Band Sound Pressure Levels Research Articles

Acoustics research of propulsion systems

The liftoff phase induces high acoustic loading over a broad frequency range for a launch vehicle. These external acoustic environments are used in the prediction of the internal vibration responses of the vehicle and components. Present liftoff vehicle acoustic environment prediction methods utilize stationary data from previously conducted hold-down tests to generate 1/3 octave band Sound Pressure Level (SPL) spectra. In an effort to update the accuracy and quality of liftoff acoustic loading predictions, non-stationary flight data from the Ares I-X were processed in PC-Signal in two flight phases: simulated hold-down and liftoff. In conjunction, the Prediction of Acoustic Vehicle Environments (PAVE) program was developed in MATLAB to allow for efficient predictions of sound pressure levels (SPLs) as a function of station number along the vehicle using semi-empirical methods. This consisted of generating the Dimensionless Spectrum Function (DSF) and Dimensionless Source Location (DSL) curves from the Ares I-X flight data. These are then used in the MATLAB program to generate the 1/3 octave band SPL spectra. Concluding results show major differences in SPLs between the hold-down test data and the processed Ares I-X flight data making the Ares I-X flight data more practical for future vehicle acoustic environment predictions.

Vibration Based Prediction of Noise in Passenger Car Gearbox Structures

In this paper, a method for predicting the contributions to noise radiated by a vehicle gearbox structure using measured vibration responses is presented. The gearbox structure is idealised as a set of flat plates. The radiation efficiency is calculated from the physical properties and edge constraints of each plate and the measured vibration response of the gearbox structure surfaces. These data were used in a simple acoustical power theory to determine 1/3-octave band sound pressure level under free field conditions for the whole and individual noise sources. The predicted individual noise sources are then used to pinpoint the exact problem area in the gearbox structure with a view to noise reduction.

Analytical Technique for Predicting Passenger Car Gearbox Structure Noise Using Vibration Response Analysis

The gear pair assembly remains one of the major noise and vibration sources in power transmission systems typically used in automotive, aerospace and industrial applications. The gearbox structure vibration and noise signatures are often dominated by several high-level tonal peaks that occur at the fundamental gear mesh frequency and its harmonics and other sources. In this paper, development of the concept of a method for predicting the contributions to exterior noise radiated by an original vehicle gearbox structure using measured vibration response. An idealization of the gearbox surface as a set of flat plates was used to calculate the radiation efficiency from physical properties and edge (end) constraints of each plate, and the vibration response of the gearbox structure surface was measured using accelerometers. These data were used in a simple acoustical power theory to determine 1/3-octave band sound pressure level under free field conditions for the whole and individual noise sources. Moreover, the prediction of the individual noise sources in order to pinpoint the exact problem area in the gearbox structure so that treatment or redesign can be carried out in correct way. Research Article British Journal of Applied Science & Technology, 3(4): 860-883, 2013 861

A study of the aerodynamic and acoustic performance of an indoor unit of a DC-inverter split air-conditioner

The aerodynamic and acoustic performance of an indoor unit of a DC-inverter split air-conditioner is simulated by computational fluid dynamics (CFD) and computational aerodynamic acoustics (CAA). Numerical predictions of the flow rate (FR) and band sound pressure level (BSPL) are verified by experimental data and used to create an improved design. The parametric influence upon the values of FR and BSPL for the unit is analyzed for various cases of geometrical parameters of the indoor unit and the cross-flow fan. Two improved designs are developed that can increase the FR for the existing prototype of indoor unit while the BSPL remains lower.

Introduction of sound material in living environment 2004 (SMILE 2004): A sound source database for educational and practical purposes

Sound sources are used in various settings of education, research, and business that are related to architectural acoustics. Especially in educational scenarios, they are very effective for students to learn theories together with the experience of listening to actual sounds related to them. Sound Material In Living Environment (SMILE 2004), a sound source database published in 2004 in DVD form (Japanese only), is the culmination of our project. SMILE 2004 is intended as a collection of sample sounds for use in lectures on architectural acoustics at universities or industrial high schools. In addition, it is useful for measurements or demonstrations for academic or practical purposes. The database contains 913 digital sound files of real sounds and impulse responses, which cover the study fields of acoustics, noise control, and soundscapes. Each sound is accompanied by such information as the 1/1 octave band sound pressure levels and a brief explanation and photographs of the situation in which the sound was recorded. In addition, sample presentations for lectures and practical demonstrations are attached as examples of the use of this database for educational and practical purposes. Details of all the above features will be introduced in a demonstration.

