Sound

Sound waves are longitudinal pressure waves.

Sound waves are made up of a series of regions of high and low pressure. The areas of high pressure are called compressions. The areas of low pressure are called rarefactions.

Wave energy travels through the material. But a particular piece of material, only moves back a forth a short distance.

Speed of Sound

The speed of sound in the air is dependent on the amount of water vapor and temperature of the air.

At normal room temperature, the speed of sound is about 343 m/s.

As a general rule, if we can count the number of seconds between seeing and hearing a lightning bolt, we can approximate the distance to it. Every 5 seconds is equal to approximately 1 mile.

Is this an accurate approximation?

Sound Intensity

Intensity = Sound Power/Area [W/m2]The sound intensity drops as a function of

the inverse square of the distance from the sound source.

Compare the sound intensity at point 1 and 2 of a 12x10-5 W sound traveling out of the speaker.

(A1 = 4 m2 and A2 = 12 m2)

Loudness

The decibel is a measurement used to compare two sound intensities.

I = 1x10-12 W/m2 is equal to 0 dB and considered the threshold for normal hearing.

An increase of 10 dB roughly doubles the relative sound intensity.

Sound W/m2 dB

Threshold of hearing 10-12 0

Whisper 10-10 20

Normal conversation 3.2 x 10-6 65

Garbage truck 10-3 90

Rock concert 1 120

Threshold of pain 10 130

Human Hearing

A high pitched noise has a high vibration frequency, whereas a low pitched noise has a low vibration frequency.

Human hearing ranges from 20 Hz to 20,000 Hz.

Infrasonic is below this range. (earthquakes)

Ultrasonic is above this range. (dogs)

Natural Frequency

All objects vibrate when they dropped or tapped against a surface.

This natural frequency of an object depends on its elasticity and the shape of the object.

When a forced vibration matches the frequency of an object a dramatic increase in amplitude occurs, this is called resonance.

Doppler Effect

Austrian physicist Christian Doppler identified frequency-shifting phenomena in 1842, and the “Doppler effect” was coined. In essence, a wave frequency, whether transverse or longitudinal, increases as a source and listener move toward one another. Similarly, the frequency decreases as a source and listener move away from one another.