The basic definition of wave motion is a transfer of energy, without transferring matter. The waves that we will be dealing with are sound waves, which are basically Mechanical Longitudinal Waves. The Mechanical part of the title means that the waves require a medium to travel through, such as the air when you are speaking. The longitudinal part describes the motion of the wave. It means that the motion of particles in the medium is parallel to the motion of the motion of the wave.
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The diagram at the right is of a simple wave. The red line represents the wave, and the blue line represents the equilibrium point in the medium. There are a number of measures that pertain to waves. One of the first things you will notice about waves is that they are repetitive in their motion. The shortest distance between two point where the wave motion is repeated is known as the wavelength, represented by the Greek letter λ (lambda). Often the points used are the points with the highest displacement from the equilibrium. In transverse waves, the points that are above the equilibrium point are the crests, and those below that point are the troughs. These are pointed out with the green lines at the right. In a longitudinal wave, such as sound, the points used are compressions: the points of the highest compression of the particles in the medium, and rarefactions: the points of highest "spreading" of the particles in the medium.
The shortest time interval in which the motion of the wave is repeated is the period (T). The frequency (f), is the number of complete vibrations the wave will complete in one second. The frequency is measured in units called Hertz (Hz). One hertz is equal to one vibration per second, and
therefore frequency and period are related by the equation:
f = (1/T)
By knowing the period or frequency of a wave, and the wavelength of a wave, you can find out the speed the wave is traveling by using the equation:
v = λ * f
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Unlike matter, two or more waves can be in the same place at the same time. Since both waves affect the medium independently of one another, the effects of the waves can be analyzed using the principal of superposition. "The principle of superposition states that the displacement of a medium caused by two or more waves is the algebraic sum of the displacements caused by the individual waves. The result of the superposition of two or more waves is called interference." (Source: Physics: Principles and Problems). When the crest of one wave meets the crest of another wave, or the trough of one wave meets the trough of another wave, the algebraic sum of the displacements leads to a displacement that is farther from the equilibrium point than that of the waves alone. This is known as constructive interference.
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One the other hand, if the crest of one wave meets the trough of the other, the displacement from equilibrium will be lees, or possibly zero, this is known as Destructive Interference. It should be noted that after the two waves have completely passed through each
other, they will be unchanged from their original form.
More accurately, the question should read how intense is the sound. While loudness changes from person to person on how well their sense of hearing is, the intensity of a sound is a definite measurement.
The intensity of a sound is given by the equation:
The typical units for measuring the intensity of sound are watts / m2. "Humans are equipped with very sensitive ears capable of detecting sound waves of extremely low intensity. The faintest sound which the typical human ear can detect has an intensity of 1*10-12 W/m2. This intensity corresponds to a pressure wave in which a compression of the particles of the medium increases the air pressure in that compressional region by a mere 0.3 billionths of an atmosphere. A sound with an intensity of 1*10-12 W/m2 corresponds to a sound which will displace particles of air by a mere one-billionth of a centimeter. The human ear can detect such a sound. This faintest sound which the human ear can detect is known as the threshold of hearing. The most intense sound which the ear can safely detect without suffering any physical damage is more than one billion times more intense than the threshold of hearing". (Source: Intensity and the Decibel Scale)
The scale that is used by physicists to measure intensity is based on multiples of 10, and is known as the Decibel Scale. The threshold of hearing the sound with the least intensity that can still be heard by a human is assigned a value of zero decibels. This correlates to an intensity of 1*10-12 W/m2.If a sound is ten times this intense, or 1*10-11 W/m2, it is given the intensity of 10 decibels (dB). If a sound is ten times greater than this, it is 20 dB, and so on.
Harmonics are also known as overtones. Harmonics are the frequencies present in the vibrating wave in a addition to the fundamental tone. Harmonics are usually talked about when talking about string instruments, and well will talk more about them on the stringed instrument page.