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 ($\text{d}\text{D}$).     

Measurement of background noise in sound-insulated rooms

Problems concerning measurement of stationary background noise levels below the dynamic limits of normal transducers are studied. The use of very sensitive large transducers is possible, but in general is restricted to rather low frequencies due to their extreme directional characteristics. Thus a 10 inch transducer may be used up to approximately 250 Hz. Two-transducer approaches based on correlation techniques cover a much wider frequency range because ordinary small transducers are applicable. Measurement errors due to diffraction or to unavoidable spacing of the two transducers generally become significant for transducer diameters in excess of one-quarter of a wavelength at the upper limiting frequency, although an exception to this occurs in the particular case of a plane progressive wave. The use of extremely small insensitive transducers is restricted by the necessity of having very impractical integration times. If measurements of levels down to ?20 dB re 20 μPa are carried out with condenser microphones in one-third octave frequency bands a practical compromise seems to be employment of $\text{1}\text{2}$ inch microphones in the range of center frequencies from 25 Hz to 5 kHz. This range may in practice be doubled (e.g., extended to 10 kHz) if measurements in the range 2·5–5 kHz are carried out with both 1 inch and $\text{1}\text{2}$ inch transducers so that corrections can be obtained for extended range measurements from 5 kHz performed with $\text{1}\text{2}$ inch transducers only.     

Investigation of the noise emitting zones of a cold jet via causality correlations

Within the framework of lighthill's acoustic analogy the causality method proposed by Ribner and Siddon is used to identify equivalent noise sources inside a cold jet. An exploration of a few cross-sections shows that a two-dimensional investigation suffices in a first approach for integrating the source function provided the upper frequency limit does not exceed a Strouhal number approximately equal to 0·5. Furthermore the transverse distribution of the source term shows the jet region located on the microphone side to be dominant; the effective diameter of the source region is comparable with that of the nozzle. It is shown that in a direction with an angle of 30° to the jet axis, the “shear noise” is dominant (about 70% of the noise measured in the same frequency range). The noise emanates essentially from the transition region and from layers located between $\text{r}\text{D}\text{= ±0·25}$ and $\text{r}\text{D}\text{= ±0·375}$. This analysis is suitable for frequency range bounded above by St = 0·54. For the direction with an angle of 45° to the jet axis comparable results are obtained in a frequency range also limited at St = 0·54. However, this range contains only 40% of the total acoustic energy. The source region of the “shear noise” (near 70% of the total energy) and that of the “self-noise” remain always in the transition region located at 4–11 D. Radially the main part of the noise originates from the layers located on the microphone side between $\text{r}\text{D}\text{= 0·25}$ and 0·375. For the direction with an angle of 60° to the jet axis the “shear noise” is no longer measurable and the calculated “self-noise” represents only a few percent of the noise measured. For an acoustically excited jet (white noise filtered between St = 0·39 and St = 0·52, 320 Pa at the nozzle) another type of correlation appears which is believed to be related to a coherent structure travelling inside the jet at 0·75 v_j. The study of the source term shows that this structure must be related to noise originating from the nozzle outlet.     

Traffic noise annoyance near light controlled intersections

Four hundred noise samples were taken at varying distances from three light-controlled intersections, from which the increments in percentile level above those predicted for the equivalent free flow case were derived. No factors other than those included in the prediction method could be discerned, and linear regression of the whole sample was used to establish the relationship between the increment, $\text{ΔL}{}_{\mathrm{<mtext>n</mtext>}}$, and distance, $\text{x}\text{(m)}$, from the intersection, e.g. $\text{ΔL}{}_{10}\text{= 3·21 ? 0·01}\text{x}$. A postal social survey with 12 environmental questions was sent to 30 subjects at each of six free and six interrupted flow sites, where 18-h noise measurements were made. A 69 per cent response was obtained. The slope of the regression line between question scores and $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$ was found to differ between free and interrupted flow, but those against $\text{L}{}_{\mathrm{<mtext>50</mtext>}}$ were similar. The ‘dissatisfaction’ score and a composite ’annoyance’ score correlated well, 0·76, but ‘dissatisfaction’ gave a slightly higher score for free flow than interrupted, and ‘annoyance’ the reverse. This suggests that $\text{L}{}_{\mathrm{<mtext>50</mtext>}}$ is a useful indicator of subjective response if both free and interrupted flows are involved. However the data also supports the use of the logarithm of percentage of heavy vehicles as an indicator of dissatisfaction in the interrupted flow case.     

