Acoustics

Acoustics is the science of the sound wave's behavior. It is based upon the interaction of the sound waves in spaces with surfaces, where the sound is reflected, absorbed, or diffracted. Psychoacoustics is the study of how humans hear and perceive sound.

Direct sound is the first wave that reaches a listener's ears, directly from the sound source.
Early Reflections are the sound waves that bounce off the walls or other objects in the room and reach the listener withing 30 ms after the Direct sound. These Early reflections combine with the Direct sound to create the entire Initial sound.
Reverberant sound is comprised of the rest of the reflections as the amplitude and time between them decreases.

Early reflections add loudness and fullness to the direct sound and help define the subjective size of the room. They also hold the most energy of the sound.

Reverb Time (RT) is defined as the time it takes a sound to decrease 60 dB.
A longer RT gives the perception of a larger and hard-surface room, as well as being farther from the sound source. An RT of 1 sec or more is considered "live", while an RT of 1/2 sec or less is considered "dry".

An Echo is a distinct repetition of the Direct sound, and therefore occurs 35ms or more after the Direct sound, so that it will not be combined with the direct sound like early reverberations are.

The Haas, or precedence effect, occurs when two identical sound waves of the same intensity come from two separate sources, but one is delayed by 30-40ms. The sound emitted by the closest location is heard first. This creates the impression that the sound comes only from that closer location. The Haas effect is what allows us to perceive sounds as coming from between two stereo speakers.

Acoustic aspects:
Warmth - achieved with lower frequencies and increased RT
Intimacy - early reflections around 15-20 ms give a sense of closeness to the source
Clarity - the ability to hear sonic details, such as lyrics and attack of notes, when there is more direct sound than reverberant
Auditory Source Width (ASW) - the perception of the sounds width versus the actual, which is affected by the room, the microphone used, and the panning

Reverb Times used for:
Speech = 0.3-0.5 sec
Pop/Rock = 0.5 - 0.6 sec
Orchestral = 1.5 - 2 sec

Studio design is based on 4 factors: Sound Isolation, Room Dimensions, Room Shape, and Acoustics. All four have the ultimate goal of lowering noise - which is essentially any unwanted sound.

Noise Criteria (NC) is a rating system used to identify background noise levels. Similar to the Equal Loudness Curves, there is a graph that shows the dB needed for every frequency level and different NC values.

Isolation - Keep noise out and the sound source from going out of the studio.
Dimensions - The dimensions of a room add coloration to the recording, as certain frequencies are reinforced through resonance. Resonance occurs when a material has the same natural frequency as the sound wave and results in vibrating together in phase so that the amplitudes increase. Remove resonance in a room by creating Height X Width X Length ratios that are not multiples of one another (ie 10 X 20 X 30).

Shape - Parallel walls reinforce sound waves, since the angle the sound wave hits the wall equals the angle of reflection and bounces right back to the opposite side. To break up these parallel reflections, design the room with non-right angle corners or rounded walls.

Acoustics - Different surfaces interact with sound waves differently - they may absorb, reflect, diffract, or diffuse the sound. Absorption soaks up the sound, which results in a lifeless, dry sound. Reflection results in a complete bouncing off of the sound from the material, giving reverberation. Diffraction bends the sound around the surface, while diffusion reduces the amplitude of a wave and spreads it in time.

Link to more information on sound diffusion
Link to more information on sound diffraction

The Sound Absorption Coefficient goes from 1.0 for complete absorption to 0.0 for complete reflection. Different materials are assigned a value in this range based on how they react to sound.

Listen to interesting binaural microphone effects here