Sound

CH 7: SOUND Sounds are a form of energy produced by rapidly vibrating objects. Sound needs a material medium for its transmission. Sound cannot travel through a vacuum. The vibrating object causes compressions and rarefactions in the medium. A receiver senses the sound by sensing the compressions and rarefactions.

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Sound is made when something vibrates. The vibration disturbs the air around it. This makes changes in air pressure. These changes in air pressure move through the air as sound waves.

The sound waves cause pressure changes against our ear drum sending nerve impulses to our brain.

This is similar to throwing a rock into a pond. Air molecules ripple through the air in sound waves like water waves rippling across a pond.

Sound Waves: Pitch Pitch: the frequency of a sound wave. The human ear is not as sensitive to pitch as it is to loudness. –Most people cannot hear frequencies below 20 Hz or above 16,000 Hz. –Most audible sounds lie in the range of 1000 to 5000 Hz. –Exposure to loud noise damages our ability to hear higher pitched sounds.

When the frequency of a sound doubles we say that the pitch goes up an octave. We can hear a range of pitches of about ten octaves. Many animals can make sounds and hear frequencies that are beyond what we can hear.

Sounds with frequencies less than 20 Hz are infrasonic while sounds above Hz are ultrasonic.

p in text p ,2,3extra 4 p ,4,5extra 2,3,6,7

Loudness To create vibrations energy is used. The greater amount of energy used the louder the sound. The strength of the changes in air pressure made by the vibrating object determines loudness.

The loudness of sound depends on the amplitude of the sound wave. The human ear is extremely sensitive to the pressure differences caused by sound waves.

THE SPEED OF SOUND Air pressure and elevation do not significantly affect the speed of sound in air. p p , p extra p.246 1,2,6-8

Notice any patterns?

THE INTENSITY OF SOUND p p , p Sound intensity is the power of sound per unit area (W/m 2 ). Sounds can be emitted with an extremely large variance in intensity and likewise humans can sense extremely soft sounds as well as loud sounds. The quietest whisper is about W/m 2 while a sound with an intensity of 10 4 W/m 2 will instantly perforate an eardrum.

The decibel scale is utilized for sound intensity and gives an easy scale to judge relative intensities. The least intense sound we can hear is given the intensity of 0 dB. For every 10 dB increase in intensity the sound increases its true intensity by 10X. The scale is logarithmic so if the intensity increases by 30 dB then the true intensity has increased by a factor of 1000X. The intensity of sound we hear depends on the power of the source and the distance between us and the source.

As the sound spreads out from its source, the concentration of power becomes less. As the distance from the source increases the amount of power is spread over a greater area. The amount of power per square meter is called the intensity of the sound.

Humans do not perceive sound intensity linearly. For us to perceive a sound as twice as loud its intensity must be ten times greater.

The scale begins (0 dB) on the softest sound that a person can hear. This is called the threshold of hearing. The scale ends at the volume that causes pain (120 dB) and is therefore called the threshold of pain.

Sound Waves: Loudness Decibels: dB, logarithmic scale of sound level. –10 dB: Barely audible. –30 dB: Quiet breathing. –50 dB: Normal classroom conversations. –70 dB: Noisy traffic. –100 dB: Fire engine horn. –110 dB: Concert/Airplane. – dB: Human Pain Threshold.

read The Human Ear p will not test on parts of the human ear

read The Reflection of Sound Waves p understand echoes and echo problems (remember to double distance) -p , p know echolocation, who uses it -know ultrasound applications

Not responsible for diffraction and refraction of waves p Not responsible for section on interference of waves p Responsible for beat frequency calculations p , p ,2 p

The Doppler Effect (p ) The apparent changing frequency of sound in relation to an object’s motion is called the Doppler effect, named after Christian Doppler ( ). If a sound emitter is moving towards a listener (or vice versa) then the listener hears a higher frequency than is actually emitted. If a sound emitter is moving away from a listener (or vice versa) then the listener hears a lower frequency than is actually emitted. The Doppler effect (Doppler shift) has been used to estimate the speed of distant stars and galaxies (using light waves) relative to our solar system. The Doppler shift is also used in police radar for speeding. You are not responsible for the equation which quantifies the Doppler effect.

The Mach Number is the ratio of an object’s velocity to the speed of sound. When flying at Mach 1, an object is flying as fast as the sound it gives off. When the object emits another sound the crest will alongside the original crest so these crests pile up, producing an area of very dense air. This intense compression of air is called the sound barrier. Extra thrust is needed to break through this barrier. Objects must be designed to cut through this dense air leading to sleek and pointy shapes. At hypersonic speeds, the crests are left behind the object which constructively interfere with other crests to create a double cone. This intense acoustic pressure is called the sonic boom.

"A "sonic boom" is heard when a plane exceeds the speed of sound. "

Did you know..... The crack of a whip is the sound of the tip breaking the speed of sound; a small sonic boom