Many hearing aid users have difficulty hearing in noisy or reverberant environments. Sometimes this is merely an inconvenience, but in a dental polyclinic setting where dentist-patient communication is critical, such challenges can be a matter of health and safety. As a possible solution to this problem, a hearing-impaired dental student evaluated the LEXIS™ FM system during practical courses and found that the technology performs well under these conditions.

The Need for Wireless FM
Improving communication success for hearing impaired people typically involves the provision of hearing aids.¹ Today’s state-of-the-art hearing aids use both analog and digital signal processing technology, and over the past decade the hearing care field has seen great improvement in the devices’ audiological efficacy and multi-microphone directionality. Despite these advances, however, hearing aid users can still have trouble understanding in challenging listening conditions characterized by background noise, reverberation, or unfavorable distance to the presenter.

As a solution, wireless communication systems have become widely used by many hearing aid users. In such solutions, a microphone is typically placed close to the talker’s mouth, and the voice signal is amplified and delivered to the hearing aid via FM radio transmission. FM stands for “frequency modulation” and is a method of encoding audio information within a radio wave by periodically varying the instantaneous frequency while maintaining a constant amplitude. Because the quality of the sound reaching the hearing aid user’s ear via an FM system depends largely on the characteristics and placement of the transmitter microphone, the listener often experiences the talker’s voice as if he or she were only inches away.

FM and Students
Students are the most prominent group within the FM-user community. This is because classrooms and lecture halls can present great challenges for speech understanding—especially at an age when language development and knowledge acquisition are critical. Distance from the teacher and varying speech levels as the teacher moves about can impair intelligibility. Noise from computers, heating ducts, and other common equipment can mask information. Rooms darkened for slide presentations make lip-reading nearly impossible.

But these problems are not limited to the classroom or to the young. Communication difficulties in automobiles, restaurants, parties, houses of worship, and other situations are well known to adult hearing aid users.

To meet the communication requirements of children and adult, nine companies—Audibel, Bernafon, Micro-Tech, Nu-Ear, Omni, Oticon, Phonic Ear, Qualitone, and Starkey—have recently developed and introduced an FM system called Lexis (Figure 1).² The basic components of the system are a transmitter with a built-in directional microphone array, plus a small receiver that attaches to nearly any behind-the-ear (BTE) hearing aid via an audio shoe.

Figure 1. The Lexis system with receiver.

Because the transmitter’s array is highly directional (8.5 AI-DI) and uses digital signal processing to maintain low-frequency performance, the objective of the following evaluation was to focus on the benefits of Lexis outside the traditional classroom environment where listening conditions are extremely variable and (potentially) even more challenging. Many listening devices show sufficient performance characteristics when being used in common situations; unfortunately, deficits can accumulate when undergoing real “road tests.”

FM System in a Dental Office/Educational Setting
Selecting an adult candidate for FM use requires establishment of need (ie, how important is communication to that individual?), activity level (ie, will the individual frequently encounter troublesome listening environments?), and comfort with technology (ie, will the introduction of a hearing aid accessory be accepted?).

For this evaluation, the author positively met all three criteria. As a 27-year-old dental student taking his practical patient course at the School of Dental Medicine at the Friedrich-Alexander-University Erlangen-Nuremberg, Germany, he needs to communicate with patients without error, is expected to perform dental examinations and procedures frequently, and is a good adopter of technology due to his technical (engineering) background.

The Friedrich-Alexander-University dental curriculum contains several clinical courses, normally performed in polyclinics with many dental chairs placed side-by-side. Dentistry is a profession under recent scrutiny for the possible hazardous impact of noise on the dentist’s hearing. Although studies have concluded that typical dental office conditions are below the limit of risk, noise from drills and other equipment can be severely disturbing for a hearing-impaired dentist—particularly when trying to communicate with a patient. Obviously, clear understanding between dentist and patient is very important to avoid any grave errors in treatment (eg, history of allergic reactions, understanding concerns about discomfort/pain, etc).

Because of the importance of having full freedom of head movement and the smallest area for possible contamination, an ear-level receiver system such as Lexis seemed ideal for the dental treatment sessions.

