Sound pressure level of a fire bell decreases by 6dB when distance is doubled.

What It Shows

The intensity of sound waves emanating from a point source changes inversely as the square of the...

A pulse of sound gets "inverted" when reflecting off the open end of a pipe, but does not get inverted when reflecting off the closed end.

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.

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...

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...

White noise from hot turbulent air turns glass tubes into awesome organ pipes.

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Standing sound waves in a glass pipe are made evident by the fountains of kerosene inside the pipe.

What it shows:

The air inside a very large glass pipe (partially filled with a fluid) is acoustically excited into a standing wave. Once resonating, the locations of the velocity antinodes inside the pipe are dramatically made evident by the vigorous agitation of the fluid, resulting in fabulous foaming frothing fountains of fluid. The velocity of sound can also be determined by noting the resonance frequency and measuring the distance between antinodes....

An aluminum rod, supported in the middle, rings for a long time in its longitudinal mode.

What it shows:

Longitudinal standing waves in solids.

How it works:

A metal rod is not unlike an organ pipe with both ends open. Holding it exactly in the middle will force the simplest, or fundamental, mode of vibration -- the ends will be free to vibrate maximally and the center will be a node. The fundamental frequency happens to be 2.26 kHz. As with a pipe open at both ends , the rod will vibrate at all the odd as well as even...

A 1.5m length of string driven at one end and fixed at the other shows standing waves for various driving frequencies.

What it shows

The fundamental is the most dramatically visible state (usually around 15Hz). It's possible to...

Obtain as many harmonics as your arm can handle.

What it shows:

Generation of a standing wave by reflection from a fixed end.

How it works:

A two person demonstration using a 2m (2cm diameter) steel spring. ¹ One party acts as the fixed end, standing holding the spring rigidly at chest height. The other sends the pulses down the spring by vigorous up-and-down movements. The frequency is adjusted to set up a standing wave from the fundamental up to whatever you're capable of (see Comments). Amplitudes of...

Longitudinal wave demo with suspended slinky.

What it shows:

Demonstration of longitudinal traveling waves in a spring. 

How it works:

The Slinky hangs with a bifilar suspension from a rigid thin-walled electrical conduit frame, which is light, strong and cheap. In total, 23 suspension points run the length of the spring; the cord is a thick cotton thread that attaches to a loop of the Slinky with No.10 fishing swivels. The layout of the Slinky and frame are shown in figure 1, but the thread has been...

Interference patterns of water waves generated by different sources at adjustable frequency.

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Large (20 feet long) Shive wave machine that can clearly show the reflection and transmission of a pulse at the boundary of fast and slow media.

What it shows:

Being so large (20 feet long), transverse traveling waves on this apparatus are easily seen by a large audience. The propagation speed of the waves is much slower than on the Shive Wave Machine, giving the audience time to process what's going on. The apparatus can be used to show three properties of waves: (1) wave speed is inversely proportional to the square root of the medium's inertia, (2) waves traveling from a...

Rods attached to metal spine; transverse wave generator shows the reflection of waves free, fixed, terminated and transition boundaries.

What it shows

Mechanical demonstration of transverse standing or traveling waves using the Shive wave machine.

How it works

The Shive wave machine consists of a series of horizontal metal rods 1.25 cm apart coupled by a torsion wire. A pulse can be sent down the machine by displacing the end rods (when doing this by hand, pull down on more than one rod as the connections are delicate and do break). The far...

Mechanical analog of a Paul Trap particle confinement—a ball is trapped in a time-varying quadrupole gravitational potential.

How it works:

A large saddle shape (attached to a plywood disk) is mounted on a multi-purpose...