Air acoustics: testimonials on use of listening devices and searchlights (1939 – 1940)

Testimonials on use of listening devices and searchlights (1939 – 1940)

The museum received a testimonial about the pre-World War II organisation of the searchlight units against air targets that were part of the Army’s Sappers Weapon (Wapen der Genie) and the way how listening devices were part of the operations. The Sappers’ “Lighting Department against air targets” consisted of multiple searchlight units. Each unit was divided into four sections with four searchlights each, one of which was the recon searchlight – listening device combination. Only after the direction of the target was recorded with the listening device, the leading light a.k.a. reconnaissance searchlight was ignited. If an air target was caught in the light beam, the other three searchlights were ignited and aimed. Additional information was retrieved from veterans of the XIIth Searchlight Department.

Conversation with Mr. J. S.: When he was 18 to 19 years old, J.S. served as a conscript in the Kromhout barracks in Utrecht. He was trained there as a listener on the two-axle Van Soest listening device in Fort bij Rijnauwen. He never saw the three-axle listening device. Blindfolded he was trained to locate targets with the listening device by aiming at a working bell that could be moved from left to right and up and down (“buzzer trajectory”). The sound source had to be followed as best as possible. Later on, exercises with listening devices and a sports plane as target took place on the moors.

Schematic to the buzzer trajectory at Fort bij Rijnauwen (courtesy of the Nieuwe Hollandse Waterlinie)

In 1939 – 1940, JS’s section was stationed in the Binckhorst Castle, The Hague, where section commander Lieutenant de Groot was called the “count”. The searchlight sets of the section were located in De Haagse Hout, in Leidschendam, in Wateringen, and in Zoetermeer. The section was assigned to the VIIth Searchlight Department against Air Targets with Ribbius as commander. According to J.S., Ribbius was a “great” man with a great social feeling. After his retirement, Ribbius gave J.S. a certificate of excellent service. This certificate helped him throughout his whole professional career.

Conversation with Mr. van R.: Van R. served in the 1st section of the VIIth Searchlight Department in 1939 – 1940. He was quartered in Loosduinen. The operational locations of the four searchlights of the section were Radio Scheveningen, the Madepolder, the Leyweg and Kijkduin, where the recon searchlight and the listening device stood. Van R. was assigned to the searchlight in Kijkduin, which also included the well-known violinist Benny Behr. He also told us that Ribbius did a great deal for his people. After the mobilisation, each department member received a silver commemorative ring from him.

Interview with G.M. van R. and J.B.: Both were assigned to the 2nd section of the VIIth Searchlight Departments with searchlights in Wateringen, Kwintsheul, Loosduinen, and Nootdorp. J.B. was stationed in Wateringen and Van R. in Loosduinen. Van R. was familiar with the three-axle searchlight that was in use in Loosduinen, but he did not know the three-axle listening device.

Interview with A. de J.: Sergeant de J. served in the 4th section, searchlight Nootdorp of the VIIth Searchlight Department in 1939/1940. The recon searchlight of Nootdorp had a three-axle listening device. Its altitude plane was determined by measuring the angle of inclination and the angle of the line that intersected with the ground level. As a soldier at the Koekamp (Malieveld), the J. operated the Electric Listening Device (radar Von Weiler) for some time. He also knows that the equipment was scrapped on the spot during the German invasion. He returned later to the spot to see if there was anything useful to find. This was not the case; the destruction was carried out thoroughly. It was known to De J. that the drawings of the radar ended up in England.
After the capitulation, De J. joined the Chief of the Electronics Department at the Central Workshop (CWP) of the PTT at Binckhorstlaan. In that position, he has built with others of the Physics Laboratory PTT the radio transmitter “Resurgent Netherlands” (due to the German invasion, the “Measurement Building of the Commission for Physical Capabilities” was moved to the PTT and became the Physics Laboratory PTT). The Germans were deluded by building the low-frequency part of the broadcast transmitter as a distribution transmitter and the high-frequency part as a high-frequency melting furnace. His department also contributed to the construction of medical equipment developed by the Physics Laboratory PTT. De J. knew both Van Soest and Gratama very well.

Letter after call in “Opmaat” of LKW, dd. 12-3-2000: “The listening device (Van Soest) was used in Tjimahi for the detection of aircraft. As soon as we had detected the target, the observation was taken over by the central command. Then the searchlight and the anti-aircraft battery were put into operation.”

Letter from N.C.M. Janssen: Janssen wrote about exercises where blindfolded people listened to a buzzer that was moved back and forth and/or up and down. This was necessary to practice the spiraling movement with the listening cones so that the sound of the target is not lost. The circular motion had to be so small that with the horizontal movement the sound in the back of the head moves from left to right or vice versa and that with vertical movement the sound intensity changes slightly in strength. From this impression, the listener had to conclude in which direction the target moved so that he could continue to track it.

