Quick notes Chapter 12 Characteristics of Sound
1.
Source of sound is a vibrating object. Energy is then transferred through a medium in the form of longitudinal sound waves. Sound then must be detected by an ear or microphone.
2.
Air is not the only medium to carry sound, but the speed of sound is different in different materials and depends some on temperature.
Examples: a.
Air—343 m/s b.
Water-1440 m/s c.
Glass-4500m/s
3.
Loudness is a sensation by a listener that is related to the intensity in the sound wave.
4.
Pitch refers to the highness or lowness of the sound and is determined by frequency.
5.
Human frequency range is 20 Hz to 20,000 Hz (audible range)
6.
Frequencies above 20,000 are considered ultrasonic –some animals can hear in these ranges
7.
Longitudinal waves are also called pressure waves because sound waves can be analyzed by changes in pressure. Pressure variation is easier to measure than displacement. In a compression the pressure is higher than normal and in rarefaction (expansion) pressure is less than normal.
8.
Infrasonic is when sound waves have frequencies below audible range, like thunder and earthquakes. These can be in resonant frequencies to the human body and cause problems.
Intensity of Sound-Decibels
1.
Intensity is the energy transported by a wave per unit time across an area perpendicular to the energy flow
2.
Intensity is proportional to the square of the wave amplitude and is measured in W/m 
3.
Human range of is from 10 -12 w/m 2 to 1 w/m 2
4.
A decibel is 1/10 th of a bel named after Alexander Graham Bell
5.
 is the sound level of any sound in terms of its intensity so
 = 10 log I/I
0
where I
0
is the threshold of hearing or 1 x -12 w/m 2
6.
Normally loudness of a sound decreases as you get further from the source and the intensity is inversely proportional to the square of the distance. This is different indoors because of reflection of sound off of walls.
7.
Intensity is proportional to the wave amplitude and we can relate the amplitude to intensity and sound level

The Ear and its Response
1.
The function of the ear is to transform vibrational energy into electrical signals which then are carried by nerves to the brain.
2.
Sound waves travel from the outer ear down the ear canal to the eardrum, which vibrates from the waves. The hammer, anvil and stirrup transfer the vibrations to the inner ear.
3.
The ear is not equally sensitive to all frequencies.
Sources of Sound: String instruments
1.
The source of any sound is a vibrating object
2.
Instruments produce standing waves
3.
Stringed instruments can vibrate with nodes only at the ends. This frequency is called the fundamental.
4.
The fundamental frequency corresponds to a wavelength that is 2x’s the length of the string.
5.
Octaves are a doubling of a frequency (C above middle c has 2x’s the frequency of middle c)
6.
When you put your finger on a stringed instrument you shorten the length of the string. The string can vibrate at higher frequencies called harmonics or overtones that have more nodes.
7.
For the same wavelength, the velocity on a heavier stringer is lower and the frequency will be lower, so pitch is lower. Pianos have different lengths of string as well.
8.
For velocity, v=  (F
T
/  )where  =m/l—hence when you tune you are changing tension on the instrument