Steve Bornstein, Ph.D., C.C.C./Audiology
Associate Professor
University of New Hampshire
Department of Communication Sciences and Disorders

 

 

Abstract

Children with Central Auditory Nervous System (CANS) Dysfunction have been observed to potentially have several deficits, such as difficulty with temporal tasks, degraded speech, time-compressed speech, and auditory pattern recognition. However, perhaps the greatest overall deficit is the ability to perceive speech when there is background or competing stimuli. One reason for this difficulty probably is that important speech perceptual cues may be missing or inconsistent, thus preventing a child with a compromised Central Auditory Nervous System from being able to develop strong perceptual saliency or phonological awareness of the speech code. One possible way of overcoming the reduction of speech cues in background noise would be the use of Sound Field Systems.  In Part 1 of this two-part article the difficulties faced by children with CANS Dysfunction, factors influencing the clarity of speech, multiplicative distortion effects, and the relationship to general speech acoustics and perception were discussed as a theoretical rationale for the use of Sound Field Systems with children who have CANS Dysfunction. In this article the speech spectrum will be discussed in more detail as well as further discussion of multiplicative distortion effects, and concepts of intrinsic and extrinsic redundancy. Research evidence supporting the use of Sound Field Systems with children who have CANS dysfunction will also be presented.  

 

 

Introduction

 

In Part 1 of this two-part article the premise was put forth that one of the significant problems faced by children with CANS dysfunction – often referred to as central auditory processing disorder (CAPD), is that audibility of significant speech cues may be missing or inconsistent when there is background noise. It is known that people with CANS dysfunction have significant difficulty in everyday life when trying to understand speech in background or competing stimuli. In addition, there are several other factors that will interfere with the clarity of speech for children with CANS dysfunction such as reverberation, distance, a speaker’s rate of speech, other acoustic distortions such as reduction or elimination primarily of high frequencies, temporal masking, the intrinsic distortion within the auditory system itself, and the anatomical and/or physiological loss of redundant pathways within the Central Auditory Nervous System (loss of “intrinsic redundancy”).  In addition, when combining these distortions the effects are significantly greater than one would predict by simply adding the effects of each distortion. This is referred to as the Multiplicative Distortion principle [1] and was discussed in great detail in Part 1.  This negative multiplicative effect becomes even more problematic when considering the intricacies of the speech spectrum and the fact that speech sound audibility may be inconsistent due to variations in the speech spectrum. While this inconsistency may not be a problem for children with typical systems, when coupled with other distortions in the environment (reduction in “extrinsic redundancy”) this presents a problem for children with CANS dysfunction who also have reduced “intrinsic redundancy” in their central auditory nervous systems.

 

Therefore the purpose of this paper is to discuss specifics of the speech spectrum and its effects on children with CANS Dysfunction especially considering multiplicative distortion effects, research evidence supporting the use of Sound Field Systems with children who have CANS dysfunction to increase extrinsic redundancy in speech, and considerations for future research.

 

 

The Speech Spectrum, Intensity Variations, and Multiplicative Distortions

 

In Part 1 of this two-part paper it was shown how distance can affect the signal-to-noise ratio, and when considering that an average intensity of background noise is approximately 60 dBA SPL in most classrooms, signal-to-noise ratios in a majority of classrooms are not enough for children with CANS dysfunction to be able to understand speech maximally. However, the situation is more complex than what was discussed in Part 1 when one considers the speech spectrum and acoustic cues for speech perception in detail. 

 

Although sound is measured in dB SPL, for educational and clinical purposes it is more useful to discuss acoustics by converting to dB HL.  First of all, although 65 dBA SPL converts to 45 dB HL, this value is not equal across frequency. Although the absolute values are relatively unimportant because speech is constantly varying, the 50% or average intensity from 250 Hz through 2000 Hz is approximately 42 dB HL, at 3000 Hz it is approximately 37 dB, at 4000 Hz it is approximately 34 dB, and at 6000 Hz approximately 30 dB [2].  However, these are the 50% points, or in other words, this is the intensity level that is reached in each frequency region about half the time. Within each frequency region the 90% confidence interval has been shown to range by approximately 40 dB. For example at 1000 Hz and 2000 Hz 10% of the time the intensity might be 60 dB HL and 10% of the time the intensity might be as low at 20 dB HL.  At 4000 Hz, 10% of the time the intensity might be as high as 48 dB and 10% of the time as low as 20 dB.  What this means is that one cannot just think about overall signal-to-noise ratios but must consider how signal-to-noise- ratios are different in different frequency ranges, and that certain frequencies may be detectable and certain frequencies might not be detectable.

