Investigation of the unpleasantness of infrasound combined with audio soundusing psychoacoustic scaling methods
Elisa Burke(1,2) , Euginia Putri Stederi(1) , Stefan Uppenkamp(2) , Christian Koch(1)(1) Physikalisch-Technische Bundesanstalt, Germany, elisa.burke@ptb.de(2) Medizinische Physik, Carl von Ossietzky Universität Oldenburg, Germany

Abstract
At many immission sites, human exposure to infrasound ( f < 20 Hz) is usually accompanied by sound in theaudio-frequency range (audio sound, 20 Hz ≤ f < 20 kHz). This gives rise to the question of whether theinteraction between infrasound and audio sound affects the quality of auditory perception. Psychoacoustic experiments were performed within the framework of the EARS 2 project of the European Metrology Programmefor Innovation and Research (EMPIR). Recent results in this project had already shown that detection thresholds for infrasound were increased when simultaneous audio sound is present. The current study deals with thehypothesis that the unpleasantness related to infrasound is changed when infrasound is presented along withaudio sound. A rating task on a numerical scale and a pairwise comparison task were conducted to quantify
and to compare the unpleasantness of (1) isolated infrasound (sinusoid), (2) isolated audio sound (sinusoid andbroadband), and (3) the combination of both, at different sound pressure levels. Normal hearing listeners aged
from 18 to 30 years participated in the hearing tests. The results should be of use to improve the understandingof the impact of combined noise on humans and their well-being in the vicinity of potential noise sources.
Keywords: Infrasound, Unpleasantness, Psychoacoustic scaling methods

1INTRODUCTION

The effect of infrasound (frequencies f < 20 Hz) on humans is a matter of current research and debate. In
the recent years, the human exposure to infrasound increased and there are more and more individuals feelingannoyed by noise with infrasound components, like road and rail trafﬁc noise and wind turbine noise (e. g.
[1, 2]).
Since human exposure to infrasound is usually accompanied by sound in the common audio frequency range(audio sound, 20 Hz ≤ f < 20 kHz), the question arises whether the interaction of infrasound and audio soundis a contributing factor for the large annoyance of environmental noise with infrasound components. Only few
studies so far have investigated the effect of combined infrasound and audio sound stimuli on human perception.
For instance, Møller investigated the annoyance of infrasound (sinusoid at 16 Hz) combined with audio sound(narrowband noise centred at 1000 Hz) by exposure in a whole-body pressure-ﬁeld chamber. He observed that
“Combinations of audio and infrasonic noise were in general given a rating close to, or slightly above, therating of the most annoying individual noise conditions” [3].
Within the research project EARS 2 (project of the European Metrology Programme for Innovation and ResearchEMPIR) our aim was to realize a profound investigation of the human perception of combined infrasound andaudio sound stimuli. We focussed on the perception of sound by means of the human auditory system andexcluded other ways of sensation, like somatosensory mechanisms. Therefore, we applied a specially developed
sound source system delivering infrasound and audio sound stimuli directly to the participant’s ear canal bymeans of a foam eartip.
Recent results in this project revealed that detection thresholds for audio sound were hardly affected by thepresence of infrasound. On the contrary, thresholds for infrasound can signiﬁcantly increase when simultaneousaudio sound is present (results of a pilot study [4], manuscript of the complete data set is submitted).
The present study takes the previous ﬁndings a step further by investigating the perceived unpleasantness, a3000

precursor of annoyance, for combined infrasound and audio sound stimuli. We wanted to investigate whether
the combination of infrasound and audio sound is rated as more unpleasant than isolated infrasound and isolated
audio sound stimuli. We were especially interested in whether this also applies for the combination of audio
sound with infrasound below threshold level, since levels of infrasound in environmental noise are often below
threshold level (e.g. [1]). In addition, the masking effect of audio sound on infrasound may affect the perceived
unpleasantness for infrasound combined with audio sound, especially, when infrasound is presented at a nearthreshold level.
We performed two different rating tasks, a numerical scale rating task and a paired comparison task, in order
to examine whether both methods provide similar results. The results discussed in this paper are an outcome ofa pilot study with six participants.

