Passive Attenuation Research Articles

Active control of sound through full-sized open windows

There is a pressing need to address urban sustainability challenges of increasing ambient temperatures and noise levels in densely-populated, high-rise cities. Solutions that utilise active noise control on open windows to reduce indoor noise levels seem promising, as natural ventilation is still maintained. Active noise control utilizes acoustic transducers arranged around the open window to generate a secondary incidence noise that destructively interferes with the real noise. The two most common techniques of transducer arrangement, distributed and boundary layouts, are investigated for the typical single-glazed sliding window. Finite element method is used to establish the control performance of the active noise control system and the passive attenuation provided by the sliding window. Based on the investigated fundamental limits of active control, the distributed layout has consistently yielded better performance than the boundary layout. The distributed-layout method can also reduce noise more effectively than a fully-glazed window. Moreover, sources distributed only in the partial opening of a simulated sliding window can attenuate noise as effectively as the fully-glazed window. The distributed-layout method is tested on a full-sized window, where the active control system has up to 16 channels and evenly distributed across the window opening. In the test with tonal sounds, the feasibility of the active control system is demonstrated. The experimental results have validated the simulation findings for normal incidence plane waves.

Optimal design of damping layers in SMA/GFRP laminated hybrid composites

This work describes the optimization of the shape profiles for shape memory alloys (SMA) sheets in hybrid layered composite structures, i.e. slender beams or thinner plates, designed for the passive attenuation of flexural vibrations. The paper starts with the description of the material and architecture of the investigated hybrid layered composite. An analytical method, for evaluating the energy dissipation inside a vibrating cantilever beam is developed. The analytical solution is then followed by a shape profile optimization of the inserts, using a genetic algorithm to minimize the SMA material layer usage, while maintaining target level of structural damping. Delamination problem at SMA/glass fiber reinforced polymer interface is discussed. At the end, the proposed methodology has been applied to study the hybridization of a wind turbine layered structure blade with SMA material, in order to increase its passive damping.

Real-time estimation of the passive attenuation for otoacoustic emission measurement probes

Despite hearing loss prevention programs in place at work, noise-induced hearing loss (NIHL) remains the first reported occupational disease. To improve the detection of over-exposure to noise, an initial proof-of-concept for field monitoring of inner-ear health using otoacoustic emissions (OAE) was developed and successfully validated in laboratory conditions. However, in real-life situations, the unsupervised placement of the OAE probes remains a challenge: proper fit of the probe in the ear canal is required to ensure adequate hearing protection of the wearer, to improve the signal-to-noise ratio of OAE measurements and to maintain the proper calibration of stimuli signals in the case of the Distortion Product OAE (DPOAE) approach used. Algorithms were recently developed to quantify, in real-time, from the OAE probe microphones and receivers signals, the passive attenuation of the OAE probes. The validation of this approach was conducted, in laboratory conditions, on five human subjects exposed to industrial and pink noise recordings at realistic levels.

Design Considerations for Robust Noise Rejection in Otoacoustic Emissions Measured In-Field Using Adaptive Filtering

Otoacoustic emission (OAE) measurement is a sensitive and effective technique to monitor changes in the inner ear potentially induced by noise exposure. However, outside a controlled testing environment, measurements are challenging since the level of ambient noise might be higher than the low-level OAE response. Therefore, an OAE system was designed, suitable to measure OAEs repeatedly on an individual worker in noisy test conditions. This system features a left and right earpiece, each equipped with a pair of miniature loudspeakers, an external and an internal microphone. In addition to the passive attenuation of the earpiece, adaptive filtering on the distortion product OAE (DPOAE) signals is used to further enhance the ratio between the measured OAE signal and interfering noise. The adaptive filtering technique uses the sound captured by the ipsilateral external and internal microphones as well as from the contralateral internal and external microphones. In this paper, the accuracy of DPOAE signals are studied using different combinations of the four microphones in single and dual adaptive filter topologies, as well as the benefits of adding a fixed transfer function in the adaptive filtering algorithm topology to estimate the acoustic path. Side-by-side comparison shows that a dual-stage adaptive filtering algorithm, using a combination of the contralateral internal microphone with the ipsilateral external microphone, is the most promising approach to denoise the DPOAE signal.

