When talking about high-fidelity sound systems, I found that people would often think that the loudspeakers are the most important part of their stereo. This may be due to the fact that loudspeakers are visually very noticeable, sometimes impressive, and that they are heavily advertised as the “key” to a good sounding system.
During my research, I realized that loudspeakers are of course important, but less than other parts of the system. For me, and I am not the only one thinking this way, the two most important parts are the source and the room. Yes, I include the room in the sound system as it dramatically influences the way we hear the sound coming out from the speakers.
The most important one would be the source, and this is why I chose to restore the best of all, namely the reel to reel tape machines. The second most important one would be the room, and this is the topic of this paper.
The room I decided to transform into a professional studio had many awful characteristics one could think of for a music room:
It is very small: 2.1m high, 3.9m long, and 3.2m wide (roughly 12m2 and 26m3)
The floor was made of ceramic tiles
The ceiling was made of glossy plastic
It had a large bow window and a glass door
In a nutshell, it sounded horrible.
The good part though was that the room dimensions relative to one another, called room ratios, were pretty good.
Let’s have a closer look at this critical aspect of room acoustics.
1. Optimal room dimensions
The room dimensions are mostly important for low frequencies, as they are quite long sound waves. For example, a 44Hz sound wave will have a length of 7.72m!
When you play music in a room, the sound waves produced will bounce on every surface of the room, and it instantly becomes a big mess of sound waves. The funny part is you want to keep that mess. Why? It is quite simple to understand: you want your room to have an as-flat-as-possible frequency response. That means you don’t want any frequency to sound louder than other ones, because in that case the sound coming from your loudspeakers will be altered before arriving to your ears.
Now, the problem with all rooms, and especially small ones, is that their inherent dimensions will favor some frequencies that “match”. When a frequency matches certain dimensions of your room, there will be creation of a standing wave or resonance. It means the sound wave will travel in the room on the same path over and over again, thus creating louder or quieter areas in your room at this frequency compared to the original sound from your loudspeakers.
These standing waves in a room are called room modes, and they can be calculated.
So how do you calculate your room modes? Some great websites have an online calculator for room modes, some others will give you the “golden ratios” for your room size.
I will take 2 examples here applied to our room dimensions:
The first one is a room ratio calculator that can be found here:
Here we can see that the yellow dot is placed on a black spot, which indicates smooth modal characteristics for the room. In other words the standing waves are relatively widespread and they are not too many.
The second one is an online room mode calculator that can be found here:
Here we can see the different room modes and that the red cross is inside the Bolt-area, which indicates an accumulation of good room ratios.
Conclusion: even if these calculations are not 100% accurate because no room is a perfect box, they clearly indicate that our room has excellent dimensions for music listening :-)
2. Low frequencies acoustic treatment
Low frequencies are difficult to tame in a small room as they are very long. It means their path through an absorbent material will be shorter than for high frequencies, and therefore very thick absorbent materials should be used.
This is why regular acoustic foam and egg packs applied on walls will not help tame low frequencies, they will only work on high frequencies.
The composition of the absorbent material is also very important, because each material has an absorption coefficient that varies for different frequencies.
I chose 10cm rigid fiberglass mats with 48kg/m3 density as this material was available here in Shanghai and has excellent absorption coefficients at low frequencies.
The most efficient way to treat a room is to place absorbent material at the corners of the room, because this is where the concentration of sound waves is maximum. Therefore I decided to heavily treat the 2 corners behind the speakers with columns of fiberglass from floor to ceiling. The base of each triangular column is 60cmx60cmx85cm. I also chose to cover the wall and bow window behind the speakers with 10cm thick panels.
For the 2 corners behind the listening position it was trickier because the room is very small and we still need space to breathe... The compromise was to build thinner columns and place them a bit away from the actual corner. Why? Because absorbent materials absorb better sound waves when their velocity is high. In fact they transform velocity into heat. Against the wall, the sound wave has no velocity but a high pressure, and as we get away from the wall the velocity increases and the pressure diminishes. We will see later that you can also work on sound pressure with different devices.
Once I had sketched up the room to understand what amount of fiberglass would be needed, we started working with my teammate Pierre.
