Things to Consider about Noise Reduction
Hums, clicks, rustles, chirps, shuffles and creaks. Just a few of the sounds that your recordings could well do without. Here we provide some tips on noise reduction. Some of the following tips should already be a part of your normal working methodology. However, even if you only learn one new trick, it will have made reading this article a worthwhile investment of your time.
1. Where possible, choose a minimum signal path. All electrical circuitry, no matter how well designed, adds a little more noise to your signal, so the less circuitry there is between source and destination, the better. Using a separate mic preamp or voice channel patched directly into a recorder input can make a huge improvement over inputting via a large mixing console. If you have to use a console, consider taking a feed from the mic channel's direct output or insert send and routing that directly to the recorder. External mic preamps are particularly useful with low-cost computer soundcards, as the mic inputs on cards (where provided) tend to be rather basic. However, you must be aware that no matter how careful you are, a cheap computer soundcard is likely to introduce a noticeable amount of noise by virtue of its proximity to other noisy circuitry inside the computer.
2. Use the right microphone for the job. A general-purpose dynamic mic is fine for close vocals, drums or instrument amplifiers, but is unlikely to be sufficiently sensitive to handle quiet acoustic instruments or more distant sounds. A good preamp will help, but ultimately, quiet or distant sounds need to be recorded using a sensitive capacitor or back-electret mic. As a rule, mics that operate only from phantom power tend to be more sensitive than mics that can run from batteries. For very difficult jobs, you'll need to combine a sensitive mic with a very low-noise mic preamp.
3. Noise generally includes hum as well as hiss. So if your system is plagued by hums and buzzes, there's a strong possibility they might be caused by ground loops. Use balanced connections where possible, and if you're getting hum problems when connecting an unbalanced source to a balanced destination, try making up a special cable, as shown in Figure 1.
4. Keep mains cables away from signal cables as they can be another source of hum. Where they must cross, try to ensure they cross at right angles, as this minimizes interference. Don't have mains cables lying alongside signal cables, especially mic-level signals or unbalanced lines. Also keep wall-wart power supplies away from audio cables, because some of these generate strong electromagnetic fields.
5. Recording levels are important in minimizing noise, particularly if you're using analogue machines. As a rule, tape recorders using any form of Dolby noise reduction, or no noise reduction at all, can be driven 3 or 4dB "into the red" on signal peaks. However, machines fitted with dbx noise reduction work better if the signals are allowed just to reach the red on peaks. Digital machines are more forgiving of low recording levels, as they have a much higher signal-to-noise ratio than analogue recorders. But it still pays to get as close to the peak level as you can without clipping, in order to improve the clarity of low-level passages and reverb tails. This is especially true when using low cost soundcards, because they tend to suffer more from noise than hardware recorders.
6. There are still people who don't understand what tape noise reduction does and as a consequence either they don't use it at all or they use it incorrectly. All the major tape noise reduction systems work on the encode/decode principle, which simply means that something happens to the signal during recording and then the opposite happens during playback to restore the original sound. The 'something' generally involves compression/expansion and equalization, so if you record without noise reduction but play back with it, the recording will sound dull and odd. There's just one rule - if you record with noise reduction on, you must play back with it on to get the correct sound.
7. Not all noise is electrical! When you're working with real musicians playing acoustic instruments, most noise problems tend to be acoustic. Here you need to pay attention to sound leaking in from outside (such as traffic noise), machinery or appliances elsewhere in the building, and, of course, noise from the players themselves. The main culprits are creaking chairs, people kicking mic-stand legs, rustling music sheets and general shuffling. Most of these can be solved by asking the players to remain perfectly silent for a few moments before and after takes. Fitting the mics with shockmounts can make a noticeable difference, especially if the room has a wooden floor.
8. Most pop vocals need compression to keep the sound even. However, for every dB of compression you apply, the noise during quiet passages is boosted by the same amount, so you really need to start off with the quietest signal you can. Even headphone spill can sound annoyingly loud after compression, especially if the singer has one phone off or is wearing acoustically open phones. Fortunately, it's usually possible to quieten the pauses using a gate or expander, and many compressors come with these built in.
9. If you don't have enough gates to go around, you can still clean up your mix by manually muting tracks when they are not needed. It helps if you have someone to assist you on the mix, and making a mute list referenced to time code or to the recorder's position counter will ensure you hit the mute buttons at the right times. This is one area where digital mixers score highly, as mutes can be programmed to operate automatically.
10. Finally, don't get too obsessed with noise - most musical instruments or performances include a natural noise element of some kind. For example, a completely clean rock guitar track would sound quite unnatural, as would a vocal performance with all the breath noises gated out. By the same token, if you're recording acoustic instruments, the players' breathing is natural, though be careful when doing overdubs, as you also multitrack the breathing!