GN Hear & Now


THE 16/4 RULE.

To achieve a high quality earmould or custom hearing instrument, it’s important that you take an accurate ear impression. A substandard impression will improperly represent the patient’s ear geometry and can result in a number of fit or acoustical compromises. ReSound recommends the use of the TruFit impression technique, which uses the 16/4 rule:

  • 16 mm from aperture = 4 mm beyond 2nd bend

  • Is the canal length > 16 mm from the concha side?

  • Does it extend 4 mm past the second bend?

  • Have you provided a full helix?

To help you measure and confirm the accuracy and suitability of your impression, ReSound recommends the use of the Impression Measurement Key. This key can be used to measure the dimensions of your impression, and confirm the length and width for an optimal custom product.

A Length: Using the 16 mm notch, measure the length of the canal from the tip to where it meets the concha bowl. The canal should fill the entire notch. It is acceptable if the canal is longer than the notch as in the photograph above.

B Width & C Width: Using the 5 mm and 10 mm notches, measure the area between the aperture and the 1st bend of the impression. Use the 5mm notch to measure the narrow width of the canal. The canal should be wider than the notch. Use the 10mm notch to measure the wider dimension of the canal, the canal should be wider than the notch.
To help you achieve a 16/4 TruFit impression, it may help to make a mark on your ear light at 16mm from the tip. Then insert the otoblock so the mark on the TruFit ear light tip aligns with the canal aperture as shown below.
In some cases it’s recommended to do an open jaw impression.

  • When ear geometry lacks retention
  • When patient has severe Temporomandibular Joint (TMJ) movement
  • When device has tendency to work out of ear
  • When patient has feedback or loose fit issues
If the impression does not meet the criteria after measuring, determine if taking another impression could meet the guidelines better. If still in doubt about the impression meeting the criteria, send it to the ReSound lab for further assessment.

Valerie Smets
ReSound Product Manager, ANZ