Click here to purchase the entire book in PDF format. Noise Reduction It's possible in some specific cases to use equalization to reduce noise in recordings, but you have to be aware of the damage that you're inflicting on some other parts of the signal. High-frequency Noise (Hiss) Let's say that you've got a recording of an electric bass on a really noisy analog tape deck. Since most of the perceivable noise is going to be high-frequency stuff and since most of the signal that you're interested in is going to be low-frequency stuff, all you need to do is to roll off the high end to reduce the noise. Of course, this is be best of all possible worlds. It's more likely that you're going to be coping with a signal that has some high-frequency content (like your lead vocals, for example...) so if you start rolling off the high end too much, you start losing a lot of brightness and sparkle from your signal, possibly making the end result worse that you started. If you're using equalization to reduce noise levels, don't forget to occasionally hit the ``bypass'' switch of the equalizer once and a while to hear the original. You may find when you refresh your memory that you've gone a little too far in your attempts to make things better. Low-frequency Noise (Rumble) Almost every console in the world has a little button on every input strip that has a symbol that looks like a little ramp with the slope on the left. This is a high-pass filter that is typically a second-order filter with a cutoff frequency around 100 Hz or so, depending on the manufacturer and the year it was built. The reason that filter is there is to help the recording or sound reinforcement engineer get rid of low-frequency noise like ``stage rumble'' or microphone handling noise. In actual fact, this filter won't eliminate all of your problems, but it will certainly reduce them. Remember that most signals don't go below 100 Hz (this is about an octave and a half below middle C on a piano) so you probably don't need everything that comes from the microphone in this frequency range - in fact, chances are, unless you're recording pipe organ, electric bass or space shuttle launches, you won't need nearly as much as you think below 100 Hz. Hummmmmmm... There are many reasons, forgivable and unforgivable, why you may wind up with an unwanted hum in your recording. Perhaps you work with a poorly-installed system. Perhaps your recording took place under a buzzing streetlamp. Whatever the reason, you get a single frequency (and perhaps a number of its harmonics) singing all the way through your recording. The nice thing about this situation is that, most of the time, the hum is at a predictable frequency (depending on where you live, it's likely a multiple of either 50 Hz or 60 Hz) and that frequency never changes. Therefore, in order to reduce, or even eliminate this hum, you need a very narrow band-reject filter with a lot of attenuation. Just the sort of job for a notch filter. The drawback is that you also attenuate any of the music that happens to be at or very near the notch centre frequency, so you may have to reach a compromise between eliminating the hum and having too detrimental of an effect on your signal.