This is the essence of what Harvey Fletcher and W. Human perception of loudness varies across the frequency spectrum. Once you wrap your head around relative values like decibels and the subjectivity of loudness in general, considering frequency adds yet another layer of complexity. So if normal conversation in a room is said to be at a level of 65 dBSPL, you might expect things to get twice as loud (around 77 dBSPL) during a Buffalo Bills football game, for example. Measuring loudness with an SPL meter will provide decibel units relative to the threshold of human hearing or 0 dBSPL. So -3 dBFS (FS = full scale and refers to digital audio systems) is essentially meaningless unless you are comparing it to the loudness perceived at 0 dBFS or unity gain. It’s important to realize that decibels are relative units, meaning they require a reference value or beginning value to be meaningful. Noticeable loudness differences are perceived at changes of 2 to 3 dB for most people. A doubling or halving of amplitude is often perceived when a change of 10 to 12 dB is implemented. As a practicing engineer, you will quickly get accustomed to the effect of changing the level on a track by 3 dB, 6 dB, 12 dB, etc. That is an incredibly small amount of pressure! Using absolute measures like Pascals to describe amplitude would be cumbersome and needlessly precise in terms of audio production, so decibels are used to describe amplitude levels and changes in relative terms. A Pascal (named after mathematician and physicist, Blaise Pascal) is a unit of measurement for pressure and the threshold of human hearing is considered to be 20 millionths of a Pascal in terms of sound pressure level (expressed as 20 x 10 -6 Pa or.

Amplitude, on the other hand, is something that can be measured precisely by instruments like a sound pressure meter. Perceived loudness is also highly dependent on frequency and the limitations of the listener. Loudness also varies across species, as anyone with a dog or cat can attest. Two individuals may perceive the same source to be at different loudness levels. A tone producing machine used by Fletcher and Munson ( source) Loudness Is a Subjective Concept

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