Volume 35, Issue 10, October 1963
Index of content:
35(1963); http://dx.doi.org/10.1121/1.1918734View Description Hide Description
The new civic auditorium in Jacksonville, Florida, consists of three major spaces: 3200‐seat auditorium, a 600‐seat theater, and 20 000 sq ft of exhibition space. The main auditorium will accommodate a variety of speech and music activities. It has a volume of one million cu ft and a design reverberation time (full) of 1.7 sec at midfrequencies. Essential to the acoustical design is a demountable plywood stage enclosure for orchestral concerts. The paper compares the predicted reverberation times for the auditorium with measurements obtained following completion of the building. The room‐acoustics design illustrates why some auditoriums require suspended sound‐reflecting panels for “first reflections” under a higher surface that forms the acoustical boundary of the space, providing the necessary volume.
35(1963); http://dx.doi.org/10.1121/1.1918736View Description Hide Description
Light coatings of the order of 0.002 g/cm2 to 0.04 g/cm2 were applied to open‐celled foams of 1‐in. thickness and 97% porosity. It was found that coatings of 0.002 g/cm2 improved the absorption over a range of frequencies extending from 480 to 1500 cps or 1980 cps. The improved absorption is attributed (in units of ρc) to the resistance approaching unity and/or the reactance approaching zero. The reactance becomes large at high frequencies with increasing coating mass per unit area, thereby causing the absorption at high frequencies to decrease. The improved absorption shifts to lower frequencies with increasing coating mass per unit area.
35(1963); http://dx.doi.org/10.1121/1.1918738View Description Hide Description
Differential thresholds for frequency were obtained from 20 normal hearing, but otherwise unselected, subjects using the method of constant stimulus differences for 6 hours. For each subject twelve difference limens (DL's) and constant errors (CE's) were calculated. The subjects were divided into two groups of ten each: group 1 received feedback, or immediate knowledge of results, in hours 1, 2, 4, and 6; group 2 received feedback in hours 2, 4, and 6. Group 1 had considerably larger DL's and CE's throughout the six hours than did group 2. Both groups showed smaller DL's the last hour (median = 4.2 cps) than on the first (7.3 cps), with no indication that learning was complete. CE's became less variable with practice, but the median CE (−1.0 cps the first hour and −0.8 cps the sixth) showed essentially no change. The effects of feedback in the last four hours were slight and largely obscured by variability and the general improvement with practice.
35(1963); http://dx.doi.org/10.1121/1.1918740View Description Hide Description
A damage risk criterion for exposure to noise is specified primarily in terms of the impairment of speech communication as the result of noise‐induced deafness. The criterion allows as permissible any noise exposure that, on the average, causes a temporary threshold shift measured 2 min after exposure, in the average normal ear, of no more than 10 dB at 1000 cps or below; 15 dB at 2000 cps; or 20 dB at 3000 cps or above. It is held that after 10 years of near daily exposure to such a noise, the average person will have a stable hearing level, relative to present‐day audiometric zero, of a similar magnitude. To determine what kinds of sounds and durations of exposure would meet this criterion, twelve ears were exposed to bands of filtered, random noise for durations ranging from 3 min to 8 h. Temporary threshold shifts for pure tones were obtained by comparing pre‐ and post‐exposure audiograms. From these data functions were obtained which show the sound‐pressure level, durations, bandwidth, and center frequency of bands of noise that cause, on the average, temporary auditory fatigue equal to the maximum allowed by the aforementioned criterion. The critical bandwidth for noise with respect to auditory fatigue appears to be slightly wider than for the frequency region covered in these tests.
35(1963); http://dx.doi.org/10.1121/1.1918742View Description Hide Description
The transfer function of the middle ear was investigated in anesthetized cats. The amplitude and phase angle of vibration of the malleus, incus, and round window for constant sound‐pressure level at the eardrum was measured by a capacitive probe. The acoustic impedance at the eardrum in the frequency range from 200 to 8000 cps was also measured. The acoustic impedance at the eardrum was found to be proportional to the ratio of sound pressure at the eardrum and velocity of the malleus from about 200 cps to 4 kc/sec, showing that the eardrum acts as a rigid piston in this frequency region. By comparing the amplitude of the malleus and that of the round window, it was found that the coupling between the malleus and the cochlear fluid is very rigid. A second‐order low‐pass function described by its Laplace transform was found to be a good approximation to the experimental data, up to a frequency of about 4 kc/sec, or approximately 4 times the cutoff frequency. In the frequency range where the eardrum acts as a rigid piston, it is thus possible to estimate the transfer function of the middle ear entirely from the acoustic impedance data in the plane of the eardrum.