Attenuating severe slug flow at large valve opening for increased oil production without feedback control signal

In the co-current flow of gas-liquid mixtures through pipeline-riser system, severe slugging is frequently encountered and manifests in significant fluctuation of flow and pressure. This can pose serious threat to production facilities. The most common method for its mitigation is by choking which unfortunately could negatively affect production. The objective of this study therefore is to develop a technique that can help stabilise the system and maximise production simultaneously.In this paper, a new general method for multiphase flow system stability analysis was proposed based on a new passive attenuation method – the intermittent absorber. A series of experiments were carried out in a 4″ pipeline-riser system which is 55m long with horizontal pipe inclined at −2° connected to 10.5m high catenary riser followed by 3m horizontal topside section. Air and water were used as experimental fluids. Numerical studies were also conducted on a 4″ pipeline-riser system to proof the concept.The results showed that the intermittent absorber possess potential for stabilising severe slug flow at larger valve opening and lower pressure. For the case studied, up to 9% reduction in the riser base pressure was reported which practically implies increased oil production.

Relative vibration attenuation by means of self-active devices constituted by permanent magnets separated by silicone layers

The relative vibrations of two small frames with different stiffness, subjected to the same oscillating excitation coming from their base platform, were investigated. The effects of the innovative coupling of standard passive attenuation and time variation of magnetic forces (self-produced by the vibration itself) were investigated. The first experimental tests demonstrate that the amplitude of relative vibration can be easily decreased up to 75% without requiring external energy. In perspective, by varying the attenuator shape and power, it is possible to use it in all the cases in which it is necessary to reduce the vibrations of a breakable or unstable structure inside a building accidentally exposed to vibration, such as a rotor inside a frame or a shelf in a museum room. Copyright © 2016 John Wiley & Sons, Ltd.

Active Dendrites and Differential Distribution of Calcium Channels Enable Functional Compartmentalization of Golgi Cells.

Interneurons are essential to neural processing because they modulate excitability, timing, and synaptic integration within circuits. At the input layer of the cerebellar cortex, a single type of interneuron, the Golgi cell (GoC), carries these functions. The extent of inhibition depends on both spontaneous activity of GoCs and the excitatory synaptic input they receive. In this study, we find that different types of calcium channels are differentially distributed, with dendritic calcium channels being activated by somatic activity, boosting synaptic inputs and enabling bursting, and somatic calcium cannels promoting regular firing. We therefore challenge the current view that GoC dendrites are passive and identify the mechanisms that contribute to GoCs regulating the flow of sensory information in the cerebellar cortex.

Pure-tone audiometry outside a sound booth using earphone attentuation, integrated noise monitoring, and automation

Objective: Accessibility of audiometry is hindered by the cost of sound booths and shortage of hearing health personnel. This study investigated the validity of an automated mobile diagnostic audiometer with increased attenuation and real-time noise monitoring for clinical testing outside a sound booth. Design: Attenuation characteristics and reference ambient noise levels for the computer-based audiometer (KUDUwave) was evaluated alongside the validity of environmental noise monitoring. Clinical validity was determined by comparing air- and bone-conduction thresholds obtained inside and outside the sound booth (23 subjects). Study sample: Twenty-three normal-hearing subjects (age range, 20–75 years; average age 35.5) and a sub group of 11 subjects to establish test-retest reliability. Results: Improved passive attenuation and valid environmental noise monitoring was demonstrated. Clinically, air-conduction thresholds inside and outside the sound booth, corresponded within 5 dB or less > 90% of instances (mean absolute difference 3.3 ± 3.2 SD). Bone conduction thresholds corresponded within 5 dB or less in 80% of comparisons between test environments, with a mean absolute difference of 4.6 dB (3.7 SD). Threshold differences were not statistically significant. Mean absolute test-retest differences outside the sound booth was similar to those in the booth. Conclusion: Diagnostic pure-tone audiometry outside a sound booth, using automated testing, improved passive attenuation, and real-time environmental noise monitoring demonstrated reliable hearing assessments.

