# Velocity measurement

1. May 25, 2010

### romesh

I have to measure the particle velocity of sound waves in water. Which instrument to use to measure particle velocity of a sound in a specific direction. I searched but I find some anemometer setups for velocity measurements. I think they are all water flow measurement methods. Is there any difference between water flow speed measurement and particle velocity measurement of sound in water? If not I can use the anemometer setup.

I asked this same question in General physics but no response(may be the was wrong place to ask).

2. May 25, 2010

### Studiot

Anemometers are not suitable to measure sound velocity.
They measure the flow velocity of a fluid which is a totally different thing.

How are you going to measure sound velocity?

Well that depends on your resources. The straightforward way is to measure the time of transit of a sound pulse between an emitter and a receiver, known distances apart. This is easily within the capability of modern electronic equipment available at high school level.

3. May 26, 2010

### romesh

Thanks very much for the reply. In fact we need to measure the particle velocity while recording the sound. Will particle velocity be different from sound velocity? I do not know if the water current velocity will be equal to the particle velocity.
I read some documents that anemometers can be used to measure the particle velocity.

4. May 26, 2010

### Studiot

What do you mean by 'particle' and 'particle velocity'?

The individual molecules posses a random distribution of velocities due to their thermal energy, in any fluid. There is a mean or average velocity at any given temperature.
The transmission of sound imposes an additional harmonic motion on the molecules. As with any harmonic motion the velocity will vary from zero to some maximum and back to zero through the cycle.

In fluid (or indeed any) mechanics the 'particles' considered are not the individual molecules, they are small units of the fluid, larger than molecules, but small enough to be considered as small from the point of view of the laws of mechanics. The mechanical properties of the fluid are described by these 'particles'. So the flow velocity will be the movement of these units, which contain many molecules.

The combination of fluid mechanics and thermodynamics allows the linkage of these properties through the specific heats of the fluid.

The Royal Naval Scientific Service did research and published a book a while back about the generation, transmission and measurement of sound in water and other liquids.

Ultrasonics by Dr P Vigoureux.

5. May 26, 2010

### romesh

Thanks for the elaborate answer. Actually I need to find the direction of sound source in water. That is the reason I wanted to measure the particle velocity. By particle velocity I mean the velocity of the sound. I read some papers that said I can measure the sound source and where sound comes from by first finding out the particle velocity. that is the reason I was running behind the term particle velocity. If I can find the velocity of the sound wave is there any way to find the sound source? And I also do not know the difference between particle velocity and sound wave's velocity.

6. May 26, 2010

### Studiot

Perhaps if you were to forget particles and velocity explain what you are trying to do then this thread could progress.

I really have no idea if this is a laboratory or undersea task or what. Acoustic positioning in 3D requires measurements to at least three known locations.

I suggest you get hold of the book

Underwater Acoustic Positioning Systems by P H Milne

This should provide all the theory you require.