# Homework Help: Question on waves

1. Jan 7, 2005

### orange

In a dispersive medium, the velocity is dependent of the wave's frequency. If you would send a signal through pulses in a dispersive medium such as water, would this not mean that the pulse will scatter?

Bye!

2. Jan 7, 2005

### Astronuc

Staff Emeritus
When you say signal, do you mean something like applying frequency moduluation or amplitude moduluation so the sound waves?

I am mostly familiar with applications of ultrasonics in which the frequencey and amplitude are invariant (fixed by source transducer), and the applications are very short range.

Over what distances would you apply this signal transmission?

Pulses (sound waves) disperse period.

IIRC, scattering occurs at distinct boundaires (such as gas/liquid or liquid/solid) or where there is an abrupt change of acoustical properties.

3. Jan 7, 2005

### Tide

Since a pulse is composed of a spectrum of frequencies and the medium is dispersive then the pulse will spread. This is not usually regarded as "scattering" which, as Astronuc properly points out, only occurs when there is an abrupt change in the properties of the medium, i.e. the change occurs on a scale comparable to or smaller than the wavelength of a given spectral component.

4. Jan 10, 2005

### orange

Thank you for your answers Tide and Astronuc. I was thinking of any distance and for any kind of wave, including electromagnetic pulses. Sorry for being unspecific.

I had this scenario going; When, lets say, submarines communicate under water, they'd probably send pulses of radio waves to each other. Since the frequencies in the signal pulse travel with different speed the pulse will be spread and maybe unrecognizable when it reaches the other sub.

The spreading of the pulse will of course be very, very small. But lets say if it were to travel many times around the globe, the pulse would be worthless, correct?

5. Jan 11, 2005

### Tide

Orange,

I don't know the specifics of dispersion by ocean water but you have the right idea. There's just one thing, however. Ocean water is electrically conducting which makes it difficult to propagate all but the longest wavelength electromagnetic radiation.