From Rhythm Track to the Mix: The Art Of Sonic Translation

ã 2002 ~ 2004 by Eddie Ciletti

Summary of this article

• Low Frequencies are not easy for our ears to perceive unless loud (Loudness Curve)
• Low Frequencies are not easy for monitors to reproduce WELL (big speakers do it better because they have the larger surface area required to move all that air)
• Low Frequencies are boundary and acoustics dependent - where you place monitors in a room can have a significant effect on what you hear.
• Directional mics have proximity effect - a low frequency boost - and as such, there is likely to be bottom that must be 'thrown away" either by high pass, shelving OR by using an omni microphone.

Getting a mix to translate to the outside world is still one of the great audio challenges, especially for beginners.  When achieved, it is often by successive approximation, overshooting the target more often than not.  When an approach works, we stick with it, even if the method is flawed. How can the process be so complicated?  Or Simplified?  The answer lies in our perception of "loudness" along with our psychological preference for using technology to "fix" rather than pursue a better understanding of sonic science. 

I started by building monitor stands that double as Bass Extenders rather than solve the problem with a subwoofer.  Click Here to View.  Near Field monitors have a low-frequency limit and while super low frequencies may be omnidirectional, I wanted full range from a stereo pair.  This is not the point of the article, except to say that in order to better judge Low Frequencies, it is important to be able to hear them.  A full-range monitoring system is essential - and can be affordable - whether you buy a system outright (8-inch woofers minimum), augment as I did OR choose the easiest and most affordable route via Subwoofer.  Just note that Subs solve some problems while creating others, the most obvious of which is calibration.  Subs should be placed equidistant between the main monitors.

THE PREMISE
Cardioid mics positioned close to the source generate lots of extra bottom.  This might be ok for some things, but it can add up  — I am calling 9dB more than necessary — a per channel build- up that makes for a lot of muck.  Problem is our control rooms are not loudness compensated and often, insufficiently treated, so that the extra bottom sounds right to our ears.

Fixing the bottom, rather than compensating with more treble, results in smoother sounding mixes.  According to the Loudness curve, our ears are some much less sensitive to bass than to treble, so treble compensation makes a recording sound more like the equalizer than "natural."  Thinning out the excess bottom leaves more room for the stuff that's supposed to have bottom.

Click here for more on the topic of Proximity Effect

All of this stuff is explained below...

SONIC SAND TRAP

All audio grasshoppers will eventually grasp that equalization can be both a seductive and deadly detour. Even seasoned veterans wrestle with the spectral balance of a mix from time to time. If a mix seems bass heavy, is it better to roll off the bass or add treble? Doing the latter tends to make a mix sound like the equalizer — twenty-four or thirty-two equalizers all with about the same bandwidth, at a similar frequency and overlapping into the ear's most sensitive frequency range. The end result is a harshness that is exacerbated by Digital’s lack of sonic forgiveness. Here’s one solution that suggests digesting the sonic food before adding spices…

To minimize the time detour often associated with the "successive approximation" approach to mixing, take the time to burn a CD early in the recording process. Doing so will reveal problems and allow the opportunity to fix tracks well before the mastering process. Burn two CDs, one flat but normalized, the other with just a little peak limiting and normalization. Play on as many systems as possible, using a MiniVan as the acid test. Take note of the position of the level control keeping in mind that higher gain will reduce the effect of the loudness compensation. What's that you say?

FIGURE-1a: The Fletcher-Munson  Equal Loudness Curve (1933)

FIGURE-1b: The Robinson-Dadson  Equal Loudness Curve (1956) The Threshold of Hearing is designated "TOH."

Our perception of Loudness varies with Frequency and Level as originally detailed by Fletcher and Munson (Figure-1a), their "ear-search" at Bell Labs in 1933 generated the Equal Loudness Curve. Figure-1b shows the results of a later study by Robinson and Dadson in 1956. Many others have contributed to a body of work that also crosses over into the Science of Perception (and masking) known as Psycho-Acoustics and as applied to such algorithms as AC-3, and MP3. Figure-2 is the response curve required to make all frequencies appear equally loud, all the way down at the ear’s Threshold of Hearing; it is the basis for the Loudness Button on consumer audio systems.

 
FIGURE-2: At the Threshold of Hearing, this is what the ear needs to hear. 
Image courtesy of Rod Nave@gsu.edu (Department of Physics and Astronomy @ Georgia State University) 

 

The Proximity Effect of Directional Microphones 
and 
Other Causes of Low Frequency Muck Build-Up in an Audio Recording.

