Originally appeared in the May 1989 issue of Installation News magazine.
Nowadays, it seems, even the terminology used in the auto sound industry is more than enough to baffle the brain and dull the senses. Buzz words like anti-aliasing, seventh-order, over-sampling, differentially balanced and others are common place in the vocabulary of almost every stereo buff. There does seem to be one exception, however, and that is the term "weighting network."
A common misconception is that the weighting network is a discount phone service for people on a diet. Seriously, weighting networks play a crucial role in the measurement of sound and understanding the role they play will more than likely prove to be a valuable asset in the future.
In order to appreciate the need for weighting networks, it is first necessary to acquire a basic working knowledge of the way we perceive sound. Most significant is the fact that our ears are not equally sensitive to all frequencies, and although they may appear to work very well, their frequency response is actually far from "flat." To complicate things even further, this variance in hearing sensitivity is more pronounced at low Sound Pressure Levels (SPLs) than at high ones.
These variations are clearly visible in the family of curves shown in figure 1. These curves, known as Equal Loudness Contours, represent the SPL required at any given frequency to create the same perceived loudness as a 1 KHz tone. For example, to give the same apparent loudness, a 40 Hz tone would have to be 15 dB higher than a 1 KHz tone at 80 dB, and almost a full 40 dB louder than a 1 KHz tone at 30 dB.
Upon closer inspection, it becomes self evident that Equal Loudness Contours are in reality nothing more than inverted frequency response curves of the healthy human ear. It is also quite clear that the frequency response of the ear tends to be "flatter" at the higher SPLs. These qualities, in addition to other complex factors, resulted in the development of various weighting networks.
These networks, commonly referred to as "A", "B", and "C" weight filters, are used to tailor the frequency response of electronic test equipment in such a manner as to simulate the ears perception of sound. In other words, it allows test instruments such as SPL meters and Real Time Analyzers to "hear" sound the same way we do.
In a typical SPL meter, the weighting networks are located as shown in figure 2. In this example, sound is picked up by the microphone and then amplified by the preamplifier section. Next, the signal is passed through the appropriate weighting network. From here, the weighted signal is then further amplified and presented to the RMS detector circuit. Finally, the results of the test are displayed on the digital read-out.
The type of weighting network employed; "A", "B", or "C", is usually selected via a switch on the control panel of the test instrument. Figure 3 represents the frequency response curves of these three internationally recognized weighting networks.
For the most part, "A" weighting is used for relatively low level SPLs in the range of 20 to 55 dB and its response curve correlates most closely to the ears frequency response at these low listening levels. "B" is used for measuring moderate SPLs between 55 and 85 dB and the "C" weighting network is used for measuring SPLs of 85 dB or more.
It is important to note, however, that any weighting network can be used at any SPL. For instance, the government uses the "A" network when measuring noise in industrial environments and at airports. In most cases, the type of network used will almost always be determined by the circumstances involved and/or a clearly defined set of rules and regulations.
Whatever the case may be, accurate measurements will almost always require a simple understanding of these networks. For, without it, a weighting network might as well be a news channel on cable TV.