Secondary Sound Sources Research Articles

Real-time active noise control with preservation of desired sound

In the active sound control problem a quite arbitrary bounded domain is shielded from noise generated outside by implementing secondary sound sources on the perimeter. The sound generated by interior sources, also known as desired sound, is supposed to be protected inside the shielded domain. If the desired sound is present, it is required to remain it unaffected by the control. This problem becomes much more complicated since the secondary sources have a reverse effect on the input data. In the current paper, a novel potential-based algorithm for active sound control is applied to attenuate noise with the preservation of desired sound. The uniqueness of this algorithm is that it realizes an active sound control via the utilization of surface potentials which have a projection property. The algorithm automatically removes the contribution of controls and desired sound from the total input field to be measured. The applicability of the algorithm is demonstrated via a series of numerical experiments. In addition, some factors such as the effect of the number of controls and sensors are also discussed.

Active Structural Acoustic Control of an Active Casing Placed in a Corner

Electric appliances used in workplaces and everyday life often generate a low-frequency noise, which affects human body systems. Passive methods employed to reduce noise are not effective at low frequencies. The classical approach to active noise control practically involves the generation of local zones of quiet, whereas at other areas the noise is reinforced. Moreover, it usually requires a large number of secondary sound sources. Hence, an active casing approach has been developed. The active casing panels’ vibrations are controlled to reduce the device noise emission. Efficiency of this method has been previously confirmed by the authors and the results have been reported in multiple journal publications. However, in the previous research experiments, the active casing was placed at a distance from the enclosure walls. In this research, the active casing is located in a corner and such placement is intentionally used to facilitate the active control system’s operation. The noise reduction performance is investigated at multiple configurations, including a range of distances from the corner and different error microphone arrangements. The analysis of both primary and secondary paths is given. Advantages and drawbacks of different active casing configurations are presented and discussed.

Acoustic impedance control for reproducing sound environment by constructing virtual wall

In order to obtain high presence, it is necessary to reproduce the sound environment of remote places. Conventionally, sound environments at remote places are reproduced by recreating a sound signal at that place. In this study, an acoustic technique that reproduces sound environments of remote places by recreating the dynamics of the environment at that place is proposed. In the proposed acoustic technique, the desired sound environment is attained by making listeners on the spot feel as if there were a wall that reflects and absorbs the sound waves. By controlling the acoustic impedance at the position where the wall is constructed, the wall is virtually built. In addition, the acoustic impedance including the virtual viscosity and elasticity that can be arbitrarily determined at the position where the wall constructed is set. As a result, by deciding the acoustic output from the secondary sound source so as to attain the desired acoustic impedance, the wall is virtually constructed to reproduce the desired sound environment.

Transparent piezoelectric film speakers for windows with active noise mitigation function

Noise pollution has a significant negative effect on psychological health and quality of life. The building envelop, particularly windows often constitute to the primary path for external noise sources to travel into the buildings. Here we have designed and fabricated transparent piezoelectric film speakers as secondary area sound source for active noise mitigation for window application with ventilation function. The performance properties of the obtained transparent speaker and its effectiveness for active noise mitigation are evaluated through numerical simulations as well as experimental measurements. The produced transparent piezoelectric film speaker exhibited enhanced overall sound pressure level (SPL) and broader frequency response range with a significantly improved SPL at frequencies below 300 Hz. Furthermore, compared to an electromagnetic speaker as a point secondary sound source, using the transparent piezoelectric film speaker as an area source resulted in more uniform noise mitigation with substantially larger averaged SPL reduction. The results presented here show the potentials and advantages of transparent piezoelectric film speakers for active noise mitigation.

Reprint of: A practical algorithm for real-time active sound control with preservation of interior sound

In the active sound control problem a bounded domain is protected from noise generated outside via implementation of secondary sound sources on the perimeter. In the current paper we consider a quite general formulation in which sound sources are allowed to exist in the region to be shielded. The sound generated by the interior sources is considered as desired. It is required to remain it unaffected by the control in the protected area. This task proves to be much more complicated than the standard problem of active sound control because of the reverse effect of the controls on the input data. A novel practical algorithm is proposed that can be used for a real-time control. It accepts a preliminary tuning of the control system. In the algorithm the only input information eventually needed is the total acoustic field near the perimeter of the region to be shielded. It includes the contribution from both primary and secondary sources. In the algorithm the noise component to be attenuated is automatically extracted from the total acoustic field. The control system can potentially operate in a real-time regime since it only requires a consequent solution of a quadratic programming problem.

