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The range of sound pressure levels or the dynamic range of human hearing encompasses a factor of
10,000,000. This enormous range of values is difficult to handle and additionally does not represent the
subjective perception of sound, since human hearing tends to use a logarithmic curve. When an increase in
loudness by the factor two is perceived as one step, four times the loudness level equals two steps. So, the
decibel is a unit of measurement that describes a level in relation to a reference quantity. To make clear
which reference quantity is meant, the abbreviation SPL (sound pressure level) is sometimes used together
withdB. Starting with a value of 0dB SPL (= 2*10
-5
Pa) for the threshold of audibility, anydB values can be
calculated by means of the following formula:
L
p
p
=⋅
20
2
1
log
L = e.g. the absolute sound pressure level indB SPL
p
1
= e.g. a reference sound pressure of 0.00002 Pa
p
2
= the sound pressure (in Pa) produced by the sound source to be calculated
log = decimal logarithm.
0
20
40
60
80
100
120
140
160
Threshold Of Audibility
Falling Leaves
Recording Studio
Quiet Apartment
Normal Conversation
"Loud" Office
Power Drill
Threshold Of Pain
Jet Engine
Machinery Hall
Sound-Pressure Level (dB SPL)
Fig. 4.1: Dynamic range of human hearing
As can be seen, human hearing has a very wide dynamic range of about 130dB, which surpasses the range
of a DAT or CD player with an approximate range of 96dB. From a physical point of view, a 3dB boost
corresponds to an increase in power by the factor 2. However, the human ear perceives a signal to be twice as
loud as before only if it is boosted by about 10dB. This will give you an idea of the range of boost or attenuation
that can be realized with the 24 filters of the FEEDBACK DESTROYER PRO. For each filter, you can apply a
boost of +16dB or a cut of -48dB, i.e. you can boost the selected frequency by the subjectively perceived
factor 3 (physically ≈40) or attenuate it by the factor 27 (physically ≈60,000)!
The sound which an EQ produces not only depends on the selected frequency and the amount of gain (ex-
pressed indB); the bandwidth of the filters also plays an important role. Here, we generally use the so-called
absolute bandwidth of a filter, which is measured from the lower to the upper cutoff frequency. Starting from
there, you can divide the absolute bandwidth by the filters center frequency to calculate the relative bandwidth.
The quality factor (Q) is simply the reciprocal value of the relative bandwidth.
4. TECHNICAL BACKGROUND