Page 394 - Industrial Ventilation Design Guidebook

5,4 VENTILATION NOISE—CHARACTERISTICS, EFFECTS, AND SUGGESTED COUNTER-MEASURES 349

At the same dB(A) level, the noise with the stronger low-trequeiK\ tea
ture was thus experienced as being significantly less disturbing I hap flu
more high-frequency noise. This result suggests that the A-weighting o\ci
estimates the contribution made by low-frequency tones to the disturbing
experienced. This could be taken to mean that the general applicabihu ut
the dB(A) level is extremely limited at times when the goal is to cjrn out
evaluations of the anticipated disturbance effects of ventilation noise ,;<HI
tain ing tones.
An investigation designed as a tone experiment was also carried out on ,\
broad-band ventilation noise. The average mid-range frequency for tht broad
band indicated by each respondent when the most acceptable and least dtu*pt
able noise levels were set reflects the situation applicable to tone exposures.
The average set mid-range frequencies for the broad-band components \\ert
129 and 456 Hz, respectively. The most acceptable noise level had a lower fr« -
quency than the least acceptable noise level for all the respondents. 1 he csti
mate of the mean values in all the inconvenience variables was sigmtu.mth
higher for the least acceptable noise level than for the most acceprablt noisi
level in all variables.
The results revealed by these investigations on the whole indicated
that the measures taken to counter ventilation noise in order to reduce
the effects on disturbance, performance, and exertion should be directed
at higher frequency components within the low-frequency range. A
greater general lowering of the dB(A) level based on measures to counter
the low-frequency parts of the ventilation noise may involve a smaller
limitation of the inconvenience effects than a smaller comprehensive low-
ering of the dB(A) level based on a measure to counter the higher fre-
quencies of the ventilation noise.

5.4.5,3 Influence Due to Exposure Period
The influence of the period of exposure on the disturbance experi-
enced due to ventilation noise has been studied both in authentic expo-
sure situations in offices and in laboratory experiments. The link
between the estimated disturbance experienced due to ventilation noise
and the period of exposure, i.e., the time during which the office person-
nel stated that they could hear the ventilation, was tested on quite a large
2
group of respondents. The whole test group was divided into two
groups, based on estimated values above or below 50 mm on a 100 mm
estimation scale. The group with a lower average disturbance experience
exhibited significantly lower experience periods (231 min) than the
group with a higher average disturbance experience (390 min). The link
between the estimated disturbance experience and the time for which the
ventilation noise could be heard points to a positive linear correlation,
according to which the disturbance experience increases in line with the
increase in the period for which it is experienced. This can be interpreted
as indicating that exposed persons become habituated to or adapt: to the
low-frequency ventilation noise only to a very small extent, or not at all.
It is felt that similar conclusions can be drawn from a recent, more sys-
tematic field study into the significance of the period of exposure prior to