Sound Waves March 22-23, 2016

The nature of sound What is a tuning fork? How are they used? How do we know that sound is a wave? Visualizing sound waves Video 1 Video 2Video 1Video 2 What kind of waves are sound? Mechanical – caused by oscillations of atoms Longitudinal – creates areas of high and low pressure

Sound Waves are Longitudinal Waves The bell in the picture is at rest, so the surrounding air is at average air pressure. When the bell is struck, the vibrating edge creates regions of high and low pressure. Forward movement of bell – generates regions of slightly higher pressure than normal Backward movement of bell – generates regions of slightly lower pressure than normal

All sound is formed in the same manner … a vibrating source creates a longitudinal wave.

Sound needs a medium – won’t travel in a vacuum since nothing to compress and expand

Frequency of Sound The frequency of sound refers to the number of oscillations per second made by atoms, and it is measured in Hz (s -1 ). Frequency = pitch Human ear can hear between 20 – Hz. Human ear can hear between 20 – Hz. Infrasonic – below 20 Hz Infrasonic – below 20 Hz Ultrasonic – above Hz Ultrasonic – above Hz Bats and dolphins use ultrasound for navigation; elephants and whales communicate over large distances using infrasound.

Loudness of Sound Which variable do you think corresponds to the loudness of sounds? Amplitude! Loudness is measured on a decibel scale.

Speed of Sound Speed of sound depends on … The medium, only! Amplitude doesn’t affect speed. Neither does frequency (though high frequency waves will have λf = v ) short wavelengths and low frequency waves will have high wavelengths in accordance with λf = v ) Sound is typically fastest in solids, then in liquids, and slowest in gases. Sound is faster in warm, humid air than in cold, dry air.

Doppler Effect The perceived change in frequency of a sound that occurs when the sound source and observer are in motion relative to each other. Represents a stationary sound source The distance between wave fronts is the wavelength. All observers will hear the same frequency Represents a source moving at v Wave fronts are still produced with the same frequency but are bunching up in the direction of travel Observer in front of the source hears a higher frequency sound Observer behind the source hears a lower pitch, lower frequency sound. This is called a Doppler shift

General Rule: If the source and the observer are moving … closer together  the perceived frequency is higher... farther apart  the perceived frequency is lower

Doppler Effect Applications Doppler shift can be used to determine the relative speed of objects. Waves are directed at an object, they reflect (bounce back) and are picked up by a receiver. The shift in frequency can be used to determine the speed of the object. Examples: ultrasound waves to show blood velocity microwaves in radar guns The light from distant stars often appears to be ‘red-shifted’ indicating that those galaxies are moving away from us.

Just For Fun Represents an object moving at Mach 1 or at the speed of sound (343 m/s; 750 mph) Observer will not hear anything until the source arrives – The pressure front will be intense Represents an object moving at Mach 1.4 or greater than the speed of sound. Observer will see the sound source pass by first before the observer actually hears the sound it creates.