Dynamic microphones are the tougher brother of condenser microphones, and are perfect for recording loud sources like guitar amplifiers and drum kits. But how does a dynamic microphone work? Some well-known classics are the Shure SM57 and SM58 – you’ll probably find these in every studio in the world!
The first microphone I ever purchased was a Shure SM58 so I could record my guitar amp at home. A dynamic mic is a key component of any studio and you should certainly look into picking up at least one model, alongside a condenser microphone to help give you a balance of recording sources.
How Does a Dynamic Microphone Work?
The diaphragm inside the microphone’s capsule is attached to a coil of wire (known as the voice coil) that is suspended in a magnetic field. When a sound wave hits the diaphragm, the coil moves and cuts across the lines of magnetic flux that is produced by the poles of the magnet.
This produces an electrical signal in the coil that is sent out of the microphone and into a preamplifier. The electromagnetic theory of induction explains how this works – when a material made of metal moves through a magnetic field, an electrical current is induced into that metal.
So when the moving diaphragm causes the voice coil to move through the magnet’s magnetic field, an electrical signal is produced which then corresponds to the sound it’s recording.
Dynamic mics are solid and durable. They can be used to record loud sounds with high sound pressure levels like drums and guitar amps. These sorts of instruments don’t really have much high-end frequency detail either, making them even more suited to dynamic microphones.
Dynamics usually have a slight bump in their mid-range frequency response, where certain frequencies are emphasised a little more during recording compared to other frequencies. This is a part of the microphone design that makes them so useful for things like guitar amps, which already emphasise these frequencies as well. The frequency bump is usually somewhere between the 1kHz-8kHz mark.
In fact, some of the classic dynamic microphones like the SM57 or SM58 from Shure, have their signature sound because of their specific mid-range frequency response.
The high-frequency response of this type of microphone tends to fall around the 15kHz area, so they’re not ideal for recording the types of instruments better suited to condenser mics. If you’re after a recording with less detail but with a full-bodied sound, go for a dynamic.
Just like condenser mics, dynamics can suffer from the proximity effect, something to always be aware of. This causes lower bass frequencies to increase in level when the sound source gets too close to the microphone, usually within a distance of around one foot.
You’ll always have the option to filter out these sorts of lower frequencies at the mixing stage, but ideally you always want to get the recording correct at source.
On-axis and Off-axis
When it comes to recording with a mic, you need to keep in mind the on-axis response and the off-axis response.
A mic’s on-axis response is how the microphone records the sound entering straight in from the front of the microphone – how it should be used. The off-axis response is when any sound (it could be from other instruments in the room or from the sound reflections of the room itself) enters the mic from other directions. For example, into the back of the microphone.
The on-axis response is what you’re looking to record. The off-axis response should be minimised as much as possible, especially in smaller home studios where the sound of the room is not usually ideal.
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Due to the way dynamic mics are built and designed, they always come with the cardioid pickup pattern. This pattern is the best to use in smaller home studios, as the recording room is usually not acoustically designed or treated.
The cardioid response is directional, so the sound is best recorded on-axis into the front of the microphone. Any sounds or room reflections entering the mic from off-axis directions will be minimised, especially if you are recording with the close-mic technique.
The transient of a microphone refers to how fast it can react to an incoming sound. The first 0-100 milliseconds of a sound in the studio can contain a lot of information. For example, think of the initial crack of a snare drum. How quickly the microphone can react and capture this initial sound is known as the transient response.
Due to the heavy moving parts of the inner diaphragm and coil, dynamics react much slower than condensers and can struggle to capture fast and sharp transients. This is one of the reasons for the rugged sound typical of a dynamic, and the lack of a high-end frequency response.
This isn’t a bad thing – this makes a dynamic microphone perfect for recording guitar and bass amps, certain drums and particular types of vocals.
Sensitivity / SPL / Noise
The sensitivity of a microphone is measured in volts. It’s a measurement of how much the microphone’s signal will need to be boosted by the mic’s preamp so it can reach to ‘line-level’ – a signal that is high enough to be processed and recorded by other pieces of equipment in your studio.
The sensitivity of a mic isn’t something you need to think about too much these days when you’re recording in your own home studio. Most professional microphones will all have high-quality sensitivity ratings.
Dynamic mics can handle very high SPLs (sound pressure levels) – some mics can handle as high as 140dB before they start distorting, which is around the mark for the threshold of pain.
This is another reason why dynamic mics are so well-suited to recording things like guitar amps. You can crank up the volume incredibly high and record all the power that an amp can produce without having to worry about the microphone distorting the signal.
The background noise generated by dynamic models is higher than with condensers, due to their physical design. This is known as the signal-to-noise ratio, and is another reason why they’re not the best choice of microphone for quieter, gentler instruments. Condensers have lower noise backgrounds, so are the better choice in these scenarios.
I have attempted to record an acoustic guitar with a dynamic mic on a few occasions in the past, only to find that the result was pretty much useless because of the background noise coming through. Try to avoid this if you can!
There are plenty of models to choose from if you’re looking for a dynamic mic, for any level of budget. Some popular models you’ll find are:
- Medium budget – Shure Beta 57A, Shure Beta 58A
- Higher budget – Sennheiser E945, Electro-Voice RE20
You can never go wrong with the SM57 or SM58, perfect dynamic microphones for guitar, bass, drums, and vocals. Every home and professional studio in the world uses the SM57 and SM58, and you’ll also find them used for live stage performances as well.
I generally recommend using dynamic microphones in a home studio when you’re trying to record the following:
- Rock vocals or loud vocals
- Guitar and bass amps
- Loud brass instruments
Dynamic mics are the perfect choice in these recording situations. Alongside one or two condenser mics, you’ll also need one or two dynamic mics in your studio to give you a great range of choices when it comes to recording.
You should now be able to easily answer the question posed at the top of this page – how does a dynamic microphone work? Dynamic microphones are tough and robust, and can easily record very loud sounds with high sound pressure levels without any problems at all.
I definitely think you should get at least one for your own studio. It’ll sit nicely alongside your condenser microphone, and with a mic of each type you’ll be able to record a great range of sounds for your songs and productions.