CLASS – IXCHAPTER - SOUND

Definition of Sound

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 there is no one there to hear it, does it make a

sound?

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

Sound is a physical disturbance in a medium.

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

What type of waves are sound waves?

Review: Sound is a mechanical wave

• 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.

Check your understanding:A sound wave is different than a light wave in that a sound wave is:a. 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.

When a tuning fork vibrates, it creates areas of high pressure (compressions) and low pressure (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 Wave

Sound as a pressure wave

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

actually a longitudinal wave.

Check your understandingA 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.

Frequency of SoundThe 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 Sound ExampleA 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.

The frequency of sound• We hear frequencies of sound as having different

pitch.• A low frequency sound has a low pitch, like the

rumble of a big truck. • A high-frequency sound has a high pitch, like a

whistle or siren.• In speech, women have higher fundamental

frequencies than men.

Frequency 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.

What about animals?

Dogs can detect frequencies as low as approximately 50 Hz and as high as 45000 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 vibrationExample: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 ScaleOn 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 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?