Talkin’ Mic Basics with Shure’s Gino Sigismondi: Choosing the Right Mic
Sometimes people ask us, "Why do you guys make so many different types of microphones?" The answer is: there are many different applications that require different form factors – handheld, headset or stand-mounted and different technologies that produce different sound qualities. But often, it comes down to what sounds right to you: it ultimately comes down to using your ears and listening. But understanding the subtle and not-so-subtle differences will help you narrow the field.
Different Transducer Types
A transducer is anything that can take one form of energy and convert it to another form of energy. That's what a microphone does.
A mic measures the variations in air pressure that we recognize as sound waves and changes them into electrical signals that can be manipulated for sound reinforcement, for recording purposes or for broadcast. The acoustic wave is converted into an analogous electrical signal. All microphones do this, but they do it in different ways. So a microphone is really just a measurement device – measuring variations in air pressure and providing a corresponding electrical signal.
As the front end of the audio system, the microphone is one of the more important elements in the signal path. If you don't capture the sound accurately before it gets into the electrical domain, there really isn't a great way to fix it later on. The more of that you do on the back end with processors and other tools, the more work is involved and the less natural it will sound. If you choose the right mic and put it in the right place, everything that follows will be that much better.
Dynamic and condenser mics are most popular types. There are other types – ribbon mics, crystal mics, control magnetic, and carbon mics, for example - but those are largely historical, so we won't cover them here.
The most popular is the dynamic mic. It's a very simple device – rugged, reliable and in most cases, not very expensive. Sound waves move a thin, lightweight diaphragm, typically a very thin layer of a Mylar®. The physical energy required to make this diaphragm move is not very great.
[caption id="attachment_10028" align="alignright" width="300"] Cutaway of a Dynamic Microphone[/caption]
The diaphragm has a coiled wire attached to it and is suspended in a magnetic field. A basic property of electricity is that when a wire cuts through a magnetic field, a current is induced in that wire. As sound waves strike the diaphragm and move it back and forth, the coil also moves back and forth in the magnetic field, inducing current and a corresponding varying voltage in the wire. Those wires go out to the connector at the bottom of the mic. Some microphones might have an output transformer to step up the impedance and provide a little more signal, some don't. That's the basic structure of a dynamic microphone.
It's a completely passive device, so there's no additional power needed to get it up and running. Plug it into your system and you're good to go. Because they are such simple devices, they're not very expensive, they're very reliable and they're hard to kill. Think SM58® and SM57.
There are some limitations, of course. They're not very sensitive. It takes more energy get that mass of the coil that's attached to the diaphragm moving so they're better for up-close applications and loud sound sources. They're not very good for miking sound sources from far away, like a choir for instance.
They're nearly impossible to overdrive. A human being can't create enough sound pressure level to overdrive a dynamic microphone. There is no way, for instance, that a singer can destroy an SM58 by singing too loud. There may be some distortion at the input of the mixer if its gain control is set too high, but the problem is not happening at the microphone. You'd have to mic the space shuttle for something like that to happen.
Most dynamic mics sound pretty good, but there's a limitation in frequency response in terms of how much high and low frequency it can pick up.
[caption id="attachment_10029" align="alignright" width="300"] Cutaway of a Condenser Microphone[/caption]
Condenser microphones are a little more complicated.
One critical difference is that the diaphragm of a condenser mic does not have the mass of a coil hanging off of it. The actual diaphragm is metalized, usually gold-layered or gold-sputtered and the diaphragm is tensioned over an air gap above a charged metal backplate.
When the sound wave strikes the diaphragm, it doesn't have to work as hard to move it because there's no mass of coil attached to it – and that's one reason why condenser microphones are more sensitive. They're designed for quieter sound sources.
The output of a condenser microphone is much lower and the impedance is much higher, so there are some additional electronics – specifically, a microphone pre-amp – that's part of the mic design. The pre-amp requires phantom power, supplemental voltage that powers up the electronics of the condenser microphone. Phantom power is typically supplied by the mixer the microphone is connected to.
If you don't provide a condenser microphone with phantom power, it simply will not work. It's a call we often receive at Shure from people who are accustomed to using a dynamic mic like an SM58 but purchased, for the first time, a $300 condenser mic. They plug it into their sound system and it doesn't work. This leads to a longer discussion of phantom power and a suggestion that they turn on their mixer's phantom power switch. It's an important detail to remember. There are a few condenser microphones that will run off a battery, but this is far less common.
