Frequency Response of Microphones
Frequency Response of Microphones
Last week we got into the Directional Response of mics. This week we talk about Frequency response.
The frequency response of a mic can tell you a lot about it and how to use it. In general, a frequency response chart tells you what a microphone puts out as opposed to what it gets in. In other words how true it is to the original sound that it's presented with.
Microphones are tested in an anechoic chamber. This is a specially constructed room just for audio testing. The room is completely dead, without any sound reflection. A speaker is set up in front of the mic that is being tested and pink noise is played (pink noise is essentially all frequencies with even energy in every octave). The mic cable is routed into a spectrum analyzer that measures the output of the mic. A frequency response chart is produced from this, usually over the 20 Hz to 20 kHz range which is the range of human hearing. Like any manufacturing process, no two mics are exactly the same, you can even get an occasional lemon. This chart helps the manufacturer keep quality control high and can let them match two mics for sale as a pair. As audio pro's we can look at the frequency response chart and get a ballpark estimate as to what the mic's all about. In a perfect world, a flat frequency response is the golden mean. In other words if the mic is putting out exactly what it gets in, then that's great! However, this world isn't perfect last time I checked, and all mics have some kind of variance from the zero line. This shows up as dips and peaks on the frequency response chart. These peaks (called bumps) and valleys are not spikes but gradual rises and falls. Condenser mics usually stray little from the zero line. Whereas dynamic mics can look like a mountain range .
The Real Difference
A lot of factors determine what makes up a microphone's frequency response, for the most part you can think of condensers as being more true to life than dynamics. This also accounts for the great price difference in the two mics. You can get a brand new dynamic for under $100, whereas a decent condenser starts at around $400 and then the sky's the limit. For instance, a refurbished U-47 tube mic can be as much as $7,000! Does this mean that a dynamic is not desirable at all? No way! There are many applications where you'd prefer to use a dynamic mic. For instance, dynamics can take more level in general and great for use in high SPL (sound pressure level) situations. Close miking of drums for instance.
Let's take a workhorse mic like the Shure SM-57 and use it as an example. Look at the frequency response chart (this is a re-draw and not from the manufacturer so it's not gospel, just meant to be an estimation). You can see that at 50Hz the mic is down -10dB. It then gradually rises to zero level at about 150Hz. After that it's flat until you get to the 4KHz to 6KHz where you have a bump that rises and settles down and then the mic drops off at the high end. Definitely not a true representation of what's going on in the real world. However, this is a good thing! First off, this mic has a cardioid directional response. (See the polar patterns explained in previous features.) This means that the mic would be directional and would cancel from the back. In addition the mic is built like a rock and is cheap to buy. Looking better all the time. The frequency response makes it especially good for snare drum. The fundamental frequency of the snare resides in the 150Hz to 250Hz range, right where the mic is flat. The presence bump at 5KHz is just where the snap of the snare resides. In addition it's rolled off low end makes it great for de-accentuating the kick drum which is very close in proximity, and the cardioid pattern makes it naturally reject the off-axis hi-hat, cymbals and toms. A match made in heaven! That's why this mic has been around for a long time and will continue to be so. At the other end, this mic would be a bad choice for miking a kick drum because of the lack of low frequency return.
A mic's frequency response chart can tell you a lot about where to use or not use a mic. We will refer to this in upcoming features and mic reviews. Stay tuned for more info along our road to better and better recording.
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