Band Random Noise Research Articles

Performing Active Noise Control and Acoustic Experiments Remotely

This paper presents a novel and advanced remotely controlled laboratory for conducting Active Noise Control (ANC), acoustic and Digital Signal Processing (DSP) experiments. The laboratory facility, recently developed by Blekinge Institute of Technology (BTH) Sweden, supports remote learning through internet covering beginners level such as simple experimental measurements to advanced users and even researchers such as algorithm development and their performance evaluation on DSP. The required software development for ANC algorithms and equipment control are carried out anywhere in the world remotely from an internet-connected client PC using a standard web browser. The paper describes in detail how ANC, acoustic and DSP experiments can be performed remotely The necessary steps involved in an ANC experiment such as validity of ANC, forward path estimation and active control applied to a broad band random noise [0-200Hz] in a ventilation duct will be described in detail. The limitations and challenges such as the forward path and nonlinearities pertinent to the remote laboratory setup will be described for the guidance of the user. Based on the acoustic properties of the ventilation duct some of the possible acoustic experiments such as mode shapes analysis and standing waves analysis etc. will also be discussed in the paper.

State transitions in the Duffing resonator excited by narrow band random noise

The Dufffing hardening spring resonator has the status of a canonical model for nonlinear vibrations. Over a limited range of excitation frequencies and depending on the degree of nonlinearity, it has three states of response to sinusoidal excitation, one of which is unstable. The system will remain stably in either of the other two states depending on the history of excitation: in a higher energy state for the frequency ascending and in the lower energy state for the frequency descending. When the sinusoidal excitation is replaced by narrow band random excitation, the author showed in a 1961 paper experimentally that the system could make transitions between these two states and argued that the fluctuating phase of the excitation would allow the source to inject or draw energy from the resonator allowing a transition from one state to the other. This presentation develops a dynamic model for the system that allows energy transmission between the source and resonator and an indicator of the state of response based on instantaneous impedance.

Computational bluff body aerodynamic noise prediction using a statistical approach

A new method for calculating the aerodynamic noise generated by bluff bodies is presented in this paper. The methodology uses two-dimensional, unsteady Reynolds averaged Navier Stokes turbulent flow simulations to calculate the acoustic source terms. To account for turbulent flow effects that are not resolved by the flow simulation, a statistical approach has been developed and applied to introduce narrow band random noise. Spanwise de-correlation of flow information is accounted for using a correction method based on a de-correlation length scale. Curle’s compact acoustic analogy is used to calculate the far-field noise. To illustrate the effectiveness of the method, the turbulent flow and noise about two test cases are calculated and compared with experimental results from the literature.

Target detection against narrow band noise backgrounds.

We studied the detectability of narrow band random noise targets embedded in narrow band random noise backgrounds as a function of differences in center frequency, spatial frequency bandwidth and orientation bandwidth between target and the immediately adjacent background. Unlike most target detection experiments the targets were not added to the background; they replaced the underlying background texture. Simulations showed that target detection probabilities could be accounted for by a simple transformation on the summed outputs of a two layer filter model similar to the complex channels model proposed by Graham, Beck and Sutter (Graham, N., Beck, J., & Sutter, A. (1992). Vision Research, 32, 719-743). Subsequently, the model was tested on the detection of camouflaged vehicle targets with encouraging results.

Roving-level tone-in-noise detection.

The detectability of tones, or of intensity increments to tones, in bands of random noise was measured for conditions in which the overall level was fixed or was randomly roved from interval to interval of every experimental trial. The purpose of the within-trial rove was to limit the usefulness of a detection strategy based on overall level or level within a single "critical band." At "supracritical" bandwidths, the functions relating masked threshold to noise bandwidth for the roved conditions were similar to those obtained when no rove was employed. At "subcritical" bandwidths, thresholds were higher in some roved conditions, but, for the largest rove, were still lower than would be predicted from arguments based purely on level detection--with one exception. A comparison of observer performance relative to the statistical limits imposed by the roving-level procedure indicated that the traditional critical-band energy-detector model could not account for the results, which are attributed to discrimination based on spectral shape or on waveshape.

Roving level tone in noise masking

The results of tone in noise detection experiments have long formed an empirical basis for the notion of the “critical band” and have led to the development of the “energy detection” model. In this study, the detectability of tones in bands of random noise was measured for conditions in which the overall level of the noise was roved randomly from interval to interval of each experimental trial. The functions relating masked tone threshold to noise bandwidth for the roved conditions were identical to those obtained when no rove was employed at “supracritical” bandwidths. At “subcritical” bandwidths, the detection thresholds were higher for the roved conditions, but were still lower than would be predicted from arguments based purely on energy detection. These results suggest that the traditional critical band and energy detection models should be modified to account for discrimination based on spectral shape or waveshape. [Work supported by NIH and NMRDC.]

