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

Construction of IIR filter for adaptive noise control system of boat
noise reduction
Haruki FUKATSU1; Kenji MUTO2
Shibaura Institute of Technology, JapanABSTRACT
Noise from boats is often loud and can reach apartments or the offices located near Tokyo’s canals. The aimof this research is to create quiet spaces in buildings using a sound damping system with adaptive noisecontrol. This system does not use a reference microphone because there is sometimes no place to put it outsidea window. We propose a tap length for an infinite impulse response (IIR) filter as an adaptive noise control
system for reducing boat noise. In this paper, we analyzed the characteristics of boat noise as well as theadaptive damping filter.We investigated the step size and the tap numbers of the IIR filter to improve theperformance of the adaptive noise control system with a digital signal processor. The results showed that boat
noise is composed of a peak frequency and some other dominant frequencies caused by the boat engine which
produces long continuous noise. We therefore designed the IIR filter to adapt to these frequencies. As a result,the system effectively reduced boat noise in a computer simulation.
Keywords: Adaptive noise control, Boat noise, Digital signal processor, Noise investigation

1. INTRODUCTION
The sound of the boats is perceived as annoying to the occupants of buildings located near a canal[1]. Many boats travel along canals from morning to night and may include tugboats, pleasure boats,fishing boats, houseboats, and others. The cause of boat noise is the boat’s engine, which emits a low
frequency sound. It would be natural to assume that reducing boat noise will improve the quality oflife for people living and working near canals.
We speculated that adaptive noise control would be effective for reducing boat noise because of
the low-frequency sound emitted by boat engines. Commercially available noise canceling earphones
and headphones use such adaptive noise control. These devices are capable of providing planar noisecontrol that covers some limited spaces; a few products can even cover an entire room. Shimitsu et al.
performed adaptive noise control simulations of noise in a ship’s cabin. A finite impulse response(FIR) adaptive noise control simulation was conducted to determine the usefulness of the adaptivenoise control system [2]. However, such a system has not been tested in a real-world situation. Inaddition, the system was limited to controlling noise inside the cabin but not sound emitted from theship toward the surrounding environment. Murao et al. studied spatial control of noise using active
acoustic shielding, which consists of a small noise-controlling speaker that uses wave interference [3].
The system is incorporated into a window of the room and can be used as a window vent, but its
presence blocks the view out of the window.
In this study, we propose a tap length suitable for an infinite impulse response (IIR) filter using as
an adaptive noise control system for reducing boat noise. This system does not use a reference
microphone because there is no place to put it. We consider IIR system to perform more effectively
than the FIR system because of the low-frequency sound emitted by boats. In this report, in order toconfirm the effectiveness of the adaptive noise system, we examined the characteristics of boat noiseand conducted a computer simulation that showed the filter length of the IIR adaptive filter is effective
in reducing boat noise inside a room.

1ma19071@shibaura-it.ac.jp

2k-muto@shibaura-it.ac.jp4334

2. IIR ADAPTIVE FILTER COMPUTER SIMULATION
2.1
METHODWe used the IIR adaptive noise control system to reduce boat noise. In order to examine IIR
adaptive noise control by computer simulation, the effectiveness of individual tap lengths to reduceboat noise was evaluated on a trial-and-error basis. IIR adaptive noise control is used to conduct
experiments in places where a reference microphone cannot be used. The of noise reduction level
depends on the accuracy of the calculation. The tap length is the number of filter coefficients. Bychanging the tap length, the accuracy and speed of the operation can also be changed. The noise
reduction level according to tap length, frequency, and type of boat were found in the frequency andtime domains. The target value of the noise reduction level was set to 75 dB or less in the dominantfrequency. The sound insulation of windows used in houses was determined based on the JIS standard.
There is a 10-dB reduction at the frequency of boat noise penetrating windows with the lowest soundinsulation. The maxium achievable noise reduction value is considered to be about 65 dB, which
includes the effect of the window as a damping material.
The IIR filter used a least mean squares algorithm [4]. The filter update equation for the IIR filter
coefficient U(n)=[ܷଵ ሺ݊ሻǡ ܷଶሺ݊ሻǡ ‫ ڮ‬ǡ ܷ୐ ሺሻ] is‫܃‬ሺ ൅ ͳሻ ൌ ‫܃‬ሺሻ ൅ ɋሺሻ‫ܡ‬ሺሻ,where time is n, the tap length of the IIR filter of adaptive filter is L, and the step size is ν. The errorsignal is e(n) and the output signal of the IIR filter is y(n)= [‫ݕ‬ଵ ሺ݊ሻǡ ‫ݕ‬ଶ ሺ݊ሻǡ ‫ ڮ‬ǡ ‫ ୐ݕ‬ሺሻ]
ሺሻ ൌ ሺሻ െ ሺሻ,‫ܡ‬ሺ ൅ ͳሻ ൌ ‫܃‬ሺሻ‫ ் ܡ‬ሺ݊ሻ,
where the desired signal was d(n).
2.2 SIMULATION CONDITIONS
A tap length suitable for boat noise was proposed for the IIR adaptive noise control system. To
determine the most effective tap length, the level of noise reduction according to tap length wasanalyzed using tugboat noise at the highest sound pressure level. Step size ν was 1.0 × 10 −13, sampling
frequency was 48 kHz, and the tap lengths of the feedback section were 2, 4, 6, 8, 10, 12, 14, 16, 20,
24, 30, and 36. We examined the difference in noise reduction level for each tap length. The frequency
response was analyzed according to the tap length.
Next, computer simulations of IIR adaptive noise control were performed for tugboats, pleasureboats, houseboat, and fishing boats using the optimal tap lengths determined from the examination.
Step size ν was 1.0 × 10 −12 and sampling frequency was 48 kHz. For each boat, the level of noise
reduction in the frequency and time domains was measured. The cruising boat was excluded because
its peak sound pressure level was less than 70 dB.
2.3 BOAT NOISE USED FOR EXPERIMENTS
The boat noises used in the simulations were audio of boat passes recorded near a canal made on
March 16, 2015 from the veranda of a room on the 10th floor of anapartment building. Recordings
were made of tugboats, pleasure boats, houseboats, fishing boats, and cruising boats. Boat noise hashigh signal strength in the lower frequency range due to the periodic nature of the sound generated
from the boat engine. Figure 1 shows the frequency characteristics of tugboats, pleasure boats, andhouseboats. Figure 2 shows the frequency characteristics of fishing boats and cruising boats.

