Noise Control Devices Research Articles

System and method for optimized active controller design in an ANR system

A tailored active noise control design method is presented that provides for improved noise attenuation performance for each individual user and improved hearing protection in a specified noise field as a function of a specified metric indicative of a noise reduction objection. Characteristics of individual users, behavior of the associated passive hearing protection, and the external noise environment are all concurrently accounted for in an automatic method for designing an active controller that limits the exposed noise level for a specific individual. The controller manufacturing process and implementation may be performed in-situ for each individual automatically. The design method may also account for actuator limitations and can be applied equally well to any passive/active noise control devices including headphones and earplugs.

Development of a System to Control Noise Transmitted Through Interior Panels of Railway Vehicles (1st Report, Study on a Method of Controlling Noise Transmitted Through a Panel Using a Module-Type Noise Control System)

In considering the application of an active noise control system to interior noise reduction of railway vehicles, the control system must respond to changes in the vibration characteristics of the interior panels during train running. We propose a new module-type noise control system, in which small noise control devices are arranged on a plane, to act against the noise transmitted through the interior panel that becomes dominant in tunnels. This noise control system controls the noise incidence to the interior panel, therefore, it is unnecessary to identify the vibration characteristics of the interior panel or noise transmitted through it, and a certain level of noise reduction is ensured irrespective of the change of vibration characteristics of the interior panel during train running. As fundamental work, we carried out a study on the application to the noise transmitted through a flat plate equivalent to an interior panel. This paper first presents an outline of the noise control system and modeling. Furthermore, the overall behavior of the system is examined by numerical analysis, and finally experimental results of the control effect are shown.

Active Helmholtz Resonator With Positive Real Impedance

The impedance of a passive noise control device is strictly positive real, if the device is installed in noise fields with weak mean flows. Passive noise control devices are, therefore, more reliable than active ones. Active control may be applied to a Helmholtz resonator to introduce electronic resonance. It will affect the impedance Zact of the resonator. A controller may be designed such that (a) Zact is small and resistive at some tunable frequencies; and (b) Re{Zact}⩾0 in the entire frequency range of interest. If criterion (a) is satisfied, the active resonator can suppress duct noise at tunable frequencies. It is difficult to design a controller to satisfy criterion (b) because parameters of the controller depend on acoustic parameters of the noise field. A new method is proposed here to design an active controller to meet both criteria simultaneously. The satisfaction of criterion (b) implies a positive real Zact and a robust active resonator with respect to parameter variation in the noise field. Experimental results are presented to verify the performance of the active resonator.

An analytical study of the effectiveness in noise control due to combination of mufflers in design of duct systems

Mufflers are effective noise control devices used in acoustical design of duct systems. The performance evaluation of a single muffler in terms of four-pole matrix approach is well known. The performance evaluated in terms of four-pole parameters and insertion loss describes the system using various components namely source, muffler, and termination. The performance of a reactive muffler is known to be frequency dependent based on its internal geometrical details. This paper presents analytical studies on the effectiveness of combining mufflers on the noise control. The studies deal with both series and parallel arrangement of the reactive mufflers. The effects of various parameters such as number of mufflers, the length of ducts associated with the system, and the muffler geometry are presented. For a given source model, simple expansion chamber-type mufflers are studied in detail in order to understand the nature of interaction in different types of arrangements. Applications of this study would be to increase the noise reduction by combining mufflers in systems such as automotive, HVAC, etc.

A comparative study of varying fan noise mitigation techniques in relation to sustainable design goals

Green building design promotes effective use of materials and energy, improved indoor environmental quality (IEQ), and enhanced occupant comfort. These ‘‘green’’ goals can occasionally conflict with common acoustical approaches used for fan noise control. A design striving for low noise levels from the ventilation system to benefit occupant comfort can inadvertently introduce elements that are contradictory to other green building objectives. For example, typical fan noise control devices introduce higher energy consumption or less beneficial indoor environmental quality. This paper discusses the acoustical, mechanical, environmental, and relative cost impacts of various fan noise control techniques.

