Improvements to a small free-field listening room

An inexpensive free-field listening room (7 ft 8 in × 5 ft 9 in × 6 ft in high) was designed and constructed with walls made of 5 inch high acoustical foam wedges backed by $\text{1}\text{1}\text{2}$ inch foam and a $\text{3}\text{1}\text{2}$ inch layer of building-grade Fiberglas. The composite wall construction creates a rugged, non-irritating surface that is highly absorbent and relatively inexpensive. Deviations from the inverse square law are less than 2·3 dB above 500 Hz. Reverberation from a 40 ms 1000 Hz pulse is 31 dB below the original pulse. Results of these two tests are compared with measurements by Marsh et al. [1] in a room of similar volume having flat 4 inch foam walls.     

1ƒ noise in copper whiskers

We have observed current-induced $\text{1}\text{?}$ noise in thin copper crystals (“whiskers”) ranging from 0.15 to 1.20 cm in length. We find that the noise power is proportional to the square of the applied d.c. voltage and has a power spectrum varying approximately as ?^-1 over the frequency range 0.5 Hz–2 kHz. The magnitude of the noise is 10²–10³ times larger than typical magnitudes reported for copper films of similar volume.     

Experimental study on the symmetric modes of vibration of a four bolt clamped square plate

The response of structural components excited by turbulent flow noise depends on the natural resonance frequencies and displacement modes of those components. An experimental program is described which determined those resonance frequencies and modes for a flat plate clamped in the middle of its four sides and also for the case when one of the four bolts had failed. Thirty-three resonances in the range 30·5 to 736 Hz have been identified for the four-bolt normal operation and forty-two in the range 12·5 to 496·1 Hz for the three-bolt accidental situation. Of particular interest to the turbulent flow noise response, the modes symmetrical to the $\text{x-y}$ and diagonal axes have been identified and classified into a two-dimensional matrix form for the $\text{mn + nm}$ and $\text{mn - nm}$ modes combination ($\text{m}$ and $\text{n}$ being the pure modes along the $\text{x-y}$ axis).     

Design of a self-calibrating simulated acoustic emission source

The use of conical piezoelectric transducers as point acoustic sources has been investigated. It has been shown that transducers based on a design originally developed at the National Institute for Standards and Technology in the USA can be used as point transmitters over the frequency range of interest in acoustic emission measurements (100 kHz to around 1 MHz). They should, therefore, be suitable for use in experiments to calibrate structures so that acoustic emission source strengths can be determined. It has also been shown that measurements of the response of the transmitting transducer backing can be used to assess the coupling efficiency, and hence to remove concerns about inconsistent coupling affecting the calibration measurements. The results indicate that the variation of the backing response with coupling is due to a shift in the resonance frequencies of the transducer with the mechanical load impedance. If other transducers can be shown to behave in a similar fashion this effect could be used to measure coupling in standard acoustic emission and ultrasonic transducers.     

Nonlinear first sound response of liquid helium at 1, 3 and 10 MHz

The first sound response of liquid helium at 1, 3 and 10 MHz is studied using piezoelectric ceramic transducers. Above specific threshold displacement amplitudes, subharmonic frequencies ($\text{?}{}_0\text{2}$, $\text{?}{}_0\text{3}$,…) and ultraharmonic frequencies ($\text{3?}{}_0\text{2}$, $\text{5?}{}_0\text{2}$,…) appear in the response spectrum. The magnitude of the threshold displacement amplitudes for the production of subharmonics suggests that vapor microbubble oscillation appears not to be the primary generation mechanism.     

渔业声学回波散射测量系统的宽频带换能器

针对目前渔业声学宽频带回波散射测量系统采用多个不同频带的换能器合成、结构复杂且不方便携带的缺点,本文提出一种使用低机械品质因数、高谐振频率的积层压电致动器作为核心压电元件的"朗之万"型宽带水声换能器。测试结果表明,该换能器适用的频率范围为20 kHz~150 kHz,在三个渔业声学常用频率38 kHz、70 kHz、120 kHz下的-3 dB波束宽度分别为20.0°、11.5°、5.0°,可以满足对常见渔业资源种类的宽带声学回波散射特征的测量要求。     