A 3.5-GHz-Band GaAs HBT Stage-Bypass-Type Step-Gain Amplifier Using Base-Collector Diode Switches and Its Application to a WiMAX HBT MMIC Power Amplifier Module

This paper describes circuit design and measurement results of a newly proposed GaAs-HBT step-gain amplifier configuration and its application to a 3.3-3.6 GHz WiMAX power amplifier module for use in customer premises equipment. The step-gain amplifier implemented using only a usual HBT process is based on a current-mirror-based, base-collector diode switches and a passive attenuator core for the purpose of bypassing a power-gain stage. The stage allows an individual design approach in terms of gain and attenuation levels as well as large operating current reduction in the attenuation state. To confirm the effectiveness of the proposed step-gain amplifier, a prototype of the amplifier was designed and fabricated, and then a WiMAX power amplifier module was also designed and fabricated as an application example of the proposed configuration to an amplifier product. Measurements are as follows. For a 3.5-V power supply and a 3.5-GHz non-modulated signal, the step-gain amplifier delivers 23.7 dBm of 1-dB gain compressed output power and 10.7 dB of linear gain in the amplification state. In the attenuation state, the amplifier exhibits 21 dBm of 1-dB gain expanded input power, -9.7 dB of gain, and 15 mA of current dissipation while keeping the gain stage switched off and maintaining input and output return loss of less than -10 dB at a 3.5-GHz band. The WiMAX amplifier operating with a 5-V supply voltage and a 64-QAM modulated signal is capable of delivering a 28.5-dBm linear output power, a 37-39 dB gain, and 15% of PAE over a wide frequency range from 3.3 to 3.6 GHz in the high-gain state while keeping error vector magnitude as low as 2.5%. This amplifier, which incorporates the proposed step-gain configuration into its interstage, enables a 24-dB gain reduction and a 45-mA large quiescent current reduction in the low-gain state.

Passive Attenuation of Modal Cavity Aeroacoustics Under Supersonic and Transonic Conditions

Open cavities exposed to a grazing flow can exhibit large pressure fluctuations, which may result in damage to stores or components carried within a weapon bay. This study investigates the use of passive resonant absorbers based on Helmholtz resonators to attenuate the problematic pressure fluctuations that arise in cavity flows, under both transonic () and supersonic () conditions. The experimental investigations indicate that resonant arrays can be used to provide attenuation levels of up to 26 dB within a cavity under supersonic () conditions. These supersonic designs were based on the properties of transonic devices, which also delivered high attenuation levels, with up to 15 dB attenuation for a single mode at Mach 0.95. A combined configuration with an array installed at both ends of the cavity, under supersonic conditions, provided attenuation levels up to 15 dB for multiple modes simultaneously, as well as a reduction in the broadband noise. This study investigates the attenuation mechanism for these devices, and discusses the effects of the high sound pressure levels, which arise within a resonant cavity, on the operation of the palliatives. A new semi-empirical model for resonant arrays under such acoustic environments is developed, and design guidelines of cavity applications are provided.

Understanding speech when wearing communication headsets and hearing protectors with subband processing.

An adaptive, delayless, subband feed-forward control structure is employed to improve the speech signal-to-noise ratio (SNR) in the communication channel of a circumaural headset/hearing protector (HPD) from 90 Hz to 11.3 kHz, and to provide active noise control (ANC) from 50 to 800 Hz to complement the passive attenuation of the HPD. The task involves optimizing the speech SNR for each communication channel subband, subject to limiting the maximum sound level at the ear, maintaining a speech SNR preferred by users, and reducing large inter-band gain differences to improve speech quality. The performance of a proof-of-concept device has been evaluated in a pseudo-diffuse sound field when worn by human subjects under conditions of environmental noise and speech that do not pose a risk to hearing, and by simulation for other conditions. For the environmental noises employed in this study, subband speech SNR control combined with subband ANC produced greater improvement in word scores than subband ANC alone, and improved the consistency of word scores across subjects. The simulation employed a subject-specific linear model, and predicted that word scores are maintained in excess of 90% for sound levels outside the HPD of up to ∼115 dBA.

Advanced hearing protection and auditory awareness in individuals with hearing loss

In-ear and earmuff-type electronic protection devices are rapidly being introduced into the marketplace and deployed in noisy industrial workplaces and military settings. In these environments, workers must be sufficiently protected from noise while being able to maintain good communication abilities and situational awareness. Features such as level-dependent attenuation or amplification, user-adjustable talk-through circuitry, noise reduction, and remote communication capabilities are commonly found in advanced devices. The benefits of these features depend on their complex interaction with the signal and noise characteristics, the hearing status and language proficiency of the workers, and the nature of the auditory task. Yet, detailed electro-acoustical specifications are rarely reported by manufacturers. Measurement standards are also lacking, though the advent of ANSI/ASA S12.42 should in part address this situation. In this paper, the characteristics of two advanced devices are reported over a test battery including measurements of the passive sound attenuation, level-dependent talk-through sound transmission, and speech recognition in noise for listeners with a wide range of hearing profiles. Electronic level-dependent attenuation provided superior speech recognition performance than passive attenuation for all groups of listeners, and often exceeded unprotected speech recognition performance.