Active control of payload fairing noise using distributed active vibration absorbers

High sound pressure inside a launch vehicle fairing during lift‐off can damage the payload. Interior levels of up to 140 dB between 60 and 250 Hz are mostly due to exhaust plume noise combined with the limited transmission loss of lightweight composite fairings and little acoustic damping in the fairing volume. Past work using passive and hybrid passive/reactive noise control devices has shown that their limitations are mostly due to packaging volume and weight penalty. The objective of this work is to design a lightweight, compact, and low electrical power active noise control system to reduce the fairing interior sound level. Hybrid active/passive actuators such as Smart Foam (Couche and Fuller, Proceedings of Active 1999, Ft. Lauderdale, FL, pp. 609–620) and Distributed Active Vibration Absorbers (Marcotte, Fuller, and Johnson, Proceedings of Active 2002, ISVR, Southampton, England, pp. 535–546) are optimized for fairing noise control. The latter have been used to increase the transmission loss of the ...

A new approach to control noise from entertainment facilities: Active control and measurement of amplified community noise

While traffic noise is perhaps the most pervasive of community noises, much of the contribution now comes from amplified sound: live music, discos, theme parks, and exercise studios. Those producing the sound or music want it loud and those not interested want to be protected against noise. Noise limits at the receiving or producing property line must be met for the minimum community acceptance. However the time‐, and perhaps the spatially‐, varying sound in entertainment facilities is often constantly modified (and maybe monitored) near the source of the sound. Hence it is hard to relate and to control the sound at the property line. This paper presents a unique noise control device. It is based on the octave band ‘‘transfer function’’ between the sound produced in the entertainment area and the noise received at the property line. The overall insulation can be measured and is input to the instrument. When a noise level limit is exceeded at the receiver, due to the amplified interior noise at the facility, the sound output of the device is automatically controlled to reduce the noise. The paper provides details of the design and possible abatement scenarios with examples.

Current status of standards for testing electroacoustic hearing protectors

Electroacoustic hearing protectors encompass several classes of electronics: level limiting, amplitude compression and active noise control (ANC) devices. Each class of protector seeks to enhance performance by overcoming insertion loss and/or improving low-frequency attenuation. Level-limiting devices turn off the amplification when the external sound level exceeds a preset threshold. Amplitude compression devices apply variable gain and limit amplification of signals above the threshold. ANC devices create an out-of-phase signal under the protector to improve the low-frequency attenuation of earmuffs. ANC devices typically perform best in continuous noise that has strong harmonic components that can be predicted and cancelled. The different methods of enhancing protector performance require separate standards to understand both the static and dynamic performance. Testing of ANC devices has further requirements which require knowledge of both passive and active electronic conditions and in noise with different crest factors. This talk will review a variety of tests of the different protector classes.

HYBRID NOISE CONTROL IN DUCTS

Presented in this paper is a hybrid noise control system used to attenuate multi-tonal noise in ducts. The hybrid system is built by integrating active feedback noise control with adaptive–passive noise control. The adaptive–passive part of the hybrid noise control system attenuates the dominant tonal noise within the duct while the active noise control system attenuates the noise at other frequencies. To achieve such a hybrid system, the constituent technology for each noise control solution is extended. In particular, an algorithm is described for tuning adaptive–passive noise control devices in the presence of multi-tone noise sources. Also, a feedback controller design methodology is presented. This methodology, adapted from the robust control literature, is easily applied to uncertain acoustic duct systems possessing significant time delays. In the system studied, the time delay arises from the non-collocation of the microphone and the control speaker. The uncertain dynamics of the duct are due to the on-line tuning of the adaptive–passive element. The extended adaptive–passive and robust noise control technologies are combined into a hybrid system. Experimental results are included as are extensions to broadband noise sources.