Modal content of noise generated by a coaxial jet in a pipe

The problem investigated was that of noise generated by air flow through a coaxial obstruction in a long, straight pipe of inside diameter, D = 97 mm. Downstream modal pressure spectra in the 200–6000 Hz frequency range were measured by a new technique [1] for orifices and nozzles of diameter d where $\text{0·03 ? (}\text{d}\text{D}\text{) ? 0·52}$. The Mach numbers of the flow through the restrictions ranged from 0·15 to choked conditions. The shape of the modal frequency spectrum was found to be determined by the frequency ratio $\text{f}{}_{\mathrm{r}}\text{=}\text{He}\text{St}\text{=}\text{U}{}_{\mathrm{i}}\text{D}\text{a}{}_0\text{d}$, where U_i is the jet velocity and a₀ is the speed of sound in the gas downstream of the restriction. This parameter is the ratio of two non-dimensional frequencies: namely, He, which controls acoustic propagation inside circular ducts, and St, which scales the jet noise spectrum shape. At low f_r(<3) the higher modes dominate the noise spectrum above their cut-off frequencies, while for higher f_r all modes are approximately of equal amplitude. The nature of large scale turbulence structures in the region of the jet near the nozzle exit may be used to explain these phenomena. The measured modal pressure spectra were converted to modal power spectra and integrated over the frequency range 200–6000 Hz. The acoustic efficiency levels (acoustic power normalized by jet kinetic energy flow), when plotted vs. jet Mach number, depend strongly on the ratio of restriction diameter to pipe diameter ($\text{d}\text{D}$). Dividing the efficiency levels by the area ratio, $\text{(}\text{d}\text{D}\text{)}{}^2$, correlated the results over a moderate range of ($\text{d}\text{D}$).     

Ultrasound pressure distributions generated by high frequency transducers in large reactors

The performance of an ultrasound reactor chamber relies on the sound pressure level achieved throughout the system. The active volume of a high frequency ultrasound chamber can be determined by the sound pressure penetration and distribution provided by the transducers. This work evaluated the sound pressure levels and uniformity achieved in water by selected commercial scale high frequency plate transducers without and with reflector plates. Sound pressure produced by ultrasonic plate transducers vertically operating at frequencies of 400 kHz (120 W) and 2 MHz (128 W) was characterized with hydrophones in a 2 m long chamber and their effective operating distance across the chamber’s vertical cross section was determined. The 2 MHz transducer produced the highest pressure amplitude near the transducer surface, with a sharp decline of approximately 40% of the sound pressure occurring in the range between 55 and 155 mm from the transducer. The placement of a reflector plate 500 mm from the surface of the transducer was shown to improve the sound pressure uniformity of 2 MHz ultrasound. Ultrasound at 400 kHz was found to penetrate the fluid up to 2 m without significant losses. Furthermore, 400 kHz ultrasound generated a more uniform sound pressure distribution regardless of the presence or absence of a reflector plate. The choice of the transducer distance to the opposite reactor wall therefore depends on the transducer plate frequency selected. Based on pressure measurements in water, large scale 400 kHz reactor designs can consider larger transducer distance to opposite wall and larger active cross-section, and therefore can reach higher volumes than when using 2 MHz transducer plates.     

Microwave spectra of propyne and its [13C] isotopic species: Refined molecular structure of propyne

The ground state microwave spectrum of propyne is remeasured between 60 and 240 GHz with a molecular beam spectrometer. These high resolution measurements allows us to accurately determine the rotational constant and all the determinable quartic and sextic centrifugal distortion constants: B = 8 545 877.12 (12) kHz; D_J = 2.9423 (13) kHz; D_JK = 163.423 (20) kHz; H_J = 0.0097 (40) Hz; H_JK = 0.935 (136) Hz; and H_KJ = 5.23 (27) Hz (95% confidence limits are shown in parentheses). The spectra of the [¹³C]isotopic species are also reinvestigated between 30 and 240 GHz with a video spectrometer. With the accurate rotational constants we derive from those spectra a new r₈ structure is calculated for the CC bond lengths: $\text{r}{}_8\text{(}\text{CC}\text{) = 1.4586 (2)}\text{A}\text{?}$ and $\text{r}{}_8\text{(}\text{CC}\text{) = 1.2066 (2)}\text{A}\text{?}$.     

Loudness evaluations of electric clock noise

The noise from consumer electric clocks was studied to evaluate loudness estimating procedures. Eight different clock noises were tape recorded for presentation to a panel of people for loudness judgments. An electronic switch enabled the recorded clock noise and a 1000 Hz tone to be presented alternately via earphones for $\text{1}\text{2}$-sec durations with $\text{1}\text{2}$-sec silences between signals. The subjects adjusted the 1000 Hz tone to match the loudness of the clock noise. It was determined that Stevens' MARK VI procedure generally underestimates the observed loudness, but is usually less than 10 phons in error. It is possible that biasing effects had some influence on this result. If ranking on a relative loudness scale is all that is desired, dB(A) measurements will suffice. However, the dB(A) measurements do underestimate the differences in loudness between the quietest and loudest clocks.     

