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Sound Experiments


Sound Experiments

Note: Some of these demonstrations have been conceived as museum displays rather than classroom activities. As such, they require specialized equipment.

Wave Characteristics

Sound travels as a compression wave through the air. The fact that the sound can travel a long distance while the air molecules themselves travel only a short distance can be demonstrated using a "Slinky". Stretch the slinky to almost its full length. have someone hold the far end of the slinky. Holding your end of the slinky in the palm of your hand, give a rapid push on the slinky, followed by an equally rapid pull. This will produce a compression pulse that travels the length of the spring.

Reflections

Attach the far end of the slinky to the wall using tape. Send a compression pulse down the slinky as in (1) above. Ask the students to watch for a reflection of the pulse coming back. Keep this demonstration in mind when performing the speed of sound demonstration.

Speed of Sound

Students stand a measured distance from a building that has a large, flat unobstructed outside wall. The distance can be measured using a roll along device used in athletics to lay out football fields. If one of these is unavailable, use as long a measuring tape as you can find. The group of students is equipped with a pair of wood blocks that can be clapped together to produce a loud sound. As the blocks are clapped together, an echo is heard as the sound travels to the wall and is reflected back toward the students. The student doing the clapping synchronizes her clapping with the echoes. The period between each clap is the time for the sound to make a round trip from the clapping hands to the far end of the coil, Another student measures the time for, say, ten claps. The speed of sound is then calculated by dividing twice the distance to the wall by 1/10th the time for the ten claps.

Sound Pipes

A set of pipes cut to different lengths is set up on a stand at ear level. The lengths of the pipes are calculated to produce sounds corresponding to the musical scale. The visitor listens through each pipe, and notices that each pipe acts as a filter, selecting sound from the random room noise that corresponds to its natural frequency. The visitor can "play" the pipes by rapidly shifting her ear from pipe to pipe. The above display demonstrates how resonance can accentuate selected frequencies. A simpler demonstration can be used for the same thing. Ask students if they have ever listened to the sound of the sea in a seashell. If possible, pass around a large seashell, and several small ones. The sound of the sea from the small shells will be a higher pitch than that from the large one. Pass around large and small jars- they will do the same thing.

NOTE: Resonance is caused when a wave returns (usually by reflection) to its starting place just in time for another wave crest to add more energy to the wave. A good analogy is a swinger on a swing. Pushing at the wrong time does nothing; pushing at the right time, even with a little force, does quite a lot. The shells or jars cause entering sound waves to reflect back toward their opening. (What's not well-known except to scientists and engineers is that the sound wave traveling back out of the jar is partially reflected back inside, apparently by nothing at all!). A standing-wave is formed inside the jar, with outside waves of the right length reinforced by addition to the standing wave pattern. This works for shells, jars, organ pipes, xylophone bars, guitar strings, the larynx, etc.

Pan Pipes

Similar to above, a smaller set of pipes can be made to the scale. Seal one end of each pipe with a suitable material (epoxy resin works well), and blow across the top of each pipe to play.

NOTE: the length ratio between adjacent pipes is the twelfth root of two. This produces 1 semitone between pipes. A set of pipes like this is analogous to a piano keyboard which doesn't distinguish between the white and black keys. To produce the sound of a "key" (e.g. the "key " of G), there are two semitones between notes everywhere except the 3rd and 4th notes and the 7th and 8th. At these two locations there is only 1 semitone between notes. Try it on a piano keyboard to verify.

Wall Resonance

The next time you are walking across an asphalt surface and a loud jet or airplane passes overhead, bend down to the ground. You may be astonished to hear that the apparent pitch of the plane rises as your ear gets closer to the ground. This also works with hard surfaced walls and lawnmower engines. Give it a try. Why does this happen? Here's my hypothesis: The space between your ear and the ground or wall defines a "pipe length" similar to an organ pipe. The reflected sound from the ground or wall combines with the sound directly from the object, and only those waves that "fit" the pipe length survive. I haven't talked about "nodes" and "antinodes" at all here, but a decent general physics book should explain all. When you understand nodes and antinodes, try to decide which is at either location: your ear and the ground.

Phase Tapping

Next time you're bored and find yourself tapping on the table, try this: Listen for the difference between the sound of one fingernail tapping, and two. Can you detect it? The sound of two fingernails tapping has a shifting "phase" effect to it, similar to the "flanging" effects musicians use. A tap is a sound pulse. The variation in sound comes from the minute difference in the spacing of the two pulses your two fingernails make. You can, with practice, vary the effect at will. Try holding your fingers so that they strike the table at different times. Can you do it with three (or more) fingers?

Sound Lens

A large weather balloon is filled with carbon dioxide, a gas through which sound travels more slowly than through air. The slowing of the sound, combined with the spherical shape of the balloon, causes the balloon to focus sound that passes through it. Students can carry on a conversation in whispers from opposite sides of the balloon

Try above with a small rubber balloon. Fill the balloon with CO2by inserting some dry ice into a balloon and tying it closed. Experiment with the quantity required so you don't burst the balloon.

Sight of Sound (oscilloscope display)

An electronic keyboard musical instrument and a microphone are connected to an oscilloscope. The visitor can listen to a sound while observing its waveform on the oscilloscope. The idea of frequency vs. pitch is discovered as the visitor plays high and low notes. The relationship between the timbre of a sound and its waveform are noticed as the visitor selects different instruments from the keyboard's control panel

Echo Machine

A microphone connected to a guitar "echo box" is in turn connected to a pair of headphones. The student attempts to read aloud from a printed text while listening to her delayed speech through the headphones. The student discovers that for a range of delay times, it is impossible to read the text due to the disconcerting echo.

Identify Source of Sound -Game

An array of 100 speakers is positioned approximately 20 feet from a control stand, where the visitor interacts with the exhibit. The speaker array is covered with a grille to prevent the visitor from seeing the speakers vibrate. An LED marks the location of each speaker. A single speaker is randomly selected by computer control, and a sound is played through this one speaker. The visitor tries to guess which location the sound is coming from. His guess is entered into the control panel and illuminates the LED at the chosen site. The computer then illuminates the LED at the correct position (or flashes the LED if the visitor has guessed correctly). The computer keeps score of correct guesses vs. type of sound. Mid frequency sounds are easier to localize than either high or low frequency sounds, due to the relationship between ear spacing and the speed of sound. Likewise, long tones are easier than short beeps.

Bat Cave (helmet-mounted sounders) -Game

The visitor's ability to visualize and navigate through a darkened environment using acoustic reflections of a helmet-mounted piezoelectric sounder is challenged by a maze of obstacles. Obstacles consist of padded walls and hanging punching bags (stuffed army surplus duffel bags).

Sound Telescopes

Mylar spheroids can be used to make giant (8 ft. dia.) dishes suitable for collimating sound, enabling a conversation to be carried out in whispers across the room. The concentration (focusing) of sound can be demonstrated on a smaller scale. Connect a small funnel to a 2ft. length of rubber tubing. Find a large shallow mixing bowl or a wok to use as the reflector. Aim the large mixing bowl toward a wind-up alarm clock across the room. aim the funnel toward the mixing bowl (this collects the reflected sound from the bowl). Listen to the sound through the rubber tube. You may be able to hear the ticking of the clock from as much as 20 feet away. If no clock is available, try using a radio with volume turned down low.

Hear Through Your Teeth

Demonstrate bone conduction by striking a fork on the table so that it rings like a tuning fork. First, try listening to the sound directly. Next, press the handle of the fork on the bone behind the ear. Finally, bite the handle for a really LOUD sound.

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