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This website briefly describes the development of the first active acoustic homing torpedo of the US Navy:

Investigation of the use of echo-ranging equipment or an "active" homing torpedo system was initiated under the auspices of NDRC in 1941 at the G.E. Co. Research Laboratory, Schenectady, N.Y. Active homing differs from passive homing in that, with active homing, the torpedo steers on the basis of the signal returned by the target through reflection of the torpedo's own transmitted signal. In mid-1942, G.E. began development of the first active homing torpedo, Torpedo Mk 32, which was physically similar to Mine Mk 24 (figure 17).

By mid-1944, the program had progressed through the successful prototype stage... About ten units were completed when World War II ended, and the project was deactivated until 1951 when Torpedo Mk 32 Mod 2 was produced in quantity by the Philco Corp. of Philadelphia, Pa.

So basically, an active homing torpedo needs to send out its own ping and listen for that specific echo, rather than other noise on other frequencies. AFAIK, all active homing systems choose a high frequency, sounding like a high ping, because all large ship screws make very low frequency noise.

Anyway, my question is what kind of electronic device can listen to only a narrow set of acoustic frequencies? It has to be a device that existed in the early 1940's.

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    \$\begingroup\$ Bandpassfilter? \$\endgroup\$ – Curd Jan 12 '18 at 13:24
  • \$\begingroup\$ No certainty, but likely a piezoelectric transducer, possibly used as both speaker and microphone. \$\endgroup\$ – Brian Drummond Jan 12 '18 at 13:24
  • \$\begingroup\$ There are many resonant devices which could be used. An LC tank so an inductor and a capacitor. Something mechanical like a weight and a spring, then interface to that using a magnetic coil (like a loudspeaker/microphone). A acoustic solution with an acoustic resonance (pipe). \$\endgroup\$ – Bimpelrekkie Jan 12 '18 at 13:28
  • \$\begingroup\$ At this age, the relevant patents and schematics should be declassified and may be even available on the internet ... \$\endgroup\$ – pjc50 Jan 12 '18 at 13:28
  • \$\begingroup\$ yes, the transducer probably is only sensitive in a narrow frequency range (the same range in which it can generate sound efficiently) \$\endgroup\$ – Curd Jan 12 '18 at 13:29
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Sonar history lesson: -

In 1906, American naval architect Lewis Nixon invented the first sonar-like listening device to detect icebergs. During World War I (1914-18), a need to detect submarines increased interest in sonar. French physicist Paul Langévin constructed the first sonar set to detect submarines in 1915. At first, these sonar sets could only "listen" to returning signals. By 1918, Britain and the United States had built sonar sets that could send out, as well as receive, sound signals. The U.S. military began using the term "sonar" during World War II. As with radar, new military applications for sonar are constantly being developed. For example, in the early 2000's, the U.S. Navy introduced a sonar system to help clear military mines.

Text above taken from here. Use of resonant circuits whether mechanical or aided by electrical filters was well-known throughout the 20th century.

US patent 609,154 made in 1898 describes the use of matched tuned circuits used in a radio transmitter and receiver. I think Tesla may have made an earlier patent regards the use of tuning. Tuning/resonance was therefore a fairly open-book since the start of the 20th century.

Piezo electricity: -

The word ‘piezo’ is derived from the Greek word for pressure. The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880. They found that pressure applied to a quartz crystal creates an electric charge in the crystal, a phenomenon they referred to as the (direct) piezoelectric effect. Later they also verified that an electric field applied to the crystal leads to a deformation of the material: the inverse piezoelectric effect. In the subsequent century, research has been performed into the development of materials with improved piezoelectric properties, enabling commercial utilization of the piezoelectric phenomenon.

Taken from here. Making a mechanical movement resonate has been known about far longer than just the 20th century - think tuning forks and pendulums etc..

