In 1991, Nabelek, Tucker, and Letowski introduced a procedure for determining acceptable noise intensities while listening to speech. This procedure has come to be known as acceptable noise level (ANL; then called tolerated signal-to-noise ratio). The premise of ANL is that some listeners do not accept hearing aids because of their inability to accept background noise. In other words, some listeners cannot benefit from hearing aids because they are unwilling to wear the device due to an inability to withstand background noise.
The ANL procedure quantifies a listener's willingness to listen to speech in the presence of background noise. To obtain an ANL conventionally, a recorded story of running speech (available from Cosmos Distributing Inc. www.cosmosdistributinginc.com/) is adjusted to the listener's most comfortable listening level (MCL). Next, background noise is added (Revised SPIN, available from Cosmos Distributing Inc. www.cosmosdistributinginc.com/) and adjusted to a level that the listener is willing to "put up with" the background noise while listening to and following the words of a story (called background noise level or BNL). The ANL, in dB, is calculated by subtracting the BNL from the MCL (ANL = MCL - BNL) (see Appendix A for ANL instructions; Note: Clinician/researcher friendly ANL instructions can be found at web.utk.edu/~aspweb/faculty/nabelek/anl.pdf). For example, if a listener's MCL is 60 dB HL and their BNL is 55 dB HL, their ANL is 5 dB. ANL is typically measured with both the speech and background noise presented at 0 degrees azimuth. Lastly, ANLs can be measured and calculated in approximately 2-3 minutes.
ANLs are not related to age (Nabelek et al., 1991; Freyaldenhoven & Smiley, in press; Nabelek, Freyaldenhoven, Tampas, Burchfield, & Muenchen, 2006), hearing sensitivity (Nabelek et al., 1991; Nabelek, Freyaldenhoven, Tampas, & Burchfield, 2006), gender (Rogers, Harkrider, Burchfield, & Nabelek, 2004), type of noise background noise distraction (Nabelek et al., 1991), preference for background sounds (Freyaldenhoven, Smiley, Muenchen, & Konrad, 2006), primary language of the listener (von Hapsburg & Bahng, 2006), acoustic reflex thresholds or contralateral suppression of otoacoustic emissions (Harkrider & Smith, 2005), or speech understanding in noise scores (Nabelek et al., 2006). ANLs are reliable and normally distributed in listeners with both normal and impaired hearing (Nabelek, Tampas, & Burchfield, 2004; Freyaldenhoven & Smiley, in press; Freyaldenhoven, Smiley, et al., 2006; Nabelek et al., 2006). Furthermore, ANLs have been measured over 3-week and 3-month time periods for listeners with normal and impaired hearing, respectively; the results demonstrated that ANLs were consistent over time for at least 3 weeks or 3 months for the corresponding group of listeners (Nabelek et al., 2004; Freyaldenhoven, Smiley et al., 2006).
In an attempt to determine the effect of speech presentation level on ANL, Franklin, Thelin, Nabelek, and Burchfield (2006) measured ANLs in at five different speech presentation levels (20, 34, 48, 62, and 76 dB HL) in 20 listeners (10 male, 10 female) with normal hearing sensitivity. The results demonstrated that ANLs were related to speech presentation level. Specifically, a 4-dB increase in speech presentation level yielded a 1-dB increase in ANL. These results indicated that ANL is related to the presentation level of the speech stimuli but that a relatively large change in speech level was required to affect an ANL. It should be noted that the change in ANL with increasing speech presentation level was not uniform among the listeners.
Relationship between ANL and Hearing aid Use
Nabelek et al. (1991) measured ANLs in three groups of hearing aid users (N = 15/group): full-time hearing aid users, part-time hearing aid users, and nonusers of hearing aids. Full-time hearing aid users were defined as listeners who wore hearing aids whenever they needed them; part-time users wore their hearing aids only occasionally, and nonusers had completely stopped wearing their hearing aids (see Appendix B for pattern of hearing aid use questionnaire; Note: Listeners who circled answers 1, 2, and 3 were full-time, part-time, and non-users, respectively). The results demonstrated that mean ANLs for the full-time users (M = 7.5 dB) were significantly lower than mean ANLs for part-time (M = 14.0 dB) and nonusers (M = 14.5 dB) of hearing aids; however, there was no mean ANL difference between part-time users and nonusers of hearing aids. These results indicated that ANL may be related to hearing aid use.
Crowley (1994) continued this research investigating the relationship between ANLs (then called "tolerated noise levels") and several other subject variables (i.e., age, gender, years of education, number of medications taken daily, employment time, pure tone average [PTA], audiometric slope, speech perception in noise, dynamic range of the hearing loss, loudness discomfort levels, the difference between the National Acoustics Laboratory [NAL] target and insertion gain, attitude toward hearing loss, and motivation to pursue hearing aids) in an attempt to predict hearing aid satisfaction and hours of daily hearing aid use. Data from 46 first-time binaural hearing aid users was evaluated. Additionally, 4 to 6 weeks post-fitting, listeners were asked to complete the Profile of Hearing Aid Benefit (PHAB; Cox & Gilmore, 1990). Significant but weak correlations were found between ANLs and PTA, dynamic range of the hearing loss, and patient attitude toward their hearing loss. Specifically, participants with better hearing sensitivity and larger dynamic ranges tended to accept greater amounts of background noise. Additionally, participants who had the most difficultly coping with hearing loss had greater difficulty accepting background noise. In subsequent data analyses, Crowley and Nabelek (1996) used regression analysis to determine the contribution of ANL as a predictive measure of the PHAB outcome. Results showed that ANLs were predictive of the Familiar Talkers subscale of the PHAB. This was the first evidence that ANLs might be able to be used as a predictor of hearing aid outcome.
Therefore, in an effort to predict hearing aid use using ANL alone, Nabelek and colleagues (2004, 2006) preformed a large scale study (N = 191) to determine the predictive ability of unaided ANL. The hearing aid users were divided into three groups based on hearing aid use: full-time users (N = 69), part-time users (N = 69), or nonusers (N = 59) of hearing aids. Using the first 50 (41 full-time users and 9 part-time users) of the 191 subjects, Nabelek et al. (2004) assessed the relationship between unaided and aided ANLs and speech-in-noise scores. The results demonstrated that hearing aids had no effect on ANLs, but speech-in-noise scores increased with the introduction of amplification. These results indicated that acceptance of noise (i.e., ANLs) and speech-in-noise scores measure different reactions to background noise. Specifically, ANLs may be used as a predictor of hearing aid use while speech understanding in noise may be used as a measure of hearing aid benefit (Nabelek et al., 2004).
Furthermore, results of the Nabelek et al. (2006) study (N = 191) demonstrated that ANLs were related to hearing aid use. Specifically, full-time hearing aid users accepted more background noise than part-time users or nonusers of hearing aids, but part-time and nonusers could not be differentiated. These results indicated that individuals who accept high levels of background noise (i.e., have low ANLs, no greater than 7 dB) are more likely to wear hearing aids on a regular basis (i.e., become full-time hearing aid users). Conversely, individuals who accept low levels of background noise (i.e., have high ANLs, greater than 13 dB) are less likely to wear hearing aids regularly (i.e., become part-time users or nonusers of hearing aids). The results further indicated that the prediction of hearing aid use for individuals with midrange ANLs (i.e., ANLs between 7 and 13 dB) is unclear. Furthermore, a prediction of hearing aid use was determined based on the listener's unaided ANL. Since part-time users and nonusers could not be differentiated based on unaided ANLs, these two groups were combined and called the "unsuccessful" hearing aid users. They were then compared to the full-time users (called "successful" hearing aid users). Regression analysis determined that unaided ANLs could predict a listener's success with hearing aids (i.e., hearing aid use/acceptance) with 85% accuracy (Nabelek et al, 2006).
Figure 1 was generated from the logistic regression analysis and shows a predicted probability of success curve. To determine the predicted probability of success with hearing aids, the listener's unaided ANL should be located on the x-axis. Then, the predicted probability of success corresponding to the unaided ANL can be located on the y-axis. In order to express the listener's probability of success in percent, the number corresponding to the listener's unaided ANL on the y-axis of the curve should be multiplied by 100. For example, if the listener's unaided ANL is 10 dB, their probability of success with hearing aids is about 50%, which is denoted by the arrows on Figure 1 (Note: This figure was adapted from Figure 2 in Nabelek, et al., 2006).
