We Finished the first part of Studio Acoustics talking about Schroeder Frequency and why we need to calculate it. In this part, I want to talk about Absortion Materials Diffusion and Reverberation.
Absorbing materials and elements are widely used in the acoustic treatment of rooms, especially the ceiling when you want to reduce the reverberated sound energy. Their use allows control of the reverberation time.
At appropriate distances from the sound source, their use allows the control of the total sound pressure level present in the environment.
The absorption of the emitted sound energy is one of the most effective methods of noise reduction when the propagation of sound occurs within closed spaces (e.g. ducts)
They are used when you intend to isolate a source from the surrounding environment with a soundproof booth.
Types of absorbent materials
1) Porous sound absorbers
2) Perforated-absorbent panels (absorption by cavity resonance)
3) Vibrating panels (absorption by membrane resonance)
4) Bass Traps
Acoustic Foam that is usually used in music studios will absorb frequencies like this:
As you can see it only absorbs well from 500hz upwards so in a studio situation what is under this frequency will remain untouched. In the picture on the Y axis, you can see the absorption coefficient(ex: 02, 03, 0.7, 1 ), and if is 1 or more that means is absorbing like 100% usually we are looking for materials that have more then 0.6/0.7 in the area we need to work on.
Rock wool – Absorbs a bit better going at about 250hz, again what is under will not get affected and usually, the most problematic parts in studios are at lower frequencies. Depending on how thick it is can go lower in frequencies but usually is used with 10cm thickness and it looks like this.
Perforated-absorbent panels: resonance absorption of cavities (Helmholtz Resonators)
These are absorption panels NOT materials, you have to build and TUNE these panels in order to work. Basically, you have to build an MDF box and seal it with acrylic and on the front panel you have to make holes, depending on how many holes and how big the holes are this box will resonate at a frequency. Here you can calculate your resonator . If you place also some rock wool inside the box the resonant frequency will look like this.
As you can see it will be less efficient at fixed frequency but with a wider band. So if we have an AXIAL MODE at 120Hz on the length(L) of the room (we talked in Studio Ascoustics part 1/3 about axial modes) you’ll need to tune them for this frequency and place them in front and back of the room. I hope it makes sense why.
Vibrating panels: absorption by membrane resonance
You will need to build a sealed box, careful if you use the wall as the back panel if is an aerated concrete wall you have to paint it because air can pass through this kind of walls and you will not have a sealed box otherwise. The front panel has to be the membrane usually made of plywood WITHOUT any holes this time the resonant frequency will be determined by the dimension fo the box and the depth. HERE you can find more info and also you have a calculator that will be very helpful.
So for the 122 Hz Mode, I will need a resonator with these dimensions.
Take some time and play with that calculator to fit different frequencies, you will see that the bigger the depth the lower the resonant frequency.
The corners of the room are where there are a lot of active bass frequencies so to be most efficient is better to use CORNER BASS TRAPS, these are usually made of Rock Wool.
Covered with textile material so the dust will stay inside, this dust is toxic so maybe will be even safer to cover the rock wool with film foil and after that with textile material for a better look.
If you have too much absorption in your room it will sound too flat and unnatural. The diffusion will give you a natural-sounding room.
Diffusion will preserve the energy of sound not by absorbing it and by reflecting it so in this way multiple sounds will work together creating a new noise instead of sounds competing against each other.
You can even make a small room sound bigger with diffusion. The human ear can pick up the reverb and the more it is the more we think the room is bigger. With a wooden diffuser, you can make your small room sound much fuller.
You can calculate your diffuser sections sizes and its frequency range by clicking HERE – ideally is good to have diffusion between 200hz and 1000/1500hz. Again like with the resonators the more depth you have the lower the frequency.
Reverberation time T60 (s) (ISO 3382 standard):
The reverberation time T is a measure of the speed with which, once the source is turned off, the sound is extinguished in the environment.
It is the time necessary for the sound pressure level at the observation point to decline by 60 dB, starting from the instant moment the source of sound is deactivated.
Parameters related to the reverberation time
Bass Ratio BR: is the ratio of the arithmetic mean of the reverberation time T 60 between the octave bands at 125 Hz and 250 Hz and the reverberation time at medium frequencies:
BR is related to the subjective criterion “warmth” of sound;
-Beranek recommends 1.2 <BR <1.25 for music.
-Barron recommends 1.05 <BR for opera
Brilliance: is the ratio of the arithmetic mean of the reverberation time T 60 between the octave bands at 2 kHz and 4k Hz and the reverberation time at medium frequencies:
– it is related to the subjective criterion “brilliance” of the sound;
-expresses the rich content of harmonics
In large volume environments, the frequencies in question are partly absorbed by the air, so when choosing the internal acoustics it is good to pay attention to the “loss of brilliance” and not end up having a “dead room”.
Don’t worry about calculations this will be automated calculated in my calculation sheet I will share for free with you at the right time.
The Optimum reverberation time in a music room should be like in the picture below if you fall under 0.2 your room will have too much absorption and that is called “dead room”.
Listening Spot and source location
The speaker position must be symmetric and from a modal point of view, the mixing station must be such as to avoid the geometric center of the room. Deviate from the geometric center on the Front – Back axis by at least 1/4 of the wavelength corresponding to the lower cutoff frequency of the woofers. Avoid that the operator’s head is halfway up (ceiling dimensioning). Please check this link HERE
Reflections of sound against walls, ceilings or floors create phase-reversed interference cancellations
In particular, if d is the distance of the front of the monitor from the rear wall, the cancellations due to phase inversion due to the rear reflections occur when the distance 2d traveled by the reflected wave is equal to the half-wavelength of the frequency fc that is canceled by push-pull, that is:
The amplitude of the cancellation depends on the energy of the reflected wave, ie the reflection coefficient of the wall and the distance. It is useless to try to equalize the response in the gap, as increasing the direct signal also increases the reflected one.
Now we fInished with the theory in the next part I m gonna do the full project of a room. I hope this was helpful – CLICK If you missed the Studio Acoustics Part 1