Here’s a thought experiment. John (J) is standing near to Mother-In-Law (M). Both are stationary with respect to Earth’s surface. M makes a comment about a perceived lack of professional ambition on J’s part, and J responds by hopping into his Supersonic-O-Cycle and accelerating away from her. Per the Doppler Effect, as J’s velocity increases relative to stationary M, the voice he hears from M will be increasingly shifted down in frequency. Now let us suppose that J kicks his cycle all the way up to the local speed of sound. It would seem now that M’s rants and raves would literally stop from the perspective of J as the perceived local frequency would reach zero, as new parts to the message could literally never catch up to J. New “information” in the message would literally be an inch behind J, caught up in a race that it could never win and where the gap could never be breached because pursuer and pursued are traveling at the same velocity. Is this fundamentally accurate? Now, J shifts up to six hundred and forty second gear and increases the throttle, accelerating to 1.01 times the speed of sound. Now it would seem that not only are M’s rants and raves not catching up to J, J is actually “catching up” to the previous state of the wave and will hear M’s message in reverse. Is this fundamentally accurate?
For a second question, let’s set up a situation where we are accelerating without bound toward a sound source. So after J has reached Timbuktu, he turns around and starts accelerating back toward M without bound. It seems in this case that M’s message would be shifted in frequency up, and the frequency would increase without bound. Is this accurate? Would anything “special” happen when J was traveling exactly the local speed of sound?
Let us assume for the sake of argument that J has super-sensitive hearing and his Supersonic-O-Cycle is arbitrarily quiet, so J can pretty much hear any sound wave that is actually present.
The problem with all that is that sound travels in the same medium as the Supersonic-O-Cycle and therefore the shockwave coming off the SOC will completely obliterate any sound waves that it comes across.
If you could overcome that problem (let’s say by engineering a very streamlined needle-like probe on the front of the supersonic-O-cycle, such that it pierces through the bowshock of the vehicle, then yes, you’d hear the sound in reverse.
There is, I think, a really easy way for an echo to be heard in reverse if the sound is reflecting off a wall at an angle and the listener is in motion parallel to the wall, but I need to draw a picture to explain it.
Yes, but the Doppler effect still applies. So at 1.01 times the speed of sound, the observed sound will be in reverse and pitch shifted down to very close to 0 (probably that infrasonic frequency that makes you crap yourself). J would need to crank the speed up to 2 times the speed of sound to hear M in reverse at normal pitch, and go even faster for the coveted MIL reverse chipmunk effect.
I experienced a really fascinating Doppler phenomenon once:
In Berlin, I was riding my bike away from a church whose bells were ringing. About 200m straight ahead was a row of houses blocking the street. As I was riding away from the source, the bells sounded slightly lower, obviously, but the row of buildings ahead created a discrete refection that I was riding towards. So this reflection sounded a little higher, resulting in two different pitches about a quarter tone apart. Sounded really weird, especially since it was completely natural.
Is this theoretical, or has a sound wave been actually observed in reverse in a supersonic experiment? I would guess that while we can travel faster than the speed of sound now, we may not have audio equipment that is sensitive enough to perform this experiment.
Let us pray we never do lest the unholy alliance of super sonic speed, highly accurate recording equipment and rock and roll music turn us toward Satan.