Showing papers on "Sound power published in 1977"

Active sound absorption

Abstract: Sound attenuation is usually obtained by means of absorbing materials. This technique, which works quite well for sounds of medium and high frequencies, is very inefficient at low frequencies, because the thickness of absorbing material necessary to produce a constant attenuation increases with decreasing frequency. There is another technique by which destructive interference is used to silence sound fields: it is active sound absorption. Hitherto, this was based on Young’s principle of interference. This paper proposes a general theory, using Huygens’ principle, for three‐dimensional sound‐wave propagation. Some experiments are also reported and discussed.

Improved acoustic system for auditory research

Abstract: A closed acoustic system, based on the principles of lumped‐element and transmission‐line acoustics, has been developed for use in acute physiological experiments on small mammals. Through the appropriate utilization of a high‐quality push‐pull electrostatic earspeaker (Stax, SRX), a low‐impedance sound source having a relatively flat frequency response and a bandwidth of over 30 kHz is obtained. The output impedance of the source is increased by means of a lumped acoustic resistance in order to match the characteristic impedance of a uniform delivery line. The characteristic impedance of the delivery line is selected so that acoustic stimuli are propagated from the source to the eardrum without the introduction of substantial amplitude and phase distortion. Sound near the eardrum is coupled to a 12‐in. precision condenser microphone through a high‐impedance uniform probe line that is terminated in its characteristic impedance over the range of higher sound frequencies. When electrical equalization of +6 ...

Cavitation noise and inception as influenced by boundary-layer development on a hydrofoil

Abstract: This paper describes measurements of noise from two-phase flow over hydrofoils. The experiments were performed in a variable-pressure water tunnel which was acoustically calibrated so that sound power levels could be deduced from the sound measurements. It is partially reverberant in the frequency range of interest. Cavitation was generated on a hydrofoil in the presence of either a separated laminar boundary layer or a fully turbulent attached boundary layer. The turbulent boundary layer was formed downstream of a trip which was positioned near the leading edge. High-speed photographs show the patterns of cavitation which were obtained in each case. The noise is shown to depend on the type of cavitation produced.

Experiments on the transmission of sound through jets

Abstract: E on the superposition of a turbulent jet flow (0

Measurement of far field combustion noise from a turbofan engine using coherence functions

Abstract: Coherence measurements between fluctuating pressure in the combustor of a YF-102 turbofan engine and far-field acoustic pressure were made. The results indicated that a coherent relationship between the combustor pressure and far-field existed only at frequencies below 250 Hz, with the peak occurring near 125 Hz. The coherence functions and the far-field spectra were used to compute the combustor-associated far-field noise in terms of spectra, directivity, and acoustic power, over a range of engine operating conditions. The acoustic results so measured were compared with results obtained by conventional methods, as well as with various semiempirical predictions schemes. Examination of the directivity patterns indicated a peak in the combustion noise near 120 deg (relative to the inlet axis).

Investigation of the surface acoustical intensity method for determining the noise sound power of a large machine in situ

Abstract: Measurements of the surface acoustical intensity have been made on a large centrifugal chiller in order to determine the major noise‐radiating areas of the machine. In addition, an examination of the coherency of these major noise‐radiating areas has suggested that the total noise power can be obtained by addition of the noise power outputs of each component. The measurements were limited to A‐weighted sound‐power levels. The feasibility of the method has been checked by calculating the radiation efficiencies of the two dominant noise radiators. The values were found to be consistent with current knowledge of sound radiation from flexural waves in plates. Similar calculations based only on nearfield pressure measurements have demonstrated the very complex nature of the pressure field surrounding this type of large machine in situ.

Binaural sound reproducing system with acoustic reverberation unit

Abstract: A binaural sound reproducing system for transmitting sound radiated from an electro-acoustic transducer through a sound wave transmission path such as a pipe to the left and right ears of a listener includes an acoustic reverberation unit comprising a mechanical-acoustic element. The sound radiated from the electro-acoustic transducer is applied to the acoustic reverberation unit to produce indirect sound with acoustic reverberation. The indirect and direct sounds radiated from the electro-acoustic transducer are mixed and transmited through a sound wave transmission path such as the pipe to the left and right ears of the listener so that the sound image felt by the listener is localized externally of the head of the listener.