 

Given that speech intensity is lower in the higher frequencies, that perception of consonants is mostly dependent upon detecting the higher frequencies, and that speech intelligibility is primarily dependent on consonant perception, it can be seen that for all children it is important to have maximum signal-to-noise ratios for consonants because consonant perception is critical for language development and learning. For example, the primary cue for the perception of stop consonants is the 2nd formant transition either to or from the steady state portion of the preceding or following vowel. The location of the energy of this 2nd formant transition is approximately around 2000 Hz, but can be significantly higher. The perception of /r/ versus /l/ is dependent on the 3rd formant transition which is around 3500 Hz. The perception of the place of articulation for the different fricatives is dependent upon hearing frequency differences (and to a lesser extent intensity differences) primarily from 3500 Hz-7000 Hz.  Sher and Owens [3] reviewed evidence showing that acoustic cues above 2000 Hz are necessary for identification of isolated words and high-frequency phonemes. Acoustic information in the 2000-3000 Hz range seems particularly critical for speech perception [4], although higher frequencies are also important. For example, /s/, which has most of its energy from about 4000-7000 Hz is the most important phoneme linguistically in that it serves as the morphological marker for many language forms such as possessives, plurals, and some past tense forms.

 

The development of language, including a strong phonological system (which is also important for reading), is dependent upon hearing the sounds of language over and over again consistently so that they become perceptually salient. In language terms this would be described as having strong phonological awareness.  Therefore, the effects of background noise can be profound because they cause a situation where the speech signal is inconsistent in terms of its audibility, particularly in the high frequency consonant region.

 

This loss of high frequency information can be considered a reduction in “extrinsic redundancy” as Bocca and Calearo first termed it [5] and as discussed by Jerger [6]. This loss of “extrinsic redundancy” might not be a problem for children who have typical Central Auditory Nervous Systems, but it becomes a problem when this reduced “extrinsic redundancy” occurs in systems where there is also reduced “intrinsic redundancy” within the CANS [5] [6].  When one considers the multiplicative distortion effects that occur with loss of audibility due to noise, combined with other factors, it is clear that for children with reduced “intrinsic redundancy” of the CANS everything must be done to increase the “extrinsic redundancy” of a spoken acoustic message.  This is essential for children with CANS Dysfunction to develop the phonological awareness skills that are crucial for language and literacy development.

 

In terms of the effects of background stimuli combined with other distortions, in the Bornstein and Musiek study cited in Part 1[7], children with learning problems and suspected auditory processing difficulties, and children with typical systems both showed a multiplicative distortion effect, although interestingly the multiplicative effect was slightly greater in children with typical systems. Children with learning problems scored 98% on a monosyllabic word test under ideal conditions.  With 60% time-compression alone the mean score decreased to 79%. With a +6 dB signal-to-noise ratio using a multi-talker babble alone the mean score decreased to 58%.  Therefore the predicted additive effect of combining both the time-compressed speech and the noise would result in a score of 39%. However, the actual score was 28%. Children with typical auditory systems also scored 98% under ideal conditions. With time-compression alone the mean score decreased to 88%. With a +6 dB signal-to-noise ratio alone the mean score decreased to 60%. Therefore the predicted additive effect would result in a score of 48%. However, the actual score was 32%.  However, regardless of whether or not there were strong multiplicative effects or not, this study demonstrated that children with learning problems and suspected auditory processing difficulties had close to perfect speech recognition scores under ideal conditions but on average were only able to correctly identify 28% of monosyllabic words when combining 60% time-compression and a +6 dB signal-to-noise ratio. Also, children with typical auditory systems scored only 32% under these combined distortions.  This data supports the premise that Sound Field Systems should be of benefit in classrooms for both children with learning problems and CANS Dysfunction as well as children with typical auditory systems.  Another interesting finding in this study was that the multiplicative effect was absent when the speech was shifted 180 degrees out-of-phase to one ear.

