Thanks Berkeman for the reply. Although I wasnt talking abt the wavelength. More like the size of the sound wave.
You see im trying to understand the use of a needle hydrophone which is used to detect ultrasound waves underwater. So I am basically trying to find out why it's so small in size. I have heard from my professors that sound waves become narrower, as the frequency increases(Huygens Principle)
A wave can't be assigned a 'size'. It can propagate any distance and all waves spread out from the nominal direction they are pointed in. The only spatial quantity is wavelength.
However, you may be referring to the 'tightness' of a beam that can be produced. That is limited by Diffraction and depends mostly on the 'aperture' or width of the source (projector or radio antenna etc). You need a wide source to produce a narrow beam.
Your needle hydrophone is small and will not be very directional. Your professor was probably talking loosely about wavelength which, for sound in water, is a lot greater than it is in air because the wave speed is so much higher.
But, when teachers tell you something that doesn't make sense to you, you should challenge them and ask for clarity. Asking about this particular confusion is fully justified.
PS Your professor is, presumably, a Biologist and not a Physicist, which could explain the problem.
I think I believed him because of the fact that when you look at the size of hydrophones; the higher the frequency, the smaller the size of the hydrophone.Which led me to think that sound at the Mhz range will be pretty small. I actually tried explaining to him that it's probably bcuz of the wavelength.
What should I be reading? So that I can understand why a hydrophone has to be needle-shaped?
Appreciate the help :)
(FYI: Im doing a project that involves measuring the power of ultrasound waves in water)