Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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Figure 4.11 Equal loudness contours
(Figure derived from a program by Jeff Tacket, posted at the MATLAB Central File Exchange)
With the information captured in the equal loudness contours, devices that measure the
loudness of sounds for example, SPL meters (sound pressure level meters) can be designed
so that they compensate for the fact that low frequency sounds seem less loud than high
frequency sounds at the same amplitude. This compensation is called “weighting.” Figure 4.12
graphs three weighting functions A, B, and C. The A, B, and C-weighting functions are
approximately inversions of the 40-phon, 70-phon, and 100-phon loudness contours,
respectively. This implies that applying A-weighting in an SPL meter causes the meter to
measure loudness in a way that matches our differences in loudness perception at 40-phons.
To understand how this works, think of the graphs of the weighting as frequency filters
also called frequency response graphs. When a weighting function is applied by an SPL meter,
the meter uses a filter to reduce the influence of frequencies to which our ears are less sensitive,
and conversely to increase the weight of frequencies that our ears are sensitive to. The fact that
the A-weighting graph is lower on the left side than on the right means that an A-weighted SPL
meter reduces the influence of low-frequency sounds as it takes its overall loudness
measurement. On the other hand, it boosts the amplitude of frequencies around 3000 Hz, as seen
by the bump above 0 dB around 3000 Hz. It doesn’t matter that the SPL meter meddles with
frequency components as it measures loudness. After all, it isn’t measuring frequencies. It’s
measuring how loud the sounds seem to our ears. The use of weighted SPL meters is discussed
further in Section 4.2.2.2.
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