What Do Sound/EM Waves Look Like.

[Gazza2907]

In textbooks etc. they are shown like the sine or cosine graphs, but this isn't what they actually look like as the travel through space is it? Surely they would be 3D e.g. a sphere of energy spreading in all directions from the source. Any info on the properties of all types of waves would be appreciated.

 

[Gerenuk]

Here is a first hint: EM waves don't actually oscillate in space. I mean there is no deviation from some equilibrium or so. Rather the electric field strength and electric field direction varies, but they are at one particular point and don't wiggle around this point.

On the other hand, sound waves are longitudinal (or in solids also transversal) waves where the oscillation is in space. There the particles really deviate from some sort of equilibrium and interact by colliding. So basically you have regions of high densities of particles and lower densities of particles.

See
http://en.wikipedia.org/wiki/Longitudinal_wave
http://en.wikipedia.org/wiki/Transverse_wave

Moreover waves can be superpositions of these sines waves, and the general superposition can have just any form. So real waves don't look like sine waves (unless you have laser).

Also note that the sine wave pictured in book is one-dimensional. A three dimensional wave will lose strength with distance as it spreads over a larger area. So you won't actually find the perfect sine curve in 3D waves.

 

[Gazza2907]

ok, but if i am thinking of a speaker, what is actually coming out of it, are there lots of individual sound waves (transverse or longitudinal) ? or it just one big pulse of energy? like this http://images.google.co.uk/imgres?i...n&lr=&safe=off&rlz=1G1SMSN_ENUK353&tbs=isch:1

 

[Gerenuk]

There isn't something real coming out. Just the molecules in front of the speaker start moving back and forth transfering their momentum to the next ones. It's a longitudinal wave.
Whenever you hear something, these sound waves continuously propagate transported by the air molecules. The speed of the propagation and the distance between high density areas is related to the pitch of the sound.

Your ear needs to catch many of these wave peaks to hear a sound. Judging but the frequency with which it receives a high air density peak, it gives you an impression of a sound.

 

[sophiecentaur]

There is just one value of sound pressure at anyone point and one mean velocity for the air molecules to be moving in that region.
Most /all sounds have a beginning (attack) and end (decay). During the time that particular sound is passing / reaching you, the molecules are (on average) moving forward and backwards, in step with the sound you hear. They will also be moving around in step with other sounds that happen to be traveling past that point and in different directions. The motions / displacements will all add up. This will always happen in a 'linear medium' and the sounds (or any other waves) will pass through or by each other without affecting each other. Likewise, in perfectly clean air (no dust), you will only be aware of light that is actually entering your eye. Light passing from A to B on a path, not involving your eye can't be seen. Again, the waves move independently of each other. The Electric and Magnetic fields at anyone point and at one time, will have just one 'resultant' value; the sum of all vectors of all waves in that region.