Transistor hearing aid Tesla type ALS 311
Producer: Tesla Rožnov, plant Valašské Meziříčí
The year of production: 1965
Power: two cell battery 2,5 V
Volume regulation: about 35 dB
Dimensions: 49 x 73 x 17 mm
Length of connecting cord: 800 mm
Weight: 55 g
Donor: Elementary Boarding School for the Hearing Impaired, Kremnica
It was used for audability improvement for moderate and severe hearing loss and allowed an air conduction hearing. A speaker served for a sound amplification. The sound was altered to an electromagnetic signal that was amplified by the transistor amplifier, relevantly modified and in the receiver changed again into the amplified sound signal. Then it was transmitted to the user’s ear canal.
A switch that was placed on the right at a front view enabled to set up the most relevant frequency characteristics according to the type of hearing loss. The switch had four positions:
Position 1 – red dot = switched off
Position 2 – white dot = the bass lowered
Position 3 – two white dots = full volume in the whole acoustic zone
Position 4 – three white dots = the pitch lowered
When listening by a microphone the switch was directed to the side of the appliance, in the „M“ position. The receiver caught and amplified all the sounds captured by the receiver’s microphone.
Listening by inductive coupling was possible everywhere it was available to receive low frequency magnetic field instead of sound waves. At this type of listening the microphone of a receiver was disconnected and an induction coil connected. The switch had to be in the „T“ position. Listening by inductive coupling was particularly suitable when calling, in cinemas organised for this puprose.
Basic model contained: an amplifier with a clip, a cord with an electromagnetic receiver including an earbud and a soft cover for transfer.