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

Annoyance of impulsive sounds – a psychoacoustic experiment
involving synthetic sounds
Ville RAJALA1; Valtteri HONGISTO11Turku University of Applied Sciences, Finland

ABSTRACT
Impulsive sound can be perceived more annoying than a steady-state sound having the same LAeq .
The difference in perceived annoyance can be compensated by adding an appropriate penalty to LAeq.
Nordtest method NT ACOU 112 describes an impulse by using two measures: level difference (DL) andonset rate (Ron ). A psychoacoustic laboratory experiment of 32 participants was conducted todetermine the penalty for different values of DL and Ron. Sounds were synthetic and periodic impulsive
sounds with two alternative spectra. Impulsive sounds were presented at 55 dB LAeq . Steady-statesounds at levels from 49 to 70 dB were used to derive the penalty of impulsive sounds. The observed
penalty values ranged between -2 and +8 dB. The observed penalty increased with increasing DL andRon. The penalty predicted by Nordtest method usually overestimated the observed penalty whenRon ≥ 200 dB/s. Therefore, the future development of Nordtest penalty calculation scheme might bejustified. Our results can be used to develop the future penalty schemes of impulsive sounds.
Keywords: Impulsive sound, noise annoyance, penalty

1. INTRODUCTION
Impulsive sound can be perceived more annoying than a steady-state sound having the same LAeq .
This suggest that LAeq does not alone represent the annoyance of an impulsive sound. The limitation
can be compensated by adding a penalty value to the measured LAeq . The penalty should correlate with
the increased annoyance caused by impulsiveness.
Impulsivity of sound can be measured by using different methods. Nordtest method NT ACOU
112 (1) describes an impulse by using two measures: level difference (DL ) and onset rate (Ron ). DL
measures the increase of sound pressure level (SPL) during an impulse. Ron describes how rapidly theSPL increases. NT ACOU 112 is already implemented in a British standard of assessment ofenvironmental sound (2) and is currently under preparation to be implemented as an ISO standard (3).
The appropriate penalty to be associated with impulsive noise measured by using NT ACOU 112
has been very little studied (4). Our aim is to determine the penalty at different values of DL and Ron of
periodic impulses with two different spectra.

2. METHODS
2.1 Overall design
The study was a psychoacoustic laboratory experiment where the participants rated the annoyanceof 74 experimental sounds. All participants rated all experimental sounds. The experimental sounds
included impulsive sounds and reference sounds (non-impulsive steady-state sounds). The
independent variable was the experimental sound and the dependent variable was the subjectivemeasure annoyance. The reference sounds were used to determine the penalty associated with theimpulsive sounds.
2.2 Participants
The participants were recruited via different university mailing lists and Turku University ofApplied Sciences. The requirements for the participant were: age within 20 – 45 years, Finnish as a1ville.rajala@turkuamk.fi; valtteri.hongisto@turkuamk.fi7697

native language, and normal hearing. It was instructed that one should not participate the experiment
during a flu or any other illness. Thirty-two voluntary persons (13 men and 19 women, mean age 29years) participated in the experiment. The participants received a 20 euro gift token as a compensation
after the experiment. None of the participants was professionally related to our research group.
2.3 Experimental sounds
The experiment was constituted on 66 impulsive sounds and 8 reference sounds. The impulsive
sounds were presented at LAeq of 55 dB. The LAeq of the reference sounds varied between 49 and 70 dB
in 3 dB steps. Figure 1 shows the one-third octave spectra of the reference sounds. The length of eachexperimental sound was 18.5 seconds.
The acoustic descriptors of the impulsive sounds were DL (7 levels), Ron (10 levels), and spectrum(2 values). Table 1 expresses the DL and Ron values included in the experiment. An impulse in theimpulsive sound had another of the two spectra named S1 and S2. S1 corresponded to spectrum ofreference sounds (Figure 1) and S2 corresponded to spectrum of white noise.
Each impulsive sound consisted of two components: steady-state wide-band background sound andimpulses. The background sound was continuously presented through the impulsive sound . The
spectrum of the background sound corresponded to the spectrum of the reference sounds. An impulsive
sound always consisted seven impulses. The impulse frequency was 2.5 seconds, meaning that an
impulse began always 2.5 seconds after the beginning of the previous impulse. The first impulse in an
impulsive sound began one second after the beginning of the background sound. The SPL of thebackground sound under the impulsive sound varied between experimental sounds to maintain thedesirable equivalent SPL of 55 dB LAeq .
The sounds were created digitally (MATLAB R2017b, MathWorks Inc., Natick, MA, USA) to beable to achieve the desirable values of DL and Ron. The reference sounds were shaped frompseudorandom noise. The impulses were created by multiplying pseudorandom noise with a function.
For the onset of an impulse the sound pressure increased linearly. After achieving the top of the onset,
the sound pressure was set to decay exponentially. The exponential decay mimics the decay of an
impulse generated by a collision of two solid objects. The created impulse was summed to thebackground sound. All seven impulses in an impulsive sound were generated by multiplying thecreated impulse and background sound six times.
The experimental sounds were played and the data was collected by using a program coded with
MATLAB. The sounds were played from a computer by using a sound card (D -audio USB Pre-Amp,Duran Audio Ltd., The Netherlands), a headphone amplifier (Brüel&Kjær ZE 0769, Denmark), andheadphones (Sennheiser HD 580, Sennheiser GmbH & Co., Germany). The SPL and the spectrum ofeach sound was individually verified using a head-and-torso simulator (Brüel&Kjær 4100, Denmark).
Table 1 – The black circles indicate the DL and Ron values included in the experiment. The experiment always