The ensemble variance of random noise in a reverberation room

The literature contains conflicting results for the ensemble relative variance of the intensity of a frequency band of random noise which has been passed through a reverberation room. Some theoretical and experimental results suggest that passing a band of noise through a reverberation room has no effect on its ensemble relative variance. Other theoretical and experimental results show that the ensemble relative variance is doubled. This paper shows that the magnitude of the effect depends on the ratio of the integrating time of the detector to the integrating time of the reverberation room. If this ratio is small the reverberation room has no effect. If the ratio is large the ensemble relative variance is doubled. This explains why conflicting results have appeared in the literature.

Dependence of acoustic attenuation of hearing protectors on incident sound level.

The relationship between incident sound level and acoustic attenuation for four types of earplug and four types of earmuff have been investigated using freshly prepared and instrumented cadaver ears. Pure tones and 1/3-octave bands of random noise in the frequency range 125-8000 Hz were employed as steady-state stimuli with sound pressure levels between 75 and 125 dB. Impulses with peak sound levels in the range 135-175 dB(P) were also presented. For the steady-state signals employed, the eight hearing protectors have been shown to have constant attenuation characteristics over the range of incident sound levels investigated. This was also the case for the six conventional protectors (with intentionally linear characteristics) for the impulse stimuli. The two intentionally amplitude-sensitive protectors provided attenuation which increased with incident sound level for impulse noises. Comparison of the protector attenuation-frequency characteristics determined for steady-state sounds shows good agreement with those obtained from subjective (threshold shift) national standard measurement procedures. It may be concluded, therefore, that the six conventional hearing protectors studied here have attenuation characteristics that are equal for incident sound levels at about 40 and 75 dB, and that they are constant for levels between 75 and 175 dB. Consequently, the results of national standard threshold-shift procedures, although measured at low sound levels, may be applied with confidence to occupations where hazardous high-level noises are present.

Bubble-enhanced nonlinear sound generation

In a previous presentation [J. Acoust. Soc. Am. 55, S51(A) (1974)] it was shown that when two pure tones pass through cavitation microbubbles the nonlinear generation of difference frequency sound is greatly enhanced. In data presented here the primary frequency sound field directly drives a ring transducer and creates microbubbles via acoustic cavitation. Data on the variation of difference frequency SPL with difference frequency, primary frequency power, primary frequency, and ambient pressure are presented. Data for primaries consisting of two pure tones and a band of random noise are shown. In general the data agree with simple theoretical predictions only at the highest power levels where the bubble field is well established. Agreement with theory is best for the dependence of difference frequency SPL on difference frequency fs in the pure tone case and spectral shape in the band limited noise case. [Work supported by Naval Coastal Systems Laboratory.]

Loudness Discomfort Levels for Pure Tones and Noise

Two studies have been conducted to measure the loudness discomfort levels (LDLs) for subjects with normal hearing. First, the LDL for a 1000-Hz stimulus was measured utilizing the method of constant stimuli, the method of adjustment, and a tracking method. The method of constants provided the most reliable judgments among subjects over six trials. In the subsequent study, LDLs were measured utilizing the method of constants for pure tones at octave frequencies from 125 through 4000 Hz and for two bands of random noise. LDLs (in dB SPL) varied as a function of the stimulus employed. LDLs were highest at low frequencies (through 500 Hz) and relatively constant for 1000, 2000, and 4000 Hz and for the wide-band noise.

Masking and Discrimination

With the same experimental technique, two sets of related experiments have been performed. In one, a band of random noise is used to mask a pure tone. In the other experiment, the intensity difference limen for the band of noise has been determined. Thresholds for masking and discrimination were obtained with help of a Békésy audiometer. A large range of bandwidths (5–12 000 cps) and five central frequencies (500, 1000, 2000, 4000, and 8000 cps) have been employed. Both sets of data agree qualitatively with data appearing in the literature. From the discrimination data, it appears that, for small bandwidths, inherent intensity fluctuations of bands of noise determine the maximum intensity discrimination. These fluctuations should influence the masking situation to the same degree. The similarity of masking and discrimination thresholds in the region of small bandwidth lends support to this prediction. Thus, it is argued why masking data should not be judged in terms of a fixed standard (as Fletcher has done) in order to arrive at values for the critical bandwidth. If one judges masking data in terms of the discriminatory power that the ear exhibits for the masking noise employed, one arrives at critical-bandwidth data much more in line with generally accepted data. The accuracy with which these can be determined is so poor that one should consider masking experiments of this kind as totally unsuited to measure the critical bandwidth.

Some aspects of sweep random vibration

Following a brief description of the basic reasons for sine-wave and wide band random vibration testing some general arguments in favour of a new vibration test technique are presented. It is proposed that the new test may, in many cases, give a better simulation of the expected vibration environments than any of the two previously used tests. The new test, normally called sweep random vibration, consists in sweeping one or more bands of random noise through the frequency range of interest; the vibration level may be changed during the sweep according to any predetermined level vs. frequency test programme and the advantages of simple servo-controlled level regulation are utilized. Methods for reducing the test time as well as possible setting-up schedules are discussed. Due to the reduction in the requirements to power handling capacity of the vibration machine and power amplifier when the new test is specified, greater test specimens can be conveniently tested on existing laboratory test equipment than if wide band random noise is used.

