The Effect of Auditory Processing on the ... - SAGE Journals

The Effect of Auditory Processing on the Development of Low Level Low Frequency Noise Criteria.

S. Benton

(1).

H.G. Leventhall

(2).

Chelsea College, University of London, U.K. (2). A tkins Research and Development, Epsom, Surrey. U.K (Received 1st November 1982)

(I).

Abstract

The role played by loudness in the assessment of annoyance is seen to effect an intensity dominated concept current in noise assessment practices. Such dominance is not supported by the complex processing nature of the auditory system. The individual is placed within a context which requires the auditory system to align the person to external stimuli whilst maintaining the production of appropriate behaviours. Development of the concepts associated with audition is a pre-requisite to establishing viable noise assessment criteria. The limitations ofpresent day criteria, with an accepted assumption of intensity as the key parameter, are accentuated when assessments are made of low level low frequency noise. Once the individual is viewed as an active processor, bodily parameters may also serve to provide indices which are derived from the amount of 'processor work'. Introduction Concern over the effects of low frequency noise has grown apace with its well documented increased incidence in the environment, Leventhall (1980), Tokita (1980). There are indications that the traditional approach to the assessment of low frequency noise has been inadequate and furthermore derived from assumptions which may well be inappropriate, if not misleading, when applied to noise assessments in general. It has been argued that the limitations of traditional criteria have become increasingly apparent with an accumulation of empirical data, Cardozo and Lieshout (1981), Benton (1982). Relevant information can, in this instance, be drawn from research in the fields of audition and low frequency noise effects. Consideration of the information supplied by these areas enables deliberation on two prime questions. Firstly, whether a deeper understanding of the functional characteristics associated with the auditory system, would direct and benefit the development of criteria for assessing the effects of noise. Secondly. to relate the type of, in this case, low frequency, noise to the response. In other words, to identify the stimulus parameters to which the auditory system is required to respond. Moreover, to consider whether the type of symptoms recorded could provide an insight into the limitations of the system under exposure, consequent upon particular stimuli (noise form). However, before considering these two fields closely, a perspective might usefully be drawn over the approach as a whole, by recapping elements of the traditional approach. The Traditional Approach The problem of accurately relating a given physical stimulus to a subjective response has remained the central problem for those involved in assessing the psycho-physical properties of noise. The intensity of a sound refers to its physical magnitude which may also be expressed in terms such as power or pressure. The subjective perception of intensity is known as loudness and is usually expressed in terms derived from equal loudness contours, Early researchers found the auditory phenomenon of loudness to be a rational starting point in quantifying the subjective effects of sound (Stevens, 1936). The discovery that subjects were able to consistently estimate relative loudness over the frequency range 20Hz-IS KHz reinforced the preoccupation with intensity. It was later suggested that these discoveries could have alerted us to a complex characteristic of audition, in this case, that of acoustic coding and recall. Yet, intensity retained its central role in the assessment of noise.

Journal of Low Frequency Noise and Vibration Vol.1, No.3, 1982

97

AUDITORY PROCESSING

When the relationship between intensity and loudness became associated with the proposition that the hearing mechanism's predominant characteristic revolved around responding to sound pressure levels (SPL) above its 'threshold' and not to those below it, a powerful noise assessment method developed. It led to a number of derived connections, of which a prime example, still predominating, is that between loudness and annoyance. For practical purposes, the most important parameter of sound was defined to be that which induced the auditory sensation of loudness, namely its intensity. The ear displays a frequency-dependent response to 'sound' and exhibits a maximum sensitivity region between I Khz-4 KHz. Therefore, consequent upon an intensity would be a subjective response; a consideration of the ear's sensitivity region would permit a level of certainty about the extent of the response. A higher intensity would elicit a greater response - if this response was annoying, it would be accentuated by a further increase in intensity. Therefore, 'noises' which are, in general, of a lower intensity and concomitantly smaller subjective response, would produce a lower annoyance/loudness value. As the hearing thresholds were less sensitive at lower frequencies, it would follow that at lower frequencies, markedly higher intensities would be required to induce comparable amounts of annoyance/ loudness. Kryter (1970) has pointed out that many concerned in the analysis of noise have taken these assumptions to mean that low frequencies are in 'character' less likely to annoy than higher frequencies. Loudness has, therefore, been assumed to be a reasonable guide to the annoyance value of sounds (see Figure I). However, doubts about this relationship have been expressed, even before the attempt to integrate data on the auditory system's processing capacity. 110 os ~

I::l.

0

..... ~

""...

Ql

~ I

a;

>

Ql

Loudness or noisiness: Method of paired comparisons Noisiness: Method of individ adjustment Loudness: Method of individ adjustment Reference band at 1000 Hz

...J Ql

!3

'" '" ... Ql

~

'0 I:: ::l 0

en '0

~

20

100

1000 Frequency -

Figure I

10000 20000

Hz

Equal loudness and equal noisiness judgements (from Kryter)

Much of the certainty surrounding this relationship originated from the similarity between loudness contours and annoyance curves. As such, the similarity has been taken to confirm the association between loudness and annoyance, at least for broadband stimuli. Reese et al (1944) produced data demonstrating this relationship. This consisted of matching experiments over a range of SPL's 94dB-64dB. It can be seen that the standard deviations increase at the lower intensity, as shown in Figure 2. Even if the assumed relationship between loudness and annoyance holds for high intensities, it diminishes with falling SPL. As the SPL falls, so should the annoyance factor, and therefore, individual differences in rating should also decrease. As can be seen, this is inconsistent with the data. The characteristic procedure employed during these tests usually revolved around a presentation of broadband stimuli. The subject had to rate the sound on a scale of annoyance, ego I to 5, with respect to a parameter. This usually consisted of the question of how annoying would the sound be if the subject heard it at home, (or in various environments) for x hours. The method ignores the influence of the auditory system which supplies information upon which the individual bases a response. It is suggested that, without including other parameters of significance to the auditory system (outlined later), which characterize its role as an active processor, the picture remains incomplete. Indeed, it may be argued that without such considerations, the predominant characteristics to which subjects responded was simply that of intensity. 98

AUDITORY PROCESSING Frequency· Hz 20

180

31M 1170 1000

.

MOO 1800 2450 3100 4000 5100 lI100

llOOO

MK

,",ound Ilr~IUitir. 1",".1 f){ ~I.nd.nl band :j, !... 1114

§ :g ;., t":

~"'5!

~..9

E"

0

8 -§~~

0

I

"t:l~ o. o.

0

0

o

-Sf-

0

~§-8 0 0

-"'~-------

0 ..l1._ _--1

"

1.5 1.1 1.0 0.8 1.0 2.2 3.7 ~~SD~ H ~~-----------_-:._--....;,,---~-....;,,--_..... 3.0

1.3

COO

;£ ~] /7.-

15

.~ ~

1C

o.

:l

.E~ 0>

Sound Ilre~lClln' 1'\"(01 IIf :oohmclllrd hlmd ~.

o

o~

- 0 "C-

o o o

o. o.

H.. dli

o

'~L~

/7'"

~~

o o

o

o -1