The placement of the probe tube is critical when performing real-ear measurements (REM) as it serves as your point of reference for all of the measurements you will perform.
The probe tube placement can be detrimental to your measurements due to poor placement, especially in the higher frequencies.
There is a challenge in placing the probe tube correctly due to varying ear canal shapes and sizes.
Here are three tips to guide you when inserting the probe tube.
1. Good quality otoscopy
Otoscopy allows you to identify the shape and size of your patient's ear canal, especially to identify any anomalies or points to avoid (wax or turns in the ear canal) when placing the tube.
2. Ensure the correct distance is allocated on your probe tube marker
The average distance for a woman is 28 mm and for a male is 30 mm.This is the distance from the end of the tube to the marker, which should ideally position the tube within 5 mm of the tympanic membrane.
When placed, the marker should then sit at the anti-tragal notch and along the floor of the ear canal. Remember – this is an average distance so good otoscopy beforehand (to check that the ear canal is not shorter than what is advised), and further otoscopy once the tube is placed, can serve as evidence for further adjustment to the tubes placement.
3. Performing a Real Ear Unaided Gain (REUG) measurement once the tube is placed to assess the placement quality
The REUG serves as a good indicator of your probe tube placement as the shape of the curve can help to identify if further adjustment of the placement is necessary. The REUG curve should begin at 0 dB Gain and then peak between 2 – 4 kHz for an adult.
If the curve shape is very low then this could indicate that the placement is incorrect due to a blockage due to the end of the tube being against the wall or in ear wax. In this scenario you may need to replace the tube or reposition it.
Finally, the curve should dip down at roughly 6 kHz and then recover. The curve shouldn't dip too much below the 0 dB Gain line (maximum –5 dB Gain) after 6 kHz. If it does, this suggests that the probe tube needs a deeper insertion.
All the above methods complement one another. It is completely fine to re-perform otoscopy, reset the marker on the tube or re-perform your REUG measurement to get the best possible probe placement. This procedure improves with experience too, so you will find yourself getting better at assessing and placing the probe with continuous practice.
Happy probe placement!
If you want to learn more about the binaural REM feature found in the REM440 Real-Ear Measurement module in the Affinity 2.0 and Callisto™ Suites, please check out this quick guide.
British Society of Audiology (2018). Guidance on the verification of hearing devices using probe microphone measurements.
Dillon, H. (2012). Hearing aids 2nd ed. New York, Thieme Medical Publishers.
Hawkins, D., Alvarez, E., Houlihan, J. (1991). Reliability of three types of probe tube microphone measurements. Hearing Instruments, 42: 14-16.
Pumford, J., & Sinclair, S. (2001). Real-ear measurement: Basic terminology and procedures. Audiology Online.
BS ISO 12124 (2001). Acoustics: Procedures for the measurement of real-ear acoustical characteristics of hearing aids.