Architectural timber slat sound absorbers are often desirable for large open plan areas. They can control excessive reverberation with a robust, hard wearing, aesthetically pleasing architectural finish.
We are planning to expand this article but for now, as this is a complicated subject, we are just going to cover some elementary points.
We have visited many spaces with poor acoustic performance only to find large areas of timber paneling. This is generally on the ceiling. Remedial acoustic works are always required. However, if some forethought was given at the design stage, an effective solution could have been designed for.
Allowing gaps between the timber so the sound could be absorbed by an acoustic material placed behind would be a cost effective solution.
It may be worth noting that acoustic slats, acoustic slots, Helmholtz resonators all sort of merge into this bracket. There are however some subtle differences.
A resonator, like blowing over a bottle top, requires a closed cavity. A resonator is usually ‘tuned’ and has a fairly narrow ‘Q’ or resonant frequency. It prefers to provide maximum absorption at this frequency with much less effective absorption either side.
On the other hand, a broadband absorber would provide effective absorption across a wide range of frequencies. Ideally all of them, between say 100Hz to 10kHz.
A panel with some reflective surface and some open absorptive gaps provides some additional reflections back into the room so can often be beneficial in atrium type spaces.
For architectural acoustic purposes it would usually be a broadband type of absorption that is required as a HelmHoltz resonator type arrangement would be rather limiting.
A resonating sound absorber also reacts slightly differently depending on a number of factors. These can be having an absorbent material placed in the cavity to ‘broaden’ the range of frequencies absorbed. The results of this mechanism can vary. It will depending on if the absorbent is touching the opening or spaced behind it.
It is a complicated phenomena, with lots of variables, and any calculations are never going to beat actual acoustic testing of such systems.
More often than not a timber backing frame will be required.
The majority of pre-made timber slatted panel systems available in the market will still require this to provide a Class A or Class B acoustic treatment finish.
The deeper the frame the better the low frequency performance will be.
Lets assume the frame is 50mm deep.
It needs to have a suitable acoustic absorbent material inserted in it. This could be rockwool, acoustic foam, acoustic fiber it would depend on potential weight loadings and ease of installation. Generally the only difference is going to be cost and the material will need to be suitably fire retardant.
To ‘hide’ the absorbent material from view between the slats it is usual to fix an acoustically transparent acoustic fabric over this framework. Most of the time this is a black material but sometimes a coloured material may be called for.
The chosen slats are then simply fixed onto the framework to cover the absorbent leaving gaps between the timbers as desired.
What is generally apparent, and should be considered, is the following points:
- The spacing of the slats, hence the percentage open area, will have negligible effect on the absorption of frequencies below around 250Hz. It is the thickness of the absorbent that defines this.
- The wider the space between the slats, hence the larger the open area percentage, the better the absorption will be above 250Hz.
- The absorption has a minimum at a frequency where the slat width is half a wavelength and starts rolling off at a quarter wavelength.
- If the slats vary in height, hence provide subtle diffusion, then absorption is better than uniform height slats.
To summarise, covering large areas with timber slats with the exact same slat depth and spacing can be a low cost and effective noise control solution. It will definitely be better than continuous timber paneling but, for more critical areas, there may be a little too much frequency specific absorption.
A modest 25% percentage open area, especially on a ceiling, will provide significant acoustic benefits if it is carried over its entirety.
An interesting research paper can be read on the IOA website here.