Threshold Sound Pressure Research Articles

Equivalent hearing threshold levels for the Etymotic Research ER-10C otoacoustic emission probe

Objective: To determine the equivalent threshold sound pressure levels (ETSPL) for a commercially available distortion product otoacoustic emission (DPOAE) probe, and to study the impact of probe fitting and eartip size on the calibration. Design: Twenty-eight otologically normal test subjects participated in the ETSPL determination for the Etymotic Research ER-10C probe. Study sample: ETSPLs were determined up to 16 kHz and were compared to the reference hearing thresholds associated with the ER-3A insert earphone. Both ‘regular’ and ‘baby’ foam eartips were used. Results: At most frequencies, no significant threshold differences were observed between the insert earphone and the DPOAE probe. However, at 1 kHz and 4 kHz, the mean thresholds for the insert earphone were generally lower than those for the DPOAE probe, suggesting systematic differences at those frequencies. Repeated calibration runs resulted in deviations of about 0.6 dB. Similar deviations were noticed when using foam eartips of different sizes up to 10 kHz. Conclusions: Knowing the reference thresholds for DPOAE probes enables measurements of (subjective) hearing thresholds and (objective) otoacoustic emissions using the same probe. Probe fitting and eartip size had negligible effect on the determination of ETSPLs. The obtained data may be proposed for inclusion in future audiometry standards.

Behavioral responses of two captive bottlenose dolphins (Tursiops truncatus) to a continuous 50 kHz tone

At present, the fundamental frequencies of signals of most commercially available acoustic alarms to deter small cetaceans are below 20 kHz, but it is not well ascertained whether higher frequencies have a deterrent effect on bottlenose dolphins (Tursiops truncatus). Two captive bottlenose dolphins housed in a floating pen were subjected to a continuous pure tone at 50 kHz with a source level of 160 ± 2 dB (re 1 μPa, rms). The behavioral responses of dolphins were judged by comparing surfacing distance relative to the sound source, number of surfacings, and number of echolocation clicks produced, during forty 15 min baseline periods with forty 15 min test periods (four sessions per day, 40 sessions in total). On all 10 study days, surfacing distance and the number of surfacings increased while click production decreased during broadcasts of test sound. The avoidance threshold sound pressure level for a continuous 50 kHz tone for the bottlenose dolphins, in the context of this study, was estimated to be 144 ± 2 dB (re 1 μPa, rms). The results indicated that a continuous 50 kHz tonal signal can deter bottlenose dolphins from an area.

AMTAS®: Automated method for testing auditory sensitivity: III. Sensorineural hearing loss and air-bone gaps

Objective: The objectives were to measure the occlusion effect produced by three earphones—circumaural, supra-aural, and insert—and to compare air- and bone-conduction thresholds obtained with manual and automated methods for subjects with sensorineural hearing loss. Design: Acoustic and psychoacoustic occlusion effects were measured with each earphone. Manual and automated, air- and bone-conduction thresholds were compared. Study sample: Occlusion effects were measured for six adult subjects with normal external and middle ears. Pure-tone thresholds were measured for nineteen ears of thirteen subjects with sensorineural hearing loss. Results: The supra-aural earphone produced the largest occlusion effects, followed by the insert and circumaural earphones. Some systematic differences in air-conduction thresholds were found for the two procedures that may be attributable to earphone differences. A large air-bone gap at 4 kHz, reported in a previous study, was replicated. Conclusions: From 0.5 to 8.0 kHz, occlusion effects produced by the circumaural earphone are sufficiently small that covering the ear does not appreciably alter bone-conduction thresholds. Air-conduction threshold differences warrant further study to determine if reference equivalent threshold sound pressure levels for the two earphones produce equivalent thresholds. The large air-bone gap at 4 kHz suggests the possibility of an incorrect reference equivalent threshold force level at that frequency.SumarioObjetivo: Los objetivos fueron medir el efecto de oclusión producido por tres auriculares – circumaural, supra-aural y de inserción – y comparar umbrales de conducción aérea y ósea obtenidos con métodos manuales y automatizados para sujetos con hipoacusia sensorineural. Diseño: Se midieron los efectos de oclusión acústicos y psicoacústicos con cada auricular. Se compararon los umbrales de conducción aérea y ósea manuales y automatizados. Muestra del Estudio: Se midieron los efectos de oclusión para seis sujetos adultos con oídos externos y medios normales. Resultados: Los auriculares supra-aurales produjeron los mayores efectos de oclusión, seguidos de los auriculares de inserción y los circumaurales. Se encontraron algunas diferencias sistemáticas en los umbrales de conducción aérea para los dos procedimientos que pueden atribuirse a las diferencias en los auriculares. Una gran brecha aéreo-ósea en 4 kHz que se reportó en un estudio previo, fue replicada. Conclusiones: De 0.5 a 8 kHz, los efectos de oclusión producidos por los auriculares circumaurales son lo suficientemente pequeños por lo que cubrir el cubrir el oído no altera apreciablemente los umbrales de conducción ósea. Las diferencias en los umbrales de conducción aérea exigen estudios adicionales para determinar si los niveles umbrales equivalentes de referencia de presión sonora para los dos auriculares producen umbrales equivalentes. La mayor brecha aéreo-ósea en 4 kHz sugiere la posibilidad de un nivel de referencia equivalente de fuerza umbral en esa frecuencia.