The regenerated noise of HVAC elements and its measurement in pipeline track

HVAC systems may spread unwanted noise that needs to be reduced. One of the components of wide range noise can be a regenerated noise which is a secondary sound source usually caused by air flowing through a device. This regenerated noise associated with airflow is indeed a dynamic function of velocity. Therefore, HVAC designers try to prevent these problems by sizing ductwork or duct configurations so that air velocity is low. Nevertheless, in certain situations, this approach is not possible and it is necessary to deal with the regenerated noise further. This article discusses an alternative way of measuring regenerated noise of various duct elements in the pipeline track without being connected to a reverberation chamber. It should be emphasized that this method is not new but information about it is very limited and therefore almost unused.

Procedures to smooth the coverage of dodecahedron loudspeakers on façades according to ISO 16283–3 Standard

In this paper, two methods for softening the directivity balloon of dodecahedron loudspeakers when used for insulation measurements in buildings were proposed and evaluated. These proposals can be useful because the dodecahedron loudspeakers do not meet the uniformity requirements of the current Standards about noise insulation of façades in buildings. The radiation patterns of commercial dodecahedrons are highly uneven above 1 kHz and produced low coverage uniformity on the façade, as required by the Standard ISO 16283–3 in acoustic insulation measurements in buildings. This study aimed to validate the use of the proposed procedures on polyhedron-type loudspeakers in accordance with the latest ISO Standard.The first proposed method recommends addition of a reflecting surface at the back of the loudspeaker to combine the main and the mirror sources for smoothing the high-frequency directivity balloon of the setup. The second method required averaging of the dodecahedron emission from different angular positions on the façade and receiving room. This method is equivalent to averaging the directivity balloon of the dodecahedron loudspeaker.The two proposed methods were tested in the paper initially by computer simulation. First, the model of a real dodecahedron loudspeaker was generated using a commercial software. Next, different smoothing proposals were applied to the dodecahedron model to generate several secondary sound sources, one for each variant of the methods. All these new sound sources were analysed with an author’s software on a replica of the ISO Standard scenario by calculating the maximum horizontal dimension of façades with different shapes that verifies the criterions of the Standard. Finally, all of the proposals were categorized and sorted according to the façade size to cover uniformly, and some suggestions were made on optimal strategies to smooth the dodecahedron coverage.As a final point, some actual measurements of the best proposed setups have been implemented.

A practical algorithm for real-time active sound control with preservation of interior sound

In the active sound control problem a bounded domain is protected from noise generated outside via implementation of secondary sound sources on the perimeter. In the current paper we consider a quite general formulation in which sound sources are allowed to exist in the region to be shielded. The sound generated by the interior sources is considered as desired. It is required to remain it unaffected by the control in the protected area. This task proves to be much more complicated than the standard problem of active sound control because of the reverse effect of the controls on the input data. A novel practical algorithm is proposed that can be used for a real-time control. It accepts a preliminary tuning of the control system. In the algorithm the only input information eventually needed is the total acoustic field near the perimeter of the region to be shielded. It includes the contribution from both primary and secondary sources. In the algorithm the noise component to be attenuated is automatically extracted from the total acoustic field. The control system can potentially operate in a real-time regime since it only requires a consequent solution of a quadratic programming problem.

Controlling 3D sound field generated by a 2D surface from its 1D boundary

As described by the Kirchhoff-Helmholtz integral equation, for a 3D space without internal sources, the inside sound field is completely determined by the sound pressure and its normal gradient on the boundary, so if only a portion of the boundary surface vibrates as the primary sound source while all the remaining portion of the surface is acoustically rigid, a sufficiently large number of secondary sound sources can be located evenly on the primary sound surface to control the 3D sound field completely. The question to be investigated in this research is whether it is possible to control the 3D sound field by only applying secondary sound sources along the 1D boundary of the 2D primary sound surface, for example, controlling the sound transmission from a door with secondary sound sources only on its frame. Simulation and experimental results will be presented to demonstrate the feasibility of the idea.

Conversion of sound radiation pattern via gradient acoustic metasurface with space-coiling structure

We designed, fabricated, and experimentally demonstrated a gradient acoustic metasurface to manipulate sound radiation patterns. The gradient metasurface is constructed on the basis of a coiling-up space in a tunable interdigitated structure, which exhibits relative refractive index in a discretized classic hyperbolic secant profile. Capable of generating secondary sound sources with desired gradient phase shifts, the metasurface shows the ability of controlling sound radiation such as by cylindrical-to-plane-wave conversion, plane wave focusing, and effective tunable acoustic negative refraction. Owing to its deep-subwavelength thickness, the metasurface may reduce the size of acoustic devices and offer potential applications in imaging and scanning systems.

