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

Equal annoyance contours at frequencies 4 – 1000 Hz
Ville RAJALA1; Jarkko HAKALA1; Valtteri HONGISTO1
1Turku University of Applied Sciences, Turku, Finland

ABSTRACT
Knowledge about annoyance caused by infrasound is limited. The purpose of our study was to determine
equal annoyance contours (EACs) at frequencies 4 – 1000 Hz. A psychoacoustic experiment was conductedin a laboratory environment for seven participants. The participants rated annoyance of 60 sine tones. Thesounds were played in 20 frequencies in range 4 – 8000 Hz with sound pressure level (SPL) corresponding toloudness of 20, 40, and 60 phon. Three EACs were determined from mean annoyance ratings. The SPL
yielding to equal annoyance increased with decreasing frequency. Up to 31.5 Hz, the EACs were more close
to each other than on higher frequencies, suggesting that a small increase in SPL can significantly increase
perceived annoyance. The results are preliminary and final contours will be published later involving a larger
number of participants, hearing thresholds, and equal loudness contours.
Keywords: Infrasound, Psychoacoustics, Equal annoyance contour

1. INTRODUCTION
Knowledge of human perception of infrasound is very limited. Many textbooks unambiguously
manifest that human cannot hear sound in frequency range below 20 Hz. However, there is vast
research evidence that the hearing range can extend at least down to 3 Hz (1, 2).
If infrasound is perceivable, it can also be annoying. Thus, it is important to study humanperception of audible infrasound. Typical psychoacoustic phenomena that are examined are hearing
threshold, loudness, and annoyance. Investigating annoyance is of primary interest since noise
annoyance is the most usual adverse health effect of sound and some countries have also published
upper limits the sound pressure level (SPL) of infrasound for one -third octave bands 10, 12.5 and 16Hz.
Pure sine tones are the simplest form of sound and thus a natural base for basic research ofperception of sound. For example, hearing threshold and equal loudness contours are usually
determined by using sine tones (3, 4).
Earlier research of hearing threshold of infrasound suggests that infrasound is audible but require
high SPL to be perceivable (1). This sets special requirements for sound reproduction devices. The
equipment must be able to produce high SPL in low frequencies with negligible distortion.
Only few studies deal with the annoyance of infrasound (5, 6). Equal annoyance contour (EAC)
describes SPL for frequencies to be perceived equally annoying. Although the concept is analogue toequal loudness contours, there is very little knowledge of EACs (5, 7). The purpose of our study was todetermine EACs at frequencies 4 – 1000 Hz.

2. METHODS
2.1 Overall design
We conducted a psychoacoustic experiment in a laboratory setting. The independent variable was
the experimental sound and the dependent variable was the subjective measure annoyance.
2.2 Participants
The participants were recruited through Turku University of Applied Sciences mailing lists. The
requirements for participants were: age in range 19 – 26 years, native Finnish language, and ability toconduct the experiment without using eyeglasses. The eyeglasses were set as an exclusion criterion
11492ville.rajala@turkuamk.fi; jarkko.hakala@turkuamk.fi; valtteri.hongisto@turkuamk.fi

due to the use of headphones in the experiment. It was instructed that one should not participate theexperiment during a flu or any other illness. The participant were not informed that the frequency
range extends to infrasound region. Seven voluntary persons (6 female, 1 male) participated in theexperiment. The participants were native Finnish speakers and their age ranged from 19 to 26 years(mean 23, standard deviation 2). The participants received a 30 euro gift token as a compensation for
their participation. None of the participants was professionally related to our research group.
2.3 Sounds
The sounds were 60 sine tones in 20 frequencies listed in Table 1. The sounds were generated byusing MATLAB R2017b (MathWorks Inc., Natick, MA, USA). Every frequency was played in threeSPLs. The SPLs were selected to correspond the loudness levels 20, 40, and 60 phon. The SPL wascalculated for frequencies 20 – 8000 Hz by using methods described in standard ISO 226 (4). For
infrasound, SPL was acquired from reference (8). The sounds were saved in standard audio file format(.wav, 24 bit, f s = 48 kHz).
Table 1 – The 20 frequencies in the annoyance testing and their SPLs.
Frequency, Hz20 phon SPL, dB
40 phon SPL, dB
60 phon SPL, dB
4
120.7
124.8
127.4
5
118.0
122.0
126.0
6.3
115.0
120.0
125.0
8
109.4
114.3
118.1
10
107.0
112.0
116.0
12.5
103.0
108.0
115.0
16
95.1
101.3
106.9
20
89.6
99.9
109.5
25
82.7
93.9
104.2
31.5
76.0
88.2
99.1
63
58.6
73.1
85.9
12543.9
60.6
75.6
250
32.0
50.4
67.5500
23.4
43.1
62.1
1000
20.0
40.0
60.0
1500
21.4
42.5
63.2
2000
18.2
39.2
60.0
3000
14.3
35.6
56.4
4000
15.1
36.6
57.6
8000
31.5
51.8
71.7
2.4 Equipment and validation of the sounds
During the experiment the participant was located inside a pressure chamber (Figure 1). Thechamber was located in an anechoic room in in Salo, Finland. The chamber was equipped with aventilation system to maintain good air quality and constant temperature throughout the experiment.
Frequencies in range 4 – 63 Hz were played by using a loudspeaker and frequencies in range 125 –8000 Hz by using headphones. The devices received signal from a sound card (D-audio USB Pre-Amp,Duran Audio Ltd., The Netherlands) connected to a computer.
The SPL was measured and verified individually for all sounds used in the experiment.
Frequency-wise SPL correction was applied to compensate the anomalies in the frequency response ofthe system. The SPL was verified in range 4 – 63 Hz by using a precision sound level meter (NorsonicNOR150), a microphone preamplifier (G.R.A.S. 26CI, Denmark), and a microphone (G.R.A.S. 46AZ,Denmark). In range 125 – 8000 Hz, the SPL was verified with a head-and-torso simulator (Brüel&Kjær4100, Denmark), a microphone power supply (Bruel&Kjaer 2804, Denmark) , and a portable
multitrack recorder (TASCAM DR-680MKII, Montebello, USA). MATLAB was used to measure andadjust the SPL to match the target values. The frequency dependent diffuse-field correction was1493

applied (Brüel&Kjær Pulse Sound Quality 15.1.0, Denmark), which compensates the amplification ofSPL at high frequencies caused by the artificial ear of the head -and-torso simulator.

Figure 1 – Overview and the dimensions of the pressure chamber used in the experiment.
2.5 The experimental procedure
Each participant rated the annoyance of all sounds in Table 1. One participant at a time conducted
the experiment by using a computer and a MATLAB based software with a graphical user interface.
The experiment constituted of 12 phases. In phase 1, the participant read and signed informationconsent form. The phases 2, 3, and 5 included loudness rating which methods and results are notpresented in this paper. Phase 7 was a hearing threshold test which results are not discussed in thispaper. Phases 4, 6, and 8 were short voluntary breaks.
Phase 9 was a rehearsal for annoyance rating to make the participant familiar with the procedureand to allow them ask any questions regarding the rating procedure. The participant rated six sounds.
The ratings given in the rehearsal phase were not analyzed.
The annoyance rating (phase 10) was conducted in close relationship to standard ISO 15666 (9).
The participants were inquired a question “How much does the sound bother, disturb, or annoy you?”
The eleven step response scale was from 0 to 10, where 10 was labeled as “Extremely annoying” and0 as “Not at all”. The participants were instructed to use the full scale and try to make their answers as
consistent as possible. As some of the sounds were close to the hearing threshold, the participants
could also respond: “The sound is inaudible”. If the participant felt that the sound was associated withother sensations than a hearing sensation, they could express it by selecting a button labeled “Auditivesensation is associated with other sensation”. The participants had to listen the sound for 5 seconds
before they were able to give the rating. The sounds were played in one of four predetermined
pseudorandom orders. The four orders were decided so that the same sound was never presented atsame point in different orders.
In phase 11 the participant described the possible other sensations associated to the played sounds.
The results are not presented in this paper. After the experiment, the participants received a gift token
and a short introduction of the goals and impacts of the conducted experiment. The participants had achange to ask any questions related to the experiment. The participants were not informed that theexperiment included infrasound. The participants stayed in the laboratory on average 2.5 hours.
2.6 Determination of equal annoyance contours
The EACs were determined by using the following method:
1. Mean annoyance of every sound was calculated by using all annoyance ratings.
2. A linear fit was determined using three mean annoyance values and their corresponding
SPL by using equation

