As most people are familiar, the sound pressures we study in acoustics are measured in decibels (dB). Decibels are measured in a logarithmic scale, based upon a reference pressure level, of usually 20 micro Pascals (or 2x10e-5 Pa). By using the equation dB=10*log(pressure/2x10e-5)^2, we can accurately translate sound levels into decibels. Another unit, more often used in reference to sound power levels, as opposed to sound pressure levels, is the bel. One bel is equivalent to 10 decibels.
When speaking of acoustics, one must be sure to differentiate between sound pressure levels and sound power levels. Sound pressure levels are recorded by microphones and other devices. This is a measurement of the amount of pressure in the air being sensed at a given location. It follows that it's value can be determined through direct experimentation. In comparison, sound power levels are a measurement of the actual energy being put into use by a given device to create noise. Because of environmental factors, and other influences, the amount of energy a device devotes to creating sound may not be equal to the actual level of the sound as it's perceived. Sound power measurements cannot be directly measured, and must be infered through other data.
There are two popular ways for scientists to perform acoustical measurements. They include a "direct method", and a "comparison method". The direct method computes sound power levels by computing an equation of environmental factors (such as room temperature, humidity, reverberation time, etc.) and sound pressure levels. A more precise implementation of this method can be found in the ISO3745 acoustics standard. The comparison method however, is conducted by measuring sound pressure levels from a reference sound source which emits a known, constant, sound power level, and then comparing that level with the sound pressure level of the object being recorded. Each way is equaly valid and accurate.
Experiments such as the two methods mentioned above are sometimes performed in reverberation rooms, or in some cases, anechoic rooms. The design of a reverberation room is to create long lasting echoes of sound waves. This helps create a highly averaged and omnidirectional sound level throughout the entire chamber. A typical example of rooms with characteristics similar to reverberation rooms are concrete tunnels, caves, etc. Anechoic rooms, such as hemi-anechoic rooms, or fully anechoic rooms are created to simulate what is called a "free field". A free field is the representation of a theoretical infinite plan, in which no sound wave reflections, or echoes, take place. In rooms such as these, the only sounds which exist are being emitted directly from the source, and are not reflected from another part of the chamber. Anechoic rooms have the characteristic of being muted, muffled, and silent.
More specialized areas of acoustics include, but are not limited to, tonal analysis, sound quality assessments, and noise control.
Subfields and related fields of acoustics: