I Know You Understand What You THINK It Does, But...

6/28/2011 4:20:44 PM
By PSN Staff
by Danny Maland

I made it all the way through high school without actually using my assigned locker. I spent four years hauling every book and supply needed for each day in backpacks and handle bags, sometimes two at a time. There was one particular year where I broke the strap on a rather nice, black leather bag that I liked.

Dad's reaction was to say, “You know, that bag just wasn't designed to carry that amount of weight.”

My teenage know-it-all reply was, “But it all fits in there! They should design the bag so that it's able to carry that volume of lead, if I want.” (It's amazing how much more we know as absolute certainty when we're younger, wouldn't you agree?)

The problem here was something called “Expectation Management.” In my mind, the bag was capable of providing enough interior volume for the books I wanted to put inside it, and I felt that it was thus reasonable that it should handle the mass of those books as well. The real issue was that I was misreading the application of the bag. It was actually designed to hold clothing, which is usually far less dense than textbooks.

Problems with expectation management happen all the time with audio, lighting, and video equipment.

A notorious expectation management problem is the one described by the myth of “underpowering.” The assertion is that Jane or Johnny User toasted their loudspeaker drivers because they drove an amplifier into clipping (or hard limiting, as is more likely in these modern times) and that they would not have burned the voice coils if they had been provided an amplifier capable of “cleanly” swinging greater voltage at its outputs.

Not true.

Every power amplifier that I've come across has been rated for a certain amount of continuous power at a certain amount of distortion. If you're willing to live with greater distortion (or greater limiting artifacts), you can drive the amp beyond that rated continuous power. The peaks of your signal will get smashed flatter than pancakes, of course, but this does not prevent the average power of the output signal from approaching that same maximum peak.

(Incidentally, this is how the “loudness war” in recorded music works. Inter-sample clipping notwithstanding, you can't master a CD with a level above 0 dBFS. You can, however, slap a limiter across the signal, and then drive the signal into the limiter to get a higher average level.)

What's killing Jane or Johnny's loudspeakers isn't that the amplifier is too small. The really deadly thing is that Jane or Johnny expects to be able to get more Sound Pressure Level (SPL) out of their loudspeakers than is actually possible. If they were willing to drive the “little” amp beyond its linear performance limits to get a desired SPL, then it is reasonable to assume that they will end up driving a more powerful amplifier to the same level of continuous output as the smaller unit. They will then cook their drivers just as fast, because the thermal stress applied to the loudspeakers will be just as bad. (They will get less distortion or limiting artifacts from the amp, of course.) They may even kill their drivers faster, as the peak voltage deliverable from a sufficiently powerful amplifier may allow Jane or Johnny to quickly exceed the mechanical limits of their loudspeakers.

Bottom line?

Jane or Johnny is in much the same position as I was with my bag of books. They expect that their sound system should be able to do something that it actually can't, and are damaging it as a result. Expectation management, then, is an inexpensive and (if genuinely received by Jane or Johnny) effective way of fixing their problem. If they can have their expectations of the system brought into line with what the system can actually do, then they are far less likely to push their system into the damage zone. Yes, some sort of system protection to guard against accidents and misuse is a good idea, and so too is a system redesign if the install just doesn't do what it ought to do. Still, the users having an appropriate mental picture of what can and cannot be accomplished with their system is key.

Expectation management also applies to purchasing decisions. I find that it mostly comes into play when quantitative metrics of a component's performance are unavailable to, or misunderstood by, a person in charge of procuring equipment. My current favorite in this area is the “LED luminaire with no photometric information.” You probably know this unit. It's the one that is described as “bright” in the marketing literature.

Expectation management comes into play as soon as that qualitative description of “bright” enters the picture. Now, I'll tell you something. I have seen the output of fixtures that (to the best of my knowledge) can deliver 20,000 lux at one meter. That is “bright” to me, and I'm pretty sure that my standards aren't too high. I am also 100% certain that quite a few of the luminaires that I've seen described as “bright” do not even come close to my definition of “bright.” What has to be watched for, though, is that a person who has not yet gotten a feel for what can be reasonably expected of given designs at given price points might very well assume that “bright” conforms to their own idea of what “bright” means. If that idea happens to line up with what the fixture can do, then that's great. If a purchase is given the go-ahead, but “manufacturer bright” doesn't line up with “expectation bright,” then expectation management will probably have to (painfully) coincide with disappointment management. Maybe even anger management.

The point that I'm hoping to make with these examples is an idea that I was not aware of when I was younger, and that is that expectation management is a component of audio and video that can really haunt you if overlooked. As I got (and still get) an increasing amount of education in the school of hard knocks, I learned that it was very easy for the idea of what a component or system of components can do to exceed the actual performance available. I also learned that most people don't “just get it.” I think that an enormous number of clients/ end users/ people-in-general see a piece of gear that's outside their sphere of experience and figure that it must be able to do magic. I also believe that disappointment in magic that doesn't happen can be a very ugly thing.

I'm a very low-level guy in this business, but at whatever level you are, I think you can derive a lot of benefit and self-respect by ensuring that expectation management is given high priority in what you do. If your own expectations are managed well, then you will likely make better decisions. If the people that you work with have expectations that are managed well, then you will likely have better working relationships. If the clients that you work for have their expectations managed well, then you will likely have much happier customers. If you're not already working expectation management into your picture of a “total quality experience,” I think it's worth giving it a try.

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