The Zen of Audibility

The dictionary says that “audible” is “that which can be heard.” Sounds simple enough. But, sadly, it isn’t.

Some rules of thumb to keep in mind are that:

• Audibility is variable, as a function of levels, listeners, program, playback system, etc.
• Audibility means different things at different points in our hearing system. What we detect at the ear-drum is dramatically different from what we consciously perceive in the brain. So, if we hear it but don’t perceive it, is it audible? Hmmmmmm . . .
• Audibility changes as a function of listener knowledge. Hearing improves when we know for sure what we’re hearing (cynics call this the “It’s easier to get a good grade on a test when you know the answers beforehand” syndrome).
• Audibility changes as a function of experience. Called the “learning effect,” repeated exposure to an effect increases our sensitivity to whatever audible threshold we’re listening for, and increases our level of annoyance.
• Subjective expressions of audibility are often magnified greatly and non-linearly, as in when we say “The difference in clarity is amazing” when the actual difference is so small that we can barely distinguish it from chance. What’s wild is that we still really believe its amazing, even when we KNOW it’s a tiny difference. Go figure!
• Subjective recognitions of small variations in audibility are often expressed in unpredictable ways, so that a sound .1 dB louder may be described as “fatter,” rather than “louder.”

So, as you can see, audibility has a big range that is both quite variable and not very easily pinned down. One approach has been to test for the Worst Case, using the most highly experienced and trained listeners with the best possible speakers, etc., in the best possible room with the best possible equipment. When this has been done, we can find out something about what one end of the audibility range for a given effect is, the end where all possible advantage lies with the listeners. The problem is, this approach often doesn’t give us relevant data. It doesn’t tell us much, if anything, about real-world applications, because it violates the Range Rule, which states: The Range of the Theory must fit the Range of the Facts. The data, in this case, only fits the Worst Case, which may be inappropriate.

The Best Case approach is even worse, because now we have to establish the least trained, least experienced listeners, listening to the worst possible speakers, in the worst possible room, etc. Hardly seems worth it. Such coarse data will prove pretty useless in predicting useful standards for success in the marketplace.

What’s A Threshold?

Meanwhile, we have to consider the meaning of the term “threshold.” When we talk about audibility, we usually mean a threshold of some physical or psychological magnitude below which we don’t hear a given effect or artifact and above which we do hear the effect. This point, in human perception, is called the “difference limen” or the JND (“Just-Noticeable-Difference”). The normal way to establish this threashold is to measure the array of listeners’ thresholds to the Effect Under Test, and then to find the average threshold, which is the threshold that approximately 50% of the listeners can hear.

So, if 50% of people begin to perceive the presence of a sound at .00002 Pascals, then we say that that magnitude is the “threshold of hearing.” And in fact, we’ve done exactly that, so that 0 dB SPL is defined as .00002 Pascals, and as the threshold of hearing.

What is often forgotten is the obvious corrolary that if only 50% of the people can’t hear below the threshold, then obviously 50% can! So it is really foolish to say, “Nobody can hear it, because it’s below the threshold of audibility.” Half the population may be able to hear it!

So, we might wish to reconsider that threshold. If we’re really critical, we might like to set the threshold at the level where 99% of the population can’t hear the difference and only 1% can. Or, if we’re feeling generous we could go the other way, finding the threshold where 99% can hear it, and only 1% can’t.

If you think carefully about this, you can see that it has big implications for the design of loudspeakers, recorders, consoles, etc. We can design to pretty easy and low standards if we define the threshold of audibility as the level at which 1% of the population can’t hear the defects. It really gets the price down! Or we can be rigorous, and design to a standard where 99% of the population CAN’T hear the defects. But then our products will be quite expensive, and may not be competitive in the market, while only providing a marginal benefit for the Golden-Eared types. It’s a quandary.

Audibility in The Real World?

So, in the real world, the question of whether or not something is “audible” is really clouded in a kind of psychological haze, and the real, honest-to-God true answer is, “It depends!” Some days you’ll hear things, some days you won’t. What you hear may be different from what someone else hears. Which loudspeaker, amp, console, etc. you use may have a big bearing on what you hear.

As a result, you need to be very cautious with claims of sound quality, particularly in advertising. When we went to 20-bit audio from 16-bit, for example, we didn’t change the maximum level of the recording (0 dBFS), we changed the minimum level to a lower level. Now, it’s worth asking: was the 16-bit level above or below the threshold of audibility? Under what conditions? How about the 20-bit level? 24-bit, anyone? Arguments abound. We hear reports ranging from “the change is totally awesome” to “it all sounds the same to me.” While this is perplexing, it does in fact reflect the range of possible perceptions. No one is necessarily lying or mis-reporting.

So, how do you cope? How do you assess the importance of “audibility” for any particular item of gear, or acoustical treatment? What’s right for you?

My own strategy has been to put on my producer’s hat, and ask the question: “Does this difference or audible artifact matter to my production vision for the project?” I first used this criterion back in 1989, when I was on a listening panel to evaluate the CBS Copycode for the National Bureau of Standards. I heard the copycode work very obviously and clearly in a couple of spots, but I wasn’t sure how big a deal it was. So, I asked myself, “If I were producing this project, and I came across this artifact during production or post, would I try to fix it, or just let it go?” In that particular case, the answer was pretty definitely “I should fix it.” So I could reasonably report to the National Bureau of Standards that I could hear a difference, and that that difference was annoying enough that as a producer I wouldn’t want to release the project with the effect present.

Now this is an interesting and useful standard. If we are mixing and we move our heads by 6 inches, the sound will audibly change. Does it matter? Not to me. But if it does to you, then that magnitude of difference is important, and should be treated, whatever practical difficulties that may present (I told you this business is obsessive!). If it doesn’t, then we can safely ignore it, saving a heap of time ‘n money into the bargain. Normally, we hear a whole bunch of small differences during production that don’t really matter to us, and we let them go, because (a) life is too short as it is, (b) we’re over budget as it is, (c) they’ll never hear it as it is, and/or (d) it won’t bother ‘em even if they do hear it as it is.

When we start looking at the world in this existentialist sort of manner, where things either “matter” or “don’t matter,” it becomes a little easier to reliably orient ourselves. We can quickly and intuitively zero in on which magnitudes aren’t really very relevant to us, abandon them and move on to more important things. When other people tell us that such a magnitude does “matter,” we can take their words with a grain of salt, asking ourselves existential questions such as “Why does it matter to him/her?” and “Is he/she really hearing what he/she thinks he/she is hearing?” and even, occasionally, “I wonder what he/she is smoking these days.”

Even if said person is a world-famous engineer/producer/artist, we don’t automatically HAVE to be concerned if we don’t hear the same differences. We need to just keep observing for ourselves, patiently, carefully and critically. Which is to say: avoid worrying about the Emperor’s New Stereo System. Work in response to what YOU hear, not to what OTHERS say they hear.

Happy audibles, as we say in the NFL!

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