35(1963); http://dx.doi.org/10.1121/1.1918744View Description Hide Description
A technique of tympanomanometry developed for study of the aural reflex was adapted for use as an objective psychophysiological method for measuring attenuation of ear protectors in high‐level sound. Four representative ear protectors (an insert, a semi‐insert, a small‐volume muff, and a large‐volume muff) were evaluated by this aural‐reflex method and by ASA REAT method. Attenuation was designated in both instances as the mean difference between threshold values with and without ear protection (threshold shift). The aural reflex with ear protection was measured only for frequencies ranging from 400 to 4000 cps due to limitations of the signal source. A comparison of the results shows that, for the frequency range used, the aural‐reflex threshold‐shift method gives attenuation data comparing favorably with other above threshold data. The absolute‐detection threshold‐shift method indicates somewhat greater attenuation than does the aural‐reflex threshold shift, the differences varying both with the type of ear protector and as a function of frequency. The results suggest that a single‐value correction factor for adjusting standard threshold‐shift attenuation data to estimate attenuation in high‐level sound may not be appropriate.
35(1963); http://dx.doi.org/10.1121/1.1918746View Description Hide Description
Changing the time separation between two trains of pulses of identical rate produces a tone whose pitch is correlated with the magnitude of the time separation. Evidence has been advanced against the notion that the time difference tone is due to a neural time‐analyzing process. Experiments were made to determine the validity of this evidence by comparing the results of pitch and lateralization phenomena produced by filtered and unfiltered pulses of different polarities. A close agreement was found to exist between the results of the pitch experiments and those made by Flanagan on lateralization of clicks. The possibility of explaining the pitch phenomena in terms of a neural time pattern is considered, and the evidence against this hypothesis found to be inconclusive.
35(1963); http://dx.doi.org/10.1121/1.1918748View Description Hide Description
An experimental study has shown that reciprocity couplers having the cross‐sectional shape of certain irregular polygons are superior to those of circular cross section. The upper limit of microphonecalibration (for type L standard microphones) without correction for wave motion in a circular cavity is about 6 kc/sec. In a cyclic pentagonal cavity having sides of 1.02, 0.61, 0.89, 0.42, and 0.75 in., respectively, and between 0.06 and 0.1 in. thick, the effects of wave motion are less than ±0.2 dB up to 10 kc/sec. Polygons of 3, 4, 5, and 6 sides were investigated, but it appears that a pentagon represents the optimum compromise between several conflicting requirements to be satisfied by the design of the cavity.
35(1963); http://dx.doi.org/10.1121/1.1918750View Description Hide Description
A taut wire, vibrating freely in a magnetic field, generates an emf. If current is allowed to flow as a result, it damps the vibratory motion. On the other hand, vibration can be caused by an applied ac current. Then, large changes in the imaginary component of the complex impedance are noted when the frequency approaches an odd harmonic of the wife's mechanical resonance. These effects have been measured and found in agreement with a recently proposed theory.
35(1963); http://dx.doi.org/10.1121/1.1918752View Description Hide Description
The acoustic power/cps radiated by a centrifugal blower may be written in the form , where D is the impeller diameter, N is the impeller speed, Q is the volume flow rate, f is the frequency of the radiated sound, and ρ and c are the fluid density and speed of sound, respectively. The form of the function F has been determined from measurements on three small centrifugal blowers that are dimensionally similar. The power radiated above some lower cutoff frequency (fa ) may be written , where is a function derived from the function F, and M is the tip‐speed Mach number, πDN/c.
The results of the analysis are compared with the sound laws for fans.
35(1963); http://dx.doi.org/10.1121/1.1918754View Description Hide Description
The velocity of sound was measured in a sample of 0.65 centistoke Dow Corning 200 Fluid. The measurements were made in an ultrasonic velocimeter, utilizing a pulse technique at a frequency of 5 Me/sec. The method of analyzing the data is given together with tables and graphs.
35(1963); http://dx.doi.org/10.1121/1.1918756View Description Hide Description
The ultrasonic absorption and velocity in liquid mercury have been measured at 90, 150, and 270 Mc/sec over a temperature range from 25° to 130°C. No variation of absorption with frequency was found within experimental accuracy. The classical value has also been computed over this range of temperatures. It has been found that the measured value exceeds the classical value, and the difference is ascribed to volume viscosity. The most probable value of η v /η s over this range was found to be approximately 0.45.
35(1963); http://dx.doi.org/10.1121/1.1918758View Description Hide Description
The attenuation of sound between 12 and 75 Mc/sec was measured in liquid sodium and liquid potassium from their melting points to 150°C. The measuredattenuation in potassium was about 10% in excess of the calculated classical value. No excess absorption was observed in sodium, and an upper limit was placed on a possible excess absorption of about 5% of the classical value.
35(1963); http://dx.doi.org/10.1121/1.1918760View Description Hide Description
Sound cavitation is due to nuclei existing in the liquid. When they are created in water, they live for a long time, and therefore their equilibrium concentration is much larger than the number of nuclei created in the time unit. Experimental investigation allows measurement of the number of nuclei present in water as a function of the sound pressure needed for their growth to visibility. The statistical approach to cavitation, through the determination of the distribution functions, gives a deeper insight into the processes involved, such as nucleation by penetrating radiation. It is also possible to explain the discrepancies of values of cavitation threshold found under different experimental conditions.
