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Comparative study of cone-shaped versus flat-panel speakers for active noise control of multi-tonal signals in open windows


Abstract and Figures

Openings on walls, such as windows or doors, facilitate the propagation of external noises to interior spaces. Active noise control is a promising technique for the reduction of low frequency noise propagating through such openings. To preserve the full functionality of the opening, especially for natural ventilation, it is required to minimize the physical size of the secondary sources. In this scenario, using compact speakers and limiting their distances from the reference microphones are critical requirements. In this paper, we present a comparative study on the noise reduction performance of an active noise control system using compact cone-shaped and flat-panel speakers in identical configurations. The performance was measured based on multi-tonal noise with frequency components below 1.2 kHz, using different setups of single reference sensor, single error sensor, and up to four secondary sources. In evaluating the measurement data, including frequency responses, harmonic distortion, radiation patterns, and noise reduction, we highlight key differences of these two types of speakers and determine their respective application contexts.
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Comparative study of cone-shaped versus flat-panel speakers for
active noise control of multi-tonal signals in open windows
Stefano Fasciani
Jianjun He
Lam Bhan
Tatsuya Murao
Woon Seng Gan
Digital Signal Processing Laboratory
School of Electrical and Electronic Engineering, Nanyang Technological University
S2-B4a-03, 50 Nanyang Avenue, 639798 Singapore
Openings on walls, such as windows or doors, facilitate the propagation of external noises to
interior spaces. Active noise control is a promising technique for the reduction of low
frequency noise propagating through such openings. To preserve the full functionality of
the opening, especially for natural ventilation, it is required to minimize the physical size of
the secondary sources. In this scenario, using compact speakers and limiting their distances
from the reference microphones are critical requirements. In this paper, we present a
comparative study on the noise reduction performance of an active noise control system
using compact cone-shaped and flat-panel speakers in identical configurations. The
performance was measured based on multi-tonal noise with frequency components below
1.2 kHz, using different setups of single reference sensor, single error sensor, and up to four
secondary sources. In evaluating the measurement data, including frequency responses,
harmonic distortion, radiation patterns, and noise reduction, we highlight key differences of
these two types of speakers and determine their respective application contexts.
With increasing knowledge and awareness of health risks associated with environmental
noise exposure, effective noise abatement is vital to safeguard public health. Based on a
WHO/European publication, noise pollution is ranked second only to air pollution as an
environmental stressor that adversely affects human health
. It has been estimated that almost a
quarter of the European Union population is daily exposed to potentially harmful noise levels
mostly due to traffic and concentrated in urban areas. The de facto noise mitigation methods are
passive structures that scatter and/or dampen acoustic waves (e.g., acoustic barriers). However,
passive techniques are ineffective at shielding high-rise urban dwellings and are not cost-effective
for land-scarce cities
Active means of noise control are primarily effective at low frequencies, typical of
transportation noise
, where passive methods are ineffective
. Active Noise Control (ANC)
relies on electro-mechanical components, such as loudspeakers, to generate an anti-noise wave
that reduces the “primary” noise, based on the superposition principle. ANC technology has seen
the most commercial success in ANC headphones, and it has also been applied in automotive,
home appliances, industrial plants and transportation
. However, in these examples, the
deployment was often limited to air ducts and exhaust pipes. Recent advances of ANC include
applications in motorcycle helmets, medical Magnetic Resonance Imaging (MRI) machines,
infant incubators, and noise control with high-directional loudspeakers
By leveraging on conditions that ANC has been known to be most effective at, the problem
of noise pollution control can be narrowed down to abating noise through wall openings. This has
been focused on windows that can be classified into two categories: closed-window ANC, and
open-window ANC. Closed-window ANC targets window fixtures and aims to reduce low
frequency sounds that double- or triple- glazed windows are ineffective at blocking
. Open-
window ANC systems allow windows to retain their full functionality, an important feature for
homes. Recent advancements of open-window systems have yielded several variations with
varying degree of noise reduction capabilities up to 10 dB
in some cases. To enlarge the
noise reduction area, multichannel ANC systems with multiple reference microphones, secondary
sources and error microphones have been proposed
. To maintain the original functionality of
domestic windows, it is essential to minimize the obstruction caused by the ANC system.
Selecting small speakers as secondary (anti-noise) sources, and reducing their distance with
the reference microphones are critical design constraints. On this note, compact flat-panel
speakers have been proposed as effective secondary sources for narrowband noise
, or as
active structures limiting the noise wave propagation
. However, the characteristic differences
between flat-panel and cone-shaped speakers are not comprehensive when these are used as
secondary sources in ANC. In this paper, we compare the performance of two specific models of
cone-shaped and flat-panel speakers, both presenting small size and thus suitable for open-
window ANC systems. The rest of the paper is organized as follows. Section 2 includes the
technical features of the speakers and characterization measurement results. Section 3 details our
ANC test bench and the experimental settings. Results and conclusions are discussed respectively
in Section 4 and Section 5.
The two types of speakers we compare in this study are the “FRS 5”, a traditional cone-
shaped speaker, and the “Heyban”, a flat-panel speaker, manufactured respectively by Visaton
and Protro. A summary of the technical specifications is presented in Table 1. Both speakers
present compact dimensions relative to the typical size of domestic windows, and therefore result
in a minimal obstruction. The electromechanical transduction characteristics of the speakers, as
well as their geometries, present evident differences. The cone speaker can be modelled with a
cylinder shape, while its flat counterpart can be represented with a rectangular parallelepiped. The
first would determine a smaller obstruction in terms of surface area, while the second minimizes
the volume occupancy, featuring an extremely thin structure.
Both speakers have a power rating of 5 W, and the impedances are 8 Ohm and 6 Ohm,
respectively. The mean Sound Pressure Level (SPL) is about 6 dB higher for the cone speaker,
which is also evident in the respective frequency response plots of Fig. 1 (left). According to the
specifications from the manufacturer, the flat speaker has a wider frequency range, especially in
the low frequency range. To better characterize the response in the low- and mid-frequency
range, which is critical for ANC applications, we measured the SPL and the Total Harmonic
Distortion (THD) of tones in the band of 100 Hz to 2 kHz at a uniform frequency step of 100 Hz.
The THD measurement covers up to the 10
harmonic. The gains of the two independent
amplifiers were calibrated to drive a white noise signal with SPL of 100 dBA at 10 cm on the
frontal central axis. The results, illustrated in Fig. 1 (right), show a lower THD for the cone
speaker, especially when near the boundaries of the 100 Hz to 2 kHz band. The flat speaker
presents particularly high THD below 400 Hz with readings above -30 dBC. With concurrent low
SPL and high THD, at 100 Hz and 200 Hz we expect poor noise reduction when using the “FRS
5” or the “Heyban” as secondary sources. This was verified in preliminary tests, and thus we
considered only tonal noise starting from 300 Hz in the experiments detailed in Section 3.
Table 1 – Speakers technical specifications summary.
Rated Power
5 W
6 Ohm
150-20k Hz
50-20k Hz
Mean SPL (1 W/1 m)
84 dB
78 dB
Harmonic Distortion
1% @ 1 kHz / 1 W
Emission Angle
180° @ 4 kHz
126 g
85 g
5(d) x 3.1(h) cm
5.5(w) x 9.1(l) x 0.7(h) cm
Frontal Surface
19.6 cm
50 cm
60.8 cm
35 cm
Back propagation and radiation patterns are among the characteristics of the speakers that
need to be considered for ANC applications. These specifications are only partially provided by
manufacturers and hence we proceeded with an additional set of measurements. Excessive back
propagation may incur a feedback from the secondary source speaker to the reference
microphone, especially when noise and anti-noise SPL are high, which interferes with the noise
cancelling algorithm. In our context, feedback interference risk is high as the speaker-to-
reference-microphone distance is limited in a compact ANC setup. The back propagation to the
reference microphone was estimated at various distances on the central propagation axis of the
speaker, in the reverse direction as shown in Fig. 2 (right). The back propagation was measured
with the speakers calibrated for generating a SPL of 90 dBA at 6.5 cm in the direction of forward
propagation on the central axis. The results illustrated in Fig. 2 (left) show that the back
propagation of the cone speaker is on average 4.8 dBA lower than that of the flat speaker. This
gap is consistent across the measured ranges from 10 cm to 80 cm. In ANC implementations, the
drawbacks of the feedback interference captured by the reference microphone can be mitigated by
inserting sound absorbing material or using a digital cancelling filter
. However, selecting the
secondary sources with a small back lobe in the radiation pattern could improve the stability and
robustness of ANC systems.
