Audiomind's Turbulent and Kismetical Rambling Mind (audiomind) wrote in electrixa,
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A Beginner Guide to Microphones

How Microphones Work

At their most basic, microphones are transducers. A transducer is an electrical device that converts energy from one form to another. In this case, the transducer is turning sound — acoustical energy — into an audio signal — electrical energy.

Most of you would know that sound is essentially fluctuations in air pressure. The component all microphones have in common is called the diaphragm. When sound waves hit the diaphragm, it vibrates, and the vibrations (which represent the fluctuations in air pressure) are turned into electrical energy (current). At the other end of the mic lead, that current is turned into the audio signal.

Granted, that’s a pretty basic explanation of how the microphone actually works, but as a musician, producer or engineer it’s all the science you really need to know about them.

Types of Microphone

There are plenty of types of microphones, all representing a different way of converting sound into signal. Each type of microphone has its own sound when compared to the other types, so knowing which is for which is just as important as knowing your mic’s frequency response. Microphone types usually get their names from their transducer type and directional patterns (for instance, you can get an omni condenser and a super-cardioid dynamic). Let’s look at the most common types of microphone you’ll be using.

Dynamic Microphone: Dynamic mics are one of the most common types of microphone. That’s because they’re cheaper, they can be used in both live and studio situations, and quite a bit hardier than most other microphones — for instance, put a condenser and a dynamic in front of the same number of spitty singers or tuba players and you’ll find the dynamic lasts longer due to its resistance to moisture.

Generally, dynamics don’t pick up as much detail as a condenser, so they’re not used in the studio as much as they are live, but they do come in handy on loud instruments such as electric guitar where condensers are only useful a few feet away. They’re also used by bands who want to get “that live sound” in the studio.

Condenser Microphone: Condenser microphones are incredibly popular, but not as common as the dynamic because they’re expensive, and they aren’t easy to use in a live situation unless they’ve been specifically designed for that — they generate feedback very easily. Condensers generally pick up a lot more detail than dynamic microphones and are better for quieter, subtler sounds. They also require 48V phantom power, where their dynamic cousins just need to be plugged into whatever’s available to receive the sound.

Condensers are great for picking up loud sounds without losing detail, though if you don’t have a wide breadth of condensers available to you, you may end up using a dynamic for those. Condensers are fragile and anything from air moisture to a bit of a bang can ruin them in no time.

You can get live condensers, such as the Rode M2. I’ve always preferred them as I find few dynamic microphones suit my voice.

Ribbon Microphone: Ribbon microphones are quite expensive, particularly fragile, and much less common than dynamic and condenser microphones. In the studio, they’re used frequently and I know one producer (misguided or not) who said he never used any mic except ribbon mics anymore. Like dynamic microphones, they don’t require 48V power, but unlike dynamic microphones, they can be damaged if that much power is fed into them, just to prove to you they really are fragile things (there are some ribbon mics being produced now that won’t be damaged if you forget to turn the phantom power off).

Ribbon microphones get their name from the the thin metal ribbon suspended in a magnetic field that picks up the vibration and turns it into a signal by magnetic induction. They’re good for a number of purposes including stereo recording and isolating an instrument in a noisy room (think drum kits).

There are other types of microphone, including the carbon and crystal microphones, but these are the three you need to know if you’re getting started in the world of recording.

Polar Patterns

The polar pattern of a microphone determines from which direction it picks up sound — or more accurately, how sensitive they are to sounds arriving from different angles, since if you sing into the back of a cardioid microphone you’re still going to hear something (even if it’s very quiet). Read on to find out about polar patterns, including what on earth a cardioid microphone is.

  • Omnidirectional: an omnidirectional microphone picks up sound equally from every angle. Omnidirectionals are great for recording choirs, a bunch of string players standing in a circle, and so on.
  • Cardioid: cardioids are the most popular polar pattern, and pick up mostly noise from a wide front area with about bit of sensitivity around the sides of the back, and almost no sensitivity at dead-center rear. They’re named cardioid because the polar pattern is heart-shaped when demonstrated in diagram format.
  • Hyper-cardioid: like a cardioid, but picks up a thinner area at the front and is less sensitive at the back.
  • Super-cardioid: has about as much as front sensitivity as the cardioid but even less rear sensitivity than the hyper-cardioid.
  • Bi-directional: these microphones pick up sound from the front and the back, but not so much the sides. Good for duets or other situations where you want to record two sound sources but exclude any others.
  • Shotgun: shotgun microphones are named so because you point them at a sound source and they won’t pick up anything but that sound source. Technically, that’s not quite true — they have some sensitivity on the sides and at the back — but it’s far less than any other microphone. They’re often used in field recording and on television, but they come in handy when you’re recording drum kits and the like where you want isolation.

Frequency Response

Just about every microphone has a frequency response chart in the manual. It looks like an EQ graph with squiggly lines showing how sensitive it is (or isn’t) to certain frequencies. A microphone that’s designed to record a kick drum will generally have slight boosts in the bass regions and cut off a bit of high-end. Be careful to check the frequency response of every mic you buy before you actually buy it. There’s no point recording a bass guitar with a microphone that has a high pass.

