Perception of changes in loudness

Abstract

Neuhoff¹ reported that “rising level tones... change (in loudness) more than falling level tones despite having the same actual change in level... indicating that direction of change is an important (and previously unaddressed) factor in the perception of dynamic loudness change”, and speculated that: “In a natural environment this over-estimation could provide a selective advantage, because rising intensity can signal movement of the source towards an organism.” Leaving aside the question of why it may not be as important for survival to detect the movement of a sound source away from an organism, we dispute the assertion that there is no prior evidence about the influence of direction of change on the degree of change in perceived loudness. This evidence does exist and shows, in contrast to the result reported by Neuhoff¹, that declining signal intensity covers a greater range of loudness than does rising signal intensity.

Main

Twelve years earlier it was reported² that tones continuously decreasing in intensity from moderate to very low levels follow a course of accelerated “softening” until the end of the downsweep, when the loudness is very much less than it would be if that final intensity had been presented alone. The underlying mechanism for this phenomenon, which was subsequently named “decruitment”³, is unknown, but arguments have been made for both central and peripheral factors. For example, it was later shown⁴ that at least some part of the effect occurs at the receptor site, as a test in the contralateral ear at the end of the downsweep showed little or no decruitment.

In these earlier studies, the loudness of sounds sweeping up, instead of down, over the same ranges of intensity yielded some evidence for loudness enhancement (or “upcruitment”) at the end of the sweep. However, the effect, where it occurs at all, is much smaller than the accelerated softening in downsweeps. It should be noted that the earlier work concerned sweep durations much longer than the 1.8 seconds used by Neuhoff. A more recent study^5,6 has shown that, although decruitment is diminished when duration is as short as one second, a 30-decibel downsweep for 1- and 2.5-second durations covers a bigger range of loudness than the comparable upsweep.

It would therefore be prudent to limit Neuhoff’s conclusions to his own test conditions and method of measuring loudness change: the larger loudness change recorded earlier for downsweeps under a range of conditions and with a variety of measurement techniques contrasts with Neuhoff’s finding based on a single duration, a single intensity range, and a single measurement procedure.

How can this apparent conflict be explained? Perhaps, as Neuhoff argues, direct judgement of “perceived change” taps a fundamentally different process from judgements of loudness obtained at the beginning and end of a sweep. But listeners have been known to make their own decisions about what to attend to in an experimental setting: although Neuhoff cautioned his subjects to avoid making a judgement of overall loudness, they may have done so; the direction of change may have an effect on such judgements that is quite different from its effect on loudness judgements made at discrete stages of a sweep. Close attention should also be paid to procedural differences that may turn out to be decisive.

Neuhoff’s results are of great interest but, in our view, they need to be considered in the context of what is already known about loudness perception for signals that are continuously changing in intensity. It remains to be seen whether his finding is of sufficient generality to warrant speculation about its role in the process of evolutionary selection.