DL , dB

consisted two impulsive sounds having same Ron and DL, and another of the two spectra (S1 or S2).

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R on, dB/s
5 10 15 20 50 100 200 400 600 800
5● ● ● ● ●10● ● ● ● ●15● ● ● ●
20● ● ● ●25● ● ● ●30● ● ● ● ●40● ● ● ● ● ●Figure 1 – The unweighted SPL, LZ,eq, as a function of frequency, f, for the reference sounds, the backgroundnoise level of the experimental room (BG), and the standardized hearing threshold (HT) according tostandard ISO 389-7 (5).
2.4 The experimental procedure
The experiment was conducted in the psychophysical test room Tuuli at Turku University ofApplied Sciences, Turku, Finland. The background noise level of the room was 23 dB LAeq (Figure 1).
One participant at a time conducted the experiment by using a computer and MATLAB based softwarewith a graphical user interface.
The experiment consisted seven phases. Before entering the experimental room, the participants
read and signed the information consent form (phase 1). The participant was informed that the scope ofthe study was to examine how noise should be measured. It was informed that the sou nds will not beloud and there is no risk of hearing damage or getting frightened. The participant entered theexperimental room after signing the consent form. The participants filled an initial questionnaire(phase 2) which gathered their background information. Each participant’s hearing ability was tested(phase 3) to ensure that the hearing was normal. In the familiarization (phase 4), the participantlistened six experimental sounds. The participant did not judge the sounds in the familiarization phase.
The participant rehearsed the rating process after the familiarization (phase 5). The rehearsal included
ten sounds and the ratings were not analyzed.
The annoyance rating phase (phase 6) represents the actual experiment. The experimental sounds
were presented for the participant in one of the five predetermined pseudorandom order s. The fiveorders were decided so that the same sound was never presented at same point in different orders. The
reference sounds were distributed among the impulsive sounds so that they were never consecutive.
The annoyance was measured according to the technical specification ISO/TS 15666 (6). Thequestion presented for the participant was: “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” and 0 as “Notat all”. The participants were instructed to use the full scale and try to make their answers as consistent
as possible. The participants had to listen the sound for 18.5 seconds before they were able to give therating. The same sound continued until the participant had responded.
After the experiment (phase 7) the participants received a gift token and a short introduction of thegoals and impacts of the conducted experiment. The participants stayed in the laboratory on average 65
min.
The ethical board of Finnish Institute of Occupational Health has accepted the research planconcerning this experiment (ETR 6/2015 5.11.2015).

