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All about FM

Understanding speech in noisy environments can be difficult for any child, but for a child with hearing loss it is even more challenging. Children need access to speech to develop their hearing, language, and learning skills. Background noise, distance from the person speaking, and reverberation (echo) are common obstacles that significantly reduce the child's access to crucial speech information. Although today's advanced hearing aids can improve the quality, audibility, and clarity of the speech signal, they cannot remove all obstacles to speech understanding. 

FM technology is the most effective way to improve speech understanding in difficult listening situations. FM systems work in tandem with the child's hearing aids by adding a special, remote microphone that can be placed much closer to the speaker. This transmits the speech signal directly to the ear, thereby limiting the influence from noise, distance, or reverberation. 

There is a major difference between hearing speech (audibility) and actually understanding it (intelligibility). Children need to be able to understand sounds. They also need to be able to differentiate between sounds and words. And FM systems can definitely help. The purpose of this booklet is to provide parents, teachers, and care-givers with comprehensive information about the invaluable benefits of these systems, both at home, at school, and at play.

The Need for FM

Children with hearing loss, however mild, rarely talk about their inability to understand family members or the teacher. If they are young and still learning language, they are unable to tell when speech is unclear or buried in background noise. In many cases they may not even be aware that they missed a question or misunderstood directions. It is well documented that children with hearing loss, as well as those with normal hearing, demonstrate difficulty in understanding speech when there is background noise, distance between the speaker and the child, and reverberation (echoes).

When noise muffles speech
Background noise is present in most listening environments including classrooms, kitchens, and sporting events. Hearing aids cannot selectively amplify only the speaker's voice; they also pick up background sounds. In many difficult and noisy situations hearing aids alone cannot make the speaker's voice clearer or even louder. Sometimes the level of the speech signal might actually be lower than the level of the background noise. 

The comparison of speech and noise levels is referred to as signal-to-noise ratio (SNR), and it represents the difference in loudness between the primary signal (i.e., teacher or family member) and the background noise. The greater the distance between speaker and listener, the less intense the sound becomes, making it more difficult for the listener to hear properly since background noise often remains the same. In other words, the lower the SNR, the more difficult it is for the listener to understand. A child with hearing loss needs the speech signal to be substantially louder than the noise - even higher than the level required by his or her normal-hearing peer in the same situation. 

How FM redresses the balance
FM technology improves SNR considerably. It picks up the speaker's voice virtually centimetres from the mouth and instantly delivers it to the ear for amplification. The distance the sound must travel from the speaker to the hearing aid is no longer an issue, as the FM microphone reduces that distance to mere centimetres.

The effect is as if the person speaking, (originally at a distance from the child and surrounded by noise), is now talking at a comfortable level directly into the child's hearing aid. The result is a clear, consistent supply of information, which maximizes speech understanding at all times.

Research demonstrates1 that when children use an FM system for a period of time, their rate of language development - especially grammatical complexity - actually matches that of their classmates. FM systems also help to reduce the delay in language so often related to hearing impairment. Another benefit is that they make children feel more secure; even when their caregivers move out of sight, they remain within hearing range. 

In short, FM technology is a highly effective tool that helps children with a hearing loss overcome the three major obstacles to speech understanding: noise, distance and reverberation.

Obstacle I: Noise

Background noise exists in most listening environments; it can emanate from both inside and outside the home or classroom. We all know how difficult it can be to follow a conversation while the television is on or the radio is playing. Required SNR graph

For children with hearing loss, noise is more than just an annoyance. The sound of a radio or television, people talking, street traffic, playground noise, feet shuffling, papers rustling, desks and chairs moving, fans or heaters all place great demands on the child's ability to hear and understand speech. 

Noise causes a general breakdown of communication and speech to such a degree that it can affect the child's performance at school. Background noise can interfere with the speech signal and either muddle it up or make it completely inaudible. And the child might miss vital directions and information given by the teacher or family member.

A number of studies2,3 have measured the intensity of background noise in classrooms. For a student with a hearing loss to receive the teacher's voice, the speech signal should be 15 to 20 dB louder than the background noise.

In many classroom settings, the teacher's voice is at a level of approximately 65 dB, and the background noise is approximately 60 dB (equivalent to the level of noisy printers in an office). This leaves a signal-to-noise ratio (SNR) of only 5 dB (65 dB - 60 dB = 5 dB). Since an SNR of 15 to 20 dB is desirable, the average classroom with a 5 dB SNR is clearly inadequate for the student with a hearing loss. Children with normal hearing also suffer when there is a poor SNR.

Obstacle II: Distance

Hearing is a sense that entails not only the loudness and intensity of sounds, but also distance or range. A child with a hearing loss has a reduced hearing range or a smaller listening bubble than a child with normal hearing.

When listening to someone talking from another room, you are using your hearing range or have a listening bubble that includes sounds of that loudness, intensity and distance. For the child with hearing loss this range is much reduced, they have smaller listening bubbles, and the distance suddenly becomes an obstacle to understanding speech.

Research has shown that a child should be within 1-2 meters of the speaker for maximum speech understanding. This is not always possible to achieve either in the classroom or at home. Also, as the distance between the listener and the speaker increases, so does noise and reverberation.

The speech signal drops 6 dB each time the distance doubles. Children in school usually sit at least one meter from the teacher. If the teacher's voice starts at 85 dB it drops to 65 dB just one meter away. This is the average level of sound for conversational speech, and it illustrates why children with hearing loss require a higher signal level.


classroom sound
classroom sound with FM
Distance can also be a challenge at home. There are many situations in which children are far away from the speaker. When playing outside or while eating dinner, factors such as distance, reverberation and noise have a detrimental affect on the speech signal. When children are out of sight, both they and their families feel more secure knowing that they can still hear each other. 

