A miniature unidirectional condenser microphone includes a diaphragm dividing a housing into first and second chambers. Each chamber communicates through a sound inlet port to the atmosphere. One of the sound inlet ports is covered by a sheet of porous, non-absorbent, hydrophobic thermoplastic material forming an acoustical resistance.
A miniature unidirectional condenser microphone having a fore and aft orientation, said microphone comprising:a miniature housing;electret diaphragm means dividing said housing into front and rear chambers;means in said housing forming a front sound inlet port for said front chamber;means in said housing forming a rear sound inlet port for said rear chamber;
Dimple means internal of said rear chamber projecting inwardly from the wall of said housing and surrounding said rear inlet port for defining a continuous ridge surrounding said rear inlet port and circumscribing an area larger than andincluding said rear inlet port; and
Sheet material supported in spaced relation to said rear inlet port by said ridge and covering the area circumscribed by said ridge for forming an acoustical resistance in the path of communication established by said rear inlet port betweensaid rear chamber and the exterior of said housing.
Miniature unidirectional condenser microphones require high quality and very small acoustical resistance elements having predictable resistance characteristics. Resistance elements of porous materials are potentially advantageous fromstandpoints of performance, cost and ease of assembly. However, potentially useful porous elements, if manufactured in thin sheets to provide the dimensional requirements suitable for a miniaturized microphone, tend to be characterized by a non-uniformporosity per unit area.