Attenuation of sound waves in ducts

Abstract: Apparatus for attenuating a sound wave propagating in a given direction along a fluid-containing duct comprises two sound sources spaced along the duct in a given direction; a sound detector between the two sources at a null point at which sound emanating only from the two sources substantially cancels; and means for utilizing the output of the sound detector including a phase-shifter and a frequency-sensitive amplifier to control the operation of the sound sources so that they emit sound in relative antiphase and at equal amplitudes and at such phases relative to the phase of the detected sound that the resultant of the sound radiations emitted in the given direction substantially attenuates the sound wave propagating along the duct.

A constant-flow calorimeter for the measurement of acoustic power at megahertz frequencies

Abstract: The calorimeter absorbs, in a flow of castor oil, a fraction of the acoustic power output of an ultrasound transducer. It has been used to measure powere of between 1 mW and 10 W at frequencies from 1-5 to 3 MHz. Examples are given of results obtained for an ultrasound therapy unit and an ultrasonic pulse--echo scanner. Sources of inaccuracy are defined. It appears that constant-flow calorimetry offers a potentially very sensitive method for the absolute measurement of acoustic power.

Experimental investigation of noise generated by large turbulent diffusion flames

Abstract: The sound power of combustion noise emitted by a natural gas fired experimental burner (1–3 MW) was measured under free-field conditions and compared with the noise emitted inside a refractory furnace. Measurements under free-field conditions show that the noise power output is proportional to the average gas velocity to the 2.8 power and to the burner throat diameter to the 5.6 power. The peak frequency of the combustion noise ranges between 125 Hz and 300 Hz depending on the swirl number but not on the firing rate. Measurements inside the furnace show the predominance of resonance phenomena at low frequencies. The actual combustion noise could be evaluated by the sound pressure level measured in the 250 Hz or 500 Hz octave band. The increase of combustion noise with firing rate is greater than under free-field conditions, which can be attributed to an increase of the flame turbulence rather than to an increase of the gas temperature. Preheating the combustion air greatly increases the sound power output as well as the peak frequency. A study of noise emitted by three industrial burners inside the furnace confirmed these results, notwithstanding the very different flame shapes obtained. Oil flames were found to be less noisy than natural gas flames when the combustion air was not preheated, whereas this difference disappeared on preheating the combustion air or using a pregasification burner.

Multimodal far-field acoustic radiation pattern: An approximate equation

Abstract: The far-field sound radiation theory for a circular duct was studied for both single mode and multimodal inputs. The investigation was intended to develop a method to determine the acoustic power produced by turbofans as a function of mode cut-off ratio. With reasonable simplifying assumptions the single mode radiation pattern was shown to be reducible to a function of mode cut-off ratio only. With modal cut-off ratio as the dominant variable, multimodal radiation patterns can be reduced to a simple explicit expression. This approximate expression provides excellent agreement with an exact calculation of the sound radiation pattern using equal acoustic power per mode.

Coherence between internal and external noise generated by gas turbine combustors

Abstract: Experiments and analysis on a gas turbine combustor unit are reported with a view in mind to separate propagated acoustic power from non-propagating 'pseudo-sound'. Analytically, it is suggested that a transition frequency will exist below which the interior pressure fluctuations are non-propagating, whereas above this frequency, of the order of 100 Hz, the noise is dominated by propagating acoustic waves. Coherence measurements are reported which show this concept to be borne out experimentally. Coherence between interior and exterior microphones is measured over a wide range of experimental conditions for a gas turbine combustor exhausting directly to the atmosphere. The purpose is to show that below a certain frequency, measurements of interior noise are not indicative of combustion noise ultimately propagating from an engine.

Measurement of sound absorption in rooms

Abstract: Existing methods for determining the sound absorption in rooms usually require some knowledge of the room dimensions. If sound‐pressure‐level measurements are made at various distances from a small source (such measurements herein referred to as a walk‐away test) the room sound absorption can be deduced provided the variation of sound pressure level with distance can be described by the simple equation SPL=PWL (sound power level)+10 log10(Q/4πr2+4/A′), which is commonly credited to Beranek. Unlike the reverberation time and estimated area methods (JSiαi), a walk‐away test can be used to determine the room absorption where either the room volume or wall areas and the absorption coefficients are uncertain or for rooms which are acoustically coupled to other rooms.