Figure 1: A regression curve displaying the predicted probability of success with hearing aids as a function of unaided ANL score (Note: This figure was adapted from Figure 2 in Nabelek et al., 2006).
Effect of Hearing Aids
The following two studies were completed to determine the relationship between some aspect/feature of hearing aids and ANL. Other studies investigating if ANLs could be improved through hearing aid technology are discussed later in this review under "Improvements in Acceptable Noise Level."
Freyaldenhoven, Plyler, Thelin, Nabelek, and Burchfield (2006) investigated the effects of venting and gain compensation on ANL in two groups of listeners. The groups were separated based on degree of low-frequency hearing loss. Group one had low-frequency hearing thresholds better than 45 dB HL, and group two had low-frequency hearing thresholds poorer than 45 dB HL. The results established that acceptance of noise was not significantly affected by gain compensation or venting for either group. Moreover, this indicated that activating gain compensation or changing vent size based on patient preference should not affect a listener's willingness to wear hearing aids.
Additionally, Freyaldenhoven, Plyler, Thelin, and Burchfield (2006) investigated the effects of monaural and binaural hearing aid use on speech understanding in noise and acceptance of background noise for 39 listeners with hearing impairment. Speech understanding in noise was measured using masked speech recognition thresholds (SRTs), and acceptance of background noise was measure using the ANL procedure. The results revealed a significant improvement in masked SRTs with binaural versus monaural amplification; however, there was no improvement in ANL with binaural versus monaural amplification for most listeners. Based on these results, the authors recommended the use of binaural amplification for most listeners because binaural amplification maximizes speech intelligibility in noise without hindering a listener's hearing aid use/acceptance. Furthermore, it should be noted that individual data analysis revealed some listeners' best monaural ANL was better than their binaural ANL, indicating that some listeners may be more willing to use amplification if fitted monaurally instead of binaurally. Individual data analysis further revealed that some listeners exhibited interaural ANL differences, indicating that acceptance of noise/hearing aids may be dependant on the fitted ear if only one hearing aid is going to be fitted.
Mediation of ANL
Harkrider and Smith (2005) examined the role of the auditory efferent system on ANL. Monotic (speech and noise presented to only one ear) and dichotic (speech presented to one ear and noise presented to the other ear simultaneously) ANLs were measured for 31 adults with normal hearing and compared to phoneme recognition in noise, contralateral suppression of transient evoked otoacoustic emissions, middle ear impedance measures, and ipsilateral and contralateral acoustic reflex thresholds. Results indicated that neither monotic nor dichotic ANLs were related to middle ear impedance measures, acoustic reflex thresholds, contralateral suppression of otoacoustic emissions, or phoneme recognition in noise. Monotic ANLs, however, were correlated with dichotic ANLs. These results suggest that ANL may be mediated by non-peripheral factors. Furthermore, since efferent activity in the contralateral acoustic reflex arc is correlated with efferent activity in the medial olivary cochlear bundle, these findings suggest that ANL may be mediated, in part, beyond the level of the superior olivary complex where binaural processing initially occurs within the central auditory nervous system.
Harkrider and Tampas (2006) measured physiological responses including click-evoked otoacoustic emissions (CEOAEs), auditory brainstem responses (ABRs), and middle latency responses (MLRs) in females with normal hearing with low (n = 6) versus high (n = 7) ANLs. The results indicated no differences between individuals with low and high ANLs for CEOAEs or waves I or III of the ABR. Differences between the two groups emerged for wave V of the ABR and Na-Pa of the MLR. These results further support that acceptance of background noise is mediated from central regions of the auditory system.