Letter from veteran W.F. K., former Sergeant: He showed the museum crew how to operate the three-axle Van Soest listening device and explained its purpose. The two-axle devices, which were in use in the Dutch army, did not have a cylindrical table but worked with a separately added angle measuring table (possibly the roundtable known to us) with a manual viewer. As the flight speeds increased over the years, the two-axle listening devices did not work properly anymore because too much extrapolation had to be made on the roundtable. The extrapolation made the observed target location inaccurate. For that reason, the three-axle searchlight and accompanying three-axle listening device were designed in the thirties. With the three-axle searchlights, the actual lamp house was in a so-called cradle on rolls.

Three-axle searchlight schematic explanation
Three-axle searchlight schematic explanation
Three-axle searchlight
Three-axle searchlight

The searchlight was set according to the altitude plane. That was the plane containing both the plane’s straight line of flight and the location of the searchlight.
When the cradle is rotated around the first axis in the direction of the line of intersection with the ground plane (= field angle), the third axis was adjusted causing the second axis to be perpendicular to the altitude plane (= angle of inclination of altitude surface, angle of position). The three-horned listening device Van Soest delivered the field angle and the angle of inclination of a target with which these two axes of the searchlight were set. At that time, the lamp was ignited and moved around the second axis. This was the so-called swaying with the searchlight in the altitude plane. The target was then met automatically.
According to K., the Sappers department of the Army was divided into three main groups:

  1. Pioneers and torpedoists (anchor symbol),
  2. Signal Corps (lightning bolt symbol), and
  3. Lighting regiment (helmet symbol).
The operational positioning of the listening device (LT), recon searchlight and secondary searchlights
The operational positioning of the listening device (LT), recon searchlight and secondary searchlights

The Regiment Lighting troops consisted of Searchlight Departments against Air Targets. Each Searchlight department was commanded by a Captain. Each of these Departments consisted of four sections each under the direction of a lieutenant. Each section had a recon searchlight – listening device combination and three tracking searchlights. The ideal situation was that the recon searchlight and corresponding listening device were three-axle devices, and the tracking searchlights were two-axle searchlights (map angle and elevation). The following diagram shows the field situation of a section (LT = listening device, verkennend ZL = recon searchlight).

After observing a target with the listening device (LT), the recon searchlight was set and the lamp ignited. Once the air target was in the light beam, the other searchlights of the section were aimed and ignited. The listening devices in K.’s department were two-axle Van Soest listening devices in most cases, but the use of the three-axle listening device Van Soest also occurred. According to K., the three-axle searchlight was also used two-axially when there was no three-axle but only a two-axle listening device. The axis of the lamp housing was then set horizontally.

In the three-axle listening device, the table-box on the first floor of the listening device was rotated around the vertical column in the map angle. A straight line was written on the screen with a pencil on the protractor indicating the angle of inclination of the altitude plane. K. no longer knew how the pencil construction worked. On the photo below the scale for the reading of the field angle of the stand-surface is shown. On the scale on the table cabinet was used to derive the angle of inclination of the altitude plane.

Position scales and reader position on three-axle listening device model Van Soest
Position scales and reader position on three-axle listening device model Van Soest
Position scales and reader position on three-axle listening device model Van Soest
Position scales and reader position on three-axle listening device model Van Soest

According to K., the three-axle searchlight was very modern in 1939 – 1940. It worked with a carbon peak lamp, the setting of which went automatically. A motor drive brought the coal tips closer together to compensate for the burning. The motor drive, however, ran a little too slow. Part of the heat from the charcoal flame was projected onto a bimetal using a small hollow mirror. If the flame became too cold, the bimetal allowed the motor to run faster until the correct temperature was reached again and the bimetal disconnected the speed-up circuit. The coal tips of the two-axle searchlights were controlled by hand.
Note: After the German invasion, all the equipment of the searchlight department Sloten (Amstelveen / Amsterdam) was handed over to the Marine Establishment in Weesp.

Further contacts with veterans via the “Opmaat” periodical

Letter from veteran K.D. S., dated 2-2-1999 and telephone contact on 10-3-1999: In April 1939, I was called as a conscripted sergeant to report to the Kromhout barracks in Utrecht for the so-called pre-mobilisation. We were transported from Utrecht to Rotterdam, where our equipment, namely searchlights, listening devices, dynamo trucks, etc., were stored in a warehouse. We were assigned to a section of the IIIrd Searchlight Department against Air Targets. Its headquarters was housed in the castle of Rhoon and was commanded by Captain Lamberts. Our war destination was Spes Bona at the Enk in Rotterdam (South).

