Although we are all familiar with the medical stethoscope used by doctors to listen to the internal condition of the patient’s heart and lungs etc., few will be familiar with the use of stethoscopes in other industries.
In 1936 British engineer Charles Edward Noel-Storr, Managing Director at Capac Co. Ltd in London, came up with an interesting variation on the stethoscopes that were available at the time.
The Capac BIN-AURAL
Capac Company’s BIN-AURAL was somewhat similar in appearance to a medical stethoscope and was used to detect faults and assess the condition of a variety of mechanical devices and structures.
The long probe called a tectoscope can be put in contact with the outside part of a machine/engine/structure and the sound from inside is transmitted back to the earpieces via a metal diaphragm. In this way the skilled mechanic can identify specific problems. Virtually no sound is picked up from outside, making it perfect for isolating faults in noisy environments such as factories and machine shops.
So what made the Capac BIN-AURAL different to its competitors?
Apart from the single tectoscope illustrated above the user could also employ a second tectoscope. This enables the operator to listen to the mechanism from 2 different points simultaneously.
A further option is also available in the form of the Tectophone.
The tectophone (illustrated below) is a small exponential horn which is screwed on to the diaphragm. This can be used to listen in close proximity to the outside of a mechanism in the particular location where faults are suspected. It can also be used in conjunction with a tectoscope to listen to the outside and inside simultaneously or to compare.
For a detailed explanation of the uses of the Capac BIN-AURAL I have reproduced below the original manual that came in the box.
CONCLUSION
In the build up to World War 2 aero engine makers such as Armstrong Siddeley Motor Limited, Bristol Aeroplane Company Limited, D. Napier & Son Limited and many other British engineering companies adopted the Capac BIN-AURAL as an effective diagnostic tool. With the aid of the BIN-AURAL, a mechanic could pinpoint the source of a sound, or listen to that sound from 2 different perspectives.
P.S. Whilst the Capac BIN-AURAL is not itself a microphone, a microphone can be easily attached, making it a great tool for gathering unusual sounds from inside engines and machines and other structures.
The following ‘collage’ recording of bits of my car engine has been made with a Sennheiser MKE2 inserted into the rubber tubing which connects to the tectoscopes.
Martin Mitchell's Microphones 