 

In the Bornstein study [8], 24 children with typical auditory systems aged 8-9 were compared with 24 adults with typical systems. Both children and adults showed a multiplicative distortion effect as a result of reduced extrinsic redundancy using time-compressed speech and a multi-talker babble, with the effect being greater in children. The adults in this study had a mean score of 99% correct on a monosyllabic word test under ideal conditions. With 60% time-compression alone the mean score decreased to 91%. With a +6 dB signal-to-noise ratio alone the mean score decreased to 62%. Therefore the predicted additive effect of combining both the time-compressed speech and the babble would result in a score of 55%. However the actual score was 38%.  The children in this study had a mean score of 98% correct on a children’s monosyllabic word test under ideal conditions. With 60% time-compression alone the mean score decreased to 87%. With a +6 dB signal-to-noise ratio alone the mean score decreased to 62%. Therefore the predicted additive effect of combining both the speech and the babble would result in a score of 53%. However the actual mean score was 30%. As in the previous study by Bornstein and Musiek [7], shifting speech in one ear 180 degrees out-of-phase decreased the multiplicative distortion and the combined effects more closely approximated an additive effect. Also, the decrease in the multiplicative effect by shifting the speech out-of-phase was more pronounced in adults, probably reflecting maturational differences between children and adults. These maturational differences include myelination, synaptogenesis, and dendritic branching [9].   It can reasonably be argued that these multiplicative effects would be greater in children with CANS dysfunction given that there would be further reduced maturation or a compromised neurological system.

 

 

Use of Sound Field Systems to Increase Extrinsic Redundancy in Speech

 

One significant way of increasing the extrinsic redundancy in speech and reducing multiplicative distortion effects, thereby providing the clearest acoustic signal possible is through the use of Sound Field Systems, using either FM or Infrared technology.  Sound Field Systems increase the intensity of a speaker’s voice thus potentially increasing the signal-to-noise ratio by various amounts, depending upon loudspeaker placement and classroom size. Reports have indicated that the increase in a speaker’s voice, and hence the signal-to-noise ratio, ranges from 8-15 dB [10]. While this is not perfect, it is an improvement.

 

There has been evidence for the benefits of Sound Field Systems with children other than those with CANS Dysfunction. For example, Rosenberg et al. [11] and Heeney [12] presented significant evidence supporting the use of Sound Field Systems for children with typical auditory systems. Rosenberg at al. [11] used both control classrooms without Sound Field Systems and experimental classrooms with Sound Field Systems (a total of 74 classrooms and approximately 2000 students) in kindergarten, first, and second grade. Students were followed for one year. Those in classrooms with Sound Field Systems showed significantly greater improvemenst in listening and learning behaviors and skills (measured by the Evaluation of Classroom Listening Behaviors, and Listening and Learning Observation).  Heeney [12] used a total of 43 control and experimental classrooms consisting of 626 students from first to sixth grade, also over the course of one year. The students using Sound Field Systems showed significantly greater improvements in listening comprehension, reading comprehension, and reading vocabulary as measured by Progressive Achievement Tests, and in phonological awareness as measured by the Sutherland Phonological Awareness Test.

 

Crandell and Bess [13] demonstrated improvement in speech perception for 20 children with typical hearing, aged 5 to 7 years, as measured by Bamford-Kowal-Bench sentences presented in a multitalker babble, listening through a Sound Field System, at distances of 6, 12, and 24 feet.

 

Darai [14] reported greater improvements in literacy for first grade children at the initial reading stage using Sound Field Systems compared with children not using the systems over a 5-month period, with bilingual children and children with learning difficulties showing the greatest improvements.

 

There have been other studies that have shown educational, listening and/or psychosocial improvement using Sound Field Technology in children with normal hearing sensitivity [15] [16] [17] [18] [19] [20] [21], as well as children with minimal hearing loss [22], children with learning disabilities and normal hearing sensitivity [23], children with developmental disabilities [24], children with Down Syndrome [25], and children with ADHD [26].