35(1963); http://dx.doi.org/10.1121/1.1918762View Description Hide Description
A study is made to determine the frequency and concentration dependence of the ultrasonic absorption in aqueous suspensions of (1) scenedesmus and diatoms, two forms of algae, and (2) artemia(brine shrimp) eggs. The measurements were made over a frequency range of 125 kc/sec to 55 Mc/sec.
The excess absorption caused by scenedesmus is found to be linearly dependent on concentration and has a value of for the highest concentration measured, 0.72 g/liter of water. The excess absorption caused by diatoms is also linearly dependent on concentration and has a value of 25×10−15 m−1 sec2 for 290 g/liter of water, the highest concentration measured. For both scenedesmus and diatoms, it is concluded that the normal concentration of these forms of algae found in large bodies of water causes negligible absorption compared to the absorption of the water itself.
The major portion of the excess attenuation due to the shrimp eggs in the kilocycle region is attributed to a scattering mechanism. Application of a theory for elasticscattering in this wavelength region (ka<1) gave good agreement with experimental results. A phase‐velocity investigation of a resonance phenomenon at 7 Mc/sec revealed a characteristic velocity dispersion.
35(1963); http://dx.doi.org/10.1121/1.1918764View Description Hide Description
Using integral equations, the scattered field due to an acoustic wave impinging on an arbitrarily shaped cylinder is calculated. The equations are solved by finite difference methods, and the results are compared with those obtained from an eigenfunction expansion for the scatteredwave from a right circular cylinder. Both nearfield and farfield results are presented for other cross sections.
35(1963); http://dx.doi.org/10.1121/1.1918766View Description Hide Description
The dispersive characteristics of axially symmetric waves in infinitely long, composite,elastic rods are explored. The characteristic equation relating frequency to the propagation constant is derived for two‐material rods in which a solid, circular core of one material is bounded by and bonded to a circular casing of a second material of different physical properties. The equation is explored numerically using a digital computer for two rods displaying different geometric and physical properties. The lowest few branches of frequency spectra are developed from the roots of the equation, and the dispersive properties of the individual modes shown are discussed. The lowest few cutoff frequencies are established for each of the two rods, and the radial displacement distribution for each frequency is established. Finally, conditions are established for the existence of equivoluminal modes similar to those shown to exist in plates by Lamé and Lamb, in solid rods by Onoe, McNiven, and Mindlin, and in hollow cylinders by Gazis.
35(1963); http://dx.doi.org/10.1121/1.1918768View Description Hide Description
When making measurements in random sound fields, for example in a reverberant enclosure, it is desirable to know what diffraction effects occur at a reflecting object such as a transducer, suspended in the enclosure. In the case of a sound field consisting of a traveling plane‐wave train, the diffraction effects for spheres and cylinders have been well studied, and various analytical results are available. Some of these results are here extended to the case of a randomly incident sound field. The potential and kinetic energy distributions in the vicinity of a rigid reflecting sphere under random incidence are derived as functions of the diameter of the sphere in wavelengths. Corresponding results for a pressure‐release boundary condition at the surface of the sphere, and for an infinite circular cylinder in a two‐dimensional random field, are also given. The general effect of boundary configuration on diffraction pattern is discussed, and the random field pressure at the vertex of a wedge‐shaped reflector of arbitrary angle is considered as an example. The diffraction effects are basically reciprocal, and the correspondence of the results with those for the reciprocal cases is discussed. Some practical applications of the results are considered.
35(1963); http://dx.doi.org/10.1121/1.1918770View Description Hide Description
A method of analysis is presented for predicting sound radiation from an arbitrary body (shell) vibrating in an infinite fluid medium. The acoustic field is described by a distribution of surface sources of unknown strength at the shell fluid boundaries, which leads to a set of integral equations. In the general case, the shell with or without an attached internal structure is described by an elastic system having a finite number of degrees of freedom. The dynamic interaction between the shell and the fluid at their boundaries is then formulated in terms of the unknown strength of the source distribution. By matrix analysis, the method has been formalized and coded in the digital computer for computation.
Numerical results of two examples are presented to demonstrate the versatility of the method, which makes possible the solution of a large class of sound radiation problems.
35(1963); http://dx.doi.org/10.1121/1.1918772View Description Hide Description
A stable platform from which to perform experiments at sea has been a long‐sought goal of sea‐going scientists. In order to perform fine‐scale experiments on fluctuations of acoustic signals in the ocean the manned buoy FLIP has been built to provide the needed stable platform for such work.
The 355‐ft‐long, 600‐ton craft is towed to station in the horizontal position, and operates in the vertical position at a draft of 300 ft with its electronics laboratory about 30 ft above the water line.
A description is given of its development, stability, and operating characteristics, including the unique flipping operation.