Building a proper control room – construction of bass traps

Hi and welcome back to the third article in this series.
After we discussed the prerequisites, came up with an acoustic design concept and did the first measurement of the naked room, it’s now time to actually start construction works.

Along the line of our acoustic design concept we will start with building a wooden frame for the bass absorbers at the wall-ceiling junction. Remember, we want these to be 40cm high and one board of mineral wool deep, i.e., about 70cm when allowing some additional space for the frame. The construction of the frame is relatively simple. One slat is screwed to the ceiling and one to the side wall. These two slats are now joined to form a cuboid. After filling one part of a frame with mineral wool it is reinforced at the bottom and sides with further slats. These also support the mineral wool boards.

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Construction of the frame for the mineral wool bass traps at the wall ceiling junction.

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Bass absorbers at the wall-ceiling junction getting their filling – rear right corner.

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Bass absorbers at the wall-ceiling junction getting their filling – front right corner.

After the wall-ceiling junction has been finished the remaining ceiling is lowered accordingly and likewise filled with 30cm of mineral wool.

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Entire ceiling lowered using a wooden frame that’s filled with mineral wool.

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Entire ceiling lowered using a wooden frame that’s filled with mineral wool.

The last step is to build the front corner traps as described in the design concept article. To achieve this each mineral board is cut into two halves. Each of these halves is then cut into two triangles of the same size which are finally stapled on top of each other. Finally the corner traps are supported by another frame of wooden slats.

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The front right corner trap as well as the entire ceiling filled with mineral wool.

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The front left corner trap as well as the entire ceiling filled with mineral wool.

In order to prevent damage from moisture the entire construction has to be thoroughly sealed using proper vapour retarder. If the construction is mounted to a concrete wall/ceiling it is best to use a humidity adaptive vapour retarder that allows the construction to desiccate back into the room when necessary. Furthermore, the vapour retarder provides a barrier for the mineral wool fibres. Please note, that it is of utmost importance to mount the vapour retarder properly, i.e., using the system-specific adhesive tape and glue as directed by the manufacturer. Failure to do so can lead to severe problems with moisture and mouldiness.

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The entire construction is thoroughly sealed with vapour retarder.

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The entire construction is thoroughly sealed with vapour retarder.

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The entire construction is thoroughly sealed with vapour retarder.

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The entire construction is thoroughly sealed with vapour retarder.

All right, now it’s time for the next measurement. Using the exact same position as for the first measurement the result looks like the following. We will keep the order of graphs the same in all articles. First is the frequency response, which already improved quite a lot, e.g., the peak that we had at 40Hz has been lowered by 10dB and the region from 100 to 500 Hz has been smoothed out dramatically. Of course there is still comb-filtering and one can also identify the slightly asymmetric response of the two speakers due to the room-asymmetry that we mentioned in the last article.

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Frequency response with installed bass traps with speakers and microphone at best position – 1/48 octave smoothing, both speakers measured simultaneously (blue curve) as well as individually (red and green curves).

Even more impressive is the change in the EDT curve. Please note that the curve has been re-scaled in comparison to the first measurement. While we had an early decay time of about 3.6s (3600ms) in the 100 – 200Hz region and more than 1s nearly everywhere else in the frequency spectrum during the first measurement, it’s now down to about 0.6s (600ms) for the worst region, i.e., the highest frequencies. The region of interest for now is the one up to 500Hz which is already looking very good apart from the lowest bass regions.

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Early Decay Time (EDT) curve.

This change in the EDT curve is of course also reflected in the waterfall plot. Here, you can also see the dramatic improvement in the bass area. One point that can also be noticed is the impact of the vapour retarder. Normally, one would expect the high frequencies to be damped quiet easily by the bass traps. However, the vapour retarder is a relatively thick film (or foil if you like) and thus obviously reflects frequencies above about 1.5kHz. We will see this statement to be confirmed by the measurements done after mounting two layers of cotton fabric above the vapour retarder.

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Waterfall plot after the bass traps have been installed.

Of course, the spectrogram shows the same facts as the waterfall diagram, as already pointed out in the last article.

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Spectrogram plot of the room with installed bass traps.

The last plot, the ETC (Energy Time Curve) is not that useful to look at yet, since we haven’t done anything to eliminate early reflections up to now. Nevertheless, we will show it here for the sake of completeness.

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ETC plot.

As you hopefully see, the construction of bass traps already helped a lot in the achievement of a proper control room acoustics. Keep in mind that in the bass region “the more the merrier” is virtually always true.

Next time we will see how mounting two layers of fabric alters the measurements and talk about what we can do to prevent overdamping in the high frequency region.

I hope you are enjoying this series and am looking forward to meeting you in the next article.

All the best,
Markus

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2 thoughts on “Building a proper control room – construction of bass traps

  1. Pingback: Building a proper control room – outline and prerequisites | orthogonalrecords

  2. Pingback: Building a proper control room – mounting fabric and difference measurements | orthogonalrecords

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