As many here might be aware. I treated my room for bass in 2021 with stellar results. However the front and side walls haven't yet been treated, due to conflicting information on use of absorbers and diffusers. So I spent quite a bit of time in 2022 reading up on the subject.
The pro's in the business, in my observation, seldom ever reveal the tricks of their trade. So it was time consuming to peice together small smatterings of information from different posts on different forums to peice the below together.
And as can be seen after reading below, the choice between absorption and diffusion is dependent on distances ( diffuser to speaker and diffuser to listener) and surface area available. Where diffusion can't be used, absorption will work. As long as its not overdone to make the room sound dead by over damping.
The purpose of treating our room :
1. To arrange the placement of our speakers and listening position in such a way, that the primary reflections ( first reflections) reach our ears 15 milliseconds after the direct sound from the speakers. Converting 15 milli seconds into distance, give us 5.1 mtrs or 17 ft.
So if "distance from speaker baffle to first reflection point" + "distance from first reflection point to ear" - distance from speaker baffle to listeners ear >17 ft. Then no treatment is required at this first reflection point.
2. However in small rooms, the above required 17 ft will not be possible. So the solution then is to treat the first reflection point, with either absorption or diffusion such that the amplitude of the reflected sound is 10 dB less than the amplitude of the direct sound from the speaker.
3. So absorbtion or diffusion at first reflection point - The recommended ratio between absorption vs diffusion vs reflection ( bare walls or ceiling) is 33%:33%:33%. And that's simply not possible in a domestic environment.
So, the idea is to use equal amounts of both. However, by equal amounts, we mean equal effective area and not equal surface area. The max effeciency of a diffuser is taken as 0. 6 NRC, where as a proper absorber designed for that same frequency can have a NRC of 1.0. Which means, for every 1 SQ.ft of absorption used, we will need 1.66 SQ. Ft of diffusion to have the same effect. Thus, diffusion will take up more surface area than absorption. Thus, absorption is recommended for smaller rooms.
4. 1D or quadratic diffusers - These are the easiest DIY diffusers with plans readily available online. Every quadratic diffuser is designed with a target frequency range in mind. The lower frequency where the diffusion starts is called as its design frequency. Let us take an example of a 5 inch deep quadratic diffuser, whose effective frequency usually starts at 900 hz. The wave length of a 900 hz sound wave is 37 cms. We multiply this by 3, to get 1.11 mtrs.
So we should not have any sound source or listener within a distance of 1.11 mtr from the boundaries of a, quadratic diffuser. A quadratic diffuser is called as a RPG - reflection phase grating. The phase of the diffused wave is offset from the direct sound. And when source or listener are within 3 wave lengths of the diffuser, then the interaction between the direct and reflected sound will cause a tingy note to the treble and will damage imaging of the system.
I have previously read that diffusers have to be placed at a minimum of 8 ft from the listener. Then recently on a different forum, a designer of studio's mentioned that he would never use a quadratic diffuser, unless the listener was at least 10 millisecs from the quadratic diffuser. That translates to 3.4 mtrs or 11.3 ft. But since he was referring to a studio where 100% acoustics are required, I would like to think that for a domestic small room 8 ft might be doable
5. 2D or skyline diffusers - Where room size doesn't permit quadratic diffusers, due to close proximity of speakers or the listener, the skyline diffusers come to the rescue. The skyline diffusers are less effective than quadratic diffusers, but don't come with a minimum distance requirement that I've seen so far. Actually skylines don't diffuse, they apparently only work via staggered reflection. So these can be used without a minimum distance requirement probably. But I would still stick with a minimum of 8 feet distance to the ear in my room, because it is possible at 2 points.
6. Another types of diffusers recommended for small rooms are poly diffusers. These look like a half cylindrical PVC pipe. And the inside can be stuffed with glass wool to absorb bass as well. These poly's don't alter the phase, and thus don't have a minimum recommended distance again.
7. I've saved the best for the last - RPG BAD - Binary amplitude diffusers - these are patented by RPG systems, and haven't seen same being offered by anyone else. These are thin, almost 1 inch thick panels, with a, mathematically calculated cuts in the baffle plate They diffuse from mid frequencies upwards and absorb bass at the same time. So these would be most ideal.
