Sonar, what determines frequency

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Discussion Overview

The discussion focuses on the factors that determine the frequency selection in sonar systems, particularly in the context of locating fish in water. Participants explore the implications of different frequencies on detection capabilities and the physical principles underlying these choices.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the choice of frequency in sonar may not significantly impact detection, suggesting that diffraction of sound waves could play a role, particularly if the wavelength is too long to reflect off small objects.
  • Others argue that higher frequencies, which have shorter wavelengths, can detect smaller fish but are limited in range due to greater attenuation, while lower frequencies can detect larger objects over longer distances.
  • A participant mentions that longer wavelengths propagate further in seawater with less attenuation, whereas shorter wavelengths provide better spatial resolution.
  • There is a discussion about the influence of fish anatomy, specifically swim bladders, on sonar returns and how this relates to frequency selection.
  • Questions are raised regarding the reasons behind the attenuation differences between long and short wavelengths and the ability of short wavelengths to resolve smaller objects due to diffraction limits.

Areas of Agreement / Disagreement

Participants express differing views on the significance of frequency selection in sonar applications, particularly regarding the trade-offs between detection range and resolution. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Limitations include assumptions about the relationship between frequency, wavelength, and object size, as well as the specific applications of sonar that may influence frequency choice.

Who May Find This Useful

This discussion may be of interest to individuals studying acoustics, marine biology, or engineering applications related to sonar technology.

Goodver
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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.
 
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Goodver said:
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
 
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
 
Last edited:
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.
 
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.
 
Last edited:
thank you

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

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