Saturation Sound Pressure Level Research Articles

Preferred Listening Levels of Children Who Use Hearing Aids: Comparison to Prescriptive Targets

The preferred listening levels (PLLs) of children with sensorineural hearing loss were elicited using conversation-level speech, heard through the children's own hearing aids. All hearing aids were fitted using the desired sensation level (DSL) method. Comparisons were made between the PLL and targets from the following prescriptive formulae: DSL version 4.1 and two versions of the National Acoustic Laboratories (NAL) procedure, including NAL revised for severe-profound losses (NAL)-RP and NAL nonlinear NAL/NL1. Results for this sample of children indicated that the PLL was similar to the DSL targets, and that, on average, NAL-RP/NL1 targets recommended less gain than that preferred by the majority of children in this study. The implications of factors such as acclimatization, test levels, and clinical procedures on these results are discussed.

Measuring the Outcomes of a National Rehabilitation Program: Normative Data for the Client Oriented Scale of Improvement (COSI) and the Hearing Aid User’s Questionnaire (HAUQ)

AbstractSelf-report outcome measures were routinely collected from 4421 adult clients around Australia. The measures used were the Hearing Aid User’s Questionnaire (HAUQ), which assessed hearing aid use, benefit, problems, and satisfaction, and the Client Oriented Scale of Improvement (COSI), which identified client needs, change in listening ability, and final listening ability in situations important to each client. Listening to television or radio and conversing with one or two others in a quiet place were the most frequently nominated needs. The benefit reported for noisy places was less than for quiet places, but very positive nonetheless. The normative data collected show a marked concentration of responses near the upper end of the scale for each of the outcome items except daily use. Consequently, correlations between the measures, although highly significant, were mostly less than 0.4. When the data were collapsed across subjects seen at each hearing center, benefit, satisfaction, usage, and problems with the hearing aids became much more strongly correlated with each other, with correlation coefficients up to 0.8. Benefit, satisfaction, usage, and the types of problems clients encountered with their hearing aids varied significantly from hearing center to hearing center. The most frequently reported problem was dissatisfaction with the quality of the subjects’ own voices (i.e., occlusion effect), followed by feedback. The problems most closely related to usage, benefit, and satisfaction, however, were the presence of feedback, comfort of the earmold or earshell, and the quality of the users’ own voices. These outcome measures appear to be most suitable for identifying needs, identifying individuals receiving markedly less than average benefit, and for finding small differences between outcomes for subgroups of the population. Abbreviations: ANOVA = analysis of variance; APHAB = Abbreviated Profile of Hearing Aid Benefit; BTE = behind the ear; COSI = Client Oriented Scale of Improvement; GAS = Goal Attainment Scaling; HAUQ = Hearing Aid User’s Questionnaire; HHIE = Hearing Handicap Inventory for the Elderly; ITE = in the ear; NAL = National Acoustic Laboratories; NAL-RP = NAL Revised Profound; SSPL = saturation sound pressure level

The National Acoustic Laboratories' procedure for selecting the saturation sound pressure level of hearing aids: theoretical derivation.

