PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Quantification of the unpleasantness of fan noise in the form ofpreference-equivalent levels
Stephan TÖPKEN1; Steven VAN DE PAR2
1,2Acoustics Division, Department of Medical Physics and Acoustics,

Cluster of Excellence Hearing4all, Carl von Ossietzky University Oldenburg, Germany

Abstract
In a previous study by the authors, a semantic differential was used to rate a broad variety of fan sounds,
which were equalized in A-weighted sound pressure level. A factor analyses of the results indicated six
perceptual dimensions and five groups of sounds. The pleasant and the unpleasant fan sounds differed mainlywith respect to the first three perceptual dimensions. A loudness analysis of the signals showed systematicdifferences in the patterns of the specific loudness for the different groups of sounds.
In the present study, loudness- and preference-equivalent levels were measured for typical sounds from thethree major groups of sounds of the previous study. The results enable an interpretation of the findings
obtained for sounds with equal A-weighted sound pressure levels in the form of level differences. It turns outthat the group of unpleasant fan sounds needs to be reduced by 10 dB compared to the group of pleasantsounds based on median overall levels. Furthermore, level differences of up to 8 dB were found between thepreference- and the loudness-equivalent levels for the group of unpleasant sounds. This difference is smaller
for the two other tested groups of sounds.
Keywords: Fan noise, Preference, Pleasantness

1. Introduction
Fans are a part of many products of daily use and, thus, fan noise is a part of environmental noisethat humans hear every day. In many cases, the technical application of fans for cooling or ventil ation
purposes is mandatory and the resulting noise can often not be avoided. Several studies alreadycharacterized fan noise based on listening tests. They showed that the evaluation of fan noise depends
not only on the loudness of the sounds but other sound characteristics play also a role (1, 2, 3, 4).
However, in listening tests exploring the perceptual space, the overall sound pressure level or loudnessof the sounds is often equalized to reduce to the dominant impact of loudness differences on theevaluation and to capture the influence of sound characteristics other than loudness more clearly (5).
In a previous study by the authors, the perceptual space of fan sounds was explored with a semantic
differential consisting of 29 adjective pairs, which were specifically composed for the characterizationof fan sounds (4). Overall, 35 different fan sounds from eight different fan manufacturers and one
research institute were rated by 45 participants in listening tests.
The application of a factor analyses to the results of the semantic differential data delivered six
perceptual dimensions. The perceptual dimensions were characterized as I pleasant, II humming/bass,III shrill, IV monotone, V reverberant and VI noise-like. Similar perceptual dimension were also foundby Feldmann et al. (1) for fan noise and by Sung et al. (2) for HVAC&R equipment. A second factor
analysis delivered five groups consisting of sounds that were described and evalu ated similarly. The
mean semantic profiles of the three most important sound groups are shown in Figure 1. Based on
their semantic profiles these sound groups were denoted unpleasant (A), humming (B) and pleasant(C). In that previous study, it was also possible to define psychoacoustic indices for a description of
the most important perceptual dimensions, which are also relevant for the evaluation of the sounds.

12stephan.toepken@uni-oldenburg.desteven.van.de.par@uni-oldenburg.de8031

Figure 1 - Mean semantic profiles of the three most important groups of sounds (A unpleasant, B hummingand C pleasant) plotted over the adjective pairs, ordered according to their loadings onto the six perceptualdimensions I-VI from the previous study (4).
To reduce prominent loudness differences and get an insight into the influenc e of the soundcharacteristics on the evaluation, the study with the semantic differential was based on sounds thatwere intentionally equalized to an overall A-weighted sound pressure level of 55 dB(A). However,the questions remains how the effects of different sound characteristics on judgments for levelequalized sounds translate to sounds which differ in original sound pressure level or loudness.
Although the concept of level adjustments is used in many standards, only rather few knowledge onlevel equivalents for differences in sound evaluations is available.
The aim of the present study is a determination of points of subjective equality (PSEs) for loudness
and for preference in separate listening experiments for selected sounds of the prior study. The PSEs
were measured as loudness- and preference-equivalent levels compared to a fixed common reference
sound as level differences on a dB-scale. In this way, differences between the pleasant and unpleasantsound groups from our prior study can be interpreted in terms of dB-differences.

2. Method
Points of subjective equality (PSEs) were measured with a two interval, two alternative forced
choice paradigm varying the level of the test sounds with a 1 -up, 1-down rule. The measurements are
based on the assumption that an increase in overall level leads to a higher loudness and makes a test
sound less preferred due to an increased unpleasantness (6). Following this assumption, the level ofthe test sound was increased, if the reference sound was louder, in the loudness experiment, or if thetest sound was preferred, in the preference experiment. The level of the test sound was reduced, if it
was judged to be louder (loudness experiment) or the reference sound was pr eferred (preferenceexperiment). The reference sound was fixed to an A-weighted sound pressure level of 60 dB(A). In
the preference experiments, the level changes were 6 dB at the beginning of an adaptive track, whichwere halved down to a value of 1.5 dB after each upper reversal point of the adaptive track. In theloudness experiments, the levels were initially changed by 3 dB, which was halved down to 1.5 dB
after the second upper reversal. The adaptive procedure terminated after four reversal points wit h astep-width of 1.5 dB in both experiments. The PSEs were calculated as a mean value over these four8032

