Noise and sound insulation — theory and practice

The employer is responsible for getting an overview of all noise sources in the working environment. The rules are in Executive Order no. 63 of 6 February 2006 on protection against exposure to noise at work; the Working Environment Authority's guidance on noise (D.6.1) lapsed on 15 December 2025. Identify whether the noise is airborne sound (e.g. a loudspeaker), structure-borne sound (vibrations in solid materials such as footstep noise or a washing machine) or noise from outside (e.g. lorries with both rolling noise and engine noise).

Do not base the assessment solely on supplier data. Take measurements directly at the workplace under normal operating conditions. Remember that noise exposure is defined as the average of the noise levels a person is exposed to over a full 8-hour working day.

Check whether the measured level exceeds the permitted limit values. Outdoors, for example, 70 dB applies in commercial and industrial areas, 50 dB in residential areas on weekdays 7–18 and 35 dB in holiday home areas at certain times. Indoors in living rooms, the limit is 30 dB during the day and 25 dB at night. The recommended maximum noise exposure at a workplace is 80 dB A.

Identify whether the problem is primarily about bass (below 500 Hz), midrange (500–1500 Hz) or treble (above 1500 Hz). Low-frequency sounds are very difficult to dampen, whereas high tones above 1500 Hz are relatively easy to deal with.

The wrong choice of material can amplify noise — loose mineral wool, for example, can amplify noise by up to 60 dB at 120 Hz.

The first priority is always to reduce the noise where it arises. Check whether the machine has worn gears, a worn belt, loose valves or leaks. If necessary, fit sound-absorbing rubber mounts under the machine to stop vibrations spreading to the base and on into the building structure.

If reducing the noise at source is not sufficient, enclose the machine in a box lined with sound-absorbing material. Make sure the enclosure has separate, adequately sized openings for air intake and air outlet, and plan now for these openings to be fitted with silencers.

A silencer (reflective attenuator) is a chamber that absorbs sound in the passage. The chamber size and the diameter of the inlet hole determine the frequency range that is attenuated. A small outlet pipe hole mainly attenuates low frequencies, while a larger hole handles high frequencies. If necessary, fill the chamber with loose or heavy mineral wool to increase absorption.

Smooth, flat surfaces reflect sound and increase the sound pressure in the room. Reduce the area of smooth surfaces by lining walls and ceiling with sound-absorbing materials. For low-frequency sounds, thick, heavy materials are used — e.g. a fibre mat of 105 kg/m³, which achieves attenuation of up to 35 dB at 4000 Hz.

Acoustic baffles are panels of sound-absorbing material hung from the ceiling. When sound hits a baffle, it is reflected on to the next one, and because of the large total surface area the noise is gradually absorbed down to an acceptable level. Baffles are particularly effective in rooms where it is not practical to cover all surfaces.

For rectangular ventilation ducts, an absorption chamber is constructed in which the sound inlet and sound outlet are not placed opposite each other. Sound that is not reflected straight through hits the back wall and bounces back and forth before leaving the chamber considerably attenuated. For low-frequency sounds, thick insulation is used at the top and bottom; for high frequencies, thinner mineral fibre boards are distributed over the entire inner surface of the duct.

Reverberation time is defined as the time it takes for the sound level to decrease by 60 dB after the sound source has stopped. A short reverberation time gives good room acoustics. Reverberation time is typically measured by firing a blank-cartridge pistol and recording how quickly the sound level falls. The shorter the reverberation time, the better the speech intelligibility — an important requirement in offices and classrooms.

If the noise cannot be sufficiently reduced by technical means, organisational measures are implemented. This may mean reorganising work routines so that employees spend as little time as possible in noisy areas. At 100 dB, the maximum permitted exposure time without hearing protection is only 15 minutes.

If neither technical nor organisational measures are sufficient to bring the noise exposure below the recommended limits, suitable hearing protection must be issued to employees. Hearing protection may also be chosen for psychological reasons, even at noise levels below 85 dB A, as prolonged exposure to noise can affect mental health.