THE PSYCHOPHYSICAL PROBLEM

According to Shakespeare, “The world is much upon us.” In fact, the world is always upon us. If we pick something up, gravity tries to make us put it down. Sometimes there is so much heat that we strip to bathing suits; sometimes there is so little that we pile on coats. Light energy falls on our eyes; if there is enough of it we can see. When air pressure fluctuations transfer energy to our eardrums, we hear. Just what is the mapping from the energies that impinge on us to our sensations of the world around us?

The field where this issue is explored is called psychophysics. It is one of the oldest and one of the most mathematically oriented fields of scientific psychology. It is also one that has a substantial practical application. Consider the following example, the case of machinery-generated noise.

Anything with a moving part generates air pressure waves. These waves cause fluctuations in listening devices, including the ear. The energy per unit area in the fluctuations is measured in decibels (db). To give some content, ordinary speech is conducted at about 60–70 db. The pain threshold for the ear is in the 115–125 db range, varying somewhat with frequency. Obviously, then, machinery noises must be held to 110 db or less.

But how much less? Muffling sounds can be an expensive process. Determining how much it will cost to reduce sound emissions by 10 db is often a straightforward problem in engineering.