reversal points. The adaptive tracks of six test sounds were interleaved in each experiment and thepresentation order of the test and the reference sound was randomized in each trial.
2.1 Procedure
The listening tests for the 11 test signals were carried out in 5 sessions on different days together
with 19 additional test signals. The results of the 19 other signals will be presented in anotherpublication. Each session started by handing out written instructions to the participant. One -half ofthe participants always did the loudness experiment first, followed by the preference experiment. The
other half carried out the experiments in opposite order. After ea ch listening experiment, theparticipants took a short break and the experimenter recorded the first impressions mentioned by theparticipant. Each listening experiment had a duration of 20 to 30 minutes and a complete measurement
session had a duration of about 90 minutes.
2.2 Stimuli
The 11 test signals were chosen as typical sounds from three groups of sounds o f an earlier study(4), covering unpleasant (A1, A2 and A3), humming (B1, B2, B3 and B4) and pleasant (C1, C2, C3and C4) fan sounds. Curves of the specific loudness for the test sounds and the reference sound for acommon overall level of 60 dB(A) are given in Figure 2.

Figure 2 - Specific loudness curves of the 11 test sounds, which were selected from the three major groupsof sounds (A unpleasant, B humming and C pleasant) from the earlier study (4), and the reference sound(REF) calculated according to the DIN 45631 for a common level of 60 dB(A).
The reference sound (REF) was based on a white noise signal, filtered by a second order high -passwith a cut-off frequency of 200 Hz and a second order low-pass with a cut-off frequency of 500 Hz.
It was designed to be a rather pleasant and neutral signal, similar to the most pleasant fan noise fr om
the prior study (C3 in Figure 2) and to have a rather high value of the index that was highest correlated
to the pleasantness judgments (4). The major difference between the reference sound REF and soundC3 are two prominent tonal components, which are also visible as peaks in the specific loudness of
C3. All sounds had a duration of three seconds. They were digitally stored with a sampling rate of
44100 Hz. The reference sound had a fixed A-weighted level of 60 dB(A). The level of the test sounds
was 60 dB(A) at the beginning of each experiment and then varied according to the adaptive procedure.
2.3 Participants
Overall, 40 volunteers (20 female, 20 male) participated in the listening test. The mean age of theparticipants was 24 years (min=20 years, max=35 years). About 30 % of the participants did not haveany experience with listening test. The other 70 % already participated in other listening test before.
All of the participants reported no hearing problems. Each participant took part in five listening8033

session on separate days and judged the 11 test signals presented here and 19 additional signals.
2.4 Apparatus
The listening experiments took place in a soundproof booth. The sounds were presented dioticallyover open headphones (Sennheiser HD650) that were driven by the headphone output of an external
audio interface (RME Fireface UCX). The measurements of the PSEs were realized with the AFC Toolbox (7) in Matlab (The Mathworks).

3. Results
Figure 3 shows the median points of subjective equality (PSEs) across participants from theloudness and the preference experiments for the eleven test sounds. All median PSEs are lower than
the reference level of 60 dB(A), which means that level reductions were necessary for all test soundsto make them equally loud or equally preferred as the reference sound. The interquartile ranges, shownas error bars in Figure 3, are considerably smaller for the loudness PSEs than for the preference PSEs
for all test sounds. Apparently, the loudness was judged more uniformly across participants whereas
the preference PSEs indicate a considerable amount of inter-individual variability. This is especiallythe case for the sounds from Group A, which was characterized as the group of unpleasant sounds inthe previous study (4).

Figure 3 - Median points of subjective equality for loudness Lloud (grey) and for preference Lpref (black) for
the 11 test sounds from the three major groups of sounds (A, B and C) compared to the reference soundwith fixed level of 60 dB(A) (solid line). Group medians across the sounds of each group and acrossparticipants are indicated as grey (loudness) and black (preference) dotted horizontal lines.
The differences between the loudness-equivalent levels L loud and the preference-equivalent levels
L pref are rather small (about 2 to 3 dB) for the sounds from the Group B (humming) and the Group C(pleasant) and the interquartile ranges overlap to a great extent for each of these test sounds. Thus,
the preference evaluations seem to be tightly linked to the loudness judgments of the sounds for thesetwo groups. For the sounds from Group A (unpleasant), the median PSEs for preference are up to 8
dB lower than the median PSEs for loudness. Apparently, an adjustment to equal loudness is not
sufficient to make the sounds from Group A also equally preferred as the reference sound. Thisdifference might be attributed to the unpleasant sound character of these test sounds, requiring larger
attenuations of the test sounds to render them equally preferred as the reference sound (6).
Within each of the three groups of sounds, the median PSEs for preference L pref for the individualsounds cover ranges of about 5 dB. For a better comparability of the groups, group medians (shownas dotted horizontal lines in Figure 3) were calculated for each participant across the respective soundsof a group individually and then across all participants. In terms of these total group medians acrossparticipants, the sounds from Group C need about 6 dB, the sounds from Group B need about 11 dBand the sounds from Group A need about 16 dB of level reduction compared to the reference sound.

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The difference between the pleasant and the unpleasant fan sounds, which was on average about 2
scale values on the first perceptual dimension of the semantic differential data (see Figure 1),
corresponds to a level difference between the group medians of about 10 dB in term the preference
equivalent levels.

Acknowledgement
The present study is part of a larger project dealing with the psychoacoustic characterization of fannoise, which was initiated by the “Forschungsvereinigung für Luft- und Trocknungstechnik FLT e.V.”
from Germany. The project is supported by the German federal ministry for economic affairs andenergy (BMWi) on the basis of a decision by the German Bundestag.

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