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for sweeps (1–2 kHz and 6–7 kHz bands) mimicking naval sonar signals

The distance at which active naval sonar signals can be heard by harbor porpoises depends, among other factors, on the hearing thresholds of the species for those signals. Therefore the hearing sensitivity of a harbor porpoise was determined for 1 s up-sweep and down-sweep signals, mimicking mid-frequency and low-frequency active sonar sweeps (MFAS, 6-7 kHz band; LFAS, 1-2 kHz band). The 1-2 kHz sweeps were also tested with harmonics, as sonars sometimes produce these as byproducts of the fundamental signal. The hearing thresholds for up-sweeps and down-sweeps within each sweep pair were similar. The 50% detection threshold sound pressure levels (broadband, averaged over the signal duration) of the 1-2 kHz and 6-7 kHz sweeps were 75 and 67 dB re 1 μPa(2), respectively. Harmonic deformation of the 1-2 kHz sweeps reduced the threshold to 59 dB re 1 μPa(2). This study shows that the presence of harmonics in sonar signals can increase the detectability of a signal by harbor porpoises, and that tonal audiograms may not accurately predict the audibility of sweeps. LFAS systems, when designed to produce signals without harmonics, can operate at higher source levels than MFAS systems, at similar audibility distances for porpoises.

Equivalent threshold sound pressure levels (ETSPL) for Interacoustics DD 45 supra-aural audiometric earphones

This paper describes the determination and results of pure-tone equivalent threshold sound pressure levels for the Interacoustics DD 45 audiometric earphone equipped with standard Model 51 cushions. The size and shape of the DD 45 transducer resembles the classic Telephonics TDH 39 earphone. Pure-tone hearing threshold measurements were performed for both ears of 29 test subjects. All audiometric frequencies from 125 to 8000 Hz were used. The data are intended for inclusion in future standardized reference equivalent threshold sound pressure levels. The results show that the DD 45 may be a good substitute for the THD 39 without the traditional 5-dB problem at 6000 Hz.SumarioEste trabajo describe la determinación y los resultados de los niveles de presión sonora para umbrales equivalente de tonos puros, con auriculares audiométricos Interacoustics DD 45 equipados con amortiguadores estándar Modelo 51. El tamaño y la forma de los transductores DD 45 se parecen al auricular clásico Telephonics TDH 39. Se realizaron mediciones de umbrales auditivos con tonos puros en ambos oídos de 29 sujetos de estudio. Se usaron todas las frecuencias audiométricas desde 125 a 8000 Hz. Los datos serán incluidos en referencias futuras de estandarización de presión sonora para umbrales equivalentes. Los resultados muestran que los DD 45 pueden ser un buen sustituto para los TDH 39 sin el problema tradicional de 5-dB en 6000 Hz.

Equivalent threshold sound pressure levels (ETSPL) for Sennheiser HDA 280 supra-aural audiometric earphones in the frequency range 125 Hz to 8000 Hz

Hearing threshold sound pressure levels were measured for the Sennheiser HDA 280 audiometric earphone. Hearing thresholds were measured for 25 normal-hearing test subjects at the 11 audiometric test frequencies from 125 Hz to 8000 Hz. Sennheiser HDA 280 is a supra-aural earphone that may be seen as a substitute for the classical Telephonics TDH 39. The results are given as the equivalent threshold sound pressure level (ETSPL) measured in an acoustic coupler specified in IEC 60318-3. The results are in good agreement with an independent investigation from PTB, Braunschweig, Germany. From acoustic laboratory measurements ETSPL values are calculated for the ear simulator specified in IEC 60318-1. Fitting of earphone and coupler is discussed. The data may be used for a future update of the RETSPL standard for supra-aural audiometric earphones, ISO 389-1.

The influence of 70 and 120 kHz tonal signals on the behavior of harbor porpoises ( Phocoena phocoena) in a floating pen

Two harbor porpoises in a floating pen were subjected to five pure tone underwater signals of 70 or 120 kHz with different signal durations, amplitudes and duty cycles (% of time sound is produced). Some signals were continuous, others were intermittent (duty cycles varied between 8% and 100%). The effect of each signal was judged by comparing the animals’ surfacing locations and number of surfacings ( i.e. number of respirations) during test periods with those during baseline periods. In all cases, both porpoises moved away from the sound source, but the effect of the signals on respiration rates was negligible. Pulsed 70 kHz signals with a source level (SL) of 137 dB had a similar effect as a continuous 70 kHz signal with an SL of 148 dB (re 1 μPa, rms). Also, a pulsed 70 kHz signal with an SL of 147 dB had a much stronger deterring effect than a continuous 70 kHz signal with a similar SL. For pulsed 70 kHz signals (2 s pulse duration, 4 s pulse interval, SL 147 dB re 1 μPa, rms), the avoidance threshold sound pressure level (SPL), in the context of the present study, was estimated to be around 130 dB (re 1 μPa, rms) for porpoise 064 and around 124 dB (re 1 μPa, rms) for porpoise 047. This study shows that ultrasonic pingers (⩾70 kHz) can deter harbor porpoises. Such ultrasonic pingers have the advantage that they do not have a “dinner bell” effect on pinnipeds, and probably have no, or less, effect on other marine fauna, which are often sensitive to low frequency sounds.

Does overexposure modify the fine structure of distortion product otoacoustic emissions

It is investigated, whether the pattern of distortion product otoacoustic emission (DPOAE) fine structure (quasiperiodic variations across frequency) is altered by an acoustical overexposure. DPOAE fine structures are determined in 16 normal-hearing humans using a high frequency-resolution and primary levels of L1/L2=65/45 dB. DPOAEs are measured both before and after the subjects are monaurally exposed to a 1 kHz tone lasting for 3 min at an equivalent threshold sound pressure level of 105.5 dB. After the exposure the DPOAE levels are shifted to lower values and recover to the initial levels within 20 min. The DPOAE fine structure pattern is analyzed by an automatic classification algorithm, which determines ripple center frequency, ripple spacing, ripple height, and ripple prevalence. For some individuals, an alteration of some of the parameters could be observed after the exposure, i.e., either a more pronounced or a less pronounced fine structure could be observed after the exposure. Across subjects, no shift of the fine structure pattern in frequency could be observed, also none of the calculated parameters changed consistently after the exposure. The results do not support the idea that the DPOAE fine structure might contain more information about the state of hearing than the DPOAE level.

Air and bone conduction evoked potential audiometry in the northern elephant seal

Elephant seals (Mirounga angustirostris) are the largest and most aquatic of the pinnipeds, spending up to eight months of the year at sea diving to depths as great as 1600 m. The pinna is absent in the elephant seal and the middle ear cavity and auditory canal are lined with a cavernous tissue, both of which are likely adaptations to deep diving. Elephant seals demonstrate a greater sensitivity to low frequency sounds than do other pinnipeds and an overall greater sensitivity to underwater sound than to airborne sound. The relative importance of sound conduction pathways in the elephant seal is undetermined, although it has been speculated that bone conduction pathways are important to underwater hearing in this species. To compare the sensitivity of the elephant seal to both air and bone conducted stimuli, auditory evoked responses were recorded in seals exposed to signals presented through headphones and via a bone vibrator. In comparison to airborne stimuli, bone conduction methods provide an opportunity to more effectively study sensitivity to low frequency sounds, but are challenged by a lack of reference equivalent threshold sound pressure levels. Future efforts should compare bone conduction and direct field audiometry results obtained within the same individual.

