Temporal gaps in noise and sinusoids

The ability of human observers to detect partially filled or completely silent intervals (gaps) was measured using a variety of different waveforms. The slopes of the psychometric functions for gap detection using broadband noise are dependent upon the amount of noise remaining during the gap. For completely silent intervals, the psychometric function covers a range of only 2 ms, but the psychometric functions for partially filled intervals are less steep. The detection of gaps in narrow-band noise (surrounded by complementary band-reject maskers) is strongly influenced by the signal-to-noise ratio. The signal bandwidth and center frequency also influence detectability. Gap detection improved as signal bandwidth increased, and detection improved when signal bands containing gaps were centered at higher frequencies. Detection of gaps in single components of a 21-component, equal-amplitude complex also showed lower thresholds as the frequency of the component containing the gap increased. Increasing the number of components in the complex that contained the gap improved the detectability of the gap, more so when the gaps were all presented at the same time (synchronous condition). Uncertainty about the temporal position of the gap within the observation interval made the gap more difficult to detect. This temporal uncertainty effect occurred for gaps in broadband noise, in narrow-band noise, and in sinusoidal waveforms.     

声屏障和有源噪声控制声屏障

概述了声屏障降低噪声的原理和常用方法，提出了存在的不足，介绍了有源声屏障的降噪性能与发展动态，提出了进一步研究的设想。     

Highway noise barrier perceived benefit

A laboratory experiment was performed in which 82 subjects judged the benefit of a noise barrier by listening to tape recordings of before-barrier and after-barrier traffic noise. These perceived benefit judgments were related by regression analysis to the barrier attenuation, the before-barrier traffic sound level, and a music background level, all of which were varied over the course of the experiment. Prediction equations were developed for barrier benefit in terms of these sound levels, their purpose being to provide a model for barrier benefit that can be used in barrier site selection and design. An unexpected finding was that barrier benefit was highest when before-barrier sound levels were lowest: i.e., subjects preferred a noise barrier that solved a moderate noise problem over an equally-attenuating barrier that only partially solved a more severe noise problem.     

Field performance of a noise barrier

This paper is based on the results of a series of measurements of the performance of full-scale noise barriers of various heights, carried out under a range of wind conditions. It relates to the effects of a long barrier in reducing the noise from localized sources, such as the peaks of traffic noise.     

Excess noise of the silicon surface barrier detectors

From our experiments the following conclusions follow:

Performance of a motorway noise barrier at Heston

This paper reports on the performance and costs of a 300 m long experimental noise barrier erected alongside an elevated section of the M4 motorway to protect existing two-storey housing which is as close as 20 m to the motorway. Measured reductions in L₁₀ values, at the façades of these dwellings, were around 8 dB(A) at first floor windows and around 4 dB(A) at ground floor windows. These measured reductions were based on the levels with no barrier but with some screening due to the elevation of the road.A wooden fence in the barrier position at the start of the experiment gave reductions of about 3 dB(A) at the façades of the nearby houses. The subjective impressions of the occupants showed a markedly favourable overall reaction to the change from the wooden fence to the experimental noise barrier.A reduction in the variability of the received noise levels from the partially screened situation without a barrier to the screened situation was found. There was no significant increase in noise levels for positions on the opposite side of the motorway.     

An active noise barrier with unidirectional secondary sources

An active noise barrier with unidirectional secondary sources is investigated in this paper, where the unidirectional secondary source consists of two closely located loudspeakers with pre-adjusted phase difference. The secondary sound field of the unidirectional sources is adjusted to maximally match the primary sound field in the shadow zone behind the barrier. It is shown both numerically and experimentally that the noise reduction performance of the active noise barrier can be improved remarkably by replacing monopoles with the unidirectional sources. The mechanism for the improvement is also investigated.     

