The piezo-electric membrane (3) and the housing (2) are so formed that they fulfil the expression f cav is less than f o, whereby f o is a resonance frequency of the piezo-electric membrane and f cav is a resonance frequency of the resonance hollow chamber. The housing comprises a main body (4) with an upper plate section (4a) having a sound emission hole (4c). It has a cylinder section (4b) of one-piece construction with the upper plate section and having an opening at one end. A cover component (5) closes the opening of the housing main body. A support section (5a) extends to one side of the main housing body and the piezo-electric membrane is so supported that it is arranged between the support section and the main housing body which also holds it. POLYMERS - The housing main body and the cover component are made of plastic.
