Sound Pressure Level Difference Research Articles

Nonlinear effects in the generation of sound by internal‐combustion engine exhausts

The exhaust waveforms and spectra of single cylinder, two‐ and four‐stroke engines have been studied under closely controlled operating conditions, to provide information on the nonlinear parameters to include in analytical treatments of mufflers. Typical waveforms at an open exhaust, obtained by signal averaging to remove cycle to cycle variations, consist of a large amplitude (2–6 psi), highly damped, sinusoidal pulse followed by a smaller amplitude (∼0.5 psi) oscillation after the port/valve closes. The amplitude of the radiated pulse is found to decrease progressively as the angle from the direction of the exhaust discharge is increased, leading to front‐back differences in sound pressure level of ∼10 dB at low frequencies. it appears that the dominant source properties can be predicted by treating the cylinder and exhaust port as a Helmholtz resonator, with a pressure dependent orifice resistance of the type described by Ingard. Measurement of the peak velocity of efflux of the exhaust gases permits the observed radiation patterns to be predicted by including convection of the simple acoustic source by the flow.

Use of ‘‘corner microphones’’ for sound power measurements in a reverberation chamber

A comparison was made between acoustic measurements conducted with microphones mounted in the trihedral corners of the 425‐m3 NBS reverberation chamber and similar measurements using microphones located in the room interior, away from the room boundaries. Measurements of broadband and discrete‐frequency sound pressure and of reverberation time were included. It was found that for frequency bands below the 200‐Hz 1/3‐octave band, the difference in sound pressure level between the corner and interior locations was, in general, more than 9 dB. In addition, the variations in the broadband steady‐state sound pressure level and in the reverberation time were much less among the corners than among the interior locations for frequencies below 100 Hz. It is concluded that for the lower frequency bands, use of the corner locations may reduce systematic errors associated with the interference patterns and provide greater measurement precision because of the lower spatial variance.

Acoustical characteristics of outlet with glass wool chamber and duct

Low‐frequency sound generated from air‐duct systems is difficult to reduce by a terminal device. We have, therefore, studied the use of an outlet with glass wool chamber and duct to increase the effective insertion loss. As the transmission loss of the chamber walls is small, low‐frequency sound energy equivalent to the sound insulation power of ceiling board can be passed into the space above the ceiling by using these units. The acoustical characteristics of steel plate chambers and flexible ducts were also studied for comparison. According to the results obtained, the power level of transmitted sound at 125 Hz is lowered by about 1 dB by the use of a glass wool chamber, and by about 12 dB in the case of a steel plate chamber. The sound pressure level difference of an auditorium ceiling is usually about 15 dB.

Clinical critical band estimation.

A modified version of the method of loudness summation, developed for clinical critical band estimation, is presented. The stimuli are noise bands centred around 1 kHz. The standard procedure includes 1) determination of the "sensitivity curve": the loudness difference between broad band noise and narrow band noise as a function of the level. For clinical use this function is necessary for evaluation of the level with the most rapid growth of loudness with bandwidth, i.e. the optimum level for sharp determination of the critical bandwidth. 2) Determination of the critical band function, i.e. the difference in sound pressure level required for equal loudness of the test noise band and the reference as a function of the bandwidth. This determination is performed at a level with maximum growth of loudness, evaluated by the sensitivity curve. 3) Critical band estimation by a mathematical model, described in another work (Bonding et al., 1978) from the data obtained. The method is analysed regarding feasibility and reproducibility.

