Constant Sound Pressure Level Research Articles

Assessment of FM systems with an ear canal probe tube microphone system.

A technique is described to measure the real-ear performance of an FM system using an ear canal probe tube microphone device. The method involves placement of the FM microphone next to the monitoring (compression) microphone of the probe tube assembly to produce a constant sound pressure level input to the FM system. With the probe tube in the ear canal, a hearing aid alone is measured with a 60 dB SPL input and the FM system attached to the hearing aid (personal FM system) is assessed with an 80 dB SPL input to account for the higher input levels that occur due to the 6 inch distance between the speaker's mouth and the FM microphone. This technique permits a rapid comparison of the real-ear response of the hearing aid and the FM system.

Effect of middle ear modification on umbo vibration. Human temporal bone experiments with a new vibration measuring system.

The effects of aditus blockage, decrease of tympanic cavity volume, and resection of the tensor tympani muscle on umbo displacement were studied in human temporal bones using a new non-contacting video measuring system. This system, combined with stroboscopic illumination, allowed measurement of umbo vibration down to 0.3 micron in the 0.1- to 4.5-kHz frequency range for a constant 124-dB sound pressure level at the tympanic membrane. Aditus blockage caused a mean increase of 5.4 dB with a peak at 2.8 kHz and a mean decrease of 3.4 dB below 1.6 kHz. Decrease of the tympanic cavity volume by 30% to 42% showed an improvement of 2.2 dB in the 1.4- to 3-kHz range and a similar loss below 1 kHz. Resection of the tensor tympani muscle produced a slight improvement of 1.7 dB at 1.8 kHz.

Development of auditory-evoked potentials in the cat. II. Wave latencies.

Brain stem and forebrain auditory-evoked potentials were studied parametrically during the first 90 postnatal days in unanesthetized kittens using tonal and click stimuli. This paper describes changes that occur in transmission time through the auditory pathway during development by analyses of the maturational time courses of latencies associated with waves of both auditory brain stem responses (ABR's) and late-occurring auditory-evoked potentials (AER's), recorded subdermally from the vertex. In response to click stimuli, ABR latencies were found to decay rapidly early in postnatal life and more slowly after the third postnatal week. Those trends were modeled as a two-stage sequential process, with a linear stage occurring between 7 and 18 postnatal days followed by an exponential stage during which adult latencies were achieved. AER latencies changes during development were less complicated, and followed a single-stage exponential time course. When threshold influences were taken into account--that is, when data were adjusted so that sensation level (SL) was constant across age--the latency-maturation curves associated with all ABR waves were adequately described by a single exponential, and latencies recorded from young animals were substantially shorter than latencies associated with the same aged animals when analyses were carried out with constant sound-pressure level (SPL) stimuli across age. In addition, the difference function, generated when isoasymptotic SPL and SL latency versus age functions were subtracted from one another, was also represented by an exponential curve, suggesting that at least two processes underlie the latency decay that occurs during postnatal development. Evoked responses to tonal stimuli throughout development were consistent with the basoapical developmental gradient that is observed anatomically.

High-Frequency Audiometry Using Precision Earphones: Reliability under Laboratory and Field Conditions

New circumaural earphones were tested in the frequency range from 100 Hz to 20 kHz and compared to commonly used supra-aural earphones. The circumaural earphone HD 230 (Sennheiser) generates test stimuli at up to 20 kHz with almost constant sound pressure levels when its pos ed on an artificial ear. The reproducibility of hearing threshold mea ed with a new microprocessor-controlled Békésy audiometer using the ne was nearly as good as under free-field conditions. The practicabil diagnostic value of high-frequency audiometry have been demonstrated er field conditions. For this application, the good sound attenuation these earphones (30 dB above 1 kHz) are important. High-frequency he ds of healthy pupils and of pupils with a history of otitis media are kHz, the hearing threshold level difference between both groups reached 20 dB.

Cross‐correlation studies of whole nerve action potential waveforms in chinchillas

A cross correlation was calculated for the compound action potential (CAP) response to a tone burst at a given reference frequency at 10 dB above CAP threshold and the compound action potential response to test signals at adjacent frequencies. The normalized cross‐correlation coefficient at zero time delay was measured as a function of the test frequency with signal level maintained at the 10 dB level above threshold. Constant CAP peak‐to‐peak magnitude and constant sound pressure level responses were also correlated with the response to the reference tone burst. Repeating the experimental procedure at several reference frequencies, the correlation coefficients formed band‐pass functions centered about the respective reference frequencies. The data were examined for a possible relationship between the bandwidths of the correlation coefficient functions and critical bandwidths and/or frequency difference limens. [Supported by the NINCDS.]