Behavioral avoidance threshold level of a harbor porpoise (Phocoena phocoena) for a continuous 50kHz pure tone

The use of ultrasonic sounds in alarms for gillnets may be advantageous, but the deterring effects of ultrasound on porpoises are not well understood. Therefore a harbor porpoise in a large floating pen was subjected to a continuous 50 kHz pure tone with a source level of 122+/-3 dB (re 1 microPa, rms). When the test signal was switched on during test periods, the animal moved away from the sound source. Its respiration rate was similar to that during baseline periods, when the sound was switched off. The behavior of the porpoise was related to the sound pressure level distribution in the pen. The sound level at the animal's average swimming location during the test periods was approximately 107+/-3 dB (re 1 microPa, rms). The avoidance threshold sound pressure level for a continuous 50 kHz pure tone for this porpoise, in the context of this study, is estimated to be 108+/-3 dB (re 1 microPa, rms). This study demonstrates that porpoises may be deterred from an area by high frequency sounds that are not typically audible to fish and pinnipeds and would be less likely masked by ambient noise.

Reference hearing threshold levels for short duration signals

Hearing thresholds for clicks and tonebursts were measured on 26 otologically normal persons (age 18–25 years) using the earphones Sennheiser HDA 200 and Telephonics TDH 39. The test signals are specified in IEC 60645-3 and in ISO 389-9. The acoustic test signals were presented both as a single stimulus and as repeated stimuli with the repetition rate of 20 Hz. The frequencies used for the toneburst were: 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz. Test signals and measurement methods were all in accordance with the recommendations given in ISO 389-9: Preferred test conditions for the determination of reference hearing threshold levels. The results are given as peak-to-peak equivalent threshold sound pressure levels (peETSPL). The results are in good agreement with other sparse results from literature and are part of the basis for the ISO 389-6 standard from 2007.

Towards a unifying basis of auditory thresholds: Distributions of the first-spike latencies of auditory-nerve fibers

Detecting sounds in quiet is the simplest task performed by the auditory system, but the neural mechanisms underlying perceptual detection thresholds for sounds in quiet are still not understood. Heil and Neubauer [Heil, P., Neubauer, H., 2003. A unifying basis of auditory thresholds based on temporal summation. Proc. Natl. Acad. Sci. USA 100, 6151–6156] have provided evidence for a simple probabilistic model according to which the stimulus, at any point in time, has a certain probability of exceeding threshold and being detected. Consequently, as stimulus duration increases, the cumulative probability of detection events increases, performance improves, and threshold amplitude decreases. The origin of these processes was traced to the first synapse in the auditory system, between the inner hair cell and the afferent auditory-nerve fiber (ANF). Here we provide further support for this probabilistic “continuous-look” model. It is derived from analyses of the distributions of the latencies of the first spikes of cat ANFs with very low spontaneous discharge rates to tones of different amplitudes. The first spikes in these fibers can be considered detection events. We show that, as predicted, the distributions can be explained by the joint probability of the occurrence of three independent sub-events, where the probability of each of those occurring is proportional to the low-pass filtered stimulus amplitude. The “temporal integration” functions of individual ANFs, derived from their first-spike latencies, are remarkably similar in shape to “temporal integration” functions, which relate threshold sound pressure level at the perceptual level to stimulus duration. This further supports a close link between the mechanisms determining the timing of the first (and other) evoked spikes at the level of the auditory nerve and detection thresholds at the perceptual level. The possible origin and some functional consequences of the expansive power-law non-linearity are discussed.

Reference zero for the calibration of air-conduction audiometric equipment using ‘tone bursts’ as test signals

The aim of this study was to determine reference peak-to-peak threshold sound pressure levels for air conduction sound transducers using groups of tone bursts as test signals. For this purpose, threshold measurements with five different earphones and a loudspeaker were carried out on groups of 25 young, otologically normal test subjects in the frequency range between 250 Hz and 8000 Hz, following as closely as possible the ISO Preferred Test Conditions. The dependence of the results on repetition rate, type of sound transducer, gender and age of the test subjects, and on reference pure tone thresholds was investigated. The results mainly depend on the reference equivalent sound pressure levels for pure tones of each sound transducer. Together with the results of another study carried out in Denmark, the data of the present study will form the basis for the International Standard ISO 389 Part 6 on reference hearing thresholds for acoustic test signals of short duration.