An investigation of the measurement of transmission loss

The determination, by means of the two reverberant room method, of the transmission loss of apertures of even very large size is shown to be practical and a new data reduction procedure is presented. The new procedure requires the measurement of sound pressure level differences in both directions and measurements of the Sabine absorption in each room. The use of apertures of large size as effective standard partitions of zero transmission loss is then possible and has allowed the investigation of the measurement technique. Theoretical analysis and measurements on nine different apertures in four different testing facilities have provided the data upon which the following conclusions are based. 1. 1. The ordinary way of determining transmission loss, for example, as stated in ISO DIS 140 may be used with reasonable accuracy for partitions providing a level difference of 10 dB or greater in each direction. However, if the sum of the level differences in each direction is less than 20 dB then consideration must be given to cross-coupling between the rooms during the course of the receiving room calibration procedure. 2. 2. For partitions and apertures which provide level differences of less than 10 dB the transmission loss may be determined by using a new modified procedure based on measurements of level differences in each direction and either (a) initial reverberation times in each room or (b) measurements of the injected acoustic power in each room during the level difference measurements. 3. 3. Experimental data suggest that for satisfactory measurement of transmission loss by means of the two reverberant rooms technique the following conditions should be satisfied: (a) the modal density in all frequency ranges should not be less than half of the reverberation time; (b) the reverberation time should not be less than 550 divided by the measurement band centre frequency; (c) the Sabine absorption should be less than 6% of the total surface area of the receiving room.

Recent field testing experience of privacy between dwellings using ASTM Recommended Practise 597

Some recent experience with field testing of privacy between dwelling units using the method of the new ASTM Recommended Practise 597 is reported and the effects of airborne flanking paths on the ratings obtained is discussed. The results of the testing experience indicate that the single number rating provided by this new test method is approximately two points less than the Noise Insulation Class Rating (NIC) obtained by grading 1/3-octave-band noise reduction measurements according to ASTM 413. It is suggested that this new single number rating obtained from A-weighted sound pressure level differences utilizing the specially shaped source and absorption test spectra be called the Privacy Index.

Model for the localization of sound in the azimuthal plane

An objective study of the interaural time difference (ITD) was performed on a manikin comprised of a head and a torso. Data were taken for both a bare and a closed torso. The measured ITD's correspond reasonably accurately at the low and the high frequencies to the computed theoretical values for a rigid sphere of an effective radius a. The theoretical ratio of the low-frequency (<500 Hz) ITD to the high frequency (>2000 Hz) ITD is 32. The measured ITD is a minimum between 1.4 and 1.6 kHz for angles of incidence, θinc, of sound between 15° and 60°. At both the low and the high frequencies the data can be expressed by universal curves when the ITD is normalized by (a/c0) sinθinc, where c0 is the speed of sound in air and θinc is the angle of incidence. Both the ITD and the interaural sound pressure level difference (ILD) show differences between measurements made with the bare torso and those with a clothed torso. These objective results support the subjective measurements of past experiments which showed that in a man there was no localization improvement below approximately 500 Hz, poor localization between 1000 Hz and 2000 Hz, and a change in the localization cue around 1400 Hz from ITD to ILD.

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.

Sound pressure levels and radiation patterns of the vocalizations of some North American frogs and toads

Sound pressure levels of the vocalizations of 21 species of North American anurans were measured in the field. In most species the peak levels at 50 cm in front of the male exceeded 100 dBre 2×10−4 μbar (Table 2). The sound pressure levels of the calls of most individuals varied by less than 2.5 dB, but intraspecific variation was significantly higher, averaging about 8.5 dB (Tables 1, 2). Sound pressure levels of mating calls were positively correlated with body size in the toad,Bufo americanus (Fig. 2); however, interspecific differences in sound pressure levels were not clearly related to interspecific differences in body size. Measurements of directivity patterns indicated that sound fields around three males ofHyla crucifer were uniform, whereas two males ofH. chrysoscelis represented directional sound sources (Table 3).

Clinical Studies on Acoustic Trauma due to Explosion

Audiometric findings of traumatic deafness caused by explosion were discussed in 18 cases. All subjects complained of tinnitus immediately after exposure to explosion and audiometric examination revealed hearing impairment in 15 cases. About four days after explosion tinnitus faded away in all cases, and after two weeks the hearing impairment was restored in most cases.In order to measure, the difference of sound pressure level in the room, the pistol shot was employed as a sound source. The data obtained in this experiment suggested that individual hearing loss was not always proportioned to the intensity of the sound. As a result, the authors deduced that individual difference of susceptibitily played an important role in the occurrence of the traumatic deafness.

Perceptual isolation and schizophrenia.