 

 

Conclusions and Future Directions

 

The American Speech-Language-Hearing Association [27] recommended that for children with sensorineural hearing loss the average ambient noise levels for unoccupied classrooms should not exceed 30-35 dBA and the S/N ratio should be greater than +15 dB. It is reasonable that at least this standard be applied to children with CANS dysfunction. This signal-to-noise ratio often cannot be reached or approximated without the use of Personal or Sound Field Systems. For children with CANS dysfunction Sound Field Systems are probably the systems of choice because the child is not signaled-out as being “different”. In addition, these systems are more economically feasible and potentially benefit a majority of other children in the classroom as well.

 

Furthermore, although this author is not aware of studies using Sound Field Systems directly with children with CANS dysfunction it is reasonable to assume that if improvements have been shown in children with normal hearing sensitivity that there would be at least similar, and probably greater, improvements, in children who have CANS dysfunction, especially when considering the acoustics of speech, reduced extrinsic redundancy in typical environments, reduced intrinsic redundancy of the central auditory nervous system in children with CANS Dysfunction, and multiplicative distortion effects.

 

Certainly more and better research investigating Sound Field Systems are needed, particularly incorporating children with CANS dysfunction. More research is needed because with few exceptions, most of the studies have lacked rigorous peer review or have used faulty research designs such as having teachers fill out questionnaires without experimental control such as using unblinded teachers, lack of control classrooms, poor description of subject characteristics, and lack of baseline data. In addition, most of the studies have examined benefits over only several weeks, or in some cases a few months of use.  To this writer’s knowledge only the Rosenberg et al. [11] and the Heeney [12] studies followed students over one year.

 

In addition more studies are needed looking more specifically at factors such as speech recognition ability and the effects of different classroom environments on potential improvements using Sound Field Systems.  There is conflicting evidence regarding the effect of classroom acoustics on the benefit of Sound Field Systems. For example, Wilson et al. [18] found that Sound Field Systems might only have significant benefits for children with normal auditory systems in classrooms where noise levels are already not excessive. Alternatively, Dockrell and Shield [19] found that there was greater improvement for children with typical auditory systems in classrooms with poorer reverberation times. Although more research is needed regarding this issue one could reasonably hypothesize that children with CANS dysfunction would benefit from Sound Field Systems even in environments where benefits are minimal for children with normal auditory systems.

 

Further research would also be beneficial investigating specific abilities that may or may not be improved in children with CANS dysfunction. For example, Dockrell and Shield [19] found that in children with typical hearing there was no difference in reading, spelling, or mathematic improvements in amplified versus unamplified classrooms. However, they did find improvements on a nonverbal measure of speed of processing and listening comprehension.  Given that temporal processing and listening comprehension are significant deficits in children with CANS Dysfunction improvements in these areas would be beneficial. 

 

Another area worthy of investigation would be the relationship between improvement, or lack thereof, using Sound Field Systems and specific types of CANS Dysfunction.  For example, Stecker [28] described the Buffalo Model of Central Auditory Processing Disorder assessment and management.  The Buffalo Model identified specific profile difficulties of Decoding, Tolerance-Fading Memory, Integration, and Organization with specific interventions tailored for each profile. Chermak and Musiek [29] also discussed identifying  different types of deficits in persons with CANS dysfunction and then formulating treatment programs based on those deficits. For example, treatment approaches would be different in some respects for individuals who show similar auditory performance decrements with competing signals, but dissimilar temporal processing abilities.

 

In summary, however, despite the limitations in studies of Sound Field Systems, when one considers the bulk of these studies in combination with what is known about speech acoustics and perception, multiplicative distortion effects, and auditory processing deficits in children with CANS dysfunction, there is a strong rationale for the use of Sound Field Systems for children who have CANS Dysfunction.

 

 

 

 

 

Steve Bornstein, Ph.D., C.C.C./A is an associate professor at the University of New Hampshire in the Department of Communication Sciences and Disorders. He earned his Ph.D. from the University of Connecticut. Prior to coming to UNH he was on the faculties at the University of Memphis and Columbia University. Steve previously was also an adjunct associate professor at Dartmouth Medical School in the Department of Otolaryngology. He has had interest, and has published, both in areas related to Central Auditory Nervous System function and Aural Rehabilitation. He currently has a developing interest in the applications of fMRI to the study of Central Auditory Nervous System function. The department at UNH has a nascent research program in brain imaging, particularly in the area of motor speech.

 

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