ponse of C- message and psophometric weighting. Now we want to distinguish between flat response and weighted response. Of course, the curves in Figure C.1 show weighted response. Flat response, regarding a voice channel, has a low-pass response down 3 dB at 3 kHz and rolls off at 12 dB per octave. An octave means twice the frequency, SO that it would be down 15 dB at 6 kHz and 27 dB at 12 kHz, and SO on. The term flat means equal response across a band of frequencies. Suppose a flat network has a loss of 3 dB. We insert a broad spectrum uniform signal at the input to the network. In the laboratory we generally use "white noise." White noise is a signal that contains components of all frequencies inside a certain passband. We now measure the output of our network at discrete frequencies and at whatever frequency we measure the output, the level is always the same. Figure C.1 shows frequency responses that are decidedly not flat. We return now to the problem of noise in the voice channel. If the voice channel is to be used for speech telephony, which most of them are, then we should take into account the annoyance factor of noise to the human ear. Remember, when we measure noise in a voice channel, we look at the entire channel. Our noise measurement device reads the noise integrated across the channel. As we said, certain frequency components (around 800 Hz or 1000 Hz) are more annoying to the listener than other frequency components. Psophometric C-message 10,000 Frequency in hertz (Hz) Figure C.1 Line weightings for telephone (voice) channel noise. LEARNING DECIBELS AND THEIR APPLICATIONS It is because of this that we have developed a set of noise measurement units that are weighted. There are two such units in use today: 1. C-message weighting, which uses the unit dBrnC, 2. Psophometric weighting, which more commonly uses the numeric unit, the picowatt (pWp) psophometrically weighted. One interesting point that should be remembered is that the lowest discernible signal that can be heard by a human being is -90