An upward looking 11-in. circular piston source in water was used to obtain a series of pressure measurements in air and water for the frequency range of 15–40 kHz. Transmission across the air-water interface showed the expected loss of 65.4 dB ± 0.5 dB for local conditions of temperature and barometric pressure. Horizontal scans across the acoustic axis showed that the beam pattern of the circular piston could be faithfully reproduced in air as well as in water. However, measurements along the acoustic axis in air departed significantly from the predictions of geometric acoustics. These results will be compared with a numerical solution of the Helmholtz equation for a distribution of sources along the surface of the water. [Work Supported by Office of Naval Research.]

Experimental measurements of the directivity patterns of an underwater sound projector operating at a frequency of 38 kHz were made with a hydrophone in the water and with a microphone in air above the water surface. Three different grazing angles of the projector beam with the water surface were used. The measured patterns in air and in water agree very closly down to the level of the second sidelobes. Making the measurements in air eliminates the problem of a surface reflection for a hydrophone near the air-water interface, allowing source level measurement without interference effects. The price for this freedom is more than 65-dB transmission loss through the interface. The measurement method is thus restricted to cases in which the source level is high enough to allow a sufficient signal-to-noise ratio at the microphone after the transmission loss. [This work was sponsored by the Office of Naval Research.]

The spreading-loss portion of the total propagation loss of sound reflected or scattered from a rough ocean bottom is not firmly established in the literature. Many investigators of bottom backscattering assume, for non-normal incidence, spherical spreading both from the source to the bottom and for the return path; this procedure yields a spreading-loss correction term of −40 logR (where R is the one-way range in yards). At normal incidence, the mirror-image solution is generally assumed for bottoms which are not too rough relative to the wavelength, resulting in a spreading loss term of −20 log (2R). To clarify the actual range dependence of the returning echo, experimental measurements were made in a laboratory tank on the echo from randomly rough model surfaces having known statistical properties. Data are available for normal incidence at 100 kHz. The echo from a plane surface was checked and confirmed to result in mirror-image behavior, with a spreading loss of 6 dB over the return path. The echo from a random rough surface having surface irregularities comparable to a wavelength fluctuated greatly with range and with small changes in the particular area of surface insonified, but an average of many range dependence curves shows an echo-amplitude decay rate of 13.5 dB for a tenfold increase in range, beginning about 2 ft from the reflecting surface.

Experimental measurements are presented for the target strength and directivity patterns of the reflections from stainless steel spherical shells filled with a mixture of carbon tetrachloride and freon (F113) in the frequency range 30–130 kHz. Five spheres were tested having nominal outside diameters of 6, 7, 10, 12, and 18 in. with varying wall thicknesses. Target strength measurements were made separately on echo components arising from reflections from the front and rear surfaces of the sphere; the focused rear-surface reflections showed target strengths up to 21 dB higher than those corresponding to the front surface, controlled by the sphere wall material and thickness, and by the index of refraction of the fluid. Subject Classification: [43]30.30.

Acoustic measurements of insertion loss and echo reduction at normal incidence were made for several materials for evaluation of their use as underwater sound transducer windows in the frequency range of 50–500 kHz. Materials tested included the commercial polyurethane products such as PR-1527, CPC-19, Scotchcast 221, Scotchcast 8, and some of these materials with talc added during curing to vary the density. Values of sound velocity are derived from the echo reduction data and allow determination of ρc values for each material. A computer model of the echo variation with frequency accurately matches the measured echo reduction plots.

In many acoustic systems, the aperture dimensions of the projecting and receiving transducers are comparable. The variation of received signal level with separation distance along the acoustic axis in the transition region between near- and farfields has been studied by several workers, but apparently little has been published on the composite directivity patterns for this case. This paper presents the results of an experimental investigation of the composite directivity patterns for a series of projector-hydrophone pairs of various ratios of apertures as a function of range from the nearfield to the farfield. Relevance of the results to several applications, including transducer calibration, acoustic communications, and range determination, is discussed. Criteria for separating the nearfield and farfield regions for both axial pressure level variation and directivity patterns are discussed. [This work was sponsored by the U. S. Navy Department.]