The author’s hearing loss was diagnosed at age 5, the result of improper treatment with ototoxic streptomycin. Audiometric evaluations reveal symmetric bilateral sensorineural hearing loss with profound hearing loss in both ears (Figure 2). Unaided speech-recognition scores are 45% for recorded monosyllabic words (Freiburg-Test) presented at 60 dB SPL. Bone-conduction thresholds measured at 250 Hz, 500 Hz, and 1000 Hz reveal only tactile responses. Acoustic reflexes are consistent with cochlear pathology.

Figure 2. Hearing thresholds of author.

The author has worn binaural hearing aids since age 6, including recent BTE Perseo models with integrated FM receivers. For the purpose of this evaluation, he additionally switched to binaural BTE Phillips D72 instruments with Lexis receivers.

Communication abilities are good under optimal listening conditions with both brands of hearing aid aids. However, difficult situations are encountered with:

• Lectures with dimmed lights during slide presentations;
• Discussions in the lecture theatre with colleagues;
• Instructions at the dental chair in the polyclinic with background noise from neighboring chairs.
• Talking to patients in the polyclinic office setting—especially children, women and soft-speaking patients in general.

The author had used a Phonak MicroVox FM system for many years with good results; however, this system reached its limits under these special conditions.

Evaluation Results
The Lexis system was evaluated during the winter term 2003/04, revealing the following observations:

• The system greatly enhances understanding in noisy clinical and theater conditions due to its variable focus control function. If background noise increases, the Superfocus mode helps to filter the important spoken information. Furthermore, discussions in the lecture theatre became easier to follow with little loss in information.

• Dentistry requires special hygienic conditions. While in many public situations, such as in cafeteria or a bar, the FM system can be placed on the table. This is a problem at the dental chair. First, the possibility to disinfect the device is an important condition for the implementation of an FM system under clinical conditions. Because of its rounded shape and plain surface, Lexis meets this requirement. The evaluation has shown no damages of the surface coating after more than 100 applications with a standard surface disinfection solvent. Furthermore, because of the silver colour it gives a very clean impression to the patient. The footing on the instrument table is secured by the special stand mechanism with the batteries integrated in the base serving as a strong abutment.

• In some situations (eg, during polishing or periodontal surgery that can produce aerosol blood generation), it is recommended that the patient place the transmitter under the napkin, securing it with the Lexis clip system. Due to the specific placement of 4-microphone array, the generation of friction noise is very low.

• The wireless connection guarantees the greatest freedom of head movement possible for the dentist and contributes to a safe dental treatment. There is no risk of loose wires dropping into the operation field.

• The author experienced little or no interference with patient-monitoring systems or when using ultrasonic scalers.

• The power consumption of both the hearing aids and the FM system was fairly low; however, there could be further improvements and size reductions using ultra-low-power circuits and chips. The past decade has seen major reductions in minimum feature size for digital CMOS processes. Although the integration of smaller devices leads accordingly to lower power-supply voltages, digital hardware still constitutes a power-hungry solution for applications such hearing aids and other listening devices. The main problem is that, in a number of situations, the computational complexity and low power consumption required in medical devices restricts the minimum size possible.³

• In contrast to similar products, the Lexis receiver features an independent microphone sensitivity control. Adjustable within ±7 dB, this control provides power resources for special situations when necessary.

• An additional stereo jack provides the possibility to connect the system with other communication devices used in a dental clinic (eg, a radio set as used in emergency service).

Summary
Lexis performed well under the special conditions of dental education and practice. The author recommends it to dental/medical students and professionals.

Acknowledgements
This evaluation was sponsored by Phonic Ear Inc by provision of material.

References
1. Dillon H. Hearing Aids. Stuttgart, NY: Thieme; 2001.
2. Phonic Ear Inc. Look to Lexis for great-sounding, easy-to-use ear-level FM. Available at: http://www.phonicear.com/ lexis.asp. Accessed: April 19, 2004.
3. Toumazou C, Dietrich S. Advanced Mixed Analogue and Digital Processing Technique—a novel design methodology for hearing aids and cochlear implants. Proceedings of the 48th International Congress of Hearing Aid Acousticians (UHA Congress); Frankfurt, Germany; 2004:161-164.