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This is an impression of an ear canal ~If your new hearing aid is fitted deep down, partly- or mostly past the second bend, there will be less likelihood of an unpleasant side-effect called OCCLUSION. This manifests itself as a sensation of your own voice sounding (typically) very loud, echoey and/or like you are talking in a barrel. Your partner or relative can experience this by talking whilst sticking a finger in one then both ears. The lesser effect of occlusion with only one finger in the ear, partly explains the once-popular habit of persons wearing only one hearing aid (ie, one can get over occlusion far more easily if wearing one hearing aid, not 2). We know a lot more now, and it is quite rare to be fitted with one aid only unless there is a very good reason to do so.Risk of occlusion increases if the ear is completely blocked AND if your LOW FREQUENCY HEARING is normal or near-normal (this is common in those of us with NOISE INDUCED HEARING LOSS). If you are getting a CIC or ITC (or a BTE with mould) AND your low frequency hearing is near-normal, it should be fitted with a VENT .
A vent can have a great influence on the sound quality of a hearing aid. In almost all fittings, a very tiny vent is advisable for pressure equalization purposes(between the ear drum and the hearing aid). Otherwise, vents are used mostly when occlusion risk is high. The size of the vent is generally linear in line with perceived risk. In some cases the vent may be very big, 4 or 5mm diameter. A vent also serves to modify low frequency amplification downwards (and in turn can emphasize the higher frequencies). Vent size increases the chance of FEEDBACK (the unwanted whistling heard from some hearing aids). Sometimes a too-large vent can solve initial problems of occlusion but can create a new problem further down the line – The hearing loss worsens, the hearing aids are turned up, and then start whistling. 
TIP: If your vent gets blocked, this will normally increase occlusion and usually reduce hearing in background noise. Use a vent cleaning tool regularly. 
Occlusion caused by custom-made shells
Look at the picture to the right. The aid to the left never sounded satisfactory. Yet, the newer aid in the middle sounded completely different. Notice the position of the sound outlets.

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