Fig. 1 – Frequency responses (left), single tone SPL and THD (right) for cone and flat speakers.
Fig. 2 – Back propagation to reference microphone. SPL (left) and measurement setup (right).
The frontal radiation pattern was measured with the speakers placed in the same structure
used in ANC experiments, which is described and illustrated in Section 3.1. The amplifier was
calibrated to drive the speakers with 90 dBA white noise at 10 cm distance on the central axis.
We took 32 SPL readings at a distance of 11 cm from the speaker, with the measurement
microphones uniformly distributed in a 4×8 matrix with a length of 52.5 cm and width of 24 cm.
The same measurement was repeated by doubling the distance to 22 cm. Results are illustrated in
the heat-maps of Fig. 3. With respect to the heat-maps, speakers were located below the
coordinates CNTR-m3 (minor offset toward m4). The differences between the two directivity
patterns are evident. The cone speaker presents a wider emission angle than the flat speaker. On
average, the cone’s SPL readings are higher at every location of 3.5 dBA at 11cm and 2.6 dBA at
22 cm. The flat speaker propagates a slightly higher SPL than the cone-shaped only on the central
axis (0.4 dBA at 11cm and 0.1 dBA at 22 cm). The differences between the maximum and
minimum SPL for the low and high measurement plane are respectively 13.4 dBA and 6.8 dBA
for the “FRS 5”, while for the “Heyban” we measured 15.2 dBA and 9.5 dBA. In Fig. 3, some
reflections determined by the ANC experimental setup are visible at positions m1. For ANC, a
wider directivity pattern has the potential to reduce the noise in a larger region, but it can also
lead to a constructive anti-noise wave, resulting in SPL increase. This depends on the specific
ANC configuration, algorithm, number and positioning of secondary sources and error
microphones. Therefore, the selection of wide or narrow radiation pattern is strictly context
specific. Our observations on back propagation and directivity pattern for the “Heyban” speaker
confirm the “sharp directivity” and “sound on both sides” features claimed by the manufacturer.
Fig. 3 SPL heat-maps approximating the frontal radiation pattern of the cone (left) and flat
(right) speakers at a distance of 11 cm (top) and 22 cm (bottom). The 32 measurement
microphones are uniformly distributed into a 4×8 matrix with size 24 cm by 52.5 cm.
The speakers are located below CNTR-m3.
Finally, we estimated the difference in response times between the two speakers. A fast
response time reduces the delay in generating the anti-noise from the moment the noise is
captured at the reference microphone. This in turn allows shorter microphone-to-speaker distance
given identical causality constraints, contributing to the compacting of the ANC setup. For this
characterization, we drove the cone and flat speakers simultaneously with an impulse while
recording the emitted sound with two independent microphones placed right in front of them. We
used a sampling rate equal to 96 kHz and linear interpolation was used to detect the inter-sample
peak position. The flat speaker was faster than the cone of about 27.5 μs, which is equivalent to 9
mm traveling distance for sound waves. The results were consistent across repeated
measurements, different microphone positions, impulse amplitudes, and test bench swap.
3.1 Configuration
To roughly approximate a scenario in which the noise wave propagates into the interior
space through a window, we built a quasi-cubic chamber with the top face open, where the ANC
system was installed. The chamber is made of plywood and the inner surfaces are covered with
acoustic absorbent material, which mitigates reflections and resonant modes. The external noise
source is emulated with a 7 inch cone speaker installed in a wooden case and placed at the bottom
of the chamber. On the open face, we installed an array of cone speakers and an array of flat
speakers. These are mounted on plastic supports, symmetrical with respect to the center of the
chamber. The supports do not obstruct the back of the speakers so that we could evaluate the
effect determined by the secondary source back propagation.
