摘要:

A new method of measuring reverberation time is described. The method uses tone bursts (or filtered pistol shots) to excite the enclosure. A simple integral over the tone‐burst response of the enclosure yields, in a single measurement, theensemble averageof the decay curves that would be obtained with bandpass‐filtered noise as an excitation signal. The smooth decay curves resulting from the new method improve the accuracy of reverberation‐time measurements and facilitate the detection of nonexponential decays.

ISSN:0001-4966    

DOI:10.1121/1.1909343

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

This paper describes the mathematical model used for computing both the frequency and angular distribution of the normal modes in rectangular rooms. The criteria adopted were computed for each half‐octave band over the first 4 octaves of normalized frequency for rooms with dimension ratios ranging from313:1to 1:1. Considerable variation in the frequency‐spacing criterion exists not only for changes in room dimensions but also from one half‐octave band to the next. No clearly defined optimum room dimension, as predicted by Bolt, emerges from this study. The angular‐distribution index is more regularly behaved with rather definite stratification apparent as a function of the room height/length ratio, when the height direction is taken as the angular reference. When both the frequency and angular criteria are combined, only a few small regions of dimension ratios appear to be good. From these regions,p= 0.69,q= 0.43;p= 0.83,q= 0.65;p= 0.82,q= 0.72, together withp= 1/21/3,q= 1/41/3, appear to be among the best. For rooms having satisfactory mode distribution, an approximate formula has been developed for determining the lowest midband frequency for which a room may be used for measurements of continuous spectrum sounds. The formula turns out to be a constant divided by the cube root of the volume, where the constant is a function of the measuring bandwidth and the number of normal modes required therein. For 20, 12, and 9 modes in 1‐,12‐, or13‐oct bands, the constants are 1150, 1280, and 1355 cps, respectively.

ISSN:0001-4966    

DOI:10.1121/1.1909344

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Contour charts are given of the sound fields near one, two, or three reflecting surfaces at fight angles, for random sound incidence. The charts show contours of the mean‐squared pressure in decibels as a function of position in the region within two wavelengths of the boundary surfaces. The boundary conditions at the reflecting planes are pressure‐reflecting (normal velocity component equal to zero) or pressure‐release (pres‐ sure equal to zero). By reciprocity, the charts also give the relative radiation resistance of a point source as a function of position near such reflectors. The formulas used are tabulated; they extend and in some places correct formulas given in an earlier paper [J. Acoust. Soc. Am.27, 247–258 (1955)]. The features of the sound fields near the reflectors are discussed, and some applications to acoustical measurements in reverberant sound fields are considered.

ISSN:0001-4966    

DOI:10.1121/1.1909345

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

The dc gradient across the reticular lamina, which can be as large as 185 mV, represents the algebraic difference between the endocochlear potential (EP) and the negative resting potential found within the organ of Corti. During asphyxia, the decline in CM paralleled closely the decline in this dc gradient. These data suggest strongly that CM represents a modulation of both dc resting potentials. The negative resting potential was examined further. Like EP, it is most probably extracellular, since it can be recorded with glass pipettes having a tip diameter of 25 μ. It decreases as a linear function of log K+concentration in scala tympani, and it appears to be much more resistant than EP to cochlear temperature reduction.

ISSN:0001-4966    

DOI:10.1121/1.1909346

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

The effect of the presence or absence of feedback, or immediate positive reinforcement, was studied at 3 performance levels: 88%, 75%, and 62% correct. TheBUDTIF(Block Lip and Down, Two‐Interval—Forced‐choice) experimental procedure was utilized. Signal‐to‐noise ratios were varied in a 104‐trial run (about 7 min) so as to maintain the desired performance level. The noise was set to 35 dB SL. One group of naive subjects was used; each subject was presented one of the 6 conditions of performance level and feedback for 5 consecutive runs. Another group of experienced subjects was presented with all 6 conditions in each of six 1‐h sessions. Threshold signal‐to‐noise ratios, intrarun variability of levels utilized, the time per run, and interrun variability were determined. No statistically significant and systematic effect of feedback was found.