Assessment of heavy earth-moving machinery noise vis-a-vis routine maintenance

Exposure to noise from various types of equipment/machinery has been identified as a critical health hazard for personnel working in the Indian mining industry. Heavy Earth Moving Machinery (HEMM) used in mines has been identified as a major source of noise, and several earlier investigators have proposed various types of retrofit measures on these machines to reduce noise. The present work was taken up with the objective of evaluating the noise generation characteristics of HEMM as influenced by periodic maintenance. Detailed noise measurements were carried out in a large open pit coal mine located in India. The effect of maintenance on noise production was assessed for ten dumpers and three dozers by measuring the A-weighted one-third-octave band sound pressure levels after the machines had been subjected to maintenance at the end of 300 hours, 500 hours, 750 hours and 1000 hours of use. Measurements were also carried out to assess the effect of maintaining specific HEMM systems noise characteristics at 1000 hours. Assessment of sound pressure levels at each periodic maintenance revealed major sound level reductions with 1000 hours maintenance. The data also identified the major noise generating systems in HEMM as air systems, exhaust systems, cooling systems and fuelmore » systems.« less

A method for evaluating automotive steering column noise

Automotive component suppliers are often called on to supply components which meet very vague requirements such as no audible noise from component. As odd as this might seem, this type of specification usually has a number associated with it. This number could be the measured A-frequency weighted sound pressure level, the total loudness, some averaged band sound pressure level, or even a vibration response level. The irony comes from the fact that most of these components do not generate noises on their own but only when subjected to intense vibration. To deal with this, component suppliers must somehow construct a measurement system which will capture these squeaks and rattles as the component is vibrated. This paper describes a measurement system and method for evaluating the noises generated by automobile steering columns. Descriptions are given of the various vibration inputs used, the equipment required, and the parameters measured.

Room noise criteria—A new approach at low frequencies

ANSI S12.2-1995, ‘‘Criteria for Evaluating Room Noise,’’ permits two different sets of criterion curves for evaluating room noise—room criterion (RC) and balanced noise criterion (NCB)—with little guidance as to the selection. These different procedures can have significant impacts on the cost of designs, especially in the low-frequency regions, since the difference in the criteria is more than 10 dB in the 16- and 31-Hz octave bands at lower levels. The authors suggest there are sufficient data to support a modification to ANSI S12.2, resulting in a single evaluation procedure based on the strengths of each of the two existing procedures. The authors suggest establishing room criterion curves for steady noise without abnormal temporal or spectral characteristics, with correction factors for noises with abnormal features. The suggested criteria for steady noise are the NCB curves based on time-average octave band sound pressure levels, and as a starting point, the correction factors for low-frequency tones and fluctuation differences between the NCB and RC curves. Analyses of currently funded research and re-examination of existing cases can determine the exact correction factors and measurement techniques to quantify these abnormal characteristics. A revised ANSI S12.2 could have basic criterion curves for steady, continuous, and broadband room noises, which could be used for design with correction factors to these basic criterion curves for sounds with abnormal characteristics. [See NOISE-CON Proceedings for full paper.]

Model scale testing of outdoor noise propagation from a power plant

A proposed cogenerating power plant of 70-MW capacity is to be located in an urban setting and will be subject to stringent environmental noise emission criteria. A four-cell forced draft cooling tower is included in the plant design. Computer modeling of cooling tower noise propagation to affected neighboring residences involved significant uncertainties due to the complex shapes of power plant building elements and existing surrounding buildings. Without a finished facility in which to perform full-scale field tests, the next best thing is a scale model test. This paper will discuss the specifics of the 1/20th scale test setup, materials, selection, instrumentation, test execution, some problems and some lessons learned. The results of the test will be given in terms of the differences in octave band sound-pressure levels between an unobstructed free-field value and the modeled in situ value, at several receptor positions. The test enabled the specification of more precise sound attenuating elements for the cooling tower fans and inlets in order to comply with the governing regulations.

Studies on noise and vibration of agricultural machines.

This paper reports on studies carried out regarding noise and vibration on a tractor and a binder which are the most commonly used for agricultural purposes. 1) The sound levels on both of a tractor and a binder were over 100 dBA, and the level on binder was larger about 6 dB than that on tractor. The 1/3 octave band sound pressure levels on a tractor exceeded the permissible criteria for 8 hr in the frequency range of 315 Hz and those on a binder exceeded it for 2 hr in the range of 630 Hz. 2) The vibration level on the handlebars of a tractor was the highest in the Y direction (117. 5 dB) and that under the seat was the highest in the vertical direction (97.5dB). The 1/3 octave band vibration acceleration levels for a tractor in the Y direction exceeded the exposure guideline for 4 hr in the frequency range of 16 to 50 Hz and those under the seat in the vertical direction exceeded the permissible criteria for 8 hr in the frequency range of 31.5 Hz. 3) The vibration level on the handlebars of a binder was the highest in the Z direction (114.1 dB) for the metal handlebars and (112. 1 dB) for the rubber grips. The band levels on a binder in the Z direction exceeded the guideline for 4 hr in the frequency range of 50 Hz and from 16 to 25 Hz. 4) We confirmed that there was sufficient danger of to hearing due to noise and vibration hazards in using both a tractor and a binder continuously for long periods of time in comparison with the exposure guideline and permissible criteria.

Relationship Between Static, Flight, and Simulated Flight Jet Noise Measurements

Jet noise data are acquired in three different forms: static, flight, and simulated forward flight. Often data must be transformed from one type to one of the other two. A number of studies exist defining the relationship between these three types of measurements. These studies are deficient because they do not explicitly consider the jet noise source characteristics or do not clearly distinguish between mean square pressure, power spectral density, and one-third octave band sound pressure level measurements. In the current study, the relationships between static, subsonic flight, and subsonic simulated forward flight one-third octave band sound pressure level measurements were defined.

Effect of thickness on airfoil surface noise

Noise emission from very small chord and very large chord airfoils was measured with eleven 0.63 cm microphones placed along a horizontal semicircle (4.57 m radius) that was centered at the leading edge of the test airfoil. The noise signals were analyzed by an automated spectrum analyzer which yielded 1/3-octave band sound pressure level spectra for each microphone, and the data were corrected to remove the effects of atmospheric attenuation and jet noise. It is found that the effect of thickness is large and must be accounted for in any fundamental airfoil noise theory that attempts to describe the noise emitted from real airfoils. Incident mean velocity gradients and compressibility must also be taken into account. The effect of thickness increases with frequency, with thick airfoils being quieter than thin ones.

Effect of operating parameters on noise emission from individual road vehicles

A noise survey has been undertaken on over 1000 vehicles operating on public roads. By the use of a remote device, vehicle velocity and acceleration have been measured and related to the instantaneous sound pressure level emission from the vehicle. The relationship is expressed in terms of A-weighted sound pressure level as a function of both velocity and acceleration. Octave band sound pressure levels are also expressed as functions of these variables. Light and heavy vehicle groups are considered separately.

Railroad and rail transit noise sources

The railroad or rail transit system design team must predict A-weighted, or preferably, octave band sound pressure levels near passing trains. This is the first step in estimating noise over any area of potential impact and in subsequent selection among noise abatement design alternatives. In this paper recently reported measurements of locomotive and railcar noise emission are reviewed and presented for use in making such design predictions. Locomotive noise is largely confined to the range of 75 to 95 dB(A) at 30 m (100 ft) for all speeds. For various railcars on tie and stone ballast track the trend of data suggest that A-weighted sound levels increase uniformly with speed over the speed range of available data, now reaching 400 km/h (250 mile/h). At a given speed these data typically have a noise level range of 15 dB. The data show an apparent lower bound for noise among steel wheel on steel rail vehicles operating today. Octave band spectra are given relative to A-levels for various locomotive and railcar types. These recent data disagree with some average spectra based on data from 1970 and earlier. Reported measurements and estimates of noise level reductions from various locomotive, railcar and track design factors are tabulated; the data exhibit considerable scatter. Two current procedures in which the above-mentioned data are used to predict pass-by noise levels and spectra are described and evaluated. It is concluded that for estimating noise level reduction due to combinations of treatments, or for estimating octave band reduction, procedures and useful data are limited. To provide more precise design information future experiments must take better account, either deterministically or statistically, of all factors which influence measured noise, including some discussed herein which cannot always be controlled by test team or design team decisions.

Methods of measuring and rating hearing protector performance.

This paper reviews various methods that have been used to measure and rate the performance characteristics of hearing protectors. Subjective and objective measurements methods will be discussed with respect to test method, environment and stimuli. A variety of methods for relating protector attenuation data to the requirements of a specific environment will be presented and discussed. The paper includes suggestions for use of this information with environmental data developed from octave band analysis or sound pressure level measured in dBA.

