Volume, Loudness and Frequency
It is generally accepted that humans can hear sounds in a range between 20Hz and 20Khz. There are, of course, exceptions where a person can hear frequencies beyond this range. But even those are not immune to the eventual deterioration of the ability to hear these sounds as a person gets older.
Our brains can handle such a wide range of pressure because they deal with volume changes as ratios rather than absolute amounts. A sound that would be absolutely jarring in a quiet setting is practically ignored in a noisy one. Since our brains hear loudness as ratios, we can’t describe how loud a sound is unless we relate it to some other sound. Scientists have agreed on standardized “other” sound, nominally the softest thing an average healthy person can hear. This is often called the “threshold of hearing”.
Reflection, Absorption and Diffusion
When coming in contact with a surface, there are three things that can happen to sound.
Sound can be reflected — it can bounce off hard surfaces – air molecules can’t relieve their pressure by moving the wall or ceiling, so they spring back the opposite direction – and the reflective waves mix with the original ones. If we hear the reflection later that about a tenth of a second after the original sound, it’s perceived as an echo. In some cases, the sound keeps bouncing between surfaces in the room; many echoes, randomly spaced are heard as reverberation. If the sound can travel a distance between bounces, and the space has lots of different spaces to bounce off, the reverb sound richer. But they can also interfere with intelligibility or fidelity, which can be one of the hallmarks of amateur sounding tracks.
Sound energy can be absorbed by the surface. Some of this absorbed sound is converted to heat while some will pass through the surface. The remaining sound that is reflected reflected, assuming there is still any, ends up getting weaker. Whereas hard and dense surface tend to reflect sound, softer fibrous materials are more effective at absorbing sound.
Sound can also scatter in different directions as it reflects off a surface with the energy remaining inside the room. This is important for evening out reflections and keeping the sound intelligible.
Understanding these three behaviors will be important in dealing with room acoustics during your recording and mixing process.