Deterring effects of 8–45kHz tone pulses on harbour seals (Phoca vitulina) in a large pool

The marine aquaculture industry suffers losses due to pinniped attacks which damage net enclosures and fish stocks. Acoustic harassment devices (AHDs) emit loud sounds which are intended to deter pinnipeds from approaching aquaculture enclosures. At present, many AHDs emit sounds in the 8-20 kHz frequency range. It is not known whether sounds of higher frequencies have a deterrent effect on seals. Therefore five captive harbour seals (Phoca vitulina) were subjected to four series of tone pulses together spanning a broad frequency range (8, 16, 32 and 45 kHz). Pulse duration was 250 ms and pulse interval was 5s. Each of the four sounds was made deterrent by increasing the amplitude. The seals reacted by swimming away from the sounds. The displacement effect of each sound was judged by comparing the animals' surface positions, and number of surfacings, during ten 45 min baseline periods with ten 45 min test periods per frequency (one frequency per day in rotation, 40 sessions in total). The seals were displaced by all four frequencies throughout the 40 trial days. The seals came to the surface more often when the test tones were produced than in the baseline periods. The initial displacement distances did not change over the 40 test days. This suggests that operating AHDs for only short periods will be more effective and less likely to result in habituation by the seals than operating them continuously. The discomfort threshold sound pressure level (SPL) was established for each of the four pulse frequencies. The acoustic discomfort threshold SPL is defined as the boundary SPL between the area that the animals generally occupied during the transmission of the sounds and the area that they generally did not enter during sound transmission. The discomfort threshold SPL may depend on the context.

Towards a Unifying Basis of Auditory Thresholds: The Effects of Hearing Loss on Temporal Integration Reconsidered

For signal detection and identification, the auditory system needs to integrate sound over time. It is frequently assumed that the quantity ultimately integrated is sound intensity and that the integrator is located centrally. However, we have recently shown that absolute thresholds are much better specified as the temporal integral of the pressure envelope than of intensity, and we proposed that the integrator resides in the auditory pathway's first synapse. We also suggested a physiologically plausible mechanism for its operation, which was ultimately derived from the specific rate of temporal integration, i.e., the decrease of threshold sound pressure levels with increasing duration. In listeners with sensorineural hearing losses, that rate seems reduced, but it is not fully understood why. Here we propose that in such listeners there may be an elevation in the baseline above which sound pressure is effective in driving the system, in addition to a reduction in sensitivity. We test this simple model using thresholds of cats to stimuli of differently shaped temporal envelopes and durations obtained before and after hearing loss. We show that thresholds, specified as the temporal integral of the effective pressure envelope, i.e., the envelope of the pressure exceeding the elevated baseline, behave almost exactly as the lower thresholds, specified as the temporal integral of the total pressure envelope before hearing loss. Thus, the mechanism of temporal integration is likely unchanged after hearing loss, but the effective portion of the stimulus is. Our model constitutes a successful alternative to the model currently favored to account for altered temporal integration in listeners with sensorineural hearing losses, viz., reduced peripheral compression. Our model does not seem to be at variance with physiological observations and it also qualitatively accounts for a number of phenomena observed in such listeners with suprathreshold stimuli.

Reducing hearing protector test time with a minimum audible pressure to field transfer function

Effective hearing-loss prevention programs should train workers to properly fit hearing protection devices. In order to minimize the training time for protector fitting, unoccluded minimum audible field threshold (MAF) for narrow-band noise could be estimated from the worker’s annual minimum audible pressure (MAP) audiogram. Murphy et al. previously reported a transfer function for 5- and 1-dB audiometry for 75 and 10 subjects, respectively [W. J. Murphy, J. R. Franks, S. L. Hall, and E. F. Krieg, J. Acoust. Soc. Am. 101, 3126 (1997)]. Significant differences at several frequencies between the 1- and 5-dB groups prompted further study of the transfer function. This paper reports MAF and MAP results for 44 subjects tested with both 1- and 5-dB audiometry and 1-dB sound field. Pure-tone thresholds were significantly different, for 1- and 5-dB step sizes. The binaural reference equivalent threshold sound pressure level (RETSPL) sound-field thresholds were compared with the RETSPL thresholds for monaural listening in a sound-field listed in ANSI S3.6-1996, Table 9. The binaural thresholds were found to be significantly higher (p<0.05) than the standard values at all frequencies except 8000 Hz.

