Lab 3: The Synthetic Psychology of Sound Localization

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Task 3: Learn about the microphone and your ear

To make our robot respond to sound we need to give it electronic ears. In this part of the lab you will build a simple electret microphone circuit using the components that came in your robot kit. Since you have two ears (and that is a critical part of how you perceive sounds arranged in space), you will build two copies of this circuit, each on a different side of your robot's breadboard.

The Ear

First, a bit about your ear. Sounds waves travel through the air and arrive in your ear canal, which is a small tube. At the end of the tube is a small patch of tissue called the ear drum. In response to the movement of the air in the ear canal, the ear drum vibrates back and forth. A set of tiny bones (the ossicles: hammer, anvil, and strriup) are connected to the ear drum and move along with it. They serve to amplify the sound waves and pass them to the inner ear.

The stirrup connects to a small curvy fluid filled organ called the cochlea. Inside the cochlea are tiny neuron with hairs on them. The hairs vibrate in various ways in respond to the resonance in the cochlear fluid.

Different areas of the cochlea pick up different frequencies of sounds partly do to their spatial arrangement. As you can see in the figure above, the cochlea wraps around itself. Cells in different parts of the cochlea thus receive different parts of wave energy from the vibration. Cells near the opening are particularly sensitive to high frequency waves. Cells near the end or "apex" respond to low frequency waves. Thus, the cochlea is the first stage of decomposing the pressure wave of the sound received in your ear into different frequency components (similar to a Fourier decomposition). In addition, the hair cell neurons in the cochlea send action potentials in response to stimulation. Thus, the ear converts sound energy (which is a physical pattern) into electrical activity (action potentials or neuron spikes).

The Microphone

Some of the principles of how ears work are similar to how microphones work, and others are quite different. The types of microphones we are dealing with are called "electret microphones" (see above for a picture). There is a nice technical description of electret microphones available here. The most important concept is that there is a very thin, permanently charged material called the "electret diaphragm." When sound waves hit this material is vibrates just like the drum in your ear. This causes very small changes in the shape of the diaphragm which effects the the voltage provided by the element. There is usually a small supporting circuit in the microphone that acts as a small amplifier (using a JFET transistor) but it isn't critical to understand. The important idea is simply that the sound energy is converted into small fluctuations in voltage.

Q4: What is a major difference between the output signal of your ear and that of a microphone (think physics and the type of information that is being encoded!)?

Return to the lab overview, or move on to task 4.
Copyright © 2013 Todd Gureckis, Diagrams and schematics of the Parallax robot come from the Parallax website, much excellent material was taken from David Heeger's course notes on sound localization