8. The first reflective point treatments, only come after bass treatment. Unless RT60 times below 400 hz are brought down below 0.5 secs. We won't be able to hear the full benefits of the panels at first reflection points.
The pro's in the business, in my observation, seldom ever reveal the tricks of their trade. So it was time consuming to peice together small smatterings of information from different posts on different forums to peice the below together.
And as can be seen after reading below, the choice between absorption and diffusion is dependent on distances ( diffuser to speaker and diffuser to listener) and surface area available. Where diffusion can't be used, absorption will work. As long as its not overdone to make the room sound dead by over damping.
The purpose of treating our room :
1. To arrange the placement of our speakers and listening position in such a way, that the primary reflections ( first reflections) reach our ears 15 milliseconds after the direct sound from the speakers. Converting 15 milli seconds into distance, give us 5.1 mtrs or 17 ft.
So if "distance from speaker baffle to first reflection point" + "distance from first reflection point to ear" - distance from speaker baffle to listeners ear >17 ft. Then no treatment is required at this first reflection point.
2. However in small rooms, the above required 17 ft will not be possible. So the solution then is to treat the first reflection point, with either absorption or diffusion such that the amplitude of the reflected sound is 10 dB less than the amplitude of the direct sound from the speaker.
3. So absorbtion or diffusion at first reflection point - The recommended ratio between absorption vs diffusion vs reflection ( bare walls or ceiling) is 33%:33%:33%. And that's simply not possible in a domestic environment.
So, the idea is to use equal amounts of both. However, by equal amounts, we mean equal effective area and not equal surface area. The max effeciency of a diffuser is taken as 0. 6 NRC, where as a proper absorber designed for that same frequency can have a NRC of 1.0. Which means, for every 1 SQ.ft of absorption used, we will need 1.66 SQ. Ft of diffusion to have the same effect. Thus, diffusion will take up more surface area than absorption. Thus, absorption is recommended for smaller rooms.
4. 1D or quadratic diffusers - These are the easiest DIY diffusers with plans readily available online. Every quadratic diffuser is designed with a target frequency range in mind. The lower frequency where the diffusion starts is called as its design frequency. Let us take an example of a 5 inch deep quadratic diffuser, whose effective frequency usually starts at 900 hz. The wave length of a 900 hz sound wave is 37 cms. We multiply this by 3, to get 1.11 mtrs.
So we should not have any sound source or listener within a distance of 1.11 mtr from the boundaries of a, quadratic diffuser. A quadratic diffuser is called as a RPG - reflection phase grating. The phase of the diffused wave is offset from the direct sound. And when source or listener are within 3 wave lengths of the diffuser, then the interaction between the direct and reflected sound will cause a tingy note to the treble and will damage imaging of the system.
I have previously read that diffusers have to be placed at a minimum of 8 ft from the listener. Then recently on a different forum, a designer of studio's mentioned that he would never use a quadratic diffuser, unless the listener was at least 10 millisecs from the quadratic diffuser. That translates to 3.4 mtrs or 11.3 ft. But since he was referring to a studio where 100% acoustics are required, I would like to think that for a domestic small room 8 ft might be doable
5. 2D or skyline diffusers - Where room size doesn't permit quadratic diffusers, due to close proximity of speakers or the listener, the skyline diffusers come to the rescue. The skyline diffusers are less effective than quadratic diffusers, but don't come with a minimum distance requirement that I've seen so far. Actually skylines don't diffuse, they apparently only work via staggered reflection. So these can be used without a minimum distance requirement probably. But I would still stick with a minimum of 8 feet distance to the ear in my room, because it is possible at 2 points.
6. Another types of diffusers recommended for small rooms are poly diffusers. These look like a half cylindrical PVC pipe. And the inside can be stuffed with glass wool to absorb bass as well. These poly's don't alter the phase, and thus don't have a minimum recommended distance again.
7. I've saved the best for the last - RPG BAD - Binary amplitude diffusers - these are patented by RPG systems, and haven't seen same being offered by anyone else. These are thin, almost 1 inch thick panels, with a, mathematically calculated cuts in the baffle plate They diffuse from mid frequencies upwards and absorb bass at the same time. So these would be most ideal.
8. The first reflective point treatments, only come after bass treatment. Unless RT60 times below 400 hz are brought down below 0.5 secs. We won't be able to hear the full benefits of the panels at first reflection points.
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