Sound

Carl Wozniak

Northern Michigan

University

Some sound facts? Sound is a mechanical wave which is created by

vibrating objects and propagated through a medium from one location to another.

A wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location.

The medium is simply the material through which the disturbance is moving;

Parts of a wave The crest of a wave is the point on the medium which exhibits the maximum amount of

positive or upwards displacement from the rest position. Points D and I on the diagram represent the troughs of this wave.

The trough of a wave is the point on the medium which exhibits the maximum amount of negative or downwards displacement from the rest position.

The amplitude of a wave refers to the maximum amount of displacement of a a particle on the medium from its rest position.

The wavelength of a wave is simply the length of one complete wave cycle. The frequency is the number of complete waves that pass a stationary point in one second.

Pitch The pitch of a sound refers to whether is is

high, like the sound of a piccolo, or low, like a bass drum. The lower the frequency, the lower the pitch The higher the frequency, the higher the pitch

Sound waves whose frequencies are outside the audible frequencies are called ultrasonic (if higher) and infrasonic (if lower).

Sound level

Loudness is related to the energy in a sound wave. We measure sound in units called decibels (db)

Ticking of a Watch 20

                   Whisper 30

Normal Speech 50-60

Car Traffic 70

           Alarm Clock 80

Lawn Mower 95

Chain Saw 110

                  Jackhammer 120

Jet Engine 130

Speed of sound At a temperature of 68°F (20°C), sound

travels at 1,125 feet/sec (343 meters/sec) through air. This is the same as traveling at 756 miles/hr (1,217 km/hr).

As the temperature rises, the speed of sound increases. V ≈ (331 +0.6T) m/s, where T = temperature °C

As density increases, the speed of sound increases.

Speed of sound When a jet flies faster than the speed of

sound it creates shock waves at the front of the plane (bow shock) and tail of the plane (tail shock).

Shock waves are abrupt jumps in the pressure, temperature, speed, and density of the fluid in which they propagate

A sonic boom, is our perception of these waves as they pass by us.

Speed of sound The most common

misconception about sonic boom is that it is the sound of the object "crashing through the sound barrier".

Speed of sound If an object or aircraft

entered the atmosphere from the vacuum of space and never decelerated to a speed below the sound speed of the atmosphere, the sound barrier would never be broken, yet observers on the ground would all hear a sonic boom, provided that they were close enough to the aircraft.

The sound barrier In the transsonic region, condensation clouds

frequently form around the object.

The clouds appear for the same reason that clouds always form, namely, that the air has cooled to the point that the ambient water vapor condenses.

Near the sound barrier

How we hear

Our ears detect and amplify the vibrations in the medium (i.e. air or water) around us.

The vibrations are measures in hertz, cycles per second.

We can hear vibrations between 20 and 20,000 hertz (Low numbers are low pitches, high numbers are high pitches.)

How we hear Vibrations strike the eardrum The vibrations are transferred and amplified by

the three middle ear bones: hammer, anvil, stirrup.

The vibrations are passed on to the cochlea. Inside the fluid-filled cochlea are many small hair-like structures that detect the vibrations.

The vibrations are turned into an electrical impulse which is passed on to the auditory nerve, which connects with the part of the brain that perceives sound.

Hearing in different media

We can tell the direction a sound is coming from because the sounds don’t reach our ears simultaneously.

Hearing in different media When we are under

water, the speed of sound so fast (3200 mph, 1400m/s) that our ears cannot separate the difference

We can’t tell the direction of a sound underwater.

Doppler effect

If a noisy object is stationary, sound waves propagate out in all directions evenly.

Doppler effect If that same object is moving, the sound

waves are compressed ahead of the object and stretched behind it.

Direction of motion

Doppler effect

The wavelengths of the compressed waves are shorter, resulting in an increased pitch, and the wavelengths of the stretched waves are longer, resulting in a decreased pitch.

Doppler effect

Doppler effect

When an object is moving supersonic, it is moving in front of the compression waves.

The sound comes from behind the object.