Will all depend on her loss and what the Audi says.
Here’s what google says so you have some idea.
DETERMINING EARMOLD VENTING & CANAL LENGTH
Advantages and Disadvantages to Venting
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.
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.
Parallel vs. Diagonal Vents
Parallel vents are generally used, because not only do diagonal vents decrease low frequency gain like parallel vents, diagonal vents also decrease mid frequency (630-1600 Hz) gain by up to 10 dB. Therefore, unless you want to decrease mid frequency gain, parallel venting should be used.
The Effect of Parallel Vent Diameter/Canal Length
The rule of thumb is that as the parallel vent size increases (1 to 3mm), low frequency gain decreases (up to 30dB) at 500 Hz and below, and mid frequency gain 630-1600 Hz increases (up to 10 dB). As canal length increases (6 to 22 mm), the effect of venting is reduced. Thus, low frequency gain decreases are less (15 dB versus 30 dB) with long canals, and mid frequency gain will decrease as well (in fact, there may be no increase in mid frequency gain at all).*
So, what does this mean to you? If you want to increase the mid frequencies, and decrease the low frequencies, you should order a short canal (6mm) and a large vent (3mm). A long canal (22mm) with a large vent (3mm) will not reduce the low frequencies as much as a short canal (by as much as 10 dB), and the mid frequencies will be reduced more with a long canal than a short canal. Pressure equalization vents (.06-.8mm) on the other hand, no matter what the canal length is, generally only decrease gain up to 6 dB below 250 Hz.*
A Venting Selection Guide
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.