Audio Blog

Choosing a Microphone Part 1

Written By: Alex Cloud, Audio Engineer

As a part of our new blog series, I was asked to pick a piece of gear here at Omega and write up a review. I thought about it a lot, and I decided to write about what is arguably the most important piece of the signal chain (aside from the talent of course): the microphone. Realizing the size of this task, I have broken this blog into three parts. This first two parts are all about microphones – what to listen for, what practical factors to consider, and how they work. The third part is about our excellent mic collection at Omega Studios and some of our Staff Engineers’ go-to choices and tips.

Choosing a microphone is the first opportunity an Engineer has to shape a sound. While that sound can later be influenced by EQ, compression, expansion, time-based effects, or any combination of processes, there’s nothing quite like the experience of hearing the right microphone, for the right instrument or voice, recorded in such a way that it needs no further manipulation to sound just right in the mix. There is no hierarchy of microphones – even the cheapest most busted-up broken mic sounds exactly right for some sources, and even the most pristine expensive finely-manufactured microphone simply doesn’t work very well for some sources. If Engineering was as easy as buying the “best” mic (or the “best” compressor/EQ/console/monitors/acoustic treatment, the “best” anything) I would simply take out a loan, buy that mic, and have a hundred hit records – and so would everyone else.

Microphone selection is actually quite nuanced, but it’s not so complex that it’s unapproachable. Microphones have measurable, audible properties. The ways these properties overlap, and how they are influenced by the real-world acoustic events you’re trying to capture are what determine a mic’s character and its appropriateness for a particular source.

 

FREQUENCY RESPONSE

The first property many engineers concern themselves with is a microphone’s frequency response. You can think of this as the microphone’s relative sensitivity to different frequencies. Some microphones are really good at capturing low frequencies, and not so good at capturing the highest frequencies. Some mics are the opposite. Imagine if you knew the exact acoustic sound pressure level of a particular frequency being played in a room, and you measured the electrical output level coming from a microphone in that room. If you compared the electrical output level to the actual acoustic level playing in the room, you’d have a reference for how sensitive the mic is for that specific frequency. Now do that for a wide range of frequencies. If you displayed all of that information on a graph, it would look something like this:

Microphone Frequency Response

Frequency response graphs such as these tell you the measured results of a process very much like the one described above. They let you know how much the microphone itself will emphasize or de-emphasize particular frequencies in the material you are recording. No microphone has a perfectly flat frequency response, and that makes some microphones more appropriate for some sources or environments.

Frequency response is clearly important information to know when choosing a microphone, but my personal opinion is that people tend to focus a little too much on this one property to the detriment of a few other important properties. Perhaps this is because frequency response is easy to understand and visualize, and we can immediately hear a difference between two mics that have different frequency responses. But really, if this was the most important factor in choosing a microphone, one could just use EQ to make any mic sound like any other mic; experience tells us that it really doesn’t work that way.

 

POLAR PATTERN

Another property that affects a microphone’s sound is its polar pattern. You can think of a mic’s polar pattern as a map of the areas where the mic is most sensitive. Another way to put it is: from which directions does the mic accentuate or pick up sound, and from which directions does the mic attenuate or reject sound? There are three main polar patterns you’re likely to encounter: omnidirectional, bidirectional, and cardioid. Omnidirectional (or “omni”) mics are more or less equally sensitive to sound coming from any direction. It doesn’t matter whether sound is coming from the front, back, sides, top, or bottom, it will be captured with the same level as if it was coming from any other direction. Bidirectional (or “figure-eight”) microphones more or less only pick up sound from the front and rear, and more or less reject sound from the sides, top, and bottom. Cardioid microphones more or less only pick up sound from the front, and more or less reject sound from the sides and rear. If you were to measure the microphone’s sensitivity to sound from different directions and then display that graphically, it would look something like this:

As you can see from the graphs, the polar patterns are not brick walls of rejection, but gradual curves with ballooning shapes – hence the “more or less” in the descriptions above.There are a few in-between polar patterns that are also useful, but not as common as the three mentioned above. Sub-cardioid is between omni and cardioid – it’s mostly omnidirectional, but slightly more sensitive on the front than the rear. Supercardioid and hypercardioid are between cardioid and bidirectional – they are better at focusing on the very middle of the front and rejecting sound from the sides and front-sides than cardioid mics are, but as a result they develop little flares of sensitivity in the rear.

It’s important to remember that the polar patterns are intended to show direction of sensitivity – not a perimeter outside of which sound is rejected. A cardioid mic will still capture sound from its rear, but that sound will be significantly quieter than sound coming into the front of the mic. Another important concept when interpreting polar pattern graphs is to remember that a graph displays information on a 2D plane, but microphones capture sound in three dimensions, like this:

OFF-AXIS FREQUENCY RESPONSE

If you combine the two properties discussed already – frequency response and polar pattern, you will begin to notice another important characteristic of microphones – the off-axis frequency response. While the polar pattern guides are helpful, having a solid line that tells you authoritatively where that microphone will pickup and where it will reject is a gross oversimplification. The truth is that most microphones exhibit different polar patterns at different frequencies, generally acting more “omni” for lower frequencies and more sharply directional for higher frequencies. Often for high frequencies the polar pattern can bloom into complex explosion-shaped graphs with numerous lobes of pickup and rejection.

It’s complicated, because it’s physics. What it means for you practically is that as you travel around a microphone, that microphone’s frequency response can change dramatically. Sometimes a mic’s off-axis frequency response is smooth and very similar to its on-axis frequency response. Sometimes it’s peaky and sharp sounding, sometimes it’s balanced but dull. Generally these anomalies are exaggerated the further you get from its “normal” on-axis angle. Off-axis frequency response is most important when recording a group of live musicians in a room all at the same time, or when multi-mic’ing a complex instrument like a drum kit, because even when using highly directional microphones there will be bleed from the other instruments/voices/pieces of the drum kit. That bleed will be subject to the off-axis frequency response. When you mix all of the tracks together, that off-axis bleed needs to combine pleasantly with the on-axis sound from the  close mics on the same instrument/voice/piece of the drum kit.

Sometimes that means you want a smooth off-axis frequency response so that the bleed and the direct sound reinforce each other. Sometimes you want an off-axis response with a sharp high-frequency drop off, so that there is little overlap between the off-axis bleed and the direct sound. It’s not so much a matter of which off-axis frequency response is “best” as it is which blends best with all of the other microphones in that specific situation. Sometimes a microphone can sound excellent on a rack tom, for example, but when you combine it with the overhead microphones the sound of the cymbals becomes harsh or odd-sounding. Solo the overheads and they sound fine, solo the rack tom and it sounds fine, the issue is the off-axis cymbal sound bleeding into the rack tom microphone and the comb filtering that occurs when you mix that with the overhead mics. And all of this points toward a key factor when choosing a microphone for a particular element of the recording – it’s not always about making that thing sound as good as it possibly can. Often, the goal is to imagine beforehand how the sound from all of the different microphones will blend together into a cohesive whole. Now factor in an entire ensemble recording, or trying to record a full band in a room that’s really too small, and it becomes a lot to worry about!

It’s also worth noting that you can take advantage of a mic’s off-axis frequency response on purpose; many people will notice when mic’ing a guitar amp, for example, that pointing the microphone directly at the speaker can sometimes unnaturally accentuate high frequencies, making it sound harsh and piercing. If that happens, try turning the microphone to the side a little bit, so that the mic’s off-axis response will roll off some of those high frequencies.

 

PROXIMITY EFFECT

Another crucial property to consider that stems from the mic’s polar pattern is its proximity effect. The easy definition of proximity effect is that sometimes when you put a mic really close to the subject there is a large increase in low frequencies. Generally, omnidirectional mics exhibit no proximity effect, and the more directional a mic is the more pronounced the proximity effect is. That’s not always a bad thing! It’s simply another factor to be aware of when deciding on a microphone and where to put it. You can use the proximity effect on purpose to thicken up an otherwise lackluster sound. Some mics are even designed to sound a little thin when distance-mic’ing, so that when close-mic’ing the proximity effect fills out the spectrum and it sounds balanced. Some microphones exhibit more of a proximity effect than others, or at different distances, or in different ranges of the low and low-mid frequencies. Many mics even have a switchable low frequency roll-off, to counteract the proximity effect when close-mic’ing.

 

TRANSIENT RESPONSE

An often overlooked aspect of a microphone’s sound that in my opinion is one of the most important is the mic’s transient response. Transient response is how quickly or slowly the microphone responds to the the onset of a sound. One’s first reaction might be to insist that a quicker transient response is always better, but that’s not necessarily true. A transient response that is too quick can make some sources seem harsh or brittle, even when there is not a lot of high-frequency content. A transient response that is too slow, however, can make some sources seem dull or bland, even when there is plenty of high-frequency content. Related to transient response is a phenomenon called “overshoot”, wherein the microphone can have too quick a transient response, which then causes the diaphragm to snap back too forcefully. This can manifest itself as a slight ringing or distortion during heavy transients, which actually has the effect of smearing the transient instead of enhancing it. This tends to be a problem more with the loudest transients than with quiet ones. So when it comes to transients, a quick transient response can make a source sound upfront or intimate, but more is not always better. And keep in mind also, an important factor is not just the transient response for the thing you are pointing the mic at, but how that fits in with all of the other sounds from all of the other microphones. Often I will choose mics with slower transient responses for elements that need to sound further in the background of the mix, and mics with quicker transient responses for elements that need to stick out and draw the listener’s attention.

 

These are some characteristics to listen for when evaluating a microphone. Check out the next blog to learn about some practical considerations you might need to factor in.

 

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