In my January column, I touched on four variables of low-frequency perception: the Equal Loudness Curve for humans; loudness controls found on consumer gear; acoustics; and the proximity effect of directional microphones. This month, we'll explore the last topic.
The 1-2-3 combo of the loudness curve, inconsistent monitoring levels and unresolved acoustic issues can (and will!) conspire to reduce the perceived Low-Frequency Energy (LFE) in any environment. Even before the listener becomes aware of this deficiency, along comes the directional microphone. Used upclose and personal (to maximize isolation), mics with cardioid and figure-8 polar response exhibit a proximity effect, a noticeable rise in LFE as the mic is moved closer to the source — a bump that makes things sound “right” and “warm,” at least for the moment.
Furthermore, multichannel recordings — where many directional mics have been used at close range — may suffer from a build-up of low-frequency “muck.” In such cases, it's always preferable to remove the muck rather than compensate by boosting other frequencies, especially those in the 2 to 5kHz range, where the ear is most sensitive. Heightened awareness of these four variables — any one of which might be your Achilles' heel — should make them easier to tame, hence this month's continuing zoom-zoom-zoom on the boom-boom-boom.
MAPS AND CHARTS
More than any single piece of audio gear, mic and speaker (transducer) idiosyncrasies in the frequency response are the most easily identifiable by ear. You should be intimately familiar with at least one of the seven microphones represented in Fig. 1 through Fig. 6. Typically, these exhibit all sorts of bumps and lumps; by comparison, electronics are almost indistinguishably flat. The charts shown here were downloaded from the Web and resized for equal amplitude (vertical) and frequency (horizontal). If you'd like a closer look, they're also posted at www.tangible-technology.com; click on “recent articles.”
To generate their published “flat” response curves, microphones are typically measured in the lab at a distance of 1 meter (39.37 inches) from the source. For directional mics, any distance closer to the source yields increasingly more “bottom” from the proximity effect. (Omnidirectional mics are not affected by proximity.) Knowing their products are used up close, directional microphone designers may incorporate a bass roll-off filter — either switched or incorporated into the mic itself — which can be parsed from the 1-meter response (although detailed proximity curves are preferred). Three of the mics investigated here have published proximity response, detailing that all the action is at 1 foot or less, reinforcing the oft-suggested advice of “moving the mic first” rather than using EQ to fix the situation.
Manufacturers are not consistent in their presentation of data, either in graphic or text form, although the Internet is much easier to update than reams of product literature. On the Net, I learned that the roll-off of the AKG C-12VR is -6 dB/octave at 100 Hz and -12 dB/octave at 130 Hz, while the Neumann TLM-170's roll-off circuit “attenuates the frequency response below 100 Hz to suppress undesired structure-borne noise.” Another example is the Earthworks SR-77, which is “flat” at 15 cm (6 inches), so obviously the response at 1 meter shows a substantial roll-off. All are helpful bits of information, but the addition of proximity curves can help identify a problem and determine whether the best solution is the built-in filter or external EQ.
I originally planned to start in familiar territory with the Shure SM-57 but instead chose the Beta 57 because it provided proximity curves at 2 feet, 2 inches, 1 inch and ⅛-inch (Fig. 1). These most important curves reflect the typical response when used under “normal” conditions, the “eating” style favored by rock performers at 2 inches or less. (I love it when I see vocalists actually work the mic, but that doesn't happen often.) At 2 feet, the response is down almost -3 dB at 200 Hz, but at 2 inches, the response between 100 and 200 Hz increases 7 dB to 10 dB, or about equal to the hyped response in the presence region (5 kHz). An inch closer adds almost 5 dB of warmth; that's the Inverse Square Law working — it's magic! If not already instinctively ingrained, this information should be helpful to recording and live sound engineers alike.
Sennheiser's e-609 and the venerable MD-421 do their chart dance together in Fig. 2. At the top left, the e-609's proximity effect at 5 cm (1.96 inches) yields a 10dB boost centered around 125 Hz. Below, the MD-421 shows the results of its five-position Music/Voice switch from “flat” to -16dB roll-off at 100 Hz. For myself and many others, the MD-421 is often a kick drum mic, based on its 10dB upper-midrange bump.
Figure 3 details the on- and off-axis response of an Electro-Voice RE-20 (top and bottom, respectively). The off-axis (leakage) response greatly affects the perceived sound of a mic. The RE-20's rejection response is particularly smooth. DPA (formerly B&K) provided the widest selection of graphs for its mics, including a highly detailed look at just the proximity response of the 4011 cardioid mic.
The 4011's response in Fig. 4 is two graphs in one: To the left are the effects of proximity from 1 meter to 0.1 meter in five steps; to the right of 1 kHz, I appended the graph with the effect of cable length on max output level (148dB SPL). This is not the response of the mic under normal use even with these cable lengths. The DPA 4001's low-frequency response seems optimized at a distance of 12 inches, where it's flat down to 20 Hz. Just under 4 inches from the source, the boost at 50 Hz (and below) is about 12 dB. Wow!
Next are the large-diaphragm condensers. Figure 5 shows the response for a Neumann TLM-170. The roll-off is about -6dB per octave starting at 160 Hz. Figure 6 shows the response of three AKG CK-12 capsules as re-skinned by Walker Microphones in Canada (519/654-0070), mounted on the vacuum tube electronics body of a C-12. The CK-12 chart is useful for several reasons. First, it is not a “publicity photo,” but rather the raw output from a measurement system. All of the other response charts are smoothed either purposefully via system options or by a graphic artist. Smoothing is fine to depict the overall character of the mic. Figure 6 shows subtle variations in three capsules, barely obvious except in the bottom two octaves. In this case, the chart was used to determine the best match for a stereo pair. All microphones have production tolerances.
THE LOW DOWN
Studying these charts made each of these mics (which I have used and abused ad infinitum) more tangible. From this research, it's clear that many directional mics are designed to be “flat” not at 1 meter, but at unique distances, predetermined by their “typical” application. I hope this article sheds some light on the power of proximity — knowing where “flat” is and, from there, realizing the dramatic difference ±4 inches can make.
It was equally interesting — surprising in some cases — to discover each mic's unique characteristics at the opposite end of the spectrum. We all know how different models respond to similar applications; we all have favorites. Hopefully, the “why” is now a little more apparent. Like the proverbial squeaky wheel, it's the odd track that gets our scrutiny, and being familiar with microphone characteristics can go a long way toward reinforcing an engineer's intuition at the moment of capture.
In our business, there are more relatives than absolutes. Whether tracking, overdubbing or mixing, the primary job is to make each channel relate to the whole, regardless of the monitoring system being used. If the process of making the puzzle pieces fit becomes a struggle — particularly when the bass needs more room — then it's the monitoring system that gets the scrutiny. Then we want absolute confirmation of the truth, but in the end, it's all relative. Isn't that something Stephen St.Croix would say? I meant to say that the final judgment rests with how it plays in Peoria.
Eddie researches, repairs and consults from the afternoon to well past midnight. From dawn till noon, he's often found attending to his toddlers' “input/output” details.