More information on acoustics

How sound moves

Sound waves radiate from a sound source. A source of this type might be a human voice, an instrument, a loudspeaker or a machine.

Sound arises when air or solid bodies (or even liquids) are made to vibrate by being “pushed”. This vibration in turn transfers itself to neighbouring “bodies”, which might be air molecules or the elementary particles of solids. In this way the sound propagates itself as a sound wave or a zone of increased air pressure.

If one of these sound waves reaches the human ear, it makes the ear vibrate, which sends the information to the brain as nerve impulses with the result that we experience the sensation of hearing.

Types of sound

We distinguish various types of sound depending on how it is produced and how it is propagated – if the frequency of these sound waves is within the range that can be detected by the human ear we speak of audible sound.

The audible range goes from low frequencies of approx. 16 Hertz (vibrations per second), i.e. deep tones, to high frequencies (high pitches) of up to 20,000 Hertz – although the perceptible range can vary enormously from one individual to another, and depends above all on the age of the listener.

Perception

In addition, sound is described with various terms depending on the form of perception. Only a "pure" sine-wave vibration is called a “tone”, and in general what we hear is a mixture of tones, which we call “sound”.

If such sounds are subjectively felt to be disturbing or are so loud that they can damage our hearing, we call them “noise”.

Propagation of sound waves

When sound spreads in the open air, the energy of the vibrations gradually declines with the increasing distance travelled.

On the other hand, some of the sound emanating from a source in a closed room strikes a receiver as direct sound (i.e. strikes a microphone or a human ear), and some of it will reach the boundaries of the room (walls, floor and ceiling) where a proportion of it is reflected. The reflected part of the sound will, in turn, strike obstacles and be split once again, and so on. This results in a sound field whose properties determine the sound of the room. Reflected sound waves are perceived some time later than the “original wave” and are a major factor in our perception of the room.

Some of the incident sound will penetrate into the boundaries of the room, where some of it will be “swallowed up”. Part of the sound will cause this boundary to vibrate and it will continue to be propagated within this body. The sound re-emerges somewhat weaker on the other side, where it is emitted again as noise.

Influencing sound

As the sound travels from its source to the receiver, and to a neighbouring room, there are various ways in which it can be influenced, to produce a variety of different effects:
Sound insulation is intended to reduce the passage of sound to neighbouring rooms or beyond and to avoid disturbances. The different parts of the sound take different paths: when the airborne sound meets the walls, ceiling and floor, it causes them to vibrate; the structure-borne sound goes directly into solid building components, spreads through them and is re-emitted at a different point.

A combination of these two parts is termed impact sound and arises when some one walks over a ceiling, for example.

The sound vibrations are lessened by being turned into different forms of energy. The sound energy is initially converted into vibrational and distortion energy, and finally into heat.

A certain room acoustic, that is, a desired property or effect of the sound field in the room, can be created by absorbing the sound. Various different absorbing claddings on the room’s surfaces are used to attenuate certain parts of the sound field.

Here, too, the sound energy is converted. It is turned into frictional and distortion energy, and again ultimately into heat.

Measuring acoustic values

The acoustic quality of a sound-absorbing measure is described in terms of its properties in attenuating airborne sound, reducing impact sound, and the internal acoustics.

These acoustic values are determined in sound measurements, and evaluated and confirmed by independent experts in accordance with the applicable standards and measuring techniques.