How Loud Can You Go?
Jan 1, 2003 12:00 PM, BY EDDIE CILETTI
This month, I'll examine how understanding and applying loudness-perception curves relates to room bass management. Our perception of loudness varies with frequency and level, as originally detailed by Fletcher and Munson (Fig. 1a) in their landmark “ear search” at Bell Labs in 1933, which generated the Equal Loudness Curve. Figure 1b illustrates 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 its application 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 to the ear's threshold of hearing (TOH) — the basis for the loudness controls found on consumer-audio systems.
MIC PROXIMITY EFFECT AND OTHER CAUSES OF LF MUCK
My research identifies several sources of added bass energy that are likely to detour engineers-in-training. (Much of this is equally applicable to those who have “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. (This topic, using the Sennheiser e-609 as an example, was covered in the October 2002 “Tech's Files.”)
Professional gear does not include “loudness- compensation” switches. At minimum, it helps to have an alternate system in the control room, defined not just as speakers but as a system, which includes 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 a typical control room environment — on systems without loudness compensation — always remember that Fig. 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. It is helpful to understand why the proximity effect's complementary bass boost might be misleading because it feels right, though it isn't necessarily the correct solution.
I suggest incorporating the loudness curve's LF portion into your listening environment. As Figs. 1a and 1b indicate, the curve is a bit of a moving target based on the level at which you like to monitor. As the level increases, the need to hear bottom decreases. But, 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 solely 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 your room of low-frequency response.
Any discussion of acoustics must include the subject of accuracy vs. 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 (aka reality), because consumer systems are anything but flat. But bigger rooms with the accepted “magical” dimensions (such as a rectangular room with a length-by-width-by-height ratio that's approximately 2×1.5×1) are inherently more fixable. Untreated, all rooms exhibit some amount of low-frequency buildup in the corners that's 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 bigger challenges, along with their proportionally smaller monitoring systems. Those with 6-inch or smaller woofers can't move enough air to create effective low-frequency impact, and a subwoofer doesn't help in the critical frequency range of 120 to 240 Hz — the two octaves that are well represented in car audio systems, especially with the loudness boost in effect.
Because 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.
Figure 3 shows a “pedestal” that I built for my Fostex NF-1A monitors in order to place the tweeter at ear level. Because I built 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 for simplicity's sake, because adding an inductor in series created a simple first-order crossover at 150 Hz. (Adding a second speaker in parallel would have boosted the level 6 dB 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 150 Hz.
After augmentation, the low-frequency response is emphasized — yet remarkably cohesive — instead of the disembodied bottom that is often associated with subwoofers that are crossed over too high. Regardless of whether this solution is applicable beyond my own personal needs, the point here is to raise awareness about the loudness curve and offer a possible solution. It is important to maintain consistent, safe monitoring levels because an increase or decrease can skew a track's or a mix's spectral perception. Higher levels can diminish the need for loudness compensation but cause ear fatigue or worse, permanent damage.
With this particular solution, 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. In a future column, I'll document the “before and after” response of the room and monitors, attempting to correlate the augmented response with Fletcher, Munson, Robinson, Dadson, et al.
Visit Eddie at www.tangible-technology.com for additional data on the NF-1A (including simple internal modifications) and other interesting readings.
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