CLASSROOM AMPLIFICATION DEVICES FOR CHILDREN

CLASSROOM ACCOMMODATIONS AND HEARING ASSISTANCE

The classroom environment

Classrooms are a critical auditory environment for children yet many do not provide favorable conditions for hearing. The classroom environment is one that must be controlled to provide favorable conditions for hearing. There are three important variables to be noted: Noise, reverberation, and distance from the teacher. Sources of classroom noise may include the children themselves, furniture noise, ventilation systems, and external ambient noise. Ambient noise levels often exceed an optimum 35 dBA (unoccupied), and hard surfaces can reduce hearing effectiveness by increasing reverberation time beyond an optimum maximum of 0.3 to 0.6 seconds (American Academy of Audiology; AAA, 2011a). Signal level and SNR decrease with distance from the signal source.  

Children require a greater SNR than adults for speech recognition. Young children require speech levels that are at least 20 dB above those of interfering noise and reverberation (AAA, 2011b). In practice, this is difficult to achieve without amplification. Consequently, even children with normal hearing may experience difficulty hearing in class. Many children with CAPD particularly have difficulty hearing in background noise. The noise level does not need to be loud to disrupt auditory input. Adults with CAPD describe how noise from a fan or  refrigerator can interfere in properly decoding speech. Some children with CAPD are overwhelmed by all classroom noise levels, becoming distressed and unable to function. Such children are sometimes withdrawn from school.

Sometimes, minor modifications to a classroom, for example, sealing obvious entry points of external noise and introduction of absorbent materials, may improve the acoustic classroom environment, but are unlikely to sufficiently improve the audibility for a child with CAPD. This

is because some children with CAPD may need amplification of the primary signal, not just an improved SNR, to hear well (see Section “Amplification”). Hearing assistive technologies (HATs) and in particular remote microphone systems can alleviate or overcome all three sources of signal degradation in the classroom: Noise, reverberation, and distance from the talker.

Other Environments

Children with CAPD have difficulty when speech is rapid or degraded by distance, acoustic conditions, or accent, when information streams are complex or lengthy, and when competing sounds are present. It follows that HAT can be helpful to them in many aspects of their lives besides the school environment. Moreover, given the positive neuroplastic changes that occur over time from wearing amplification (Friederichs and Friederichs, 2005; Hornickel et al.,

2012), children with CAPD should be encouraged to use their HAT as much as possible.

Amplification

Terminology

The majority of recent studies of amplification for children with CAPD have used remote microphone hearing aids, with body- or head-worn receivers, which receive a signal from a microphone worn by the speaker. The transmission medium has typically been frequency modulation (FM). Hearing systems of this type are usually referred to as “personal FM systems.” This term is ambiguous, because it refers to accessory FM systems used by wearers of conventional hearing aids or cochlear implants. Furthermore, FM is increasingly being replaced by digital modulation (DM) technology. From the point of view of advocacy as well as accuracy, use of the term “remote microphone hearing aids” reinforces the point that children with “central deafness” require amplifying hearing aids in much the same way as do children with peripheral hearing loss. Until remote microphone hearing aids become recognized as simply another type of hearing aid they remain classified as assistive listening devices (ALDs) or, in more current terminology, a type of hearing assistive technology (HAT).

CLASSROOM AMPLIFICATION SYSTEMS

Classroom amplification systems, also referred to as sound distribution or sound field systems, provide amplification of the teacher’s voice through loudspeakers. Their efficacy is variable, depending in particular on the room acoustics. Classroom amplification systems typically improve SNR by 3 to 5 dB, but may worsen SNR in classrooms with very poor acoustics. Adaptive systems which increase the amplification as the noise level increases can achieve better than 5 dB. Portable desktop systems in which a small loudspeaker is placed on the desk of an individual child provide a slightly better SNR, perhaps 10 dB, for that child. Remote microphone hearing aids can provide at least 20 dB improvement in SNR. A meta-analysis by Schafer and Kleineck (2009) comparing speech discrimination in noise with various FM systems in trials involving cochlear implant users showed no significant improvement with sound field systems but 17% improvement with desktop systems and 38% improvement with personal direct auditory input FM systems.

CANDIDACY FOR AMPLIFICATION

It is sometimes mistakenly assumed that only children with CAPD who complain of difficulty hearing in noise, or who score poorly on a speech-in-noise test, will benefit from remote microphone hearing aids. In fact, research results and clinical experience indicate that nearly all children with CAPD show classroom benefit from personal amplification as long as the classroom teacher is cooperative. Results range from children whose hearing ability in class is instantly transformed through to those in whom benefits are more subtle and slower to manifest. There is no known predictive test of degree of benefit to be derived from amplification

(though the Hornickel et al. study reported above shows an interesting correlation between initial inconsistency of the brainstem response and subsequent benefit). However, recommendation of amplification only for children with abnormal scores on tests of hearing in noise undoubtedly denies potential benefit to many children.

Electroacoustic verification of remote microphone hearing aid. Upper curve(vertical hatches) represents audibility of the amplified pathway. Lower curve (horizontal hatches) represents audibility of the unamplified pathway through the open ear canal.