Triboelectric Effects

What it shows:

As long ago as 600 B.C., the Greek philosopher Thales knew that amber, when rubbed, would attract bits of paper and other light objects. Many other substances have this same property and can be electrified by rubbing. The kind of electrification (positive or negative) depends on the substances used.


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Musical Bottle

A beer bottle becomes a Helmholtz resonator when air is blown across its mouth.

musical bottle

Semimusical Blocks of Wood

What it shows:

Drop a piece of wood on the floor and listen to the sound it makes. It may sound like noise, but it also makes a "semimusical" sound which is so poor in quality that one would be hard pressed to call it musical. Yet it is not pure noise because the sound contains a series of regular impulses that have a pitch. This may be demonstrated by dropping wood bars (one by one) onto the floor — a musical scale or tune is easily recognized.

How it works:

The tuned wood (oak) bars are 6½" long and 1" wide with thicknesses...

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Siren Discs

What it shows:

Demonstrate musical intervals, the relation of pitch to frequency, and autocorrelation in psycho-acoustics.

How it works:

A 25 cm diameter metal disk has a number of concentric rows of regularly spaced holes. When rotated at a uniform speed while blowing air at a row of holes, a musical note is produced by the sequence of regular puffs of air issuing from successive holes. The frequency is determined by the speed of rotation and the known number of holes.

The numbers of holes in the successive rows are 24, 27...

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What it shows:

The effect of length, tension, diameter, and kind of material on the pitch of a vibrating string is demonstrated. One may also show the harmonics of a vibrating string.

How it works:

The sonometer is a long hollow wooden box along the top of which are stretched one or more strings rigidly attached to the box at one end, with provision at the other for changing their tension. If there is just one string, it's known as a monochord. The monochord illustration is from John Tyndall's book entitled Sound, (...

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Organ Pipes

Selection of single organ pipes, open and close-ended, to blow through.

organ pipes

Doppler Tuning Forks

Run towards the blackboard carrying a tuning fork...

What it shows:

Waves emitted from a moving source are Doppler shifted to higher frequencies when moving towards the observer, and shifted to lower frequencies when moving away from the observer. In this situation the source is moving away from you, but the raised frequency sound is reflected back interfering and causing beating.

How it works:

All you need is a tuning fork (say 896Hz, see comments), a reflective surface like a blackboard, and plenty of room to take a run at...

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Doppler Whirler

A high-pitched alarm on the end of a rope is whirled about the head.

What it shows:

Doppler shift of sound emitted by an object moving in a circular orbit, with the pitch clearly changing as the object move towards, away or perpendicular to the line of the observer. Useful as an analogy to the redshift and blueshift of spectral lines from a rotating astronomical source such as a planet or binary star system.

How it works:

We have a Powerhorn™ Security System buzzer attached to a 1.5m length of nylon cord. Swing it in a...

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Acoustic Horn

What it shows:

As a passive amplification device, the exponential horn is amazing. Using a "talking" greeting card as a feeble source of music, the intensity of the sound gets amplified by about 18 dB when the greeting card is coupled to the horn ... a dramatic effect.

How it works:

The multicellular horn is a cluster of eight smaller exponential horns, each with a small mouth to avoid beaming in a large frequency range, but together they form a sector of a sphere large enough to control directivity at low frequencies — the...

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Refraction of Sound

Balloons filled with helium, CO2, or SF6 act as diverging and converging lenses, respectively.

What it shows:

A balloon, filled with a gas different from air, will refract sound waves. A gas denser than air turns the balloon into a converging lens and a lighter gas makes it a diverging lens. An air-filled balloon has little effect.

How it works:

The refraction phenomenon occurs whenever waves travel from one medium to another in which the velocity of the wave changes. The amount of refraction at...

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Double Sound Source Interference

What it shows:

Two loudspeakers, separated about 1.7 meters emit the same tone of frequency 500 Hz and produce a pattern of constructive and destructive interference.

How it works:

At this frequency, the successive positions of constructive interference (maximum intensities of sound) occur approximately every two meters at a distance of 10 meters (which is roughly the middle of the lecture hall). The separation of maxima would be about 2.3 meters at 440 Hz. One way to make the interference pattern evident to the students is to...

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Ring of Fire

ring of fire

What it shows:

In explaining the electron orbits in the Bohr atom, de Broglie's principle of particle wave duality allows you to treat the electrons as waves of wavelength nλ = 2πr where r is the radius of the orbit. Then the only orbits allowed are those which are integer...

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