Subject: Re: JND
From: "Tim Altom" <taltom -at- simplywritten -dot- com>
To: "TechDoc List" <techwr-l -at- lists -dot- raycomm -dot- com>
Date: Tue, 5 Sep 2000 14:02:19 -0500

> I've not heard of JND, but I do know that most (if not all)
> human senses have a logarithmic response, not a linear
> response. That matches your statement of "The lower the
> stimulus, the smaller the JND." Scales that measure
> sensory response are also logarithmic, for example decibels
> and density (photographic, not chemical).
> Enjoy!
> --- John Bell
> jbell -at- siebel -dot- com

You're right about some JNDs being logarithmic. That makes sense, as the
objective of a creature in the wild is to recognize changes, not absolute
conditions, so a creature may well evolve a logarithmic scale and sacrifice
some precision to get range. A decibel is actually a rather large delta, and
it was originally thought that the JND was one decibel. Decibels rise
logarithmically. Now it's known that the early research was somewhat flawed
and that a quarter-decibel is closer to the actual JND. Still, even a
quarter-decibel is a lot of variation. To have that be a single discernible
"step" means that our hearing is not able to discriminate very
takes a large step to make us say "hmmm...louder..." We could doubtless have
much better sensitivity if we didn't rely on such large steps for our JNDs,
but that would narrow the total bandwidth we could hear.

However, that speaks only to intensity or amplitude, not to other factors.
For instance, the difference between shades of orange isn't logarithmic, and
can vary a good deal from person to person. The difference between
closely-matched flavors differs widely, too. There is similarly a range of
responses for "touch tests" applied to the skin, in which caliper points are
touched to the skin to see how widely spaced they must be for the subject to
feel two points instead of one.

Further, the JND for any sense is dependent on a lot of collateral factors.
In print, for example, using thin white letters on a yellow background
vastly increases the JND. In psychological terms, this is sometimes known as
"figure and ground" that it's harder to pull the figure out of the
ground clutter. The easier the ground and figure can be told apart, the
smaller the JND. The same principle applies to hearing,'s easier to
pick up a quarter-decibel tone delta if you don't have Motley Crue blasting
on the stereo.

The JND has had a lot of research done on it, but it's restricted mostly to
the psychology community. It's interesting and informative, but there are so
many factors to it that not a lot of ink has been expended making it work
for a living.

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