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    \$\begingroup\$ Yes but that text does not explain what device was used to listen to a specific narrow range of frequencies. I looked at your link and searched for "resonant", no hits. If it really is a resonant circuit, your source does not say so. \$\endgroup\$ – DrZ214 Jan 12 '18 at 13:41
  • \$\begingroup\$ See my additions regarding tuned circuits used on antennas and mechanical resonation. \$\endgroup\$ – Andy aka Jan 12 '18 at 13:52
  • \$\begingroup\$ See also this useful document about the history of sonar transducers: google.com/… \$\endgroup\$ – Andy aka Jan 12 '18 at 14:22
  • \$\begingroup\$ This guy did a lot of work on radio and sonar transducers: en.wikipedia.org/wiki/Reginald_Fessenden \$\endgroup\$ – Andy aka Jan 12 '18 at 14:27
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As a former sonar engineer, I can tell you that all sonar transducers have mechanical resonant frequencies. For passive sonars, for which a wide operating frequency band is desirable, the hydrophones are designed to have a resonant frequency well above the operating frequency. In that way, their receiving response is relatively flat until the resonant frequency is approached.

For active sonars, efficiency of transmission is important so transducers operating at their resonant frequency are used. Since they are resonant devices, they have a mechanical Q which determines their useful bandwidth. Most sonar transducers have Q's in the range of roughly 5 to 20. For a torpedo application, in which size is important, the operating frequency will be high to keep the transducer size small.

A resonant transducer, due to mechanical considerations, will be roughly the size of the wavelength of its resonant frequency. Thus a transducer resonant at 5 kHz will be about 1 foot in size while a 50 kHz transducer will be about 1 inch in size. Thus torpedo sonars usually operate at frequencies near 50 kHz. As far as detecting the received signal, 2 factors come into play. One, the receiving response of the transducer is also bandlimited (transducers, due to mechanical reciprocity, have the same frequency response for receiving as for transmitting). Thus the transducer effectively filters out much of the unwanted signals present in the ocean environment. Two, the preamplifier circuitry that receives the transducer signal has bandpass filtering included so that is also rejects signals outside the operating band of the sonar.

In the 1940's it was certainly within the state of the art to build such circuits (e.g. the proximity fuse which greatly increased the performance of anti-aircraft guns during World War II used miniature vacuum tubes in a radar system that operated at much higher frequencies).

To summarize, it was the combination of a resonant sonar transducer and band limited amplifiers that allowed the creation of a homing torpedo.

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    \$\begingroup\$ A few paragraph breaks would be nice, Barry. \$\endgroup\$ – Transistor Jan 12 '18 at 17:40
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    \$\begingroup\$ @Transistor no room for paragraphs on sub-marines. \$\endgroup\$ – Trevor_G Jan 12 '18 at 19:29
  • \$\begingroup\$ @Transistor Sorry about that. I tend to ramble some times. Trevor got it right about the room on submarines. There is none. I've had to sleep on torpedo racks with a very thin air mattress (yes, the torpedoes had been removed.) \$\endgroup\$ – Barry Jan 12 '18 at 21:46
  • \$\begingroup\$ Just curious, but 50 kHz is way above human perception (20 Hz - 20 kHz), so you could not hear a ping from that. That ping you hear in the movies could of course be hollywood fiction, but I've heard Germans describe it in interviews from youtube, (tho I think that was not from a torpedo but rather from the British ASDIC). \$\endgroup\$ – DrZ214 Jan 13 '18 at 3:19
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It does indeed seem to have been piezoelectric.

http://www.piezo.com/tech4history.html :

The first serious applications work on piezoelectric devices took place during World War I. In 1917, P. Langevin and French co-workers began to perfect an ultrasonic submarine detector. Their transducer was a mosaic of thin quartz crystals glued between two steel plates (the composite having a resonant frequency of about 50 KHz), mounted in a housing suitable for submersion. Working on past the end of the war, they did achieve their goal of emitting a high frequency "chirp" underwater and measuring depth by timing the return echo. The strategic importance of their achievement was not overlooked by any industrial nation, however, and since that time the development of sonar transducers, circuits, systems, and materials has never ceased.

Same article goes on to describe better ceramic and ferroelectric material development for sonar use in the second world war.

"sensitive to a narrow range of frequencies" implies a resonant structure, and the above quote does describe the sensors as resonant at around 50 kHz.

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