Hi again! Welcome back to the fourth article in this series.

Today it’s time to see what difference it makes if two layers of cotton fabric are mounted above the construction that we already measured last time. Since that task is quiet simple, today’s article will be relatively short.
The primary reason we want to put some fabric on top of the bass trap construction is a purely aesthetical one. We simply do not want the vapour retarder and it’s imprint to be seen trough the slits in the wooden board construction that is planned on top of the bass traps. Since we used a cotton fabric that is relatively thin and the imprint of the vapour retarder would still shine through a single layer, we used two layers of fabric.

Two layers of cotton fabric mounted on top of the construction – front wall.

Two layers of cotton fabric mounted on top of the construction – rear left corner.

Two layers of cotton fabric mounted on top of the construction – ceiling.

Two layers of cotton fabric mounted on top of the construction – ceiling.

A side effect of this approach is, that the fabric also alters the acoustic response of the room, as does every change that you make to the room. Let’s see how strong the effect is. Looking at the frequency response curve we can only spot slight differences. That’s perfectly reasonable since one shouldn’t expect a drastic change in frequency response by just two layers of thin cotton fabric.

Frequency response after the cotton fabric has been mounted; speakers and microphone at best position – 1/48 octave smoothing, both speakers measured simultaneously (blue curve) as well as individually (red and green curves).

The EDT curve, as well as the waterfall and spectrogram plot, however, show that the fabric changed the decay time of the high frequencies. Everything above about 1.5 kHz is now damped considerably more than without cotton fabric. This is exactly the effect that we already mentioned in the last article. Frequencies in this range had obviously been reflected by the relatively thick vapour retarder. Nevertheless, the two layers of cotton seem to be thick enough to dampen those frequencies some more. This in turn helps to bring down the decay time to a maximum of about 400ms (0.4s) throughout the entire frequency spectrum.

Early Decay Time (EDT) curve after fabric has been mounted.

Waterfall plot after the fabric has been mounted on top of the bass traps.

Spectrogram plot of the room with installed cotton fabric.

Changes in the ETC are actually not worth mentioning since we still haven’t done anything specific to the “real” early reflections up to now,e.g., a treatment of the side-walls.

ETC plot.

In the course of treating early reflections via side and cloud absorbers and due to the fact that a big couch for future clients is planned at the back end of the room, we need to think about not over-damping the room in the high frequency range. This will be achieved by mounting wooden boards across the construction that we built so far. These boards will have a certain width and will be mounted with a certain slit between each other. These two dimension then define a limiting frequency. All frequencies below this limiting frequency are diffracted into the absorber construction, whereas all frequencies above the limit are reflected by the wooden boards.

More on this construction and it’s effect on the room response will be the topic of next weeks article.

All the best,
Markus