Acoustic absorption of a decorative painting

A new colour or patina look

There can be various reasons for wanting to paint an acoustic wall or ceiling covering:

The covering is soiled after many years of use and you want to freshen it up after cleaning
In a monument, you want to imitate the pattern of the original vault
A painting or fresco to be applied by an artist
Contemporary designers often use a patinated look by using specific mortars or sponge techniques
You simply want a new colour

But how do these modifications affect your acoustic system?

Acoustic absorption and porosity

Sound pressure sets air molecules in motion. The vibrating air particles penetrate an acoustically absorbent surface and rub against the edges of the material pores: sound is converted into heat by friction. Acoustic walls and ceilings are therefore porous by nature and it is essential to preserve their porosity as much as possible. Overpainting lavishly is not a valid option.

zoomin porositeit — Porositeit akoestisch stucwerk

Acoustic absorption and specific air resistance

The sound absorption is officially measured in a laboratory according to EN ISO 354 on a test surface of 12m² in a reverberation chamber. On a project basis, the specific air resistance Rs can be measured more quickly in a round tube with a diameter of 10 to 15 cm, before and after painting a sample of the acoustic surface. The measurement standard is then EN ISO 9053-1:2018. In the tube, an air flow with velocity v (m/s) is pushed through the material and the pressure drop p (Pa) over the sample is measured. Rs is then simply the ratio p/v (Pa.s/m). There is a direct relationship between Rs and the acoustic absorption of the material.

There is an optimum for the value of the specific air resistance. If the value is too low, there is insufficient resistance and there are too few pores to obtain high friction and ditto sound absorption. This is, for example, the case with open woven fabrics or metal mesh. If the air resistance is too high, the air particles cannot penetrate sufficiently into the material, which also results in low sound absorption.

Acoustic plasterwork and stone vault

In a listed building, we want to increase the acoustic comfort in a swimming pool with a stone vault. The planned acoustic plasterwork is therefore to be painted partly with a joint pattern like brick (A) and partly with light sponge work (B).

The brick pattern increases the air resistance of the untreated sample by 1317 Pa.s/m (A).

The sponge technique over the entire surface brings an increase of 1872 Pa.s/m (B).

In both cases, the original sound absorption decreases from αw = 0.8 to 0.7. The loss occurs at frequencies higher than 1500Hz.

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