Passive room treatments
From DRC
Passive room treatments mean things like:
- Bass traps
- Sound absorbers
- Sound diffusors
- Helmholtz resonators (reduce sound at one specific freq)
- Room contruction techniques
- Speaker and listener positioning
All of these things can be used to (sometimes dramatically) improve the listening experience.
Do they work?
Without a doubt we must first point out that traditional room treatment is far more powerful and a potentially superior solution to digital room correction.
However, it's not an exclusive option. We can combine both of these solutions and reap the advantages of both
Pros
- Fixing the problem at source, not trying to "pre-mangle" the sound so that after the room "re-mangles" it comes out the way we wanted at the beginning.
- The improvements are available on a much wider listening area than that provided by electronic correction.
Cons
- This is seriously non-trivial stuff to design.
- You need treatments which are designed and specific for your room (and often placed in very specific locations)
- For bass treatment these devices are by necessity quite large and potentially intrusive (low SAF)
- It's very easy to make things worse not better (although the same is true with DRC)
- Hard to find good quality contractors who can design and build the required devices for you
- If you are installing these things after the fact (ie can't build them into the room design), and your partner doesn't like "hifi clutter", then you may be seriously restricted as to whether you can implement these treatments at all.
So the main point to bear in mind is that these traditional room treatments can be large and bulky unless implemented in the room when it is constructed. DRC on the other hand is trivially easy to implement and basically just requires you to plug in another box. However, DRC is correcting things AFTER the sound has been "distorted" by the room
Dr Earl Geddes on room treatment
Earl Geddes is the co-author of "Audio Transducers" an extremely comprehensive book on audio matters which also goes into high quality. room construction techniques. He is also known for his research on horn based loudspeakers, these are much more highly directional than traditional loudspeakers and this can diminish a number of problems due to room issues. I asked him to summarise his view on tradional room treatments
Earl on room treatments
Thanks. I really wish that I did have time to delve into this further but I don't. I will have to say out front that I do deal with these topics in depth in my books and so I would reference those for a more in depth discussion.
General room construction - whats bad, what causes the problems
The biggest problem that I see is rooms that are dead at HF and live at LF - the exact opposite of what you want and exactly what you get if you liberally place typical damping material around the room. This causes LF booming and spatial smoothness problems and a lack of presence or spaciousness at the HF.
Speaker positioning - why it matters
There are two aspects here. The first and most important is imaging - dominantly a HF issue. Speaker design and placement almost completely determine the systems imaging, the room is a secondary effect and nothing else matters. Good speakers will image poorly if poorly placed and poor speakers will sound poor no matter how they are placed. All too often I see people buy good speakers which are then placed by their wife's. I like wife's as much as the next guy, but if I'm going to spend a lot of time and money on speakers then I want them placed correctly. The interaction between the room and the speakers is complex and covered in my books as well as a free white paper on my web at http://www.gedlee.com/new_loudspeaker.htm I see no reason to repeat that discussion here.
At low frequencies, assuming the woofers can be located separately from the main speakers, it's a statistical problem. The variation of sound levels (about the mean response) with frequency at a point and spatially around the room will go down as 1/N where N are the number of sources. In my setups I use 15" woofers for the three main speakers and down-mix a lot of LFE to those channels - they can handle it with ease (done in AC3 playback). Then I place a sub in a corner somewhere and another sub as far away from the first as possible, but not in another corner. Now with 5 woofers the sound field variation will be 1/5 that of a single sub-woofer - without doing anything to the room. If the room is well damped at LF then the spatial response and the frequency response at any point are extremely smooth. The subs need not be large since there are a couple of them and they are only smoothing out the response not carrying the whole low end. I use 12's (2). I also mix them up with ported and closed box designs. (R&L - closed, center ported, 1 sub closed other ported.)
Bass problems - how to tackle them. Resonators, traps, absorbers?
I think that this was answered above. Resonators and absorbers are exactly what I do use, except that I build them into the room boundaries rather than add them in as fixtures. A freely suspended damped wall (see my texts) need only be a few inches thick and will add a whole lot of LF damping. In fact, done right, no one would ever even know that it was there.
Floor, ceiling, wall and other furniture reflections - the problem and some solutions
I am coming to the conclusion that any nearby reflections and diffraction from the speakers, off the cabinets, walls, bookshelves, stands, whatever, are detrimental to the imaging and sound quality. These nearby reflections must be minimized. Easily done if the speakers are behind a curtain and nothing else is back there except damping material on all sharp edges and the back wall. This is the ONLY place that I use much damping, with one exception, the side walls - small tapestries, otherwise the room is stone or wood. Even the furniture is not heavy and well damped. Its very easy to over-damp a room at HF. Carpet is evil, wood floors look better, last longer, and sound better. My ceilings are wood - suspended. A small throw rug can kill the first floor bounce. The ceiling bounce has not offended me as much as lateral reflections and this is well documented in the literature. Remember that with directional speakers all of these early reflection problems are minimized.
Now after the first arrival, behind the listener, there should be a lots of diffraction surfaces etc. to diffuse the sound.
Bang for buck - what to do first, what to add to the to do list for later
Well to me, if you don't have good high directivity loudspeakers everything else is just frosting on a poor cake. Once you have a good set of controlled directivity loudspeakers, correctly placed, LF absorption is first, then minimize the early reflections and diffractions nearby to the speakers. Actually, compared to the cost of a good set of CD loudspeakers, everything else is just frosting on the cake. Only now the cake is worth sampling.
In Tulsa at the GPAF (see http://greatplainsaudiofest.com/2005/), where my speakers were first shown, everyone else was complaining about the poor rooms. I thought my speakers actually sounded pretty good, seeing as there were no room treatments and no choice in placement. In essence the speakers dominate the situation and getting those right has to be first. After that you do what you can to the room.