We built all the absorption panels frames from plain wood, and used different thicknesses of felt to cover them. Fiberglass is not a very friendly material to work with, so if you decide to order some please protect yourself! A face mask and gloves are the absolute minimum: this thing is full of very tiny glass splinters that you don’t want to breathe or get into your skin…
I chose felt material as a cover for the panels as it is breathable, it has an irregular surface, and finally the look is rather nice. Breathability is the most important because you want the sound waves to get into your absorbers to get absorbed :-)
Once all the broadband absorbers were in place, it was time for measurements.
The purpose here is to place an omni-directional microphone at the intended listening position, and use a room analysis software to understand what is the room acoustic response. The principle is very easy to understand: you play a range a tones or frequencies through your loudspeakers, and you record what the microphone “hears” at the listening position.
The software will then show you if the sound level (in dB) is the same, higher or lower than what it should be. In other words the software will enable you to visualize the frequency response of your room compared to the sound level coming out of your loudspeakers. And this is why we want a flat frequency response: in that ideal case every sound coming out of the loudspeakers will arrive at the same level to your ears.
The results were quite encouraging, but one huge problem remained: the 44Hz mode. The number one resonant frequency for this room is 44Hz, and even 100kg of absorption panels could not tame it enough. So what could we additionally do to tame it?
4. Face to face with 44Hz
This issue forced me to go further in my research in room acoustics. I think I found a good solution that I will share with you now. Do you remember I wrote earlier that you can work on sound waves velocity and pressure? Well here I will show you how to work with sound waves pressure.
How? With a Helmholtz resonator! Any closed volume with an hole in it is a Helmholtz resonator. When you blow air at the bottleneck with the right angle and speed and it starts to make a noise and vibrate, you have activated a Helmholtz resonator. This kind of device is really useful for taming precise frequencies.
How does it work? A Helmholtz resonator acts like an air spring. The volume of air inside the cavity (in our previous example inside the bottle) will be compressed when you blow air at the hole, and the air at the entrance of the cavity will act like a mass. The air inside the cavity will compress, and then decompress to go back to normal pressure. If you excite the mass at the right frequency, the mass will enter in resonance.
Great, but how do we use this in a music room? The other thing you need to know is that a Helmholtz resonator will send back the same frequency but with reverse phase. A sound wave has a phase, it goes up and down, up and down, etc. If this sound wave excites the resonator, the latter will send back the same frequency but in inverted phase: down and up, down and up, etc. And acting this way, it will cancel the resonant frequency. The bigger the volume, the stronger it works.
To go a bit further, you can insert inside the volume some absorbent material in order to widen the range of frequencies it will work with, at the expense of efficiency at each of them.
I found some online calculators for such resonators but they revealed to be inaccurate. My first prototype was a complete disaster due to that, so I went back to the original formula.
The correct Helmholtz resonator calculation formula is:
The Helmholtz resonators were placed at the back of the room, near the corners. And their effect is very noticeable. It basically solved the 44Hz problem!
Here is the first prototype that did not work at all:
And here is the second one, effectively tuned to 44Hz:
5. Dealing with high frequencies
Dealing with high frequencies is quite straightforward at first view. You want to apply absorption material at every first reflection points between the loudspeakers and your listening position. Why? Because the loudspeakers send sound waves in many directions including walls, floor, and ceiling. The listener will get the direct sound from the loudspeakers, but also the same information with a slight delay coming from the walls, the floor, and the ceiling. This will cause a focus issue that we don’t want.
In the studio, we applied acoustic foam panels on the side walls, we applied felt on the entire ceiling, and we installed a thick rug on the floor.
We also pimped the ugly air conditioning with felt, just for glory :-)
That was for absorption. Now let’s talk diffusion. If you put too much absorption in a room, it will sound dull and therefore the music you play will not sound natural. The tricky part in dealing with high frequencies is to find the good balance between absorption and diffusion.
What is diffusion? A diffuser is a device with irregular surface you can place in your room. Its major function is to… diffuse sound. Basically the sound waves will bounce on it and come back with a phase and/or direction shift. Diffusion is used to give more life to your room and to make it sound bigger than it is in reality.
After several trials, I placed one diffusor in the center behind the loudspeakers, and 2 bigger ones behind the listening position. I found it gave a great tonal balance to the room.
Building a high-fidelity music studio turned out to be a complex and rewarding project. The result is a room where people can enjoy music in ideal conditions, and it makes you want to share with others this immense pleasure for the senses.