ISSN:0001-4966    

DOI:10.1121/1.1909347

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Masked thresholds were measured with an operant tracking method at 4 frequencies between 1000 and 8000 cps and at 3–5 levels of wide‐band noise. As in humans and cats, masked thresholds in the rat increased linearly with noise level. Critical ratios were calculated from the masked thresholds and found to be 10 or 11 dB greater than those in man and 5 to 6 dB greater than those in cats. Greenwood's functionΔfcb = 10ax+b, relating critical bandwidths to position on the basilar membrane, was shown to fit reasonably well the data of this experiment. The constant relation shown by Fletcher between critical ratios and frequency DL's, previously found to apply to man and to cats, appears to apply to rats also.

ISSN:0001-4966    

DOI:10.1121/1.1909348

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Several hundred young men were given careful audiometry before beginning duty in noise of 105–110 dB SPL at one or more of the octaves 300–600, 600–1200, 1200–2400 cps. These men were then given the same audiometric examination intervals up to five years. Less than 15% of ears had permanent threshold shifts (PTS) of more than 20 dB at any frequency. Trend curves extrapolated over log time predict a median PTS of 8 dB at 4 kc/sec for 10 years' exposure. The PTS actually found was thus 20 dB less than predicted by the ASA Z‐24 Committee report; the 4‐kc/sec TTS2for an 8‐h exposure to this noise was over 60 dB; use of the 4‐kc/sec TTS2index would thus vastly overpredict both the actual PTS of 8 dB and the Z‐24 Committee prediction of 22 dB. Ear defenders as actually worn by this population reduce median PTS by no more than 5 dB. It is concluded that a damage‐risk criterion of 100 dB SPL at any of the relevant octaves would be conservative, protecting at least 850/0 of young healthy ears from PTS of over 20 dB at any frequency.

ISSN:0001-4966    

DOI:10.1121/1.1909349

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

This paper presents data for output sound powerPOof wind instruments relation to input powerPIsupplied by the player.PIwas calculated aspV̇, wherepequals mouth pressure and V̇ air flow rate through the instrument.POwas calculated from sound‐pressure level and measurements of reverberation time in a live room of known volume. A part of the data was obtained in a room of unknown characteristics; from 15 comparable measurements on 8 different instruments in both the live and the unknown room, data were obtained that allowed calculation ofPOalso from other experiments in the unknown room. Measurements were made on single notes, played bothppandff, on each instrument; one low and one high note on the scale of each instrument were chosen. The ratioPO/PI, representing the mechanical efficiency of wind instruments as sources of sound power, varies from less than 0.001% to about 2%. It appears to increase with increasingPIand, in some instruments, with frequency. The consistent results obtained for 3 different flutes played by one performer suggest that the variability noted in the other data at least partially reflects individual differences in mechanical efficiency.

ISSN:0001-4966    

DOI:10.1121/1.1909350

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Using the 100% relative‐humidity technique for controlling the mole function of water vapor, the Napier (relaxation) frequency of oxygen has been measured as a function of the mole fraction of various water vapors, consisting of pureH2O, pureD2O, and three mixtures thereof. PureH2Ogives, as is well known, a quadratic inH, the mole fraction; pureD2Ogives a linear relation down to 0.7×10−3. The mixtures show a linear relation above 1.5×10−3and a curvilinear approach to zero below that. Calculation indicates that the heterogeneous molecule HDO is far more effective as a thermal equilibrator for oxygen than either of its homogeneous parents. Quantitatively, with (f/p)min cps per atm andhin mole fraction ×103, the results are:100%H2O:(f/p)m = 7+183h+132h2;50%D2O:(f/p)m = −664+1100h;80%D2O:(f/p)m = −490+908h;92%D2O:(f/p)m = −250+585h(aboveh= 1.3);92%D2O:(f/p)m = 7+275h+100h2(belowh= 1.3);99.8%D2O:(f/p)m = −85+385h.

ISSN:0001-4966    

DOI:10.1121/1.1909351

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

The problem of an acoustic wave incident on a rigid sphere “loaded” in a narrow azimuthal region is considered. The solution appears as the superposition of the field scattered by the unloaded sphere and that radiated by an active slot coincident with the load, with the radiation strength of the slot related to the loading characteristics in the combined problem. By varying the loading impedance, wide degree of control over the scattering behavior of the sphere can be achieved even when only “passive” loads are permitted. Numerical results are presented.

ISSN:0001-4966    

DOI:10.1121/1.1909352

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP