Instructables

How To Make a Simple iPhone Amplifier

video How To Make a Simple iPhone Amplifier
You can make a simple, passive iPhone amplifier with a bowl or deodorant cap.  This video shows how and why it works.
stubbsonic10 months ago
I think the theory that is mentioned in the video is a little misleading. Sound waves from the iPhone® speaker propagate (fancy word) in all directions. Placing the speaker in a bowl not only causes the waves that would have been directed away from the phone to be bounced back-- to join the waves already headed toward your ears-- but it also focuses them (because the bowl is concave). In other words, it acts like a lens to focus the waves reflection into a narrower beam.

The deodorant cap seems to do two things, both resonating and reflecting/focusing. But I'd be curious to know exactly why the vibrations emanating from the iPhone® speaker are not as loud as with the cap on, since the cap also propagates the same sound energy in all directions-- and in fact it loses some of that energy as the cap is not without it's kinetic losses.
HowToLou (author)  stubbsonic10 months ago
You are right about increased focus, but the gain amplitude gain comes from matching the acoustic impedance of the speaker to that of the air, as the video demonstrated with the water table. See http://en.wikipedia.org/wiki/Megaphone.
Thanks for that info. I'm going to have to study "acoustic impedance" (I've not heard of it until now).