Characteristics of several industrial noise environments

In order to further define in the literature the characteristics for different noise environments existing in industry over 2000 work stations in eleven different industries were investigated. The data measured included dB(A) sound levels and octave band sound pressure levels. These data are analyzed with respect to mean slope, mean slope as related to dB(A) sound level, flatness, bands of concentrated acoustic energy (or pure tones) and the correlation between the measured dB(A) sound level and that predicted by using the measured octave band sound pressure levels.

Sound radiation from randomly vibrating cylinders of finite radius, with applications to textile‐loom noise reduction

Recent studies of textile‐loom noise suggest that the picking stick is the major contributor to the overall noise level produced by an automatic textile loom. This has led to research directed towards a better understanding of how sound is generated and radiated by the picking sticks. The present paper is concerned with the radiation of sound from randomly vibrating beams of circular cross section. Vibrations on the beam are considered as two traveling waves moving in opposite directions. The result is then averaged over all possible phase differences, assuming all to be equally probable. Expressions for the radiation efficiency are compared with previous results based on a modal approach. Predictions of the octave band sound pressure level based on the theory and on vibration data satisfactorily compare with experimental measurements.

Statistical characteristics of some standard reverberant sound field measurements

The space and time fluctuations of 1 3 - octave band sound pressure level and reverberation time measurements in hard-walled chambers have been investigated experimentally with the aim of determining characteristic probability density functions and typical variances. The basic random variables observed (that is, the mean square pressure and the reverberation time estimates) are acceptably described by a gamma distribution given in a paper by D. Lubman. A specific sampling procedure has been used for the experiments: the samples were taken in randomly chosen discrete positions and a time averaging performed in each position so that the time variances thus obtained were much smaller than the spatial variances. Such a sampling procedure leads to the statistical characteristics of the random variables being independent of any specific equipment used. With the aid of the basic probability density function, statements of precision with regard to time and space fluctuations can be made from the data obtained in this way. Confidence intervals for standard measurements of sound pressure level, reverberation time, sound pressure level difference, sound power level and transmission loss are presented. The typical variances in various possible situations are not always well-known, which means that one has to use estimated values of the variances in the derivation of the respective confidence statements. This fact causes some uncertainty, and one must regard the confidence statements thus obtained as approximate only. From the results obtained in these investigations, it appears that the degree of this uncertainty would be quite acceptable in a normal measurement situation.

Noise Generated by a Power Loom

An investigation was made to survey the actual noise level generated by a power loom, devise some method to reduce the noise level on the power loom and explore the possibility of controlling the noise generated by looms in a factory.A power loom was placed in a room, 494M3 in volume, of the workshop attached to our university. The mean value of six room-constants obtained in various octaves band measured 152m2. The noise level and the octave band sound pressure level were measured with a sound level meter, an octave band filter and a level meter. Equal-level contours of the noise level and the sound pressure level were drawn with these instruments. The noise level at a height of 150cm reached a maximum of 90phons.The noise within about 2 meters from the loom was louder on the diagonal line, but at 2 meters or a longer distance from the loom it was regarded as a point source. The frequency characteristic of the sound pressure level showed a peak value in the 1600-3200c/s band. Sound power in the band was 11 milliwatts.A piece of gum was stuck on the surface of the stopper and a spring was replaced with a suitable one to reduce impulsive. Then the noise made by the picking mechanism decreased by 4-5dB in a high-frequency range.After the gearing was insulated with a cover, the noise radiated by the gearing in the driving part decreased by 4-8dB in a middle and a high-frequency range. This indicates that the noise generated by a power loom can be reduced considerably by a complete adjustment of the loom and by insulating the sound source.

Progressive Wave System with Electro-Pneumatic Transducer

This paper describes a Progressive Wave System, 1 in. × 1 in. in cross section, employing one electro-pneumatic transducer in the frequency range 37.5–1200 cps. The unit is capable of generating sound pressure levels in excess of 160 db over-all sound pressure level. High-frequency energy is generated by conventional compression-type dynamic transducers. The high-frequency energy is combined with the low-frequency energy in a mixing unit which consists essentially of a 45° acoustic mirror. By means of appropriate slots, the mirror is made transparent in the low-frequency range and reflective at the higher frequencies. Design data on the acoustic mirror and measurements of over-all and octave band sound pressure levels, as well as variations in sound pressure over the cross section of the tube, particularly in the transition range from the plane to the progressive wave condition, are presented. The capability of the system when four electro-pneumatic transducers are employed also is discussed.