Tampas and Harkrider (2006) continued the investigation of the effects of auditory evoked potentials on ANLs. In addition to the ABR and MLR, cortical, auditory long latency responses (LLRs) were obtained from two groups of females: one with low ANLs and one with high ANLs. Furthermore, ANLs were measured at three speech presentation levels (35, MCL, 70 dB HL) for the two groups of listeners. The results revealed no differences between the two groups for the early waves of the ABR, yet significant differences existed between the two groups for waves III and V of the ABR and the MLR and LLR peaks. The results further demonstrated that the ANL growth rate for the two groups was not uniform, and the groups differed on some of the physiological measures. Specifically, females with low ANLs demonstrated a slower ANL growth rate with increasing speech presentation level. Additionally, the ABR latencies and LLR amplitudes were significantly different for the two groups. These effects supported the theory that acceptance of noise is mediated from central regions of the nervous system. More specifically, it was hypothesized that acceptance of noise is mediated, in part, by cortical inhibition.
Improvements in Acceptable Noise Level
In 2005, Freyaldenhoven, Nabelek, Burchfield, and Thelin investigated the suitability of the ANL procedure for assessing the benefit of directional microphone technology in 40 adults, who utilized binaural hearing instruments with omnidirectional and directional capabilities. Directional benefit was evaluated using front-to-back ratios (FBRs), masked SRTs, and ANL, and was calculated as the directional score minus the omnidirectional score. The results demonstrated that all three measures improved by 3 dB when utilizing hearing aids with directional microphones. These results indicated that ANL may be an alternative method for measuring directional benefit in hearing aids. These results further indicated that ANLs can be improved when the signal-to-noise ratio at the eardrum is increased.
Mueller, Weber, and Hornsby (2006) further investigated the use of hearing aid technology on ANLs by measuring the effect of digital noise reduction (DNR) on acceptance of noise in 22 listeners with hearing impairment using the Hearing in Noise Test (HINT). Each listener was fit with a behind-the-ear Siemens ACURIS Model S (Siemens Hearing Instruments Inc, Piscataway, NJ), employing 2 different types of DNR algorithms: (1) modulation-based and (2) adaptive, fast-acting DNR, similar to the Wiener filter. These DNR algorithms operated simultaneously and independently in 16 channels. The results demonstrated that listeners accepted significantly more noise when using hearing aids with DNR capability turned on (mean improvement = 4.2 dB) versus DNR turned off. These results indicate that within the constraints of the DNR algorithms and test conditions employed in this study (i.e., utilizing steady-state noise from the HINT test), DNR can significantly improve acceptance of noise, which may result in improved ease of listening in background noise.
The effect of stimulant medication on acceptance of background noise was examined for 15 normal hearing female college students with ADHD/ADD by Freyaldenhoven, Thelin, Plyler, Nabelek, and Burchfield (2005). ANLs were measured at three speech presentation levels (20, MCL, and 76 dB HL) in two experimental sessions: a medicated session and an unmedicated session. The results demonstrated that as speech presentation level increased, ANL also increased for listeners with normal hearing; these results were in agreement with data obtained by Franklin et al. (2006). The results further revealed that ANLs improved while the listeners were on stimulant medication for the treatment of ADHD/ADD. These results indicated that listeners with ADD/ADHD can accept more background noise when taking stimulant medication for the treatment of ADD/ADHD and provided the first evidence that pharmacological intervention could manipulate ANLs.
Summary
In summary, ANL is a procedure used to quantify listeners' willingness to listen to speech in the presence of background noise. ANLs can be obtained in approximately 2-3 minutes and are reliable and consistent over time in both listeners with normal and impaired hearing. ANLs are not related to speech perception in noise scores and do not change with the introduction of hearing aids, indicating that ANLs can be measured before hearing aids are fitted and used as a predictor of hearing aid use. Most importantly, ANLs are related to a pattern of hearing aid use and can predict success with hearing aids with 85% accuracy. Additionally, ANLs are thought to be mediated beyond the level of the periphery at the level of the central auditory nervous system in listeners with normal hearing. Lastly, research has shown that ANLs can be improved through the use of pharmacological intervention in listeners with normal hearing and through the use of directional microphones and noise reduction algorithms in hearing aids for users of amplification. Readers who are interested in additional information on ANL should look for an upcoming live e-Seminar on Audiology Online in March of 2007. This event will be recorded and available through Audiology Online after the live presentation date.
References
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