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Clap for light; Peek to live

Q: Some electronic instruments are sensitive to sound. For example, clap and a lamp glows, or a bell rings. What is the device responsible for that? How do they work? (Rajesh, Tirupattur, India)

A: The responsible device is a simple transducer that you probably well know — a microphone. It converts sound energy (for example, a clap) into output electrical energy, such as a current for lighting a lamp.

How a dynamic microphone works: Sound moves the diaphragm and its attached coil of wire. This makes the wire move perpendicular to the field of a magnet. The force of the magnetic field on the charges in the wire induces a current in the wire that images the sound pressure variation. [Rod Nave ©, HyperPhysics, Georgia State University]

All microphones do the same basic job. A thin flexible diaphragm senses (actually, resonates with) the sound wave. The wave’s crests and troughs push and pull the thin metal or plastic back and forth. Then microphones convert diaphragm motion into an electrical signal that varies in amplitude, mirroring the diaphragm’s back-and-forth movements. Different types of microphones use different methods to convert the varying movement to a varying signal.

A dynamic microphone’s diaphragm, for instance, moves an attached coil of wire past a magnet to induce current flow in the wire. See diagram. A sound wave moves the diaphragm, which moves the wire coil in the magnet’s magnetic field, which causes current to flow in the wire.

OK, that’s how the electronic device (the microphone) converts sound to an electrical signal. But, to turn on a lamp by clapping, we need two more parts — a filter and a switch. The filter’s the tricky one.

"Each sound has a frequency signature," says Tom Jakobs, engineer at InvoTek, Inc. See figure. A clap — a short burst of loud noise — has a distinctive signature.

The filter must pass that desired signature and reject all other noises such as a cough or a bark. Commercial clappers make a person clap twice to get a double-clap signature decidedly different from such single-burst sounds as a door slam.

The top signature is the waveform of a single clap. The bottom one is the waveform of a person saying “Ah.” [Tom Jakobs, InvoTek, Inc., used with permission]

Once the filter has identified the double clap, other circuitry conditions and amplifies that signal into a trigger signal. The signal trips a relay that closes a switch and the lamp comes on.

The Voice Switch, which InvoTek makes, helps severely disabled people who are also unable to speak clearly but can make voice sounds. The device recognizes their garbled speech as a human voice, filters out other environmental noises, and triggers a relay to turn on a light or even control a computer, says Jakobs.

Further Reading:

InvoTek, Inc: Sound Switch

Wikipedia: Microphones

HyperPhysics:  Piezoelectric effect by Rod Nave

HyperPhysics: Crystal microphone

Q: Do birds close their eyes when they sleep? (Mary, Someplace, Earth)

Yes. All birds we’ve studied close their eyes almost all the time they’re asleep.

Dozing Arctic loon, on a nest. [Curtis Carley, U.S. Fish and Wildlife Service]

Birds have various sleep levels, though, somewhat like us. Throughout deep sleep, birds close their eyes. But, during light sleep, some birds (for example, ducks) will open their eyes for a quick look-see. Other birds (such as hawks and falcons) keep them closed during all sleep levels.

In 1983, zoologist Dennis Lendrem from the University of Nottingham, UK, spied on dozing ducks. He learned that sleeping ducks peek while asleep about once every two to six seconds. The more threatened they feel, the more they glance around.

Ducks in small flocks peeked more often. So did those in exposed positions. When displaying bright feathers to attract a mate, males also peeked more. Apparently, they felt conspicuous and therefore threatened.

That’s it. Even peeking birds close their eyes and keep them shut about 90 percent of their sleep time.

Further Reading:

Animal Behavior: Sleeping and vigilance in birds, Field observations of the mallard, by Dennis William Lendrem, 1983, 31, 532-538.

Cook County, Forest Preserve: Sleeping birds, by George W. Dunne and Roland F. Eisenbeis.

Physiology & Behavior: Sleep patterns in the bird Aratinga canicularis

Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. The Birder’s Handbook, a field guide to the natural history of North American Birds. New York: Simon & Shuster, Inc.,1988.

(Answered June 3, 2005)