In our ANC experiments, we used a configuration that includes 1 reference microphone, 4
secondary sources, and 1 error microphone (1×4×1). The speakers are distributed uniformly over
two arrays with a length of 40 cm. The reference and error microphones are respectively
positioned 20 cm below and 10 cm above the central axis of the speakers array. The primary
source speaker is also located along the same vertical axis. The geometry of the chamber, the
positions of speakers, and the positions microphones are illustrated in Fig. 4, in planar top and
side views, and a 3D model. Pictures of the described setup are shown in Fig. 5. In preliminary
ANC experiments, detailed in Section 3.3, we used a 1×1×1 configuration with the primary
source, error and reference microphones located on the vertical axis of the single active speaker.
Fig. 4 – From left to right: planar top view, planar side view, and 3D model of the chamber, with
details on the location of the primary and secondary speakers, reference and error
microphones, for the cone speaker setup (top) and flat speaker setup (bottom). Length is
expressed in millimeters.
Fig. 5 Experimental setup with cone speakers, flat speakers, primary speaker, reference and
error microphones installed in the chamber for emulating an open window ANC setup.
The array of measuring microphones is visible in the right image.
Primary and secondary speakers were driven with a Dayton Audio MA1240a multichannel
class AB amplifier. The gain of each independent channel was manually calibrated for each
speaker. For the reference and error microphones, we used the G.R.A.S. 40PH high-sensitivity
condenser microphone. Experiments and measurements were conducted in a room with acoustic
absorbent material on walls, floor and the ceiling (partially). The ANC controller, as well as the
operator, was outside the measurement room.
3.2 Implementation platform and algorithm
The ANC processing was implemented on a National Instruments PXIe platform. The
microphone signals, including those used for SPL measurement, were acquired with the
multichannel data acquisition module PXIe-4499, equipped with high-precision Analog-to-
Digital Converters (ADC). The speakers were driven, through the MA1240a amplifier, with the
multichannel output module PXI-6733. The ANC algorithm was implemented in LabVIEW
running on the PXIe-8135 embedded controller. We worked with a sampling rate of 8 kHz,
sufficient for addressing noise in the low-mid frequency band considered in these experiments,
and a buffer size of 128 samples.
For this comparative study, we implemented the Filtered-X Least Mean Square (FxLMS)
with a feed-forward ANC structure. The length of the adaptive filters was set to 100
taps and the fixed step size was set to 5×10
for single tone noise cases, while we used
respectively 1000 taps and 1×10
step size for the multi-tonal noise cases. The length of the
secondary path filters is 100 taps and the coefficients were estimated offline using the least-mean-
square (LMS) algorithm.
The ANC causality constraints are determined by the distance between reference
microphone and secondary sources. In our compact setup, this distance is limited and the
resulting constraints are tight. These cannot be satisfied in our implementation based on a general
purpose operating system. The minimum input-output latency is high due to the high-quality
ADC and also the buffering that allows real-time computation. In this study, however, we are
addressing only periodic noises with invariant phase and amplitude sinusoidal components. In
this scenario, the feed-forward ANC structure can reduce the SPL without causality. The excess
of latency between reference microphone and secondary speaker can be modelled with a phase
shift of the periodic noise signal, which is compensated in the adaptive filter through the error
microphone feedback signal.
3.3 Test cases and measurement setup
To compare the noise reduction performance of cone and flat speakers, we measured the
SPL reduction over a large set of test cases. In the preliminary test, we used a 1×1×1 ANC
configuration with the following noise signals: single tone at 300 Hz, 600 Hz, 900 Hz, and 1200
Hz; dual tone at (300, 500) Hz, (800, 1100) Hz, and (500, 1200) Hz. For the 1×4×1
configurations, in addition to the test cases listed above, we also considered the following multi-
tone noise signals: (300, 600, 900) Hz, and (300, 600, 900, 1200) Hz.
For each test case, SPL readings with fast time weighting (equivalent to 125 ms intervals)
were taken with the ANC system enabled and disabled. In the results, these are averaged over 10
separate readings. We used an array composed of 8 40PH microphones, placed in 8 different
positions. A total of 64 measurement points were distributed on two parallel planes, at the vertical
distance of 11 cm (labelled “LOW”) and 22 cm (labelled “HIGH”) from the secondary source.
