sound

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SOUND

K Warne

http://www.warnescience.net/




Sound• Think of how a drum makes a noise.

• All sounds are produced by vibrating objects.





Sound• All sounds start with a vibration.

• The vibrating object causes compresses and stretches out

(rarifies) the air around the object.

• These pressure fluctuations then spread out from the object in

three dimensions.

High Pressure

CompressionLow Pressure

RarefactionVibration

Waves spread

out in three

dimensions





The speed of Sound

speed = distance

time

x

t = time taken for sound to travel distance x

The speed of sound is the distance covered by a sound wave in one

second.





Experiment to measure the speed of sound

Apparatus:

– Loud noise - visual! (starter pistol)

– Stopwatch, Pencil, notepad

– “Clickometer”

( ( ( ( ( ( ( ( (





Measuring the speed of SoundTime (s) Speed (m/s)

Measurements:

Distance = …….. m

Speed = distance/time

AVERAGE SPEED m/s

Write up:

Aim

Theory

Apparatus

Method

Results

Analysis Graph

Conclusion

Evaluation



Speed of Sound.xls



Measuring the speed of SoundTime (s) Speed (m/s)

0.9 169

0.94 161.8

0.3 507

0.45 338

0.6 253.5

0.5 304.2

0.37 411.1

Speed = distance/time AVERAGE SPEED = 306.3m/s

Write up:

Aim

Theory

Apparatus

Method

Results

Analysis Graph

Conclusion

Evaluation



Speed of Sound.xls



Practical Write UpAim - State what you were trying to prove or discover.

Theory - Explain (briefly) the underlying theory - include a hypothesis (what you think about the practical) and a prediction ( what you think will happen) if appropriate.

Apparatus - list everything used - in detail.

Method - Describe what was done in steps and in detail.

Results - List all results in a neat table including units.

Analysis - Analyze your results to achieve your aim - calculations, graphs etc. All working must be shown clearly.

Conclusion - What do your results show - have you achieved your aim.

Evaluation - Evaluate the results - are they consistent? If not why do you think they vary? Do any of your results seem anomalous (out) how does this affect the overall result if you ignore it? Are there improvements you could suggest to make your results better?





Practical Write UpConclusion - Speed calculated 552 m/s.

Evaluation - Our results are not accurate. Actual value 330 m/s. Our percentage error = (error/actual value)x100 = 67% Our speed is too high.

• Our distance was calculated twice with and an accuracy of 30cm.

• Percentage error (distance) of

% error (s) = (error/actual value)x100 (0.30/131)x100 = 2.29 %

• Percentage error (time)

% error (t) = (error/actual value)x100 (0.05/0.25)x100 = 20 %

• Obviously our time measurements are causing the biggest error. This is largely due to the fact that the time measurements are very small (0.3) of a second and so any error is a large percentage of the value being measured.

• There is also reaction time delay when starting and stopping the stopwatch - that occurs TWICE in each reading!

• As speed is calculated by dividing distance by time and our speed value is too high - our time values must have been too small!

• To get more accurate results we need to - - -





Transmission of Sound• Sound is transmitted as a Longitudinal Wave

• It is a series of Compressions & Rarefactions

• Solids - more dense - particles closer, Liquids - less dense -greater spaces, gases - least dense - large spaces.

• Sound transmitted better in solids, then liquids then gases.

• Factors affecting - wind, air pressure

Air compressed - high pressure

Air rarified - low pressure

Vibrating

Object

• Sound cannot travel through a vacuum.

• If there is no air to carry the sound wave there can be no sound.

Direction of movement





Resonance• All objects possess their own natural frequency at

which they can vibrate.

• Any vibration at that natural frequency, in the same medium, will cause the object to vibrate.

1. Singer generates

vibrations at the natural

frequency of the wine

glass.

2. Sound waves transmit the energy to the glass.

3. The glass begins

to vibrate - and can

shatter if the volume

and frequency are

right.





Characteristics of Sound• Reflection - sound is

reflected off surfaces in the

same way as other waves.

• Reflected sound is called an

echo.

• Echo location - bats, sonar

• Multiple reflections -

acoustics - reflected

sounds must be absorbed. (curtains or reflecting surfaces.)

S =

depth

Distance = speed x time

Depth (s) = distance/2

Distance =

2xs





Refraction of Sound

• Sound travels faster

through warm air.

• The sound waves

refract at night to

follow the earths

surface.

• Sounds can be heard

better at night.

ground

At Night

Warm air

Cold air

ground

Daytime

Cold air

Warm air

Sound is

refracted away

from the earth.





Sound Volume

Soft

Sound

The volume of a sound wave is determined by the amplitude of the vibration.

Small Disturbance – small differences in pressure

Large Disturbance – very high and very low pressure

Loud

sound

Air

pre

ssure

Air

pre

ssure

Low

amplitude -

air pressure

does not

change much.

High

amplitude -

air pressure

changes

drastically.





Sound Pitch

High

Pitch

Sound

The pitch of a sound wave is determined by the frequency of the vibration.

Low

Pitch

sound

Air

pre

ssure

Air

pre

ssure

High pitch is caused by high frequency.

Low pitch is caused by Low frequency.





What is a Decibel…Practically Speaking• The level of sound the ear first begins to recognize (threshold of hearing) has

been determined to be .0002 microbars.

• The small amount of pressure causes the ear membrane to move less than the diameter of a single atom!

• A sound loud enough to begin to create a sensation of pain (threshold of pain) represents a sound pressure that is 3,162,300 times more than the threshold of hearing! These are the limits in our range of hearing.

• if we used the microbar to describe sounds, the numbers would be very cumbersome. In addition, it would be difficult to comprehend any comparisons.

• Early acousticians came up with a simple method of comparing two sounds. A sound that was perceived to be twice as loud as another was said to be one Bel greater in sound level.

• The Bel was used as a unit of comparison. It is not a unit of measure. Its namesake, Alexander Grahm Bell, was a pioneer in the science of audiology (the study of human hearing).

• t soon became apparent this unit of comparison was not very useful in describing the difference between similar sounds. A small unit of comparison, the decibel, was established. One decibel (1 dB) is one-tenth of a Bel. Since a decibel is one-tenth of a Bel, then 10 decibels (10 dB) would equal one Bel. In other words, a sound that is twice as loud as another sound could be described as being 10 decibels (10 dB) louder. By definition, one decibel (1 dB) represents the smallest change in volume a human ear can perceive. The average ear, however, can only detect a 3 dB change





How Loud?





Ear





Ultrasound





Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides (with all the gaps filled in) as well as

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www.warnescience.net

(paste into your browser if link above does not work)

Have a look and enjoy!

Keith Warne






sound

Education

speed of sound speed

transmission of sound

speed of sound apparatus

sound waves

reflected sound

conclusion speed

speed value

percentage error time