Sonar, what determines frequency

[Goodver]

The principals of SONAR is that transmitter sends a sound signal which bounces backwards from the object to receiver.

hence, as far as i understand there should not be any difference which freuqency to use.

or it is a matter of diffraction of sound wave, like if wavelength is too long then small objects will be covered by the wave and will not reflect?

what influence and why have different frequencies for sonar?

thank you.

 

[SteamKing]

The principals of SONAR is that transmitter sends a sound signal which bounces backwards from the object to receiver.

hence, as far as i understand there should not be any difference which freuqency to use.

or it is a matter of diffraction of sound wave, like if wavelength is too long then small objects will be covered by the wave and will not reflect?

what influence and why have different frequencies for sonar?

thank you.

Sonar has broad application, and your post is somewhat unclear about which of these applications you are most interested in.

However, this article discusses the different frequencies preferred for use in Echo Sounding, one application for which sonar is used:

http://en.wikipedia.org/wiki/Echo_sounding

 

[Goodver]

sorry about that,

suppose I am talking about SONAR system to locate fish in water.

I am interested not in which frequencies are used, but WHY such particular frequecies are used, preciaselly which factors determine which frequency to use

 

[Baluncore]

If the fish is small compared with the wavelength it will be hard to see.
Higher frequencies have shorter wavelengths so can see smaller individual fish.
Higher frequencies are attenuated more so they cannot see as far.
Low frequencies, with long wavelength, can see only whales and big shoals of fish.
Low frequencies can see into the bottom sediment at greater ranges.

 

[olivermsun]

Briefly speaking, longer wavelengths (lower frequencies) will propagate further, with less attenuation, in seawater. Meanwhile, shorter wavelengths (higher frequencies) will provide better spatial resolution.

A practical constraint is that, to keep a tight beam pattern, the transducer width has to increase in proportion with the wavelength.

Animals like fish typically have swim bladders which cause them to act sort of like big air bubbles. In this case there's some dependency on the resonant frequency of the swim bladder, but in general the sonar returns tend to be much stronger than you'd otherwise expect.

 

[Goodver]

thank you

why long wavelengths attenuate less than short ones?
why short wavelengths can 'see' smaller objects?

 

[Baluncore]

why long wavelengths attenuate less than short ones?

Absorption of sound in seawater.
http://resource.npl.co.uk/acoustics/techguides/seaabsorption/

why short wavelengths can 'see' smaller objects?

Imaging using waves is fundamentally limited by diffraction.
http://en.wikipedia.org/wiki/Diffraction-limited_system