They're more sensitive to environmental conditions and they're more expensive than dynamic mics because there are many more internal electronic components in their design. But on the flip side, they're more sensitive and offer a wider frequency response so they're more natural sounding. However, due to the active electronics that are part of condenser mic design, it is possible to overload or cause distortion in the microphone. Some condenser microphones are equipped with a "pad" that can be engaged to reduce the sensitivity of the microphone when used with loud sound sources.
This can be divided into two categories – and really, it's just about how the microphone sounds:
Shaped Response – can take many different forms.
The X-axis in this diagram shows the frequency of human hearing, from 20 Hz to about 20,000 Hz. The Y-axis shows the output level of the microphone. You can look at the different frequencies to determine how much signal that particular mic is putting out. You'll notice at some frequencies, the output of the mic is lower or less sensitive and on others, the output of the mic is higher. This can provide an advantage in certain scenarios.
For example: If you're looking at the 2-6 KHz range, the SM58 mic is more sensitive and has more output. This is good because this is the range of most human speech where consonants can be heard. Consonants define speech intelligibility. In a church application, the message is the most important thing – so it's important to have a microphone with good sensitivity in this range.
Now, look at response in the range below 100 Hz. The response drops off pretty dramatically. In the case of the human voice, that's OK unless you're trying to mic a bass singer in a gospel quartet. What happens in that range is mostly unwanted noise, wind noise, handling noise, vibration, so if you have a mic that rolls off a lot of that, it's beneficial for cleaning up the overall sound quality. Response below 100 Hz is usually unnecessary unless you're miking a grand piano, bass drums or the occasional bass singer.
Flat Response - is just what it sounds like. The output of the microphone is pretty much the same across all frequencies.
It will pass everything along, whether or not it's needed or desired. It's a very natural sounding and very uncolored frequency response. For acoustic instruments, for example, where you don't want to alter the sound in any way, a flat response mic might be the best choice.
Which response you need really depends on what you're miking. A wide-ranging flat response mic will pick up sounds that you don't necessarily need and it won't color the sound coming out of it.
Directional Response- This is how the microphone responds to sounds coming at it from different directions. There are two categories:
Omnidirectional – sound coming from all directions
Uni-directional – sound coming from one direction
Bi-directional is another, less common category that refers to a mic that picks up sound from two directions, but we'll focus on two that you are most likely to encounter.
Omnidirectional ("omni") mics are sensitive to sounds coming from all directions. They have a coverage angle of 360o so it doesn't matter where the mic is pointed. The response will be the same. Omnidirectional microphones are good for speech applications, as lavalier or headset microphones. In this case, they offer the most "uncolored" response (see proximity effect below), and since you don't have to worry about picking up the drum kit, the lack of off-axis rejection isn't really a concern.
Unidirectional ("uni") mics take on a couple of different variations, the most popular of which is the cardioid pattern. It has a heart-shaped pickup pattern; that's where the "cardio" comes from. When you look at the diagram, you'll see that there's very little pickup 180o off the center.
The cardioid pattern is designed to capture the sound source you want to capture and reduce pickup of everything else, since it effectively rejects off-axis sound. On a stage with a lot of sound sources and a lot of noise, it's very beneficial compared to an omni which will tend to pick up everything. Since the cardioid mic is less sensitive to other sounds, like the sounds coming out of loudspeakers, it allows you to get more gain before feedback than you would with an omni.
Like everything else in audio, there are some trade-offs. One of these is proximity effect, something that every unidirectional mic exhibits. That's the boost in low frequencies as you move closer to the microphone. Sometimes people like this effect and other times that bass response will muddy things up. Omni mics don't product this effect since the frequency response is the same no matter how far the sound source is from the mic itself. Cardioid mics are also more susceptible to handling, wind noise and vibration.
Supercardioid and hypercardioid are even more directional. There's even greater rejection at the sides but a little bit more pickup in the null area (at the back of the microphone). The overall sensitivity to ambient sound is less than even a cardioid mic. An experienced vocalist in your church can really benefit from this type of tight polar pattern, but a less experienced singer who moves the mic around in a theatrical fashion will run into problems.
Keep in mind that there's never a one-size fits-all option. It all depends on what sounds best for your application.