有源声屏障中误差传感器的位置优化

有源声屏障利用有源控制系统提高声屏障低频段的降噪效果。有源控制系统中误差传感器的位置对整个系统的降噪效果有较大的影响。通过数值模拟和实验研究误差传感器的位置优化问题,得出了有源控制系统中误差传感器摆放位置的两条结论:(1)所介绍的三种摆放中,误差传感器的位置在次级声源的正上方时,有源控制系统在屏障后方声影区引入的新增插入损失最好,特别是对于距离屏障较远的区域;(2)当误差传感器的位置在次级声源的正上方时,误差传感器与次级声源间的距离存在一个最优距离使得屏障后方声影区的衍射声得到最好的降低。     

The return wall as a traffic noise barrier

The attenuation of road traffic noise by a number of finite barrier configurations is investigated using scale modelling and computer simulation. It is found that the accepted UK prediction technique gives a consistently conservative estimate of barrier performance. It is shown that the use of a return wall as a noise barrier can be advantageous in certain circumstances.     

Performance of a noise barrier within an enclosed space

The present study involved experimental, theoretical, and numerical analyses of the insertion loss provided by rigid noise barriers in an enclosed space. The existing classical diffuse-field theory may be unable to predict the actual sound pressure level distribution and barrier insertion loss for indoor applications. Although predictions made by the ray tracing method at high frequencies are reasonably satisfactory, the method is computer-intensive and time-consuming. We propose a new formula that incorporates the effects of diffraction theory and the reflection of sound between room surfaces. Our results indicate that the present formula provides more realistic and practical predictions of the barrier insertion loss than existing approaches.     

Equivalent level of railroad noise with and without barrier

This paper presents models for predicting the equivalent level, $\text{L}{}_{\mathrm{<mtext>eq</mtext>}}$ of railroad noise. For open terrain two cases are considered: with and without ground effects (second section). On the assumption that, at a given point, $\text{L}{}_{\mathrm{<mtext>eq</mtext>}}\text{= L}{}_{\mathrm{<mtext>eq</mtext>}}{}^{\mathrm{*}}$ is required, the problems of barrier length, height and position are discussed (third section). Examples of the application of presented theories are given.     

Investigation of a single barrier discharge in submillimeter air gaps. Nonuniform field

Pulse characteristics of single barrier discharges as well as parameters of charges accumulated on the surface of a dielectric under the atmospheric pressure in the “needle-(0.1–2.0)-mm air gap-polymer barrier-plane” system are investigated. It is found experimentally that for the positive polarity of the needle, the voltage for the discharge initiation is higher than in the case of the negative polarity by ～25–35%. The reversal of the needle polarity from negative to positive increases the amplitude of the discharge current and the accumulated surface charge by ～1.5–3 times. For the positive polarity of the needle, the discharge is governed by a streamer mechanism, while for the negative polarity, the discharge is initiated by the formation of a single Trichel pulse. The single pulse regime is observed for the discharge current up to a certain electrode gap d _CR. For the positive needle and for air gap width d _air > d _CR ≈ 1.5 mm, a multipulse burst corona is formed, while for the negative needle and d _air > d _CR ≈ 0.9 mm, a damped sequence of Trichel pulses evolves in the system.     

Active noise control with linear control source and sensor arrays for a noise barrier

A study is conducted on minimizing the sum of the squared acoustic pressures with a linear array of control sources and a perpendicular linear array of error sensors, placed above the top of a noise barrier. Particular angular orientations, with respect to the center of the barrier top, and spacings of the linear arrays of control sources and error sensors result in moderate to significant additional reduction of the acoustic pressure in the shadow zone. Visual inspection of the sound pressure field, with and without active noise control, found that uniform and significant additional insertion loss can be generated near the barrier. Numerical simulations were conducted to test the proposed method. For separations between control sources and error sensors much less than a quarter wavelength of the primary noise disturbance, results show that the angular orientation, of the combined linear control source and sensor arrays, is a weak factor for acoustic pressure reduction in the shadow zone. Weak angle dependence serves as an advantage to the proposed method, which yields uniform performance for any angular orientation. An angular orientation involving the alignment of the furthest error sensor with the first diffracting edge of the barrier and the primary source was observed to perform well for a variety of frequencies, since the spacing between error sensors and between control sources is of the order of a quarter-wavelength. Improved noise control in the shadow zone of a barrier is achieved by the use of two control sources and angular orientation as mentioned above. Further spatial extension of the area of reduced acoustic pressure is possible by utilizing an increased number of control sources.     

