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Recording Equipment

This article, Microphone Basics - Part 2, discusses more common microphone designs, their characteristics, and some advantages and disadvantages of each, as well as some microphone placement techniques involving stereo pairs of mics. Recording engineers can gain a significant improvement in their results, through a solid understanding of the principles surrounding microphone selection criteria, and microphone placement, relative to a sound source, when they possess a firm knowledge of the design of each type of mic, and intended applications for them.

continued from Part 1

Microphone Basics - Part 2

Cardioid mics are tremendously useful, where it is important to get a good recording of a sound source, while excluding as much as possible of another source positioned to the rear or rear-sides of the mics’ axis, or the imaginary line which runs through the centre of the mics’ capsule or diaphragm. Such off-centre sounds are said to be off-axis. These sounds might be from another performers’ instrument, in a live session, or perhaps, a monitor speaker, on stage. A quality cardioid mic will reject the majority of these off-axis sounds, making a clean recording or gig much easier. However, cardioid mics, like every perfect thing on the planet, have their tradeoffs. When a cardioid mic is placed close to a sound source, it will exhibit what is known as proximity effect, meaning that low-mid and bass frequencies will be significantly boosted in the program, which may make a singer sound nice and warm and fat, but might make an acoustic guitar become intolerably muddy and formless. Further, when used in pairs or groups, cardioids are very sensitive to phase cancellation issues, in which tiny variances in the sound pickup times of each mic, when the otherwise-identical signals are combined in mono, create a filtering effect which is unstable, usually acoustically uncomfortable to listen to, and which, in extreme cases, can almost totally remove the affected tracks from a mix, just as surely as directly erasing them would. This phenomenon is discussed in greater detail elsewhere on this site.

Omnidirectional, the second common polar pattern mentioned, has the inverse tradeoffs to the cardioid design – here, the mic sounds transparent and natural, almost anywhere that you place it around the source, without proximity effect to consider, and a carelessly-placed mic pair will still exhibit barely any unpleasant phase issues, but, on the flip side, omni mics will pick up sound from anywhere around them, so, isolating an instrument in a group just won’t work out with omni mics. If you need isolation, you’ll need cardioids, or a separate recording space.

Bipolar mics are essentially two cardioid mics, placed back-to-back inside one microphone body, allowing sounds to be picked up from one side of the mic, or the other, but not from in between the two – in fact, bipolar mics reject audio at their exact mid-point, or null point, better than any other mic pattern's “dead” side can.

So, what to do? One way to use cardioid mics in a pair, so that there is a minimum of phase trouble, is to use what is known as an X-Y mic pair, a common coincident pair in stereo micing. This technique will work with any pair of cardioid mics, but will be most efficiently done with two cardioid condenser mics, with small-diaphragm mics working the best of all. To make this pair, or any mic placement, for that matter, listen, first, to your sound source – for example, an acoustic guitar – and choose the sweetest sounding spot you can find on the instrument, while the player is doing the same type of performance that they will be recording. Remember that mics hear sound very specifically, much like you'd see things, looking through a paper towel tube at the world, and so, they don’t exactly take the whole environment into account, the way our ears do. If you were to listen to the same mic placed at ten different spots, two inches apart each, in a line across the front of a guitar, for example, the tone coming from the mic at each of the ten spots would have a slightly, or possibly significantly different quality from the others. So, find the optimum position on the source, and then take the first mic, and place it about 10 cm in front of this great-sounding spot you have found, at about a 45 degree horizontal angle to the front of the source/instrument, rather than pointing the mic straight at it. Now, place the second mic exactly over top of the first mic, but at the opposite (mirror) angle to the first one, so that the pair now has i) both capsules in virtually the same space, with a small vertical gap between them, to avoid their bumping into each other during recording, and ii) about a 90 degree angle between the two axes of the mics. The actual value of this angle, in practice, can range from about 70 degrees, to about 110 degrees, outside of which you will either encounter a very narrow and vague stereo image, or uncontrollable phase cancellation problems. By placing the two capsules in the same physical space, as we have done, here, audio energy from the source will now strike both of them at exactly the same time, eliminating time-related phase issues between the mics. The X-Y pair is probably the most widely-used stereo mic pattern, due to its ease of setup, and its virtual freedom from phase problems. Try it out, and see for yourself.

Another simple trick, is the use of two omnidirectional mics, in what is called an A/B pair. Here, the two mics are simply placed at two “sweet” spots in front of the source, but very close to the same distance each from it, again, to minimize differences in audio arrival times to the two mics. If you need to use cardioids to do this, consider the 3:1 Rule, which is explained in more detail elsewhere on the website.

Go to Part 1 or Part 3 of this article.

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Microphones

Gefell Small Diaphragm Condenser Mics

Gefell SMS2000

SMS 2000 Modular Studio Mic System

The SMS 2000 miniature condenser microphone system consists of microphone amplifier and exchangeable capsules with cardioid, hypercardioid and omnidirectional polar patterns, making up the M 200, M 210 and M 270 microphones. This allows a universal application in professional recording and sound reinforcement.

The newly developed, transformerless circuit design highlighted by remarkably low intrinsic noise and high output capability substitutes the previous transformer. The module system is adjusted to optimum reproduction properties, to allow for particularly clean and colourless sound reproduction.

Contact Rock Shop Pro Audio for information and pricing on the SMS 2000 and Gefell products.

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Recording equipment, recording consoles, mixing consoles, acoustics and systems design from...

Audient, D&R, ATC, Magix, Eventide, Gefell, Earthworks, Bryston, JBL, Lexicon and more