About sound and its production, propagation and variations

The basic physical properties of sound

Sound is usually said to be pressure waves that propagate through a medium which can be air, another gas, liquid or solid medium. What happens when sound is produced and carry on is as follows:

Something is vibrating back and forth and presses repeatedly at the medium.

For each time a part of the medium is pressed together, so that the pressure in that part increases.

The compressed part of the medium will then press at the nearby parts so that the those are pressed together, while the first part widens again and loses pressure.

In this way parts after parts of the medium are pressed after each other in a specific direction at first, but gradually the pattern spreads in all directions.

Since the process repeats, you will get patterns of increasing and decreasing pressure in the medium that move along.

Since the variation in pressure has the same direction as the direction of propagation, sound waves are longitudinal waves.

The amount of pressure one can measure at any point at a specific time is called the amplitude. The amplotude is measured like pressure in other circumstances, for example Newton per square meter.

Each repetition from neutral but rising pressure to maximum anplitude, down again to neutral presssure, firther down to minimum amplitude and again back to neutral pressure, is called a period.

How much of a period a point in space at a given point in time has gone through, is called the face of a period. The face is often measured in a number from 0 to 360, where 0 is the start and 360 the end of the period. The face number is thus measured like a agle in degrees. But it can also be measured in radians.

The length of a period is called the wavelength. How many periods that are passing through a point in space in each time unit is called the frequency, often measured in periods per second, or hertz.

How long it takes a period to pass at each unit of time is called the sound velocity or sound speed, often measured in meter per second.

You have the following simple mathematical relation between these measures:

frwquency=sound velocity/wave length


strength= k x maximun amplitude x frequency

where k is a constant that is related to the medium and the shape of the amplitude variation in each period.

A sound can propagate for very long distances, but since it spreads in all direction, the amplitude will decrease the longer you get from the source. But the movement within a sound wave will gradually push at the molecules and atoms in the medium in a disordered fashion too. This will heat up the medium, depleating the sound for energy, and eventually the sound is converted to heat.

Variants of sound

A maximally fit human ear can hear sounds between 16 hertz, which is 16 periods in a second to 24000 hertz, but most humans are not able to hear sounds of so large frequency span. Sound with lower frequency like these are called ifrasound. Even though infrasound cannot be heared, it can be felt like virbrations and even seen if the amplitude is strong enough.

Sounds higher that the audible are called ultrasound. Ultrasound can sometimes be felt like heat, and may produce other sensations in the body. Since ultrasound of high frequency and high amplitudes can carry a lot of energy, it can produce strong physical effects, for example heating liquids to the boiling point or set fire to things.

If the frequency through a period is totally smooth, the sound waves are called sine wave sound. The amplitude in a sine wave is in each part of each period is proportional to the sine of the face of the period where the face is measured like a angle between 0 and 360 degrees.

The frequency of the sound is often called the tune.

Most sound waves are not smooth. They are usually a blending of sound waves with different requencies. The lowest frequency is usually regarded as the main tune or simply the tune of the sound. Usually the higher frequencies are usually called overtunes.

The overtunes are often a whole number multiple of the main tune, which means that each period of the main tune gets internal variations, each consissting of a whole period. Overtunes with this pattern is called harmonic overtunes.

The pattern with the frequency of the main tune, the frequencies of the overtunes, and the relative amplitude strength of the overtunes in comparison to the main tune, is what determines how different sounds are heared differently.

Most music instruments give sound with a number of harmonic overtunes, each with a certain strength.