Hybrid linear and matrix acoustic arrays

This work describes a new technique for constructing both linear and matrix array transducers. It used completely separated piezoelectric elements whose vibration modes have been studied experimentally. Each piezoelectric element has a $\text{λ}\text{4}$ matching plate and backing, and so it forms a separate transducer. The array is formed by an assembly of these transducers.The technique shows some interesting features such as the possibility of reducing the dispersion of the electroacoustic characteristics of the single elements to the required value, mechanical strength, and the possibility of periodical maintenance.     

Pulsed excitation of luminescence in dihydrated oxalic acid

Luminescence has been excited by an a.c. electric field in oxalic acid dihydrate at temperatures 273, 283 and 293 K. The voltage and frequency dependence of the emitted light flux has been studied. The brightness (B) has been observed to increase with voltage (V_rms) and frequency (v). The relation b = b₀ exp(-b/V^{$\text{1}\text{2}$_rms)} describes the voltage dependence of brightness quite accurately at frequencies greater than 500 Hz. The brightness wave forms at different voltages and frequencies of the applied field have also been studied. One primary and one secondary peak have been observed in each half cycle of the applied sinusoidal voltage.     

Electro-optical characteristics of anthracene doped by its halogen derivative

Electroluminescence in anthracene doped by its derivative 1-chloroanthracene at dopant concentrations 10^?3, 10^?4 and 10^?5 mole % has been reported with and without matrix. At low frequency of the exciting electric field the variation of the brightness is linear with the voltage but at higher frequencies (500–2000 Hz) the relation $\text{B = B}{}_{\mathrm{O}}\text{exp}\text{[?}\text{C}\text{V}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{]}$ fits better with the observations. The decrease in the electroluminescence brightness with increase in the dopant concentration suggests the trapping action of dopant energy levels where nonradiative recombination predominates.     

Heterodyne measurements of 12C16O laser frequencies and improved Dunham coefficients

Earlier measurements of absolute frequencies of ¹²C¹⁶O laser lines by heterodyning with second harmonics of a ¹²C¹⁶O₂ laser have been extended to 15 more lines by use of the rare isotopes ¹²C¹⁸O₂ and ¹³C¹⁶O₂. Interband difference frequencies for 20 pairs of ¹²C¹⁶O lines have also been measured by beat frequency techniques. These data together with 1438 other previously reported frequency measurements in the microwave and infrared regions have been analyzed to determine coefficients of a Dunham series $\text{E}{}_{\mathrm{vJ}}\text{=}\text{Σ}{}_{\mathrm{m,n}}\text{Y}{}_{\mathrm{mn}}\text{(v +}\text{1}\text{2}\text{)}{}^{\mathrm{m}}\text{[J(J + 1)]}{}^{\mathrm{n}}$ for the CO electronic ground state. Transition frequencies predicted from the derived coefficients are substantially more accurate than earlier tabulations, having estimated uncertainties of the order of 2 MHz in the strongest laser bands. The coefficient Y₃₁ has been determined for the first time.     

The microwave spectrum of 14NH3 in the v = 000011 state

The frequencies and assignments of 50 lines in the pure inversion spectrum of ¹⁴NH₃ in the 0001¹ vibrational state are reported in the microwave frequency region 18–53 GHz and in selected regions up to 58 GHz.The J = 0 inversion frequency, K-type doubling constant K, l = 2, ?1 and molecular dipole moment in this state are 32 904.7 ± 2.0 MHz, 1.958 ± 0.040 MHz and 1.459 ± 0.002 D, respectively, where model inadequacies are included in the uncertainties of the first two parameters. The dipole moment measurements for this and the ground state are in excellent agreement with Stark laser measurements. An expression containing the effective l-type doubling constant is obtained from the combination of frequencies $\text{[ν(1, 1, 1) ? ν(1, 1, ?1) ? ν(2, 1, 1) + ν(2, 1, ?1)]}\text{8}\text{= 10 361.894 ± 0.004}\text{MHz}$. A preliminary value for the l-type doubling constant is 10 655 ± 20 MHz.     