Each plane has 32 SPL reading points uniformly distributed in a 4×8 matrix with a length of 52.5
cm and width of 24 cm. The displacement of the measurement points is illustrated and labelled in
Fig. 5. For each measurement distance, the microphone array was positioned on the top and
parallel with the central axis of the secondary source array (labelled “CNTR”), shifted 8 cm
towards the outer of the chamber (labelled “OUT8”), shifted 8 cm and 16 cm towards the center
of the camber (labelled “IN8” and “IN16”). For the 1×1×1 experiments, the flat and cone
speakers enabled are located right below the measurement location CNTR-m3.
The ANC implementation described in this section, including the chamber emulating the
open window and the ANC algorithm, has room for improvement be improved. This work aims
to compare the performance of different compact speakers in a specific ANC application.
Therefore, we are focused on the noise reduction patterns, rather than on the absolute noise
reduction capability. Our setup can thus be considered as a solid test bench for evaluating cone
and flat speakers in identical operating conditions.
Fig. 6 From left to right: planar top view, planar side view, 3D model of the 64 measurement
points for the cone speaker setup (top) and flat speaker setup (bottom). Length is
expressed in millimeters.
The results of the test cases are presented in Table 2 separately for the cone-shaped and flat-
panel speakers, including average, maximum, and standard deviation of the SPL reduction for
each measurement plane. Results are grouped per ANC configuration and noise type. The heat-
maps of Fig. 7 and Fig. 8 illustrate the SPL reduction patterns over the 64 measurement points.
As expected, the highest SPL reduction values are always achieved in the region
surrounding the error microphone, which is located approximately at CNTR-m3 for 1×1×1 cases,
and between CNTR-m4 and CNTR-m5 for 1×4×1 cases. For the single secondary source with
single tone, we observed that the maximum SPL reduction along the vertical axis of the speaker is
3 to 4 dBA higher with the cone speaker. Moreover, the cone speaker yields an overall better
noise reduction on the higher measurement plane. A roughly similar pattern can be found with
dual tone noise. The SPL reduction is lower (about 1.5 dBA) and presents a more scattered
pattern with the single flat speaker, while a more uniform and better reduction of the noise SPL is
evident with the cone speaker. In this configuration, the wider directivity pattern of the cone
speaker has shown its advantage.
When using four speakers as secondary sources, the noise reduction obtained with cone and
flat speakers are more similar and uniform across the different noise cases used in our
experiments. At the lower measurement plane, the cone speakers provide a slightly better SPL
reduction but higher deviation, indicating a less regular spatial noise reduction. The scenario is
inverted at the higher measurement plane. The setup with flat speakers resulted in a slightly better
noise reduction as well as deviation. Increasing the number of the secondary speakers from 1 to a
line array of 4, higher SPL reduction (values above 8 dBA) is obtained in an extended spatial
region, along and across the central axis of the speaker array. Although the speakers are placed at
close distance in the ANC array, the wider directivity pattern of the cone speakers did not incur
any interference detrimental for the SPL reduction. In this configuration, the directivity pattern
differences between cone and flat speakers have negligible impact.
The noise reduction results obtained at 300 Hz show that the cone speaker outperforms the
flat speaker of an average 3 dBA and 2 dBA respectively in the 1×1×1 and 1×4×1 configuration.
This difference is compliant with the low SPL and high THD of the flat speakers at low
frequencies, as reported in Fig. 1. Therefore, the cone shaped setup is more suitable in ANC
systems that abate noises with lower frequency components.
In the various test cases and experimental settings, we used identical and fixed values for the
amplitude of tonal noise and for the gain of the amplifier driving the primary speaker. The noise
SPL was close to the limit after which the back propagation of the secondary speakers introduces
a fatal feedback to the reference microphone. This limit differs for the two speaker models. The
setup using the cone-shaped speakers, which present a lower back propagation, can handle tonal
noise up to 4 dB higher than the flat speakers. Finally, during the experiments, we noticed that the
setup with flat speakers usually converges faster. This observation was consistent across all test
cases, but not verified with any systematic measurements.
Table 2 Summary of the SPL reduction results for all test cases, with average, maximum, and
standard deviation for each case on the LOW and HIGH measurement planes.
