SoundA Mechanical wave

The Nature of a Sound Wave

Sound originates when a body moves back and forth rapidly enough to send a coursing wave through the medium in which it is vibrating. Vibrations in the air travel to our ears and cause our eardrums to vibrate. Air molecules collide transmitting energy away from the sound source.

http://www.youtube.com/watch?v=IIoB6xV5Z8A

http://www.youtube.com/watch?v=3zoTKXXNQIU

http://www.avatar.com.au/courses/PPofM/sound/wave.mov

http://www.youtube.com/watch?v=f66syH8B9D8

This is a spherical wave. It consists of one condensation and one rarefaction. The wavefront is a continuous spherical surface in which the variations for all parts of the surface have the same phase.

Speed of sound in air

At 0oCthe speed of sound is 331.5m/s

The speed of sound increases by 0.6m/s for every 1oC increase in temperature.

This is because the molecules have so much more kinetic energy; they move faster.

Mach number

Mach number compares the speed of sound to something else.

Mach 1 = the speed of soundMach 2 = 2 x the speed of sound

Properties of Sound

FrequencyRelated to pitchHigh like a violin, low like a bass drum

Audible frequencies 20-20000Hz (average)We are most sensitive to sounds 2000-3000Hz

Sounds >20000Hz are ultrasonic(dog whistle)

Sounds <20Hz are infrasonic(heavy machinery and earthquakes

Very loud sounds have a slightly lower pitch

Natural frequency

The natural frequency of an object is the lowest frequency at which it will vibrate when it is allowed to vibrate freely.

Example: pendulum

QualityThe quality of a sound is the property

that enables you to differentiate BETWEEN a piano or a flute.

Pure sounds vibrate at one natural frequency, while rich sounds vibrate at many natural frequencies at the same time.

A FLUTE MAKES A PURE SOUND WHILE A CELLO’s sound IS RICH.

Sound intensity

Sound intensity is related to loudness.Loudness is related to energy and

therefore, amplitude.Loud sounds have a larger amplitude

than quiet sounds.

Some sound thresholds

Relative intensityThreshold of hearing 0 decibelsThreshold of pain 120 decibelsPerforation of eardrum 160 decibels

The doppler effect

The doppler effect is the change in observed frequency or wavelength of a sound due to the motion of the source or observer.

Even though the pitch and loudness of an ambulance siren is the same at all times, it sounds different as it approaches you and passes you.

http://www.youtube.com/watch?v=-Zu5SGllmwc

A source makes waves that center around the location where it makes the sound.

aS the source moves, the new sound waves center around that location.

This explains the functionality of radar detectors, ultrasound, sonar of bats…

sonic booms

When the source of a sound is in motion, subsequent sound waves can “catch up” to those made before it.

This catching up can create an area of overpressure (because of constructive interference) and an area of rarefaction (low pressure). The extreme difference between high and low pressure creates a shock wave known as a sonic boom.