Explicit form of f.m. distortion products with white-noise modulation

When a frequency-modulated wave passes through a network whose phase or amplitude characteristics vary non-linearly with frequency, distortion terms appear as both frequency and amplitude modulation of the output wave. If the characteristics are expressed as power series, these distortion terms appear, to first order, as products of powers of time derivatives of the unwanted frequency modulation. When the frequency modulation may be simulated by a band of random noise (as in multiplex telephony carrying large numbers of channels), the spectra of the distortion products can, in principle, be described by simple algebraic functions of the characteristics (i.e. the minimum and maximum frequencies and the r.m.s. frequency deviation) of the modulating noise band.Except in certain special cases, the derivation of these algebraic formulae by straightforward analytical methods becomes prohibitively tedious for distortions of order much above the second. However, once the formulae are found, the insertion of numerical values for particular cases is straightforward. In the present paper it is shown how the problem can be reduced to the repetition of a number of standard operations which can be carried out using a digital computer. The technique is illustrated by application to fourth-order distortion appearing in the amplitude modulation, generated by terms in the amplitude characteristic up to sixth degree. Even in such an apparently simple case as this it appears from the literature that the closed form of the distortion formula has not hitherto been obtained. This example is of direct practical interest since, for example, the amplitude characteristic of a maximally-flat-amplitude triple-tuned circuit is of sixth degree in the region around the midband frequency. With a minor modification, the resulting formula also applies to fourth-order distortion appearing in the frequency modulation, owing to terms in the phase characteristic up to sixth degree.Use is made of a discontinuous contour integral applied to a similar, but somewhat simpler, case by Bennett;1 a closely analogous course can be followed using the more recently developed theory of generalized functions,2 but in this particular problem the contourintegral method is more economical.Formulae for the various orders of distortion in the top channel due to amplifier and discriminator characteristics are given in tabular form.

Fluctuations of Scattered Noise

The coefficient of variation of scattered sound is shown to be a decreasing function of number of frequency components. The limit is found and the distribution of the rms value of scattered narrow band random noise is shown to be a Dirac delta function.

The measurement of random monochrome video interference

ONE OF the important factors in the design and maintenance of television transmission circuits is random interference, sometimes referred to as random noise. Much of this type of interference originates in the input stages of amplifiers but it may also originate in the line or in other parts of the transmission path. These noise components are greatly modified by circuit equalization which may vary from a few decibels (db) to possibly 27 or more db per octave. Thus the transmission engineer is confronted with the problem of evaluating random interference having different energy levels, per cycle, over the television bandwidth. Before noise evaluations can be made, information must be obtained by subjective tests on the interfering effect of broad, narrow, and mixed bands of random noise distributed throughout the television band.

The Output Signal-to-Noise Ratio of a Power-Law Device

An nth law device is fed by a sinusoidal signal and narrow band random noise whose spectrum lies close to the signal frequency. Its output spectrum is different from zero only in the vicinity of harmonics of this frequency. The ratio R′ of output signal power to output noise power in the vicinity of the mth harmonic is computed in terms of the input signal-to-noise power ratio R. In general, R′(R) involves a couple of hypergeometric functions, but for small R we have R′≅Γ2[(m+n)/2+1]Rm/(m!)2Γ(n+1) for m≠0, R′≅Γ2(12n+1)/[Γ(n+1)−Γ2(12n+1)] for m=0. For large R we have R′≅2R/(m2+n2).

The cardiogram in clinical diagnosis and prognosis. I. Theoretical considerations.

The relationship between incident sound level and acoustic attenuation for four types of earplug and four types of earmuff have been investigated using freshly prepared and instrumented cadaver ears. Pure tones and 1/3-octave bands of random noise in the frequency range 125-8000 Hz were employed as steady-state stimuli with sound pressure levels between 75 and 125 dB. Impulses with peak sound levels in the range 135-175 dB(P) were also presented. For the steady-state signals employed, the eight hearing protectors have been shown to have constant attenuation characteristics over the range of incident sound levels investigated. This was also the case for the six conventional protectors (with intentionally linear characteristics) for the impulse stimuli. The two intentionally amplitude-sensitive protectors provided attenuation which increased with incident sound level for impulse noises. Comparison of the protector attenuation-frequency characteristics determined for steady-state sounds shows good agreement with those obtained from subjective (threshold shift) national standard measurement procedures. It may be concluded, therefore, that the six conventional hearing protectors studied here have attenuation characteristics that are equal for incident sound levels at about 40 and 75 dB, and that they are constant for levels between 75 and 175 dB. Consequently, the results of national standard threshold-shift procedures, although measured at low sound levels, may be applied with confidence to occupations where hazardous high-level noises are present.