Sound and Acoustics

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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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 its...

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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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