Loudspeaker Power Response by Gene DellaSala – Audioholics ©
Power Response is the sum of the total radiated acoustic output of a loudspeaker as measured in a sphere around the speaker at several incremental intervals on- and off-axis in the far (reverberant) field. This measurement essentially captures the total sound emitted by a loudspeaker at all frequencies, in all directions, and is therefore thought by proponents of this measurement method to be more representative of how a speaker will sound in an acoustically well-balanced listening environment than by what can be inferred from a simple on-axis anechoic frequency response measurement.
The majority of today’s top speaker designers seem to engineer their speakers to have a very uniform, “correct” (no major peaks or dips) on-axis near-field frequency response, with a minimum of interference-producing protrusions on the baffle and correctly-aligned vertical drivers (M-T above the woofer, not side-by-side). This optimizes the speaker’s response close up—minimal comb filtering and interference between the Mid and Tweeter in the horizontal plane in the crossover overlap region—while preserving the speaker’s inherent far field power response characteristics.
Important Design Considerations
A speaker designed to have proper, correct, uniform response relatively close-up can still have an excellent far-field power response. Let’s say, for purposes of this example, you have a 10” 3-way speaker with a 4 ½” midrange and a 1” dome tweeter. The crossover points are 500 Hz and 2500 Hz. These crossover points use the drivers in frequency ranges where they do not become directional (a 10” woofer is ‘good’ to around 1300 Hz, and a 4 ½” mid is ‘good’ up to around 3kHz), so the speaker has wide dispersion. If the drivers are all in a vertical line, then any serious crossover region interference (or acoustical interference) between the drivers will be limited to the vertical plane, which is less important for listening area coverage (and listener positioning) than the horizontal plane, since listeners can be spread out several feet laterally on the sofa and chairs, but their seated ear positions are likely to be all within a tight 6” or so window vertically.
But that very same speaker with the same drivers and crossover points laying on its side will have a markedly different near-field frequency response when off-axis horizontally, since its drivers are now horizontally-arrayed and all the interference between the drivers is now thrown into the horizontal plane. So, things may be OK at 0 degrees, but at 15, 30, 45, 60, etc degrees off horizontal axis in the near field (the way people sit, from one side of the room to the other), the horizontal speaker will sound quite different from its vertical counterpart, as the radiation patterns from its drivers overlap, canceling and reinforcing each other’s outputs—in the critical horizontal plane.
Good engineers from the major ‘name’ speaker companies know this, which is why virtually every single serious music speaker from good speaker companies have their drivers aligned vertically, so their horizontal nearfield response is smooth and interference-free. Makes no difference whether they’re aluminum mids or beryllium or Kortec tweeters or paper or polypropylene woofers, sealed, ported, transmission line or passive radiator. Good engineers put their drivers in a vertical line to minimize the midrange-treble acoustical interference and interference between drivers in the horizontal plane.
- vertical arrangement of drivers produces a smoother and more uniform off-axis nearfield horizontal response
Note that the far-field response of our hypothetical 10” 3-way speaker will be much the same, whether it’s standing up vertically or lying down horizontally (assuming it’s at roughly the same height with respect to the listener’s ears and with respect to the room’s major reflective boundaries). In the far or reverberant field, it’s the speaker’s total acoustic output that counts, and that will be the same, whether it’s mounted vertically and horizontally.
So the takeaway is this: You can design a speaker that performs well and is nicely behaved in the near field without sacrificing its reverberant field behavior. But the reverse is not necessarily true—a speaker that ignores near-field aberrations, driver placement issues which create acoustic interference, baffle/grille protrusion interference issues, etc. to concentrate solely on far-field performance can sound much worse in the near or critical field than one designed to perform well at all distances. The AR-3a—a very famous speaker known for its smooth far-field response—was humorously criticized by a well-known audio critic as having a “cacophony of near-field phase and interference issues”—because of its non-vertically-aligned drivers and obtrusive, interference-producing decorative cabinet molding.
Some 20 years after the 3a’s introduction, AR came out with a vastly-improved model called the AR-78LS, which used very similar drivers (a 12” sealed woofer, and dome midrange and tweeter units like the 3a, but now with ferrorfluid cooling which the 3a’s drivers didn’t have). However, the 78LS’s drivers were all in a neat vertical line and the M-T was a Dual-Dome unit so they acted as one driver with virtually no acoustical interference either horizontally or vertically. Additionally, the baffle/grille was totally free of any acoustic obstructions. Result? Unlike the 3a, the 78LS had superb near-field and far field response. With intelligent design choices and an awareness of basic acoustic information, speakers can perform exactly the way the designer intends, without any surprises.
The “rules” of directionality, dispersion, how much/little a speaker will engage the room by virtue of its dispersion, the degree of audibility of a speaker’s first-arrival sound in the near, critical, and reverberant fields are known bits of information.
A given designer may subscribe to any of the following design philosophies:
“First arrival—smooth on-axis FR— dominates the listener’s perception!”
“Far-field power response as a determinant of a speaker’s perceived tonal balance is a discredited, outdated concept!”
“Wide-dispersion loudspeakers convey a greater sense of ‘air’ and three-dimensionality than narrow-dispersion speakers,” etc.
Regardless, there is no reason the designer can’t achieve exactly what they intend to accomplish.
Conclusion
Both comb filtering (acoustic interaction between widely-spaced multiple sources) and acoustical interference (the acoustic aberrations that emanate from a single source employing multiple drivers) are acoustic artifacts that can impact the nature of reproduced sound as it is perceived by a human listener with two ears. Although the audibility of these phenomena when listening to complex, highly dynamic source material played in a fairly “live” environment while in the critical or far field may be somewhat limited, they are, nonetheless, audible, sometimes detrimentally so. Dealing with potential acoustical interference issues of multiple drivers in the same loudspeaker cabinet is something any serious designer should be concerned about and not just brushed off as a measurement artifact that doesn’t have real world implications. The important thing is for speaker designers to have clearly-defined goals (near- vs. far-field optimization, to what degree they feel wide high frequency dispersion is important, etc.) and to intentionally take the awareness of these phenomena into account when developing their loudspeakers.
Acknowledgements
I would like to personally thank the following people for their contributions and/or peer review of this article, all of whom are true experts in their respective fields. Their contributions enabled us to make the most comprehensive and accurate article possible on the very complex topic of loudspeakers cabinets dealt with herein.
- Paul Apollonio, CEO of Procondev, Inc
- Paul Ceurvels, Design Engineer, Atlantic Technology
- Steve Feinstein, Audio Industry Consultant
- Shane Rich, Technical Director of RBH Sound
- Mark Sanfilipo, Audioholics.com Resident Speaker Expert and Writer
- Dr. Floyd Toole, PHD & Chief Science Officer of Harman