Microphones With Switchable Polar Patterns
| FIG. 1 - Circuit of a condenser microphone with electrically switchable polar characteristics.|
| FIG. 2 - Polar patterns obtainable from a microphone as shown in Fig. 1 by adding or subtracting two cardioid patterns.|
In previous columns, we discussed the different microphone polar patterns, omnidirectional, bidirectional, and directional (which included cardioid, supercardioid, and hypercardioid). Each of the mics discussed produced only one of these particular polar patterns.
But mics can also be designed to produce more than one polar pattern. A switch on the body of the mic allows the user to select one of the provided polar patterns.
But how do they do that, asked Jeff Koscho, senior systems engineer for Video Networks Inc. He wrote: "I just read your pet peeves article. In it, you mentioned a difference between omnidirectionals and directional mics. It is consistent with what I've learned, taught and lived with through the years: slits behind the microphone diaphragm allow the sound to enter and create acoustic cancellations.
"What I'd like to know, and hopefully you can provide some insight, is how do the switchblade-patterned microphones work? The switch is an electrical device, but the change in pickup patterns are due to acoustic principles. I asked a manufacturer's rep this one once, and he didn't have an answer for me."VARIABLE PATTERNS
Let's try to answer this question here.
In general, for a microphone to have different polar patterns it must have more than one diaphragm element. The electrical outputs from each of the elements, which result from the impinging acoustical sound pressure levels, are added or subtracted from each other, thus forming new polar patterns.
Now let's get more specific. A common way of creating different polar patterns in one microphone is to mount two condenser cardioid elements back to back inside the mic housing.
A common construction method is to create a single element with two diaphragms (one facing the front of the mic, the other facing the rear) with a common backplate. The mic body is also designed with appropriate slits and internal acoustic delay elements as part of the cardioid design.
Remember that the condenser mic uses a capacitor for each element. One of the two parallel plates of the capacitor is actually the diaphragm, which moves in the presence of sound waves, and the other is a fixed backplate. This type of mic requires a polarizing voltage.
The polarizing voltage to the front element is fixed, but the voltage to the rear element is variable. A common method is to switch this voltage, and that is what the selector on the mic body is doing. The voltage to the rear element can also be continuously variable with a potentiometer. The element's sensitivity is proportional to the applied polarizing voltage.
The electrical signal (AC) outputs of each element are summed together to produce the final output of the mic.
Let's use a mic with a switch. If it's set to the cardioid position, the voltage to the rear element is switched off, so that only the front element contributes any signal. Since this element is designed to be a cardioid, then the polar pattern of the entire mic will be a cardioid.
If the polar pattern switch is set to omni, it switches in a DC polarizing voltage to the rear element that is equal in value and polarity as that of the front. Now, both elements will produce signals each with a cardioid polar pattern. When the two cardioid patterns are added together they produce an omnidirectional pattern.
If the polar pattern switch is set to figure-8 (bidirectional), it switches in a DC polarizing voltage to the rear element that is equal in value but opposite in polarity to that of the front. This means that the signal from the rear cardioid element is subtracted from the signal from the front cardioid element, resulting in the figure-8 pattern.
This type of construction is used on such microphones as the Shure KSM44, and the Neumann U87 Ai.
The Neumann TLM127 has two more polar patterns available-a wide-angle cardioid (between the omni and cardioid) and a hypercardioid (between the cardioid and figure-8)-plus the three mentioned above. This is done by applying a voltage (either positive or negative) that is lower than that applied to the front element. This lowers the sensitivity of the rear element, and it thus produces a lower output, that when added to or subtracted from the output from the front element, produces these intermediate polar patterns.A FEW SWITCHABLES
In addition to the switch on the mic body, many Neumann mics can be remotely controlled with standard phantom powering on standard shielded twisted-pair audio cable.
The AKG C 414 B-XL II is another example of a mic that has five switchable polar patterns-omnidirectional, wide cardioid, cardioid, hypercardioid, figure-8. An optional remote control is also available.
The AKG C 4000 B has these three switchable polar patterns-omnidirectional, cardioid, and hypercardioid.
(The mics mentioned above are only a sampling of those that have switchable polar patterns.)
The Shure KSM9 takes a different approach to producing multiple polar patterns. This mic can be switched to either cardioid or supercardioid.
Michael Pettersen, director of applications engineering at Shure explained how the KSM9 is constructed:
"The KSM9 has two electret condenser elements; one element is in front of the other. Both elements face forward and are contained within a single housing. The front element is acoustically tuned half-way between cardioid and supercardioid. Via a double-pole switch, the rear element is connected in parallel to the front element.
When the rear element is connected to have the same polarity as the front element, the resultant pattern is cardioid. When the rear element is connected to have the opposite polarity as the front element, the resultant pattern is supercardioid. Using this approach, the sensitivities of both patterns are nearly identical."MIXING IT
Another approach to creating variable polar patterns is to bring out the signals from the different microphone elements and mix them externally, like on separate channels of an audio mixer.
The Josephson Engineering C700A consists of an omnidirectional element and a figure-8 element. Its C700S model has an additional side-facing figure-8 element that provides not only the polar pattern itself, but allows the direction of the pattern to be changed. Here, the polar math works like this: omni-plus-figure-8 (equal levels and polarity) equals cardioid. At the front, the level is double that of the individual elements.
Changing the levels of the two elements relative to each other with an audio mixer, provides a wide range of polar patterns. Changing the polarity of the figure-8 results in patterns facing the opposite direction (like a reverse-cardioid, for example.)
The side element of the C700S, allows the polar pattern to be "steered" in different directions. This means that the maximum response of the mic doesn't necessarily have to be at 0 degrees on axis. It could be at 30 degrees, -45 degrees, as two examples, or at other angles depending on the level and polarity of the side element mixed compared with the omni/figure-8 combination.
Microphones with switchable or variable polar patterns are useful additions to one's microphone collection. They provide a certain amount of flexibility for use with different sound sources and recording environments, and you can learn a lot about the characteristics of different polar patterns under these different circumstances.