Most of us understand the basic premise of sound and vibration, even if we do not know what it is. I am certain that you will know sound is a wave and that by banging two objects together we can create sound waves. To go further than this will take you into the study of acoustics, the science of sound.
What is sound I hear you ask. In layman’s terms sound is the effect noticed by humans and animals when waves travel through the air, into the ear and converted by our brains. Our brains interpret the difference between the normal air pressure and the change in air pressure caused by the wave, which we hear as sound.
These sound waves that are moving through the air are not difficult to visualise. When you throw a stone into a calm pond you can notice ripples moving along the surface of the water. These surface waves are not too dissimilar to the waves moving through the air in that they radiate outwards in all directions from a disturbance in the air (rather than the water).
This wave phenomenon occurs because air particles are pushed out the way by energy dissapating from a sound source into the air. These air particles then knock into one another much like pushing over a row of dominoes.
Not all sound sources project a simple pattern of waves like the ripples on a pond as some might have complex shapes and sound associated with them. For simplicity we will use the simple “point source” to explain the movement of sound in this article, which can be seen below.
Although the ripples on water and the image above provide a neat example of the propagation of sound in two dimensions, we must understand that the air and space around us are three dimensional. This can be a little trickier to visualise but the image below gives an example of sound waves propagating in 3D.
Frequency and wavelength
Whilst these two words might sound scary and bring up bad memories from secondary school, they are quite easy to understand. Put simply the frequency of a wave correlates to the pitch of a sound, where the frequency increases, the sound usually becomes higher pitched. Frequency is measured using (Hz – e.g. 1000hz = 1000 waves per second).
In more technical terms the frequency of a sound is determined by how many times a second a wave repeats itself. To measure where one wave begins and another wave ends, we use something called wavelength. The wavelength is measured using distance units such as meters(e.g. 1000hz sound wave has a wavelength of 0.343 meters). This is measured from a set point on a repeating wave and is the length between the start and the next successive set point. Usually the part of the wave where the particles are most dense (compression) or the part where the particles are least compact (rarefaction) is used.
Sound Pressure Level
Changes in air pressure are measured and described using a term you may be familiar with; Pascals (Pa). The quietest sound that a human can hear is very minute at 0.00002Pa depending on the frequency of the sound. On the other end of the spectrum a noise source that is above 20Pa is defined as the threshold of pain. Sound that is louder than this will be painful to the ears and likely cause hearing issues if not stopped soon afterwards.
You may have noticed that the difference between these two pressures is quite large. This is obviously quite confusing and long winded to write down, especially when maths is being used. As such a compression of these numbers is often used by acousticians and everyday people alike – decibels (measured in dB).
Using a logarithmic expression such as decibels, we can have it so that every time a zero is added to the sound pressure we add ten decibels. This means that 0.00002Pa is 1dB and 20Pa is now 120dB.
We hope that this short, layman’s introduction to sound and acoustics has been informative and useful.
Some examples of sound pressure levels found in everyday life are shown in the infographic below:
This is the first post in a series that we are calling “Practical Acoustics”. The series is there to teach acoustics in a way that is easy to understand and not to overwhelm people with an interest in the subject with mathematics and strange terms.
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