The early investigators in the field of perceptual isolation were at McGill University, especially Bexton et al. (1954). Since then there have been many subsequent works and papers on the subject. In general, published results of this type of experiment can be divided into two groups. Sometimes marked sequelae to sensory deprivation have been described (Bexton et al., 1954 and Goldberger et al., 1958). Such disturbances included thinking, imagery and time appreciation, together with delusional and hallucinatory phenomena. On the other hand, at Princeton University, Vernon et al. (1956), under very carefully controlled experimental conditions, found virtually no sequelae. A paper published from this hospital (Smith and Lewty, 1959) described a silent room which was designed, constructed and standardized to a mean sound pressure level difference of 80 decibels. (Measurements made at octave intervals from 80-10,000 c./sec.) In this particular room 20 volunteers spent varying periods from 5 hours 50 minutes to 92 hours 20 minutes under conditions of partial and complete sensory deprivation. We used a room so as to mitigate the physical effects of reduced somatic activity and fur gloves were used instead of the more common cardboard cylinders, which after a period of time produce a great deal of skin irritation and indeed abrasions, and introduce a great deal of unnecessary complications into a study of pure sensory deprivation.

Comparison of Threshold Data Obtained by the Method of Limits and the Method of Adjustment

One of the major areas of emphasis in contemporary audiology is the development of audiometric techniques for dealing with clinical problems related to permanent hearing impairments. However, few studies have been conducted which attempt to establish the relationship of threshold values obtained under the various testing conditions. The present paper describes the results obtained for 105 subjects monaurally tested in an anechoic chamber on a modified ADC audiometer and a Békésy-type audiometer. A comparison of test results for nine frequencies between 250 cps and 8000 cps for each ear shows statistically significant differences in mean threshold sound pressure levels for nearly all frequencies, with the ADC values lower in most instances. Likewise, statistically significant differences in the variability of threshold values were obtained at all but one frequency below 2000 cps, with variability less in the ADC values. There also appears to be a general tendency for the product-moment correlation between ADC and Békésy threshold values to increase with an increase in the frequency of the testing tone. These findings obtain whether the Békésy-type audiograms are read using a “midpoint curve” or a “peak curve” to provide the threshold data. Although no significant differences in the variability of “midpoint” and “peak” Békésy threshold values were obtained at any frequency for either ear, all differences in mean threshold sound pressure levels are statistically significant. The results of the study clearly reveal the effects of testing method on mean thresholds of hearing and indicate the need for the standardization of procedures in large-scale hearing tests. This work was supported by the U. S. Air Force.

Measurement of Difference in Loudness Between Typing Noises

It is difficult to ascertain the absolute loudness of typing noise by objective measurement. However, it may be practical to use such measurement to determine the loudness level difference in a comparison of the sounds from two typewriters. The requirements that must be met by an accurate measurement technique depend on the degree of accuracy required. Information pertaining to the accuracy necessary has been obtained by employment of two loudness matching procedures over a period of several months. The purpose of the tests was to determine the difference in sound-pressure level of typing noise necessary to enable a person to identify reliably the louder of two typewriters operated simultaneously in a quiet background. It was found that under the conditions adopted for the investigation, differences in sound-pressure level had to be nearly one decibel before they could be recognized 50 percent of the time. Characteristics of typing noise other than pressure level are discussed. The opinion is expressed that these will, in many cases, not cause significant measurement error, and that difference in loudness between typing noises can often be measured with practical accuracy by means of a sound-level meter.

Measurement of Difference in Loudness between Typing Noises

It is difficult to ascertain the absolute loudness of typing noise by objective measurement. However, it may be practical to use such measurement to determine the loudness level difference in a comparison of the sounds from two typewriters. The requirements that must be met by an accurate measurement technique depend on the degree of accuracy required. Information pertaining to the accuracy necessary has been obtained by employment of two loudness matching procedures over a period of several months. The purpose of the tests was to determine the difference in sound pressure level of typing noise necessary to enable a person to identify reliably the louder of two typewriters operated simultaneously in a quiet background. It was found that under the conditions adopted for the investigation, differences in sound pressure level had to be nearly one decibel before they could be recognized 50 percent of the time. Characteristics of typing noise other than pressure level are discussed. The opinion is expressed that these will, in many cases, not cause significant measurement error, and that difference in loudness between typing noises can often be measured with practical accuracy by means of a sound level meter.