In the beginning there was sound...! G'day welcome to the second of our meandering series, following the signals long journey from the stage to the speakers. This time around we will take a look have at how mics respond to sound coming from different directions….and how they are designed to do it. In live-sound, with high stage-volumes from many sound sources, it is important that mikes be Directional, so that they only pick-up what they are pointed at, while 'rejecting' other sounds. For example, on a Drum Kit there are separate mikes on the various types of drums & cymbals. If each mic was picking up sound equally from all directions, the sound we would end up with would be a 'mish-mash' of all the drums, and sound like the proverbial...!! By using directional mics we can get separation between the sounds/instruments. We can then use the mixing desk to weave our magic spell. We can EQ and position each sound where ever we want it in the stereo field, add FX like reverb to specific instruments …the fun stuff of mixing. For this technique to be effective, good separation must be achieved between microphones, so that each picks up the sound from the source to which it is assigned, with the minimum possible 'spill or bleed' from adjacent sounds. The other benefit of mic separation is that it helps us to isolate, and keep a handle on, feedback.

Microphone Response Patterns.
A microphone's polar response pattern is a picture of how a mic responds to sound coming from different directions. Standing directly in front of the microphone is said to be On-Axis, with "0 0" as the reference point. Points not directly in front of the mic are referred to as being X 0 Off-Axis.

The grid lines of the Polar Response Graph are in a circle, North and South Pole…doh!! These lines represent the strength of the sound, measured in Decibels (dB). The outer line - 0dB - tells us that the output of the mic is equal to what went in. Any dips from a smooth circle tell us where, from the On-axis position, the mic does not pick up sounds as strongly, showing us the mics 'directionality.' The points on the graph where the mic picks up the signal least, is called the 'null' points of the mic.

Frequency Response.
The trade off in reducing feedback and getting separation of the sounds is that all directional mics have a different frequency response off-axis than they do on-axis. The better quality mics maintain their on-axis frequency response characteristics over a wider pick-up area.

Cardioid mics also suffer from handling, rumble, wind and 'proximity effects.' The proximity effect - aka 'bass boost' - is what give us that classic deep and rich 'FM radio voice-over sound.'

It is typical for mic specification sheets to include Frequency Response Curves to indicate the mic's 'sensitivity' at various frequencies. The more frequencies that are shown on these graphs, the better. That's the only way to get an idea how they sound in-the-real-world.

Cardioid Microphones.
The typical on-stage vocal mic - e.g. the legendary Shure SM-58 - is designed to pick up sound from in front of the mic, and reject the sound that comes from behind it. These are called cardioid mics, as their response pattern is shaped like an upside down heart.

This ability to reject background noise helps us to use higher levels of sound reinforcement and foldback before feedback takes-off. This is crucial when there are foldback wedges blaring away right underneath the mic position. ["Like Feedback through the Foldback, these are the Gigs of our lives !!" ] When the vocalist is using a cardiod mic, the null point of the mics polar response is aimed at the wedges, picking up that sound the least.

Cardioid mikes - aka, Unidirectional mics - are made to be directional by the use of 'ports' within the mic. Off-axis sounds from behind the mic enter these slots and eventually reach the back of the mic's diaphragm, as well as striking the front of the diaphragm. These ports are designed so that the off-axis sounds cause opposite but equal pressure at the front and the rear of the diaphragm. The end result of all this is that the sound from behind the mic is cancelled out by 'Phase Cancellation', rejecting the sound. [Relax we'll look at microphone phase relationships in the next part of this series.]

Hyper-cardioid Microphones.
These are a variation of cardioid mikes with even greater directionality and having an even tighter response / rejection pattern. They have their greatest rejection at 450 each side behind the mic. ( Tecnically, 1350 and 2250 off-axis.)

To an even greater extent than standard cardioids, Hypercardioids mikes trade-off an uneven Frequency Response for a tighter Off-axis rejection pattern. Unless their is a real need for a tight Pick-up Pattern, {ie. more rejection Off-axis} it is better to use a standard cardioid mike.

An Omnidirectional Mic is sensitive to sound coming from all directions. They are the simplest in design of all mikes and in theory, offer the best frequency response of any pick-up pattern. 'Omnis' are rarely used in Live Audio due to the fact that they cannot be positioned or 'aimed' to pick up specific instruments, while rejecting others. This makes them very susceptible to feedback problems, as well as making it impossible to EQ or control individual instruments within the mix.

As we have seen, directional mics are essential for live sound or any type of multi-mic situation. By using mics with a cardioid or hypercardioid, we can use the mics polar response pattern in our favour. We position the mic to capture the instrument we want, while minimising unwanted spill from the other nearby instruments.

Next time we'll look at how to keep mics in tip-top condition.

In the meantime, if you've got any audio questions give me a yell. Send me an email or post a message on roadtalkz. I'm not the font of all knowledge, but if I don't know the answer to your
questions, I know enough guru's who will!!

See-ya-later.

Willy T.

mailto:willyt-downunderdesk@roadogz.com

This is extracted from the book:
"LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA
Systems." © 1998 WBN Thurbon Giraffe Communications. ISBN 0 646
36408 1.
Available via email or at all good Music & Book shops.