What is sound?
Sound is created by a vibrating object. The vibrating object displaces particles in a medium (eg air/water). These particles in turn displace other particles and this continues through the medium. This propagation of particle displacement is called a sound wave.
A vibrating object will vibrate a number of times per second. A new set of propagating particles will occur each time the object vibrates and so each vibrating object will create a number of sound waves per second. This is referred to as the wave frequency and is measured in Hertz (Hz). The number of sound waves per second (frequency) determines the pitch of the sound. High frequency vibrating objects create high pitched sounds and low frequency vibrating objects create low frequency sounds. The human ear can detect sound waves between 20 and 20,000 Hz.
The intensity or loudness of a sound depends on how much each displaced particle is moved by the vibrating object. This is referred to as the amplitude. Particles that are displaced a relatively large amount (large amplitude) create a loud sound and those moving only a small distance a quieter sound. As particles that are displaced further have more energy, intensity of sound is measured as Watts/meter2.
Human ears can detect sounds over a huge range of intensity and to make figures easier to handle intensity is more commonly represented as a logarithmic scale. This is the decibel scale. The faintest sound the human ear can detect is called the threshold of hearing. This is assigned a sound level of 0 decibels. A sound 10x more intense is 10 dB. A sound 10x more intense than this is 20dB, 10x more intense (ie 1000x) is 30dB.
So, an object vibrating with a larger amplitude creates a more intense sound which means a louder sound is detected by the ear. Perception of loudness by the ear/brain is not quite in the same order as intensity. So, a 20dB sound is 100x more intense than the threshold of hearing but this does not mean that we hear it as 100x louder. Each 10dB increment is perceived by the ear as about twice as loud. So a 20dB sound is perceived as 4x as loud as the same sound at the threshold of hearing. Below are some examples.
Rustle of Leaves: 10Watts/meter; 10dB;Relative loudness = 2
Conversation: 10,000Watts/meter; 40dB; Relative loudness = 16
Vacuum Cleaner: 100,000,000W/m; 80dB; Relative loudness = 256