717,090. Microphones; piezo-electric receivers. HEIMANN, W., [trading as PHYSIKALISCH-TECHNISCHE WERKSTATTEN July 23, 1952 [July 23, 1951; July 23, 1951; July 23, 1951], No. 18697/52. Addition to 705,450. Class 40 (4) [Also in Groups XXXVIII and XL (b)] In an installation to enable the driver of a vehicle to hear horn signals from behind (see Group XXXVIII) a carbon microphone positioned near the tail of the vehicle and a piezo-electric receiver placed near the driver are resonance-tuned to a frequency range which includes normal horn frequencies, i.e. about 2500 c/s to 3000 c/s. The microphone assembly (Fig. 1) comprises a pick-up trumpet 1 carrying at its apex a housing 2 for a carbon microphone 3 pressed against a thin rubber washer 9. This is mounted within a protective housing 4 by means of two rings 6 one of which is attached to the trumpet 1 and the other to the housing 4 with the interposition of three or more vibration-damping blocks 5. Preferably the housing 4 is coated internally, and the trumpet 1 externally, with a sound-absorbing material. In a modification, Fig. 2 (not shown), the trumpet 1 is carried on blocks 5 mounted at its larger end. Alternatively the trumpet 1 may be rigid with the housing 4 and the whole mounted on the vehicle chassis by a suitable vibration-damping block. A coarse-meshed grid 7 prevents entry of stones. Spaced gauze screens 8 protect the microphone from water splashes and are arranged so as not to lie at sound nodes. Instead of or in addition to screens 8, a thin water-repelling diaphragm may be provided. A cone of maximum diameter. 20 cms. and slant height 20 cms. is stated to form a satisfactory trumpet. The receiver (Figs. 4 and 5) comprises a piezoelectric crystal 15 driving a cone diaphragm 21 and resonating at 2500-3000 c/s. The crystal 15 is secured in a semi-circular slot 17 in a housing 14 by means of three small rubber feet 16 and its free corner is attached to the diaphragm 21 of aluminium foil by means of a member 18 of material having a low softening temperature and low thermal conductivity, e.g. a vinyl resin. This member 18 has a slot for attachment to the crystal and is shaped to fit the apex of the cone diaphragm 21, part of it projecting through a small hole in the manner of a rivet, Fig. 6 (not shown). This end is deformed by heat, e.g. by a soldering iron, to form a rivet head and securely hold the diaphragm. The housing 14 is closed by a trumpet member 25 which is attached by a simple snap action of a slot 27 over small protuberances on the outer surface of the housing 14. A slightly elastic ring 28 serves to hold the diaphragm 21 and a gauze grid 26 in position. The cavity behind the diaphragm 21 may be tuned to resonance by an adjustable piston (not shown) which varies the volume of the housing.