Microphones - Which ones, when, and why?

Intro

By and large, microphones all do the same job: convert sound into electricity. There are a few different mechanisms which can achieve that, with varying degrees of success. Here, I'll be discussing each of the major types of microphone (condenser, dynamic, ribbon), with an overview of how they work, what that means in terms of sound, and how each might be applied. I'll refer to particular microphones that I use regularly.

Dynamic Mics

Dynamic microphones are probably the most common in the world, with models like the Shure SM58 and SM57 being seen on stages all over the world. A typical example is physically rugged and can usually handle high SPLs with virtually no distortion.

Dynamic mics work on the same principle as a speaker, but operated in reverse. There's a coil of wire suspended in a magnetic field, and attached to the coil is a diaphragm (typically thin plastic). Sound hits the diaphragm, which vibrates in sympathy. The coil moves in the magnetic field, and produces a voltage which is sent out to the mixing desk.

Because the plastic diaphragm must be light, it tends to be thin and have relatively little damping. As a result, the diaphragm itself can exhibit some resonances, typically through the kHz range - the EV RE320, for example, shows a very strong resonance at 5kHz. The result tends to be that a typical dynamic mic will have a fairly "colourful" mid-high range which will need careful work by the manufacturer to make sure the sound is smooth and natural. Sennheiser did a great job with their e935 and e904 models, as did Beyerdynamic with their M201TG, which all feature a clean and natural sound and a smooth kHz range.

Since the coil adds considerable mass to the diaphragm, dynamic microphones tend to fall off quickly above 10kHz or so, meaning they might miss out on some "air" or "sparkle" compared to other microphones.

The directional pattern is achieved by allowing some sound to hit the rear of the diaphragm, after being delayed. The idea is that the acoustically-delayed sound will allow sound wrapping around the front to "catch up". The net result is the pressure ends up applied equally at both sides of the diaphragm, and so the diaphragm won't move and no signal wil be produced. Different manufacturers have had varying degrees of success, but the approach does have its limitations: the "ideal" polar pattern shape, cardioid/super/hyper/whatever typically only occurs over a fairly narrow frequency range. Electro Voice, for example, typically show the polar pattern at 500Hz, and it always looks perfect. However, it's important to review the polar pattern over a range of frequencies, as this will show how well the microphone's directionality will hold up in use.

Live sound typically requires microphones that can handle high SPLs (since everything tends to be close-mic'd) and be physically rugged (since mics often get dropped by singers or hit by drummers), so dynamic mics are often used on-stage.

Condenser Mics

Condenser mics are usually seen in studios, where high-quality sound capture is required. They work by making the diaphragm itself electrically conductive (typically using a very thin layer of gold on a mylar sheet), and then applying an electrical charge to a metal plate behind the diaphragm. The result is a capacitor. When the diaphragm vibrates due to air pressure, the value of the capacitor changes (the value depends on the distance between the plates), and electrical charge is effectively "pushed" around. That signal is passed through a very sensitive preamp before being send out to the mixing desk.

Sound-wise, condenser mics often feature a wider bandwidth than a dynamic mic. A good condenser mic will reach up to 20kHz, and the low-frequency response tends to be more extended, too. The extra high-frequency extension is typically perceived as "extra detail", which is often desirable.

Since the built-in preamp can only have so much headroom, condenser mics have a maximum SPL which they can endure before electrical clipping occurs. NB - the capsule itself isn't responsible for that clipping, it's the circuitry afterwards that put an upper limit on things. The inclusion of a "pad" switch helps matters - by reducing the signal levels entering the circuitry, the microphone can experience higher SPLs without clipping.

Small-diaphragm condensers (typically diaphragm is 12mm diameter or smaller) usually exhibit the cleanest and flattest sound, as well as the best-maintained polar pattern, which makes them the first choice for picking up acoustically large sources: drum kits, choirs, orchestras. Frequent readers will note that I favour Beyerdynamic MC930 mics for those purposes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Large-diaphragm condensers (25mm and up) are often seen in studios. Part of the reason, I suspect, is because they look cool. Since they have a larger diaphragm, more electricity is converted to sound, and the downstream circuitry typically needs less gain to achieve a healthy signal level. As a result, a large-diaphragm condenser can have an exceptionally low noise floor.

Since there's (literally) more room to work, it's possible to place two microphones back-to-back. By combining their signals in different ways, a variety of pickup patterns can be achieved - the SE4400a has four options, ranging from figure-of-8 to omni, making it useful in a wide range of situations.

The down-side of the larger diaphragm is that the off-axis response typically suffers, in a similar way to speakers - when the frequencies get high, the wavelengths become short enough that the capsule itself becomes acoustically large, disrupting the sound it's supposed to be detecting.

By and large, condenser mics tend only to be used in a few select spots for live sound, such as drum overheads. That doesn't have to be the case, though: with careful design, condenser mics can be tuned to handle the rigours of stage use. The Beyerdynamic TG-X930 is an excellent example - that mic set off as an MC930, but the engineers added a multi-stage grile to make sure plosives (the bassy "pop" sound that happens on the letter P or B when someone is speaking, especially close to the microphone) are kept at bay, and adjusted the preamp so that it would be happy at 150dB. By comparison, the MC930 with the -15dB pad engaged maxes out at 140dB.

Ribbon Mics

Ribbon mics are technically a subset of dynamic mics: both rely on electromagnetic induction to produce electrical signals. However, the similarity ends there. Ribbon mics use a very thin aluminium foil suspended between two powerful magnets. As the foil moves, voltage is produced within the foil itself. This voltage tends to be exceptionally small, so a transformer is required to step it up to a more useful level.

Due to the mechanical layout of these mics, ribbons are pretty much exclusively figure-of-8 in their pickup pattern, and that pattern is often very consistent across the frequency range. Beyerdynamic produce a couple of models that aren't figure-of-8, but that's much more the exception than the rule.

The ribbon itself tends to be lightly corrugated and has to sustain its own weight. As a result, they must be stored vertically to ensure the ribbon doesn't gradually sag until it moves out of the magnetic gap. They're also very sensitive to low-frequency overload - putting one at the port of a kick drum would stretch the ribbon out immediately.

Additionally, ribbons (being fig-8) have lots of proximity effect, and often have a subdued treble range, with some EQ required for a natural sound.

After all that, they still have their up-sides. The polar patterns are typically excellent, and while the frequency response may tilt down towards the treble, ribbons tend to be resonance-free in that range. ie, where dynamic mics often add some colouration, ribbons usually do not. As a result, it's safe to use EQ to bring the treble back into line.

I keep a couple of different ribbons here. First is the SE VR1, which actually has a fairly strong treble response - it certainly isn't lacking in that regard. Thanks for the well-controlled pickup pattern, I like to use one of these as the side mic in an M/S array with a Beyerdynamic MC930 as the "mid".

The other ribbon is a 1950s Cadenza ribbon mic. It sounds like a vintage ribbon, with a rolled-off treble and strong lower-midrange. Sometimes, though, that's exactly what's needed. Plus, it looks cool.

Conclusion

In conclusion, all mics do the same job - convert sound into electricity. How well each one does that can vary hugely within each type of mic - a good dynamic mic is preferable to a bad condenser, for example. So, given a Box O' Random Mics to work with, I would try to use those mics which are a good example of their type, rather than sticking resolutely to existing stereotypes of condensers for studio, dynamics for live, etc.

​​​