Sound

Definition of SoundDefinition of Sound

Sound is a wave created by vibrating objects and propagated through a medium from one location to another.

Sound is a wave created by vibrating objects and propagated through a medium from one location to another.

If a tree falls in a forest, and If a tree falls in a forest, and there is no one there to hear there is no one there to hear

it, does it make a sound?it, does it make a sound?

Based on our definition, there Based on our definition, there ISIS sound in the forest, whether a sound in the forest, whether a human is there to hear it or not!.human is there to hear it or not!.

Sound is a Sound is a physical physical disturbancedisturbance in a medium. in a medium.

Sound is a Sound is a physical physical disturbancedisturbance in a medium. in a medium.

A person to hear it is not required. A person to hear it is not required. The medium (air) is required!

What type of waves are sound waves?

• The sound wave is transported from one location to another by means of particle-to-particle interaction.

• If the sound wave is moving through air, then as one air particle is displaced from its equilibrium position, it exerts a push or pull on its nearest neighbors, causing them to be displaced from their equilibrium position.

• Since a sound wave is a disturbance that is transported through a medium via the mechanism of particle-to-particle interaction, a sound wave is characterized as a mechanical wave.

Review: Sound is a mechanical Review: Sound is a mechanical wavewave

A sound wave is different than a light wave in that a sound wave isa. produced by a vibrating object and a light wave is not. b. not capable of traveling through a vacuum.c. not capable of diffracting and a light wave is.d. capable of existing with a variety of frequencies and a light wave has a single frequency.

Check your Check your understanding:understanding:

When a tuning fork vibrates, it creates areas of When a tuning fork vibrates, it creates areas of high pressure (compressions) and low pressure high pressure (compressions) and low pressure (rarefactions).(rarefactions). As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.

Graphing a Sound Graphing a Sound Wave.Wave.

Sound as a pressure wave

The variation of The variation of pressurepressure with with distancedistance is a useful way to is a useful way to represent a sound wave graphically. But remember – sound is represent a sound wave graphically. But remember – sound is

actuallyactually a longitudinal wave. a longitudinal wave.

A sound wave is a pressure wave; regions of high pressure (compressions) and low pressure (rarefactions) are established as the result of the vibrations of the sound source. These compressions and rarefactions result because sounda. is more dense than air and thus has more inertia. b. waves have a speed that is dependent only upon the properties of the medium.c. can be diffracted around obstacles.d. vibrates longitudinally; the longitudinal movement of air produces pressure fluctuations.

Check your understandingCheck your understanding

The vibrating object that creates sound could be the vocal cords of a person, the vibrating string of a guitar or violin, the vibrating tines of a tuning fork, or the vibrating diaphragm of a radio speaker. As a sound wave moves through a medium, each particle of the medium vibrates at the same frequency. This makes sense since each particle vibrates due to the motion of its nearest neighbor. And of course the frequency at which each particle vibrates is the same as the frequency of the original source of the sound wave.

Frequency of SoundFrequency of Sound

A guitar string vibrating at 500 Hz will set the air particles in the room vibrating at the same frequency of 500 Hz, which carries a sound signal to the ear of a listener, which is detected as a 500 Hz sound wave.

Frequency of Sound Frequency of Sound ExampleExample

• We hear frequencies of sound as having We hear frequencies of sound as having different different pitchpitch..

• A low frequency sound has a low pitch, like A low frequency sound has a low pitch, like the rumble of a big truck. the rumble of a big truck.

• A high-frequency sound has a high pitch, like A high-frequency sound has a high pitch, like a whistle or siren.a whistle or siren.

• In speech, women have higher fundamental In speech, women have higher fundamental frequencies than men.frequencies than men.

The frequency of soundThe frequency of sound

Animation showing Frequency and Pitch

Frequency of SoundFrequency of Sound• The human ear is capable of detecting

sound waves with a wide range of frequencies, ranging between approximately 20 Hz to 20 000 Hz.

• Any sound with a frequency below the audible range of hearing (i.e., less than 20 Hz) is known as an infrasound.

• Any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound.

Ultrasound?

Ultrasound is a medical imaging technique that uses high frequency sound waves and their echoes. The technique is similar to the echolocation used by bats, whales and dolphins.

How it works: Ultrasound

The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.

The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).

Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.

The reflected waves are picked up by the probe and relayed to the machine.

The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue and the time of the each echo's return (usually on the order of millionths of a second).

The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image like the one shown below.

What about animals?Dogs can detect frequencies as low as approximately 50 Hz and as high as 45 000 Hz.

Cats can detect frequencies as low as approximately 45 Hz and as high as 85 000 Hz.

Frequency and music

Certain sound waves when played (and heard) simultaneously will produce a particularly pleasant sensation when heard. Such sound waves form the basis of intervals in music.

For example, any two sounds whose frequencies make a 2:1 ratio are said to be separated by an octave and result in a particularly pleasing sensation when heard. That is, two sound waves sound good when played together if one sound has twice the frequency of the other.

Loudness

Intensity

Intensity: the rate at which a wave’s energy flows through an area

Sound intensity depends on Amplitude Distance from source

Measured in decibels (dB)

Loudness is sort of like intensity, but…

Loudness is Subjective! (This means it depends on the person who is hearing it.)

Loudness is a personal, physical response to the intensity of sound.

As intensity increases, so does loudness, but loudness also depends on the listener’s ears and brain.

Intensity is caused by the Amplitude of the vibration

Example:A vibrating guitar string forces surrounding air molecules to

be compressed and expanded.The energy that is carried by the wave is imparted to the

medium by the vibrating string. The amount of energy that is transferred to the medium is

dependent on the amplitude of vibrations of the guitar string.

If more energy is put into the plucking of the string, then the string vibrates with a greater amplitude. The greater amplitude of vibration of the guitar string thus imparts more energy to the medium, causing air particles to be displaced a greater distance from their rest position.

The Decibel Scale:

The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine . In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference!

The Decibel Scale

On the decibel scale, the smallest audible sound (the threshold of hearing) is 0 dB.

A sound 10 times more powerful is 10 dB.

A sound 100 times more powerful than near total silence is 20 dB

A sound 1,000 times more powerful than near total silence is 30 dB.

Intensity (Loudness) is Intensity (Loudness) is measured in decibels:measured in decibels:

SourceIntensity

Level# of Times

Greater Than TOH

Threshold of Hearing 0 dB 100

Rustling Leaves 10 dB 101

Whisper 20 dB 102

Normal Conversation 60 dB 106

Busy Street Traffic 70 dB 107

Vacuum Cleaner 80 dB 108

Large Orchestra 98 dB 109.8

Walkman at Maximum Level 100 dB 1010

Front Rows of Rock Concert 110 dB 1011

Threshold of Pain 130 dB 1013

Military Jet Takeoff 140 dB 1014

Instant Perforation of Eardrum 160 dB 1016

 

Check your understanding

A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?