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Figure 1. Frequency characteristics oftugboats, pleasure boats, and houseboats.

Figure 2. Frequency characteristics offishing boats and cruising boats

3. RESULTS
To obtain an effective tap length for IIR adaptive noise control by computer simulation, tap lengths
were examined. Figure 3 shows the noise reduction level for each tap length of the IIR adaptive filter.
For tap lengths of 2 and 36, there was a noise reduction of 8 and 14 dB, respectively. The noise
reduction level increased according to increase in tap length. Figure 4 shows the frequency analysis
results for each tap length of the IIR adaptive filter. The signal before noise control was activated isthe original sound pressure level, and the signal after noise control was activated shows the signal
corresponding to tap lengths of 20, 24, 30, and 36. There were differences in the corresponding
dominant frequencies due to differences in tap length. From these results, a tap length of 30wasconsidered to be appropriate for boat noise because when the sound of the tugboat used in the research
was 85 dB, a tap length of 30 was well below the target value of 65 dB or less. We attempted tominimize both the tap length and computational noise level. We applied a tap length of 30 andobserved the noise reduction level for each boat. Figures 5–12 show frequency characteristics andnoise reduction levels before and after activation of adaptive noise control for each type of boat. Thesystem was able to reduce the noise of tugboats, pleasure boats, houseboats,andfishing boats by 14,16, 18, and 16 dB respectively. The adaptive noise control signal converged in 0.1 s. As a result, thereduction target value of 70 dB was reached for each type of boat.

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Figure 3. Noise reduction level for tap lengths
2–36

Figure 4. Frequency characteristics for each tap lengths 20–36

Figure 5. Frequency characteristics ofFigure 6. Noise reduction levels before

tugboat noise

and after adaptive noise control fortugboat noise

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Figure 7. Frequency characteristics ofFigure 8. Noise reduction levels before

pleasure boat noise

and after adaptive noise control forpleasure boat noise

Figure 9. Frequency characteristics ofFigure 10. Noise reduction before andhouseboat noise

after adaptive noise control for houseboat
noise

Figure 11. Frequency characteristics ofFigure 12. Noise reduction level before

fishing boat noise

and after adaptive noise control for fishingboat noise

4. Conclusion
Computer simulations were performed to check whether the IIR adaptive filter could adapt to boat
noise. Frequency analyses were performed to reduce the boat noise. In addition, the performanceaccording to tap length of the IIR filter was confirmed to correspond to the dominant frequency of theboat noise, meaning that the boat noise was a sound emitted at a low frequency with a small numberof dominant frequencies. The noise reduction level increased as the tap length increased. We4338

considered that a tap length of 30 was effective for reducing boat noise. According to the results ofcomputer simulation, the noise four types of boat could be reduced to the target value of 75 dB or less.
Based on these results, we can conclude that IIR adaptive noise control is effective for reducing boatnoise.

REFERENCES
1. K. Muto, T. Akahira, A result of 24-hours measurement of small boat noise in residential area along
canal,12th Western Pacific Acoustics Conference 2015, Singapore,Proc. of WESPAC 2015, P8000017,
pp.221-224, 7 December, 2015.
2. S. Ishimitu, M. Yoshimura, Study on Active Control System of Ship Interior Noise, Jounal of the JIME,Vol.37, No.9, 2002, pp67-74.
3. T.Murao, M.Nishimura, K.Sakurama, S.Nishida “Basic Study on Active Acoustic Shielding: phase 6
improving the method to enlarge AAS window-2”, Proc. of INTER-NOISE 2014
4. B. Widrow㸪S. D.Stearns㸪Active signal processing㸪Prentice-Hall㸪1985.

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