Optimal use of absorption in a room
It seems to be common to cover walls, or large parts of them, in relatively thin absorption (9 to 15mm thick). This may be because it is heavily advertised, seems to be lower cost or because it means less paintwork needs to be done. More than likely though, it is done out of ignorance. The thought that more is better is a fallacy for two reasons;
It is not a linear addition. If reducing the reverberation time on the room to half it’s current level requires 5 square metres (for example), the next 5 square metres also halves modified level i.e. the first five square metres goes from 100% to 50% and the second five square metres goes from 50% to 25%. So there is an optimal amount that yields the best reduction in reverberation time square metre (or dollar).
Thinner materials absorb more high frequency than low frequency. Over use of thin materials results in rooms that feel and sound muffled and dead. The single figure RT60 may be low but the speech transmission index will still be poor. Ultimately over use of thin materials has not solved the acoustical problem and cost a lot.
The graph below shows the change in reverberation time from an empty room to that with 5, 10 and 15 square metres of absorption. Clearly, the first 5 square metres has the greatest effect. The room is 4 x 3 x 2.4m
The far smarter option is to use a lesser amount of thicker absorber. Most commonly 50mm thick absorbers are used because they absorb from high frequency down to the low end of the spectrum of speech. This creates a balanced reverberation time that sounds natural. The cost is similar to overusing thin materials because a lot less area is used even though the 50mm thick absorber is generally more expensive than the thin one.
As shown below, classroom hard has no absorption. Classroom thin is covered in thin absorption. Classroom thick is covered in thick absorption. Classroom nice is a moderate amount of thick absorption. Although the instance of covered in thick absorption may seem attractive in terms of speech transmission index, it would feel uncomfortably dead.
The reverberation times for the instances above are shown below. Again, classroom nice is obviously the room of choice as it would feel comfortable to be in.
In summary, it is far better value to use a lesser amount of thicker absorption. It will result in better speech transmission and a more comfortable feeling. The cost should be similar. If the room is used for any type of performance, music or A/V like teleconferencing or videoconferencing, this becomes even more important.
This may look simple but it produced this feedback;
I thought I would give you some feedback on our recent purchase of your acoustic tiles. We installed them a few weeks ago now and everyone is honestly shocked at how well they work. The room has gone from an echo filled, noisy box to a nice and (almost eerily) quiet conference room. Everyone's voice boxes and ears are thankful.
Thanks for your advice and service.