The proliferation of hearing screening programs has led to the identification of an increasing number of infants and children who require amplification. In an attempt to bring greater accuracy and objectivity to the challenging process of selecting and fitting amplification for the pediatric population, the real-ear-to-coupler-difference (RECD) measurement procedure has been developed.1
Since infants and children have much smaller ears than adults, they bring considerable variation with them to the amplification process. In addition, variations in middle ear impedance and residual ear canal volume can significantly influence the adult RECD. Therefore, it is important to measure the RECD, on both adults and children, and utilize it in the amplification process whenever possible.
WHAT IS AN RECD?
The RECD is the difference, in dB across frequency, between the SPL measured in a 2-cc coupler and the SPL measured in the patient's ear. This difference measure is used in some amplification fitting protocols (e.g., DSL [i/o], NAL-NL1) to capture the effects of the patient's occluded ear canal acoustics. Obtaining the patient's “acoustic signature” is helpful in converting HL to ear canal SPL to accurately select a hearing aid. It is also used in calculating the predicted REAR (real-ear aided response) which is used when verifying hearing aids in a coupler. The RECD is a very useful measurement, especially for selecting and fitting hearing aids for infants and young children.
HOW IS IT MEASURED?
The following steps describe a general procedure for measuring an RECD.1 For a description of how to conduct the procedure on specific probe-microphone equipment, contact the distributor or consult the DSL v4.1a manual.2
- Measure the dB SPL of a test signal in an HA-2 coupler using an insert earphone that has been calibrated using 2-cc RETSPL (reference equivalent threshold sound pressure level) values.
- Place a probe tube in the patient's ear and couple a foam tip, impedance tip*, or personal earmold to the insert earphone.
- Place the tip in the patient's ear, and measure the ear canal dB SPL using the same test signal.
- The difference between the two measurements across frequencies is the RECD.
Some probe-microphone equipment will automatically subtract the two measures to give the RECD across frequencies. It has been shown that clinical RECD measures are accurate, reliable, and efficient.3–5 Once you have had some practice, you can complete the procedure in less than 5 minutes, and the values collected can be used throughout the amplification process.
WHAT DOES AN RECD LOOK LIKE?
Obtaining an accurate RECD measurement starts with learning what a typical RECD looks like. Figure 1 shows an average RECD for an adult with normal middle ear status, when measured with a foam or impedance tip. Typically, the values are positive across frequencies, and increase in the high-frequency region:
- To convert from the real ear to the coupler, subtract the RECD.
- To convert from the coupler to the real ear, add the RECD.
By convention, positive RECD values indicate the extent to which levels measured in the real ear exceed levels measured in the coupler for the same test signal. Values in the low-frequency region will generally be in the range of 0 dB to 10 dB and increase up to 20 dB in the high-frequency region. Infants and small children have much smaller ear canals than adults, so the values will be larger. In other words, smaller volume, greater SPL, and thus greater RECDs. The general shape of the RECD is the same for both children and adults, but the values are different within and between these populations.
HELPFUL HINTS TO OPTIMIZE YOUR MEASUREMENT
Now that you know what an RECD should look like, you can attempt a measurement using your probe-microphone equipment. If this is your first attempt at a measurement, it is helpful (and much less frustrating) to try one on a colleague or a cooperative adult patient. As you become more confident with the procedure, try measuring it on an older child then move to toddlers and infants. This will result in confidence and success. The following steps outline some hints that will help you obtain an accurate RECD measurement.
(1) Proper probe-tube placement
Mark the probe tube to the appropriate insertion depth: 31 mm for adult males, 28 mm for adult females, 15 mm to 25 mm for infants and children.
When you are inserting the probe tube, the mark should stop at the intertragal notch. The insertion depth marks are to guide you in placing the probe tube to within 5 mm of the eardrum. This can also be done by measuring 5 mm from the medial tip of the patient's earmold (see Figure 2).
(2) Always use otoscopy
(a) Before placing anything in the patient's ear canal. This helps you to determine the shape and length of the canal, and establish if there is any cerumen blockage.
(b) After placing the probe tube in order to ensure appropriate insertion depth. Some adult ear canals slope upwards like a hill. This can be used as a landmark if you place the probe tube at the crest or top of the “hill” pointing to the eardrum. Be careful not to move the probe tube when using the otoscope.
Apply earmold lubricant (e.g., Otoease, Otoferm, etc.) to the portion of the tube that will be inserted into the ear canal. Be careful not to go right to the end, as the lubricant may plug the tube. The lubricant will help keep the probe tube resting on the floor of the ear canal. In addition, applying some lubricant to the foam tip or earmold will reduce friction when inserting the tip in the ear canal while the probe tube is in place. It will also help to ensure that the tube does not move further into the ear canal.
When the probe tube is in place, you need to insert a foam tip or earmold carefully without altering the position of the tube. When inserting the earmold or foam tip into the ear canal, stabilize the probe tube at the intertragal notch with your little finger. Use the thumb and index finger of the same hand to insert the mold/tip.
(5) Don't hesitate
When performing this measurement on a toddler, give the child a toy mirror to look in and hold. Tell the child to watch you in the mirror to “make sure I'm doing it right.” This will engage his/her attention, reduce anxiety, and help keep his/her head relatively still.
For infants who are are sedated for an ABR you can measure the RECD while they are still asleep. Otherwise, have the parent hold the infant and proceed.
For both infants and children, stabilize your hand against the child's cheek and/or head when inserting the tube or insert/mold so that sudden movements will not catch you by surprise. Also, make sure you are familiar with your equipment and the procedure before trying to measure an RECD on an infant or young child. If you are confident, the child will be less anxious. Once the tube and mold/tip are in place, it is okay if the child moves. As long as the patient is not vocalizing during the measurement and the tube and tip do not get dislodged, it will not affect the measurement.
(5) Troubleshoot your measurement
If the equipment you are using allows, check the real-ear portion of the RECD before you “accept” it as your measurement. Look for negative values in the low frequencies and roll-offs in the high frequencies. The next section will describe some possible causes of inappropriate RECD measurements, and some ways to remedy the problems.
ENSURING ACCURATE MEASUREMENTS
It is important to have an accurate measurement of your patient's ear canal SPL, since it will be used at various stages in the fitting process. The coupler measurement should be the same every time you do it, assuming your equipment is stable.
You can measure the coupler response portion of the RECD first thing in the morning. This way, it will be done before your patient arrives and it gives you a chance to make sure your probe-microphone equipment is functioning appropriately. When the probe tube and foam or impedance tip are situated in your patient's ear, start the test signal and wait. Check the accuracy of your measurement while the signal is on. But, before accepting the measurement, ask yourself the following questions and try the solutions:
- Are the high frequencies rolling off around 2000 Hz to 3000 Hz? If yes:
- Possible causes:
- Earmold measurement: If you are measuring the real-ear portion using the patient's personal earmold, the increased length of the earmold tubing (compared with a foam tip) will roll off the high frequency response above 2000 Hz, in comparison with a response measured with a foam tip. Therefore, this measurement would be acceptable in the high frequencies (see Figure 3).
- Foam tip measurement: If you are measuring using a foam tip, your probe tube may be too shallow.
- Re-insert the probe tube to within 5 mm of the tympanic membrane and remeasure.
- Do I have negative values between −1 dB and −9 dB in the low-frequency region? If yes:
- Possible causes:
- Earmold measurement: The probe tube may be causing some of the low-frequency sound to escape from around the earmold. Also, the earmold may have a vent larger than 1 mm, which will cause sound to leak out.
- Foam tip measurement: The foam tip may not be fully expanded in the ear canal or the size of the foam tip is too small. Also, the foam tip may not be inserted deep enough into the ear canal. In all cases, low frequency sound will leak out.
- Use earmold lubricant (e.g., Otoease, Otoferm, etc.) on the foam tip or earmold to create a better seal around the ear canal. Plug the medial side of the earmold vent when doing the measurement. Also, if you have the appropriate size of foam tip, make sure the most lateral end of the tip is flush with the opening of the ear canal and the foam has completely expanded (see Figure 4).
- Do I have negative values between −10 dB and −15 dB in the low-frequency region (seeFigure 5)? If yes:
- Possible cause:
- Earmold or foam tip measurement.
- The patient may have a perforated eardrum or a myringotomy tube in place.
- Perform an otoscopic examination and check acoustic impedance results. It is normal to see extreme negative values in the low-frequency region when a tube is in place or there is a perforation in the patient's eardrum.
- Do I have increased positive values in the low- and mid-frequency region? If yes:
- Possible cause:
- The patient may have middle ear effusion. The increased mass and stiffness of a fluid-filled ear will cause increases in the RECD in the low- and mid-frequency regions compared with a measurement obtained in an ear without middle ear effusion.6 When a patient has middle ear effusion, the RECD results are more variable, making it even more important to obtain this measurement in this situation.
- Check acoustic impedance results. It is normal to see increased positive values in the low- and mid-frequency regions when the patient has middle ear effusion.
The real-ear-to-coupler-difference measurement is used to capture an individual's occluded ear canal acoustics for the purposes of selecting and fitting amplification. Obtaining an accurate measurement is important for matching the appropriate electroacoustic characteristics of your patient's hearing aids. Following the guidelines discussed above will help you obtain more accurate and reliable RECD measurements. The Quick Reference Guide below is intended to help you remember the useful tips for optimizing your RECD measurements.
Support for this project was provided by The Ontario Research and Technology Consortium. I am grateful to Dr. Richard Seewald, Susan Scollie, and Jane Joyce for their assistance in preparing this report.
1. Moodie KS, Seewald RC, Sinclair ST: Procedure for predicting real-ear hearing aid performance in young children. AJA
2. Seewald RC, Cornelisse LE, Ramji KV, et al.: DSL v4.1 for Windows: A software implementation of the Desired Sensation Level (DSL [i/o] Method for fitting linear gain and wide-dynamic-range compression hearing instruments. London, ON: National Centre for Audiology, 1997.
3. Scollie SD, Seewald RC, Cornelisse LE, Jenstad LM: Validity and repeatability of level-dependent HL to SPL transforms. Ear Hear
4. Seewald RC, Moodie KS, Sinclair ST, Scollie SD: Predictive validity of a procedure for pediatric hearing instrument fitting. AJA
5. Sinclair ST, Beauchaine KL, Moodie KS, Feigin JA, et al.: Repeatability of a real-ear-to-coupler difference measurement as a function of age. AJA
6. Martin HC, Westwood GFS, Bamford JM: Real ear to coupler differences in children having otitis media with effusion. Brit J Audiol