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Hearing Aid Verification
Frequently Asked Questions
What about negative RECDs?
 Negative RECD values may result from several sources. The first and most common possibility is the result of a loose-fitting earmold or an earmold with a large vent. In this case, amplified low-frequency information leaks out around the canal or through the earmold vent. However, unamplified low frequencies may also enter the ear canal through these same sources. Negative RECD values resulting from venting are most commonly seen below 1000 Hz. Errors in predicted real ear amplified gain may occur with large negative RECDs (greater than -5 dB), especially when the gain of the hearing instrument is minimal at the same frequencies where the negative RECD values occur. See Hoover, et al (2000) for more information.
A second source of negative RECD values is the presence of a patent tympanostomy tube or TM perforation. In this case, the RECD measurement may include the resonance characteristics of the entire middle ear space, creating a negative RECD value in the low to mid frequency region between 750 and 1500 Hz (Martin, et al, 1997) . Because negative RECD values may be present at higher frequencies than seen with venting, it is less likely to result in estimated real ear gain errors. Figure 1 below contrasts the measured RECD values for an intact eardrum (Left ear, left panel) with an eardrum that has a perforation (Right ear, right panel).
Figure 1. Intact versus perforated eardrum
In situations where the probe tube is not inserted deeply enough into the ear canal, negative RECD values may occur in the higher frequencies above 3000 Hz. Figure 2 below illustrates negative high frequency RECD values resulting from short probe tube insertion.
Figure 2. Improper probe tube insertion
When all RECD values are negative (as seen in Figure 3 below), a blocked or crimped probe tube may be the cause. The tube could be blocked by wax or by incorrect placement of the tube against the ear canal. A tight-fitting earmold can cause enough pressure to crimp or close the probe tube.
Figure 3. Blocked tube probe
Otoscopic inspection of the ear canal prior to measuring the RECD should indicate when some situations such as tympanic membrane perforations or wax are present. The audiologist should troubleshoot the probe tube and its placement in the child's ear canal when RECD results are unexpected.
Why not just use average RECD data instead of measuring the RECD?
Although age-related RECD averages exist, repeated studies of RECD results in children (Feigin, et al, 1989; Bagatto, et al, 2002) have indicated that inter-subject RECD variability is high across age. Individual variability of measured RECD values ranges from 10-14 dB across frequencies for all ages studied. A 10-14 dB difference between an average RECD value and an individually measured response could mean the difference between audibility and no audibility of speech sounds for some children. It could also mean the difference between a safe versus an excessive real ear saturation response level.
Age-related averages do not take into account the unique characteristics of an individual earmold such as canal length, tightness of fit, or venting, which all can affect the real ear response. In addition, previous average data has been grouped into 12-month age categories. Therefore, the average RECD for a 4 month-old would be the same as the average RECD for an 11 month-old. More recent data from Bagatto, et al (2002) has provided more specific age categories for infants, but different test systems and software applications may still be applying the older, larger age categories.
What about middle-ear disease?
Middle ear effusion and tympanic membrane perforations are the most common middle ear conditions that affect the RECD response. TM perforations were discussed above, with the effect being a negative RECD value in the low to mid frequency range. On the other hand, middle ear effusion with increased stiffness of the TM often creates larger positive RECD values across the frequency range tested. (Martin, et al, 1996).
How often does the RECD need to be repeated?
Most pediatric guidelines recommend that the RECD be repeated whenever the child has an earmold change. When this recommendation is applied to infants and very young children, it can mean frequent RECD measures as new earmolds are needed as often as every 2-3 months when infants are growing rapidly. Using one ear's measured RECD as the predictor for the second ear can be an efficient compromise when faced with such frequent retesting. At older ages, RECD measures should be repeated at least annually.
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