Helicopter Transmission Vibration and Noise Reduction Program

Abstract: : A combined analytical and test program has been performed to develop a method for analytically determining the vibration and noise characteristics of a helicopter transmission. This effort included formulation of the necessary analytical method, validation of this method through direct comparison with test data, and use of the method to predict the effects of various transmission design changes. The analytical method formulated in this program makes use of available techniques for predicting gear-mesh-induced excitations. These techniques have been expanded to include a more rigorous treatment of spiral bevel and helical gear induced mesh excitations. Response of the dynamic system is predicted using a coupled torsion and bending analysis of the gearshafts, and includes the effects of bearing and case dynamics. Predicted case surface response is used directly to calculate radiated sound power.

Noise Characteristics of a Can-Type Combustor

Abstract: The effects of flow rate, combustor temperature rise, inlet temperature, nozzle exit diameter, and duct length on spectral content, directionality, radiated sound power, and thermoacoustic efficiency are deduced experimentally using an 8-in.-diam combustor. Even at a high exit velocity of 900 fps, the combustor-on noise is seen to dominate jet noise. Internal and far-field spectra are similar below 1000 Hz (broadband with spikes at longitudinal duct resonance frequencies). Directionality is weak except at exit velocities in excess of 550 fps. At a given nozzle exit temperature and diameter, the internal overall sound pressure levels appear to be related linearly to the far-field levels. The thermoacoustic efficiencies vary from 10 ~ to 10 ~ 4 .

Optical sound recording system with inversely polarized sound signals

Abstract: An optical sound recording system of modified variable area type, wherein a light beam, preferably a laser beam, is pulse-width modulated in response to sound signals and deflected at a high frequency beyond audible range and then recorded on a sound track of cinema film. The pulse-width modulation of the light beam is performed in such a manner that a carrier wave having the same frequency, phase, and wave form to those of the deflection of the light beam and being clipped on both sides of amplitude by a pair of sound signals polarized inversely with each other and biased positively and negatively is used for gating the light beam. Consequently a high quality optical sound recording system unrelated to the carrier waveform used for the pulse-width modulation can be realized.

Qualification procedures for free-field conditions for sound-power determination of sound sources and methods for the determination of the appropriate environmental correction

Abstract: For determination of sound power of sources by the ’’method of enveloping measurement surfaces,’’ the test environment should provide a measurement surface which lies (1) outside the nearfield of the sound source under test and (2) inside a sound field free of undesired sound reflections from room boundaries or reflecting objects near the source. Methods to check the free‐field conditions and to qualify a given measurement surface for an actual source under test are (1) the absolute comparison test using a (small) calibrated reference sound source, (2) the relative comparison test using a small test sound source which radiates broadband noise that remains essentially constant during the measurement, and (3) the reverberant test, which requires measurement of reverberation time. Method 3 is only applicable in closed spaces (rooms). Methods 1 and 2 may be used in rooms and outdoors. Methods 1 and 2 require replacing the source under test by the reference sound source or test sound source in the test site. I...

Simultaneous measurement of sound velocity and sound attenuation in liquids by a correlation method

Abstract: The correlation between the input and output signals of an ultrasonic ransmitter–receiver system with two quartz transducers was used for the simultaneous measurement of sound speed and sound absorption in liquids With a correlator built from a semiconductor heterodyne detector (active mixer) followed by a low‐pass filter, the space‐dependent cross‐correlation function of the signals, which depicts the sound field in the liquid, was recorded on a x‐t recorder by continuous variation of the transmitter–receiver distance First tests with the apparatus yielded a reproducibility of the sound velocity of ±01%, that of the sound attenuation was within ±3%

Improving flow characteristics of sticky or granular materials - using loudspeakers to direct sound waves at specified frequencies at material in silo

Abstract: In cleaning off, loosening and improving the flow characteristics of granular, small piece or thick flowing materials in chambers, vessels, silos, etc. sound waves are used which transmit oscillations to the particles of the materials. This leads to a reduction of internal friction and to a release in tension. The sound is generated by electro-magnetic or electro-mechanical loudspeakers and is beamed from them. The frequency of the sound may be varied in accordance with the operating requirements, with the sound volume being similarly variable. The current requirements of the installation can normally be supplied by the domestic supply of 220 Volts, 50 Hz. The installation may be used as a stationary or a mobile operating unit.

Analog sound velocity calculator

Abstract: An analog apparatus for determining the velocity of sound at a selected position in a liquid media is provided with a means for representing the conductivity, temperature, and depth of the position in the form of analog voltages, and is further provided with an analog calculating means receiving each of the voltages for calculating the velocity of sound as a function of conductivity, temperature, and depth. The sound velocity may be calculated at a succession of positions to provide a sound velocity versus depth profile of the liquid media.