# How would reversed sound sound like in real life?

Might be an odd question, but recently I read up on how sound works. From Wikipedia:

In physics, sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as air, water or other materials.

Which got me thinking on how fake reversed sound in an audio software must be. Imagine someone knocking on a wooden door. From what I understand, the sound you hear comes from the wooden door vibrating from the impact of the fist. If you record it and reverse the sound in an audio software, it can't be an accurate representation of how it would sound if it was reversed in real life: it would have to (somehow) mimic the vibration of the wooden door and play that backwards, which is obviously impossible.

So if I'm right, how would such a sound actually sound in real life if it was reversed?

• Think about this: if I take two people and inbetween them, send some sound wave towards one, if I reversed that wave and sent it the other way, previous-tail first as the new head, what would the person receiving that hear?
– user95137
Aug 14, 2017 at 18:40
– user163104
Aug 14, 2017 at 18:51
• What would "reversed" mean "in real life?" How would you propose to really reverse a sound? Aug 14, 2017 at 18:52
• Probably related: physics.stackexchange.com/q/192448/25301 Aug 14, 2017 at 19:27

The sound wave that a computer uses is a measure of amplitude and frequency of the high and low pressure zones of the sound wave detected at a point in space.

Think of the audio recording as you sitting still listening to the knock at the door. If you had a sensitive pressure measurement device that could respond at very high frequency, you could measure how the pressure changes over time.

That would give you the equation for your sound wave at one point in space over the course of time. The computer then does it's magic and powers a speaker to vibrate with those amplitudes and frequencies to recreate that particular sound when you want it.

If you somehow reversed time and stood at that particular point, the sound waves should theoretically go through the same amplitudes at the same frequencies, just in the opposite order. This is also what audio software does to the captured wave to reverse it.

Theoretically they should sound the same.

The issue is that you can't reverse sound waves. The vibrations spread in every direction, not just to the point where you measure them. They dissipate, so to reverse them, all the other vibrations and heat and movement caused by the sound would have to be undone. Reversing sound is unphysical; but if we could, I see no reason why it wouldn't be like the audio software.

The challenge with this question is that it requires one to suspend the laws of physics and invent new ones. There's no concept of "reversing a sound" in real physics. Sound always travels in the direction sound travels. Because of this, we would need to define what it means for a sound to be reversed. The answers would vary based on the definition.

To provide one such definition, consider the case of simply reversing the vibrations. To do this, we're going to have to locally reverse entropy, which is legal (you can't reverse the effects of entropy globally, but you can do it locally).

In such a definition, the first thing you would notice is your eardrums getting warm. This is the thermal energy that was dissipated from your eardrums as they stopped vibrating from the sound. In the normal sound direction, this occurred after the sound, but in the reversed version we need to have it precede the sound, because we'll need its energy in the next step.

Next, a spontaneous oscillation forms on your eardrum. This oscillation was forced by the vibrations in the air in the forward direction, but must be spontaneous in the reverse direction.

These vibrations produce vibrations in the air which join vibrations from all over the room, focusing in on the wood. These vibrations would eventually knock on the knuckles.

You would hear exactly what you hear in "synthetic" reversed sound, because all you would hear is what the oscillations in your eardrum tell you that you hear. In this case, they are spontaneous oscillations that are the exact reverse of the sound you heard in the forward direction.

Oh, and the knuckles are really confused. As far as the hand would be concerned, it would feel like the door reached out and smacked it across the knuckles. That's what happens when your run physics backwards.

• Oh good, I didn't waste my time in the answer. We went into detail on the complimentary parts of this it looks like. You dealt with what reverse sound looks like, I tried to explain how we measure the sound in the first place.
– JMac
Aug 14, 2017 at 19:31
• Just to complicate it a bit more, every other object that picked up the sound wave would also have to do the same to actually run the situation in reverse. Quite unlikely to reverse that much local entropy :P
– JMac
Aug 14, 2017 at 19:37
• @JMac Good! I was totally handwaving away the fact that somehow we were getting an electrical signal out of the eardrum. That's a really dicey region where some things have the effects of entropy reversed, but not all. I handwaved that part, while you focused on it! Aug 14, 2017 at 19:38
• @JMac Yep, everything would have to do the same process. I'd say its quite unlikely to reverse that much local entropy, but the usual arguments suggest that the creation of life indeed reverses an astonishing amount of entropy, so the idea isn't totally unfounded. Aug 14, 2017 at 19:39
• Somewhere out there is a planet that had one sound-based event reversed.
– JMac
Aug 14, 2017 at 19:40