The Myth of Microphone Reach
One common misconception is that directional microphones reach like a zoom lens on a camera – that you can take your viewfinder and focus on something far away and bring it closer. Microphones don't work that way.
Sound waves are much longer than light waves and microphones are not able to bend those waves to bring them closer. Microphones don't have a reach associated with them. What that means is that you need to get the mic as close as possible to the sound source for a couple of reasons:
- The microphone is not going to go out and isolate a particular sound.
- Sound waves follow the inverse square law. That says that the energy of a sound wave drops as it spreads out in space. Every time you double the distance between the sound source and the microphone, you lose 6 dB of signal, which is quite a bit. If I move the microphone one foot away, the drop is sound is noticeable. If I move it from 1 foot to two feet away, that's a 12 dB drop which will be perceived as more than half as loud. So be aware that when you're moving microphones further and further away, you are losing a lot of the direct signal.
What this graphic shows is that there's a certain amount of noise and reverberation in any given room. That's a concept known as critical distance which is the distance at which the direct sound of what you're trying to mic and the ambient noise and reverberation become equal. When your microphone is beyond that critical distance, you'll hear all the ambience in the room at a level equal to the direct sound. It's the sound that some people describe as being in the 'bottom of a barrel' or sounding like a 'tin can'. Every room will be different and if you don't want to have to calculate what the critical distance is for every worship space, just try to remember to keep the microphones as close to the sounds sources as you can.
Another phenomenon you may experience is comb filtering which is where the audio signal takes multiple paths to reach the microphone, possibly reflecting off a tabletop or a lectern and having those reflections combined back in the microphone itself. When that happens, the frequency response graph looks like a comb – that's where the term comes from. It has a very hollow, phase-y sound that's not very natural and can really be distracting at times. It's another reason to keep the microphone close to the sound source, so that the direct sound will be much louder than the reflected sound. It's also a good argument for longer gooseneck microphones in lectern application because it keeps the mic further away from surfaces and closer to the speaker's mouth.
You can also experience electronic comb filtering. This happens when there is more than one microphone picking up the same sound source. It can easily happen in a choir application. When the same sound source goes to two different microphones and those mics are combined back in the mixer, you end up with the same comb filtering frequency response effect.
How you deal with electronic comb filtering is by following the 3-to-1 Rule. It's a good rule to remember in sound applications where more than one mic is being used. It states that for every unit of distance from the mic to the sound source, the next microphone should be three times that distance away.
It's a common problem in many sound systems. But it's not the fault of the microphone. Feedback results from the interaction of all the components in the sound system.
Here's what's happening: the sound source goes into the microphone and the microphone signal goes into an amplifier and then a loudspeaker where it's made louder. That same sound comes out of the loudspeaker and is picked up by the microphone again – it forms an audio loop that results in the sound or sounds we know as feedback. You can't buy a microphone that "doesn't have any feedback in it".
Tips for avoiding feedback:
- The way to combat feedback is to keep the microphone as close to the sound source as possible.
- Keep the mics as far away from the loudspeakers as possible. If you can keep them separated from the loudspeakers, it's less likely that they will pick up the sound and create a feedback loop.
- Lower the speaker output.
- Move the loudspeaker farther away from the microphone. Each time this distance is doubled, the sound system output can be increased by 6dB.
- Move the loudspeaker closer to the listener. Each time this distance is halved, the sound system output will increase by 6dB.
- Use in-ear monitoring systems in place of floor monitors.
- Acoustically treat the room (if possible) to eliminate hard, reflective surfaces like glass, marble and wood.
Some people think using unidirectional microphones will solve their feedback problems, but it's actually less effective than many of the suggestions above. EQ can also be used and room acoustics are also a factor, but in most cases, following the first three tips here will go a long way in reducing feedback problems.
ABOUT GINO SIGISMONDI: Gino Sigismondi has been active in the music and audio industry for nearly twenty years. Currently managing the Systems Support department, Gino brings his years of practical experience in professional audio to the product training seminars he conducts for Shure customers, dealers, distribution centers, and internal staff. He is the author of the Shure educational publications "Selection and Operation of Personal Monitors," "Audio Systems Guide for Music Educators," and "Selection and Operation of Audio Signal Processors."
Gino spent several post-college years as a live sound engineer for Chicago-area sound companies, nightclubs, and local acts. He continues to remain active as a musician and sound engineer, expanding his horizons beyond live music to include sound design for modern dance and church sound.