Sound Waves

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 2 What kind of waves are sound? Mechanical – caused by oscillations of atoms Longitudinal – creates areas of high and low pressure Tuning fork -  An acoustic device that has a two prongs (tines) formed into a U shape from steel. Each tuning fork is of a different size and resonates at a specific constant frequency or pitch; used as a standard in order to tune musical equipment to. Strike a tuning fork against the desk to allow students to observe the sound Have students watch the video link to observe the motion of the tines as they are dipped into water in slow motion Ask students what this video shows about the vibration of the tines and what the energy from their movement does to the environment around them. Answer: The vibration of the tines transfers kinetic energy to the material surrounding them. We hear the energy in the form of sound, we can see it in the form of splashing on the water’s surface in the video.

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 What are high pressure areas called? - compressions What are low pressure areas called? - rarefaction

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

Remember! 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 Bats and dolphins use ultrasound for navigation; elephants and whales communicate over large distances using infrasound. Human ear can hear between 20 – 20 000 Hz. Infrasonic – below 20 Hz Ultrasonic – above 20 000 Hz

Loudness of Sound Which variable do you think corresponds to the loudness of sounds? Amplitude! Loudness is measured on a decibel scale. Decibel scale is logarithmic; each 10 db increase is perceived as twice as loud. 30 minutes of exposure to anything 90 db or louder can cause permanent hearing loss. 30 seconds of exposure to anything over 120 deb or louder can cause permanent hearing loss.

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 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

If the source and the observer are moving Doppler shift is used for radar guns. Doppler shift is also used for EM waves … the ‘red shift’ of most galaxies indicates that most galaxies are moving away from us … supporting the idea that 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. The red shift observed in distant galaxies (obviously) doesn’t come from reflected light. Instead, we observe that the light is redshifted compared to the light emitted from burning the same fuels (He and H) on Earth. Some galaxies are blue shifted but the overall pattern of Doppler shift indicates that the universe is expanding.

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. Bullet shows a shock wave just in front of it as it travels at the speed of sound Picture of a fighter jet just as it breaks the sound barrier… white condensation cloud is generated from extreme pressure in the air in front of the jet and low pressure behind the jet – cloud forms because the water vapor that is really hot in front of the jet cools quickly immediately behind the jet as it passes. In the bottom, a Mach cone is formed, this is the source of the sonic boom Bullet traveling at more than twice the speed of sound with a noticeable Mach cone Jet in Supersonic flight Sonic boom = loud explosive sound caused by jet moving at supersonic speeds