This paper presents the derivation of a procedure for prescribing the saturation sound pressure level (SSPL) of hearing aids. The procedure is designed to be used with either measured values of loudness discomfort level (LDL) or with hearing threshold values alone. SSPL needs to be low enough to prevent the hearing aid from causing loudness discomfort to the aid wearer but high enough to prevent the hearing aid from being excessively saturated by speech. The maximum SSPL likely to be acceptable can be predicted by measuring LDL or by estimating LDL from hearing thresholds. The minimum SSPL likely to be acceptable can be predicted by calculating, for any particular hearing loss, the amount of gain likely to be needed and hence the SSPL needed if the speech input signal is continuous discourse at an overall level of 75 dB SPL. The midpoint between the minimum and maximum acceptable SSPL values is defined as the optimal or prescribed SSPL, and the three frequency average (3FA; 500, 1000, and 2000 Hz) value of this can be predicted from the 3FA hearing thresholds. Alternatively, the SSPL prescription at each frequency can be prescribed on the basis of the hearing aid gain at each frequency. For either method, the SSPL prescription needs to be increased for people with a conductive component to their hearing losses. The SSPL prescription, when referred to a 2-cc coupler, needs to be decreased for infants, for deeply inserted hearing aids, for multichannel hearing aids that limit SSPL separately in each band, and possibly for nonlinear hearing aids. The 3FA SSPL prescribed for persons with a sensorineural hearing loss increases linearly from 89 dB SPL for normal hearing to 107 dB SPL for a person with a 60 dB HL 3FA loss, and then linearly again to 139 dB SPL for a person with a 120 dB HL 3FA loss. The procedure predicts that the acceptable range of SSPLs is very wide for people with mild losses but is vanishingly small for people with profound losses.

Hearing loss, speech, and hearing aids.

Modern hearing aids permit adjustment of a number of electroacoustic parameters, among them frequency response, saturation sound pressure level, and various aspects of compression. Relatively little is known, however, about how the electroacoustic characteristics of hearing aids affect the information-bearing properties of speech. Even less is known about how hearing aids might alleviate or exacerbate the effects of impaired hearing. This article reviews current knowledge in three areas: (a) characteristics of mild/moderate hearing loss, (b) information-bearing aspects of speech, and (c) the relation between electroacoustic characteristics of hearing aids and the speech signal. Concluding suggestions are made regarding the implications of the current data for selecting hearing-aid characteristics.

Loudness Discomfort Level and Reliability as a Function of Instructional Set

Loudness discomfort levels (LDLs) for a 12-speaker babble were measured in 20 subjects with normal hearing sensitivity for two instructional sets, listening in the sound field. One instructional set asked the listener to indicate when they would choose 'not to listen for any period of time'. The second instructional set asked the listeners to indicate when they 'would choose not to listen for 15 minutes or longer'. A simple up-down adaptive procedure was used to measure LDLs. Data were gathered on both instructional sets for all listeners. LDLs for each instructional set were repeated on four separate occasions within a range of 1-3 weeks. The average LDLs for the instructions utilizing the criterion of choosing not to listen for any period of time were 9 dB higher than the criterion of choosing not to listen for 15 minutes or longer. The reliability of LDLs for both instructional sets was high, ranging from correlations of 0.90-0.97 for the criterion of choosing not to listen for any period of time, and ranging from correlations of 0.62-0.89 for the criterion of listening for 15 minutes or longer. The criterion of choosing not to listen for any period of time resulted in slightly more reliable measures. These results support the importance of considering the instructions given to a person to measure LDLs, and hence for where to set the saturation sound pressure level of a hearing aid. Also, with appropriate methods, LDLs can be reliably measured in the sound field.

Hearing aid saturation and aided loudness discomfort.

Clinical measurements of the loudness discomfort level (LDL) are generally performed while the subject listens to a particular stimulus presented from an audiometer through headphones (AUD-HP). The assumption in clinical practice has been that the sound pressure level (SPL) corresponding to the sensation of loudness discomfort under AUD-HP conditions will be the same as the corresponding to LDL with the hearing aid. This assumption ignores the fact that the distortion produced by a saturating hearing aid could have an influence on the sensation of loudness. To examine these issues, 5 hearing-impaired subjects were each fit with four linear hearing aids, each having a different saturation sound pressure level (SSPL90). Probe-tube microphone measurements of ear canal SPL at LDL were made while the subjects listened to continuous discourse in quiet under aided and AUD-HP conditions. Also using continuous discourse, real-ear coherence measures were made at various output sound pressure levels near LDL. All four hearing aid types produced mean LDLs that were lower than those obtained under AUD-HP conditions. Those hearing aids with higher SSPL90 produced significantly higher LDLs than hearing aids with lower SSPL90. A significant negative correlation was found between real-ear SPL and real-ear coherence. Quality judgments made at LDL indicated that sound quality of hearing aids with higher SSPL90 was preferred to that of hearing aids with lower SSPL90. Possible fitting implications regarding the setting of SSPL90 from AUD-HP LDL measures are discussed.

Consistency of Electroacoustic Characteristics Across Components of FM Systems

In the absence of national or international electroacoustic standards for the evaluation of Frequency Modulated (FM) amplification systems, it becomes important to know the variability one may expect across similar models. Evaluation of thirty FM systems of the same model obtained from three different educational sites was performed to determine the variability that may occur as a result of the receiver, lapel microphone, or neckloop. There was a range as great as 20 dB in high frequency average saturation sound pressure level and equivalent input noise across receivers, lapel microphones, and neckloops. These results highlight the need for regular electroacoustic monitoring of not only the FM transmitter and receiver, but also the individual components, such as the lapel microphone and the neckloop.

Electroacoustic Performance of Direct-Input Hearing Aids with FM Amplification Systems

The purpose of this paper was to provide audiologists practical information regarding the use of direct-input hearing aids with FM systems. The degree to which the electroacoustic characteristics of direct-input hearing aids were preserved when used with two different personal FM systems was evaluated. Although there were limited changes in saturation sound pressure level and harmonic distortion, there was an increase in equivalent-input noise and full-on-gain. The frequency response curve of the aid + FM condition relative to the aid alone differed in the high frequencies for the two FM systems. Aids with equivalent gain values had varying amounts of gain when tested with the FM systems set at a constant volume control setting. This variability in the electroacoustic response of a given aid when coupled to a personal FM system suggests that one may not assume characteristics of the hearing aid are maintained when it is used with an FM system.

Loudness discomfort levels in children.

Loudness discomfort levels (LDLs) traditionally have been used to set the saturation sound pressure level (SSPL) or maximum output of a hearing aid. Many procedures have been used to obtain LDLs for adults; however, no systematic study has been conducted to determine if LDLs could be obtained reliably for children. In the present study, LDLs were measured on 20 hearing-impaired children aged 7 to 14 years using a modification of a procedure described by Hawkins, Walden, Montgomery, and Prosek (Ear Hear 1987; 8: 162-169). Test-retest reliability measures were obtained for 8 of the 20 children, and this modified procedure was found to provide reasonably reliable results. Data from the group of 20 children also were compared with similar data obtained from 20 hearing-impaired adults. These results revealed no systematic differences in LDLs between the two groups, suggesting no a priori reason to limit the maximum output of a hearing aid for a child in this age range below the levels that are appropriate for adults. Poor correlation between LDLs and hearing levels for both age groups indicate a need for determining LDLs on an individual basis whenever possible.

Matching commercial hearing aids to prescriptive gain and maximum output requirements.

Insertion gain and saturation sound pressure level (SSPL) characteristics of 71 behind-the-ear hearing aids were evaluated in a computer-assisted analysis as acceptable matches to prescriptive insertion gain and SSPL values derived by a variation of the Prescription of Gain and Output, or POGO, technique (McCandless & Lyregaard, 1983). Stringency criteria in a primary analysis varied from +/- 6 to +/- 16 dB and +/- 2 to +/- 8 dB for gain and SSPL, respectively. Three audiometric contours and associated loudness discomfort levels were used in the simulation. Findings indicated that the relationship between tolerances and the number of aids selected as optimally appropriate varied substantially as a function of audiometric configuration. Minimum gain and SSPL tolerance values of +/- 12 dB and +/- 4 dB, respectively, were required to provide a reasonable choice of instruments for a variety of such configurations. Further analyses shed light on the feasibility of using alternative stringency criteria and on the clinical utility of hearing aid selection using small clinic samples.

Audiometric earphone discomfort level and hearing aid saturation sound pressure level for a 90 decibel input signal (SSPL90) as measured in the human ear canal.

The acoustical problems involved in matching the saturation sound pressure level for a 90 dB input signal (SSPL90) of a hearing aid to individual discomfort level were investigated. The real ear SPL (RE/SSPL90) produced by a supra-aural earphone used when measuring uncomfortable loudness (UCL), and RE/SSPL90 produced by three different hearing aids at 90 dB SPL input, were measured for nine subjects, using a miniature microphone technique, and compared to the corresponding coupler levels used when matching hearing aid maximum output to UCL. It was found that a hearing aid often gives about 5 dB, and sometimes 10 dB, higher RE/SPLs than the earphone, if the hearing aid output levels, as measured in a 2-cc coupler (IEC126), are equal to the earphone output levels as measured in a 6-cc coupler (NBS9A). It is recommended that a safety margin of at least 5 dB be used in the preliminary fitting when matching hearing aid SSPL90 to the patient's UCL, converted to dB SPL.

Electroacoustic characteristics of personal FM systems.

Electroacoustic characteristics of various personal FM system configurations were obtained through the use of a Zwislocki ear simulator and KEMAR. Saturation Sound Pressure Level 90 and frequency response curves were obtained with different hearing aids in the microphone mode and then compared to the response obtained when the hearing aids were coupled to a FM receiver via neck loops and direct input. In addition, hearing aid orientation to and distance from the loop, clothing effects, internal noise levels, FM receiver volume control taper, and the effect of different FM receivers were investigated. The results indicated substantial microphone-teleloop response differences. Only minor differences were observed in the microphone-direct input comparison. Other variables were found that significantly affect the use of these personal FM systems. The results are discussed in terms of the implications for clinical use.

The use of construct validity to establish predictive relationships between hearing aid performance and speech understanding

Factor analyses were used to group 42 hearing aid electroacoustic dimensions. The resultant groups included harmonic distortion, gain, saturation sound pressure level, bandwidth, and regularity of frequency response. Representatives of these groups were entered into multiple regression analyses with listener scores obtained on a continuum of psychoacoustic materials. Results indicated that psychoacoustic scores are influenced by a hierarchy of electroacoustic dimensions and that the hierarchy may be quite different for different materials. This information can be used in a battery approach which produces a constructed validity for establishing prediction between hearing aid performance and speech understanding.

Speech recognition performance at loudness discomfort level.

The purpose of this experiment was to determine the relationship between the loudness discomfort level (LDL) and maximum speech recognition performance for listeners with sensorineural hearing loss. Performance-intensity functions were obtained for 30 listeners with mild to moderate sensorineural hearing impairment at five speech levels from -18 to +4 dB re: the LDL. Recognition scores were obtained for three speech materials. For all speech materials, recognition scores at levels below the LDL were equivalent to or higher than scores obtained at and above the LDL. These results support the contention that the LDL represents a sound pressure level above which no improvement in speech recognition occurs, and thus, might reasonably be used as the appropriate level for the saturation sound pressure level of a hearing aid.

Test-retest stability and effects of psychophysical methods on the speech loudness discomfort level.

It is generally agreed that the saturation sound pressure level of a hearing aid should not exceed the patient's loudness discomfort level (LDL) for speech. This study investigated (1) the stability of the test and retest LDLs when obtained on different days using McCandless's and Berger's instructional sets and (2) the effects of a psychophysical method (adjustment versus limits) on the speech LDL using McCandless's instructions. Good reliability was observed since one-half of the listeners had test-retest differences of 2 dB or less and nearly all subjects obtained LDLs within 8 dB. The mean LDLs for the respective methods of adjustment and limits were 86.8 and 92.9 dB SPL, and statistically significant. Additionally, a significant interaction between the two methods was observed. The LDL measurement is also discussed in relation to other tests used to evaluate hearing aid performance.