Highway noise barrier location for maximum benefit/cost

A benefit/cost analysis of noise barrier location was conducted, which suggests that (a) for flat terrain, or terrain in which the roadway and receivers are equally elevated but separated by a ditch or other depression, the best barrier location is near the receiver, (b) for roadways raised above the right-of-way by 4 ft (1·2 m) or more, the best barrier location is probably near the highway, but this should be verified by detail design, (c) for a depressed roadway, the best barrier location is near the receiver, and (d) for terrain in which the roadway and receivers are separated by more elevated terrain, barrier location must be studied in detail—for elevations of the intermediate terrain that exceed the grade elevation by 2 ft (0·6 m) or so, the best barrier location may be the raised intermediate point.     

Detection of partially filled gaps in noise and the temporal modulation transfer function

Results of experiments on the detection of silent intervals, or gaps, in broadband noise are reported for normal-hearing listeners. In some preliminary experiments, a gap threshold of about 2 ms was measured. This value was independent of the duration of the noise burst, variation of the noise level on each presentation, or the temporal position of the gap within the noise burst. In the main experiments, the thresholds for partial decrements in the noise waveform as well as brief increments were determined. As predicted by a model that assumes a single fixed peak-to-valley detection ratio, thresholds for increments are slightly higher than thresholds for decrements when the signal is measured as the change in rms noise level. A first-order model describes the temporal properties of the auditory system as a low-pass filter with a 7- to 8-ms time constant. Temporal modulation transfer functions were determined for the same subjects, and the estimated temporal parameters agreed well with those estimated from the gap detection data. More detailed modeling was carried out by simulating Viemeister's three-stage temporal model. Simulations, using an initial stage bandwidth of 4000 Hz and a 3-ms time constant for the low-pass filter, generate data that are very similar to those obtained from human subjects in both modulation and gap detection.     

An example of in-plant noise reduction with an acoustical barrier

Existing theories for predicting the distribution of sound intensity in rooms and the performance of acoustical barriers are applied in designing a barrier to reduce noise in an industrial environment. The combination of the theories is found to predict the sound pressure level in the barrier shadow zone reasonably well.     

Investigation of a single dielectric barrier discharge in submillimeter air gaps: Uniform field

The characteristics of a single discharge in 0.01–0.2-cm air gaps are investigated under atmospheric pressure between the metal electrodes and in the presence of a polymer barrier on one of the electrodes. The experimentally determined surface charge density distributions are satisfactorily described by a 2D Gaussian function with the proposed parameters. In the uniform field, the breakdown voltage weakly depends on the presence of a dielectric barrier. At the same time, the change from the positive to negative polarity of the metal electrode leads to a manifold increase in the discharge current and in the surface charge. The breakdown occurs under considerable overvoltage; the estimates indicate the streamer mechanism of the breakdown.     

Effect of Schottky-diode barrier capacitance on low-frequency noise of microwave detector

It is shown experimentally and theoretically that low-frquency-noise sources localized in the transition region and in the base resistance of a Schottky diode change their contribution to the retified-current noise by an order of magnitude or more under the influence of the barrier capacitance: the more the barrier capacitance shunts the junction at the high frequency, the higher the detector's noise level. Fluctuations of the barrier capacitance caused by fluctuations of the electron-trap occupation in the space-charge region do not manifest themselves in the noise of the studied detectors.St. Petersburg State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 5, pp. 407–420, May, 1992.