Criteria for designing multiple-sprung masses for wideband noise control

In a companion paper the characterization of a multiple-sprung masses design for wideband noise control is presented. The characterization is largely conducted in terms of the point impedance of a set of sprung masses. The sprung masses in the set are collectively acting at a point. In that companion paper the dependencies of the joint impedance on the number of sprung masses, the modal overlap factors, the total mass, and the resonance frequency distribution are of particular interest. In the present paper this characterization of a set of sprung masses is utilized to define a number of design criteria that determine the potential viability of the set as a noise control device. In the final analysis, the device must be assessed in an in situ setting. In that setting the device is tested in terms of the overall gain. The determination of the overall gain requires, in addition to the joint impedance of the device, the impedance of the hosting master structure at the point of attachment. (The device is meant to control the noise in the master structure.) It is argued that although the overall gain is the final judge, the potential viability of a device may be a useful critique of the intended performance of the device. In this vein, a viability and a criterion of promise are defined to assist with the design processes of this noise control device. In assessing the viability and in satisfying the criterion of promise, the desired characteristics in a set of sprung masses can be judicially selected.

Computational methods for designing a silator—A low-frequency resonator element

The silator is a vacuum bubble resonator element which is used as a noise control device and has a low-frequency performance. It consists of an evacuated, lens-shaped, convex cap of sheet metal. The capsule volume can be made as small as desired, since, because of the vacuum inside, there is no air stiffness. The effect of a silator is comparable to that of a Helmholtz resonator, but only a hundredth of the latter’s installation volume is needed. Broadband noise control can be achieved by a set of silators tuned to different frequencies. The principles of this silator technology in aeroacoustics were established by Bschorr and Laudien in 1992. The resonant characteristics of such a device are very sensitive to its geometrical properties. The plasticity of the material, the metal forming, and the remaining stresses in the convex cap also influence its effectiveness. The major development problem lies in the extremely small manufacturing tolerances. The tolerances decrease with increasing size and frequency. In this paper, computational methods and results are presented to yield the final design of the silator’s shape with a very low resonant frequency. The calculations were performed using the finite-element program MSC/NASTRAN for Windows.

Multi-purpose apparatus for evaluation of noise transmitted through air ducts

The noise generated in HVAC systems by fans or air turbulence is propagated along ducts and, through duct walls, can be irradiated in rooms; moreover duct lines can easily transmit the sound produced in noisy environments from one room to another, vanishing the sound insulation of walls which they are passing through. The knowledge of sound insulating properties of duct walls is fundamental for a good system design, including unitary attenuators and active noise control devices. In this work a multi-purpose apparatus for evaluation of noise transmitted through air ducts in a reverberant room is presented. This apparatus allows one to measure, with the same equipment, the sound power level emitted from a short length ducts specimen in a reverberant room, as well as linear sound attenuation and break-in and break-out transmission loss. A further application of the same apparatus is the measurement of sound insulation and normal incidence impedance of small plane elements of isotropic or composite materials.

Entertainment park noise control devices

A big entertainment park is operating with noisy attractions near the Garda lake, a pleasant land where people go not only for amusement but also to relax, especially during holiday time. Noise measurements have been made during different periods of daytime and a noise map has been developed taking into account the difference of level between the top of the noisiest attraction and the windows of the most exposed house. Starting from this computerized model, and knowing the emission spectrum of the most relevant sources, it has been possible to design some noise control device useful for reducing the noise exposure level restricted to the exposed houses alone: This was possible combining earth barriers and a hot windstream.

Active Reduction of Radiated Noise From a Baffled Piston Using a Volume Velocity, Feedforward Controller

A new active noise-control technique has been developed for control of low-frequency sound generated by vibrating surfaces which is based on minimizing the volume velocity. Noise reduction is achieved by distributing an array of control devices over the surface of the radiating structure. Each device consists of a motion-sensing mechanism, an analog control circuit, and a loudspeaker. The loudspeaker is driven such that it reduces the volume velocity of the radiating structure within its close proximity. This paper briefly presents the theory behind this approach as well as controller design issues. Finally, a discussion of the experimental verification of this concept using a 10-in. uniformly vibrating circular plate (i.e., a baffled piston) and a single noise-control device is given. Broadband (50–500 Hz) sound reductions in the range of 10–20 dB were achieved over a wide spatial area.