Loudness Scaling Revisited

The present work was undertaken in an attempt to evaluate whether it is reasonable to expect that categorical loudness scaling can provide useful information for nonlinear hearing aid fitting. Normative data from seven scaling procedures show that the individual procedures relate the perceptual categories differently to sound level and with a substantial between-subject variance. Hearing-impaired data from four studies demonstrate that the inverse slope of the loudness function varies linearly with hearing loss and with a constant variance. In relation to hearing aid fitting, the slope can, in most cases, be predicted from the hearing loss with an accuracy within the range of a normal finetuning. For the fitting of nonlinear hearing aids, the statistical properties of both normal and impaired loudness functions are equally important. The present analysis strongly suggests that categorical loudness scaling cannot, in general, provide significant information for the fitting process.

High-frequency (8–20 kHz) reference thresholds for audiometry

ANSI (S3.6-1996) and ISO (CD 389-5, 1996) have reported interim high-frequency (8–16 kHz) reference equivalent threshold sound pressure levels (RETSPLs). Since the interim RETSPLs were derived using very limited data, there is a need to increase the data base for specifying high-frequency RETSPLs. Thus, this study determined high-frequency (8–20 kHz) thresholds for 50 young adults having normal low-frequency (0.25–8 kHz) hearing across four test sessions, separated by at least one day, using Sennheiser HDA 200 and Koss HV/Pro earphones. The mean thresholds increased as frequency increased, especially at 16 kHz and above, and were similar to the ANSI and ISO interim RETSPLs. However, the standard deviations were very high and the range scores were very wide for each earphone at each frequency. This would indicate that specifying high-frequency RETSPLs may not be feasible due to very high intersubject threshold variability. On the other hand, the intrasubject threshold differences across each possible test session comparison were found to be very repeatable and within a clinically acceptable range. This would indicate the usefulness of high-frequency audiometry for monitoring patients having or suspected of having otopathology. [Work supported by the Pennsylvania Lions Hearing Research Foundation.]

Integration time for short broad band clicks in echolocating FM-bats (Eptesicus fuscus).

Vespertilionid FM-bats (four Eptesicus fuscus and one Vespertilio murinus) were trained in an electronic phantom target simulator to detect synthetic echoes consisting of either one or two clicks. The threshold sound pressure for single clicks was around 47 dB peSPL for all five bats corresponding to a threshold energy of -95 dB re 1 Pa2*s. By varying the interclick interval, delta T, for double clicks it was shown that the threshold intensity was around -3 dB relative to the threshold for single clicks at delta T up to 2.4 ms, indicating perfect power summation of both clicks. A threshold shift of -13.5 dB for a 1 ms train of 20 clicks (0.05 ms interclick interval) confirmed that the bats integrated the power of the stimuli. At delta T longer than around 2.5 ms the threshold for double clicks was the same as for single clicks. Thus, the bats performed like perfect energy detectors with an integration time of approximately 2.4 ms. This integration time is an order of magnitude shorter than that reported for bats listening passively for pure tones. In our setup the bats emitted sonar signals with durations of 2-3 ms. Hence, the results may indicate that while echolocating the bats integration time is adapted to the duration of the sonar emissions.

Normal hearing levels for an ER‐3A insert earphone

Equivalent threshold sound pressure levels were obtained on each ear of 48 normal hearing adult subjects from 125–8000 Hz using TDH‐49P earphones, referenced to a NBS‐9A coupler, and ER‐3A insert earphones, referenced to a HA‐2 coupler. At each frequency (N = 9), the thresholds were significantly higher for the TDH‐49P than for the ER‐3A; however, the test versus retest and right versus left ear thresholds were not significantly different. The unadjusted ER‐3A thresholds were in agreement with the reference equivalent threshold sound pressure levels (RETSPLs) reported by ISO [ISO 389, DAD 3, 1990]. After correcting for the subject's deviation from normal TDH‐49P hearing levels and coupler (HA‐2 to HA‐1), the ER‐3A thresholds were also in agreement with interim REISPLs reported by ANSI [ANSI S3.6‐1989, Appendix G]. A complete data base concerning normative ER‐3A thresholds will be presented and discussed in reference to the results of the present study, ISO and ANSI RETSPLs, and for future standardization.