Psychoacoustics

Sound Perception

Our experience is that the human ear is the most effective tool for defining whether a sound is tolerable and acceptable to an individual.

This said Sound Level Meter readings do have their place and are helpful – particularly from a health and safety perspective; however, in terms of noise loudness, and how intrusive it is, the crux is what will be heard by those working nearby.

To aid our understanding it will be beneficial to look more closely at both loudness and psychoacoustics (the study of sound perception).

Loudness

In acoustics, loudness can be defined as 'a subjective term for the sensation of the magnitude of sound'*. It is not therefore to be confused with Sound Pressure Level (SPL) which is the technical term for 'how loud things are' and is most commonly measured in Decibels or dB(A).

Quiet rack manufacturers give their soundproofed enclosure a performance rating in SPL. As previously stated, these ratings do have their place, but they are not helpful in defining loudness.

Psychoacoustics

The sensitivity of the human hearing system to audible frequency bandwidth is not flat. This means that we don't hear all sound frequencies equally.

This is a very important point to bear in mind when comparing quiet rack performance. Two cabinets may perform identically, in terms of SPL noise reduction, however, to the listener; one will be more irritating than the other.

This is because the human ear is 'tuned' to be more sensitive to mid-range frequencies (1-4 kHz) and less sensitive at high and low frequencies.

So, even though two cabinets have similar SPL ratings, if one is less effective in absorbing mid-range frequencies than the other this cabinet will be perceived as being louder and more intrusive to the hearer.

* Reference: www.owenscorning.com