# Focusing sound waves

#### ayushmorx

Is there any way to focus a sound wave BEHIND an acoustic lens i.e. the source and receiver are on different sides of the lens? Will it result in a significant loss of sound energy?

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#### LURCH

How about a megaphone? Probably not what you're looking for, but I think it at least proves the principle.

#### nasu

Some ultrasound focused transducers work like what you describe.
They have a lens to focus the ultrasound produced by a source attached to the back of the lens.

#### sophiecentaur

Gold Member
How about a megaphone? Probably not what you're looking for, but I think it at least proves the principle.
A megaphone (horn) is more of an impedance matching device than an acoustic lens. Its diameter is too small to produce a particularly narrow beam.

#### sophiecentaur

Gold Member
An optical converging lens works by introducing long delays in the light path through the centre and short delays for the path through the edges. The result is that parallel 'rays' that arrive at all parts of the lens arrive 'in step' at a point (called the focus) near the lens. This is Fermat's Principle. (Look it up). Light travels slower in glass than in air so a converging lens has a thick middle and thin outer (convex)

With an acoustic lens, you have a choice of structure. Sound travels faster in solids than in air so you can use a very low density solid concave lens (less delay at the rim than at the centre will cause the sound to converge). Alternatively, you can use a honeycomb structure (like a handful of drinking straws) where the centre holes are long and the edge holes are short. Sound travels through the holes slower than in air so this involves a convex structure to get a converging effect.
Look at the Google Images for Acoustic Lens and you will see both types of structure in many different forms.

#### ayushmorx

So basically if I make something which looks like an optical concave lens, but I make it out of a metal, I will solve my problem? Since sound travels faster in solids than in the fluid medium, it will focus the sound waves?(Assuming Fermat's principle holds for sound waves for relatively short distances)

Also, Will the waves being focused arrive in phase(assuming they were in phase from a single source before hitting the lens?)

#### sophiecentaur

Gold Member
So basically if I make something which looks like an optical concave lens, but I make it out of a metal, I will solve my problem? Since sound travels faster in solids than in the fluid medium, it will focus the sound waves?(Assuming Fermat's principle holds for sound waves for relatively short distances)

Also, Will the waves being focused arrive in phase(assuming they were in phase from a single source before hitting the lens?)
Not a metal - a very light weight foam, which has been designed for the purpose. If you use metal, most of the sound energy will be reflected and not transmitted through; you need a material with a not too dissimilar impedance to that of the air.
Alternatively, a concave structure with tubes of varying lengths. You would need to find the speed of sound in a tube of that diameter and then work out the time differences to give waves in step from each tube to arrive at your chosen focus. A bit labour intensive and you would need to find out the details from other successful designs or else do a lot of measurements. If you have a good dual beam scope, you could probably find the speeds for various tubes by looking at phase shift. A bit lumpy but easier to achieve than having to find / select suitable foam material. (Low loss will be the problem, I think)

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