[RCduck7]

Starting on which frequency range are first reflections not pleasant for the listener. Does one need absorption only for the tweeter or also for a mid/bas unit?

[jtcf]

High frequencies are directional and of short length and are easily damped by sidewall treatments. The mids and bass get progressively longer and need (naturally) heavier thicker treatments. The high frequencies that bother our ears dwell in the "presence region" between 10 - 2 kilohertz. Google "frequency range of instruments charts" to see some excellent graphics of where various instruments and vocals are located on the frequency spectrum.

[richidoo]

Starting on which frequency range are first reflections not pleasant for the listener.

100Hz. 1st reflections from front wall and floor blurs midbass frequencies, causing muddyness and over thick, chesty tone. Common problem when speakers are positioned too far out into the room to "improve imaging." Bass band wavelengths are too long and energetic to damp by absorption.

Does one need absorption only for the tweeter or also for a mid/bas unit?

Diffusion is better for mid and high frequencies than absorption. Too much spot absorption mixed with untreated surfaces makes a room sound weird. But some small absorption spots can be useful at the first reflection points of the listening position when the budget doesn't allow for that much diffusion.

For lower frequency reflection problems, consider strategies like positioning speakers closer to the reflecting wall to reduce the delay time for less smearing, and to raise the frequency up into a less noticeable band. Electronic active absorbers and passive membrane absorbers are much more efficient than traditional FG on-wall absorbers. Newer residential construction has thinner walls that easily pass bass freqs, while concrete basement walls hold bass in.

[RCduck7]

I am asking this because on my right side i have an open bookcase as a roomdivider. I will put drawers in it and have the possibility to put these drawers at 1 feet, 2 feet or 3 feet high. These drawers will have a reflective surface. I don't like putting these drawers in the bookcase as low as 1 feet unless there is an audible benefit. At 2 feet it is about the same height as the mid/bas unit and at 3 feet it is the same height as the tweeter. I would like to put these drawers 2 feet high.

[dfw]

Whenever you treat for side reflections, you want to make sure your absorb a wide bandwidth of frequencies.  You don't want to just absorb the high frequencies because that will change the frequency content of the reflection in relation to the direct sound.  You either want to absorb most of the sound or none at all.  Make sure you use an absorber at least 4 inches thick.  Don't use 1 inch or 2 inch absorbers because they will allow lower frequency sound to reflect and will color the sound in your room.

[RCduck7]

Thanks for your comment dfw. 

[WGH]

Stereophile has an excellent technical article about reflected sound and absorption. Basically what everyone has been doing for years is wrong.
I wish I had the room to experiment with PolyFlex Diffusers but there is a window with blinds at the first reflection point on one side and nothing on the other because it's an L-shaped space.

The article and the A/V Room Service site is packed with information and is thought provoking.

NWAA Labs: Measurement Beyond The Atomic Level
https://www.stereophile.com/content/nwaa-labs-measurement-beyond-atomic-level

You need a big space to measure the effects of reverberation times of a room with and without absorption.
"At very low frequencies," Ron interjected. "NWAA Labs can measure accurately down to 25Hz because it resides in a floating room inside a floating room, separated from the outside world by roughly 25' of concrete. The room's background noise at 1000Hz is an astounding –43dB!"




A few tidbits from the article:

Absorption is the most common treatment used in listening rooms. The measurement of absorption was first described by Wallace Sabine, who compared the reverberation times of a room with and without absorption. This difference was then converted to units of absorption using this formula:

A = 0.9210(V*d/c)
where
A = equivalent absorption area in m2,
V = volume of reverberation room in m,
c = speed of sound at ambient temperature and humidity in m/s,
and d = decay rate in dB/s.

Most of what we think we know about absorption is wrong! Absorption calculations in use today can err by as much as 85%. The biggest error concerns how important the area of absorption is in the calculation of how much absorption is needed.


"DeGrandis has done some unbelievable research in the field of diffusion," Ron said. "He's come up with a computer program that allows him to simulate what happens to a design when he changes parameters. "We've done the same research with diffusion, where shape is again key. I'm sorry, but almost 90% of what's out there, theory-wise, is BS. For example, you can't use a block's length in a 'skyline-style' diffuser to determine the frequency range that it affects."


So what does work?

PolyFlex diffusers. The only drawbacks I see is they are ugly and too cheap. An acoustically transparent box around them fixes the ugly part, then they are no more intrusive as absorbing or scatter panels that may or may not work.
https://avroomservice.com/

 

[Letitroll98]

A fabulous article with shocking conclusions.  So those guys with 1/2 round construction tubes had it right all along.  Thank goodness I didn't get motivated to fix my listening room before now.  If it becomes widely accepted it will upend the entire industry.  Will near field placements take over?

[robertom]

Very interesting article!
Anyway, if I'm reading it correctly (and I'm not sure I am, since I'm not a professional in room treatment), what the author says is not so shoking as it may seem. For example, about diffusion, he is saying essentially two things: the first is that the range of diffused/scattered frequencies is calculated incorrectly in common measurements criteria, and the second is that the most effective diffuser type for him is the 'poly' (and the reasoning behind is understandable). About absorption, he says almost the same thing: a lot of industrial measurements of materials (for example: batts of mineral wool) are inaccurate, and moreover the shape of the panel largely influences effectivenes of absorption (with the best shape being a star! this is really new for me!).
  But we should consider, first of all, that the best acoustical engineers in the world have always relatively relied on the manufacturer measurements of materials (absorbers) and even on their own calculation of diffusion (diffusers); they always measure (and hear) the effect of everything directly in the room they are treating. In other words, they know measurements and theoretical behaviour of a device can be wrong or inaccurate, so they always measure the actual effect they have on the room. They measure the room, not the device(s). After a while, of course, they know that a certain project is going to work for a certain kind of room. Every acoustic engineer always has (it is just a little example) his preferred materials (a certain model of mineral wool, for example). It's partly a science, partly an art.
 Moreover the article does not talk about bass frequencies (below Schroeder frequency) treatment, and it seems to confirm what is widely known, that is that a diffuser (poly or other types) do not have any influence on bass frequencies. I can understand poly instead of absorption in the early reflection points, but nowhere the article seems to suggest that you can treat bass frequencies with polys. I think that membrane and helmoltz resonators are always the way to go, for bass frequencies. Unless my interpretation of this excellent article is flawed somehow.
 And moreover: how can a poly be effective at, let's say, 200Hz? The acoustical engineer that treated my room used absorption in the first reflection points, not diffusion. And I think that the main reason is exactly this: maybe a poly can deviate most of the reflected sound from your ears (the article is surely right on this point), but it would be almost 'transparent' for frequencies below (let's say) 1000Hz, maybe something lower in the best case. A 200, 400, 500Hz frequency almost certainly would 'ignore' the poly, would pass it and bounce from the wall back to the listener ears. All of this, of course, from my very limited knowledge of the subject, but are things that are considered pretty basic in room treatment.