Road traffic noise attenuation by belts of trees

Measurements were made at a number of sites of road traffic noise propagating through belts of trees and bushes and above grass-covered ground, respectively. The belt widths were between 3 and 25 m. The distance from the road to the front of the belts also varied from site to site. The microphones were placed 1·5 m above the ground. A comparison between attenuations obtained, expressed as differences in equivalent constant A-weighted sound pressure levels, L Aeq , showed no significantly higher attenuation values for propagation through belts of trees than for propagation above grass-covered ground. Only in the frequency range above 2 kHz were attenuations significantly higher through the belts of trees and bushes. The belts of trees selected consisted mainly of deciduous trees and bushes between 5 and 10 years of age. Such types and widths are representative of what could often be used in normal urban situations in an attempt to provide practical noise reduction. According to the results of this investigation, however, these do not significantly reduce L Aeq 1·5 m above the ground. Planting of belts of trees and bushes between roads and dwellings might influence the environmental quality of residential areas due to nonacoustic factors or reduce nuisance due to spectral changes not affecting L Aeq . This has not been investigated.

Speech audiometry and fitting of hearing aids in noise.

Conventional speech audiometry, performed in a noise-free testing room, often gives only little information on the patient's impairment of speech perception in daily life. A sensitized form of free-field speech audiometry is suggested which simulates everyday speech perception and, at the same time, supplies comparable and reproducible results. The test material consists of phonetically balanced sentences. From separate loudspeakers, a standardized environmental noise with a constant sound pressure level, 50 dB(A) is admixed. By this technique, certain handicaps in speech perception, undetected by conventional test procedures, can be revealed. Above all, the results are in good accordance with the subjective assessment of disability. A standardization of 'handicap speech audiometry' based on these principles is therefore suggested. The comparative hearing-aid fitting in noise improves the final selection with special regard to practical use.

Cochlear mechanics, nonlinearities, and cochlear potentials.

Cochlear microphonic (CM) magnitude functions are shown from various turns of the cochlea at constant sound pressure level (SPL), constant stapes displacement, and constant stapes velocity. The similarities of these curves with basilar membrane displacement functions are considered. The influence of CM nonlinearity on the shape of the plots is discussed, and it is demonstrated that the response is extremely nonlinear in a narrow frequency range above the best frequency of the electrode location, while at other frequencies it is much more linear. All nonlinear phenomena that are demonstrable in cochlear potentials, namely harmonic distortion, summating potential generation, and interference effect, are primarily concentrated in the aforementioned nonlinear frequency region.

An investigation of the noise field produced near a half-inch diameter steam jet

The near noise field of a 0·5 in. diameter unchoked steam jet was investigated, using a nozzle installation and supply line designed and constructed for the purpose. The overall sound field (25 to 40,000 c/s) and the sound fields for three representative frequencies ( 1 3 - octave bandwidth) were plotted as contours of constant sound pressure level within 60 diameters of the jet nozzle exit. These noise fields formed the basis for discussion of the jet noise sound fields and in particular, the directional trends of the noise fields at various frequencies were investigated. The noise radiation decay rates in different directions were estimated and an approximate radial limit for the beginning of the inverse square law was fixed. To find the variation of sound pressure level with jet Mach number, the distribution of the velocity exponent was plotted as a contour map. The characteristics of each sound field were compared with previous work by other investigators; particular attention was paid to the different media used. All the characteristics of the sound fields investigated were compatible with both the quadrupole and simple source theories for the generation of sound, although some of the results were contradictory to statements made in other reports.

Use of Sensation Level in Measurements of Loudness and of Temporary Threshold Shifts

The hypothesis that equal sensation levels (SL) of stimulation give rise to equivalent loudnesses in normal observers is disproven by showing that recruitment often occurs near threshold even in normal ears. Since neither constant sound pressure levels (SPL) nor constant SL's including 0 dB SL (threshold itself), can be assumed to produce equal loudness, it is suggested that observers be equated at the most comfortable listening level (MCL).

AN EXPERIMENTAL STUDY OF THE ACOUSTIC IMPEDANCE OF THE MIDDLE EAR AND ITS TRANSMISSION PROPERTIES.

The acoustic impedance at the eardrum and the cochlear microphonic potential at constant sound pressure level at the eardrum were measured in anesthetized cats and rabbits. It was found that the inverse of the impedance (admittance) and the cochlear microphonic potential at constant sound pressure are proportional over a large frequency range. In addition, the effects of opening of the middle-ear cavities and of variation of the air pressure in the cavity, as well as of activity of the middle-ear muscles were studied. In further experiments, the impedance of the eardrum itself and of the middle ear with the cochlea disconnected was measured.

Transfer Function of the Middle Ear

The transfer function of the middle ear was investigated in anesthetized cats. The amplitude and phase angle of vibration of the malleus, incus, and round window for constant sound-pressure level at the eardrum was measured by a capacitive probe. The acoustic impedance at the eardrum in the frequency range from 200 to 8000 cps was also measured. The acoustic impedance at the eardrum was found to be proportional to the ratio of sound pressure at the eardrum and velocity of the malleus from about 200 cps to 4 kc/sec, showing that the eardrum acts as a rigid piston in this frequency region. By comparing the amplitude of the malleus and that of the round window, it was found that the coupling between the malleus and the cochlear fluid is very rigid. A second-order low-pass function described by its Laplace transform was found to be a good approximation to the experimental data, up to a frequency of about 4 kc/sec, or approximately 4 times the cutoff frequency. In the frequency range where the eardrum acts as a rigid piston, it is thus possible to estimate the transfer function of the middle ear entirely from the acoustic impedance data in the plane of the eardrum.