SMALL ACOUSTIC-ABC

Optimal room acoustics are the result of a complex interaction of various factors that are influenced by both the physical characteristics of the room and its intended use. To improve the acoustics of a room, it is crucial not only to understand the physical principles of sound propagation, but also to consider the individual perception of sound and the effects of different acoustic phenomena. A basic understanding of the most important acoustic terms, as presented in the "Little Acoustics ABC", is essential in order to select the right materials to optimize room acoustics.

Building acoustics vs. room acoustics

If we now look at the effects of sound on people in a closed room, we can see the following fundamental difference:

In the field of building acoustics, the question of how sound can be prevented from penetrating a closed room, i.e. sound insulation, is therefore one of the issues addressed. Room acoustics, on the other hand, is the study of the propagation of sound within closed rooms and attempts to research the means by which the propagation of sound inside the room can be optimally influenced, often by means of sound absorption and targeted reflection or diffusion.

Human hearing

Our human ear perceives fluctuations in air pressure, which are referred to as sound waves and are triggered by a sound event. The pitch of a sound event is determined by the frequency of the sound ƒ, i.e. the number of oscillations per second, described by the SI unit Hertz [Hz]. The lower the frequency, the greater the respective wavelength of the sound wave, whereby the human ear perceives frequencies from approx. 20 Hz to 20,000 Hz. In acoustic planning, all parameters must always be considered frequency-dependent in order to ensure clean and meaningful planning.

Not all audible signals cover the entire frequency range of human hearing. Human speech, for example, extends from approx. 125 Hz to 8 kHz. This range is therefore particularly important for planning room acoustics. The frequency composition of a signal results in the characteristic timbre of the signal.

A sound event must also have a certain volume in order to be perceived by the ear at all. This is known as the hearing threshold, which is also frequency-dependent. The human ear is most sensitive to sounds in the range between 500 Hz and 4 kHz, while sounds in the bass range below 100 Hz are only perceived at all at high volumes.

Reverberation time

The most important measure when considering room acoustics is the reverberation time T. This parameter is used to describe the time it takes for a sound event to decay to one millionth of its original energy, i.e. to lose 60 dB in level.

If a sound event is produced in a room, the sound waves spread more or less spherically throughout the room, depending on the directional characteristic of the sound source. Only part of the sound energy reaches the listener directly. A large proportion of the sound energy reaches the listener with a delay via reflections from the room surfaces. The more hard surfaces there are in a room, the more frequently the sound wave is reflected in the room and the more reflections reach the listener, making the reverberation time longer. The reverberation time can therefore be reduced and regulated by introducing sound-absorbing surfaces.

Different reverberation times are sought for different types of use, depending on the room volume:

Sound absorption

To reduce reverberation in a room, sound-absorbing materials must be used. So-called porous absorbers are often used, i.e. materials with a certain porosity, such as textiles or open-pored foams. The incident sound energy in such materials is converted into heat by friction and diffraction effects within the material and thus "absorbed". Membrane absorbers (also known as plate transducers) or Helmholz absorbers, which absorb the incident sound energy according to a different physical principle, are used less frequently. The property of how well a material can absorb sound is specified with the dimensionless value α (sound absorption coefficient). The following applies:

• α = 1 corresponds to 100% absorption
• α = 0 corresponds to 0% reflection

The ability of different materials to absorb sound is strongly frequency-dependent, which is why sound absorption in the reverberation chamber is also measured and specified as a function of frequency. To make it easier to classify materials, an average value can be calculated from the frequency-dependent sound absorption coefficient, which is then assigned to a sound absorber class:

When measuring the sound absorption coefficient in the reverberation room, the type of installation is also decisive for the measured value. The measured absorption values of acoustic curtains can therefore not be given in general terms, but should always be given in connection with the respective test setup. We measure our curtains as standard with a 100 mm distance from the wall and both 0% and 100% pleat allowance.

Sound insulation

In the field of building acoustics, the sound reduction index of a building component is particularly important. This indicates the extent to which the incident sound is prevented from propagating. In comparison to absorption, this is not about reducing reflections (and therefore the reverberation time) within a closed room, but about reducing the volume between two parts of a room or separate rooms. The sound insulation of a building component depends heavily on its weight and the composition of the materials.

The sound reduction index R is given in dB, i.e. in the same unit as the sound pressure level. Doubling the sound pressure corresponds to a measured level increase of 6 dB. However, the perceived loudness of a signal depends on many other factors, such as the duration of exposure, the frequency or the spectral composition. A subjectively perceived doubling of volume corresponds to a level difference of approx. 10 dB.

Sound pressure level

The physical quantity used to characterize the strength of sound events is the sound pressure, measured in Pascal [Pa]. The human ear can perceive a very wide range of pressure fluctuations in the air. Between the hearing threshold (approx. 20 μPa) and the pain threshold (20 Pa), there is a factor of 1:1,000,000. For a clear representation, the sound pressure is given as a ratio to the hearing threshold, which also corresponds more closely to the human auditory impression. This results in the unit decibel [dB] for sound levels.

Flow resistance

As described in the chapter on sound absorption, the impinging sound in porous absorbers, which include most curtains, is achieved by frictional effects in the material. In order to enable such friction, the so-called flow resistance must be in a range between 500 and 1500 Pa s/m. If the value is significantly lower, the material can be described as sound permeable; if the value is significantly higher, a large proportion of the sound energy is either reflected or passes through the material without further absorption of the sound energy.

The flow resistance provides an indication of the acoustic properties of a material, regardless of the installation condition. However, the actual acoustic properties of a component must always be considered in connection with the installation on site, for which the sound absorption coefficient is measured.