2Materials and methods

2.1 Setup
The infrasound and audio sound stimuli were each generated by separated sound sources, both were realized
as electrodynamic loudspeakers mounted in damped wooden boxes. Tubes coupled the sound sources to thesame audiometric ear tip (E-A-RTone/ E-A-RLink, Standard Insert Foam Eartips), which was used for monaural
presentation of the acoustic stimuli. The infrasound and audio sound signals were digitally generated at 96 kHz
sample rate in MATLAB and they were fed into separate channels of an external soundcard (RME Fireface
UC). The outputs were connected to ampliﬁers (type Beak BAA 120 for infrasound signals, type Tira BAA 120
for audio signals).
During the hearing tests, the sound source system and the participant were located in an anechoic room providing a sufﬁcient low-background sound level even in the infrasonic frequency range. The audiometric eartip wasinserted into the participant’s right ear and the contralateral ear was occluded with an earplug. A mouse and acomputer screen were placed in front of the participant.
The sound pressure level of the audio sound stimuli were calibrated with an IEC 60318-4 [5] occluded earsimulator (Brüel & Kjær 4157, with ear canal extension DB 2012), whereas the sound pressure level of theinfrasound stimulus was calibrated with a 12 low-frequency pressure-ﬁeld microphone (Brüel & Kjær 4193,with UC0211) coupled to a cavity with a volume of 1.3 cm3 , which is approximately equivalent to the volumeof the human ear canal.
2.2 Stimuli
In the unpleasantness rating task, 26 different stimuli (Table 1), including isolated infrasound, isolated audio
sound, and combined infrasound and audio sound stimuli, were used. A sinusoid at 12 Hz was applied as
infrasound stimulus, and a sinusoid at 1000 Hz and a bandlimited pink-noise stimulus (broadband) with the frequency range between 250 Hz and 4000 Hz were used as audio sound stimuli.
The stimuli were presented at different sound pressure levels. The sinusoid at 12 Hz was presented at 95 dB SPL
and 110 dB SPL (Table 1, No. 1 - 2, No. 15 - 26), which represents sound pressure levels around and clearly
above the average threshold level reported in [6, 7] that is values between 90 dB SPL and 93 dB SPL for a sinusoid at 12.5 Hz. Since the sound pressure levels of environmental infrasound noise are often below thresholdlevel we were especially interested in whether audio sound combined with infrasound at a level below threshold is more unpleasant than isolated audio sound. Therefore, the infrasound stimulus was also presented at80 dB SPL when it was simultaneously presented with audio sound (No. 9 - No. 14). The sinusoid at 1000 Hz
was presented at 20, 50, and 70 dB SPL. The broadband stimulus was also presented at three sound pressurelevels (21, 36, and 53 dB SPL) having the same loudness as the sinusoidal stimulus at 1000 Hz according to [8].
The duration of each stimulus was set to 2 seconds to keep the total duration of the pairwise comparison task
cycle with 650 pairs of stimuli within an acceptable range for the test person. The same duration of the stimuli
was applied to the numerical scale ratings so that the results of both procedures remain comparable.
Cosine-squared windows with a ramp duration of 0.25 seconds were applied to stimulus onsets and offsets. The3001

broadband stimulus was digitally pre-shaped to equalise the frequency response of the audio sound source.
Table 1. Stimuli for the unpleasantness rating tasksNo.
12345678910111213141516171819202122232425
26

Infrasound:
Stimulus
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz
12 Hz

Level [dB SPL]
95
110
80
80
80
80
80
80
95
95
95
95
95
95
110
110
110
110
110
110

Audio sound:
Stimulus
1000 Hz
1000 Hz
1000 Hz
Broadband
Broadband
Broadband1000 Hz
1000 Hz
1000 Hz
Broadband
Broadband
Broadband1000 Hz
1000 Hz
1000 Hz
Broadband
Broadband
Broadband1000 Hz
1000 Hz
1000 Hz
Broadband
Broadband
Broadband

Level [dB SPL]205070
213653205070213653205070
213653205070213653

2.3 Psychoacoustic methods
Two different psychoacoustic scaling methods were applied to investigate the unpleasantness for combined infrasound and audio sound stimuli (stimuli, see Table 1). For each participant, a pairwise comparison task wasperformed as the ﬁrst experiment, and, after that, a 11-point-numerical rating task was conducted. The experiments were implemented in MATLAB.
2.3.1 Pairwise comparison unpleasantness task
In the pairwise comparison task, the participants were asked to compare the unpleasantness of all combinationsof the 26 stimuli listed in Table 1, excluding the comparison of two identical stimuli. Each measurement trial
consisted of the presentation of one pair of stimuli and, simultaneously, two panels labelled "1" and "2" weresuccessively highlighted in red colour on the computer screen in front of the participant. Then, the participant
was asked to indicate which stimulus was more unpleasant for him/her by clicking on the respective panel.
Each comparison of one pair of stimuli was repeated once, but in reverse presentation order. So in total 650
comparisons (26*25) were conducted. The order of the comparisons was pseudo-randomized.

