Audiology Lecture Notes

Hearing Instrument Shells

• Full Shell:
  • Fills the external ear and ear canal.
  • Used for severe to profound hearing losses.
  • Provides the best acoustical seal.
• Skeleton Mold:
  • Has a cutout in the mold.
  • Offers good retention without filling the entire ear canal.
  • More cosmetically appealing than a full shell.
• Half Shell:
  • Like a shell, but the upper portion is removed.
  • Sits in the bowl of the ear but lacks the helix area.
• Semi Skeleton:
  • Removes the rear rim of the earmold.
  • Features a locking extension at the bottom and top for canal and helix lock.
• Cross, Freefield, Janssen Molds:
  • Leave the canal unoccluded.
  • Minimize low-frequency amplification.
  • Used for open-fit, thin-tube fit, and RIC (Receiver-In-Canal) hearing aids.

Custom Molds for RICs and Thin Tube BTEs

• Custom molds aren't limited to tiny canal hearing aids.
• Reasons for using custom molds:
  • Retention: Needed if there is an anatomical reason for additional retention.
  • Significant Hearing Loss: For higher power receivers, a full shell mold ensures a better seal, directing more sound into the ear and preventing leakage.
• When ordering, specify the desired mold; don't assume the manufacturer will send a tiny canal mold.
• Clinicians are in charge of fittings and should choose the best fit based on the individual's hearing loss.
• Use the manufacturer's recommended dome for the hearing aid.

Modifying Vinyl Ear Molds

• Modify with caution, as the buffing wheel may remove additional material due to the mold's softness.
• For small modifications, skip the Dremel and go straight to the buffer.

Silicone Ear Molds

• Allergy-free, comfortable, and provide a good acoustical seal, especially for high-gain fittings.
• More difficult to modify than other materials.
• Follow the earmold manufacturer's recommendations for modifications, including specific bits and speeds for grinders or Dremel tools.

Hard vs. Soft Materials

• Soft ear texture (flaccid pinna): Use a harder material.
• Firm ear texture: Use a soft material.
• Ears tend to get softer with age.
• Some materials start hard and soften with body heat, offering a compromise between ease of insertion and comfortable seal.
• Order forms indicate the material type, including options like a solid acrylic body with a flexible canal.

Tubing

• Inside and Outside Diameters:
  • Frequency response is affected by the inside diameter.
  • Feedback resistance is affected by the outside diameter.
• Common Sizes:
  • Number 13 tubing.
  • Available in medium, thick, and double-wall options.
• Lifespan and Maintenance:
  • Tubing shrinks, discolors, and stiffens over time, impacting acoustic output.
  • Typical tube life is 3-6 months.
  • Check tubing at every office visit.
• Replacement Procedure:
  • Measure the new tubing on the patient's ear to prevent it from becoming too short over time.
  • Insert the earmold with extended tubing, attach the hearing aid, measure, and cut to size.
  • This task is done in the office by the professional.
• Retubing Techniques:
  • Pressure fit (pull-through).
  • Gluing.
  • Tube locks (integrated locking mechanism).
• Moisture Issues:
  • Water droplets in the tubing can block sound, especially in humid or hot environments.
  • Patients may report a "dead" hearing aid.
• Dry Tubing:
  • Resists moisture but is stiffer and more difficult to work with.
  • The benefit may outweigh the inconvenience.
• Laser SLA Molds:
  • Require industrial-strength superglue.
  • Do not use a reamer, as the mold can shatter.
• Tube Locks:
  • Available in various shapes and materials (brass or plastic).

Venting

• Affects frequencies below 1000 Hz.
• Diameter vs. Low Frequencies:
  • The greater the vent diameter, the greater the roll-off in low frequencies.
• Open Fit:
  • Gives up control below 1000 Hz.
• Occlusion:
  • The more occluding the fit, the more low-frequency control you get.
• Most hearing aids have some kind of vent, primarily for pressure relief and equalization.
• Important for patient acceptance, as it addresses the feeling of being plugged up.
• Venting is less crucial than before in shaping the overall response curve due to digital technology, but comfort remains important.
• Parallel Venting:
  • Most common type.
  • Modifies low-frequency response without affecting highs.
• Diagonal Venting:
  • Used for unusually small ear canals.
  • Can affect high frequencies and is more prone to feedback.
• External Vent:
  • Effective for ear drainage and reduces feedback.
• Fixed vs. Select-a-Vent (SAV) / Mini-Vent Plug (MVP):
  • Fixed vent: Has a set size.
  • SAV/MVP: Allows on-the-fly adjustments without sending the aid back to the manufacturer.
  • Helpful for addressing feedback issues as ears change with age.
• Venting Chart:
  • Pressure vent: 0.04 inches (1 mm).
• Mostly Open Vent (MOV):
  • A large vent option, such as the ReSound MOV, can be useful when needed.

Ear Canal Resonance and Ear Molds

• Ear molds that occlude the canal impact the ear canal's natural resonance.
• The natural resonance of the ear canal is based on it being an open tube.
• Adjust the hearing aid's frequency response to compensate for this impact.
• Horn Effect:
  • A large change in the diameter of the sound tubing in the earpiece system will improve high frequencies.

Ear Mold Modifications

• Modifications are sometimes needed for comfort or to address feedback issues.
• In-Office Modifications:
  • Reduce or remove a helix curl.
  • Reduce the canal length.
  • Create an external vent (trenching the vent).
  • Remove eye spots.
  • Drill out vent holes to make them bigger.
  • Change the style (e.g., shell to half shell, skeleton to canal).
• Ensure that modifications do not position the sound bore into the canal wall, as this can cause feedback or block the sound.
• The sound bore should be aimed directly at the tympanic membrane (TM).
• Check with the earmold manufacturer about recommended tools and speeds.
• Verification with REM (Real Ear Measurement):
  • Any changes to an acoustic fit must be verified using REM and may require reprogramming.
  • This applies to domes as well as earmolds.
  • Changing dome types (e.g., open to closed) impacts low-frequency gain, which may need adjustment.
  • REM allows you to see the impact of changes and reprogram the hearing aid accordingly. Anytime an acoustic element is changed always consider and perform REM. Domes, Molds, anything that will affect the acoustic output of the hearing aid.