1×1×1 1×4×1
Single Dual Single Dual Multi
Cone Flat Cone Flat Cone Flat Cone Flat Cone Flat
average 3.78 3.95 3.85 2.66 3.99 3.98 4.26 4.37 2.56 2.03
maximum 11.23 8.21 8.25 9.81 10.66 10.31 11.63 10.50 12.65 10.26
deviation 2.53 2.41 2.06 2.08 2.54 2.83 2.64 2.55 3.07 3.34
average 4.72 3.20 5.06 3.53 4.08 4.65 5.10 4.83 3.48 3.24
maximum 10.27 6.99 7.70 6.33 8.36 10.12 9.03 10.06 7.48 8.43
deviation 2.06 1.45 1.59 1.59 1.93 2.45 2.04 2.39 2.31 2.62
Fig. 7 SPL reduction heat-maps for cone speaker (left) and flat speaker (right) averaged over
the single tone cases (top), and dual tone cases (bottom) for the 1×1×1 ANC
Fig. 8 – SPL reduction heat-maps for cone speakers (left) and flat speakers (right) averaged over
the single tone cases (top), dual tone cases (middle), and multi-tone cases (bottom) for
the 1×4×1 ANC configuration.
We presented experimental settings and discussed the results of a comparative study on the
noise reduction performance of an ANC system based on compact cone-shaped and flat-panel
speakers. In identical configurations, we found that the cone speaker performs better in reducing
the noise at low frequencies, despite its slightly worse nominal low-frequency response as
compared to the flat speaker. The cone-shaped speaker outperforms the flat-panel type also in
single secondary source experiments, providing noise reduction in a considerable larger and
more uniform region. Measurements with ANC setup including the array of four closely spaced
secondary sources showed almost identical performance between the two types of speakers.
Therefore, the use of the cone speaker may be advantageous in ANC setup with larger spacing
between secondary sources, as for installations on larger windows and wall openings. The
shorter response time of the flat speakers allows realizing smaller ANC units, though a more
effective acoustic absorbent material is required to isolate the reference microphone from the
higher back propagation. In future works, we will use the same setup to investigate noise
reduction performance with multiple reference and error microphones, and also extend the study
to narrowband noise.
This material is based on research/work supported by the Singapore Ministry of National
Development and National Research Foundation under L2 NIC Award No. L2NICCFP1-2013-7.
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... To enlarge the noise reduction area, multiple-channel ANC (MCANC) systems with multiple reference microphones, secondary sources and error microphones are required (8,18). To maintain the original functionality of domestic windows, it is essential to minimize the obstruction caused by the ANC system (19). In These previous studies on open window ANC reveal that given a typical size of a window, the number of ANC units increases dramatically, which complicates and increases the space and cost on the controller design, deployment, and maintenance. ...
... A low-passed white noise (cutoff at 2205 Hz at fs = 44100 Hz, variance set as 1) is used as the primary noise, where the first 4 seconds are used for adaption with step size set as μ = 0.001, and the fifth second is used for evaluation of noise reduction using equation (8). (7) Two simulations conducted include simple impulse responses with single delay and amplitude determined by the distance, and actual impulse response measured from a real setup (19). The NR results using simple impulse responses are plotted in Figs. 6, and 7. When there is no variation (i.e., amplitude variation = 0, delay variation = 0, and position variation = (0,0) ), the NR performance among these three methods are quite close. ...
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The rapid growth of the industry has a major effect on the environmental noise pollution, and it ranks second to the air pollution that adversely affects the human health. Passive noise control techniques are impractical and very expensive for low-frequency noises. To solve such acoustic problem active noise control has been studied since early 20th century. Active noise control is based on the principle of superposition: i.e., it mitigates the unwanted noise by generating an anti-noise having the same amplitude but opposite in phase. In this paper, we present the physical classification of the existing literature on the basis of both noise source and quiet zone characteristics. Examples are point to point, point to zone, zone to point and zone to zone. We focus on the developing trends of active noise control in the last decade and discuss latest add-on features and multi-channel active acoustic shielding for open windows.
... This assumption stems from the free-field case [24], where deviation in propagating modes between 2D and 3D simulations only occurred at normalised frequencies kw larger than π 2 , at which control is poor for all angles of incidence. Successful translation into a practical ANC system requires further investigation into the cost function to be minimised, the error sensor placement, the quality of the reference signal, and finally the signal processing algorithm [27,28] and hardware used [29]. ...
