At the Microphone Input and Preamp

Meanwhile, we have to be concerned about what is happening at the microphone. Microphones have self-noise, which is usually expressed in terms of equivalent Sound Pressure Level. Right now, for instance, I am studying two condenser microphones, one of which advertises equivalent noise of 15A dB SPL and the other of which advertises 27 dBA SPL (note that both specs are A-weighted, which is a fair thing to do in this case). This specification tells you that the measured noise floors of the microphones are equal to acoustic levels of 15 and 27 dBA SPL respectively.

So, if you encounter an acoustic level similar to either of those levels you’ll be able to hear the mic noise. If your acoustic noise floors are significantly above those levels, you might never get down to the mic noise. The problem is that the mic noise will be predominantly high frequencies while the acoustic noise will be predominantly low frequency. The A-weighting specification helps to minimize this difference, but you can still figure that until the A-weighted room noise is a good 6 dB louder than the mic noise, you’re going to be able to hear the distinctive hiss of the mic noise.

At the same time, your microphone has some kind of sensitivity specification. This is usually specified as the number of milliVolts the mic puts out when you stimulate it with a 1 Pascal signal, which is nerd-talk for 94 dB SPL. The two mics I was referring to above have sensitivity specs of 16.3 mV (which is -36 dBV) and 2.2 mV (-53 dBV) respectively.

These specs are very useful to know, because they suggest how much gain you are going to need amplify the signal to 0 VU (about 1 volt) for a 94 dB SPL acoustic sound, which is a pretty common loud sound. That spec (-36 or -53 dBV) also suggests the amount you are going to amplify the mic signal and electrical noise as well as its acoustic and mic noise floor. The less gain you need, the quieter your signal will appear to be. If you are recording a low level sound, or recording from some distance, you really need all of the electrical level out of the microphone that you can get.

Managing Mic Levels in recording

Attenuators (Pads) on mics

On the other hand, for a microphone placed in a kick drum, your biggest problem will be microphone overload. Typical maximum levels for a microphone are up in the 140 dB SPL range, but it is nonetheless possible to drive both condenser and dynamic mics into, er, non-linear behavior, particularly with some of our sillier rock ‘n roll moves.

Many condenser mics include a passive attenuator or “pad,” which is placed between the capsule and the amplifier stage in the microphone. This is very handy for high-noise environments, because if your source overloads the amp stage in the mic, there is no way to correct for it further down the audio path.

Mic pads and Mic Attenuators

Further down the signal path, at the first point in the console, we get to the mic preamp. It usually includes another passive attenuator coming just before the gain stage plus a negative feedback trimmer to the mic preamplifier called “mic trim” that controls the actual amount of gain the mic preamp will provide, typically over about a 40 dB range.

The attenuator is a passive resistor network and doesn’t affect signal behavior except to reduce amplitude. The mic trim, on the other hand, is modifying the mic preamp circuit and so you want to be think about it and use your ears as you tweak it. As a rule of thumb, the more negative feedback, the lower the distortion and broader the bandwidth. This suggests that you should reduce gain with the mic trim pot first and the attenuator only if you need more attenuation than you can get with the mic trim. However, use your ears here. Mic preamps and their “sound” is a touchy subjective business, and you may find that you prefer the trim to be wide open (maximum gain) for your favorite sound. Like I said, use your ears.

So the rules of thumb are:

• to minimize noise, use as little gain as possible, which means either making the sound source louder, moving closer to the sound source, or using a more sensitive mic.
• to minimize overload, make sure you aren’t overloading the mic itself. If you are and it’s a dynamic, change to another mic. If it’s a condenser, switch in the attenuator on the mic.
• use the mic trim to regulate level to tape, leaving the main fader at unity gain (also called “design center”). If you can’t turn it down far enough, then use the attenuator. Check with your ears that this gives you sound closest to what you want.

Fun and Profit Time

The most important issue here is to keep in mind that the dynamic range of the music we actually work with is a lot smaller than the dynamic range we have available, in spite of the noise/distortion limits we have to tip-toe between. But, even though the music may have a limited range, we have come to expect the surrounding audio window to have a noise floor that is virtually inaudible and free from overload distortions.

This means careful listening, some reasonably careful measuring now and again, and very thoughtful handling of these initial stages of gain in the recording process, which are both the largest gain stages and probably the most critical.

Remember, the dynamic range of the audio chain is limited by how close to overload you let the signal get in any stage and how high you let the noise become in any given stage. This includes the acoustic noise of your recording room, which often won’t be nearly as good as the hypothetical specs I’ve offered up to you here.

Choose mics with an eye to their sensitivity as well as their sound and manage their gain at the mic trim pot, not at the main fader. Use attenuators only when needed, not as a matter of course. And try, during set-up, to plan for using the least amount of gain possible. Don’t use your most sensitive condenser on the snare and your really insensitive dynamic on the room-miked alto flute track!

Even with modest gear, you can obtain really fine, quiet, clean sounds by just keeping track of where your levels are at every point in the chain.

Happy pads!

Dave Moulton is still trying to figure out if up is really louder. His dogs assure him that in either case louder is better!