Beamwidh

[Sture Hultqvist]

Hi


I have recived many questions by mail an one common


is how to calculate beamwidh. Easy for you that


knows of course, but here is a thumbrule:


Calculate the wawelenght for the actual frequency.


Sound of speed is around 1500m/s. For 500KHz


you have 1500/500 = 3 mm.


Then divide this with the lenght (size) of your


transducer for example 350 mm and you have


the angle in radians:


3/350 radians = 0.5 degrees (rough).


The elements in the array should not be spaced more


than 1/4 wawelenght but it is not critical.


Sture

[bob]

Re: Beamwidh

Sture,


Again, great info!


I was wondering about the vertical beam width.. Ideally, I guess you would want a 90 degree vertical beamwith, which, according to your calculations above, would mean an element width of about 6.5mm.


I think you told me that your transducers were not that wide. Is this because you wanted less than 90 degree beamwith?


Perhaps the other dimensions are constrained by the element having to be resonant at a certain frequency. How do we design an element for a certain resonant frequency?


Also, I have seen a number of different sidescan units with different resonant frequencies. It seemed that the lower frequency models had a longer range, but the higher frequency models had better resolution. Do you think there is an good value we should use when fist starting out?


Thanks again!


-Bob

[Sture Hultqvist]

Re: Beamwidh

Vertical beamwidh:


The thumbrule is not perfect but if the


aperture size is close to the wawelenght you will


have a angle close to 1 radians, around 56 degrees.


500KHz and 3 mm will be close to this.


Since this is the -3dB point you will have wider


beam at the -6 db point and so on. In practical use


you will have a very wide beam for strong signals


close to your towfish, something like 180 degrees.


For range and frequency I post separatly.


Sture