View Full Version : Acoustic Detectors.

12-23-2005, 10:57 PM
These were posted a few days ago in alt.binaries.pictures.military by Rah O'hara. And all commentary is his.

They are acoustic detectors, for use against enemy aircraft. Most date from the 20 and 30's in the immediate prewar years, before Radar was put into widespread use.

I knew about most of these sets already but found them fascinating nontheless, and thought you all would appreciate them.


Professor Mayer's topophone: 1880
It was devised to assist navigation in fog.


There are two horns in the horizontal plane, and two in the vertical plane.


A four-horn acoustic locator again, in England: 1930s.
Once more there are three operators, two with stethoscopes linked to pairs
of horns for stereo listening. The exact method of operation is currently
unknown, but I suspect was as follows: the man on the left adjusts the
mounting elevation until the aircraft noise is apparently central, while the
chap on the right adjusts the bearing for the same result. The man in the
middle reads bearing and elevation from dials and transmits it by telephone
to the air defence system where the results from several locators can be
combined to triangulate the target, and give its approximate height and


The impressive array of Japanese war-tubas belong to at least two acoustic
locators mounted on 4-wheel carriages. It is a little difficult to work
exactly what is connected to what, not least because the background appears
to have been erased by some unsubtle retouching, but I think that the format
is the same as the British model; there are two horns in a horizontal plane,
and on one side of the mounting there are two more in a vertical plane.
To the right, one of the figures is the Japanese emperor Horohito. Behind
him are the AA guns intended to be used in conjunction with the locators.
The only Japanese gun that I have found documented as being used with a
sound locator is the Type 88 dual-purpose AA/coast-defence 75mm; there is
not enough visible detail to verify that these are the guns shown in the
picture, but they look about the right size.


Acoustic locator on trial in France: 1930s.
This remarkable machine is an acoustic locator based on hexagons. Each of
the four assemblies carries 36 small hexagonal horns, arranged in six groups
of six. Presumably this arrangement was intended to increase the gain or
directionality of the instrument. OPnce again there are three operators.


Like the British and French versions, the RRH was also composed of four
horns, two to determine bearing, and two for elevation, arranged in a ring.
The two lateral horns have a horizontal bar across their mouths.


The RRH acoustic locator with operators at their posts.
The RRH could detect targets at distances from 5 to 12 km, depending on
weather conditions, operator skill, and the size of the target formation. It
gave a directional accuracy of about 2 degrees.
It had a crew of three - traverse aimer on the left seat, elevation aimer on
the right seat and a dial-reader/talker in the middle. The rolled-up
material above the operators' heads could be unfurled to provide shelter.

The curved things visible under the ring are the rear of the horns.


The German RRH acoustic locator again.
This gives a better view of the rear of the horns, curved for compactness.


A US Army sound locator for a mobile searchlight unit.
The locator and control station were connected by cables to the searchlight
and a mobile electricity generator.

Note there are three horns and not four, the one in the middle here being
shared between the horizontal and vertical planes.

These locators continued to be deployed when radar sets were introduced, in
the hope of convincing the Germans that the US searchlight battalions were
still dependant on acoustic location.


A US Army sound locator in use.
This photograph was dated January 1943, and was presented by the American
media as being current equipment. This was another piece of misinformation
as radar sets were already in widespread use for searchlight control at that

Note the large diameter acoustic tubes leading to the operator's headset.


An early four-horn system, date and nationality unknown.
Unfortunately I have no information at all about this photograph. The
uniform suggests the First World War to me, as the operator appears to be
wearing puttees, but this is far from certain. All very much conjectural.


An acoustic locator dish in Kent, England: built 1928.
This 30-foot high dish is located at Greatstone, Kent. The small concrete
hut in front housed the operators. The vertical mast in the centre carried
the acoustic pickup tubes. A static dish can be much larger than a fully steerable horn, giving more
acoustic gain and the possibility of detecting aircraft at greater ranges.
The pickup tube could be moved sideways to "steer" the direction of maximum
sensitivity by a limited amount.


An acoustic locator wall at Greatstone, Kent: built 1930.
A mirror has to be much larger than the wavelength of what it is reflecting
to work efficiently. This 200-foot wall was a later development designed to
concentrate audio wavelengths in the 15 to 18 foot range, which were not
handled effectively by 20-foot and 30-foot dishes.

The wall could detect aircraft at 20 to 30 miles distance. This may not seem
impressive, but in aircraft interception every second is valuable. With its
later microphone installation the wall had a bearing accuracy of 1.5

12-24-2005, 05:27 AM
Great post!
Those concrete ones are the most intersesting! Were they used operationally? I mean radars were there already.

12-24-2005, 05:35 AM
Very interesting stuff. Thanks for posting. http://forums.ubi.com/images/smilies/25.gif

12-24-2005, 05:38 AM
I see that 'Bus' is back LFII. Merry Christmas mate. http://forums.ubi.com/images/smilies/25.gif

12-24-2005, 05:42 AM
And to you, mate. http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

12-24-2005, 06:15 PM
Sorry don't know anything more about them that what was posted.

I know that the small mobile ones on the allied side continued to be used for a few years, say into 1943 in an attempt to deceive the enemy.

12-24-2005, 06:57 PM
Boy, I would have hated for a swarm of bees to fly inside one of those horns when you were listening!

Really though, I bet flying insects were troublesome for these things. One bee directly in the horn would probably sound like an He-111!

12-24-2005, 09:14 PM
Interesting post http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif I read somewhere about people digging "listening" pits to give them a form of local warning against aircraft attack.

12-25-2005, 01:31 AM
Kudos waldo, that was definately an interesting piece of work you've researched. I haven't read an interesting post at this forum in what seems like years.

12-25-2005, 09:06 AM
There must be some kind of sound filtering device installed. Otherwise, imagine if someone yelled near those machines when they were operational... Poor operators http://forums.ubi.com/images/smilies/59.gif