Norval
AVForums Grandmaster
Since it seems there has been a lot of interest in the topic on Room Acoustics lately, I decided to type up this part on room acoustics.
Fist of all I?m not a specialist by a long shot, but I have done some reading and asked some questions, and have a little experience in music as well as speaker building and technical. Most of this info comes from articles on the net, and based on my own experimentation, experience and knowledge on Radio principles, (as the radio and sound are not that far apart), I?ve slammed together this peace.
There are ideal room sizes and ratios and info on it available on the net, but most of us have an existing room, so I?ll concentrate on looking at that.
There are a few things that we want to control, namely:
a) Reverberation time: This is the time that a signal takes to fades away. Generally accepted is the time that it takes for a signal to drop down with 60dB. Therefore referred to as RT60, and in listening rooms the aim would be to have it fairly short, around 0.3 to 0.5s. Reverberation time can easily be controlled by either diffusers or absorbers. Diffusers might be the ideal since too many absorbers might tend to make a room too dead.
Solution: Book shelf filled with books of different sizes. CD racks, or any furniture. Egg holder shaped foam (but this is ugly and expensive.
b) Reflections: The aim here is to stop reflections that reach the ear in 20 to 30ms. Longer reflection times can be processed by the brain as reflections and could sound quite nice. But the shorter reflections affect the stereo image. Here I?d say absorbers are the answer. Look at first or early reflection points first. If the listening position is close to the rear wall you will definitely put absorbers there, but if the listening position is 1.5m or further diffusers will work better. The reason for is quite simple, when sitting close to the wall you will receive short reflections which is unwanted.
Solution: The most cost effective solution is semi rigid fibreglass. This can be put behind a painting, or covered with nice fabric and put against the wall in a decorative manner.
c) Room modes: The most neglected part, mainly because it?s not so easy to control. On the HF side things should be sorted out with diffusers and absorbers used to control reverberation time and early reflections. The problem lies on the LF side, and unfortunately a clap test won?t reveal problems in the bass region.
Solution: Bass traps are the answer for this problem. There are three options;
i.) Semi-rigid fibreglass in the corners of the room. I have made two wide-band bass traps using pink Aero-Lite, and compressed a 150mm to 50mm, with a piece of plywood behind it and softbord in front with a 30mm gap to the wall. The one is used in a corner of the music room, where it works quite well. The reason for the thickness is to absorb more bass, so is the reason for the gap behind it and the addition of the softbord. This works well in the corners because the air gap is larger which means lower bass absorption. I used Pink aero-lite because I had it. The better stuff to use would be Semi rigid fibreglass as well. Take note that one or two inches of semi rigid fibreglass against the wall will not be effective in the bass band. It needs to be thicker, or spaced a distance from the wall, and will therefore work better in the corners.
ii) The second option would be to use a Helmholtz resonator. I know Cleansound has got some at his place, but I have no experience with it myself. The disadvantage of a Helmholtz resonator is that it is effective in a narrow band. But if a room has a bass resonance at a specific frequency it?s the way to go. It?s also more efficient than rigid fibreglass below 150Hz. A Helmholtz resonator is basically a ported box that resonates at a certain frequency. The resonating box requires energy, thus ?absorbing? the frequency of interest.
iii) The panel resonator would be the third option. Sometimes this is also referred to as a Helmholtz resonator, but for purpose of explanation I would refer to it as different things. I?ve got a panel resonator in the music room as well, build for around 80Hz, and effective to over 120Hz. It is not clear whether the Helmholtz or panel resonator would be better, but from what I gathered it six of the one and half a dozen of the other. The advantage of a panel resonator is that it could be installed against a flat wall or even against the ceiling, where as the Helmholtz resonator would be aimed at the room corners.
A panel resonator is only a heavy panel (like plywood), with a cavity behind it, and some damping inside. The resonance is based upon the density of the panel, and the depth of the cavity.