Loudness Discomfort Level for Speech

This study was undertaken to compare the speech loudness discomfort levels (LDL's) with two instructional sets which have been proposed for saturation sound pressure level selection of hearing aids. The phraseology recommended by McCandless and by Berger was presented to normal-hearing and hearing-impaired listeners. The normal-hearing subjects obtained mean LDL's of 94.6 and 111.9 dB SPL for these respective instructions, which was statistically significant. The hearing-impaired listeners also showed LDL's with Berger's instructions (114.7 dB SPL) to be significantly higher than with McCandless' instructional set (109.3 dB SPL). Consequently, this investigation suggests that these two instructional sets may lead to substantially different saturation sound pressure levels. Further studies are needed to determine the most appropriate phraseology for LDL measurement, including the assessment of speech intelligibility at various saturation sound pressure levels. Another instructional set was constructed which (1) includes an explanation to patients of the purpose and importance of the test, (2) requests listeners to indicate the upper level they are "willing" to listen as opposed to the level they are "able" to listen, (3) instructs patients to search thoroughly around their LDL before making a final judgment, and (4) contains a statement that the LDL should be made with the understanding that the speech could be listened to for a period of time. Whatever instructions are used, clinicians are advised to interpret their LDL's very cautiously until validational studies are available.

Effects of speech materials on the loudness discomfort level.

Clinicians have used speech stimuli when measuring the loudness discomfort level (LDL) to determine the upper intensity limit for test stimulus presentation, and to select the saturation sound pressure level for an individual's hearing aid. Because little research has investigated the effects of speech stimuli on the LDL, this study was undertaken to compare LDLs using six commercially available speech materials on 120 normally hearing listeners. Our comparisons showed no significant differences between the mean scores for any of the speech stimuli. These findings suggest that any differences in the mean LDLs among studies probably are not attributable to the speech stimuli. The intrasession reliability of LDL measurement was also assessed using a modified method of limits procedure with 2-dB increments and instructions stressing initial discomfort. It was concluded that examiners probably could attain a high degree of reliability by simply averaging the results of two ascending trials, because 95% of these test/retest differences did not exceed 6 dB. Our findings were integrated with previous studies in terms of: (1) test stimuli; (2) listener experience; (3) instructional set; and (4) psychophysical method. This discussion points to many unanswered questions and concludes that the LDL should be interpreted very cautiously. Moreover, it is suggested that the stimuli selected for LDL measurement should reflect the examiner's purpose.

TTS resulting from hearing-aid usage

Two experiments were conducted in which a normal-hearing young adult (the author) wore an ear-level moderate-gain hearing aid under various listening conditions. In the first experiment, a somewhat artificial experimental design was employed which attempted to maximize the risk involved. In the second experiment, amplification was worn in a more realistic environment (a college classroom). Considerable amounts of temporary threshold shift (TTS) were observed in both experiments despite a maximum exposure duration of only two hours. In the initial experiment, for instance, TTS exceeded 40 dB immediately following “exposure” and over 24 h were required for complete recovery. The amount of TTS was dependent primarily upon saturation sound pressure level (SSPL), duration of “exposure,” and frequency of the test signal.

A longitudinal examination of harmonic distortion in hearing aids.

This study examined the harmonic distortion levels of 1170 hearing aids submitted to the Veterans Administration for contract evaluation during the last six fiscal years. Measurements were made, with a 75 dB SPL input, at 500, 700, and 900 Hz and at the frequency at which maximum distortion was found. The gain of each aid was set with a 62.5 dB SPL input to that point at which distortion at all amplified frequencies was less than 10%. Based upon average gain and average saturation sound pressure level, each aid was placed in one of three power categories. The results show that the levels of maximum distortion appear to be inversely proportional to the rated power of the aids. Also, at all frequencies, the levels of harmonic distortion found in aids during the last two years generally equaled or exceeded those found in 1962, the first year for this study.