Sound

Imagine an entire room is filled with ping pong balls, and in the middle of the room is a big paddle. Shake the paddle and observe what happens:

As we keep shaking the paddle back and forth what we are doing is creating a longitudinal wave. Molecules of air behave like tiny Ping-Pong balls.

• The Origin of Sound:

All sounds are produced by the vibrations of material objects. In a piano, violin or guitar, a sound wave is produced by vibrating strings.

The source of all sound waves is vibration. The original vibration stimulates the vibration of something larger or more

massive. Then a disturbance is sent through a surrounding medium, usually air.

The frequency of the vibrating source equals the frequency of the sound waves produced.

The subjective impresion of the frequency of a sound is called pitch.

(infrasonic) 20 Hz Audible frequencies 20,000 Hz (ultrasonic)

• Sound in Air Produce a sound and each particle moves back and forth along the

direction of motion of the expanding wave.

This process is better understood with the following example:

Open the door and this pulse of compressedair is called a compression.

Quickly close the door and this process iscalled rarefaction.

Again:”It is not the medium that travels across the room, but a pulse that travels.”

On a much smaller but more rapid scale, this is what happenswhen a tunning fork is struck.

• Media that Transmit Sound Most sounds you hear are transmitted through the air. But sound

also travels in solids and liquids – Examples, floor, metal fence, etc. A sound is transmited louder and faster by the metal than by the air.Solids and Liquids are generally good conductors of sound – much better than air.

Sound cannot travel in a vacuum.The transmission of sound requiresa medium !!!

• Speed of Sound

Sound is much slower than light. The speed of sound in dry air at 0°C is about:

330 m/s or 1200 km/hr or 750 miles/hrQuestion:

How far away is a storm if you note a 3-second delay between a lightning flash and the sound of thunder?

• Loudness The intensity of sound is proportional

to the amplitude of a sound wave. It can be measured by instruments such as the oscilloscope, shown in the figure.

Loudness, on the other hand, is a physiological sensation sensed in the brain. It differs for differentpeople. But it is related to sound intensity.The unit of intensity of sound is the decibel (dB) where zero (0) dB is at the threshold of hearing for a normal ear.

• Natural FrequencyWhen any object composed of an elastic

material is disturbed, it vibrates at its own special set of frequencies, which together form its specialsound.A natural frequency is one in which minimum energy is required to produce forced vibration. It is alsothe frequency that requires the least amount of energy to continue this vibration.

This natural frequency depends on factor such as theelasticity and shape of the objects.

• Resonance When the frequency of a forced vibration of an object matches the

object’s natural frequency, a dramatic increase in amplitude occurs. This phenomenon is called resonance.A common experience illustrating resonance occurs on a swing. When pumping a swing, you pump in rythm with the natural frequency of the swing.More important than the force with which you pump, is thetiming.

Resonance is not restricted only to sound waves, it also occurs with mechanical vibrations. Troops must “break step” when crossing bridges.The Tacoma Narrows Bridge in 1940 is another example of destruction due to mechanical resonance.

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• Interference Sound waves, like any waves, can be made to interfere. Interference

occurs for both, transverse and longitudinal waves. Interference affects the loudness of sounds.

Destructive sound interference is a useful property in antinoise technology.

Jackhammer noise Microphone Microchip mirrors wave Headset

Also pilots use noise-cancelling earphones.

• Beats. An interesting and special case of interference occurs when two tones of

slightly different frequency are sounded together.A fluctuation in the loudeness of the combined sounds is heard: loud, faint, loud, faint, loud, and so on.

Maximum amplitude of the composite wave occurs when both waves are in phase, and minumum amplitude occurs when both waves are completely out of phase.

• Concept Summary

Sound waves are produced by the vibration of material objects.

– A disturbance in the form of a longitudinal wave travels away from the vibrating source.

– High pitched sounds are produced by sources vibrating at high frequency, while low pitched sounds are produced by low-frequency sources.

Sound waves consist of traveling pulses of high pressure zones, or compressions, alternating with pulses of low-pressure zones, or rarefactions.

– Sound can travel through gases, liquids and solids, but not through vacuum.

– Sound travels fastest through very elastic materials, such as steel.

Every object vibrates at its own set of natural frequencies.

– When an object is forced to vibrate at one of its own natural frequencies, resonance occurs and the sound becomes much louder.

Like any waves, two sound waves can exhibit interference and make sound louder or softer.

– Rapid changes in loudeness, known as beats, occur when two tones very close in frequency are heard at the same time.