摘要:

A large‐display sound‐level meter has been constructed as an experiment in public education. The indicator is an array of lights so arranged that the sound level produced by a source is displayed as an illuminated column. In a roadside installation, motorists see a thermometer‐like display ahead and can compare the level produced by their own vehicles with the maximum level permitted by legislation, as shown by vehicle silhouettes. The device registers from 70–100 dBAin 2‐dBAsteps with an accuracy approximately equal to one increment of the scale.

ISSN:0001-4966    

DOI:10.1121/1.1913031

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The city is treated as a plane surface with many identical sound sources (motor vehicles) randomly distributed over its area. The mean energy density at any point in the plane is expressed in terms of the individual source strength, the average number of sources per unit areaN, the atmospheric absorption constant α, and a shielding factorFassociated primarily with obstacles in the transmission path. To obtain the steady‐state (median) energy density, a central cell containing a single discrete “local” vehicle is identified and treated separately from the rest of the distribution. Graphs and tables of steady‐state level and energy density as functions ofNand α are given for the homogeneous infinite city, the city of finite size, and the traffic‐free zone within a city. The theory indicates that the spreading of urban noise is determined by a characteristic distance with a typical value of 0.25 km. The observed octave‐band sound‐pressure levels from 31.5 to 4000 Hz at one location in Ottawa are compared with calculated levels based on statistical data for vehicle source strength, estimates of vehicle density, and known atmospheric absorption constants. The differences are consistent with a shielding factor of 15 dB which is in good agreement with measurements of sound transmission in urban areas reported by others. The shielding factor has an effective value substantially independent of frequency.

ISSN:0001-4966    

DOI:10.1121/1.1913032

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Measurements were made in order to evaluate the pressure level of sound generated by a grille in an airflow. A streamlined probe microphone, which is covered by a solidified glass fiber of high density, was used for detection of noise existing in the airflow. Distributions in a flow duct of the turbulence‐fluctuated pressure of the airflow and of the output level of the microphone were measured and then compared with sound transmission characteristics specified in the duct. It was found that the distribution of the microphone output agrees with the sound transmission characteristics, but not with the fluctuating pressure of the airflow. As a result, the levels of the sound pressure generated by a grille were about 50 dB (re2 × 10−4μbar) at frequencies greater than 200 Hz, with a maximum value of 65 dB at 1250 Hz, for a flow velocity of 8.4 m/sec. It was also found that the output of the probe microphone produced by the turbulence‐fluctuated pressure of the airflow can be reduced to 40 dB below the turbulent pressure level by a covering of this type.

ISSN:0001-4966    

DOI:10.1121/1.1913033

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The paper deals with the problem of active sonar signal design for best range‐estimation accuracy at a time not necessarily coincident with the instant of measurement. Thus, velocity as well as range information, is important. For a single sonar ping a matrix of second‐order statistics of range and velocity errors is obtained for the case of ambient and/or reverberation noise interference. These error statistics are related to a generalized signal‐to‐noise ratio, and simple expressions are obtained for their relative magnitudes. For a particular type of linear FM signal it is shown that these relative magnitudes are independent of target or scatterer motion and of the relative importance of reverberation and ambient noise. The results are extended to the case of several sonar pings separated in time, and simple expressions are obtained for the over‐all error statistics. If two pings are to be used, it is shown that in many cases the first ping should be designed simply to measure target velocity, while the design of the second ping should make use of this velocity measurement and strive at an accurate measurement of range.

ISSN:0001-4966    

DOI:10.1121/1.1913034

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

An underwater acoustic surface‐reflection channel is treated as a linear random time‐varying filter. Thus complete characterization of the channel reduces to a statistical description of its stochastic system functions. This paper presents the results of measurements made in a model tank with a wind‐driven surface in terms of the channel impulse response, transfer function, bifrequency function, and their ensemble averages. The impulse response is taken as the primitive because it permits evaluation of the transfer and bifrequency functions without need to resort to multiple measurements at different frequencies. Both the time‐ and frequency‐spreading characteristics of the channel are clearly evident in the system function data. It is shown that, given a single set of impulse response measurements for each geometrical configuration of interest, it is an easy matter to study the frequency response and frequency‐spreading behavior of the channel over the bandwidth of the acoustic pulse. The impulse response itself, of course, yields the time‐spreading behavior of the channel directly. Furthermore, favorable comparisons are made between impulse response derived data, and data obtained by single‐frequency measurement techniques.

ISSN:0001-4966    

DOI:10.1121/1.1913035

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The normalized variance due to the finite integration time of instrumentation used for determining the space‐time average of the sound‐pressure level in a reverberant room is given by 19/BTI, in (dB)2, whereBis the bandwidth of noise andTIthe integration time of instrumentation. This result is smaller by a factor of 2 than an earlier result reported by Andres [Acustica16, 271–294 (1965). Transl.: Bolt, Beranek and Newman, TIR‐10 (1968)]. This factor may account for the difference between the result found by Andres and the results reported by Kurtovic [Ann. Telecommun.16, 254–261 (1961)], Schroeder [J. Acoust. Soc. Amer.46, 277–283 (1969)], and Lubman [J. Acoust. Soc. Amer.44, 1491–1502 (1968)] for the normalized variance of the sound‐pressure level.

ISSN:0001-4966    

DOI:10.1121/1.1913036

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1913037

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1913038

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1913040

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP