Interior Acoustics is about the airbone sound inside rooms, i e hows it propagates and interacts with the room’s surfaces and objects. But it’s also about our perception of the room’s acoustic properties.
At Room Temperature sound propagates with 344 m/s (1,250 km/h), which results in many reflections at walls or objects in a normal-sized room before a sound wave is damped below the hearing threshold. Inside the room we can only affect the sound wave at its reflections in walls or objects. Three things can happen with the incoming sound energy:
Reflection. A Hard Surface, Such As Concrete, Glass Or Wood, Acts As A Mirror For The Sound Wave And Thus Reflects It.
Absorption. A Sound Wave Can Propagate Into A Porous Material Where It Is Transformed Into Heat By Viscous Friction.
Scattering. The Sound Wave Is Reflected In An Unordered, Almost Random, Way.
All of theses three processes are important in creating the acoustics of a room.
Most practical objects includes all three, but to a varying degree. A completely flat glass surface has very little absorption and scattering, its acoustic characteristics is dominated by pure reflection. An upholstered sofa, on the other hand, is dominated by absorption and scattering due to its softness and shape.
The Acoustic properties of a room should be designed to support the activity in the room. Good room acoustics are seldom noticeable. Since there are so many different activities that can take place in rooms there are no such thing as universally good room acoustics. Each activity has its own challenges, and consequently its own acoustic design. To help acousticians in design of room acoustics a number of measures have been developed, and the most common ones are mentioned in the following.
The most common measure in interior acoustics is the reverberation time.
The reverberation time can be said to be a measure of a room’s echo since it is the time that it takes for a sound to decrease with 60 dB from its original sound level. The reverberation time (T) can be calculated simply from the room’s volume (V) and absorption area (A) through Sabine’s formula, T = 0.16 V/A
The absorption area can be understood as the amount of “perfect” sound absorber inside the room and is commonly measured in m2S (square meters Sabine). A very important assumption for Sabine’s formula is however that all the room’s surfaces must have equal sound absorption, or that the room must have a diffuse sound field. A diffuse sound field can be understood as a completely unordered or random sound field. A practical consequence is thus that a room must have a large amount of scattering properties, or a large amount of diffusion, to be suitable for use of Sabine’s formula.
Additionally there are many other acoustic measures that can be used when designing room acoustics, e g STI (Speech Transmission Index), C50 or C80 (Clarity) and G (Room strength).