Microphone Miscellanea Part 2
By Phil O'Keefe |
Microphone Miscellanea - Part 2
Microphone sensitivity, microphone preamplifiers, and connecting your microphones
by Phil O'Keefe
Now that you've read Part 1 of this 3-part series ( if not, be sure to check it out right here on HC), you should have a basic idea about impedance, levels and SPL. But what about topics like microphone sensitivity, mic preamplification and connecting your mics? While continuing to keep Part 1's "hey, I'm a musician, not an engineer - just tell me what I need to know to connect and use this thing properly" ethos firmly in mind, let's look at those subjects.
Microphones don't all respond the same to sounds - even when placed the same distance from a single sound source, different mic models will often have different output levels because some are more sensitive than others. The more sensitive a microphone, the better it is at converting changes in acoustic pressure into an output voltage.
Internationally, sensitivity is normally measured in millivolts per pascal at 1 kHz, although the older American standard of 1 V/Pa is often used instead. A Pascal is a unit of measurement that pertains to pressure (abbreviated Pa) and is usually stated as being referenced to 94dB SPL. The more sensitive the mic, the higher the output voltage will be when it is "listening" to a 1 Pa sound source, and the less amplification it will need from an external mic preamp. With the American standard, the output level in decibels is used for comparisons. For example, using the American standard, you might see a specification like the one for the Shure SM57 moving coil dynamic microphone, which is -56 dBV/Pa (which is equal to 1.6 mV) 1 Pa = 94 dB SPL. This is a less sensitive microphone than Shure's SM81 small diaphragm condenser microphone, which has a sensitivity rating of -45 dBV/Pascal (5.6 mV) (1 Pa = 94 dB SPL).
Most condenser mics will have stronger output signal levels compared to moving coil dynamic mics, which is one of the reasons I picked a dynamic mic (the SM57) and a condenser mic (the SM81) for these comparisons. Most ribbon microphones have notoriously low sensitivity - even lower than most moving coil dynamic mics - and therefore have low output levels that place very high demands on the microphone preamplifier to deliver sufficient gain.
Since the output level of most microphones is much lower than the line level that many hardware devices are designed to receive, that level needs to be increased with a device called a microphone preamplifier ("preamp" for short).
You'll find mic preamps in all sorts of musical equipment, from mixing boards to computer audio interfaces to dedicated external microphone preamp units such as the Black Lion Audio B12A MkII shown in Figure 1. These are available in single channel, stereo or two-channel and even in 4-8 channel formats. Microphone preamps are also an essential element of multi-processor Channel Strips, which usually include a preamp, compressor and equalizer in one housing. While the features and amount of amplification provided by mic preamps can vary considerably, all are designed to bring the low mic-level output signals from your microphones up to line level.
Most microphones with tube electronics come with their own separate power supply unit because tube circuits require more voltage and current than what can be provided by a phantom power supply. Unlike dynamic microphones, they typically need two cables to connect them - a specialty multi-pin cable that runs from the power supply to the microphone (this carries power to the mic's electronics and audio from the mic back to the power supply), and a regular XLR male to XLR female "mic cable" to route the audio out from the power supply to your audio interface, mixing board, or mic preamp. -HC-
Have comments or questions about this article? Then be sure to join the discussion in this thread right here on Harmony Central in the Studio Trenches forum!
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.