Chapter 26: Sound

The Origin of Sound  All sounds are produced by the vibrations of material objects  Pitch – our subjective impression of sound  A young person can normally hear pitches with frequencies from 20 to 20,000 Hz; as we grow older, this range shrinks  Infrasonic – sound waves with frequencies below 20 Hz  Ultrasonic – sound waves with frequencies above 20,000 Hz

Sound Waves and Frequency

Sound in Air  Compression – a pulse of compressed air  Rarefaction – pulses of lower pressure  Most sound waves are combinations of compressions and rarefactions which create a longitudinal wave

Sound in Air

Media That Transmits Sound  Most sounds you hear are transmitted through the air  Sound also travels through solids and liquids  Solids and liquids are generally better conductors of sound than air, sound waves travel faster in solids and liquids  Sound cannot travel through a vacuum (there is nothing to compress!)

Bell in a Vacuum

Speed of Sound  You hear thunder after you see lightning (evidence that sound is much slower than light)  The speed of sound in dry air at 0ºC is ~330 m/s (1/ the speed of light)  For each degree increase in temperature, the speed of sound increases by 0.60 m/s (at normal room temperature of 20ºC, sound travels at 340 m/s)  The speed of sound in a material depends not on the density, but on its elasticity (ability to change shape in response to an applied force)

Loudness  The intensity of sound is proportional to the square of the amplitude of a sound wave  Sound intensity is objective and can be measured by an oscilloscope  Loudness is physiological sensation sensed in the brain  The unit of intensity for sound is the decibel (dB), after Alexander Graham Bell  Starting with zero at the threshold of hearing for a normal hear, an increase of each 10 dB means that sound intensity increases by a factor of 10  Human hearing is approximately logarithmic

SourceIntensity Level # of Times Greater Than TOH Threshold of Hearing (TOH) 1* W/m 2 0 dB10 0 Rustling Leaves1* W/m 2 10 dB10 1 Whisper1* W/m 2 20 dB10 2 Normal Conversation 1*10 -6 W/m 2 60 dB10 6 Busy Street Traffic1*10 -5 W/m 2 70 dB10 7 Vacuum Cleaner1*10 -4 W/m 2 80 dB10 8 Large Orchestra6.3*10 -3 W/m 2 98 dB Walkman at Maximum Level 1*10 -2 W/m dB10 Front Rows of Rock Concert 1*10 -1 W/m dB10 11 Threshold of Pain1*10 1 W/m dB10 13 Military Jet Takeoff1*10 2 W/m dB10 14 Instant Perforation of Eardrum 1*10 4 W/m dB10 16

Forced Vibration  Forced Vibration – the vibration of an object which is made to vibrate by another vibrating object that is nearby  The mechanism in a music box is mounted on a sounding board (the object being forced to vibrate); without the sounding board, the sound would be almost inaudible  The vibration of guitar strings in an acoustic guitar follow the same principle

Forced Vibration

Natural Frequency  Natural Frequency – an objects own special set of frequencies, which together form its special sound  The natural frequency depend on factors such as the elasticity and shape of the object  Bells and tuning forks vibrate at their own characteristic frequencies  Even planets and atoms vibrate at one or more natural frequencies

Natural Frequency

Resonance  Resonance – occurs when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude occurs Resonance  In order for something to resonate, it needs force to pull it back to its starting position and enough energy to keep it vibrating

Interference  Sound waves, like any waves, can be made to interfere  Interference affects the loudness of sounds  Destructive interference of sound waves is usually not a problem, because there is enough reflection of sound to fill in canceled spots; although, “dead spots” are often evident in poorly designed arenas  Destructive sound interference is used in anti- noise technology

Interference Patterns

Beats  Beats – the periodic variation in the loudness of sound  Beats can be heard when two slightly mismatched tuning forks are sounded together; when the forks are in step, the sound is at a maximum, when the forks are out of step, the sound is at a minimum

Beats

Assignment  Read Chapter 26 (pg )  Chapter 26 Assessment # (pg )