Occlusion Revisited.

Hearing Aid Forums Post About Occlusion and Venting

The degree of occlusion experienced will be down to a few different physical factors:

1 The pathology of the ear canal and the surrounding structures.
2 The depth of the fitting.
3 The degree of low frequency loss.
4 The level and pitch of your voice.
5 The venting of the mould.

People with a pathology that transfers sound well from the vocal chords, will struggle with more occlusion than others.
If the tip of the hearing device goes beyond the point in the ear canal that is reverberating with your own voice then it's less likely that occlusion will be experienced.
Typically people with over 60 dB (in the lower frequencies) losses don't get occlusion as they simply don't hear their unamplified voice anyway. Common high-frequency loss or Presbyacusis, tends to have a milder low frequency component which is often only 20dB or less - this leads to significant awareness of occlusion.
There's an association with a resonance at 500Hz which seems to be where occlusion is most prevalent. If you don't make a 500Hz sound at any great level, its possible you won't experience the same degree of occlusion as someone who does.
Moulds need to be vented properly: as a rule of thumb the following apply -
At low frequency losses greater than 60dB - No venting is needed - though it might be handy to have a pressure vent in there for equalisation.
At low frequency losses 40-60dB, venting around 2 mm is desirable.
20-40dB - venting of 4mm makes things sound a lot better.
0-20dB - As open as possible.

You can see why RIC open fittings are doing well compared with the older more occluded fittings, simply in terms of wearer occlusion comfort they win out due to better venting.