Hi again and welcome back to this blog post series on building a proper control room.

We started this series with a short Teaser as well as some thoughts on the prerequisites in the first article. This week, in the second article, we will have a look at the acoustic design concept that we employed for our control room here at Orthogonal Records.

Our room is 4m long, 3.5m wide and 2.5m high and therefore quiet small for building a proper control room. However, as such a room-size should be in the ballpark of most home recording possibilities, it serves as a great example. We will see in the course of this series how even such a small room can be transformed into a proper (semi-) professional control room.

In order to get a first impression of the room’s potential Bob Gold’s Room Modes Calculator can be very helpful. When entering the above dimensions into the calculator, the following important information can be drawn from the results:

- Room Modes and groups:

Depicted on the keyboard of a piano, the room modes of our example room look like this

The most important information here is the grouping. A group is a range of modes with less than 5% difference from one modal-frequency to another. The wider a group is, the better. Groups made up of at least 5% of the critical spectrum are considered safe. This criterion is colour-coded by the calculator, where dark green is best and red is worst.

The example room is already quite good here. - Room Ratio according to R. Walker (BBC) et al.

Everything alright here, too. - RT60 recommended values

These values are based on the nearest known room ratio. Important is the second block (Control Room Recommended) and therein the last line that tells us to try and keep the RT60 value between 200ms and 400ms throughout the whole frequency spectrum. - Bonello criterion

This criterion, formulated by Oscar Bonello in 1981 is another method to evaluate the ratio of a room’s dimensions. It analyzes the first 48 room modes and plots the number of modes in each one-third of an octave. If the curve increases monotonically the ratio of the room-dimensions is considered good. We have a small plateau here, but overall it looks quiet good. - Schroeder frequency

The transition frequency between the low- and the mid- to high-frequency regime for a given room. In the low-frequency regime the wavelength is comparable to the room’s dimensions and thus room modes dominate.

It is calculated as 134Hz for our example and will be important for designing a proper back-wall diffuser.

All in all one can state that our example room is quite good to start with, according to the results of the room modes calculator.

The acoustic concept that we plan to employ is basically a Live End Dead End (LEDE) approach with little pieces of a Reflection Free Zone (RFZ) concept for the front corners.

LEDE is an empirical concept for stereo mixing environments where the speaker end of the room is dampened, while the listening end is treated by a diffuser, thus kept lively. The goal is to achieve an adequate initial time delay gap, i.e., enough time difference between the arrival of the direct sound from the speakers and any reflections of the room. This gap should be longer than 20ms, without killing the reverberation of the room completely.

LEDE has been proven to be very foreseeable and is therefore a relatively safe approach for DIY projects.

The optimization strategy is based upon the accumulated knowlegde in the german speaking recording.de room acoustics forums and consists of three main steps:

- Bass absorption
- Treatment of first/early reflections
- Diffusion

All critical steps in that approach are checked and documented by measurements.

A sketch of the construction of the bass absorbers can be found in the following picture.

Bass frequencies carry a lot of energy, so to properly treat this range of the spectrum one needs massive absorbers. Here the entire ceiling is lowered by 30cm using a wooden construction, while the circumferential parts (70cm from the side walls) are lowered by 40cm. The whole construction is filled with mineral wool. Additionally, the front to side wall corners are treated with so called super chunks. Those are corner traps where mineral wool is cut into triangular form (here a right angled triangle with legs of 40cm and 60cm) and stapled to fill the entire height of the room. At the end everything is properly sealed with a humidity adaptive vapour retarder and covered with cloth. For such bass traps one has to be careful to choose the right type of mineral wool. The important coefficient is the specific flow resistance, which has to be greater or equal to 5kPa*s/m² for massive bass traps. As this construction together with early reflection absorbers and a large couch that is planed at the back wall of the room would kill way too much high end, it is covered with wooden boards. Those boards are 19.5cm wide and separated by a 2cm gap in order to absorb frequencies below about 1700Hz, but reflect frequencies above this threshold. High frequencies are absorbed very easily. Thus inappropriate treatment can easily over-damp the high frequency region resulting in a dead sounding room.

Furthermore, the super chunks in the front corners are angled such that a reflection free zone emerges at the (presumable) listening position.

In the second step, early reflection absorber (broadband absorbers) will be mounted at the points of first reflection on the side walls and the ceiling. These are not sketched in the picture above, because their position is determined by the precise location of the listening spot (sweet spot) that is also determined by measurements. More on that in a further article.

The last step will be to install a proper diffuser at the back wall in order to raise the initial time delay gap even more. In a further article we will calculate such a diffuser using dedicated (free) software and analyze its properties.

Somewhere in between those steps one has to think of building proper (heavy) monitor stands and a studio desk as well as think about where and how to place screens and appliances such as a mixing console in order to deal with early reflections of the desk. Those reflections can be very intriguing as we will see later on.

That’s it for now! I hope you enjoyed the series so far and am looking forward to meeting you again in the next article of this series, which will deal with the first actual measurements.

All the best,

Markus

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