Vibrations_and_Sound

Gap-fill exercise

Fill in all the gaps.
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Vibrations and Sound

Sound is a wave motion. We know this because sound undergoes

• (echoes)
• (you can hear around corners)
• ( Sound can be heard better on a cold day than a warm day)
• Interference

The speed at which sound travels through a medium depends on the of that material.

To demonstrate Interference of sound

Using a Signal and

1. Walking slowly to the speakers, you will notice the of the sound and at regular intervals.

2. This is because sound waves from the two speakers will interfere both and , along the .

You must make reference to a signal generator or sound from each speaker having the same .

To show that Sound needs a medium to travel through

1. Set up the Bell-Jar – the bell can be heard .
2. Remove the air from the Bell-Jar using a pump.
3. Result: While the bell can still be seen to be ringing, the sound gets until eventually nothing can be heard.


Natural Frequency

The Natural Frequency of an object is the at which the object will if to do so.

Every object has its own natural frequency, although some objects will also vibrate at multiples of this natural frequency.

Factors which determine the Natural Frequency of a Stretched String.

f =
ℓ =
T = Tension

• Frequency is proportional to the length of the string: f ∝ 1 / ℓ

• Frequency is directly proportional to the of the tension in the string; f ∝ √ T,

• Frequency is inversely proportional to the square root of the per unit of the string; f ∝ 1 / √ µ .


Resonance

Resonance is the of energy between two which have the same frequency.

To Demonstrate Resonance
• Use two tuning forks and a -board.
• Start one fork , place it on the sound-board and notice the sound.
• Place the tuning fork on the sound-board and then stop the tuning fork from vibrating.
• The second fork can now be heard.

Why

The vibrations were passed from the first tuning fork via the sound-board to the second tuning fork.

Examples of Resonance:

Washing Machines at a particular speed

Ovens.

The Frequency Limits of Audibility are the and frequencies that can be heard by a human ear.

The range is 20 Hz – Hz.

Sound Intensity

Sound Intensity is defined as per unit area.


I = P /

The unit of Sound Intensity is therefore the per metre (W m -2)

Given that the sound-wave expands equally in all directions (like a spherical balloon being blown up) then it follows that the area it is passing through is the surface area of a : Area = 4πr2.

Threshold of Hearing is the sound intensity by the average human ear at a of 1 KHz.

Its value is 1 x 10-12 Wm-2.

Sound Intensity Level

Human hearing falls roughly in the range 10-12 Wm-2 (the Threshold of Hearing) to 1 Wm-2

The size of this range is and is very impractical.

We therefore use a different set of numbers which basically compact the Sound Intensity scale.

These numbers are called Sound Intensity levels and are measured in (dB)

Examples of Sound Intensity Level

Whisper = 20 dB iPod at max volume = 100 dB Threshold of Pain = 130 dB Perforation of Eardrum = 160 dB

Note: Doubling the Sound Intensity increases the Sound Intensity Level by 3dB.


Decibel Adjusted dB(A) Scale

The decibel scale is used because it is adapted to the ear’s response
.
The ear is most sensitive to frequencies between Hz and 4000 Hz and the Decibel Adjusted scale takes this into account.

The meter used by environmental engineers has a decibel adjusted scale.

Speed of Sound in different media

In general, the speed of sound of sound in a solid is greater than in a liquid, which in turn is than that for a gas.

Fundamental Frequency of a string

A string vibrating with an at its centre and a at each end ( and no other nodes or antinodes ) is vibrating at its fundamental frequency

Harmonics

Frequencies which are of the fundamental frequency f are called .

The basic frequency is called the fundamental frequency or first , is the second harmonic etc.

Overtones

Frequencies which are of a given frequency are called .

If f is the first frequency, then is its first overtone; 3f is its second overtone etc.

Characteristics of Notes
1. Loudness: The loudness of a note depends upon the of the sound wave

2. Pitch: The pitch of a note depends upon the of the sound wave

3. Quality: The quality of a note depends upon the number of present in the note and the strengths of those different overtones

This explains why a middle C note sounds different when played on different musical instruments.

Acoustics may be defined as the science of

Reduction of noise using interference

Noise can be reduced using destructive interference eg in jackhammers. Electronic microchips produce mirror-image wave patterns of the sound.

This is fed to headphones, so to cancel out the loud noise for the operator, while enabling him to still hear co-workers voices!

Mandatory Experiments:

• Measurement of the speed of sound in air.
• Investigation of the variation of fundamental frequency of a stretched string with length.
• Investigation of the variation of fundamental frequency of a stretched string with tension


In a closed pipe numbered harmonics will be present.

In pipes open at both ends harmonics are present.