Sound Lecture 5

Goals  Understand Standing Waves  Gain an understanding of fundamental sound concepts such as intensity and beats  Learn about applications such as musical instruments

Standing Waves  Using the superposition principle from the previous lecture, we can build standing waves  Think of a guitar string that bounces up and down, which is a transverse wave

Waves on a String  We can see from the previous figure that the largest wavelength allowed is twice the length of the string  The smallest frequency is called the fundamental frequency, all other frequencies are whole number integers of the fundamental frequency

Sound and Waves on a String  Sound is produced by a vibrating string when the string disturbs the air around it  The sounds waves that are produced by the string have the same frequency as the string vibrations  The wavelength of the sound is typically not the same as the string wavelength

Sound Waves  The speed of the sound wave in air depends upon the temperature of the air  Note that the temperature in this equation must be in Kelvin's

Lets try an Example  We know a string vibrates with frequency 500 Hz and the speed of sound produced by the string in air at 0 degrees is 331 m/s. What is the wavelength of sound produced by the string at 23 degrees?

Sound Produced by a Pipe  When one blows into a musical instrument such as a flute, longitudinal waves are produced that cause sound waves  The fundamental frequency depends upon whether the pipe is open at both ends or just one

Pipe Open at Both Ends

Pipe Open at One End

Loudness  The loudness of a sound is a complex subject  Not only is it due to the energy in the wave but the way the brain perceives it  Perceived loudness is approximately proportional the logarithm of the amplitude

More on Loudness  The loudness we measure is based upon a reference value  The reference chosen is typically the threshold of hearing, or the lowest intensity a person with excellent hearing can perceive  I o = 1.00 x W/m 2  For every factor of 10 increase in intensity, the intensity level increases by10 dB

Lets try and Example

More Sound Concepts  Timbre measures how purely sinusoidal the sound wave is  Pitch is the perception of frequency  Beats are the superposition of waves into a new waveform with periodic function

The Doppler Effect  When a sound source, such as an ambulance, moves towards or away from us, the frequency we perceive is different than the frequency produced by the source  This effect is most noticeable when the ambulance changes from moving towards us to away from us or vice versa

Moving Sources  The frequency the observer measures can be related to the frequency of the source by the following

Additional Notes