# Sound waves question

1. Dec 27, 2015

### kapital

Why does sound waves propagate trough space with areas of lower and higher pressure, why does not pressure just equalize without waves? Thanx for answers.

2. Dec 27, 2015

### sophiecentaur

It takes time for the pressure to 'equalise' between different regions and, by the time air has flowed into a low pressure region, the source (loudspeaker, for instance) has produced another high pressure region. It's this delay that causes the wave. If things were instant, the pressure at one end would always be the same as the pressure at the other.
The fastest waves are EM waves (much faster than any other waves - about a million times faster than sound in air) so, over short distances and with low frequencies, the propagation seem instant.

3. Dec 27, 2015

### kapital

Okay, what about sound, when two object just collide? It is still a wave, but there is only one source(one collision) of sound?

4. Dec 27, 2015

### sophiecentaur

If you're talking about a single impulse then the same thing applies. After any part of the air has been disturbed it will return to its equilibrium position and the disturbance will be occurring further along. The disturbance cannot just 'disappear' because energy is always conserved. (The amplitude of the sound will gradually decrease as the wave spreads out or due to friction within the medium (solids and liquids raather than ideal gases)

5. Dec 28, 2015

### kapital

So if only one collision happens is like when you throw something in water and only one circle is create, here only one pressure change happen?

6. Dec 28, 2015

### Vedward

The water wave on the water surface is a transverse wave, the pressure change is perpendicular to the direction of travel. Sound waves are longitudinal with pressure change in the direction of travel. Also water is an incompressible medium while air is compressible. There are sound waves in water which are much different from ripples on the surface.

7. Dec 28, 2015

### sophiecentaur

The simplest sort of wave to describe is what you get with a long straight channel with vertical sides and deep water. You launch the wave with a dam across one end and push it inwards, rapidly and don't move it back. That will launch a single impulse (if you get it right) which will propagate along the channel, unchanged except for friction losses. This link shows a single solitary wave being set up.
Your example is not the easiest case to describe but what happens is along the following lines. In most cases, where there is a lot of water, the object will make a large hole as it enters the water, pushing water aside. That is the first wave, which will propagate outwards in a ring pattern. The water will then flow into the hole, as the object sinks and will form another peak, which will fall, too and a second wave will propagate outwards. Depending on the size of the object, you may get other smaller waves but they will gradually die down in amplitude as the energy gets dispersed.
Google solitary waves (Images) to get dozens of examples of waves. You'll find that most of the images show continuous waves rather than solitary waves.