Connectors

Most microphones use an XLR lead and plug in on the male end, though you can buy some crappy microphones that’ll plug in to a 1/4 inch jack. Steer clear of those — stick with XLR microphones or you’ll be climbing below the very worst of the recording quality microphones.

What’s important between the mic and the pre-amp is whether your lead is balanced or unbalanced. Unbalanced leads won’t do anything to stop noise. Balanced leads essentially run the signal down two wires, one with the phase flipped, so that at the other end you can combine the two signals and any noise that was introduced in the lead itself will disappear as it will be out of phase.

Electrical Current Level

The level of current a microphone generates determines gain at the other end of the lead. Mic level is a tiny amount of current, whereas line level and instrument level are quite loud as they are. When you plug a microphone into a rack in the studio or a mixer in a live situation, the first thing you need to do is amplify the signal. This is done when you are setting up your gain structure. The goal is to get all signals to line level or unity gain so they can be mixed relative to each other as easily as possible.

In a nutshell: don’t skip pre-amplification! It’s a common newbie mistake to go buy a microphone without buying some sort of pre-amp. Do your research and get something that makes your mic signal sound good — lively, loud and noise-free.

Protecting from Plosives and Wind

There are a few ways to stop plosives and pops from your vocalist or wind noise in general, but they all boil down to: put something between the microphone and the sound source (the wind is a sound source for the purposes of this tutorial). Sure, we could crap on about reducing noise by putting the singer on a funny angle or any number of tricks to get rid of plosives, but at the end of the day, some sort of mesh or fabric is going to have to go in front of the microphone. Don’t even try to record without something.

The best solution is a pop filter. You’ve no doubt seen these in use, even if only in a music video. It’s a circular fabric or metal mesh that sits in front of the microphone and clamps onto the stand. I prefer metal pop filters as I find the fabric can take some of the high-end frequencies off the top — it’s not always noticeable but I prefer to be able to take the sizzle out of a sound myself.

That said, a fabric pop filter is far better than the poor man’s solution I used years ago: sticking a sock over the microphone itself. Never had my sound clip due to a plosive, but it sure does sound muffled to my ears these days!

Buying a Microphone

So now you know how microphones work and what types of microphone exist. There are a few things to look at when you actually go and purchase a microphone. The first thing to know is what the microphone will be used for. Will it be for vocals? Guitar? Drums? Piccolo (if anyone even plays those anymore)? Or will it be an all-rounder? Be forewarned that you can’t get an all-rounder microphone — you can only get a mic that works on more sound sources than another mic.

The first factors to decide on are ones we’ve already discussed:

  • Type of mic — will you be recording live or in the studio? If you’re recording in the studio, do you want something that can tolerate the beating a loud, distorted electric guitar through a big stack will put it through? What about a metal screamer? The rule generally goes: dynamics for live situations, condensers for the studio, unless the sound is loud, in which case you go for the dynamic anyway. There are probably more exceptions to the rule than there are actual cases where you’d follow it, so do your research properly and don’t flame me if you buy a dynamic vocal mic for the stage that you hate.
  • Polar pattern — which polar pattern you want depends on so many factors. If you’re in a live situation, I would go for cardioids and mics that isolate. Maybe you want to pick up the room or multiple sound sources in a studio, in which case you’d go for something with a more open directional sensitivity.
  • Frequency response — I think that the flatter the microphone’s frequency response, the better, but perhaps you want a live mic that has a bit of extra kick in a sound source’s dominant range (this is what the SM58 does for vocalists). Keep in mind that you can’t change a microphone’s inherent response, but you can accentuate frequencies later with a bit of EQ, hence why I think flat response is the best response.

Other factors to consider:

  • Impedance — microphones are either high impedance or low impedance. I won’t go into the details of impedance, because frankly I sometimes have a hard enough time getting my head around it myself and it’s certainly not important for making great music — but generally speaking you want to get a microphone with lower impedance. High impedance microphones are cheaper and they’re fine if you’re not using a ridiculously long cable, but if you’re playing a stadium and want to run around with a twenty meter cable, it becomes more important to get a low-impedance mic and a low-impedance cable to reduce noise and interference.
  • Noise cancelling — some microphones have features to help control noise, such as suspending transducer components to isolate unwanted vibrations.

The number one factor to consider when purchasing a microphone is sound quality. Above all else, try the microphone on the sound you want to record with it and see how it compares to other microphones in your budget range.

Even with the same polar pattern, transducer type and frequency response, one microphone will sound better than the other. Tone is supposedly a matter of overtones and matching frequency responses should provide matching sounds, but microphones that are built better simply sound better. Don’t listen to anyone who tells you that two microphones will sound the same because they have the same specifications — it’s not true!

That’s all you need to know in order to know quite a bit about microphones. Next time we’ll look at microphone technique and placement.

Tags: microphones, recording, sound design
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