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2.3.2 11-point numerical unpleasantness rating task
In the 11-point numerical unpleasantness rating task, the participant successively rated the unpleasantness caused
by each stimuli (listed in Table 1) by means of a 11-point numerical scale according to the ICBEN recommendations [9]. The scale displayed on the computer screen ranged from "0" labelled "not at all unpleasant"(translated into German: "überhaupt nicht unangenehm") to "10" labelled "extremely unpleasant" (translated into
German: "extrem unangenehm"). The unpleasantness rating was repeated once for each stimulus, so that in total
52 unpleasantness ratings were performed. The presentation order of the stimuli was pseudo-randomized
2.4 Participants
Six listeners aged between 18 and 30 years participated in the hearing tests. None of them had experience withpsychoacoustic experiments with infrasound prior to this study. All of the participants were otologically normal
as tested by an otoscopic examination and by standard audiometry. They had hearing thresholds better than
15 dB HL for the right ear for pure tones between 125 Hz and 8000 Hz.

3RESULTS AND DISCUSSION

The numerical scale ratings of unpleasantness were averaged across the two repeated ratings for each stimuli.
The boxplot of the averaged numerical ratings of the participants to each stimulus are illustrated in Figure 1.
The left panel shows the results for isolated infrasound, for the isolated broadband stimulus, and for infrasoundcombined with the broadband stimulus. The right panel shows the results for isolated infrasound, for the isolated sinusoid at 1000 Hz, and for infrasound combined with the sinusoid at 1000 Hz. The colour of the boxplot
indicates the audio sound level of the stimuli and the position of the boxplot on the x-axis indicates the infrasound level of the stimuli.
11-point-numerical scale
AS (Broadband) level:
no AS
21 dB SPL
36 dB SPL
53 dB SPL

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Infrasound (12 Hz) + 1000 Hz
average unpleasantness (num. scale)average unpleasantness (num. scale)

Infrasound (12 Hz) + Broadband

AS (1000 Hz) level:
no AS
20 dB SPL50 dB SPL
70 dB SPL

1086420

no IS

8095110
IS sound level / dB SPL

no IS8095110
IS sound level / dB SPL

Figure 1. Boxplots of average unpleasantness ratings of six participants using a 11-point numerical scale fordifferent stimuli combinations for infrasound (IS) and audio sound (AS) (list of all stimuli, see table 1).
To analyse the results of the pairwise comparison task, it was calculated for each stimulus how often on average
the participants rated the particular stimulus as more unpleasant than the other 25 stimuli. Thus, the results lie

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in the range between "0" (minimal unpleasantness) and "25" (maximal unpleasantness). Figure 2 shows theresults of the pairwise comparison task illustrated as boxplots for each stimuli arranged in the same way as theresults in Figure 1.
Unpleasantness pairwise comparison task

AS (Broadband) level:
no AS
21 dB SPL
36 dB SPL
53 dB SPL

2520151050

Infrasound (12 Hz) + 1000 Hz
average unpleasantness (pair. comp.)

average unpleasantness (pair. comp.)

Infrasound (12 Hz) + Broadband

AS (1000 Hz) level:no AS20 dB SPL
50 dB SPL
70 dB SPL

2520151050

no IS8095110
IS sound level / dB SPL

no IS8095110
IS sound level / dB SPL

Figure 2. Boxplots of average unpleasantness ratings of six participants based on a pairwise comparison task
for different stimulus combinations of infrasound (IS) and audio sound (AS) (list of all stimuli, see table 1) .
Comparing the distributions of the boxplots in Figures 1 and 2, it can be seen that the results of the pairwise
comparison task and of the numerical scale ratings are consistent. There was a signiﬁcant positive correlation between both procedures (Pearson’s correlation coefﬁcient ρ = 0.86, Spearman’s rank correlation coefﬁcientrs = 0.87). When speciﬁc stimulus conditions with many data points at the lower limit on the numerical rating
scale (stimuli No. 1, 6, 12, 18) were excluded from the analysis, then, the correlation coefﬁcients changed toρ = 0.82 and rs = 0.83. Taking into account the good agreement between both measurement procedures theresults of both procedures are discussed together in the following.
It was found that, in general, the unpleasantness of the isolated stimuli increased with increasing sound pressurelevel as expected, and this applies to both, infrasound and audio sound stimuli. Comparing the unpleasantnessof the audio sound stimuli, the sinusoid at 1000 Hz was rated more unpleasant than the broadband stimulus at
the same loudness. That was also to be expected since tonal sounds are usually perceived as more unpleasantthan non-tonal sounds. The stimuli being rated as most unpleasant were the infrasound stimulus at 110 dB SPL
and the sinusoid at 1000 Hz presented at 70 dB SPL alone or combined with audio sound and infrasound, respectively.
Another important ﬁnding of our study is that the stimulation with isolated audio sound and with audio soundcombined with infrasound at a level below threshold (12 Hz at 80 dB SPL) resulted in the same unpleasantness
rating. This ﬁnding might indicate that environmental noise with inaudible infrasound components, is not signiﬁcantly more unpleasant than noise with no infrasound components.
When infrasound was presented at 95 dB SPL, which is slightly above the average threshold level according to[6, 7], it seems that the simultaneous stimulation with audio sound presented at a low and medium level (broadband: 21 and 36 dB SPL, sinusoid: 20 and 50 dB SPL) slightly increases the perceived unpleasantness comparedto the unpleasantness of respective isolated infrasound and isolated audio sound stimuli. On the contrary, wheninfrasound at 95 dB SPL was combined with the audio sound stimulus at the high level (broadband: 53 dB SPL,3004

sinusoid: 70 dB SPL) the perceived unpleasantness corresponds more closely to the unpleasantness of the isolated audio sound stimuli. A possible reason might be the masking effect of audio sound on infrasound. Recent
results of this project had shown that infrasound threshold were masked by audio sound when presented at a
sufﬁcient level (results of a pilot study [4]). Due to the masking effect the infrasound stimulus presented ata near threshold level might be inaudible or hardly audible and, therefore, the unpleasantness of the combined
stimuli corresponds to the unpleasantness of the isolated audio sound stimulus.
When infrasound was presented at a clearly audible level of 110 dB SPL simultaneously with audio sound, theunpleasantness of these stimuli were dominated by the unpleasantness of the infrasound, since the unpleasantness of the combination seems to be equal to the unpleasantness of isolated infrasound stimulus. Moreover,the unpleasantness of the stimulus combination of the sinusoid at 1000 Hz with the infrasound stimulus at
110 dB SPL might even increase with increasing audio sound level compared to the unpleasantness of the isolated infrasound stimulus. However, this effect could not be observed for the broadband stimulus combined withhigh-level infrasound.
Comparing the results of our study with the results of Møller’s study [3], we have to point out that we usedother measurement procedures and we used monaural presentation by an insert-earphone sound source system
instead of a whole-body pressure-ﬁeld chamber like Møller. Nevertheless, the results are in good agreement
since both studies showed that the combination of infrasound and audio sound are rated as equal, or slightly
more, annoying than the most annoying isolated stimuli.

4CONCLUSION

Within this pilot study, ﬁrst trends were reported for the unpleasantness of infrasound combined with audio
sound. The results of the two different unpleasantness rating tasks are consistent. Furthermore, the results ofthis study suggest that the interaction of infrasound presented at a level above threshold combined with audio
sound affects unpleasantness ratings. On the contrary, our data indicate that if there is any effect of belowthreshold infrasound on the unpleasantness ratings for combined infrasound and audio sound stimuli, this effectmust be very small. However, the experiments are to be carried out with a considerably larger number ofparticipants to validate these ﬁndings.

ACKNOWLEDGEMENTS
Financial support from the European Metrology Programme for Innovation and Research (EMPIR) is gratefully
acknowledged. The EMPIR is jointly funded by the participating countries within EURAMET and the EuropeanUnion.
The authors would like to express their gratitude to all participants.

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