Active noise control through open windows is a noise mitigation technique that preserves natural ventilation in dwellings. Designing a practical open window active noise control system requires the knowledge of the physical limits on the attenuation performance. Of the numerous variables to be optimised, it is the control source configuration (quantity and position) that ultimately defines the maximum attenuation attainable by an active noise control system. The physical limits are characterised here by systematically investigating the performance of different physical arrangements of control sources, using a two-dimension simulation model based on the finite-element method, which includes the diffraction effects of the window. The simulations reveal that the best attenuation is achieved by placing the control sources away from the edges of the window. It also shows that the plane of control sources can be placed centrally with respect to the depth of the walls, for practical implementation with minimal performance degradation. The simulated attenuation as a function of frequency and window width, for different angles of noise incidence, can be used to provide an estimate of the number of control sources, based on the desired level of attenuation. This estimate helps to determine the configuration with the minimum number of control sources required for different scenarios, before a more detailed system design is undertaken.
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Spatial audio reproduction is essential to create a natural listening experience for digital media. Majority of the legacy audio contents are in channel-based format, which is very particular on the desired playback system. Considering the diversity of today’s playback systems, the quality of reproduced sound scenes degrades significantly when mismatches between the audio content and the playback system occur. Primary ambient extraction (PAE) is an emerging technique that can be employed to solve this pressing issue and achieve an efficient, flexible, and immersive spatial audio reproduction. This thesis investigates the performance of existing PAE approaches and develops new techniques that not only improve the performance, but also enhance the robustness of PAE approaches in dealing with more complex signals encountered in practice. Objective and subjective evaluations validate the feasibility of applying PAE in spatial audio reproduction systems (as also highlighted in the headphone system).
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Environmental noise (also known as noise pollution) is a prevalent feature of any urban soundscape. Of the numerous environmental noise sources (e.g., aircrafts, road traffic, railways, industries, and construction), the World Health Organization (WHO) has identified road traffic noise as one of the main contributors to urban noise pollution. Passive noise control (PNC) methods, where physical media are used to “shield” a listener from noise sources. Even though PNC methods are effective at damping noise over a large frequency range, they are less effective at the lower frequencies due to the thickness of media required. Therefore, active noise control (ANC) methods may hold the key to a practical noise mitigation solution for protecting the health of an ever-increasing urban population. ANC methods have been shown to be more spaceand cost-effective at attenuating low frequencies and are becoming increasingly realizable due to the recent development of efficient algorithms and powerful lowcost processors. Moreover, an ANC system retrofitted to open-windows may potentially attenuate low-frequency traffic noise while still allowing natural ventilation.
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Noise pollution is a growing environmental concern. It is caused by a varied number of sources and is widely present not only in the busiest urban environments, it is also pervading once natural environments. The adverse effects can be found in the well-being of exposed human populations, in the health and distribution of wildlife on the land and in the sea, in the abilities of our children to learn properly at school and in the high economic price society must pay because of noise pollution. The European soundscape is under threat and this report sets out to quantify the scale of the problem, assess what actions are being taken and to scope those that may need to be considered in the future, in order to redress the problem
Conference Paper
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Acoustic noise problems become more and more serious with increasing use of industrial and medical equipment, appliances, and consumer electronics. Active noise control (ANC) has been studied to solve such acoustic noise problems. ANC is a technique based on the principle of superposition, i.e., an antinoise with the same amplitude and opposite phase is generated and combined with an unwanted noise, thus resulting in the cancellation of both noises. However, ANC is still not widely used owing to the effectiveness of control algorithms, and to the physical and economical constraints of practical applications. In this paper, we briefly introduce some fundamental ANC structures, and focus on recent advances on signal processing algorithms, implementation techniques, challenges for innovative applications, and open issues for further research and development of ANC systems.
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The World Health Organization, supported by the European Commission’s Joint Research Centre, is issuing this technical document as guidance for national and local authorities in risk assessment and environmental health planning related to environmental noise. The principles of quantitative assessment of the burden of disease from environmental noise, the status of implementation of the European Noise Directive, and lessons from the project on Environmental Burden of Disease in the European countries (EBoDE) are summarized, together with a review of evidence on exposure response relationships between noise and cardiovascular diseases. Step-by-step guidance is presented on how to calculate the burden of cardiovascular diseases and sleep disturbance. The limitations and uncertainties of estimating disability-adjusted life years and the usefulness and limitations of noise map data are discussed.