Spes Bona building, Enk, Rotterdam
Spes Bona building, Enk, Rotterdam

S. was the commander of a recon searchlight, a listening device and a viewing device located on a meadow near the Charloische Lagedijk in Rotterdam (South). His listening device was a deviant model with an angle measuring table (the so-called flat table) on which a viewer (binocular) was attached. This probably explains that the three-axle searchlight is in the two-axle position as shown on the picture. After the capitulation of the Netherlands after the German invasion, by order of the occupying forces, all military material was handed in on the site of the former Apollo factory in Schiedam.

Three-axle searchlight in two-axle position
Three-axle searchlight in two-axle position

Letter from veteran J.W.G.H. B.: In 1938, Sergeant B. operated as a listener the three-axle listening device. For exercises, the listening device was moved from the Kromhout barracks to Fort Rhijnauwen. The first practices were with a moving buzzer followed by training with an aeroplane. During the pre-mobilisation in 1939, he did not see the three-axle listening devices anymore.
The enclosed photos taken by him are photographs of three-axle listening devices. The first photo shows a three-axle listening device and a two-axle searchlight ready for transport; the other photo shows the searchlight section with the accompanying equipment in the Kromhout barracks: a three-axle listening device, a three-axle searchlight, and two ordinary two-axle searchlights.

Two-axle searchlight and three-axle listening device in transport mode
Two-axle searchlight and three-axle listening device in transport mode
Two-axle searchlight and three-axle listening device in transport position at the Kromhout barracks in Utrecht (approx. 1939-1940), a three-axle listening device (left) and three-axle searchlights.
Two-axle searchlight and three-axle listening device in transport position at the Kromhout barracks in Utrecht (approx. 1939-1940), a three-axle listening device (left) and three-axle searchlights.

Two-axle searchlight and three-axle listening device in transport position at the Kromhout barracks in Utrecht (approx. 1939-1940), a three-axle listening device (left) and three-axle searchlights.

Below is a picture of the full motorcade of a searchlight section. Behind the first truck we see a three-axle searchlight and behind the third truck a listening device that protrudes above the truck. That will have to be a two-story three-axle.

Searchlight section on transport (sections with horse-towed searchlights existed elsewhere in The Netherlands according to reports)
Searchlight section on transport (sections with horse-towed searchlights existed elsewhere in The Netherlands according to reports)

The next picture is of a three-axle searchlight that is in the two-axle position (adjustable in map angle and elevation).

Tweeassig zoeklicht en drieassig luistertoestel in transportstand
Three-axle searchlight ready for operations


Letter from veteran J.M. P.: In 1938 – 1939, I was 11 – 12 years old. A few kilometres away from the Airport Waalhaven in Rotterdam stood a searchlight and a listening device operated by Sappers on a meadow behind our parental home. The unit was billeted in a school on the Slotboomstraat, Charlois, Rotterdam. We met with the soldiers when we brought some “oliebollen” and chocolate milk to them on New Year’s Eve. The listening device had four horns (probably a Miriaphon) that came together in a square box. The soldier who controlled the listening device had headphones on his head. He turned the device so that he heard the targeted plane most clearly, then he read the altitude and azimuth coordinates. He passed them on to the searchlight with which the target plane was then illuminated. They then passed on the coordinated to the anti-aircraft gun that was placed about a kilometre further away in the polder.

 

Letters from veteran K. v.d. E., dd. 03-06-1999 and 27-7-1999: At the end of 1939, our family (parents and children) lived in Rhoon, a few kilometres south of the former airport “Waalhaven”. I was 9 years old at the time. After the mobilisation, a searchlight was placed on a meadow behind our house by the Sappers from the Waalhaven. The setup included a mobile generator and a listening device. During the day the listening devices were hidden from plain sight in wooden sheds. Each evening, a group of engineers from the Sappers came to set up and test the devices outside. Then they went for sleep on straw bags in a school building, except the guard posts. This went on until May 1940. Occasionally there was an alarm (English planes were already flying to Germany). The listening device had a kind of truncated windmill blades (this suggests that the device was a Miriaphon which was in use by the Sappers). I think that the device contained four sensitive microphones that could be moved in all directions. They turned all four at once. In this way, the sound of aircraft and the direction of the aircraft could be accurately determined. The listening device was helpful in directing the searchlight. A battery of heavy anti-aircraft guns was located close to the airport Waalhaven.

Listening device Miriaphon
Listening device Miriaphon