Noise control device for a solenoid-actuated valve

First Page

Accuracy of dynamic modulus measurements made with the dynamic mechanical thermal analyzer

The accurate determination of the complex moduli of viscoelastic materials is of great importance for the efficient design of many noise control devices. It is often the case that a design, in conjunction with its acoustic performance requirements, leads to a narrow band of acceptable dynamic modulus for the viscoelastic material. In such cases knowledge of the accuracy and reliability of the measurement apparatus is essential. Dynamic complex shear modulus is measured at Carderock Division Naval Surface Warfare Center (CDNSWC) with the Dynamic Mechanical Thermal Analyzer (DMTA), manufactured by Polymer Laboratories. An attempt to quantify the absolute accuracy of the DMTA measurement has been made through the consideration of errors including the accuracy of the beam solution used to infer the complex modulus, compliance of the sample mounts, measurement noise, temperature control, thermal stress, Poisson effects, and the implementation of time‐temperature superposition. With these considerations, an ad hoc error analysis is implemented to quantify absolute error bounds expected for DMTA measurements of a hypothetical material. These results are of direct interest to the ASA Committee on Standards S2WG79, ‘‘Characterization of the Dynamic Properties of Viscoelastic Polymers,’’ which has initiated round‐robin testing among committee participants. CDNSWC results from the DMTA tests of three round‐robin materials will be presented.

The silator—A small volume resonator

A new resonator element, called a silator, has been investigated for use as a noise-control device. A silator consists of an evacuated lentiform, convex cap of sheet metal. The effect of a silator is comparable to that of a Helmhotz resonator. Broadband noise control can be achieved by a set of silators tuned to different frequencies.

How ASA can assist the federal government in noise control

ASA has a great opportunity to assist the federal government in promoting noise control, but has done little to exercise this opportunity over recent years. Industrial noise, aircraft and other transportation noise, and noise effects are probably the areas where ASA members can share their knowledge and expertise most effectively. They can testify at congressional hearings and send written materials to interested members of Congress. They can also supply information and testimony at many stages along the rule-making process and can assist in the implementation of these rules once they are made final. Federal employees are often unaware of improvements in instrumentation and new noise control methods or devices. Inspectors who cover many occupational hazards cannot know about all of the available noise control options without assistance from professionals in acoustics. The ASA needs to develop a method of alerting its members about the specific congressional committees, federal agencies, and activities where acoustical expertise is needed. For the greatest effectiveness, ASA and its Noise Committee should be prepared to take a stand on important noise control issues.

PdC4a. The active control of sound reflection/transmission coefficients using piezoelectric composite materials

Abstract An active noise control device has been fabricated using a 0–3 piezo-composite sensor and actuator. This device has been used with a fast feedback electronic system. The reflection and transmission coefficients have been measured in a pulse tube. The energy was reduced by up to 0. 5 in active noise reduction mode and increased by up to 0. 17 in active noise enhancement mode from 1. 5 to 8. 5kHz. With an air backed noise control device, the echo reduction was increased by up to 12dB from 1 to 8kHz.

Applications for active noise control: What has been achieved and what can be achieved

Active noise control is currently undergoing an explosive growth throughout the world. This rapid development is occurring because the concept allows improvement in existing noise control devices, often with potential improvements in size, weight, volume, and cost. It also permits new solutions to noise control problems that were previously difficult or unsolvable. The technique is particularly attractive because, for many applications, noise may be reduced at the listener's position without physical modification of existing noise sources or their arrangement. Active noise control seems particularly well suited to the reduction of low-frequency noise which, for technological reasons, cannot easily or conveniently be attenuated with existing hardware. Because this new concept applies to such a wide variety of industrial, commercial, and military needs, a large number of specific applications are now under development or proposed for development. This paper describes the current status of active noise control technology. It describes applications that have already been realized as well as the possibilities for future development.