Acoustics

Room Acoustics
Acoustics refers to the effects the room surfaces, gear, furniture, volume and dimensions have on sounds produced within that space. The acoustics determine the amount and types of distortion, reverberation, flutter, frequency dependance, "coloring" and other anomalies that alter the sound from instruments and speakers. The acoustics will also determine how difficult it will be to record and mix music.

Reflections

Reflection off walls produces echos (and reverberation, flutter, distortion, etc.) which we don't want but speaker grille cloth is very open which is absolutely necessary. To achieve balanced acoustics in listening, mixing and recording studios it is necessary to tame the typically massive amount of reflections created by flat, hard walls and corners. Reflections cause the sound energy to "build up" due to phase addition at room resonant frequencies and the only way to control those reflections is by adding absorption.

Absorption
Acoustic absorption is simply the loss of sound energy by transforming it to heat; the frequency of heat is simply too low to hear. To do this, a sound pressure wave must physically move material so it loses energy by means of simple friction. Hard surfaces, like walls, simply reflect the sound with very little loss. Open (porous) materials let the sound pass through without effect and so have very little loss. Absorbers dampen these reflections so all we hear is the original source from the instruments or speakers, not reflections off walls, ceiling or objects which arrive later.

Room Volume
Most acoustic spaces acoustically "feel" no larger than they are and often "feel" much smaller. This is because of all the reflections. Concert halls and outdoor stages are much larger but also "feel" much larger acoustically. To get a small space to "feel" bigger than it is (or at least as big as it is) requires acoustic "treatment". This simple means eliminating (or at least substantially reducing) reflections which produce either non-uniform frequency response of the room (one or many different frequency are louder or softer than others) and eliminating (or substantially reducing) acoustic "hot" and "cold" spots in the room where specific frequencies dramatically add or cancel -- making the music from the speaker "muddy", bass too "boomy" or the music distorted. So, where are the problems?

Walls
Generally large flat areas, walls easily reflect sound. You hear sound from the speaker, but you also hear that sound as reflected off one or more walls -- but the two do not arrive at your ears at the same time. The two sound pressure waves can add, cancel or anything in between and that effect changes with frequency. It's "messy" to describe and it sounds "messy" as well. So, absorption is added to kill the reflection so you only hear the sound direct from the speaker.

Corners
Think of corners as the line created where two flat surfaces meet; not just the point where two walls and the ceiling or floor meet. Yes, the wall-ceiling-wall point is a corner, and is sometimes called a "tri-corner", but the 8 other wall-wall, wall-ceiling and wall-floor corners have a lot more surface area. This is important -- acoustic treatment is all about surface area. A 9x10x8 (feet) room has a surface area of 720 (sq-ft) and if a corner is defined to be 1 foot either side of a wall-wall intersection line, the 8 corners have 128 (sq-ft) or nearly 18% of the total surface area.

Corners are acoustically important because the Bass frequencies (300 Hz and below) tend to "concentrate" in corners because of the rectangular geometry. This makes corner the ideal location for more efficiently absorbing the excessive bass response of small rooms. So, how much sound energy must be absorbed?


Modes
Room modes are just idealized acoustic frequencies at which the length of the sound wave, or some integer or simple fractional ratio of that length, matches some room physical dimension. Sound travels at about 340.3 meters/sec or about 1,115 feet/sec. If a room were 11.15 feet long, the first, fundamental mode between the two parallel end walls would be 100 Hz. But there would also be modes at 200 Hz, 300 Hz, 400 Hz, etc. -- but not all equally effective or efficient. If you imagine sound like a billiard ball, and a room as a 3D billiard table, there can be many reflection paths where the sound bounces back to where it started and is in "sync" (same phase) so the sound pressures will be reinforcing. It is now easy to realize that some reflection paths can be at least twice the distance between opposite corners. In the 9x10x8 room, this could be about 15.6 feet between "tri-corners" with a reflective path about 31 ft -- which could easily produce a room mode at 71 Hz or its half harmonic of 35 Hz. Clearly, corners are an issue for Bass response exaggeration and need absorption.

Ethan Winer's company, RealTraps, has a free software app (called ModeCalc) for download. Entering in this Vary Small Studio room dimensions produces the following plot. Without much difficulty it is easy to see that room reflection modes start below 60 Hz and that the room is densly packed with resonance modes from 200 - 500 Hz. This room clearly needs absorption to tame all these reflections.



Dead or Alive
If all flat surfaces and all corners are covered with absorbers that would kill the room acoustics and it would sound "lifeless". Lack of room resonances does help attain "un-colored" recordings. However, the objective is a "balanced" room which 1) sounds natural, 2) sounds larger than it really is, 3) that gives no added "coloring" and one which serves multiple uses -- recording, mixing and critical listening. While there's no expectation that these goals are truly achievable with any Very Small Studio, it's still useful to have clear goals.

LEDE vs RFZ
While not the acoustic cure-all solution, in the 70s Don and Carolyn Davis described a new approach for studio design -- Live-End-Dead-End.  The concept was to use heavy absorption at the speaker end (the "dead end") of the room and diffusion reflection at the listening end (the "live end") of the room. In the 80's, Cox and D'Antonio contributed a modification based on placing the listening position about 1/3 of the way into the room from the "Live-End". This listening position should be a Reflection Free Zone.

On the surface, these concepts are logical. Place enough absorption around the sound source end (the speakers) to prevent the listener from hearing both the source and reflection of the source arriving very close in time to each other. Side walls, between the speaker end and the listening position, should have good broadband absorption -- specifically at the "first reflection points". Reflections from the "Live-End", behind the listening position, should be delayed sufficiently in time, and spatially and temporally diffuse, so as not to confuse the listener's sense of localization.

Solution
The solution for the Vary Small Studio includes corner bass absorption, wall and ceiling absorption and wall diffusion opposite the speakers. The small room size will impose many limitations, but the approach is still valid and, as measurements will show, this strategy works.