Improving speech intelligibility in active hearing protectors and communication headsets with subband processing

Parallel subband processing, in which the full bandwidth of environmental noise and a communication channel are processed separately in contiguous, restricted frequency bands, has been proposed as a means to improve speech intelligibility in noise for active hearing protectors and communication headsets (Bernstein et al., Int. J. Ind. Ergonom., in press). An active, adaptive feed-forward control structure has been employed to improve the audibility of sounds in the communication channel of a circumaural hearing protector / headset while active noise reduction (ANR) is used to complement the passive attenuation of the device from 50 to 800 Hz. The communication channel subbands have been implemented as octave bands from 125 Hz to 8 kHz. The performance of the device has been evaluated in a diffuse field when worn by human subjects. Word intelligibility in industrial noise was evaluated when the active system was not operating, when the device was operating as a fullband ANR system with fixed communication channel gain, and as a subband ANR system with adaptive gain of the communication channel signal to improve the speech signal-to-noise ratio. A significant improvement in speech intelligibility can be obtained with the subband system. [Work supported by NIOSH 5R01OH 008669.]

Improving speech understanding in communication headsets: Simulation of adaptive subband processing for speech in noise

Speech communication headsets are necessary for many high-noise environments to maintain interaction between individuals and facilitate safe working conditions. However, current hearing protection devices intended to protect hearing health can impede speech communication or expose persons to sound pressure levels (SPLs) that could lead to excessive noise exposure if a communication signal is presented improperly. This paper explores an adaptive subband communication algorithm, based on a delayless subband active noise reduction architecture, intended to adjust the communication channel gain to provide an appropriate speech signal power in relation to the instantaneous environmental noise power. The method monitors SPLs underneath the ear cup of a communication headset to provide a target speech signal-to-noise ratio without exceeding safe noise exposure thresholds. A series of computer simulations derived from a real-world communication headset model are used to compare the method developed with a traditional passive attenuation headset and a commercial active noise reduction design. The simulations demonstrate the ability of the adaptive subband communication algorithm to adjust automatically the speech signal gain for improved intelligibility while maintaining healthy noise exposure levels. Relevance to industryThe electro-acoustic performance of an active speech communication headset is explored by simulation. The concept integrates a subband active noise control algorithm with an adaptive gain control structure to improve speech intelligibility in a noisy environment. The concept automatically selects appropriate communication channel gain levels without exceeding hearing damage thresholds or requiring user input, and is directly applicable to a practical device.

Adaptive feedforward control for active noise cancellation in-ear headphones

Noise can be disturbing, stressful or even harmful. Headphones with active noise cancellation (ANC) can enhance the user’s comfort, especially when travelling. On a plane or a train, in the street or at work, these headphones give the possibility to reduce unwanted noise. The range of ANC headphones on the market is constantly increasing. Circumaural and supra-aural headphones with different control strategies have been available for a long time; over the last few years the product lines have been expanded to in-ear headphones. These headphones already have quite a good passive attenuation and are equipped with feedforward control for active noise cancellation. The best results in attenuation are achieved by semi-adaptive digital controls, which choose the best filter depending on the noise spectrum and can be manually adapted to the user. A fully adaptive control has already been proven to be very effective in aviation headsets and other ANC applications. Besides the market analysis of ANC headphones we would like to present an adaptive feedforward control for in-ear headphones and highlight the advantages compared to a static feedforward control.

An indirect hybrid sound transmission loss controller

The control of sound transmission through panels is an important noise control problem in the aerospace, aeronautical, and automotive industries. The trend towards using lightweight composite materials that have lower sound insulation performance is a negative factor regarding low frequency transmission loss. Double-panel partitions with the gap filled with sound absorption materials are often employed to improve the sound insulation performance with reduced added weight penalty. However, in the low frequency range, the strong coupling between the panels through the air cavity and mechanical paths may greatly reduce the sound transmission performance, making it even lower than the performance of a single panel in some frequency ranges. In this work, an experimental investigation of a new kind of hybrid (active/passive) acoustic actuator is presented. The idea consists of replacing the acoustic absorption material by a hybrid actuator aiming at improving the transmission loss at low frequencies without altering the passive attenuation. A prototype of the system is tested in a plane wave acoustic tube setup. Different kinds of SISO feedforward control implementations were used to attenuate the sound power transmitted through the hybrid active–passive panel using an error microphone or a particle velocity sensor placed downstream with respect to the sample panel. Measurement results of the transmission loss with active and hybrid attenuation are presented and discussed.

