by Pablo Bellinghausen —
Before getting into the more practical aspects of microphones, it’s important to know a bit about how they work. There are many introductions to mics out there, but they are often rather dry and theoretical, whereas others, in an attempt to keep things simple, miss some technical elements that are actually very important when purchasing and using mics in the real world.
We’ll try to go through the essentials here in a clear and concise way that will hopefully be useful for beginners and experienced users alike.
What is a microphone?
In theory, a mic is simply a sensor that transforms sound waves into an electrical signal – it’s really as simple as that.
Their technical term is “acoustic-to-electric transducer”, since they convert energy from one kind to the other. In practice, the thousands of different models costing from pennies to several thousands of pounds show that it isn’t all that straightforward, however.
The main element of a microphone is a capsule, containing a diaphragm, or membrane, that is physically moved back and forth by the sound waves. This movement is converted into an electrical signal and then amplified by internal circuitry. The most common mic varieties, differing in diaphragm design, are condenser, dynamic and ribbon.
The most common microphone variety in many studios is the condenser type. They are clean-sounding, accurate and will pick up a lot of low-volume detail, which can either be a good or a bad thing; it’s a common occurrence for home studio users to clearly hear the neighbours having a chat in the background of a recording!
This is a great advantage when working in controlled studio rooms; good-sounding rooms are picked up beautifully by high-quality condensers, giving a sense of organic realism to recordings. Most classic recording microphones are of this variety.
The Neumann KM 184, like the C414 above, is a condenser mic
Condensers are great for the highest sound fidelity, but their tendency to be rather fragile and their sensitivity to quiet sounds up can make them problematic in stage applications. Some older models can also be susceptible to overloading at loud levels, leading to nasty distortion.
A lot of improvements have been made in this technology in the last few years and there are some fantastic, rugged “live condensers” nowadays, but they will always end up picking up more extraneous sounds than good dynamic mics, which can easily cause feedback (that familiar high-pitched whine that is heard when a mic is placed in front of a speaker playing the same signal back). Condensers are best in quiet stages, far from any loud drums or amps, and ideally for musicians using in-ear monitoring.
The Sennheiser e965 is an amazing live condenser, but it’s not for everyone
Condensers need power, both to get the capsule to work and to drive the internal circuits. This is done via phantom power (usually 48 volts), normally supplied by the mixer or audio interface’s built-in mic preamplifier.
Some vintage-type premium models will use valve (or “vacuum tube”) circuitry and will be sold with a separate power supply unit. Valve mics will usually have a warmer, more complex sound which can be very flattering on many sources, particularly vocals.
Condensers will come in lots of shapes and sizes; one of the main distinctions is the size of the diaphragm. Technically, smaller diaphragms tend to have better transient response and fewer internal resonances and are in general more accurate than large ones, particularly for sounds reaching the capsule at an angle. However, there is just something subjectively nice about the sound of a well-crafted large diaphragm condenser, enhancing close-up vocals and instruments and making them sound “larger than life”.
The Telefunken CU-29 is a large-diaphragm tube condenser, shown here with its power supply
There is a particular type of condenser called electret, which is actually the most common variety outside of pro audio; it is found in most computers, telephones and portable recorders. Their capsule is permanently charged (or “pre-polarised”), so it doesn’t need phantom power; the microphone’s internal circuitry still does, but it can use a much lower voltage. Most consumer mini-jack mic inputs will supply 5 volts of “plug-in” power through the “ring” bit of the connector.
Electrets can be very small and very cheap to manufacture (good reasons as to why they’re so common) but until not that long ago they were all pretty bad! Consumer ones still tend do be rather poor, and many will lose their charge and stop working after a few years. However, electret technology has been refined to the point where high-quality models can sound as good as classic condenser models, and in use there is little difference nowadays.
Cheap consumer model & Audio-Technica AT4033: two electret mics of very different quality
The main difference in use between electrets and “true condensers” will be with headsets and lavalier “tie” microphones, which need to be fed a voltage no higher than about 10 volts; specific adaptors are therefore required when plugging them into an input with 48V phantom power; plugging it directly can damage the mic.
Dynamic microphones are far more rugged, being both resistant to impact and moisture; this makes them ideal for stage use. One of the most common models, the Shure SM58, is renowned for its durability and can be seen being subjected to some rather intense stress tests on the following (pretty funny) video by the Scandinavian magazine Studio:
Dynamic microphones will also tend to pick up a lot less extraneous noise than condensers, which besides helping immensely in cramped stages also makes them great for recording voice and instruments in rooms with poor acoustics. If rejection of sounds of any kind is a high priority, then a dynamic microphone is often a good choice.
Audix OM7: One of the “kings” of feedback rejection
The downside is that very small nuances in the original sound are often lost by all but the very best dynamics. The capsule is a lot heavier, which makes it harder to respond to very quick changes in volume (transient response). Compared to a condenser, the sound loses some detail and “liveliness” which cannot be recovered through tone adjustments; it does however impart a sound that works great for some vocals, and is very flattering to certain kinds of percussion, “fattening” the sound of toms, for example.
High quality dynamic microphones are also the norm for broadcast due to their rejection of poor room acoustics (often a problem in cramped radio booths), warm, flattering low-frequency response, and non-fatiguing, pleasing treble.
The Beyerdynamic M 99 is a great dynamic for broadcast and spoken word
There is also type of dynamic that uses a thin metal ribbon as a diaphragm. These used to be the standard studio microphone during the thirties, and they have been in use in broadcast and music studios ever since, most notably by the BBC. Although their popularity started to wane as soon as classic German condensers started to get a hold on the market in the fifties, there has lately been a resurgence of interest for them, and several innovative models have come out in the last few years.
Their sound is similar to dynamic microphones, but with a smoother and slightly more natural mid and treble response; most models will be rather dark-sounding but they retain detail that most dynamic mics will not; newer ones have been designed to be a lot brighter to make them more versatile. Most will pick up sound from the front as much as from the back, which can make them a bit awkward for positioning, but there are models that will try to enhance their directivity. We will go through directivity and polar patterns in detail in our next instalment.
Coles 4038 and sE VR1 ribbons: traditional vs modern
Old ribbon mics have a very low level output and are quite fragile; the ribbon itself can break if the mic is dropped, and is susceptible to blow out if phantom power is applied to it. Newer models are a lot more resistant, and their output level will often rival that of condensers, whether through innovations in the ribbon’s design or through integrated amplification circuits. Despite all these improvements however, ribbons are still a very unusual sight outside recording studios or broadcast booths.
Next time we will look at some of the technical characteristics that define the sound of a mic, such as the frequency response, polar pattern and proximity effect, as well as how to read the measurement graphs that represent them. Until then, happy recording!