Pretty clever title, eh? My research identifies several sources of added bottom that are most likely to detour engineers-in-training. This is just as applicable to those having "treble issues." To start, all unidirectional (cardioid) microphones are only "flat" at one-meter from the sound source. Move them in close and the low-frequency proximity effect makes things warm and intimate. The curves for the Sennheiser e-609 are shown below.

The response curves for the Sennheiser e-609 dynamic cardioid microphone are typical of a directional microphone's low frequency proximity effect.

HEY BUD: Wanna fix a muddy mix? 

No professional gear has any Loudness Compensation so at minimum it helps to have an alternate system in the control room, defined not just as "speakers" but as a system including the amp that powers them. It does not help to turn "off" the loudness button — because this does not reflect the consumer environment. Loudness for automotive systems is typically a default boost that is reduced as the volume control is increased.

When working in the typical Control Room environment — on systems without Loudness Compensation — always remember that Figure-2 represents what the ear wants to hear at the Threshold of Hearing. Using Cardioid mics at close range without bass roll-off combined with loudness compensation on consumer equipment results in a double bass boost. Now understand why the Proximity Effect’s complementary bass boost might be misleading, because it feels right yet is not necessarily the correct solution. 

It is my suggestion to incorporate the low-frequency portion of the Loudness curve into your listening environment. As Figures-1 (a and b) suggest, the curve is a bit of a moving target based on the level at which you like to monitor. As the level is raised the need to hear bottom is lessened yet, as a health consideration, monitoring at excessive levels over longer periods of time causes hearing fatigue and damage.

NOTE: It was once acceptable to tune a room using Equalization. I am not talking about graphic equalizer abuse — as was too often the case — but a simple, gentle curve. If you don’t feel comfortable incorporating EQ, then part of the solution is to address acoustic issues that might be robbing the control room of low frequency response.

ACOUSTI-FIX

Next comes the fork in the road, accuracy or reality? A truly accurate room would be a large space with large monitors working in a comfortable range to fill the space. Such an optimal sonic environment might not divulge the problem areas for consumer systems, a.k.a. reality, because consumer systems are anything but flat. But bigger rooms of the accepted "magical" dimensions are inherently more fixable. Untreated, all rooms exhibit low frequency build-up in the corners that is reflected back to the listening area causing destructive cancellation. These areas can be treated to trap and absorb, reducing cancellation and improving bass response at the listening position. 

Smaller rooms create greater challenges along with their proportionally smaller monitoring systems, those with 6-inch woofers or less can’t move enough air to create effective low-frequency impact and a subwoofer doesn’t help in the critical frequency range of 120Hz to 240Hz. These are the two octaves that are so well represented in car-audio systems (especially with the Loudness boost in effect). 

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Click for Fostex NF-1A Review
FOSTEX NF-1A on custom-built Bass Extender / monitor stand
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FIGURE-3:  The Fostex NF-1a on its custom pedestal / bass extender

Since I prefer to listen at low levels, the decision to go for a "curve" in my room seemed logical and necessary, rather than attempt to achieve a "flat" response or use an Equalizer to create a curve I chose… AUGMENTATION Figure-3 shows a "pedestal" I built for the Fostex NF-1A monitors to place the tweeter at ear level. Since building a cabinet and not just "a stand," the idea quickly evolved into adding an extra woofer to extend and augment the low-frequency response. The crossover options were either active or passive, I opted for the latter just for simplicity’s sake — adding an inductor in series created a simple crossover at 150Hz. (Simply adding a second speaker in parallel would have boosted the level 6dB up to the crossover point with the tweeter.) If all goes as planned, I expect to measure as much as a 6dB boost somewhere below 150Hz.

After augmentation the low frequency response is emphasized, yet remarkably cohesive, instead of the disembodied bottom that is often associated with subwoofers crossed over too high. Regardless of whether this solution is applicable beyond my own personal needs, the point was to raise awareness of the Loudness Curve and offer a possible solution. It is important to maintain consistent, safe monitoring levels because an increase or decrease can skew the spectral perception of a track or mix. Higher levels may diminish the need for loudness compensation but cause ear fatigue or worse, permanent damage. 

In summary, I split the difference between accuracy and reality, choosing a bass boost through augmentation rather than attempting EQ and potentially overdriving the 6-inch woofers (or my ears). A future article is planned to document the "before and after" response of the room and monitors attempting to correlate the augmented response with with Fletcher, Munson, Robinson, Dadson et al. That ends today’s lesson as well as your course outline options for the year (and then some).

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