摘要:

A wide range of artificial impulsive‐type signals was measured with an impulse sound level meter to determine the relationship between the known (energy) average signal level and the observed meter reading for various settings of the meter time constant and frequency weighting network. The artificial signal consisted of tone bursts superimposed on continuous background noise. The tone bursts lasted from 1 to 100 msec, were repeated in intervals from 2 to 2000 msec and had an rms level 10–50 dB above that of the random background noise. The latter was either pink noise or was shaped to resemble typical community noise. For impulse repetition rates greater than 1 pulse/sec, the impulse sound level meter reading on the average within ±1.3 dB of the energy average signal level, provided the meter was set onA‐weighting “slow,” the upper or maximum meter deflection was used, and this meter reading fell on the lower parts of the meter scale. Errors for lower pulse rates are also discussed in this paper. [This work was supported by the U. S. Environment Protection Agency.]

ISSN:0001-4966    

DOI:10.1121/1.2003389

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Subway fan and vent shafts can be significant sources of noise impact to both the adjacent community and to patrons in the subway stations. For fan shafts, the transmission of airborne noise through the shafts and surface openings to adjacent buildings and the transmission of noise through the subways to station platform are intrusive. For vent shafts airborne noise transmissed from trains passing by in the subway below to surface openings can be intrusive. The primary methods available for the reduction of fan and train noise propagated out shaft openings and fan noise propagated into the stations are (1) the use of acoustical absorption material on the interior surfaces of fan rooms, fan and vent shafts and tunnel walls and ceilings, (2) the use of sound attenuators [silencers] attached to the fans, (3) the use of specially constructed splitters in the fan and vent shafts or fan rooms, and (4) the use of acoustical louvers.

ISSN:0001-4966    

DOI:10.1121/1.2003394

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Ultrasonic velocity and absorption measurements have been made as a function of magnetic field in both single crystal and polycrystallineTb0.3Dy0.7Fe2. This highly magnetostrictive material exhibits a huge ΔEeffect as previously reported at sonic frequencies. [A. E. Clark and H. T. Savage, IEEE Trans. Sonics Ultrason. SU‐22, 50–52 (1975).] ΔEis the difference in Young's modulus between the magnetized and the unmagnetized stateE0. In a polycrystalline bar sample, we measured an increased extensional velocity as a function of applied field (to 6 KOe) which corresponded to a ΔE/E0of nearly 100%. A peak in the low field relative attenuation, with a corresponding velocity minimum, has been observed. These measurements were made at room temperature over a frequency range of 0.05 to 0.25 MHz. Bulk shear and longitudinal velocity measurements have been made from 1 to 21 MHz in both single crystal and polycrystalline samples. Measurements of sound propagation along [110] show that the crystal is elastically isotropic. Taking the density to be 9.25 g/cm2, at 12 MHz we calculateC11= 11.7,12(C11 − C12) = 3.84, andC44 = 3.84(× 1011 dyn/cm2). [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2003403

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Previous calculations of elastic wave reflections by cavities in solid materials have not included the effects of wave attenuation even for materials known to exhibit significant losses. Calculation without losses indicate the asymptotic approach of the scattering to a limiting value at someka. The inclusion of losses modifies this behavior. Cases will be shown for reflections belowka= 10 and for materials with small and large losses.

ISSN:0001-4966    

DOI:10.1121/1.2003407

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The natural way for a disturbance to propagate over the surface of a smoothly curved, fluid‐loaded elastic body is in the form of a series of damped circumferential (creeping) waves. Mathematically, the process is most conveniently described by a sum of normal modes, each characterized by a wavelength that fits the body's circumference an integer number of times. We demonstrate that any such mode will resonate at all those “eigenfrequencies” where the mode velocity coincides with the speed of one of the creeping waves. For an elastic cylinder, the 180° sound‐scattering amplitude is shown to possess marked minima at many of the eigenfrequencies, whose spacing over a sequence of modes thus determines the group velocities of circumferential pulsed signals. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2003413

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Experimental measurements have been made of the acoustic field produced by sound scattered at the brass‐water interface of an echelette grating. Theoretical calculation of the reflection coefficients for the various orders of scattered waves have been completed by J. DeSanto. Schlieren photographs are used to indicate the angular dependence of the scattered field and hydrophone measurements of the acoustic field are compared with the theoretical model.

