|Ian Mertes, an audiology doctoral student, performs real ear measures
Frequency-lowering is the generic term used to refer to current technologies that take high-frequency input signals—typically considered to be speech sounds—and deliver these sounds to a lower frequency region for improved speech understanding. The concept is not new—but the potential for success is.
Earlier schemes (e.g., vocoders, slow playback, etc.) were considered to be innovative trends at the time, but the current potential for success lies in the availablity of the digital-processing chip that accompanies most current hearing aids, allowing for real-time manipulation of the incoming signal. The two methods for accomplishing this manipulation are frequency transposition (Kuk, 2009) and frequency compression (Glista et al., 2009).
The high-frequency sounds that hold much of the discrimination and clarity in speech sounds are often the least audible for persons with hearing loss, regardless of age. Congenital and genetic factors can result in high-frequency hearing loss in infancy, as can presbycusis and noise exposure in adulthood. In each of these cases, the loss of sensory cells begins in the base turn of the cochlea, a region dedicated to the perception of high-frequency sounds, and then gradually spreads to the mid and apical turns, which are most relevant to the perception of human speech. Although there are many other documented causes of sensory hearing loss (e.g., ototoxicity), many of these causes also result in sloping, high-frequency loss configurations. Making that high-frequency information once again audible and useful is both exciting and noteworthy.
Verification is Key
There is a catch—clinicians using either frequency-lowering approach must verify that the method is successful for each patient. Audiologists cannot assume the fitting algorithm that is pre-loaded or automatic in the hearing aid fitting software is effective for each individual candidate (see "A Dozen Pairs of Hearing Aids"). Most clinicians agree that the majority of first-fit algorithms fall far short of meeting any prescriptive goals. As noted in the manufacturers' forum at the 2009 ASHA convention, they are first-fit, not final-fit algorithms. The audiologist must ensure that the fitting is adequate to provide ample audibility of the intended speech inputs. There is only one way to do so, and that is though real ear measures (REM).
Although a very recent survey indicates the number of practitioners using REM is increasing (ASHA, 2008; Kochkin et al., 2010), many audiologists still are not convinced of its value. Even more are not yet aware of the capabilities of REM tools for assessing frequency-lowering hearing aids. An example is shown in Figure 1 [PDF]. Thresholds are plotted in blue, which is referred to as an SPL-ogram (sound pressure level) in this upside-down format. The audibility of an isolated 5000 Hz 1/3-octave band of speech is shown, with the frequency-lowering algorithm both activated and not activated. The software in the measurement tool allows for this visualization of effectiveness by reducing the 1/3-octave band levels in a long-term-average-speech spectrum (LTASS) above 1000 Hz, resulting in a distinct "cavity" between 1000 Hz and the selected high-frequency band.
It is clear to both the audiologist and the patient that the frequency-lowering scheme was set appropriately, because the pink band (with frequency lowering) is above threshold and the green band (without frequency lowering) is not. This information can be obtained for four high-frequency 1/3 octave bands centered at 3150, 4000, 5000, and 6300 Hz in less than 10 minutes. For the patient in this example, real-world success will be known with time. The audiologist can be confident, however, that the frequency-lowering algorithm was fit to provide adequate opportunity to hear and use the high-frequency sounds that were previously inaccessible.
Research Explores Effectiveness
More research is needed to answer questions about the effectiveness of frequency-lowering schemes. For a particular hearing loss, is frequency transposition superior to frequency compression? These technologies are marketed for milder hearing losses, particularly in children, but what happens when the processing scheme is altered after several years of acclimatization to the novel stimulation? How does bimodal application with a different processing strategy on the other ear, such as the use of a cochlear implant, affect adjustment and success? What is the long-term impact of frequency transposition on speech production?
Although the impact of frequency lowering on speech and language development in children is a bit more difficult to assess, two efforts are noteworthy: Teresa Ching at National Acoustic Laboratories in Australia has been following two groups of young children, one group using a frequency-lowering scheme, and the other group using more traditional amplification. Her results will be available in about a year. In the United States, a five-year study at University of Iowa, University of North Carolina, and Boys Town National Research Hospital, funded by mild-to-severe hearing loss. A significant number of these children were fitted with frequency-lowering devices prior to their enrollment in the study, providing an opportunity to assess speech, language, educational, psychosocial, and other outcomes for this recent intervention scheme.
Both studies offer an opportunity for a clearer understanding of the benefits and/or limitations of this recent trend in hearing aid processing. As is always the case, the evidence base for establishing best-practice guidelines takes time to mount. In the meantime, the clinician is still charged with ensuring that the hearing aid is fit to provide appropriate audibility. Real ear measures provide both visual verification and documentation that audibility was optimized. Without such information, we are unable to accomplish the task for which we have been trained—management of hearing loss.