Obstacle III: Reverberation

Another obstacle to speech understanding is reverberation - more popularly known as echo. The reflection that sound makes when it 'bounces' off a surface can actually mask, or muffle the main signal. It can reduce the clarity of speech, decrease the signal-to-noise ratio (SNR) and make speech understanding more difficult.

Sounds reverberate in all rooms; some more than others, depending on the construction and furnishings. Hard walls, high ceilings, glass windows, and uncarpeted floors common to many classrooms and kitchens reflect sounds much more than carpeted floors and acoustically treated walls and ceilings. 

Basically, the sound bounces off the hard surfaces (e.g., glass and tile) instead of being absorbed as by soft furnishings such as carpets and pillows. 

Reverberation is the time it takes for a sound to lose its intensity by 60 dB (e.g. from 60 to 0 dB). This should be no higher than 0.3 to 0.4 seconds for people with hearing loss. Many studies have demonstrated that the higher the reverberation time (greater than 0.6), the more difficult it becomes to understand speech. Measurements taken in normal classrooms log an average reverberation time of 0.8 seconds, which is twice the recommended level. This makes understanding the teacher even harder. Reverberation can also be high at home in places like the kitchen, bathroom, and laundry.

Improving the listening conditions can help your child's speech perception. Both at home and in the classroom you can decrease the reverberation time and improve the acoustics by using soft, absorbent materials such as carpeting, pillows, and curtains.

Solution

Let us look more closely at how FM systems help to overcome barriers such as noise, increased distance, and reverberation.

Less noise
FM systems reduce noise even more effectively than the special, directional microphones in today's hearing aids. They also create a higher signal-to-noise ratio (SNR). By providing more information, FM systems improve speech understanding and enable children to develop their language skills.

Less distance
Where distance is concerned, FM systems can bring the sound closer to the listener without losing its energy and thus improve speech understanding. Since the speaker's mouth is approximately fifteen centimetres away from the microphone, physical distance is no longer an issue.

Less reverberation
Where reverberation is concerned, FM systems help to overcome the detrimental effects by delivering the sound directly to the child's ear. The direct delivery prevents the echo from masking the speaker's signal and making it less intelligible. FM systems can nevercompletely eliminate background noise or reverberation, but they can substantially reduce the effects. They also offer high-quality amplification of the teacher's or family member's voice to the child.

Other benefits
FM systems help the child to remain attentive at school and at home. They also help to reduce fatigue. Teachers and family members can also relax their vocal effort; being wireless, the FM system allows the speaker to move freely around the room while talking, without having to speak loudly. The child also gains a higher level of freedom and independence. 

Finally, the FM system offers an opportunity to learn incidentally through experience and interaction, as opposed to having to be explicitly taught.

At Home

Children with a hearing loss need consistent access to the spoken language, not just in the classroom, but throughout the day. Domestic life offers countless opportunities for learning language - from sorting the laundry to making cookies. But many reverberant and noisy situations remain, in which communication can be difficult.

At home around the dinner table, the noises of the dishes, cutlery and people talking all compete with each other. This puts everyone - particularly the child with a hearing loss - at a disadvantage. In a car the engine noise coupled with the noise from the traffic can also make hearing and understanding difficult - especially if the child is seated in the back.

The whole family can benefit from using FM systems at home. During tutorial sessions an FM system improves the signal-to-noise ratio (SNR) so that background noise becomes less distracting. Listening to the TV or to a speaker (e.g., at Sunday school) also becomes easier, since the sound is delivered directly to the child's ear. 

FM systems help the child hear and understand directions and requests in places such as the grocery store, in the car, at the zoo or in the play-ground. They can even be helpful for leisure activities - whether it's piano lessons, soccer practice, gymnastics or computer games. Family vacations or gatherings are also more enjoyable when the child has constant access to the speech signal.

At School

Hearing loss of any type and degree effects a child's ability to learn language and acquire knowledge.

The classroom can be classified as an 'auditory-verbal' environment where children are expected to learn by listening. 70-90% of the information provided is given verbally. Visual aids are not always available and teachers often move from one child to the next, further impacting the distance between student and teacher. Children also often work in groups, limiting the opportunity for favourable seating arrangements. Since children spend 75% of their day engaged in auditory activities they need clear and consistent access to their teachers and classmates.

Classrooms are noisy; many studies report background noise levels between 55-75 dBA, equivalent to the level of noisy printers in an office. The recommended maximum level is 35 dBA. FM systems are designed to increase the loudness of the teacher's voice to improve the SNR and eliminate the detrimental competition of noise. When used in a typical classroom environment, FM systems deliver auditory information to the child, easily and consistently.

Benefits for children of all age

Experience shows that children of all ages with varying degrees of hearing loss benefit from FM systems. Hey help to improve language development and academic performance and they also help to reduce fatigue.

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References

  1. Moeller, MP., et al. (1996). Longitudinal Study of FM System Use in Nonacademic Settings: Effects on Language Development. Ear and Hearing, 17, 28-41.  
  2. Bess, FH. , Sinclair, J. S., & Riggs, D. (1984). Group amplification in schools for the hearing-impaired. Ear and Hearing, 5, 138-144.
  3. Crandall, C., & Smaldino, J. (1994). An update of classroom acoustics for children with hearing impairment. The Volta Review, 96, 291-306. 

Information provided by Oticon. Reproduced with permission.
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15-Nov-2015 7:31 PM (AEST)