Hearing Aid Verification Using Probe-Microphone Measures
One of the most obvious differences between children and adults is size. More specifically, a pediatric audiologist must consider the effect that a smaller sized ear canal volume has on the intensity of sound delivered to the ear. At birth, the average ear canal is less than 14 mm in length from the ear canal entrance to the tympanic membrane. The ear canal grows rapidly in the first few years of life, but does not approach adult-like values until approximately 7 years of age. Because the overall ear canal volume is significantly smaller in infants and young children, a custom earmold will take up proportionately more of that volume when it is place. For an infant, this results in a significantly smaller residual volume between the tip of the earmold and the tympanic membrane. For a fixed sound pressure level produced by a hearing instrument, the levels will be substantially higher in this smaller residual ear canal space.
Figure 1. RECDs (in dB) measured with custom earmolds and plotted as a function of age (in months) and frequency. Data for four frequencies are plotted in each panel as follows: a) 500 Hz, b) 1000 Hz, c) 2000 Hz, and d) 4000 Hz. The solid line indicates the regression represented by the prediction equation derived from the data at each frequency. Used with permission.
Probe-microphone measures have been used routinely with adults since the early 1980s. Generally a signal is presented via a loudspeaker and ear canal SPL is measured as a function of frequency. Even though many adults can provide detailed feedback regarding the adequacy of their hearing instrument settings, probe-microphone measures can help the audiologist fine tune the fitting by providing valuable information regarding the match to target values, the existence of irregularities in the frequency response, and real-ear maximum output. With a cooperative adult, probe microphone measures can be obtained in both ears within a few minutes. Probe microphone measures also can provide an estimate of speech audibility across a range of input levels using a variety of signals, including real speech.
Traditional probe microphone measures are less likely to be successful with infants and young children due to movement and vocalizations. In 1994, Moodie et al outlined a modified clinical procedure that is appropriate for this population. Figure 2 illustrates the real-ear response of a hearing aid measured in an infant ear (upper curve) and a standard 2 cc 3 coupler (lower curve). Note that the SPL is higher in the infant's ear and that this difference varies across frequency. The difference between these two curves is the Real-Ear-Coupler Difference or RECD which is shown in Figure 3. Moodie (1994) and Seewald (1991) reasoned that if the RECD could be determined for a given ear, those values could be added to the coupler measures of any hearing instrument to predict the real-ear gain or SPL for that individual ear.
Figure 2. Real Ear and Coupler Response
Figure 3. Real Ear and Coupler Difference
Most commercial probe microphone systems now allow for measurement of the RECD. These procedures vary slightly across manufacturers, but the principles are the same. Two examples of hardware requirements are shown below.
Audioscan, illustration used with permission
The first step in measuring the RECD is to couple an insert-type of earphone to a standard 2 cm3 coupler. A broadband signal is introduced via the insert earphone and the SPL as a function of frequency is measured within the coupler.
Step 1. Coupler Response
Next, the probe microphone is placed in the ear canal of the child adjacent to the earmold. The insert earphone is connected to the earmold tubing and the SPL is measured again. The difference between these two curves is calculated to obtain the RECD. With practice, RECDs can be obtained for most children in as little as 3-5 minutes per ear.
Step 2. Real Ear Response
Figure 4. Measured RECD on a 10 month-old
Because the signal is delivered directly to the ear via the earphone, small head movements have little influence on the response. However, if excessive movement and/or vocalization affect the response, the measure can be quickly repeated. The RECD as a function of frequency can be mathematically added to the coupler response of any hearing instrument to estimate the real ear amplified response and maximum output. This means that all subsequent measures of hearing-instrument performance can be made in the coupler without any additional measures completed on the child. Because the child's cooperation is no longer required, the hearing instrument settings can be changed, multiple measures can be completed and the appropriateness of different types of circuits can be compared easily.
Step 3. Hearing Instrument Evaluation
Hearing instrument technology continues to advance rapidly. A primary focus of current processing strategies is to provide compression amplification across multiple frequency channels, causing the instrument gain and frequency response to change in reaction to varying input levels and conditions. Because of the changing nature of the hearing instrument response, manufacturers and leaders in the field of Audiology strongly recommend that hearing instruments with advanced technology be evaluated using real speech or speech-like signals at a variety of input levels. The response of a hearing instrument should be evaluated at multiple input levels including average speech (60-70 dB SPL - see figure 1 below) and soft speech (50-55 dB SPL - see figure 2 below). Additionally, saturation response with a loud pure-tone input of 90-100 dB SPL and loud speech inputs (75 dB SPL - see figure 3 below) should be evaluated.
Figure 5. Verifying Average Speech Inputs
Figure 6. Verifying Soft Speech Inputs and Maximum Real Ear Response
