During recent years, cell phones have become wildly popular. According to the Cellular Telecommunications & Internet Association, cell phone market penetration in the US reached 55% and above 80% in many European countries. Cell phones are an integral part of the everyday life of people in many societies.

Hearing-impaired people, however, are often excluded from using cell phones. Besides feedback issues, which are also prevalent when using landline phones, an important reason for difficulty of use is the radiation of electromagnetic interference from the cell phone. When a digital cell phone communicates with a base station in a network, it sends electromagnetic waves in a pulse pattern which is picked up by the hearing aid and causes a buzzing noise. The level of that noise is dependant on the distance between the cell phone and the base station (the longer the distance, the higher the energy of the pulse pattern sent out by the cell phone), and on the susceptibility of the hearing instrument to electromagnetic interference.

The buzzing noise is not only annoying, but also degrades speech understanding over the telephone. A recent study which examined speech understanding with cell phones in hearing-impaired listeners showed a massive degradation of speech intelligibility compared to landline phones.¹ Speech understanding dropped from 62% over the landline phone to only 26% using a cell phone under the same test conditions. When using a loopset (which is carried around the neck and transmits the incoming speech to the hearing instrument’s T-Coil), speech understanding was still slightly worse than when using a landline phone.

Utilizing Wireless Technology
A recently introduced FM transmitter, called SmartLink, from Phonak offers a technical alternative for hearing-impaired cell phone users. The cell phone is connected to the FM transmitter via wireless Bluetooth technology.² If the cell phone rings, a ring tone is sent to the FM receivers attached to the personal BTE hearing aids, and the voice of the caller is sent to the hearing aids. The user’s own voice is picked up by the FM transmitter and sent to the cell phone. The call is answered and hung up with the FM transmitter. The cell phone can be anywhere within the Bluetooth range and does not need to be touched except to dial a number.

The potential of this approach to improve speech understanding and satisfaction with cell phones was assessed in the following clinical trial.

Study Method
A total of 19 experienced hearing aid users participated in the trial. All subjects except one used binaural hearing instruments. The age of the subjects ranged from 11-72 years (mean: 41.2 years), and their hearing losses ranged from moderate to severe (PTA: 74 dB). Some of the subjects had their own cell phones, which they mostly use for exchanging short text messages, as understanding speech over the phone was generally too difficult for them.

Figure 1. Average pure-tone audiogram and standard deviation.

The trial consisted of speech tests in the research facility and practical experience in daily life. Two different speech tests were conducted: the Monosyllabic German Rhyme Test,³ and the Oldenburg Sentence Test.⁴ The latter was conducted at 65 dB with added broadband noise at 7 dB SNR, which represents a typical situation for using a cell phone (which often takes place in public and noisy locations). The speech test material was presented from a loudspeaker and picked up by a landline telephone receiver which was fixed at a distance of 0.5 m from the loudspeaker. The subjects sat in another room where the loudspeaker could not be heard.

The signal was transmitted to the cell phone (Nokia 6230) over a dial-up telephone connection. The level of the speech test was 65 (75) dB at the receiver of the landline phone which resulted in 63.5 (71) dB at the speaker of the cell phone measured with a Brüel & Kjær artificial head. The speech tests were conducted in 4 conditions in randomized order:

1. Unaided;
2. Own hearing instruments;
3. With additional Nokia Loopset LPS-4 (own hearing instruments set to telecoil position);
4. With additional FM transmitter (own hearing instruments set to FM).

In Conditions 1 and 2, the subject used their preferred ear for using the phone; in Conditions 3 and 4, the speech was presented bilaterally.

Results
Figure 2 shows the speech test scores for the Rhyme Test³ in quiet. As expected, unaided performance at 65 dB input level was quite poor (median: 0% speech understanding), but with large inter-subject differences. With hearing aids alone, the median speech test score was 43%. The loopset increased speech understanding to 69%, and the FM transmitter allowed for 81% speech understanding. Compared to using their hearing aid alone, all subjects except one benefited from using the FM transmitter. All differences were significant (except unaided vs. own HA, due to the large inter-subject variations). At 75 dB input level, speech understanding was higher in all conditions, but order and significances remained the same.

Figure 2. Speech test results in quiet with 65 dB input (median values and 25%/75% percentiles).

Figure 3 shows the speech test scores for the Sentence Test in noise which was conducted in two conditions only. Compared to using the hearing aids alone, the SmartLink FM transmitter allows for clearly improved speech understanding in noise (paired t-test, p<.0001). In 16 of the 19 subjects the criterion which has been set for testing the cell phones in daily life (at least 50% speech understanding at 65 dB input over the cell phone in the best condition) were met.

Figure 3. Speech tests results in noise at 7 dB SNR (median values and 25%/75% percentiles).

The subjects also kept diaries of situations where they used the cell phone, and rated subjective speech understanding and sound quality. For 2 weeks, they used their cell phones together with the FM transmitter, and for another 2 weeks without FM transmitter. In total, 154 and 127 individual cell phone calls with and without FM transmitter were described and rated, respectively. Only 2 of the 16 subjects could not use the cell phone without the FM transmitter at all, and they were not included in the statistical analysis. In terms of subjective speech understanding, listening effort, and perceived loudness, phone calls using the FM transmitter were rated significantly better than those without (p<.05).

Figure 4 shows the results for subjective speech understanding over the cell phone using the FM transmitter, and the results support the data from the measured speech test scores. Despite the predominantly severe hearing losses, speech understanding over the phone was rated quite positive. In terms of handling, there was no significant difference between using the cell phone with or without FM transmitter. However, the absolute ratings for cell phone handling showed high intersubject variation. While the younger subjects had no problems in handling the devices, some of the elderly reported difficulties (where it had no effect whether an additional FM transmitter was used or not). This suggests that, as with hearing instruments, clear instructions and counseling are necessary to allow for maximum benefit of these devices.

Figure 4. Subjective rating of speech understanding with cell phones and FM transmitter in daily life.

The study suggests that the SmartLink FM transmitter allows for substantial improvements in speech understanding when using a cell phone. It provides hearing-impaired individuals better access to mobile communication—an integral tool in modern societies.

References
1. Sorri M, Piiparinen P, Huttunen K, Haho M, Tobey E, Thibodeau L, Buckley K. Hearing aid users benefit from induction loop when using digital cellular phones. Ear Hear. 2003;24(2):119-132.
2. Ingrao B. Bluetooth technology: Toward more wireless hearing care solutions. Hearing Review. 2005;12(1):26-27, 88-89.
3. von Wallenberg EL, Kollmeier B. Sprachverständlichkeitsmessungen für die Audiologie mit einem Reimtest in deutscher Sprache: Erstellung und Evaluation von Testlisten. Audiologische Akustik. 1989;38:50-65.
4. Wagener K, Brand T, Kollmeier B. Entwicklung und Evaluation eines Satztests für die deutsche Sprache I: Design des Oldenburger Satztests. Z f Audiologie. 1999;38(1):4-15