Sound

Sound Waves  Sound waves are longitudinal waves.  The source of a sound wave is a vibrating object.  Only certain wavelengths of longitudinal waves in the air can be detected as sound.

Sound Waves  Sound waves require matter in order to propagate.  They cannot travel in a vacuum  The speed of sound is different in different materials.  The speed of sound is also somewhat dependent on temperature.

Speed of Sound  Air (room temp) – 343 m/s  Air (0° C) – 331 m/s  Helium – 1005 m/s  Water – 1440 m/s  Steel – about 5000 m/s  Concrete – about 3000 m/s

Loudness  Loudness is related to intensity.  Loudness itself is not a measurable physical quantity.  It depends on the listener.  Intensity is measurable, however.

Loudness  Sound intensity levels are usually given on a logarithmic scale.  The unit on this scale is the decibel (dB)  β = 10 log (I/I ₀ )  β = sound level (in dB)  I = sound intensity  I ₀ = base sound level

Loudness  The base sound level used (I ₀ ) is usually what is know as the “threshold of hearing”  This is the minimum intensity that is audible to a good ear.  1.0 x 10ֿ¹² W/m²

Sound Intensities  Threshold of hearing – 0 dB  Whisper – 20 dB  Talk, at 50 cm – 65 dB  Busy street traffic – 70 dB  Loud rock concert – 120 dB  Jet plane at 30 m – 140 dB

Pitch  Pitch is related to frequency of a sound wave.  A high pitch corresponds to a high frequency.  The audible range is the range of frequencies that a normal person can hear.

Pitch  Frequencies between 20 Hz and 20,000 Hz are audible.  The threshold of hearing varies for different frequencies.  It is easiest to hear sounds between 2000 and 5000 Hz.

Pitch  Frequencies too high to hear are ultrasonic.  Some animals can perceive sounds up to 100,000 Hz (bats)  Frequencies too low to hear are infrasonic.

Beats

Beats  Occurs due to interference between two sound waves that are close in frequency.  The sound level rises and falls as time passes.  This regular change in intensity is a beat.

Doppler Effect  The Doppler Effect is the apparent change in frequency of a wave due to relative motion between the observer and the sound source.  Has many uses.