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Frequency and Mic Polar Patterns

April 22, 2011

Despite the seemingly impenetrable set of numbers on a mic’s data sheet, there is a wealth of information that even casual users can understand and that will increase their intuitive awareness of what the transducer can offer. It all starts by spending a little time staring at the frequency-response and polar-pattern charts.

Typically, the information from these two diagrams is derived from sound captured at a distance of a meter from the mic element. However, anyone who has placed a directional mic—one with a cardioid or figure-8 pattern, for example—closer than three feet from a subject knows that you get a boost in the bass frequencies as a result of the proximity effect. So even though the frequency response chart only represents a mic’s potential sound reproduction at a specific distance, it nonetheless gives you a gestalt of that transducer’s behavior that you will measure against your experience.

But there is another side to the frequency response that you’ll want to pay attention to: how it affects the pickup pattern.

Top of the Charts

Like the frequency response chart, a polar plot tells only part of a microphone’s story. The directionality of the majority of mics these days isn’t consistent throughout the frequency spectrum: The polar pattern changes based on the wavelength it’s picking up, as well as the size and shape of the mic element and housing. Typically, a mic’s pattern will become more directional when it is capturing higher frequencies, which have shorter wavelengths, and less directional with lower frequencies, which have longer wavelengths.

This is clearly visible if a plot of the products’ performance across the spectrum is provided with the mic. Because Shure provides this information, I’ve selected two charts from its new Beta 181 line—the omni and cardioid plots. Keep in mind that the Beta 181 uses interchangeable capsules that are small and side-address; a mic’s physical design plays a big role in how the pickup pattern changes with frequency.

The nominal measurement you’ll see for most microphones is based on a tone, or set of tones, played at some distance from the mic—typically 1kHz at a distance of 1 meter. As you can see in Figure 1, the Beta 181 cardioid capsule gives you a near-perfect cardioid pattern at 1kHz, with the strongest output at 0 degrees (directly in front of the mic) and a gradual decrease in level as you approach 90 degrees off-axis. By the time you get 180 degrees off-axis, the output has decreased by more than 20dB, which is very significant. However, it’s worth noticing that the rear pick-up is not completely zero, and it rarely is with studio mics. Although, theoretically, the null point at 180 degrees of a typical cardioid mic has no output, there is indeed signal present, though it is made up primarily of low frequencies at very low levels. But for practical purposes, the drop of 20dB in the rear results in a recognizable cardioid pattern.

Next, note how the pattern changes at two octaves below 1kHz. The resulting increase in output at 180 degrees creates more of a subcardioid pattern, while the levels at the front and side remain consistent.

The biggest surprise for me when I first saw this plot was how the pattern transformed at higher frequencies. At 2.5kHz, the shape begins to move toward hypercardioid, then reverts back to cardioid at 6.5kHz before changing into a hybrid shape at 10kHz that approaches a figure-8.

Now let’s examine the omnidirectional capsule. The omni shape remains completely stable at frequencies below 1kHz and very close to its intended shape at 2.5kHz. By the time it gets to 10kHz, however, the output from the sides decreases well over 10dB, with slightly less decrease in output at the rear, yielding a shape similar to the cardioid at 10kHz.

Although these changes in shape look dramatic when plotted, they aren’t inherently bad; a mic’s response is part of what lends the transducer its character. But by understanding, say, the polar response of the omni capsule at all frequencies, you can judge whether it’s the right mic for a given situation.

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