Modeling, analysis, and validation of an active T-shaped noise barrier

With ever-increasing land traffic, abatement of traffic noise using noise barriers remains significant, yet it is a challenging task due to spatial competition with other infrastructure. In this study, a deep insight into the diffraction characteristics of acoustic fields near noise barriers of various geometries and surface conditions was achieved using numerical simulations. A T-shaped passive noise barrier with acoustically soft upper surfaces was demonstrated to outperform other candidates in a middle- or high-frequency range. Based on attributes of the acoustic field diffracted by T-shaped barriers, an active control strategy was developed to revamp the T-shaped barrier, in which a filtered minimax algorithm was established to drive the secondary sound sources. This algorithm resulted in more uniformly distributed residual sound fields than a filtered-X least mean square algorithm. Performance of the actively controlled barrier was evaluated at different positions and spacings of secondary sound sources and error sensors, leading to a series of optimal criteria for the design of active noise barriers. A prototype was fabricated and validated experimentally, manifesting particular effectiveness in insulating low-frequency noise, supplementing well the capacity of a passive T-shaped barrier which is effective in the middle- or high-frequency range.

Experimental Study on an Active Noise Control System for Turboprop Driven Aircraft

In order to find a solution to the control of low frequency noise in the cabin of turboprop driven aircraft, an experimental active noise control (ANC) system is introduced in the paper, which consists of vibroacoustic field analogue subsystem, acoustic field measurement subsystem and acoustic barrier exciter subsystem. Effects of both different spaces between the primary sound source and secondary sound source of the ANC system and different frequency noises on noise-reduction result are investigated based on the experimental platform. Results of the experiment show a significant potential of the ANC method for the control of low frequency noise in the cabin of turboprop driven aircraft.

Control of Tonal Fan Noise Using Flow Induced Secondary Sound Sources Generated by Air Jet Actuation

To reduce the tonal noise of axial fans loudspeakers are typically used to generate the required anti-phase sound field. The space requirement and the weight of the loudspeakers inhibit practical application of this method. In the present study the secondary sound field is generated by perturbing the flow in the blade tip regime by using high speed air jets. It has been shown, that this approach is very effective in controlling higher-order mode sound fields at blade passage frequency and its harmonics. The focus of the present work is to study the effect of various nozzle parameters on the generated secondary sound field. From the experimental data presented it can be concluded that when using circular nozzles an injection angle of α = 45° relative to the casing wall combined with an injection direction of β = 90° relative to the main flow (against the impeller rotation) are most efficient in reducing the tone levels. While it is principally possible to control acoustic modes at the blade passing frequency as well as at its harmonics, the injected mass flow rates and circumferential nozzle positions necessary for reducing two different blade tone harmonics are generally not the same. However, when using slit nozzles the differences between the circumferential nozzle positions where significant level reductions of two dominant acoustic modes can be observed can be reduced down to 4% of a stator vane pitch. Future studies will involve two rings of injection nozzles at close axial distance to reduce two dominant blade tone acoustic modes simultaneously. Finally, experiments are also planned with a model fan stage of a modern high bypass ratio aircraft engine.

Ventricular Fold Phonation: Viscoelastic Properties and HSDI

Objective: Determine the functional biomechanical properties of the human false vocal fold mucosa at phonatory frequencies, in comparison with those of the true vocal fold cover; and to quantify the vibration patterns of ventricular fold phonation with HSDI (high speed digital imaging). Method: Viscoelastic shear properties of false fold specimens were quantified as functions of frequency, with measurements of elastic shear modulus (G′) and dynamic viscosity (h′) at up to 250 Hz using a simple-shear rheometer. Ventricular fold phonation of three subjects was analyzed by HSDI (high speed digital imaging) through kymography and laryngotopography. Results: All specimens showed similar frequency dependence of their viscoelastic properties, with G′ gradually increasing with frequency and h′ monotonically decreasing with frequency. G′ and h′ of the false fold mucosa were generally higher than those of the true vocal fold cover, although the differences did not reach statistical significance. Kymographic and laryngotopographic analysis of high speed digital images revealed vibration of the false fold mucosa for subjects undergoing ventricular fold phonation. The vibratory frequencies were lower than typical fundamental frequencies for phonation. Conclusion: These findings showed false folds could function as a secondary sound source. These preliminary data suggested that the false fold mucosa could serve as replacement material for patients with unrepairable vocal fold cover damage.

