In the first part in this series I started to share how my current studio was designed and built, learning from previous mistakes. In this article we are going to delve into the maths of acoustics and get deep into spreadsheets.
Everything Absorbs Sound
Every type of material has a coefficient of absorption and if you do some digging you can find data and it is usually listed out in octave bands as a number less than 1.0. There is one from Sengpielaudio run by Eberhard Sengpiel, Berlin.
Then it is possible to do the maths to calculate how much of each material you need to absorb the different frequencies. In a good studio design you are looking for even RT60 figures in each frequency band. RT60 is the time it takes for the reverb to decay to 60dB below the initial sound level. The problem with just using foam, as Dan said in his article, is that although you can easily mop up the reverb in the higher bands, because they are not very good at absorbing sound in the lower bands, the reverb at lower frequencies is left untamed.
You also need to take into consideration the room reasonances. These are frequencies where a full wavelength (or multiple wavelengths) will fit between two surfaces. Similarly there will be other frequencies that will be attenuated because there is half a wavelength is left over between the walls.
But again all this is maths and spreadsheets, with formulas built in, are very good at calculating all of these variables. The other great advantage of spreadsheets is you can change one cell and the whole spreadsheet will recalculate.
Helmholtz To The Rescue
To handle the key room resonances I needed treatment that was tuned to absorb specific frequencies and I choose two solutions -Helmholtz resonators and LF Panel Absorbers. Again spreadsheets are available so you can find the correct design to handle the problem frequencies in my studio. The calculators I used, have long gone, but here are links to similar calculators on line, like the Helmholtz slot based resonator and the LF Panel Absorber. There is more info here too on the acoustic.ua site. It is a Russian site but there are a set of English pages. It also has an online variant of my RT60 calculator spreadsheet that you can see below.
In my research I came across a number of spreadsheets, into which, I could enter the room size and other critical information, like what I was planning to put on each surface and see how that would affect the RT60 for the octave bands as you can see from the screen grab above.
As I work a lot with speech in audio post I wanted a room with at RT60 of around 0.25 seconds across each band. So you can see by choosing different products and solutions and plugging them into the spreadsheet I could instantly see the impact on the RT60 values for the different bands. The Acoustic Project Company has a similar calculator on their site.
I built a Helmholtz Resonator and positioned it on my back wall, (see the picture above) having designed it to handle some of the higher resonances in the room. Behind my monitors I adopted a two pronged approach. I built panel absorbers again to handle the lower room resonances and then covered them with acoustic foam tiles, checking all the time with my calculator that I wasn't putting too much in and over dampening the higher frequencies.
It is also very important that the room is acoustically symmetrical so because the left wall had a large window with reflective glass, i didn't add any absorption to the right hand wall.
Racks & Cupboards
As my room had a tradition fireplace with a chimney breast and two alcoves I used the depth of the alcoves to accommodate a computer cupboard on one side, which had the squeakiest door until I oiled it and on the other side a 19" rack from floor to ceiling to take most of my equipment. On both sides I used the remaining space to create bass traps by packing them with mineral wool and finishing them with foam tiles.
The video above shows you the cupboard and how it keeps the noise out as well as how the ventilation describes earlier is configured.
Because with all the equipment, the room was going to warm up fast, so I dispensed with the radiator, but I knew I was going to need both ventilation and cooling otherwise I would melt and/or pass out due to lack of oxygen or overheating and we will look at these in the last part of this series.