Recording - June 2012 - 34

For the past two months, Fundamentals has focused on microphone basics, including the differences between condenser, dynamic, and ribbon designs. We also discussed how polar patterns are constructed and used. This month we’ll finish up our microphone survey with a look at several other important mic attributes.

As discussed last month, sound at the front of a directional microphone reaches the rear of the diaphragm later in time. While this phase relationship is crucial in constructing a directional polar pattern, it has a side effect of progressively increasing higher frequency response through phase-summation. The time difference of the two paths makes higher frequencies louder in level. Not wanting a microphone with uneven frequency response, manufactures compensate for this progressive treble boost by damping the diaphragm’s sensitivity to high frequencies. With damping, the mic sounds flat. The caveat to the damping solution is that the mic is only flat with not-close sources. The reason for this is that sound gets softer the farther it is from a source. Following the inverse-square law, sound expands spherically and loses four times its pressure level with each doubling of distance. When a microphone is several feet from a source, pressure has already experienced significant loss, and is no longer decaying so rapidly that several inches makes a big level difference between the front and back of a microphone capsule. When a mic is placed close to a source, however, the inverse square law puts greater level at the diaphragm front than at the farther-away diaphragm rear. The aforementioned capsule damping, tuned to reduce the build up of high frequencies on both sides of the diaphragm, effectively accentuates the low-frequency energy found louder at the front of the capsule. The mic now has greater bass response from up close.

Chapter 6: Introduction To Microphones — Part Three (From Proximity To Plosives)
By Eric Ferguson Microphone polar patterns can be grouped into two categories. Omnidirectional mics, also called “pressure” mics, pick up sound from all directions, while directional mics, also called “pressure gradient” mics, selectively focus their attention toward one or more bearing. Within this second category, a variety of directional patterns exist, from the simply constructed bi-directional to the complicated cardioid. Regardless of directional pattern, all pressure gradient mics work by responding to the difference in sound pressure between front and rear of their diaphragms. This reliance on pressure difference yields an important frequencyresponse byproduct: proximity effect. I like it when you’re close You’ve heard the result of proximity effect—it is what makes announcers sound manly and deep-chested. It is the tendency for directional mics to exhibit greater low-frequency response at closer distances. Most directional mics, when placed closer than about two feet from a source, are far from flat, with bass boosts greater than 10 dB common. Why does this happen? Unfortunately, the explanation is complicated, so those of you afraid of the sausage-making details may want to sleep through the next three paragraphs. But do go to Recording’s website where we demonstrate the proximity effect in the first of our bass-miking videos, at www.recordingmag.com/videos/videoDetail/12.html. Make sure you listen at full volume through full-frequency speakers. Small computer speakers can’t let you hear the low-end differences.
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Proximity effect and you While understanding the physics behind proximity effect requires intellectual effort, appreciating its tonal contribution is intuitive. We have all experienced the thick, low frequency boost that occurs when a microphone is close to a vocalist. In the early days of electrical mics, crooners such as Bing Crosby embraced the effect to their advantage. By singing close to newly introduced (and directional) ribbon microphones, Crosby and others pioneered the rich and sexy vocal sound we now take for granted.

Understanding the physics of proximity effect requires intellectual effort, but appreciating its tonal contribution is intuitive.

http://www.recordingmag.com/videos/videoDetail/12.html http://www.recordingmag.com/videos/videoDetail/12.html

Recording - June 2012

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