Oscillations and Waves

Beats

Two tuning forks with similar frequencies; one fork is variable in frequency to tune beating.

What it shows:

The interference of waves from two tuning forks of slightly differing frequencies gives rise to beating, that is, a modulated wave of frequency.

νb = (ν1 - ν2)

How it works:

Using two tuning forks of 256Hz, with one of the pair having small clamps (see figure 1) attached to the fork's limbs. These alter the fork's resonant frequency, and adjustment of the clamp...

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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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Bell Plates

What it shows

Bell plates are polygonal-shaped flat pieces of sheet metal which, when held in the hand and struck with a beater, produce a pleasant, sustained, slightly bell-like tone. Compare this to any arbitrary shaped piece of metal which produces a "clunk" when struck. The sound of the bell plate depends strongly on its shape and even the most modest change in the symmetry (like snipping off a corner) or proportions will make it go clunk when struck.

How it works

Why does a particular shape ring so well,...

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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...

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Sonometer

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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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...

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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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Doppler Turntable

Two speakers, one at each end of rotating platform; beating due to frequency shift of speakers travelling in opposite directions.

What it shows:

Doppler shifting of sound to higher frequencies occurs when a source is moving towards the observer, and shifted to lower frequencies when the source is moving away. Here two sources emitting the same frequency when stationary rotate on a turntable. With one source moving towards you and one away, the Doppler shifted waves interfere to create beats.

How it works:

Two 1.5W 8Ω...

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Doppler Ball

Plastic Wiffle Ball with built-in shriek to throw past (or at) your audience.

What it shows:

Waves emitted from a moving source are Doppler shifted to higher frequencies when moving toward the observer, and shifted to lower frequencies when moving away. This audio demonstration is also a useful analog to the optical red shift and blue shift exhibited by astronomical sources moving relative to the Earth.

How it works:

A plastic Wiffle-Ball™ is filled with foam padding to protect an enclosed mini-speaker 1 and...

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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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