Comparison of Speech Intelligibility Measures for An Electronic Amplifying Earmuff and An Identical Passive Attenuation Device

The purpose of this study was to identify any differences between speech intelligibility measures obtained with MineEars electronic earmuffs (ProEars, Westcliffe, CO, USA) and the Bilsom model 847 (Sperian Hearing Protection, San Diego, CA, USA), which is a conventional passive-attenuation earmuff. These two devices are closely related, since the MineEars device consisted of a Bilsom 847 earmuff with the addition of electronic amplification circuits. Intelligibility scores were obtained by conducting listening tests with 15 normal-hearing human subject volunteers wearing the earmuffs. The primary research objective was to determine whether speech understanding differs between the passive earmuffs and the electronic earmuffs (with the volume control set at three different positions) in a background of 90 dB(A) continuous noise. As expected, results showed that speech intelligibility increased with higher speech-to-noise ratios; however, the electronic earmuff with the volume control set at full-on performed worse than when it was set to off or the lowest on setting. This finding suggests that the maximum volume control setting for these electronic earmuffs may not provide any benefits in terms of increased speech intelligibility in the background noise condition that was tested. Other volume control settings would need to be evaluated for their ability to produce higher speech intelligibility scores. Additionally, since an extensive electro-acoustic evaluation of the electronic earmuff was not performed as a part of this study, the exact cause of the reduced intelligibility scores at full volume remains unknown.

Modeling of a distributed device for simultaneous reactive vibration suppression and energy harvesting

Distributed devices or treatments are a mainstay of passively suppressing structural vibrations. In some cases, piezoelectric materials were included within the devices to utilize them as actuators. Interest in energy harvesting encourages a reassessment of these devices, using the piezoelectric materials as a means to convert input vibrational energy into electrical power. A numerical model of one such device, exhibiting mass-spring-damper dynamics, attached to a vibrating host structure is described and validated against 3D finite element analysis. The model is utilized to evaluate the simultaneous goals of passive vibration attenuation and energy harvesting of devices on a lightweight, clamped panel. The objectives are found to be partly in opposition, particularly when the total mass of the added devices becomes more substantial. This feature is widely neglected in employing mass-spring systems as energy-harvesting devices, where the mass of the device is insubstantial relative to the main structure. As a result, compromises or alternatives may be explored to achieve both goals: the implementation of materials exhibiting greater electromechanical coupling, minimization of total applied mass to the host structure, and retuning of attached devices to exhibit natural frequencies not exactly equal to those of the host structure.

SUPRA−AURAL HEADPHONE NOISE REDUCING

An earphone for a supra-aural noise reducing headphone, with a front cavity that includes a foam portion and an open passageway. The foam portion supplements the volumetric dimension of the passageway to improve passive attenuation.

A 5‐GHz energy‐efficient tunable pulse generator for ultra‐wideband applications using a variable attenuator for pulse shaping

SUMMARYA new energy‐efficient tunable pulse generator is presented in this paper using 0.13‐µm CMOS technology for short‐range high‐data‐rate 3.1–10.6 GHz ultra‐wideband applications. A ring oscillator consisting of current‐starved CMOS inverters is quickly switched on and off for the duration of the pulse, and the amplitude envelope is shaped with a variable passive CMOS attenuator. The variable passive attenuator is controlled using an impulse that is created by a low‐power glitch generator (CMOS NOR gate). The glitch generator combines the falling edge of the clock and its delayed inverse, allowing the duration of the impulse to be changed over a wide range (500–900 ps) by varying the delay between the edges. The pulses generated with this technique can provide a sharp frequency roll off with high out‐of‐band rejection to help meet the Federal Communications Commission mask. The entire circuit operates in switched mode with a low average power consumption of less than 3.8 mW at 910 MHz pulse repetition frequency or below 4.2 pJ of energy per pulse. It occupies a total area of 725 × 600 µm2 including bonding pads and decoupling capacitors, and the active circuit area is only 360 × 200 µm2. Copyright © 2011 John Wiley & Sons, Ltd.