Finite element three-dimensional analysis of the vibrational behaviour of the Langevin-type transducer

The vibrational behaviour of the Langevin transducer is usually analysed using 1D Mason model which is valid when the lateral dimensions of the transducer are smaller than a quarter wavelength at the fundamental longitudinal resonance. In this work a 3D finite element analysis of the Langevin transducer's behaviour operating in the underwater sonar range of frequencies (30-140 kHz) is presented. Several samples with total length greater, comparable to, and smaller than the diameter of the transducer are analysed. For each sample, the resonance frequencies in the observed frequency range are computed and compared with those obtained experimentally from the measurements carried out using several in-house built prototypes. For the most important aspect ratios the resonance displacement distributions are presented and discussed. The results obtained permit to gain insight into the coupling phenomenon between thickness-extensional and radial modes and suggest that in practical applications transducers with diameters comparable to or greater than total length of the active stack can also be used. The trade-off between the efficiency and power handling ability for different designs is also discussed.     

Flavour and superunification

We consider the possibility of a non-trivial embedding of $\text{(}\text{10}\text{+}\text{5}\text{)}\text{SU}\text{(5)}$ families of spin if$\text{1}\text{2}$ left-handed fermions in a combination of irreducible massless supermultiplets of N extended supersymmetry. We demand the whole spectrum of spin $\text{1}\text{2}$ states to be anomaly free with respect to SU(N). This turns out to be a necessary condition for the absence of anomalies at the SU(5) level. We find two classes of models, with spin $\text{1}\text{2}$ fermions in SU(N) representations associated to one- and two-column Young tableaux, respectively, in which each irreducible massless multiplet occurs at most once. These two classes of models lead to a nontrivial family generation due to supersymmetry. For N = 8 extended supersymmetry, they give at most three and five families, respectively. The first class of models is more natural in the way it excludes SU(5) exotics. The same analysis is extended to the massless multiplets that can be obtained from bilinear composite fields of the (preonic) elementary fields of N extended supergravity. We prove that the generation of families requires the repetition of massless multiplets and that ($\text{10 +}\text{5}$) SU(5) families can only be generated in pairs. General properties of multilinear composite operators of the preonic fields are given and the rôle of massive representations to classify towers of operators with definite spin is pointed out.     

The 341-nm band system of pyridine N-oxide: Assignment of the out-of-plane fundamentals

Analysis of the 341-nm bands of pyridine N-oxide vapor has been extended to the out-of-plane modes of the molecule in the $\text{X}\text{?}{}^1\text{A}{}_1$ and $\text{A}\text{?}{}^1\text{B}{}_2$ states. Data for the ground state are obtained primarily from infrared and Raman measurements; using this information, the excited-state out-of-plane manifold is then built up by the assignment of overtone, sequence, and cross-sequence bands observed in the electronic absorption spectrum. These assignments lead to a complete set of a₂ and b₁ fundamental frequencies in the $\text{A}\text{?}$ and $\text{X}\text{?}$ states, though some questions cannot be fully resolved, and the uniqueness of the assignments is briefly discussed.     

Ground state spectroscopic constants of isothiocyanic acid,HNCS, from its microwave and millimeter wave spectra combined with far infrared data

The microwave and millimeter wave spectra of isothiocyanic acid, HNCS, in the ground vibrational state have been investigated in the frequency region 8–300 GHz. The a-type R-branch transitions have been assigned up to J = 25 and K_a = 4, and the a-type ^qQ₁ branch transitions up to J = 45. No b-type transitions could be identified in the frequency region covered. The far infrared data reported by Krakow, Lord, and Neely [J. Mol. Spectrosc., 27, 148 (1968)] were combined with our millimeter wave data in order to determine reliable spectroscopic constants. The rotational Hamiltonian, Watson's formalism with S reduction, has been extended empirically to higher order to facilitate the fitting of the large centrifugal distortion effects. The obtained constants are:

$\text{A = 1357.3 GHz; B = 5883.4627 MHz; C = 5845.6113 MHz; D}{}_{\mathrm{J}}\text{= 1.19393 kHz; D}{}_{\mathrm{JK}}\text{= ?1025.37 kHz; D}{}_{\mathrm{K}}\text{= 51.57 GHz; d}{}_1\text{= ?13.781 Hz; d}{}_2\text{= ?4.59 Hz.}$

The ¹⁴N quadrupole coupling constant has also been determined: χ_aa = 1.114 MHz.     

A high-precision measurement of the stark shift of the sodium D1-line

Using laser—atomic-beam spectroscopy and a special field arrangement the Stark shift of the sodium D₁-line was determined to be 48.986 (112) kHz (kV/cm)^?2. Using the literature value for the scalar polarizability of $\text{2}{}^2\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, 39.7 (8) kHz (kV/cm)^?2, the polarizability of $\text{2}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ was derived to be 88.69 (81) kHz (kV/cm)^?2.