Tech Topic: When Eyes And Ears Don’t Agree
Putting the role of measurement into perspective
By
Pat Brown
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In the previous issue (October 2003 Live Sound), I addressed some
of the caveats of measuring sound fields in enclosed spaces. The
conclusion was that the eyes and ears do not always agree when it
comes to sound quality. |
We humans base our concept of reality upon the evidence presented by the
five senses to the brain. Our concept of “blue sky” or “green grass” is
the result of the tuning of our visual system and the programming of our
brains. If our visual system were tuned to sense a different part of the
electromagnetic spectrum, then our perceptions about reality would change.
Our sense of sight is programmed with base-line reference values from
the day we’re born. We know what the sky should look like, and we know
what the grass should look like. As far as we know, “green” and “blue”
mean the same thing to everyone.
This is why we can walk into an electronics retailer, gaze at a wall of
television sets, and pick out the one that has the “best” picture. The
best one is the one that conforms the closest to our base-line programming
of what certain colors should look like.
NO “ABSOLUTES”
Our hearing system has no such set of “absolute” references. Most of the
sound events that we hear each day are man-made. Depending upon your walk
in life, the sounds that you hear most often are probably different than
the sounds that I experience. Only those who have undergone special sensory
training have a concept of what the “correct” sound of a violin, piano
or any musical instrument is - and even that is controversial among the
“experts”.
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The realm of sound is a subjective world, one that is devoid of absolute standards that form benchmarks for what we perceive. I may feel that a Gibson guitar sounds the “best,” and you may feel that a Fender sounds better. There is no absolute benchmark to justify either position, so it
simply ends up being a matter of opinion. |
Measurement provides the standard by which we can justify an opinion of
what sounds “best”. Because loudspeakers ideally are reproducers of sound
(as opposed to producers), and since we can measure what goes in (electrical
energy) and what comes out (acoustical energy), we have a benchmark by
which to judge the accuracy of the loudspeaker.
If a loudspeaker “sounds good” when playing back a Rob Zombie CD, yet
its measured magnitude response looks like the Swiss Alps, then we can
brand it as “inaccurate” with good authority. Accuracy is not necessarily
a prerequisite for (subjectively) “good” sound.
The human auditory system is the most powerful analyzer that we have at
our disposal. It is a two-channel, frequency-dependent, localizing data
collection system with on-board algorithms that give it subjective perception.
Even the most powerful analyzers can only emulate a few of these characteristics.
TWO SHORTCOMINGS
The importance of the listening process in tuning sound systems should
not be underestimated. In spite of its strengths, our hearing system has
two serious shortcomings with regard to adjusting the sound system’s response:
it is not calibrated, and it is not consistent.
The lack of calibration means that we cannot listen to a sound and state
with accuracy how loud it is. At best, we can offer a subjective impression
of “pretty loud”, “ear-splitting”, etc.
Even a highly trained listener has difficulty identifying an absolute
sound level to within 3 dB of its true level, which is a power ratio error
of 2-to-1. So, if you guessed that the SPL were 87 dBA, and you were off
by 3 dB, you missed by a 2-to-1 power ratio - a pretty large error! Absolute
level measurements are trivial for analyzers - most of which can measure
sound levels to within a fraction of a decibel.
The response of the human auditory system is not consistent. It changes
with exposure, which means that after you listen to loud sounds for a
while, the characteristics of your hearing system change.
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The stapedius tendon (try saying that three times fast!) attaches
to the bones of the middle ear and dampens their motion when your
ears are exposed to loud sounds over a span of time. So, what sounds
fine early in the show is not likely to sound the same by the end.
This “threshold shift” is nature’s way of protecting us, but as
with all protection mechanisms, it can be circumvented. |
A good night sleep will “reset” our hearing system and render us capable
of critical listening. Analyzers have no such malady. They don’t get tired
as the day wears on. So when the analyzer and your brain start to disagree,
it may be time for a rest.
REALITY DISTORTION?
We work in an industry where a lot of money is made by distorting reality.
You can drop a thousand dollars on a processor that essentially increases
the harmonic distortion of your system or fills its response with deep
notches, and feel like you have improved its sound quality.
Yet no one buys a processor for a television that makes the grass look
blue or the sky look green. When the goal of a system is accurate sound
reproduction (this isn’t always the goal) then accurate has no meaning
unless a benchmark exists. This is where measurement comes in.
Consider the following scenarios:
1) You’re at an airport and you hear a perfectly awful announ-cement come over the sound system. Why did it sound bad? The initial reaction is usually to blame the loudspeaker, since it is where the bad sound came from. But a good loudspeaker that is fed bad program material will still sound bad. Perhaps the problem is an overdriven amplifier, or poor micing technique on the part of the talker. How would one find out?
Once again, we return to measurement. If I feed the loudspeaker a known stimulus and it can reproduce it with good fidelity, then the problem lies elsewhere in the system. The process is repeated until the offending component is found, which could ultimately be a gate agent with a bad head cold. A sound system is only as good as its weakest link, and measurement is necessary to test the links.
2) A manufacturer may complete a run of loudspeakers, and find that two picked at random for a listening test sound dramatically different. Which one is the most “correct?” A measurement can provide the answer. Loudspeaker manufacturers measure each loudspeaker that comes off the line to assure that it falls within a set of tolerance values that were established during the model’s design.
Only measurements can verify that the replicas are identical to the original. No one wants to buy a loudspeaker whose only validation of performance was “Bill’s ears in Quality Control”. Bill may have been tired when my loudspeaker rolled off the line!
3) You’ve been called to tune a sound system which the client complains lacks “presence”. Most people would start boosting the high-frequency tone controls or the house equalizer to “restore” it.
But what if the system simply lacks the bandwidth to reproduce full-range music? A mixer with all of the high-frequency tone controls fully clockwise and a “smiley face” on the house EQ is probably deficient in bandwidth. Either that, or the sound guy just retired from 20 years on the road as “monitor engineer” for a heavy metal band. Some simple measurements can reveal whether the system is capable of what you are asking it to do.
4) The congregation at a local house of worship complains about poor intelligibility from the house system. Three different people have been consulted about the problem, and each of them suspects a different cause. Now how do we really get to the bottom of this?
The answer, of course, is measurement. It’s possible to spend a vast amount of time and money “fixing” the wrong problem. None of us would submit to surgery because our doctor suspects that we need it. We rely on X-Rays and CAT scans to reduce the risk of an incorrect diagnosis.
ORDER THE COMBO
Most sound system chores require a combination of listening and measurement.
One without the other can yield completely unsatisfactory results. The
two used together can quickly bring a system to its fullest potential,
and also reveal the shortcomings of a sound system that might be addressed
by equipment upgrades, changes to room acoustics, etc.
Those that don’t measure are operating in a world without references where
“anything goes”. This approach is fine for purely artistic endeavors,
but sound system design and implementation is first an engineering practice.
Loudspeaker selection and placement is mostly science, while selecting
what color to paint them is mostly art.
“Mostly” leaves room for the other, but points to the dominant process.
Measurements assure that the science has been satisfied, paving the way
for the artistic use of the sound system.
Pat and Brenda Brown own and operate Syn-Aud-Con, conducting training seminars around the world. For more info go to www.synaudcon.com
November 2003 Live Sound International