You can’t just look at the measurements —You gotta know what they mean.
It feels like my perpetual task in life is to get up on my soapbox and dispel myths and tell the truth about getting righteous sound pressure levels to the audience. What I want to do is weave the story of speaker-cabinet dispersion, power handling and sensitivity and attempt to explain why certain “grades” of speaker are pitched to various sound persons at different levels of the business.
First, there is the thoroughly professional sound pro, who has a decent budget and a discriminating need for road-ready long-throw cabs to handle big halls and outdoor festivals. Below this are your local-to-regional soundco types with stack ‘em-up cabinet needs for modest venues with moderate crowds to please. Then, there is the vulnerable anklebiter pitched a bill of goods by the music stores for MI-grade speakers with not a lot of emphasis on performance specs and a lot of selling of the visual appeal.
Line Array Disclaimer
I want to exclude line arrays in this diatribe for now, because I want to keep the math simple with point source cabinets and non-coupled performance for square-law acoustic power loss. Line arrays attempt to put a good amount of the audio frequency spectrum in the near-field effect for only 3 dB loss per distance doubled instead of the usual 6 dB square-law loss of sound pressure levels (SPLs). And, of course, all the line arrays still need subwoofer support, and those low frequencies still obey the 6 dB-per-distance-double rules because the wavelengths get to a couple meters long.
Dispersion
Speaker cabinet dispersion is defined as horizontal and vertical beamwidths of various frequency bands, where the edges are half the acoustic power (-3dB) of the forward direction. This is usually the first parameter rookies ignore when looking at speakers. A cabinet with a 90 by 40-degree horn is fine for coffee houses and small VFW halls, but has little use when you gotta rock the house a hundred feet back from the stage. A good mid-throw horn with 70 by 50-, or 60 by 40-degree dispersion is a better choice, as the narrower flare typically increases the sensitivity to get those wispy vocal consonants and cymbal crashes to the back of the venue. If your venue is a huge outdoor festival or arena, it is likely that the 40 by 20-degree long-throw horn will be used to cast as much sensitivity hundreds of feet away from the speaker locations.
Beyond the narrow dispersion/increased sensitivity axiom, the narrower sensitivity allows you to cluster speakers in splayed horizontal and vertical arrays so that each cabinet has its unique designated area to throw audio into. This means more speakers and amplifiers, which is handy, as the speaker drivers only have limited SPL generation capability via the power handling and sensitivity multiplier to get the RMS/Program/Peak SPL ratings per driver. If we could violate the laws of physics, we would all love the flux-capacitor-driven single-speaker cabinet with a wide dispersion and killer loudness for a quarter mile.
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Lead by Example
Now it is time to make the point numerically and visually. Figure 1 shows three grades of speaker cabinets that generically show grade characteristics. I chose these anonymous types not because they are favorites of mine, but because they are middle-of-the-road examples of the grades.
In previous Theory and Practice columns, I have covered power-in to power-out SPL calculations ad nauseam, but it still bears repeating. The transfer function is the sensitivity number, so that electrical-watts-in multiplied by the sensitivity equals acoustic watts. Taken into the decibel scale, dB watts plus the sensitivity in dBs SPL equals acoustic SPL output.
So, for easy math, 100 watts in (20 dB watts) is added to the 100 dB/watt at 1 meter speaker sensitivity at a given frequency band to become 120dB SPL at one meter reference distance from the speaker measured down the main dispersion beam of the speaker. In reality, most speaker manufacturers measure their speakers a few meters away and calculate the effective one-meter reference sensitivity and SPL capabilities. This is because SPL attenuation falls at a 6 dB per distance double. In the above math, 120 dB SPL at one meter drops to 112 dB SPL at two meters and 106 dB SPL at 4 meters. So loudness (sound pressure) to masses means the best audio power into the speakers you choose, plus the efficiency of the speaker within the dispersion beam provided by the sensitivity rating.
Now back to Figure 1, the professional mid- to long-throw box with 12-inch horn-loaded driver and a coaxially mounted 1.4-inch high-frequency driver provides a very nice 109 dB sensitivity rating. And with 600 watts (27 dB watts) program power input, puts out a respectable program 136 dB SPL at one meter. With your typical rock concert levels expected to be in the 90 dB to 118 dB SPL in the audience seating areas, the chart shows that this speaker will be capable of this range from 25 to 500 feet away.
Looking at the mid-priced, mid-quality speaker box (prosumer grade) with double 12-inch speakers front loaded with a 1.4” high-frequency driver; a whole different story develops. The average 97 dB SPL sensitivity and the beefy 2400 watt power handling should do pretty well at putting power to the people. But note the much poorer sensitivity being made up by four times the power input, compared to the professional speaker cabinet. The net program SPL is an okay 130 dB at one meter, and puts a rock concert SPL experience from 13 to 250 feet away at best if you’ve got the 2400 audio watts to heat up the voice coils.
Now voyage into the anklebiter kingdom with a bargain single 15-inch front-loaded driver cabinet with a 1-inch horn (MI grade). The typical 94 dB sensitivity and 500 watt program power input yields a 121 dB program SPL with a rock concert experience from 5 feet to about 100 feet. As you can see, you are not likely to have outdoor or large venue capability with this speaker system. But I still see a lot of ‘biters trying to by stacking speakers together in hopes of extending the throw. Even with perfect cabinet coupling, it would take about four of these MI grade cabinets to come close to the Figure 1’s example of a fully driven prosumer cabinet.
Summary
The big point of the column this month is that, when loudness is required, speaker efficiency trumps power handling and size by lesser speaker cabinets. While the professional box is going to cost a lot more than the prosumer and MI speaker cabinet offerings, that money is thrown into cabinet ruggedness so the cabinets can survive a couple of years in air-ride tractor-trailer rigs between concerts. So, if you want righteous loudness for big club or outdoor gigs, start looking for the 102 dB-or-higher sensitivity cabinets. These typically will have horn-loaded drivers and have a lot of weight in wood cabinetry in their construction. And they are likely to have a $2000 or higher list price per cabinet.
Obviously, professional cabinets are not for everyone’s budget or need. But use sensitivity specs first, before power ratings and other eye candy. Shop the manufacturer’s Web sites first, because the best speakers for you are not likely to be found on a retailer’s showroom floor.
E-mail Mark at marka@fohonline.com.
