Sound

Sound Waves

A sound wave is an example of a longitudinal wave. This means that the medium oscillates parallel to the movement of the Energy.

In the case of sound, the medium is the material that the sound is travelling through, which in most examples will be either in air or water.

What is a Sound Wave ?

To create a sound wave, something must cause the medium around it to vibrate. This vibration causes the medium to be compressed (squeezed together) or rarefied (spread out), as can be seen in the diagram above.


Visualising a Sound Wave

As a sound wave is a longitudinal wave, it can be hard to describe based upon the terms used in the last section.

However, if the sound wave is recorded by an oscilloscope, the signal generated can be represented by a transverse signal.

The image below shows a sound wave as represented by an oscilloscope:-

Amplitude and Frequency, Pitch and Volume

In music, the concepts of pitch and volume are clear descriptions of sound.

Both of these can be described using the correct Physics terms:-

1. Volume - How loud or quiet a sound is . In Physics, Volume is defined as Amplitude.

2. Pitch- How high or low a Sound is . In Physics, Pitch is defined as Frequency.

Volume and the Decibel Scale

In Physics, the loudness of a sound is measured using the unit - Decibel (dB). The diagram below shows the Decibel scale, with some example sounds:-

Note - The Decibel scale is a logarithmic scale, not a linear scale. This means that an increase of 10 Decibels gives double the Volume.

Sound and the Range of Human Hearing

As explained earlier, in Physics the Frequency of a sound wave (or any wave) is measured in Hertz (Hz). Sound can be generated in a wide range of Frequencies, however, only certain Frequencies can be heard by humans.

Sound Frequencies that we can hear are known as the range of human hearing.


" The range of human hearing is 20 Hz - 20,000 Hz "


Sound below 20 Hz is known as Infrasound. Sound above 20,000 Hz is known as Ultrasound.

Infrasound

Sounds with a Frequency less than 20 Hz

Elephants use Infrasound to communicate over several miles, detecting the waves with their feet.

Infrasound has been shown to given people the sensation of unease or even fear.

Whales use Infrasound to communicate with each other over hundreds of miles.

Ultrasound

Sounds with a Frequency greater than 20,000 Hz

Bats, Insects and Cetaceans (Whales and Dolphins) use Ultrasound to measure distances or detect prey.

Ultrasound can be used to image safely within the Human body, for example observing an unborn baby.

As you age, your ability to hear the high Frequency sounds is reduced. Young people can hear high Frequency notes that older people cannot.

The Speed of Sound

The speed of sound can be calculated in two different ways within the school. Both are based on the speed calculation seen previously:-

This issue with measuring the speed of sound experimentally is that the speed of sound is very fast. In order to measure the speed of sound, either large distances or computer timing is required.


Measuring the Speed of Sound (Manually)

By observing a loud distant event, it is possible to measure the speed of sound. This is due to the difference between the speed of light and the speed of sound. The speed of light is so large that for basic experiments in National 5, it can be assumed that light travels instantaneously between places. By observing a distant loud event, the light would reach you before the sound would, allowing the travel time of the sound to be measured.

The image above shows two examples of loud events that shows the difference between the speed of sound and the speed of light.

Fireworks - The light of the firework reaches your eye, and a fraction of a section later, you hear the bang of the explosion.

Lightning - The flash of a lightning strike reaches your eye, and several seconds later, you hear the rumble of thunder.

To measure the speed of sound, a person stands several hundred meters away with a set of cymbals. When the distant observer sees the cymbals hit together they start timing, when they hear the sound, they stop the timing. The distance between the two people is then measured, and the speed of sound in air can be calculated using the formula above.

Problems with manual measurement - Reaction time

The above method for measuring the speed of sound has a major flaw, the Time measurement is very short. This means that your reaction Time has a large effect on your measurements.

Reaction Time is a measurement of "how long it takes between an event and you reacting to it".

Your reaction Time depends on a lot of factors, including:-

1. How much sleep you have had.

2. Your age.

3. Distractions.

4. Alcohol/drug use.

5. Whether you are reacting to a sound or light.

Measuring the Speed of Sound (Computer Aided)

By removing the human element from the timing by replacing it with a computer, this reaction Time can be eliminated. This will give a much more accurate value for the measured speed of sound. The equipment below can be used to measure the speed of sound in the lab:-

To measure the speed of sound, a loud sound is made to one side of the kit. When the sound wave reaches the first microphone, the computer starts a timer, stopping when the sound wave reaches the second microphone. The distance between the two microphones is then measured, and the speed of sound in air can be calculated using the formula above.

Measuring Distance using Sound

Lots of animals can use sound to understand the World around them. Certain animals can use reflected sound to build up an image of where they are, this is called Echo-Location. It is also the principle behind how RADAR and SONAR work.

Echo-Location works by sending out a pulse of sound and measuring how long it takes for the echo to return.

If the speed of sound is known, this time period can then be used to calculate distance, using the formula below:-

The Distance found using this method is not the distance to the object, it is the total distance travelled by the sound (to the object and back again).

To find the distance to the object, the distance calculated must be halved.


Example 1

In the diagram below, a Bat at point A sends out ultrasonic sound pulses towards an Insect, then detects the reflected sound waves 0.15 Seconds later.

If the speed of sound in air is 340 m s−1 , calculate how far away for the bat the Insect is (distance d).


S = 340 m s−1

d = ?

t = 0.15 s


s = d / t

d = s x t


d = 340 x 0.15

d = 51 m (total distance sound travelled)


Distance to Insect = 51 / 2

Distance to Insect = 25.5 m