Good acoustics

An important part of a good and healthy indoor climate is efficient sound absorption. It is an important factor in schools, kindergartens, sports centres and work places where the noise level can be considerable.

A high level of noise over a short or long period of time can influence the well-being of those who use the facilities and lead to health conditions. Worst case, it could lead to irreversible hearing damage.

Troldtekt is known – and renowned – for its excellent sound absorption properties, which help to reduce noise and create good acoustics in every room.

Below you find some of the most important terms from the field of general acoustics.

Learn about the most important concepts within room acoustics.

We normally differ between sound and noise.

Sound is compression waves that travel through the air. We normally distinguish between sound and noise. We call pleasant, ordered waves sound but categorise chaotic obtrusive waves as noise.

Scientifically speaking, sound is defined in the following way:

“Elastic molecular fluctuations in the air or other media that create a chain reaction by putting the nearest particles in motion. The sound will then spread at a certain propagation velocity which is called the speed of sound in the medium in question.”


Frequency expresses how many times a sound wave oscillates per second. Frequency is measured in hertz (Hz). 100 Hz corresponds to 100 oscillations per second. Bass sounds have low frequency while treble sounds have high frequency.

Frequency is measured in hertz (Hz): 1 Hz = 1 vibration per second.

The low-frequency bass sounds have long, soft sound waves with few vibrations per second, while a treble sound has many vibrations per second.

Low-frequency bass sound High-frequency treble sound

For instance, the wavelength in air for a sound wave with a frequency of 20 Hz is 17 m, but the same wavelength for a high-frequency treble sound wave with a frequency of 20 kHz is only 1.7 cm.

This means that the sound wave is inversely proportional to the frequency.

Sound pressure

Sound intensity is described using decibel (dB) units. The higher the sound pressure, the higher the decibel value.

If dB measurements are to be comparable, they must be made at the same distance from the sound source. 

Reverberation time

The reverberation time is the time, measured in seconds, that passes from when a sound source is interrupted until the sound dies out. It is the character and acoustic properties of the surrounding surfaces that determine how sound waves behave and propagate.

When a sound wave hits an obstruction, it is reverberated. If the surface is hard and even, the sound wave will be reverberated directly, while a soft and uneven surface will absorb part of the sound waves and delay or muffle the reverberation.

Normally, a long reverberation time is undesirable as the echo effect of the first sound waves mixes with the subsequent sound waves. The result is a noisy and resonant acoustic image in which it is difficult to distinguish individual sounds.

With a short reverberation time, the acoustic image becomes more precise and sharp, and the speech intelligibility is increased significantly.

Sound absorption

Short reverberation times can be achieved by using materials which are good at absorbing sound. This property can be described using an absorption coefficient – specified using the Greek letter a (alpha).

Hard and rigid materials with a smooth surface often have a low sound absorption value, while materials with a soft, uneven or porous surface typically have a high sound absorption value.

The higher the proportion of the room’s total surface area that absorbs the sound, the less reverberation there will be in the room. Therefore, it is important to select a material with a high sound absorption value for large surfaces.

In this way, it is possible to regulate and reduce the reverberation time to achieve a good acoustic environment with a clear acoustic image.

The ceiling will usually be the easiest surface to acoustically regulate, since it is both large and accessible.

Speech intelligibility

The speech intelligibility in a room depends on the reverberation time and the background noise.

If a room has a long reverberation time, spoken words will not die out before the next words reach the listener. This results in poor speech intelligibility, and it will be difficult for the listener to understand what is said.

However, if the sound is absorbed, resulting in a short reverberation time, this provides a good acoustic environment and a high level of speech intelligibility.


Background noise impacts on well-being

A report from the Danish National Research Centre for the Working Environment found that teachers who teach at schools with poor acoustics are less satisfied with their job than teachers at schools with good acoustics. The study found that the most important factor for teachers' dissatisfaction was long reverberation time, which is known to result in a noisy, chaotic acoustic environment.

107 teachers at ten schools in Copenhagen participated in the study, and the desire to change job was six times greater among teachers who taught in rooms with long reverberation times. If the sound level was high for at least one quarter of the working time, teachers were also less motivated and experienced greater fatigue.

Poor acoustics also influenced teachers' perceptions of class unity. The study found that teachers working in classrooms with long reverberation times perceived the social climate in the class as more competitive and aggressive and less relaxed and comfortable.

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