SOUND Sounds are a form of energy produced by rapidly vibrating objects. Sound needs a material medium for its transmission. Sound cannot travel through a vacuum. The vibrating object causes compressions and rarefactions in the medium. A receiver senses the sound by sensing the compressions and rarefactions.

The amplitude of a sound is its loudness. The amplitude of sound in air depends on the size of the pressure changes in the air. The frequency of sound is often referred to as pitch (however this is subjective). Young people can hear a wide range of sound, from 20 Hz to Hz. Sounds with frequencies less than 20 Hz are infrasonic while sounds above Hz are ultrasonic. p in text p ,2,3extra 4 p ,4,5extra 2,3,6,7

THE SPEED OF SOUND Air pressure and elevation do not significantly affect the speed of sound in air. p p , p extra p.246 1,2,6-8

Notice any patterns?

THE INTENSITY OF SOUND p p , p Sound intensity is the power of sound per unit area (W/m 2 ). Sounds can be emitted with an extremely large variance in intensity and likewise humans can sense extremely soft sounds as well as loud sounds. The quietest whisper is about W/m 2 while a sound with an intensity of 10 4 W/m 2 will instantly perforate an eardrum.

The decibel scale is utilized for sound intensity and gives an easy scale to judge relative intensities. The least intense sound we can hear is given the intensity of 0 dB. For every 10 dB increase in intensity the sound increases its true intensity by 10X. The scale is logarithmic so if the intensity increases by 30 dB then the true intensity has increased by a factor of 1000X. The intensity of sound we hear depends on the power of the source and the distance between us and the source.

read The Human Ear p will not test on parts of the human ear read The Reflection of Sound Waves p understand echoes and echo problems (remember to double distance) -p , p know echolocation, who uses it -know ultrasound applications Not responsible for diffraction and refraction of waves p Not responsible for section on interference of waves p Responsible for beat frequency calculations p , p ,2 p

The Doppler Effect (p ) The apparent changing frequency of sound in relation to an object’s motion is called the Doppler effect, named after Christian Doppler ( ). If a sound emitter is moving towards a listener (or vice versa) then the listener hears a higher frequency than is actually emitted. If a sound emitter is moving away from a listener (or vice versa) then the listener hears a lower frequency than is actually emitted. The Doppler effect (Doppler shift) has been used to estimate the speed of distant stars and galaxies (using light waves) relative to our solar system. The Doppler shift is also used in police radar for speeding. You are not responsible for the equation which quantifies the Doppler effect.

The Mach Number is the ratio of an object’s velocity to the speed of sound. When flying at Mach 1, an object is flying as fast as the sound it gives off. When the object emits another sound the crest will alongside the original crest so these crests pile up, producing an area of very dense air. This intense compression of air is called the sound barrier. Extra thrust is needed to break through this barrier. Objects must be designed to cut through this dense air leading to sleek and pointy shapes. At hypersonic speeds, the crests are left behind the object which constructively interfere with other crests to create a double cone. This intense acoustic pressure is called the sonic boom.

Good websites for, interference, Doppler Shift and Breaking the Sound Barrier. sition/waveSuperposition.html ml