Experimentally recorded farfield echoes from linear frequency-modulated pulses incident on precision aluminum spheres in water are compared with computed echo forms. Good agreement was found using different pulse lengths and swept bandwidths. Spheres having ratios of internal to external radius of 0.95, 0.8, and 0.0 were studied, with either air or water in the hollow interior. Multiple periodic secondary echo components observed under some pulse conditions are interpreted in terms of circumferential waves and internal reflections.

Acoustic measurements have been made on two models of a new jet engine noise suppressor, using J-57 engines in tests at Kelly Air Force Base, Texas, and Miramar Naval Air Station, California. Data are presented showing noise reduction of 10 to 30 db for the engine at military power.

Field measurements of airborne sound-transmission loss involve several practical difficulties and limitations as compared to the laboratory standard test procedures for such measurements. Data from field measurements are valuable to architects and acoustical consultants, in spite of these limitations. A workable procedure for field measurements is described, along with a discussion of the difficulties. Measured transmission loss data are presented for double plaster walls on cork isolation, double plaster walls on concrete slabs, double concrete block walls, double brick walls, single concrete block walls, and a floor slab with resiliently suspended plaster ceiling.

Measurements of the effectiveness of simple expansion chamber filters in reducing low-frequency fluid-borne noise transmission in a water-filled piping system are presented and compared with theoretical design curves. Values of attenuation comparable to or greater than those predicted by plane wave theory for gaseous flow mufflers were found. The configurations tested included single chambers with lengths from 2 to 10 ft and double-section filters with chamber lengths of 5 and 7 ft. An area expansion ratio of 14 was used for all the tests in a piping system built of 2.73-in. i.d. tubing. Maximum attenuations of 20 db or more were measured for single chambers and 30 to 40 db for double-section filters. The effects of standing waves in the piping at either end of the filter are considered. Measurements were made both with and without water flow in the system.

A 3700-seat, general purpose auditorium was included in the convention center for the City of Austin, Texas. The hall is designed with removable, upholstered seats on the main floor, with the hall opening out into the exhibition area so that the auditorium proper can be converted into a dining area for 5000 people. The acoustical problems in this type of all-purpose design are discussed and measurements presented on the reverberation time, echo structure, and articulation in the building. Since the auditorium is also used as a concert hall, subjective remarks by various musicians using the building also will be presented. Some particular problems in the use of a large auditorium by Broadway theater casts will also be discussed.

Acoustic filters are being developed for attenuating liquid-borne noise in piping systems. The filters investigated are analogous to RLC type electrical filters, using the series inertances of short lengths of piping together with lumped impedance shunt elements to produce low-pass filtering action. The construction of several 2- and 3-stage experimental models is shown, along with noise reduction data measured in a laboratory water pumping system using a centrifugal pump as the noise source. (This work was supported by the Bureau of Ships.)

Acoustical measurements and design considerations are discussed for two churches with circular floor plans. One was carefully planned with the acoustics in mind and has proved successful. The other was built with a parabolic concrete dome; a thin sprayed absorbent lining inside the dome was counted on to eliminate sound focusing effects. Echograms taken at several positions inside the church illustrate the resulting severe echo problems.

Measured values of noise reduction achieved between classrooms, teaching studios, and practice rooms for four new music buildings are presented. Data on double plaster walls set on cork isolation, and on concrete alone are presented, along with data on double and single concrete block walls.

Acoustic measurements were made in the classrooms of twenty-two different school buildings in a study sponsored by the Acoustical Materials Association. Interior acoustical treatments in the rooms included full ceiling treatment, peripheral treatment, and no treatment. Measurements included reverberation time, definition, percentage articulation, transmission loss and noise reduction between rooms, followed by a questionnaire survey of the teachers who use the classrooms. The questionnaire returns indicated some interesting teacher impressions which will be reported, although not enough teachers were polled to treat their answers in a statistical manner.

Data are presented showing room noise levels from residential air-conditioning equipment measured in a number of homes in Austin. The measurements cover a range of equipments of different types, mountings, cost, and age.

Acoustical measurements are presented showing transmission loss achieved between classrooms in several public schools of very recent design in the Southwest U.S.A. Several types of ventilating systems employing natural air circulation only were measured and their effectiveness evaluated. Minimum isolation requirements for public school classrooms are discussed.

Acoustical measurements are presented on teaching studios and practice rooms of several music buildings located at four southwestern universities. Data were taken on a wide range of wall construction and recommendations on room response and acoustical isolation between rooms are presented.