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(1)
𝐴 = 𝑘𝐿𝑖 + 𝑏,
where k and b are the coefficients of the linear fit and Li is the SPL. The fit was calculated
for all 20 frequencies. Figure 2 shows an example of the fit for frequencies 4 and 1000 Hz.
3. Annoyance of 1000 Hz at SPLs 20, 40, and 50 dB was selected to be the basic points of theEACs. The annoyance values corresponding to these levels (A20 , A40, A50 ) were calculated
by using equation (1).
4. SPL corresponding to annoyance A20 , A40, and A50 was calculated in all frequencies byusing equation(2)
𝐿 = (𝐴𝑖 − 𝑏)/𝑘,where b and k are the linear fit coefficient of the frequency and Ai =(A20 , A40 , A50 ). Figure 2
shows an example of SPL calculation for frequencies 4 and 1000 Hz.

Figure 2 – A visualization of the linear fit to the mean annoyance for frequencies 4 and 1000 Hz. The
arrows demonstrate how the SPL corresponding annoyance A20, A40, and A50 is calculated. The annoyancevalues were A20 = 4.1, A40 = 5.5, and A50 = 6.8.

3. RESULTS
The calculated mean annoyance values as a function of frequency are shown in Figure 3a. Figure 3bincludes the three EACs in frequency range 4 – 1000 Hz calculated according to the method describedin Section 2.6.

4. DISCUSSION
4.1 Findings
High mean annoyance values were achieved for frequencies 2000 Hz onwards. This suggests that in
our experiment high frequency sounds were usually perceived more annoying than infrasound.
SPL yielding equal annoyance increases with decreasing frequency. For infrasound our EACs have
SPL values over 100 dB. The three EACs are much closer to each other in frequencies below 31.5 Hz
than in frequency region 125 – 1000 Hz. This suggest that a small increment in SPL in low frequencies
can increase the perceived annoyance rather much.
We only calculated the EACs in frequency range 4 – 1000 Hz because it was not possible todetermine EACs reliably above 1000 Hz.

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Figure 3 – a) The mean annoyance ratings of all sounds in the experiment as a function of frequency. The
annoyance response scale was from 0 to 10. b) Three equal annoyance contours as a function of frequency.
Equal annoyance contour describes the SPL where the sounds are perceived equally annoying. The hearing
threshold is composed from references (3) and (10).
4.2 Comparison with other studies
Andresen and Møller (5) have published EACs in frequency range 4 – 1000 Hz. They reported
annoyance in frequencies 4, 8, 16, 31.5, and 1000 Hz, while we reported annoyance contours by using
15 frequencies. Their EACs with base point at 20 dB and 40 dB in 1000 Hz can be compared to ourcontours. They found equal relationship that smaller frequencies require higher SPL to be perceived
equally annoying as high frequencies. Further, they describe that the EACs are very close to each otherin infrasound. We noticed the same phenomenon. However, our curves had higher SPL than those ofreference (5). The difference was about 15 dB for some frequencies. Further research is needed toinvestigate the reason for the difference. One must note that our results involve only seven participants
and the uncertainty can be high.
4.3 Future research
Annoyance of infrasound with SPL closer to hearing threshold (under 20 phon) should be examined.
EACs are very close to each other in infrasound region. This suggest that small increments in SPL ininfrasound can have a significant impact on perceived annoyance. Thus, infrasound with SPL slightly
over the hearing threshold can be annoying.

5. CONCLUSIONS
An experiment was conducted to determine equal annoyance contours (EACs) in frequency range
4 – 1000 Hz. EACs were calculated for three levels, having basepoints at SPL of 20, 40, and 50 dB at
1000 Hz. SPL yielding equal annoyance increased with decreasing frequency. From 31.5 Hz and below
the EACs were much closer to each other than on higher frequencies, suggesting that small increase in
SPL in low frequencies can increase annoyance significantly. The results were produced from data ofseven participants and are preliminary. The achieved EACs are preliminary and final contours will be
published later involving a larger number of participants, hearing thresholds, and equal loudness
contours.

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