And that?s it for now, sorry for the long boring post. ;D
I'll keep quiet now :-X
Fist of all I?m not a specialist by a long shot, but I have done some reading and asked some questions, and have a little experience in music as well as speaker building and technical. Most of this info comes from articles on the net, and based on my own experimentation, experience and knowledge on Radio principles, (as the radio and sound are not that far apart), I?ve slammed together this peace.
There are ideal room sizes and ratios and info on it available on the net, but most of us have an existing room, so I?ll concentrate on looking at that.
There are a few things that we want to control, namely:
a) Reverberation time: This is the time that a signal takes to fades away. Generally accepted is the time that it takes for a signal to drop down with 60dB. Therefore referred to as RT60, and in listening rooms the aim would be to have it fairly short, around 0.3 to 0.5s. Reverberation time can easily be controlled by either diffusers or absorbers. Diffusers might be the ideal since too many absorbers might tend to make a room too dead.
Solution: Book shelf filled with books of different sizes. CD racks, or any furniture. Egg holder shaped foam (but this is ugly and expensive.
b) Reflections: The aim here is to stop reflections that reach the ear in 20 to 30ms. Longer reflection times can be processed by the brain as reflections and could sound quite nice. But the shorter reflections affect the stereo image. Here I?d say absorbers are the answer. Look at first or early reflection points first. If the listening position is close to the rear wall you will definitely put absorbers there, but if the listening position is 1.5m or further diffusers will work better. The reason for is quite simple, when sitting close to the wall you will receive short reflections which is unwanted.
Solution: The most cost effective solution is semi rigid fibreglass. This can be put behind a painting, or covered with nice fabric and put against the wall in a decorative manner.
c) Room modes: The most neglected part, mainly because it?s not so easy to control. On the HF side things should be sorted out with diffusers and absorbers used to control reverberation time and early reflections. The problem lies on the LF side, and unfortunately a clap test won?t reveal problems in the bass region.
Solution: Bass traps are the answer for this problem. There are three options;
i.) Semi-rigid fibreglass in the corners of the room. I have made two wide-band bass traps using pink Aero-Lite, and compressed a 150mm to 50mm, with a piece of plywood behind it and softbord in front with a 30mm gap to the wall. The one is used in a corner of the music room, where it works quite well. The reason for the thickness is to absorb more bass, so is the reason for the gap behind it and the addition of the softbord. This works well in the corners because the air gap is larger which means lower bass absorption. I used Pink aero-lite because I had it. The better stuff to use would be Semi rigid fibreglass as well. Take note that one or two inches of semi rigid fibreglass against the wall will not be effective in the bass band. It needs to be thicker, or spaced a distance from the wall, and will therefore work better in the corners.
ii) The second option would be to use a Helmholtz resonator. I know Cleansound has got some at his place, but I have no experience with it myself. The disadvantage of a Helmholtz resonator is that it is effective in a narrow band. But if a room has a bass resonance at a specific frequency it?s the way to go. It?s also more efficient than rigid fibreglass below 150Hz. A Helmholtz resonator is basically a ported box that resonates at a certain frequency. The resonating box requires energy, thus ?absorbing? the frequency of interest.
iii) The panel resonator would be the third option. Sometimes this is also referred to as a Helmholtz resonator, but for purpose of explanation I would refer to it as different things. I?ve got a panel resonator in the music room as well, build for around 80Hz, and effective to over 120Hz. It is not clear whether the Helmholtz or panel resonator would be better, but from what I gathered it six of the one and half a dozen of the other. The advantage of a panel resonator is that it could be installed against a flat wall or even against the ceiling, where as the Helmholtz resonator would be aimed at the room corners.
A panel resonator is only a heavy panel (like plywood), with a cavity behind it, and some damping inside. The resonance is based upon the density of the panel, and the depth of the cavity.
And that?s it for now, sorry for the long boring post. ;D
I'll keep quiet now :-X
