a range of compression wave frequencies to which

a range of compression wave frequencies to which the human ear is sensitive

Sounds are produced by vibrating matter. 1. reeds 3. membranes 2. strings 4. air columns Sound is a mechanical wave (longitudinal). It will not travel through a vacuum.

• PITCH = The impression about the frequency of Sound • high pitched – high frequency (ex: piccolo) • low pitched – low frequency (ex: fog horn) • The frequency range for normal human hearing is. between 20 Hz and 20, 000 Hz. (for younger people, older people lose the higher frequencies) • Infrasonic or subsonic = Sounds below 20 Hz • Ultrasonic = frequencies above 20, 000 Hz

Range of Some Common Sounds

Intensity Range for Some Common Sounds

Compression waves travel through air or along springs. These waves travel with areas of compression and rarefaction. The medium does not travel from one place to another, but the pulse that travels.

Sound wave requires medium Any matter will transmit sound, whether it is a solid, liquid or a gas. However; sound cannot travel through a vacuum. No Sound DING VACUUM

The velocity of sound in air depends on the air temperature. The speed of sound in dry air is 331. 5 m/s at 0 ºC. This speed increases with temperature: about 0. 6 m/s º for every 1 C increase in temperature.

Speed of Sound • The speed of sound in dry air at 00 C is about 330 m/sec. (1200 km / hr) [or ~. 000, 001 x the speed of light of 300, 000 km / sec] • So air at room temperature (~20 o. C) is ~340 m/sec. • QUESTION : . How far away was the strike if there is a 3 second delay between the lightning flash and the sound of the thunder? • 340 m/sec x 3 sec = 1020 m, over 1 km (~2/3 mile) away

Sound generally travels fastest in solids and slowest in gases, but there are some exceptions. Medium Velocity (m/s) Air 330 Carbon dioxide 260 Helium 930 Hydrogen 1270 Oxygen 320 Water 1460 Sea water 1520 Mercury 1450 Glass 5500 Granite 5950 Lead 1230 Pine wood 3320 Copper 3800 Aluminium 5100

Speed of Sound • The speed of sound in a material does NOT depend on its density (mass per unit volume [g/cm 3 ]). • The speed of sound in a material DOES depend on the elasticity of a material. • Elasticity = the ability of a material to change shape in response to an applied force, then resume its original shape when the force is removed. • Steel is elastic, putty is inelastic. • Sound travels 15 times faster in steel than in air and about 4 times faster in water than in air.

LOUDNESS • The intensity of a sound is proportional to the square of the amplitude of the sound wave. (i = ka 2) • Loudness is measured in decibels (d. B) MICROPHONE VIBRATING LOUDSPEAKER • 1 AMPLITUDE OSCILLOSCOPE 10 100 • The decibel scale is logarithmic, increasing by factors of 10 1000

LOUDNESS • • • • TABLE of Loudness Levels SOURCE OF SOUND LEVEL (d. B) Jet Engine (from 30 m) 140 Threshold of pain 120 Loud rock music 115 Old subway train 100 Average factory 90 Busy street traffic 70 Normal speech 60 (Shshshshhh!) A library 40 Close Whisper 20 Normal breathing 10 Hearing threshold 0

Forced Vibration • Sounding boards are used to augment (increase) the volume (amplitude) of a vibrating object (like a string). STRINGS SOUNDING BOARD

Natural Frequency • Everything vibrates, from planets and stars to atoms and almost everything in between. • A NATURAL FREQUENCY is one at which minimum energy is required to produce forced vibrations • and also requires the least amount of energy to continue this vibration

Resonance – when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude of the vibrations occurs. • For example, a swing, or the hollow box parts of musical instruments are designed to work best with resonance. • In order to resonate, an object must be elastic enough to return to its original position and have enough force applied to keep it moving (vibrating)

A resonant air column is simply a standing longitudinal wave system, much like standing waves on a string. closed-pipe resonator tube in which one end is open and the other end is closed open-pipe resonator tube in which both ends are open

A closed pipe resonates when the length of the air column is approximately an odd number of quarter wavelengths long. l = {(1, 3, 5, 7, …)/4} * With a slight correction for tube diameter, we find that the resonant wavelength of a closed pipe is given by the formula: l = 4 (l + 0. 4 d), where is the wavelength of sound, l is the length of the closed pipe, and d is the diameter of the pipe.

An open pipe resonates when the length of the air column is approximately an even number of quarter wavelengths long. l = {(2, 4, 6, 8, …)/4} * With a slight correction for tube diameter, we find that the resonant wavelength of an open pipe is given by the formula: l = 2 (l + 0. 8 d), where is the wavelength of sound, l is the length of the closed pipe, and d is the diameter of the pipe.

Interference • Sound waves interfere with each other in the same way as all waves. • Constructive interference - augmentation • Destructive interference - cancellation

Beats • BEATS - A periodic variation in the loudness of sound. . (faint then loud, faint then loud and so on … ) • What is the frequency when a 262 Hz and a 266 Hz tuning fork are sounded together ? • The 262 Hz and 266 Hz forks will produce 4 beats per sec. and the tone heard will be halfway between at 264 Hz as the ear averages the frequencies.