Tech's Files: Do You Hear What I Hear?OPTIMIZING THE LISTENING POSITION IN YOUR RECORDING STUDIO 10/01/2009 8:00 AM Eastern
This month, I'll explore the relationship between high-frequency drivers and their next of kin (midrange or woofer) — essentially, how to optimize your monitor listening position.
Back in the day, a friend and I spent hours in local hi-fi shops checking out gear we couldn't afford. As geek teens attempting to help the adult world to get it right, one of our favorite pastimes was discovering miswired stereo systems.
These days, allowing time for such creative technical distractions may be harder to justify, but the process can still be very rewarding. My desktop has a pair of 3-inch “full-range” drivers that are well above average as a system, but rather forward in the presence region and a bit shy from the low-mids on downward. This is an impractical range for a subwoofer to cover so I augment each channel with a 4-inch driver in a cube cabinet. Some might think this is a waste of time, but geek diversions have a way of turning into moments that can be taught.
While adjusting the physical placement (see Fig. 1), sliding the orb from front to back would vertically steer the sound up or down. This is a mechanical phase adjustment that changes the arrival time between the drivers and the ear — an easy “lab exercise” that's worth sharing. Speaker designers use this technique to optimize the phase between the tweeter and its mate. In live applications, phase is electronically manipulated to steer line arrays. After the fact, we do it instinctively — the audio version of preening in front of a mirror — by bobbing up and down, and shifting left and right to find the sweet spot.
Fine-tuning the monitor orientation assumes that the majority of the control room acoustics have been tamed. There's no shortage of books and articles about minimizing reflections, de-coupling monitors from the console meter bridge or pedestal, creating a symmetrical listening environment and living within the sweet spot. Each loudspeaker system is unique, and therefore each should have a specific orientation.
For example, take the Yamaha NS-10M, a unit that nearly everyone admits is neither accurate nor a studio monitor. It makes sense that the NS-10M should be vertically oriented so left-to-right console excursions don't change the woofer/tweeter relationship. That said, the majority of users seem to prefer the sideways (bookshelf) orientation. Being off-center exaggerates the NS-10M sound that we don't like, which is especially problematic on a large-format console but less of an issue when “mouse-mixing” (i.e., staying in one place). You will find that the sweet spot width (focus) varies from monitor to monitor — some are wide, some are narrow.
There's some merit to the majority's NS-10M solution, although it contradicts geek intuition. It is one solution and there are alternatives, as this simple test may prove. The sound source should be pink noise, which is very good at emphasizing any comb filter effects.
To optimize monitor placement, listen to one monitor at a time while shifting position from tweeter-center to the center of the next driver (woofer, in many cases, but midrange driver for other monitors). There should be one region that yields the smoothest, clearest frequency response. Conversely, if the drivers are miswired, there will be a null where optimum should be.
If our results concur, “NS-10M optimum” places the ear in-between the two drivers, so keep this in mind if you try the vertical orientation. As the tweeter is offset, the vertical center is offset to that driver and not the woofer. The comb filter effect between woofer and tweeter was less on the NS-10M than I expected, but was more obviously “wrong” if listening from the woofer's center. The “power focus” (the strength of the signal in-between the drivers) was pronounced, almost hypercardioid.
The low-frequency response seems to benefit most from the bookshelf orientation (tweeters on the outside and offset high, rather than low). The NS-10M is bass shy below 200 Hz, emphasizing the familiar midrange honkyness. Now I know why horizontal placement works for so many.
Reading the Manual
I don't review or buy monitors with any regularity, so I'm not sure how many manufacturers take the time to power-assist their customers. That said, a trip to a local mastering house was yet another educational diversion. Greg Reierson, at Minneapolis-based Rare Form Mastering, has a pair of PMC MB1-XBD main monitors. By itself, the MB-1 is a three-way system, but is augmented for larger spaces by a duplicate low-frequency driver in a separate cabinet. This rather tall system puts the midrange driver at ear height when standing — the tweeter is above that — contrary to what you might think of as optimum when sitting.
As shown in Fig. 2, PMC provides very specific details to optimize its monitor for your listening pleasure. It is a teachable moment that can be extrapolated to other monitors. At minimum, we should all go back to our monitor manuals to see if, in our unpacking haste, we missed something. If no information was provided, then you now know how pink can work for you.
The Alt-Universe Tape Measure
Genelec's Acoustic Tape Measure is another example of a manufacturer's effort to improve the monitor-listener experience, especially in smaller control rooms where nearby wall reflections can hurt bass and low-mid response. It outwardly resembles a standard measuring tape, but the tape's top side is calibrated in quarter- and full-wavelengths between 115 and 10k Hz. The bottom side is a centimeter/inch rule. The instructions are written on the tape with an example.
Consider the first two bass guitar octaves. Open E-1 = 41.2 Hz; E-2 = 82.4 Hz (also open E on guitar); and E-3 = 164.8 Hz. Small rooms and small monitors don't support low frequencies very well; the low-frequency limit on Genelec's 6010A monitor is 74 Hz (hence its companion 5040A sub). Thankfully, bass is a harmonically rich instrument, especially when the cabinet is miked, so let's consider E-3, the end of the second octave at 164.8 Hz.
A monitor's “face” is 659.24 Hz away from the wall behind it — that's 20.57 inches = 0.522 meters = 1/4-wavelength of 164.81 Hz. The round-trip, with reflection off the wall, is 329.62 Hz, now the half-wavelength and the reverse polarity of 164.81 Hz. For those without the tape, there's a useful wavelength/distance calculator at www.mcsquared.com/wavelength.htm.
Two waves of opposite polarity will cancel rather than combine. This is why the distance between the monitor and the wall behind it can interact in a negative way, messing up the bass response. If you play bass (or want to hear all the bass notes), those two octaves are critical. The small-room rule of thumb is that monitors should be as close to the wall as possible; the distances to avoid are 17 inches (around 200 Hz) to 43 inches (78.75 Hz).
Tweak Party, Anyone?
Like any social relationship, getting accurate sounds from your monitors requires an investment in thought and time. This can result in a more focused and 3-D sound, reduced ear fatigue and an improved end result with less guesswork. So why not invite a few friends over tonight and tweak away?
For more Eddie Ciletti, visit www.tangible-technology.com.