Dynamics of twinning in natural α-quartz

Finite amplitude internal friction experiments in α-quartz are described and interpreted in terms of the theory of nonlinear anelasticity. The theory predicts a linear relationship between the driving force of the excitation and the period of the automodulation at its onset. This relation is substantiated by experiments performed with a natural α-quartz reed vibrating in flexure at resonance frequencies between 65 and 170 Hz in the temperature range of 162° and 224°C. The data suggests that Dauphiné twinning in α-quartz causes the automodulation governed by an activation energy of 92$\text{kJ}\text{mole}$. This activation energy characterizes short distance oxygen diffusion.     

On the prediction of impact noise,V: The noise from drop hammers

In earlier papers in this series, the concepts of “acceleration” and “ringing” noise have been studied in relation to impact machines, and values of radiation efficiency have been obtained for the various types of structural components. In the work reported in this paper the predicted and measured noise radiation from a drop hammer, both in full-scale and in $\text{1}\text{3}\text{-}\text{scale}$ model form, were examined. It is found that overall noise levels (L_eq per event) can be predicted from vibration measurements to within ± 1·5 dB, and to within ±2·5 dB in one-third octave bands. In turn this has permitted noise reduction techniques to be examined by studies of local component vibration levels rather than overall noise, a method which provides considerable enlightenment at the design stage. It is shown that on one particular drop hammer, the noise energy is shared surprisingly uniformly over four or five sources, and that when these have been reduced, the overall noise reduction is severely limited by the “acceleration” noise from the “tup” or “hammer” itself. As this is difficult to eliminate without a basic change in forging technology, it follows that “tup” enclosure or modification of the sharpness of the final “hard” impact are the only means available for any serious noise reduction. Also indicated is the reliability of using model techniques, suitably scaled in frequency and impulse magnitude, in developing machinery with impact characteristics.     

Turbulence measurements in a resonance tube

Experimental investigations of acoustically induced turbulence in a resonance tube have been performed. Frequency (f) and sound pressure level (I_p) effects have been studied. Measurements were made at various spatial locations on loops and nodes. Sampled data were processed to estimate the characteristics of turbulence. It is found that the acoustically induced turbulence appears when I_p exceeds 160 dB under the experimental conditions of f = 680–2740 Hz and I_p = 160–166 dB. The turbulent spectrum (F) and the wave number (κ) are found to satisfy a power law F ∝ K^s with $\text{s ? ?1·6}\text{to}\text{? 2·1}$. The r.m.s. turbulent velocity ($\text{u}\text{?}$) is experimentally found to have an $\text{I}{}_{\mathrm{p}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence, yet is relatively insensitive to the variation of f. Throughout the whole measuring range of f and I_p, the rate of energy dissipation per unit mass (ε) is estimated to be in the order of 10⁶–10⁷cm²/s³.     

Phonon observations from 1ƒ noise measurements

From mobility fluctuation $\text{1}\text{?}$ noise parameter measurements five phononic energies corresponding to LO, TO vibrational modes as well as TA + S₂ or O, TO + TA, TO + O combined modes were identified. They stand for phonon replicas of non-radiative recombination processes. It is thus demonstrated that the carrier-phonon interaction is the source of $\text{1}\text{?}$ noise in semiconductors.     

Resonant light absorption and plasmon tunability of lateral triangular Au nanoprisms array

Optical characteristics and electric field distribution of triangular Au nanoprism in a unit and units array under polarized light irradiation were systematically studied by numerical simulation with finite difference time domain method. It is found that the plasmonic properties of the triangular nanoprism are dominated by the electric polarization rather than the wave propagation. The triangular nanoprism presents similar optical response with a strong dipole band under different wave propagations if the electric polarization vectors are parallel to the triangular cross section. The lateral triangular Au nanoprisms array possesses a large tunability of the plasmonic properties contributed from the combined influence of inter-particle distance, particles size, polarization angle and even environmental medium. From the plasmon band shift versus the refractive index, ultra-high local surface plasmon resonance sensitivity (509.96 nm/RIU, figure of $\text{merit}=5.55$) is reached at $\sim 850\text{nm}$, making this array promising for biochemical sensing applications.     