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2.5 Analysis
The annoyance penalty caused by impulsivity was determined by using the method developed by
Oliva et al. (7). The penalty of an impulsive sound is determined by using the annoyance ratings of thereference sounds. The penalty value k of an impulsive sound (LAeq = 55 dB) is the number of decibels
that should be added to the SPL of the reference sound (LAeq = 55 dB), so that the reference soundwould be perceived equally annoying as the impulsive sound.
The penalty calculation is executed in five steps:
1. The mean annoyance ̅𝑦̅̅r𝑖̅ for every reference sound is calculated from all ratings given for thereference sound.
2. The mean annoyance 𝑦̅ for every impulsive sound is calculated from all ratings given for theimpulsive sound.
3. Annoyance A is defined as a first order linear fitting to ̅𝑦̅̅r𝑖̅ and LAeq of reference sounds:(1)
𝐴 = 𝑎 ∙ 𝑥 + 𝑏,
where x is the A-weighted SPL, and a and b are the coefficients of the fitted line.
4. The A-weighted SPL corresponding 𝑦̅ (the apparent SPL) is determined using the coefficientsfrom the fitted line:
𝐿Aeq ′ = (𝑦̅ − 𝑏)/𝑎.
(2)
5. The penalty k is achieved by reducing the actual LAeq of the impulsive sound from the apparent
A-weighted SPL:
𝑘 = 𝐿Aeq ′ − 55 dB.
(3)
NT ACOU 112 (1) defines the penalty by using predicted prominence P(4)
𝑃 = 3 ∙ lg(𝑅on ) + 2 ∙ lg(𝐷𝐿 ).
The penalty is calculated by using the formula:(5)𝑘𝑁 = 1.8 ∙ (𝑃 − 5),
for 𝑃 > 5.

3. RESULTS
Figure 2 shows the penalty values k calculated according to Section 2.5.

Figure 2 – The observed and the predicted penalty values, k, for the impulsive sounds calculated according tosection 2.5. Horizontal red line indicates the penalty of 0 dB.

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4. DISCUSSION
4.1 Findings
The calculated penalty values increased with increasing DL and Ron . The penalty was depended on
spectrum for some impulsive sounds. For some sounds, NT ACOU 112 penalty calculation model
suggests greater penalties than we achieved. The penalty predicted by NT ACOU 112 usually
overestimated the observed penalty when Ron ≥ 200 dB/s for both studied spectra. Therefore, the future
development of NT ACOU 112 penalty calculation model might be justified.
4.2 Comparison with preceding studies
An impulse can be described by using multiple different measures. Previous studies of annoyanceof impulsive noise have used different measures for impulses, making the comparison of our results toprevious ones complicated.
So far, NT ACOU 112 (1) has acquired very little research interest although many countries havestated a penalty to be applied for impulsive sounds. As far as we know, our experiment is the firstexamining the effect of DL and Ron on annoyance after reference (4) which has probably afffected theNT ACOU 112 penalty calculation scheme.
As can be observed from Figure 2, NT ACOU 112 (1) suggested penalty for our sounds for
Ron ≥ 20 dB/s and DL > 10 dB. The NT ACOU 112 limit is in good accordance with our results forimpulsive sounds having spectrum S1. However, with spectrum S2 penalties were achieved with lower
DL and Ron than NT ACOU 112 suggests. This finding suggests that the penalty estimation model could
take spectrum into account as well, not only DL and Ron. The importance of spectrum on annoyance has
been found in an earlier study (8). It seems justified that annoyance caused by a sound could depend on
several specific features of sound.
4.3 Limitations
It is challenging to determine an ultimate penalty value for impulsive sound in general, because
impulsivity is not a stationary phenomenon. Our experiment was a laboratory study with homogenous
synthetic sounds. A future experiment would be useful to investigate penalties of real sounds. It isinteresting if penalty should be determined by using more measures than DL and Ron .

5. CONCLUSIONS
An experiment was conducted to examine the penalty of impulsive sounds having different DL, Ron,and spectra. The impulsive sounds were synthetic with varying DL and Ron and two different spectra
were also investigated. The penalties increased with increasing DL and Ron . The penalties weresometimes different for different spectra. The penalty predicted by Nordtest method usuallyoverestimated the observed penalty when Ron ≥ 200 dB/s. Therefore, the future development of
Nordtest penalty calculation scheme might be justified. Our results can be used to develop the futurepenalty schemes of impulsive sounds. Further research is also welcome since the annoyance penalty of
impulsive sounds has been very little investigated. Especially real impulsive sounds could be studied.

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 Finlandand collaborating companies and associations. We want to thank the funders for making this study
possible. We express special credit to Petra Virjonen for advice and MATLAB-codes that were helpful
in many phases of the study.

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