ISSN:0001-4966    

DOI:10.1121/1.2003416

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Experimental studies by independent investigators have indicated that low‐frequency acoustic propagation can be affected by ocean‐bottom refracted energy. However, the total energy returned from the bottom is also composed of water‐sediment interface and sub‐bottom reflected energy and it is often times difficult to determine the significance of the sub‐bottom refracted energy. Deconvolution processing techniques applied to wide‐band explosive‐source experimental data have aided in time‐isolation of the various bottom‐interacted arrivals so that signal travel times could be determined. With the use of a ray‐tracing model which incorporated a sediment sound‐speed gradient, accurate identification of sub‐bottom refracted arrivals has been achieved. Spectral analyses of these data have further revealed that sub‐bottom refracted energy can exceed reflected energy by as much as 5–10 dB at frequencies below 500 Hz. This effect has been observed for diverse geographical areas of the North Atlantic (shallow basins, abyssal plains) where sediment structures vary considerably. As a consequence, acoustic properties of the ocean bottom cannot be correctly inferred at low frequencies by a Rayleigh reflectivity model. [Work supported by NAVSEA.]

ISSN:0001-4966    

DOI:10.1121/1.2003426

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

In 1972 the Acoustical Insulating Materials Association sponsored a program of research by Tracor, Inc., to develop guidelines for optimum design and use of open‐plan classrooms, particularly in regard to the effective use of acoustical absorption. This program utilized literature study, visits to open‐plan schools, and computer simulation of large rooms to develop performance criteria and procedures for evaluation. One very important conclusion from this study was that less than 10% of existing classrooms were considered acoustically excellent, and two‐thirds were just tolerable or worse. Fortunately, the features which distinguished the good rooms were conspicuous and definable. In 1975, the highly technical report from Tracor was rewritten in different language for use by teachers, school administrators, and architects. Engineering data has been reduced to simple tables, charts, and worksheets. Recommendations for accomodation to existing classrooms have also been incorporated. The Guide is now ready for publication and distribution.

ISSN:0001-4966    

DOI:10.1121/1.2003428

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The accuracy of transmission loss (TL) determinations depends in part upon the precision of measured noise reduction (NR). This is controlled by the spatial standard deviation of sound pressure level (σ,dB) in both source and receiving rooms. National Research Council is obtaining experimental results to test a theory proposed by Lubman and Bowers [J. Acoust. Soc. Am.44, 359(A), 1968] according to which σ is a joint property of the room and the source spectrum. A progress report is given, and measurements taken in one‐third octave bands are shown to be in reasonable agreement with this theory.

ISSN:0001-4966    

DOI:10.1121/1.2003438

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The area of low‐frequency sound sources has received considerable emphasis in the past several years. Of particular interest is the development of small, lightweight low‐frequency transducers that have broadband capability. One particular low‐frequency transducer that fits into this category is the PHD (post hole digger). The PHD is cylindrical in shape with a diameter of 0.3048 m, a length of 0.4572 m and has an in‐air weight of 36.3 kg. The driving mechanism for the PHD is a transverse variable reluctance magnetic motor. The unit must be pressure compensated and requires large input currents (15–30 amp) below 30 Hz. The PHD has recently been tested at Lake Pend Oreille to a depth of 61 m. The response of the unit was measured as a function of operating depth during the Pend Oreille tests. At the 61 m depth, the maximum source level that it was possible to obtain was 168 dBre1 μPa at 1 m. This limit was imposed by the amplifiers used during the tests and is not a limit of the transducer. [Work supported by NAVSEA 06H1.]

ISSN:0001-4966    

DOI:10.1121/1.2003445

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP