Response of Our Ears To Sound

We have all heard really loud sounds. A jet taking off, a dragster or funny car blasting away from a dead stop, or an explosion all generate large amounts of sound energy. The perceived loudness of this energy by our ears depends on the particular frequency we are addressing and the intensity at which we perceive or hear that frequency. To interpret this frequency/intensity ratio and put it in a form that more closely resembles actual human hearing, we go to what is termed the Fletcher-Munson loudness curves.

The Fletcher-Munson curves take frequency and intensity and apply this data to a predetermined domain of measurement. The Fletcher-Munson frequency start point is 1,000 cycles. The F/M loudness scale determines that the ear is the most reactive in the frequency range that starts with 3,000 cycles and goes through 4,000 cycles. This loudness scale also shows that the threshold of hearing is more reactive at lower frequencies. For example, the threshold of hearing at 60 cycles is 48 db higher than at 1,000 cycles.

Loudness of any sound is a ratio of the perceived magnitude of that sound energy by the live organism it encounters. The units or intervals the loudness scale uses must reflect real human reaction points. The units on the scale must match common human hearing experience and also match the sensation magnitude. The scale also must be constructed so that when the units on the scale increase by a certain factor the sensation magnitude of human hearing increases proportionately. If the units are quadrupled then the corresponding human hearing sensation must also quadruple.

Pitch is defined as frequency that reacts with the medium in which it is transmitted in. When sound travels through the air in a room, we have the frequency produced by the sound source and we also have that frequency reacting with the air and producing another sound. It is source sound and air sound combined. Pitch is not an objective quality but rather a subjective one. Pitch is the subjective quality humans assign to a sound in order to place it in its appropriate position on the music scale. There is a measurable difference between frequency and pitch.