Ray‐tracing prediction of sound‐pressure and sound‐intensity fields in empty and fitted rooms

A Monte‐Carlo ray‐tracing model has been adapted to the prediction of sound‐pressure and sound‐intensity fields in rooms with surfaces of arbitrary surface impedance, and containing parallelepiped obstacles. Phase changes due to propagation distance and wall reflection were accounted for. Diffraction around obstacle edges was modeled by the Unified Theory of Diffraction. The model was also extended to include the out‐of‐phase secondary sound source of a single‐channel, global active‐noise‐control (ANC) system. The new model was validated in comparison with predictions by reference models (e.g. FEM) and with the results of experiments in an anechoic chamber and in real rooms. It was used to predict the effectiveness of ANC. This paper discusses the development of the new models, the results of the validation tests and ANC predictions.

104 距離減衰率を改変する音場制御に関する研究(騒音の制御,騒音・振動評価・改善技術)

In recent years, with the never ending progress in technology related to audio visual equipments and PC, etc., there is an increasing trends of using the reproduction sound of contents such as music and speech even for household purposes. Since all the phenomena occurring in nature need to be considered in relative frame of reference, a pleasant and enjoyable sound to the listener may appear as a noise to the people (non-listener) around the owner of sound player. Therefore, the aim of the present work is to develop a sound field control technology which confined the sound field to the listener only. In this regard, the author first focused his attention to the relation between the vibration area of the speaker and the attenuation ratio. Based on that is proposed a method to change attenuation ratio so that sound pressure level remain same in the near field, but its level reduces in the far field. In this technique, the primary and secondary sound source of different vibration area is interacted by using anti-phase of the primary sound source. Based on the experiment and theoretical estimation the performance of the present technique is verified.

Secondary actuation and error sensing for active acoustic structure

A typical approach to active control of sound radiation or transmission from vibrating structures involves active structural acoustic control (ASAC) and active noise control (ANC), which introduce respectively force input and compacted sound source to apply on or be close to the vibrating structure. However, for the ASAC approach, arrangement for secondary force and error sensor is heavily dependent upon the properties of the primary structure and acoustical space; for the ANC approach, a large number of compacted secondary sources are required. Hence, in this paper, based on distributed secondary sound source and near-field error sensor, active acoustic structure is proposed to construct adaptive or smart structure as a versatile module or element for controlling sound radiation or transmission at low frequencies. First, a theoretical model based on a minimization of the total sound radiation from the primary and secondary panel is established, after which, taking into consideration the relationship between the vibration modes pattern and sound radiation characteristics for secondary panels, optimal arrangement for the secondary panels is examined in detail. Finally, a near-field pressure-based error sensing approach is presented, based on two kinds of object function, and active control of sound radiation is performed.

Reduction of Axial Turbomachinery Tonal Noise by Means of Flow Induced Secondary Sources Using an Extremum-Seeking Control Technique

In conventional active noise control experiments, loudspeakers are used to generate the secondary anti-phase sound field to be superimposed destructively with the sound waves radiated from the primary source. In the present study, aerodynamic sound sources are used to actively control the tonal noise of an axial fan. This is achieved by disturbing the flow field around the blade tips in such a way that additional periodic forces are set up which in turn form the secondary sound sources. To disturb the flow, air is blown into the blade tip region through the casing wall. The resulting aerodynamic sound sources are adjustable in both amplitude and phase. Results for closed loop control of the blade passing frequency using an extremum-seeking control technique are presented.

737 平板スピーカを用いた壁面透過音のアクティブ吸音ユニットの開発

An acive sound absorbing device unit of the transmission noise through a wall, using a plate planewave speaker as the secondary sound source, was developed toward effective suppression of aircraft interior noise. The controller was designed using reference accelerometer attached on the wall, and based on the Filtered-x LMS algorithm, its system identification was in advance performed so as to minimize sound level at one observation point, and then act as an off-line controller. At first, control effects were estimated by the numerical analyses using the system model identified from the actual experimental devices. Then, active sound absorbing experiments were performed with band limited white noise applied to the unit, and it was shown that enough control effects were obtained at plural observation points located in three dimensional space.

738 能動軽量遮音セルの二次音源性能向上に関する基礎研究

738 能動軽量遮音セルの二次音源性能向上に関する基礎研究