Sound waves
Where can sound travel?
Sound waves have to travel through a medium. They can't travel through a vacuum (a place where there is no solid, liquid or gas, such as in the "space" between planets and stars.)
Sound waves can travel through the air (which is a gas), such as when a person speaks to another person in a room.
But sound waves can also travel through solid and liquid mediums. Examples of these include: sound travels through a wooden door when a person outside knocks on the door and you hear it inside; sound travels through water (you can hear sounds when you swim under water).
In fact, sound waves travel faster when the medium is denser; it travels faster in liquid compared to through a gas (eg the air); it travels even faster through a solid!
Visualising a sound wave
As we said above, Sound travelling through the air consists of a series of compressions (where air particles are squashed together) and rarefactions (where air particles are spaced further apart).
Unfortunately it's not easy to visualize such a sound wave.
To make it easier we could ask a person to speak into a microphone connected to an oscilloscope.
The microphone converts the sound wave, produced by the person speaking, into an electrical signal which can be viewed on the oscilloscope.
When we do so, this is what we see on the oscilloscope screen:
The compressions (areas of high pressure) are the peaks; the rarefactions (areas of lower than normal pressure) are the troughs.
Using this pidture of a sound wave we can easily describe the wave.
Describing a sound wave
We use the words : amplitude, frequency and wavelength to desribe a wave.
1. Amplitude
The Amplitude of the wave is the maximum displacement of the wave from its rest/undisturbed position. So, the amplitude of our wave is indicated by the following :
Notice the letter a, used to represent Amplitude.
The amplitude can equally be indicated on the negative side of the wave, as follows :
Since amplitude is a distance, it is measured in metres, m.
In our everyday world, we don't often refer to the "amplitude" of a sound wave; instead we refer to its volume and it can be measured with a sound meter which gives the volume in a unit called a decibel, dB.
The larger the amplitude of a sound wave, the larger its volume; its a louder sound.
The smaller the amplitude of a sound wave, the smaller its volume; its a quieter sound.
So amplitude and volume have a very similar meaning.
2. Frequency
The frequency of a wave is the number of complete cycles of a wave in one second..
To illustrate Frequency we need to add a few numbers to our x-axis as shown below :
So, we can see that there are 2 complete cycles of the wave occuring in 1.0s. Hence, for this wave the Frequency, f is 2.
We could say, the Frequency, f = 2 cycles per second
but the preffered unit is the hertz, Hz, so we say:
The Frequency, f = 2 Hz.
In our everyday world, we don't often refer to the "frequency" of a sound wave; instead we refer to its pitch.
The larger the frequency of a sound wave, the higher its pitch.
The smaller the frequency of a sound wave, the lower its pitch.
So frequency and pitch have a very similar meaning.
The wave above with a frequency of only 2 Hz is an extremely low frequency / pitch, so low that a human would not hear it as a sound wave.
The lowest frequency that humans can detect as sound is 20 Hz (that would be 20 complete wave cyles in one second).
The highest frequeny that humans can detect as sound is 20,000 Hz (that would be 20,000 complete wave cycles in one second, a very high frequency!
The range of frequencies that a person can hear is known as the audible range for humans.
Other animals have different audible ranges, eg dogs can hear frequencies much higher than our 20 KHz limit.
3. Wavelength
The third word that is used to help to describe a sound wave (or any wave, in fact), is its wavelength.
The wavelength of a wave is the distance travelled by one complete wave cycle. If we visualise a sound wave travelling from a persons mouth, the wavelength would be the distance travelled by one wave.
To show this on a graph we would need to change the x-axis from time to distance travelled, as shown below.
The following diagram shows how we measure the wavelength of a wave:
One full wave cycle is from the first green dot to the second, so the distance between them is the wavelength, and the symbol for it is the greek letter λ.
Another way to show wavelength is the distance from the peak of one wave to the peak of the next, or from the trough of one wave to the trough of the next. See the following diagram:
Since wavelength is a distance it is measured in metres, m just like amplitue.
Wavelength and frequency are inversely related, meaning that as one increses the other decreases. So....
A long wavelength is equivalent to a low frequency / pitch.
A short wavelength is equivalent to a high frequency / pitch.
For a human the lowest frequency that we can hear is equivalent to a long wavelength of about 17m.
For a human the highest frequency that we can hear is equivalent to a very short wavelength of about 17mm.
The speed of sound
Sound travels very fast!
For example, through the air it travels about 330 metres in one second. So that's a speed of 330 m/s.
For comparison Usain Bolt, one of the world's fastest humans, takes 31 s to run 300 m.
But the speed of sound through the air is "slow" compared to its speed through a liquid or a gas.
Through a liquid such as water, sound travels at about 1500 m/s. This is why fish and whales rely more on their sound sensing ability than on their eyesight.
Through a solid such as concrete, sound travels at about 3700 m/s. That is 3.7 km in one second. That is fast!
However, if we compare the speed of sound to the speed of light, we would have to conclude that sound is not so fast after all.
Light travels at 300,000,000 m/s. That is 300 million metres in one second! So a sound wave through the air is a million times slower than light.
Sound waves can be reflected, transmitted or absorbed by different materials
Reflection of sound
The best example of reflection of a sound wave is an echo.
This occurs when a sound wave travelling through the air hits a relatively hard object; in the case of an echo the hard object is usually a distant mountain at the end of a valley.
So reflection of sound occurs when a sound wave strikes a relatively hard or dense object. The harder the better for a good echo.
A good example of making use of an echo is when a ship sends out a sound wave to work out the depth of the water. The diagram below shows a sound wave reflecting off the sea bed; knowing the time taken for the reflection to occur and the speed of sound in water, allows the captain to calculate the depth of the water.
This technique is known as echo sounding.
Transmission of sound
The fact that sound can be transmitted, meaning to pass through, is obvious when we consider that a person speaking is heard many metres away. The sound is being transmitted through the air.
Sound can also be transmitted through a liquid such as water and through a solid such as concrete; we discussed this above.
The only thing that sound will not be transmitted through is a vacuum, also mentioned above. A good example of the use of this knowledge is modern double glazed windows; they consist of two layers of glass with a vacuum in between; this considerably reduces sound passing through the window.
Absorbtion of sound
Although sound can be transmitted through a solid, a liquid or a gas, it doesn't get transmitted perfectly. For example, when a person speaks we do hear them a few metres away, but if we walk further and further away we will notice that the amplitude (or volume) of the sound will decrease. The air is absorbing the sound wave more and more as the distance from the speaker increases or as the "thickness" of the air gap increases.
However, air is not really a good absorber of sound. The best absorbers of sound are soft materials.
An air gap filled with wool or polystyrene will absorb sound quite well; they also won't reflect the sound back to its source.
When a sound wave meets an absorber, the energy of the sound wave is transferred to the absorber; a good absorber will take all of the energy from the sound wave such that no sound will pass through or be transmitted through the absorber.
When energy is transferred from the sound wave to the absorber the absorber actually heats up! This, however, would be hard to detect.
Now it is time for you to have a go at a few questions:
