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

Annoyance penalty of amplitude-modulated sound
Valtteri HONGISTO1; Petra VIRJONEN1
1

Turku University of Applied Sciences, Turku, Finland

ABSTRACT
Amplitude-modulation (AM) appears in various kinds of noise sources. The nature of periodic AM soundcan be described by e.g. AM frequency, AM depth, and equivalent spectrum of sound pressure level (SPL).
There is little research concerning the effect of these factors on annoyance. Our purpose was to determine
the penalty of periodically AM sounds for a range of AM frequency, AM depth, and spectrum of equivalent
SPL. Forty participants rated the annoyance of 70 AM sounds and 22 reference sounds. The three independent
variables of AM sounds were AM frequency (7 levels, from 0.25 to 16 Hz), AM depth (5 levels, from 1 to 14dB), and SPL spectrum (2 alternatives). All AM sounds were played at 35 dB LAeq. Reference sounds wereplayed at levels from 29 dB to 49 dB LAeq. They enabled the determination of penalty of AM sounds. Allsounds were created from pseudorandom noise. The results showed a significant effect of all three
independent variables on annoyance. The effect of both AM frequency and AM depth was very large. Thepenalty values varied from -1 to +12 dB. The results can be used in the development of penalty schemes ofAM sounds.
Keywords: Psychoacoustics, annoyance, amplitude modulation

1. INTRODUCTION
Many regulations involve a penalty, k [dB], to be added to the measured or predicted equivalentA-weighted SPL, LAeq [dB], in order to counteract the negative effect of the specific feature onannoyance. The specific features mentioned in many regulations so far are impulsivity and tonality.
Besides sound level, impulsivity, and tonality, a fourth special property of sound, which can beauditorily sensed, is amplitude modulation (AM). Two reviews (1,2) suggest that a penalty should beapplied to AM sound of wind turbines because of the additional annoyance caused by AM. AM meansthat the amplitude of a carrier sound varies with time. If the variation is periodic, the frequency ofAM sound, fm [Hz], can be defined. The frequency can be constant or randomly changing. AM exists
in the sound of, e.g. sea waves reaching the coast, pulsating road traffic noise, wind turbine noise,helicopter, ventilation, and low-speed engine. The strength of AM can be described by Dm [dB], whichis the difference between the maximum and minimum value of the A-weighted SPL.
Several psychoacoustic experiments have studied the annoyance of AM sounds (3, 4, 5, 6, 7). Theysuggest that AM increases the annoyance of sound if a certain level of Dm is exceeded. Most of theabovementioned references have focused on the AM sounds of wind turbines. Because AM can befound also in other present or future sources of environmental noise, it would be important to study
the annoyance of AM sound with a generic study without having strong connection to any realistic
sound source.
Our purpose was to determine the annoyance penalty of AM sound as a function of the modulation
frequency and modulation depth. The study was conducted for two different spectra.

2. MATERIALS AND METHODS
We conducted a psychoacoustic laboratory experiment with forty voluntary participants (mean 27years, 28 female, 12 male). The independent variable was the experimental sound (later: sound) andthe dependent variables were loudness and annoyance. However, we focus on annoyance in this study.
1valtteri.hongisto@turkuamk.fi; petra.virjonen@turkuamk.fi

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Each participant rated the annoyance of 92 sounds using an eleven-point response scale of ISO/TS
15666 from 0 (Not at all) to 10 (Extremely much).
The experimental sounds consisted of AM sounds and reference sounds. Reference sounds wereused to determine the penalty. The AM sounds were created as combinations of three independent
variables: modulation depth (5 levels), modulation frequency (7 levels), and spectrum (2 levels).
Altogether, we had 5x7x2 AM sounds, i.e. 70 sounds. All AM sounds were presented at an overalllevel of 35 dB LAeq . The reference sounds were presented at levels from 29 to 49 dB LAeq in 2 dB steps.
Reference sound had the same equivalent SPL spectrum as the AM sound but AM was absent. Because
we had AM sounds with two different spectra, we had also two sets of reference sounds.
The experimental sounds were produced synthetically (MATLAB). First, broad-band pseudorandom noise was weighted to achieve the two spectra S1 and S2 (Figure 1a). Spectrum S1 representsa typical non-tonal wind turbine noise and spectrum S2 represents urban road traffic noise. However,the sounds were generic since they were created from pseudorandom noise.
These two spectra acted as the carrier waves for the AM sounds. The amplitude of the carrier wave
was varied sinusoidally with modulation index m:110 ⁄⁄101where Dm [dB] is the modulation depth. The carrier wave was multiplied with quantity Q𝑚𝑄1𝑚 ∙ sin 2𝜋𝑓 𝑡

to induce the modulation in the steady noise. Here, fm [Hz] is the modulation frequency, and t [s] istime. Time was discretized by
1 ,𝑡0, 1, 2, … , 𝑛
where ns is the number of the samples, and fs [Hz] is the sampling frequency (44.1 kHz). The carrier
wave was modulated with only one modulation depth at one modulation frequency. Twenty-secondlong sound samples were created, and they were looped until the participant gave the response. Thesounds were presented within frequency range from 25 to 5000 Hz.
60

10S1S250

8y = 0.874x - 0.4312
R² = 0.9794

7Annoyance

40Lp,Z [dB]

Ref. sounds
AM soundLinear (Ref. sounds)

930
20

65
4
32

10

k=9.8 dB

1f [Hz]

(b)

49

47

45

43

41

39

37

35

33

29

4000

2000

1000

500

250

125

63

31.5(a)

3100LAeq [dB]

Figure 1. (a) Unweighted SPL, Lp,Z, of spectra S1 and S2 versus frequency, f. (b) The principle ofdetermining the penalty. The AM sound in this example has spectrum S1, Dm=8 dB, and fm=8 Hz.
Before the experiment, the properties of the sounds were measured so that they met the desired
properties with respect to overall level, overall spectrum, D m, and f m. The measurements were madewith head-and-torso simulator since the sounds were presented to the participants with headphones.
The experiment was conducted in the psychoacoustic laboratory of Turku University of Applied
Sciences in Turku during spring 2017. The background noise level was 21 dB LAeq . The experiment7694

consisted of 12 phases. Phase 1 was the information consent form. The experiment was supported by
the ethical board of Finnish Institute of Occupational Health. Phase 2 was an initial questionnaire
which is not fully reported here. Phase 3 was the hearing threshold test. All participants had normalhearing. Phase 4 was the familiarization to the sounds. The participants could listen to some soundexamples but they did not rate them yet. Phase 5 was the rehearsal of loudness rating for some soundexamples. The results were not analyzed. Phases 6 and 7 were loudness rating blocks for twoalternative spectra, respectively. It should be noted that both spectra were presented in different blocks
but the order of the spectra were counterbalanced between participants. Phase 8 was a short break.
Phase 9 was the rehearsal of annoyance rating for some sound examples. The results were not analyzed.
Phases 10 and 11 were annoyance rating blocks for two alternative spectra, respectively. Both spectra
were presented again in different blocks and the order of the spectra were counterbalanced between
participants. Phase 12 was feedback and reception of 20 euros gift token.
The penalty was determined according to Figure 1b. The method was developed in Refs. (8) and(9). The annoyance penalty k of an AM sound is determined by estimating the LAeq of a reference
sound which produces the same annoyance as the AM sound under investigation. The effect ofvariables on annoyance was determined using repeated measures ANOVA. The analyses wereconducted for only 35 participants since five participants were removed to fulfill the normality criteriaof ANOVA.

3. RESULTS AND DISCUSSION
We found a statistically significant effect of spectrum (p<0.003), modulation frequency (p<0.001),
and modulation depth (p<0.001) on annoyance. The annoyance penalty values are shown in Figure 2.
The effect of spectrum was small and complex compared to the effects of f m and D m.
14141212Dm:

10k [dB]

14 dB

2 dB
84 dB

8 dB
2 dB

468 dB
14 dB
42

2008
6-20.25(a)Dm:

101 dB

0.5

12fm [Hz]48-20.25

16(b)

4 dB
1 dB

0.5

124816fm [Hz]Figure 2. The annoyance penalty k as a function of modulation frequency, f m, and modulation depth,
D m. (a) Spectrum S1. (b) Spectrum S2.
The largest penalty was around 12 dB for both spectra S1 and S2. The largest penalty values wereobtained with the largest value of D m and with f m from 4 to 16 Hz. The statistical significance of thepenalty values are not reported here but typically values k < 1.5 dB were not statistically significant.
That means that the annoyance for an AM sound with k < 1.5 dB was not statistically significantly
different from the annoyance of the reference sound presented at 35 dB.
Penalty increased systematically when f m increased from 0.25 to 2 Hz. Within 216 Hz, thedependence of penalty on fm was not clear. It seems that the annoyance does not change much within
this range.
We found also negative penalty values when f m was small. Some of negative penalties were also
statistically significant. Sounds with negative penalty values sounded like sea waves on the coast.
Sounds with natural cues are often rated less annoying than steady-state pseudorandom sounds

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(reference sounds) but this is only speculation.
Several previous studies have investigated the annoyance of AM sounds (3-7). Our study is among
the most generic studies since our study is not dealing with any specific sound type and the range off m and Dm was exceptionally large. Hafke-Dys et al. (4) conducted a study which was very close to
our study (overall level was 45 dB LAeq , f m ranged from 0 to 4 Hz, D m ranged from 0 to 9 dB, fivedifferent spectra were included). Our findings on the behavior of annoyance agreed well with theirresults. However, Hafke-Dys et al. did not present annoyance penalty values. Our study is among thefirst studies which presents the annoyance penalty as a function of a broad range of f m and D m.
Scientific penalty schemes for determining the penalty of AM sound have been requested by Refs.
(1) and (2). Our data can be helpful in assessing the penalty that could be given due to the AM ofindustrial noise when f m and Dm is known. However, it should be noted that our study was a laboratory
study with synthetic sounds where the AM was periodic and constant. The results shall be applied tothe assessment the annoyance penalty in field conditions cautiously because the nature of AM in realsettings contains many properties which may affect annoyance, such as varying f m and Dm. D m canalso depend on frequency of sound, i.e. some frequency bands involve stronger AM than the others.
Furthermore, other specific factors of sound, such as tonality, impulsiveness, or specific spectra may
affect the annoyance as well or even more than AM. Therefore, our study would benefit from a followup experiment involving real sounds recorded in the field.

4. CONCLUSIONS
We conducted a laboratory experiment where the annoyance penalty of synthetic amplitude
modulated wide band noise was determined for a broad range of modulation depth (1 to 14 dB) andmodulation frequency (0.25 to 16 Hz). The penalty values ranged from -2 to +12 dB. The penalty
depended strongly both on the modulation depth and the modulation frequency. Our data can be usefulin the development of penalty schemes.

ACKNOWLEDGEMENTS
The study was part of Anojanssi –project which was funded by Business Finland, Turku Universityof Applied Sciences, Ministry of Environment Finland, Ministry of Social Affairs and Health Finland,and collaborating companies and associations.

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