Why can't we see sound waves?

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Why can't we see sound waves?

Mostly because we can't see Air

Because compressional waves in air(the definition of sound) do not result in the emission of light; nor do they cause light to bend as the light passes through them.

You can't?

Hang on, what are you really asking here? We can "see" sound waves! If you were going to imagine an evolved device which is designed to "see" sound waves, then you would imagine an ear! The fact that we don't say that we can "see" sound waves with our ears is just to do with how we use language.

Or maybe you are asking why a device which is designed to "see" light (an eye) cannot also "see" sound? Well, that's because they are two different types of phenomena. Light is really particles called photons, but sound is just a rippling disturbance in matter.

would a schlieren setup not produce images of the density changes in air?

You can sometimes see a visual distortion caused by shock waves from explosions. I think there would be similar visual effect with sound waves (refraction of light due to changes in air density), it's probably too subtle and moves too fast for us to be able to see them.

[youtube]http://www.youtube.com/watch?v=_EKSFLkcOq8&feature=player_detailpage#t=17s[/youtube]

Our eyes are constructed to sense photons, not pressure differences in the air.

ruveyn

That requires the visual interpretation of the output of an instrument. Air density changes are not registered directly on the eye.

ruveyn

well yes they can, the shockwave of an explosion does refract enough to make it immedieataely visible.

i do agree however that for common sound sources we cant, also the thread itself didnt have much context.

Only if there is enough light to see the refraction effects of a compression shock wave. It cannot be seen in the dark.

ruveyn

obviosuly that would be true.

There is no appreciable difference between a shock wave and a sound wave, except perhaps by defining the former as having a single large compression wave and the latter as a steady vibration maintained for at least several cycles.

In any case, any kind of sound in the air that won't cause irreparable damage to your inner ears if not your kidneys doesn't do anything appreciable to create a light pattern our eyes can detect. However, sound waves will indeed cause smaller electromagnetic waves to refract as they travel through them, and an advanced enough eye and brain might see like the Predator, or whatever alien thing saw sound waves in whatever movie that was.

BUT: we can see sound waves travelling through solid objects. Look closely at a guitar string as it vibrates, or a large speaker cone at a club. When you pull a rubber band taut, you can see the twang it makes physically as well as hear the twang by sticking a finger you're holding it with in your ear.

Better yet, and to put a finer point on what Declension said, sound as we hear it is an image of events through time. A two-dimensional voiceprint is a good approximation of what the brain is contending with in the cochlea. However, our brains translate this raw data into useful information like the identity of a particular voice or the amount of doom a thud portends. At this point, many people - synaesthetes - actually see pictures of the sounds they're hearing, sorted out from the two complex streams of data hitting our ears. You could be one of us and not realise it for some time.

Musicians do well to consider the sounds they send through their instruments - where they go, how they bounce off of certain wooden or metal parts, and so on. When you pluck a guitar string, a set of frequencies starts resonating in it dependent on where along the string you've plucked it. The more you've seen things like this, the more clearly your brain can assemble the video of that process initializing and continuing, until you have more or less a 3D map for all the sound that's going on in a room or even a big open space.

And then there's Evelyn Glennie, the barefooted Scottish concert percussionist who is profoundly deaf and hears the sounds around of the orchestra - quite precisely - through her feet. Her documentary, Touch the Sound, I recommend. Using the skin as a hearing apparatus, though, gives us a real two-dimensional image. It's the same two-dimensional image you use to see yourself ferreting around for keys in your pockets.

Always listen closely!

There are some significant differences -- such as that the velocity of shock waves can greatly exceed the speed of sound in the medium -- but for the purposes of this thread, yeah, those differences probably aren't significant.