Active acoustic shielding (AAS) is a system that can attenuate a sound passing through an open window. An AAS is constructed from a number of AAS cells set in an array having an approximately co-located microphone and speaker system. The concept of AAS was previously demonstrated and some simple simulations and experiments were performed. Moreover, an AAS window with four AAS cells was proved to be effective for not only a single stable noise source but also multiple noise sources and moving noise sources. However, the AAS window could only attenuate noise with frequencies from 500Hz to 2 kHz, and could not attenuate noise below 500 Hz, which is dominant in construction sites and daily life. Therefore, in this paper, a new AAS window consisting of two different AAS units in one window is proposed. One of the units attenuates low-frequency sound and the other attenuates high-frequency sound. These units are controlled independently. The new AAS window was fabricated and experimentally evaluated in an anechoic room. It was found that the new AAS window reduced noise by 5 to 15 dB in the frequency range from 300 Hz to 2 kHz over a wide area in the room.
Active acoustic shielding (AAS) is the system that can attenuate a sound passing through an open window. AAS is constructed from a number of AAS cells set in an array with having an approximately collocated microphone and speaker. Each AAS cell is individually controlled by a single-channel feedforward method. The concept of AAS was proposed by the authors and its feasibility was demonstrated in the previous report by performing some simple simulations and experiments. In this study, an AAS window with four AAS cells in a small open window was manufactured and installed in the door of a test room. Its noise-reducing performances were measured for noise from outside. The effects of multiple noise sources, moving noise sources and reflected sound in the room were also examined. As the results, the AAS window is demonstrated to be able to attenuate not only normal incident sound but also oblique incident sound in the frequency range from 500Hz to 1.5 or 2kHz. Moreover, noise reduction is obtained over a wide area in the room. Additionally the AAS window is also effective for multiple sound sources and moving sound sources.
The use of a transparent thin film speaker in a home window can provide both an invisible audio playback device and an active noise cancellation system. However, several key challenges need to be addressed for development of such a system. A traditional feedforward active noise cancellation system uses direct microphone measurements for both reference and error signals. Such a system can degrade both the audio playback and the noise cancellation performance. Hence a wave separation algorithm is used in this paper for separation of the audio playback component from the external noise component for the reference signal. Further, an online adaptive secondary transfer function estimation method is used for accurate removal of the audio component from the error signal. Two approaches are attempted for the secondary transfer function estimation – use of source audio signals and use of additive white noise. The developed algorithms are evaluated in a scaled cabin equipped with a window and a transparent thin film actuator. Experimental results show that the developed system can preserve the auxiliary audio sound while cancelling external noise effectively. The system based on the use of source audio signals for secondary path estimation is found to yield the largest cancellation of external noise while producing the least amplitude of control inputs that cancel desired audio signals.
An active window system to reduce the exterior noise sources, such as traffic noise and construction noise which enter rooms through open windows used for natural ventilation is proposed. The proposed system uses a feedforward control method for active noise control so as not to place the sensors and control sources inside the interior space of the building. For global noise reduction throughout the interior room, the control gains for feedforward control are calculated to minimize the total acoustic power of the new source, which is combined with the noise source corresponding to the open window and control sources on the window frame. The performance of the proposed system for directional exterior noise is confirmed with a scale-model experiment. The experimental results show that the proposed system can achieve a noise reduction of up to 10 dB for the entire room of the scale model regardless of the direction of the incident wave.
This paper describes an experimental investigation of an actively controlled double-glazed window. It is the second of two companion papers of which the first treated results obtained employing adaptive feedforward control. Herein, the outcome using adaptive feedback control is presented. This adaptive feedback controller has been tested in different configurations, i.e. fully and partially connected controllers. The differences between fully connected controllers with few filter coefficients and partially connected controllers with many filter coefficients are discussed. Additionally, tests with different traffic noise examples have been performed showing the ability of the actively controlled window to enhance protection against traffic noise.