摘要:

The low‐frequency electroacoustic sensitivity of piezoelectric polyvinylidene fluoride (PVF2) film attached to a cylindrical tube substrate is derived for three boundary conditions. Sensitivity expressions are developed for both the PVF2film attached to the inside or outside surface of the tube for each of the three boundary conditions. Experimental verification of the theory has been obtained for the capped composite polymer cylinder, with close agreement between the theoretical and measured sensitivities.

ISSN:0001-4966    

DOI:10.1121/1.384983

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

An introduction to three oral papers on low‐frequency transducers presented to the 97th meeting of the Acoustical Society of America in session WW on 15 June 1980 in Cambridge, Massachusetts.

ISSN:0001-4966    

DOI:10.1121/1.384984

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Scaling of a conventional transducer design to very low frequencies usually leads to greater power than needed and greater size than is considered tolerable. Normally, therefore, scaling guidelines are abandoned, and very low frequency transducers are miniaturized, that is, made with dimensions much smaller than the sound wavelength. The resulting low radiation resistance leads to high mechanicalQ, poor power‐to‐weight ratio, and poorly behaved driving‐point impedance. By employing electrical equalizers and acoustoelectrical feedback, the designer can achieve broad bandwidth in spite of the highQ, but an oversized driving amplifier is required. Transducers that operate at full ocean depths are usually liquid‐filled. The stiffness of the interior liquid increases the transducer size and tends to raise itsQand lower its coupling factor. A Helmholtz resonator configuration is often advantageous in this situation. For more moderate depths, compliant tubes or compressed gas may be used to reduce the stiffness of the interior. However, depth capability is always obtained at the expense of acoustic performance.

ISSN:0001-4966    

DOI:10.1121/1.384985

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

We are reporting our experience with the use of high‐pressure guns at many depths to generate impulsive underwater sound in the fundamental frequency range from 10 to 200 Hz. Air guns are not often used outside of seismic exploration, but they have potential as cheap, energetic, impulsive sources of low‐frequency sound. A simple analysis shows that the fundamental frequency of waveforms increases faster with increasing depth than predicted by the Rayleigh–Willis equation. In addition, the acoustical efficiency of air gun sources is found to decrease with increasing depth, falling sharply as the ambient pressure becomes a significant fraction of the initial gun pressure. Experimental waveform and energy source level data verify these analytical results. Finally, data obtained from arrays of air guns are presented. These data demonstrate that air guns may be successfully arrayed and that the internal impedance of the air gun source is high enough that each air gun resembles a constant source strength (volume velocity) source, resulting in more radiated energy than expected from constant energy sources for some spacings.

ISSN:0001-4966    

DOI:10.1121/1.384986

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Use of flextensional transducers has been limited historically by an inability to design them accurately and by their apparent unsuitability for phased or conformal arrays. Reliable design requires development of cost‐effective, relatively accurate, and preferably algebraic mathematical models. However, most early analytical research centered around large computer models; these were generally accurate, but too complex and expensive to provide important physical insight. Recent work on an equivalent circuit for Class IV flextensional transducers produced unrestrained equations of the required accuracy for peak radiated power, resonant halfpower bandwidth, resonant frequency, and the optimum power‐bandwidth product design that represents the smallest, lightest, Class IV transducer for any acoustic requirements. This paper briefly summarizes the analytical background of the model; recent experimental work; application to several sonars, including both expendable and ship systems; and the ultimate capabilities of Class IV devices.

ISSN:0001-4966    

DOI:10.1121/1.384987

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A new technique for measuring the sound velocity difference in two liquids is presented by using interdigital transducers in contact with the liquids. The phase difference between two delayed signals obtained at the output transducers, corresponding to the sound velocity difference, is converted to a dc voltage via a circuit with a function of phase comparation. As an example, the difference between the sound velocity in solutions of NaCl and that in water is given as a function of concentration. Basic design considerations for the device and experimental performance are given.

ISSN:0001-4966    

DOI:10.1121/1.384988

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Observations of ambient noise in the shallow, tropical Timor Sea north of Australia have shown a predominance of noise of apparent biological origin over much of the audio frequency band. Four of the most commonly occurring sounds which were responsible for the prevailing noise from 100 Hz–2 kHz and which do not appear to have been observed elsewhere, are described. These sounds are quite distinctive in character. Three of the sounds show several harmonics with fundamental frequencies in the range 70–220 Hz, and have durations of 2–4 s. In two of these sounds, the harmonics show a regular modulation of their amplitude, and in one of the sounds, the harmonics rise in frequency by about one octave over the duration of the sound. The fourth sound is nonharmonic and consists of a train of pulses containing four shorter pulses. The mean source levels of the sounds are estimated to lie between 127 and 150 dBre1 μPa at 1 m.

ISSN:0001-4966    

DOI:10.1121/1.384989

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

As a follow‐up to two previous studies, we examine the feasibility of dividing the bandwidth of a speech signal while keeping the time dimension intact. We utilize the real and imaginary parts of the short‐time Fourier transform to describe contiguous bandpass segments of the signal spectrum. We implement a frequency division that maintains unchanged the Hilbert envelope of the waveform for each bandpass segment. We show that the resulting signal can be usefully confined to one‐half the original bandwidth. Transmission can be effected either by an analog channel of one‐half bandwidth, or by digitally encoding at one‐half the conventional Nyquist rate. Reanalysis at the receiver and complementary multiplication of the spectrum parameters reconstruct the signal to the original bandwidth. Analysis, transmission, reanalysis, and reconstruction processes are not transparent. They provide an output of less than typical toll quality, but of reasonably good communications quality. A potential use for the method might be the simultaneous transmission of voice and data over a single voice circuit. We also show that the division and multiplication technique can be used to modify the time dimension of the speech signal (i.e., compress or expand the time scale) while keeping the frequency dimension intact.

ISSN:0001-4966    

DOI:10.1121/1.384990

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Between 29 May and 11 June 1978, an NSF‐sponsored US delegation visited the Soviet Union as part of a US–USSR scientific exchange program in the area of natural language and speech processing. Laboratories in four cities—Moscow, Novosibirsk, Leningrad, and Kiev—were visited. This paper describes those activities relating to speech processing, especially speech recognition.

ISSN:0001-4966    

DOI:10.1121/1.384991

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Linear prediction is considered with respect to a nonlinear frequency scale obtained by a first‐order all‐pass transformation. The predictor can be computed from a frequency‐warped autocorrelation function obtained from the power spectrum or by a direct linear transformation of the original acf. Three numerical procedures are compared. Alternatively, the predictor can be determined from a covariance matrix or (adaptively) from continuously formed correlations, suitably defined according to the all‐pass transformation. Prediction‐error minimization and spectral flattening are no longer equivalent criteria. In the synthesis part of a vocoder or APC system, no inverse transformation is required, since the direct form of the analysis and synthesis filters can be modified so as to immediately realize the warped transfer function. Single‐word intelligibility is compared for a predictive vocoder on a ’’Bark’’ scale and a linear frequency scale. The Bark scale yields results around 90% even at predictor orders of 5 to 7. More possible applications have been given previously by other authors.

ISSN:0001-4966    

DOI:10.1121/1.384992

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP