70db is start of a severe loss.
Venting Advantages:
Reduce unnecessary low frequency gain and output
Allow unamplified sound to enter the ear canal
Reduce occlusion effect
Relieve the feeling of pressure in the ear
Reduce moisture build-up in the ear canal.
Venting Disadvantages:
Too much venting can combine with the ear canal cavity to produce a Helmholtz resonator effect (increased gain in the 250-1000 Hz region), resulting in an echo or barrel effect when the patient speaks.
Feedback. As vent size increases, acoustic leakage increases, and therefore the probability of feedback increases.
In general, if the hearing loss below 1000 Hz is 25 dB or less, an open mold (large vent (3.0 mm) or acoustic modifier) is recommended. If the hearing loss is 30-45 dB in the low frequencies, an acoustic modifier or medium vent (2.0mm) is recommended. If the hearing loss is 50-60 dB in the low frequencies, a small vent (1.0 mm) or pressure vent (.06-.88 mm) is recommended. When the loss exceeds 60 dB, we usually do not put in a pressure vent, unless the patient is occluded or their voice bothers them. When in doubt, put in a pressure vent, and plug it if it causes problems with feedback. The “Rule of Thumb” is that it is easier to plug a vent than it is to drill a vent. We use cushion grip, a denture adhesive, to plug vents. However, before plugging the vent with cushion grip, which is hard to get out of a vent, plug the vent with Fun Tak or clay to make sure the patient is not bothered by a plugged vent. If plugging the vent does not bother the patient (occlusion, resonance in his voice, etc.), then plug the vent with cushion grip. Note, we generally plug the lateral portion of the vent, since patients feel less occluded than when we plug the medial portion of the vent. However, if feedback is still an issue after plugging the lateral portion of the vent, try plugging the medial portion of the vent at the tip of the canal. Generally, feedback is less when the medial portion of the vent is plugged. If you need to remove cushion grip from the vent, just heat it with a hair dryer. Cushion grip can be obtained at most drug stores.