The invention relates to a hearing aid comprising an acoustic input aperture and an acoustic output aperture, and a cover element covering the acoustic input aperture. An elastic element is disposed on the cash box bracket and provides the female connector with a thrust for matching the male connector tightly. The snowplow rotating body is composed of three short rotating bodies. The desired percentage of glycol in the effluent is obtained by initially setting the position of one or more modulating valves provided on each of separate water and glycol lines and then employing a refractometer feedback loop to modulate the flow in one or both of the input lines.