Unit Three: Sound

Unit Three
Sound

Vocabulary
Echo- when sound has reflected off a surface and travelled back
Reverberation- when a sound is reflected off of a close surface, as in a small room, the echo will seem to be a lengthening of the original sound
Natural frequency of vibration- the natural oscillation of any object

Sound as a Wave
- Sound waves are longitudinal waves.
- Sound waves require a medium to travel.
- FREQUENCY determines the pitch of a sound. A wave with a low frequency will produce a low sound, while a high sound has a high frequency.
- WAVELENGTH also affects the pitch (as wavelength affects frequency).  A wave with a longer wavelength will have a lower sound, while a shorter wavelength produces a higher sound.
- AMPLITUDE affects the volume, the larger the amplitude the louder the sound.

The Physics of Sound by David Suzuki

Speed and Transmission of Sound
- Sound travels fastest through solids because the particles are closest together.
- Sound is produced by a vibrating source such as a drum being struck
- It is transmitted by particles bumping into each other causing compressions and rarefractions.
- Sound is heard by the wave hitting the eardrum and causing it to vibrate.
- Mach number: the ratio of the speed of an object to the speed of the sound

Speed of Sound
We explored how sound was transmitted and through what type of matter it travelled fastest. Sound is transmitted by particles bumping into each other, creating compressions and rarefractions.  In an experiment with dominoes, we found that dominoes set up closer together fell faster than those set up further apart.  This can be related to sound in saying that those substances with particles closer together (solids) will transmit sound faster than those with particles further apart (gases).  In a solid, there is less space between the particles for them to travel to bump into the next one, and so it can be concluded that here the sound will travel faster.

Formulas
The speed of sound in air is:

v = 332.0 + (0.6xT) m/s
v = velocity (m/s)
T = temperature

The Ear

Eardrum - forced into vibrations with the same frequency as that of the source by successive compressions and rarefractions from the ear canal
Hammer, anvil, and stirrup - transmit vibrations of the eardrum to the inner ear, mechanically amplifying them
Eustachian tube - opens during swallowing or yawning, equalizing air pressure in the middle ear.

Pinna - protects the eardrum from direct injury and helps the hearer identify the direction of the sound
Ear Canal - amplifies frequencies of sounds
Cochlea - waves transmitted around it cause microscopic hairs to vibrate
Auditory nerve- sends mechanical motion converted to electrical signals by microscopic hair cells to the brain

Sound Intensity Lab
This lab explored how sound intensity increases exponentially. Sound intensity is measured in decibels (dB).  The threshold of pain the point where sound becomes so intense that pain is felt, at about 130 dB.  The threshold of hearing is the lowest sound a person can hear. We used grains of rice to represent sound intensity.  One grain of rice was represented one dB.  When increased to 10 dB, the amount of grains of rice was increased by a factor of ten (1x10).  This shows us that the scale of intensity is not linear, but exponential or logarithmic.

Doppler Effect
The Doppler Effect occurs when the source of sound moves relative to an observer (or the observer moves relative to the source) and the frequency of the sound seems to change.
Ex. train passing by

Resonance in Air Columns
We can create standing waves of sound in an air column just as we can create them in a slinky.  If the wavelength of the sound relates to the length of the pipe in a particular way the sound intensity will increase significantly.