This page is a simple simulation of hearing aid mould features and volume control action (assuming a linear hearing aid). The volume control has a range of -10 to +15dB. The controls allow you to select vents and horns of different sizes and add an ear-hook damper to see the effects of these features. At certain frequencies any sound escaping from the ear-canal will reach the microphone 'in-phase' with the incoming sound and, if the gain at that frequency is high enough, will cause feedback. Likely feedback frequencies are indicated by red dots. Each of the controls will affect whether feedback occurs (e.g. horn boosts high frequencies, the vent allows more sound to escape back to the microphone).