In a conversation on the Podcast I referred to my thoughts on how the speed of a microphone is a major factor in choosing the appropriate mic for an instrument. Russ was intrigued by this as it's not something he'd come across before and asked me to elaborate on what it was that I meant.
What Is The Speed Of a Microphone?
Talking about the speed of a mic is closely related to its frequency response. Any conventional microphone has a diaphragm, some kind of surface which is suspended in the air and can move in response to the pressure variations which make up sound waves. This thing which gets pushed backwards and forwards by the air has mass and therefore has inertia. Like a teenager, it often responds slowly and with some degree of reluctance...
The effect the mass of the diaphragm has on frequency response is widely understood. Dynamic microphones have a tiny coil of copper on the back and this adds the mass which results in a reduced top end compared a condenser mic which doesn't have a heavy voice coil. Because of this condensers have a superior top end response (let's leave ribbons out of it for now).
So Condensers Are Best?
A question I used to get asked a lot when I taught recording techniques was that if condensers outperform dynamic mics, why do people use dynamic mics? I used to get frustrated at the widely repeated idea that dynamic mics are cheap and tough whereas condensers are fragile and expensive. Maybe in the 70s but these days I think neither of those are true. My standard response was that dynamics are popular because they are slower. Because they are slower they "knock the corners off" spiky sounds and as a result they are well suited to digital recording which captures these corners far better than tape ever could. A fast mic and a fast recording medium might end up too spiky!
Transient Response
To reproduce high frequencies a mic's diaphragm has to change direction quickly. Once the physical inertia becomes too great, the output from the mic becomes different from the sound it is supposed to be capturing. The mic is too "slow" to keep up. Many sounds don't contain important information at the top end of the spectrum and these are potentially good candidates for using slower microphones but the thing which can be overlooked is that a transient, the speed with which a sound starts, is also a rapid change in direction. It's a high frequency sound, it just doesn't repeat itself so isn't perceived as "high frequency" as such. This is why I've tended to refer to the "corners" of the sound.
Snapping Sticks
To capture a nice example I arranged a trio of mics with different speeds:
Neumann KM 184 - Small diaphragm condensers are "fast", hence their excellent top end response.
Shure SM58 - A dynamic mic which is relatively slow.
Home made Subkick - A rewired speaker driver, the speaker cone is very heavy making this an extremely slow microphone
At first I tried a handclap but was disappointed at how slow the transient was. I wanted a really fast sound to show the differences most clearly. I settled on snapping a dry stick from the garden. If you see the results below you'll see how the leading edge of the snap is progressively less steep with a heavier diaphragm. This snap was a good one, it clipped the mics but that's not important, it's the rise angle we're interested in. The screenshots show progressively greater levels of zoom on the sound of the snapping stick. The KM184 is blue, the SM58 is green and the subkick is purple.
What are your thoughts about the speed of a microphone? What sources do you favour dynamic mics for and to what extent is that because of transient response? Share your thoughts in the comments below.