Room Treatment For "Small Rooms" - Part Two

Room treatment is mostly about room modes and reflections. (Sound-proofing is taken to be out-of-scope since it needs building work.)

Room modes first. These are sound waves that bounce back and forth between the walls, or floor and ceiling, because the wavelengths fit the dimensions just right. This is where boomy bass comes from.

Small rooms are held to be a lost cause because they have many, many of these resonating frequencies. That may be true, but there's a VERY important qualification: those room modes only matter if any of them correspond to one of the 88 frequencies used in the music you are most likely listening to.

Yep. There are zillions of frequencies that could be used to make music, but almost all Western Music is made with 88 of them. Here's a list. You will notice the only ones that are whole numbers are the A's from 55Hz upwards. All the rest are given to five (5) decimal places, in a scheme called twelve-tone equal temperament (which is a music theory rabbit-hole all its own). Western musical instruments are mass-produced to reproduce those notes. The chances of your room having a resonant frequency corresponding to some random note like F# above low-C (say) are approximately zero.

And if you do, all you have to do is move the speakers either back or forward a couple of centimetres (front-to-back resonance), or closer or further apart a couple of centimetres (side-to-side resonance), and it will disappear. (This is called positional equalisation.) It will not to be replaced by one on another note because a) the difference in wavelengths between any of the 88 notes is more than a couple of centimetres, and b) the resonance is between the speaker and the back or side walls, not between the front and back walls, which would be a room resonance, and your speaker is not mounted in the walls. (And even if it was, the point remains the same!)

If you have a floor-firing subwoofer, you can't fix a room mode like that, because the way the sound waves come from the subwoofer mean the resonance will be from floor-to-ceiling. Should a piece of music contain a hefty belt of 73.4 Hz D or 36.7 Hz D, both of which will pass into my subwoofer, I get a resonance. However, only five-string double-basses and instruments with names starting 'octocontra' ever get down to 36.7 Hz, leaving me with exactly one note that can trigger that resonance, and that's still way down low even for a string-bass. And no, very few pieces of music are written to include octocontra-anythings, and most orchestras would either not perform them, or use the programme or sleeve notes to apologise for the missing instrument. The lower you cross over to your subwoofer, the fewer possibilities for resonance you have.

How about all those reflections? According to the Master Handbook of Acoustics

Our hearing mechanism integrates spatially separated sounds over short intervals, and under certain conditions tends to perceive them as coming from one location.... in an auditorium, the ear and brain have the ability to gather all reflections arriving within about 35ms after the direct sound, and combine...them to give the impression that the entire sound field is coming from the direction of the original source, even though reflections from other directions are involved...This is called the Precedence Effect, Hass effect, or law of the first wavefront.

In more familiar terms, the ear has a buffer about 35ms deep. At the speed of sound that's 12m. I am two metres from my speakers. Any sound along a path strictly less than 14 metres from speaker to ear will have its sound combined with the direct sound from the speakers. That's all the first reflections in my room. So in a "small room", first reflections do not appear as separate sound sources. Instead, those reflections give the sound a sense of spaciousness which is greater as the power of the reflections increases. Reflections have to be quite loud before they are perceived as echoes. (In my "small room", the first reflections are travelling something like 3m to reach me, so they are 4/9 (inverse square law) as powerful as the direct sound, which leads to a drop of 3dB in volume and whatever absorption I get at the wall. Every little helps.)

Reflections good - sometimes. Too many and too loud, and the sound image will lose sharpness or you will get echoes. Too few and too quiet and the sound will feel muffled and dull. The trick is to get the sound quality you like.

Those with "small rooms", read on.