How does one know when to stop putting in room treatments?

In many ways it is very subjective. Subjective in the sense that the amount of treatment follows the intended use for the listening environment. One can go for as little as 2 panels if the existing space is already pretty dead. e.g. heavy drapes, heavy carpeting, lots of plush furniture, drywall. In high rise condominiums typically finished with CHB and poured concrete all around upwards of 30% surface coverage is not unheard of. In other words if one is trying to do Mickey Hart's drum kit at home, get those panels and bass traps ready. If it's Julie London perhaps not anywhere near as many.

Ultimately what at least I shoot for is neutral vocal presentation or what I refer to as perceived flat in the midband. If the voice becomes chesty then there probably is too much high frequency absorption. We all know what the voices of friends and relatives sound like so the test is if after treatment I can speak softly with someone I know across the room while being able to hear what the other person is saying very clearly. Naturally he has to sound like himself as I would hear his voice in a wide open space and not get morphed into Barry White. That's the first and most important hurdle. This simple test shows that mids and highs attenuation is fairly even, the lack of smearing of consonant sounds shows that reverberation has been decreased and the noise floor has dropped as well in the process. Small ensemble recordings with little bass should improve greatly at this point. As bass output increases, then things get more complicated. As nodes become excited the room can be acoustically overloaded. In extreme cases of overload, bass in the room actually diminishes only for it to boom in an adjacent room or hallway. The accumulated pressure seeks an exit in effect turning that hallway into a horn. The addition of bass trapping becomes mandatory for systems with high output low end capability in rooms where the longest dimensions are less than half of the wavelength of the lowest frequency that the system is capable of. One can look at a room as the inside of a speaker cabinet. Just like the inside of the speaker cabinet excess energy needs to be damped lest the cabinet itself resonate. Depending on many factors including building materials and construction room resonance can actually begin at what might seem benign levels. The common approach is to convert that excess energy into heat by way of the little fibers in the treatments that slows down the excited air molecules through friction. The second most common technique is to break up the pressure wave like quadratic diffusers do where force is distributed over a wider area and cancellations occur at the areas of deflection inside and around the cavity mouths. Leaving only a portion reflected directed back to the listening position.

I believe the question is however how can one tell if he's gone overboard. Well for one, if speaking voice or music no longer sounds natural it's time to take some down little by little. If your 30Hz capable floorstander starts sounding like a two way monitor then perhaps there might be too many bass traps.

So all things being equal, in the end it is up to the individual to decide how much reflected sound he wants in his room along with the direct sound. Being a purely personal decision. IMO there is no better method than trial and error the first time out or contracting a professional whichever comes cheaper

caesar said:

Reading the superb Realtraps website, one is led to believe that more absorption is better. But when does one stop? Does one really want a flat measuring room? Is this a "subjective" decision? When does one start switching the absorptive treatments to diffusion?

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First, I divide the audio range for rooms at about 300 Hz. I don't think it's possible to have too much absorption below 300 Hz. At least not in a typical domestic size room. A concert hall or even a smallish recital hall can sound good with longer LF decay times. But in a home-sized living room the decays will be too frequency-selective to sound good.

Above 300 Hz you can have a little ambience, a lot, or none. What's best depends on the size of the room. In a small room all of the reflecting surfaces are very close to the speakers and to your ears, so the reflections are both loud and "early." To my ears, small room ambience sounds lousy, so it's best to mostly kill it.

When to add diffusion depends more on budget than on the room requirements. Good diffusors can make a small room sound larger, and can avoid other mid and high frequency problems without absorbing sound energy. But good diffusors are complex and thus expensive, whether you buy commercial or DIY. Absorbers and diffusors both solve the same basic problem of troublesome reflections, so it's not like some room must have diffusion.

--Ethan

Apropos to other forum threads, this topic seems to scream out for actual measurements to

a) determine the type of treatment and

b) Determine actual results from incremental treatments

How can one determine what room treatments are appropriate to each unique room/system combination without measurements?

It is very computation intensive JD. With a ground up build there is nothing to measure yet. There are however tried and tested design principles that the architect/acoustician can draw on to start with to get as close as possible to predetermined design targets. In very small rooms where distance from source to boundary to listening position fall within the precedence effect time window you really will get ugly room tone as Ethan says above. This requires percentage wise even greater surface coverage. It is not an exact science yet though so even in spaces 50 sq/m and above there can be very large discrepancies between target frequency responses. Most of the time these come in the form of suck outs. Now to determine these discrepancies, IMO there is nothing better than a high resolution RTA. These did not exist in the days the great concert houses were built so points for trial and error but the way I see it, I could eyeball distance from fairway to pin but given a rangefinder I'd use the latter, never mind if I'm an amateur and can't hit the exact distance with my clumsy swing. The added confidence is always a good thing especially in the sense that now you have a way to see if you got what you paid for whether the treatments are DIY, aftermarket or custom built.

A theory as to why these discrepancies appear despite knowing the absorption coefficients of practically every building material is that the basic assumptions do not take into account the points where sound escapes. Discrepancies are often smaller in rooms specified to have lower sound transmission. Like water seeking out cracks on it's way to find it's own level sound will find any way out. The typical culprits are air conditioning ducting, seams in doors and windows and any physical connection boundaries have to the outside world. Rooms specified for low sound transmission go as far as minimizing the latter creating what is closest to a closed system hence hew closer to the basic design precepts.

A well designed and built room will accommodate many different types of speakers and in almost every instance will allow these speakers to sound much better than an untreated room. This manifests itself not just in frequency response but also in a lowering of thermal distortion and even fatigue. Every lowering of 3dB of the noise floor means you need approximately half the amplifier power to dig out the low level details. Talk about a great way to ensure both happy and safe long term listening I guess what I'm saying is that a room need not be designed specifically for a specific loudspeaker. Even if one were to go from a point source to a dipole resulting changes in treatment arrangement will be very minor.

JackD201 said:

I... I guess what I'm saying is that a room need not be designed specifically for a specific loudspeaker. Even if one were to go from a point source to a dipole resulting changes in treatment arrangement will be very minor.

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I'd beg to differ, as one needs quite different treatment between those two. For instance, my setup of dipole line source MartinLogans with an aggregate 72 square feet of radiating area needs totally different room treatments than a monopole point-source setup in the same room. If I replace my monoliths with a KEF UniQ speaker setup, I'd have way to much high and mid-bass absorption in my room.

Now, below 300hz, I'd agree, not much difference, as bass is the biggest headache in samll rooms, and if well treated for that range, then not much difference between speakers. But above that, it really depends.

If almost all the treatments are up in the ceiling soffits, a-la Superchunks, is there any chance of over treatment?

I would assume not, as you're covering a pretty small surface area