The bonded water molecule3ĪII: 180° Flips and diffusion

The proton spin-lattice relaxation time, T₁, is measured as a function of temperature in α -(COOH)₂·-2H₂O, K₂HgCl₄· H₂O and LiCHO·H₂O. The relaxation is caused by 180° flips of the water molecules about their 2-fold axes and good agreement is obtained between calculated and observed values of T₁. Empiricly the flip rate follows a classical Arrhenius equation: P· exp $\text{(? ΔH}\text{(RT))}$. A literature survey of values of P and ΔH obtained from similar investigations on other hydrates is given. The survey shows that the preexponential factor, P, is a function of the activation enthalpy, ΔH. P increases from 10¹² to 10¹⁷ Hz when ΔH changes from 2 to 17 $\text{kcal}\text{mole}$. Using a dynamical rate theory as formulated by Feit, we find the flip rate is given by: K₂· √(ΔH)· exp (K₁ΔH)· exp $\text{(?ΔH}\text{(RT>))}$. This expression can be fitted to the observed data using K₁ = 0.69 $\text{mole}\text{kcal}$ and K₂ = 2 × 10¹¹ Hz · $\text{(kcal}\text{mole)}$$\text{?1}\text{2}$. Thus both the frequency factor, K₂√ (ΔH), and the entropic factor, exp (K₁ΔH), have been obtained for flipping water molecules in hydrates. The values of K₁ and K₂ are shown to be physically reasonable.     

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.     

NMR in the quadrupole regime: Measurement of quadrupole and chemical shift parameters in single-crystal paradibromobenzene

The exact theory for the frequency of transition between the two lowest levels of a spin $\text{I =}\text{3}\text{2}$ nucleus experiencing a large asymmetric electric field gradient, an applied magnetic field, and an anistropic chemical interaction was presented in an earlier paper. Using the assumption that the quadrupolar and chemical shift tensors have the same principal axis system, the Hamiltonian was solved exactly — analytically for the applied field aligned along each of the three axes of the quadrupolar principal axis system, and numerically for arbitrary orientations.This theory is reviewed here and applied to our room-temperature experiments in single-crystal paradibromobenzene. The self-consistent least-squares fit to the field-dependencies and simultaneously the angular dependence (rotational pattern) of the resonance frequency was performed using the literature value for the pure quadrupole frequency $\text{ν}{}_{\mathrm{Q}}\text{(1 +}\text{η}{}^2\text{3}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 223·8}\text{MHz}$. The fit yielded values for the quadrupolar asymmetry η = 0·0461 ± 0·0004 and the chemical shift components σ_x = ?0·001 ± 0·001, σ_v = σ_z = 0·000 ± 0·001. Our value for η is in good agreement with values determined by other methods; it and our shift values are consistent with the information obtained by this method using a powdered specimen.The process of using the NMR signal itself to align the specimen yielded sufficient information for an unambiguous determination of the Euler angles of orientation of the crystal in its mounting within ± 0.6°.     

Measurement of the branching ratios for the decays J/ψ → ϱπ and J/ψ → γη′

Decays of the J/ψ (3.1) resonance into final states with two charged hadrons and two photons are investigated. Branching ratios for the decays J/ψ → ?π and J/ψ → γη′ are determined to be

$\text{Г(J/ψ → pφ)}\text{Г(J/ψ → all)}\text{= (1.0± 0.2) ·10}{}^{\mathrm{?2}}\text{,}\text{Г(J/ψ → γη′)}\text{ГJ/ψ → all)}\text{= (2.0± 0.7) ·}{}^{\mathrm{?3}}$

Upper limits for the same decay modes of ψ′ (3.7) are also determined.     

Infrared and raman spectra of barium nitrite monohydrate

The low temperature infrared absorption and room temperature laser Raman spectra of polycrystalline Ba(NO₂)₂ · H₂O and its deuterated analogue are studied. The strongly bonded H-atom of the water molecule makes a highly bent H-bond while the weakly bonded H-atom exhibits a slightly bent (or possible bifurcated) bond consistent with recent structural data; the H-bond enthalpies are estimated to be 3.3 and 2.1 $\text{KCal}\text{mole}$ respectively. The wagging, twisting and rocking modes of the water molecule have been assigned using several well known criteria. The force constants of these modes have also been computed. The librational splittings observed at low temperature are consistent with polarization data, and are being attributed to infrared active factor group components.     

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.     

