Chapter 26

Sound

Sound is a form of energy that spreads out through space.

Some Facts

• All sounds originate in the vibrations of material objects.

• Sound is a longitudinal wave.– Amplitude is in the same direction as

propagation.

• Sound needs a medium to propagate.

Classifying Different SoundsThings vibrating at different

rates give us different pitches of Sound.

•Pitch-The note the sound makes. –Low pitch Bass note–High Pitch Treble note.

•We measure Pitch using Frequency.

–Low pitch low Frequency–High pitch High Frequency.

At a Rock and Roll Show

Or maybe just the mega car audio system

Mo’ bass!

Range of Sound heard.

• Humans can hear sounds with frequencies from 20Hz to about 20,000Hz.

• Cats can hear up to 40,000Hz.

• Bats can hear up to 100,000Hz.

Loudness (Amplitude)

• The Loudness of sound is measured in Decibels. (dB)

• A sound of 6 dB is twice as loud as 0dB.

Waves in a Medium• The speed of a wave depends on the

medium it propagates through.

The Speed of Sound can Change.

• The speed of sound in dry air at 0°C is about 330m/sec.

• The speed of sound in a gas depends on the temperature of the gas and the mass of the particles in the gas.

• Sound travels about 15 times faster in steel than in air, and about 4 times faster in water than in air.

Some Math

•Sound travels at 340 m/sec.

•A good low bass note of c# has a frequency of about 34Hz.

•What is the wavelength of the note?

Compression and Rarefaction

• Longitudinal waves pull and push the medium that transmits them.

• We call this compression (push) and rarefaction (pull).

Natural Frequency• Every object vibrates at its own natural

frequency.• Most things from planets to atoms and almost

everything else vibrate at one or more natural frequencies.

• A natural frequency is one at which mini- mum energy is required to produce forced vibrations.

• It is also the frequency that requires the least amount of energy to continue this vibration.

Resonance• An object resonates

when there is a force to pull it back to its starting position and enough energy to keep it vibrating.

Think of how you swing in a swing.

Resonance occurs whenever successive impulses are applied to a vibrating object in

rhythm with its natural frequency.

Objects do not have to touch to resonate.

Forced Vibrations

• A forced vibration occurs when an object is made to vibrate by another vibrating object that is nearby.

• Sounding boards are an important part of all stringed musical instruments because they are forced into vibration and produce the sound.

Interference

When constructive interference occurs with sound waves, the listener hears a louder sound.

When destructive interference occurs, the listener hears a fainter sound or no sound at all.

Beats

• When two tones of slightly different frequency are sounded together, a regular fluctuation in the loudness of the combined sounds is heard.

• Guitarists regularly use this to tune their guitars.

• Interference is important in setting up your speakers.