Probe microphone measurements: 20 years of progress.

Probe-microphone testing was conducted in the laboratory as early as the 1940s (e.g., the classic work of Wiener and Ross, reported in 1946), however, it was not until the late 1970s that a "dispenser friendly" system was available for testing hearing aids in the real ear. In this case, the term "dispenser friendly," is used somewhat loosely. The 1970s equipment that I'm referring to was first described in a paper that was presented by Earl Harford, Ph.D. in September of 1979 at the International Ear Clinics' Symposium in Minneapolis. At this meeting, Earl reported on his clinical experiences of testing hearing aids in the real ear using a miniature (by 1979 standards) Knowles microphone. The microphone was coupled to an interfacing impedance matching system (developed by David Preves, Ph.D., who at the time worked at Starkey Laboratories) which could be used with existing hearing aid analyzer systems (see Harford, 1980 for review of this early work). Unlike today's probe tube microphone systems, this early method of clinical real-ear measurement involved putting the entire microphone (about 4mm by 5mm by 2mm) in the ear canal down by the eardrum of the patient. If you think cerumen is a problem with probe-mic measurements today, you should have seen the condition of this microphone after a day's work! While this early instrumentation was a bit cumbersome, we quickly learned the advantages that probe-microphone measures provided in the fitting of hearing aids. We frequently ran into calibration and equalization problems, not to mention a yelp or two from the patient, but the resulting information was worth the trouble. Help soon arrived. In the early 1980s, the first computerized probe-tube microphone system, the Rastronics CCI-10 (developed in Denmark by Steen Rasmussen), entered the U.S. market (Nielsen and Rasmussen, 1984). This system had a silicone tube attached to the microphone (the transmission of sound through this tube was part of the calibration process), which eliminated the need to place the microphone itself in the ear canal. By early 1985, three or four different manufactures had introduced this new type of computerized probe-microphone equipment, and this hearing aid verification procedure became part of the standard protocol for many audiology clinics. At his time, the POGO (Prescription Of Gain and Output) and Libby 1/3 prescriptive fitting methods were at the peak of their popularity, and a revised NAL (National Acoustic Laboratories) procedure was just being introduced. All three of these methods were based on functional gain, but insertion gain easily could be substituted, and therefore, manufacturers included calculation of these prescriptive targets as part of the probe-microphone equipment software. Audiologists, frustrated with the tedious and unreliable functional gain procedure they had been using, soon developed a fascination with matching real-ear results to prescriptive targets on a computer monitor. In some ways, not a lot has changed since those early days of probe-microphone measurements. Most people who use this equipment simply run a gain curve for a couple inputs and see if it's close to prescriptive target-something that could be accomplished using the equipment from 1985. Contrary to the predictions of many, probe-mic measures have not become the "standard hearing aid verification procedure." (Mueller and Strouse, 1995). There also has been little or no increase in the use of this equipment in recent years. In 1998, I reported on a survey that was conducted by The Hearing Journal regarding the use of probe-microphone measures (Mueller, 1998). We first looked at what percent of people dispensing hearing aids own (or have immediate access to) probe-microphone equipment. Our results showed that 23% of hearing instrument specialists and 75% of audiologists have this equipment. Among audiologists, ownership varied among work settings: 91% for hospitals/clinics, 73% for audiologists working for physicians, and 69% for audiologists in private practice. But more importantly, and a bit puzzling, was the finding that showed that nearly one half of the people who fit hearing aids and have access to this equipment, seldom or never use it. I doubt that the use rate of probe-microphone equipment has changed much in the last three years, and if anything, I suspect it has gone down. Why do I say that? As programmable hearing aids have become the standard fitting in many clinics, it is tempting to become enamoured with the simulated gain curves on the fitting screen, somehow believing that this is what really is happening in the real ear. Additionally, some dispensers have been told that you can't do reliable probe-mic testing with modern hearing aids-this of course is not true, and we'll address this issue in the Frequently Asked Questions portion of this paper. The infrequent use of probe-mic testing among dispensers is discouraging, and let's hope that probe-mic equipment does not suffer the fate of the rowing machine stored in your garage. A lot has changed over the years with the equipment itself, and there are also expanded clinical applications and procedures. We have new manufacturers, procedures, acronyms and noises. We have test procedures that allow us to accurately predict the output of a hearing aid in an infant's ear. We now have digital hearing aids, which provide us the opportunity to conduct real-ear measures of the effects of digital noise reduction, speech enhancement, adaptive feedback, expansion, and all the other features. Directional microphone hearing aids have grown in popularity and what better way to assess the real-ear directivity than with probe-mic measures? The array of assistive listening devices has expanded, and so has the role of the real-ear assessment of these products. And finally, with today's PC -based systems, we can program our hearing aids and simultaneously observe the resulting real-ear effects on the same fitting screen, or even conduct an automated target fitting using earcanal monitoring of the output. There have been a lot of changes, and we'll talk about all of them in this issue of Trends.

Predicting directional hearing aid benefit for individual listeners.

The fitting of directional microphone hearing aids is becoming increasingly more routine, and this fitting option has proven to be a successful method to improve speech intelligibility in many noisy listening environments. Data suggest, however, that some hearing-impaired listeners receive significantly more directional benefit than others. It is of interest, therefore, to determine if directional benefit is predictable from identifiable audiologic factors. In this report, we examined whether the slope of audiometric configuration, amount of high-frequency hearing loss, and/or the aided omnidirectional performance for a speech-in-noise intelligibility task could be used to predict the magnitude of directional hearing aid benefit. Overall results obtained from three separate investigations revealed no significant correlation between the slope of audiometric configuration or amount of high-frequency hearing loss and the benefit obtained from directional microphone hearing instruments. Although there was a significant, negative relationship between aided omnidirectional performance and the directional benefit obtained in one study, there was considerable variability among individual participants, and nearly all of the listeners with the best omnidirectional hearing aid performance still received significant additional benefit from directional amplification. These results suggest that audiologists should consider the use of directional amplification for patients regardless of audiogram slope, high-frequency hearing loss, or omnidirectional speech intelligibility score.

Making sense of directional microphone hearing aids.

We have witnessed a large increase in the availability of directional microphone hearing aids over the past few years. Directional microphone technology is now available in analog, digitally controlled analog, and digital hearing aids, and has been implemented into both behind-the-ear and in-the-ear styles. This Short Course reviews basic design differences across directional microphone hearing aids. A number of different laboratory and clinical evaluation methods used for assessment of both electroacoustic and behavioral directivity are then reviewed. In addition, the potential impact of test conditions such as room reverberation and type and position of competing noise(s), on listener performance when fit with directional hearing aids are considered. Recommendations and suggestions relating to the clinical and laboratory assessment of directional hearing aids are provided.

Delayed hearing loss after neurovascular decompression.

We report two unusual cases of delayed hearing loss after neurovascular decompression of structures within the cerebellopontine angle. In the first case, the patient noted a unilateral hearing loss 3 weeks after undergoing vascular decompression of the trigeminal nerve for tic douloureux. This gradually improved over an 18-month period. In the second case, the patient awoke on the 4th day after vascular decompression of the facial nerve for hemifacial spasm with a bilateral hearing loss that has remained unchanged after the onset. These are examples of delayed acoustic dysfunction occurring with a shift in surgically freed vessels and may have been induced by newly directed neurovascular compression or distortion.

Surgical and prosthetic restoration of binaural hearing in an 88-year-old man.

We report the restoration of binaural hearing in an 88-year-old man who was experiencing significant communication difficulties. His preoperative air conduction thresholds had revealed a pure-tone average of 55 dB for the left ear and 107 dB for the right, and he was dissatisfied with the benefits provided by his BICROS hearing aid arrangement that had been fitted to his left ear. The combined effects of surgery and a hearing aid offered a 70 to 80 dB improvement for his right ear and restored the communication advantages of binaural hearing.

Decision making in rehabilitative audiology.

With the emphasis on quality assurance and patient satisfaction in health care, decision making in rehabilitative audiology is a critical issue. Traditionally, rehabilitative decisions have been based on questionable assessment procedures and unproven treatment methods. In this paper, current strategies for decision making with hearing-impaired adults will be presented. Discussion will focus on nonacoustic factors influencing hearing aid fitting, the decision processes necessary for management of the hearing-impaired adult, and expanded uses of self-assessment inventories of hearing handicap.

Comparative study on cytochrome P-450 of yeasts using specific antibodies to cytochromes P-450alk and P-450(14DM).

The occurrence of cytochrome P-450(14DM) (lanosterol 14 alpha-demethylase) and cytochrome P-450alk (long-chain alkane terminal hydroxylase) in various yeast strains was determined with immunological procedures. Cytochrome P-450(14DM), which is constitutive or housekeeping enzyme playing an essential role in ergosterol biogenesis, was found in all yeast strains so far tested. Cytochromes P-450(14DM) from different species of yeast were immunologically different, although they may have had a few common antigenic sites. In contrast, cytochrome P-450alk was detected only in the alkane-assimilating yeasts.

Effect of monaural hearing aid use on dichotic test results for individuals with high-frequency hearing impairment.

It is well documented that the majority of individuals with normal hearing have a right ear advantage for dichotic speech material. There is evidence, however, that individuals with flat sensorineural hearing loss demonstrate a left ear advantage after they have used amplification on the left side. The present study examined whether these findings could be extended to a population with high-frequency hearing impairment. The subjects were administered dichotic consonant-vowel syllable tests prior to a monaural hearing aid fitting, and the tests were repeated after 1 month and again after 6 months of hearing aid use. The results revealed a group right ear advantage prior to the hearing aid fitting, although there was considerable individual variability. No consistent changes in the test scores were shown to be associated with monaural hearing aid use in either the right ear or the left ear.

Effect of amplification on tinnitus among new hearing aid users.

A tinnitus questionnaire was completed by 200 new hearing aid users. The results indicated that 62% experienced tinnitus, with 43.5% reporting frequent or continuous presence. Thirteen of the 200 patients rated their tinnitus as severe. Approximately one-half of the respondents with tinnitus reported that their hearing aids provided either partial or total relief from tinnitus. If the tinnitus was rated severe, partial instead of total relief was the most commonly reported effect. A few individuals indicated residual inhibition. A subjective evaluation of the aids' performance revealed that relief from tinnitus was frequently rated an important aspect of the user's satisfaction.

Clinical considerations in the use of time-compressed speech.

The four lists of the Auditec of St. Louis recording of the 60% time-compressed NU 6 test were presented at 32 dB SL to 28 normal subjects and 28 subjects with sensorineural hearing loss. Significant list effects were found, with only lists I and IV being equivalent for both groups. A significant learning effect, greater for the normal subjects, also was present. Clinical recommendations are made regarding list and practice effects and considerations for use of this test in the presence of peripheral hearing impairment.

Effects of audiometric configuration on speech-band thresholds in sensorineural hearing loss subjects.

Pure-tone and speech-band thresholds were compared for adults with varying degrees of sensorineural hearing impairments. The results demonstrated that the speech-band stimuli underestimated the degree of hearing loss in the high frequencies (2000 to 4000 HZ) for individuals with sloping audiometric configuration. Probable reasons for the threshold discrepancies and clinical implications of these findings are discussed.

Performance-intensity functions as a predictor for binaural amplification.

It has been suggested that a generalized central auditory deficit which is often present in the aged population might be a contraindication for successful hearing aid use. The present study examined the relationship between measured central auditory deficit and binaural versus monaural aided performance. Subjects were adults, aged 65 or older, with bilateral sensorineural hearing impairments. Unaided phonetically balanced monosyllabic word (PB) and Synthetic Sentence Identification (SSI) performance-intensity functions were obtained followed by aided monaural and binaural speech recognition measurements. A direct relationship between measured central deficit (PBmax-SSImax) and binaural versus monaural superiority was not found because most subjects showed no significant differences for the PB/SSI or binaural/monaural measurements. The best predictor for binaural superiority was found to be the slope of the performance-intensity function, with significant correlations obtained for both the PB and SSI materials.

The effects of various front-to-back ratios on the performance of directional microphone hearing aids.

Twenty-four sensorineural hearing-impaired adults were evaluated using four directional microphone hearing aids differing only in front-to-back ratios. The speech material utilized was the Synthetic Sentence Identification Message Competition Ratios of 0, -10, and -20 dB. The primary signal was presented from a 0 degree azimuth with the competing message was presented from a direct overhead location. The results revealed a systematic improvement in speech understanding as the size of the front-to-back ratio increased. This relationship was not significantly affected by the difficulty of the listening situation.

Clinical rationale for using an overhead speaker in evaluation of hearing aids.

A variable that has received little attention in the psychoacoustic evaluation of the hearing aid is the position of loudspeakers with respect to the listener, particularly the azimuth of the loudspeaker, which is used for presenting the competing message. In the past, a variety of locations have been used, some of which can bias the outcome of the evaluation. For this reason, this article suggests the use of an overhead speaker to deliver the competing signal. The overhead placement provides a neutral location that is highly desirable for making reliable repeated speech performance comparisons. In addition, the overhead speaker can be easily adapted to the testing environment while it produces the effect of surrounding the listener with the competing signal.