Toroidal hydromagnetics

The complete set of hydromagnetic equations is transformed into Poisson equations and equations of motion for flux densities and their associated variables. The toroidal components of the vector potential A and of the momentum density $\text{a}\text{≠}\text{πv}$ are represented by the po loidal flux densities Ψ and Ψ, respectively, for which the equations of motion are derived. The poloidal components A_⊥ and a_⊥ are represen ed by the potentials atΦ, U and φ, u, for which we obtain Poisson equations in the poloidal plane. Thus one has to solve two Dirichlet and two von Neumann problems at every time step. The source terms of the four Poisson equations define the remaining four variables, namely, $\text{Λ = ▽ ·}\text{A}$,$\text{Ω=(▽×}\text{A}\text{)ζ/R}$, $\text{λ=?·}\text{a}$, and $\text{ω=(?×}\text{a}\text{)ζ/R}$, for which equations of motion are also derived. In the limit of small toroidicity ? we look fo r a selfconsistent scaling of the equations with v_⊥～ε. But the curl of v_⊥×B in Faraday's law creates a toroidal plasma component of B which is one order of magnitude larger than in the case of a low β equilibrium; therefore, the motion becomes fully three-dimensional. Finally, an artificial pressure law is needed to balance the lowest order of the Lorentz force. The conclusion is then that the scaling laws previously used are not applicable for toroidal geometry, and that the effort to obtain numerical solutions is not dramatically higher than without using any scaling law.     

Infrared-radio frequency double resonance observations of pure rotational Q-branch transitions of methane

The $\text{F}{}_2{}^{\mathrm{(2)}}\text{← F}{}_1{}^{\mathrm{(2)}}$ and $\text{F}{}_2{}^{\mathrm{(2)}}\text{← F}{}_1{}^{\mathrm{(1)}}$ transitions of the J = 7 levels of the ground state of CH₄ have been observed by infrared-radio frequency double resonance using the 3.39 μ HeNe laser line. The transition frequencies are 423.02 ± 0.02 MHz and 1246.55 ± 0.02 MHz, respectively. Using these frequencies and the splitting of the E and F₂ levels of the J = 2 state calculated from the molecular beam magnetic resonance spectra of Ozier, the centrifugal distortion constants are derived to be D_t = 132933 ± 10 Hz, H_4t = ? 16.65 ± 0.2 Hz, and H_6t = 10 ± 1 Hz. The J = 15 E⁽¹⁾ → E⁽²⁾ microwave transition is predicted as 14150 ± 9 MHz.     

High-n ideal and resistive shear Alfvén waves in tokamaks

Ideal and resistive MHD equations for the shear Alfvén waves are studied in a low-β toroidal model by employing the high-n ballooning formalism. The ion sound effects are neglected. For an infinite shear slab, the ideal MHD model gives rise to acontinuous spectrum of real frequencies and discrete eigenmodes (Alfvén-Landau modes) with complex frequencies. With toroidal coupling effects due to nonuniform toroidal magnetic field, the continuum is broken up into small continuum bands and new discrete toroidal eigenmodes can exist inside the continuum gaps. Unstable ballooning eigenmodes are also introduced by the bad curvature when β > β_c. The resistivity (η) can be considered perturbatively for the ideal modes. In addition, four branches of resistive modes are induced by the resistivity: (1) resistive entropy modes which are stable with frequencies going to zero with resistivity as $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$; (2) tearing modes which are stable (Δ′ < 0) with frequencies approaching zero as $\text{η}{}^{\mathrm{<mtext>3</mtext><mtext>5</mtext>}}$; (3) resistive periodic shear Alfvén waves which approach the finite frequency end points of the continuum bands as $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$; and (4) resistive ballooning modes which are purely growing with growth rate proportional to $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{β}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ as η → 0 and β → 0.     

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.     

The spectrum of combustion-generated noise

A calculation of the energy release rate resulting from the combustion of propane-air mixtures is presented and the result is used to calculate the far field noise spectrum for an open flame by using appropriate Fourier transform techniques. The results illustrate the broad band nature of combustion noise and show that, for the range of parameters indicated, the peak frequency in the $\text{1}\text{3}$ octave band is in the range 400–1000 Hz. The results also indicate that the shape of spectrum is influenced by the time history of the heat release rate and the turbulence intensity and length scales; on the other hand, the peak frequency is a function of the heat release per unit mass of fuel which is essentially the same for hydrocarbon fuels.