Generally, Alberich coatings are not well covered in sources. One of the main topic encountered are the problems of attachment to the hull, due to poor glues. These issues are not limited to the Uboat arm, and even Virginia-class nuclear submarines are known to have shed their tiles during deployment. Further, the effect in reduction of source strength is greatly under-appreciated. Most literature assume a 10%-15% reduction of effect but actual sources may indicate a different picture:
Experimental resonance trials have empirically demonstrated the potential to reduce sound pressure and reflected sound strengths. In a simple experimental setup, a wire suspended in water was used to create gas bubbles and the water tank and was illuminated through active sound strength with the resultant throughput measured accordingly, Results showed that complete annhiliation of sound force was possible, if the gas bubbles were distributed according to x=lambda/4. These experiments were conducted at the Acoustic department of the Technical University of Charlottenburg / Berlin in 1938 (W. Kuhl).
As the Eigenfrequency of the holes was 50 KHz, the small holes created no problems of their own because it was known that allied active Sonar gears used the frequency range of 9-17 KHz. Consequently, the requirement was formulated to create a surface coating for submarines, which reduces -as best as possible- sonar strength through a resonance body. The results of the chosen formula with air bubbles of two separate sizes inlaid in an artificial rubber matrix with two layers is shown here (again, from Meyer-Oberst 1952):
The target strength as measured by "REFLECTED SOUND STRENGTH" was greatly reduced as a consequence of this coating:
Prof. Meyer´s results lead to the development of anechoic coating for Uboats under the term "Alberich". Considerable effort was required by IG Farben to develop a produceable glue and rubber, which satisfied the operational requirements and reduced the source strength in the intended frequency range of 9-17 KHz. The average reflected sound strength was just below 10% the db of an untreated surface.
U67 was chosen to be tested. Therefore the Type IX Uboat was docked in Wilhelmshafen in mai and june 1941 and received full surface coating with these two-layer rubber mats. During the subsequent ride from Wilhelmshafen to Kiel, several of the rubber tiles fell off and approx. 25% of the surface area had to be fixed and re-glued. Subsequent sonar trials with Strahl off Apenrode and off Norway in a deep-water Lo Fjord were ambigous. It was difficult to exactly quantify the effect due to vastly varying sound conditions in these trials. However, it was determined that both, the self noise level of the submarine and the source strength under S-Gerät illumination was lower but the effect was larger for the latter. Yet, because loose tiles could generate recognizable hydrophone-effects, the Alberich coating of U-67 was removed in August, before the boat went to operational cruise.
Instead UD-4, ex dutch O-26, received Alberich coating in 1941 and was ready for trials by dec. 1941.
As with U-67, UD-4 underwent problems with loosening of tiles. The effect was traced down to surface cruise of these Uboats, so that wave-interaction caused an fatigue effect on the rubber tiles. An improved glue was devloped by IG-Farben in 1942 and U-470 (type VIIc) was covered by Alberich coating subsequently.
Apparently, the boat yielded much better results in adhesion of tiles. The coating was tested over three days by the UAS off Norway, which showed a reduction of source strength from 100% reflected sound strength of an uncoated boat illuminated by S-Mob device to just 17% reflected sound strength of the Alberich coated U-470 (Rössler 2006:133).
As an indication for an uncoated type VIIc, compare HMS GRAPH (ex U-570)´s proxy (optical) and actual acoustic measurements, which had a peak beam-on target strength of 25 db (NAVMAT P-9675):
These results of exceedingly dampened target strength of Alberich coated submarines (by GRAPHs example above, 17% of 25db max =4.25 db) triggered the coating of several more U-boats.
(to be continued...)
Experimental resonance trials have empirically demonstrated the potential to reduce sound pressure and reflected sound strengths. In a simple experimental setup, a wire suspended in water was used to create gas bubbles and the water tank and was illuminated through active sound strength with the resultant throughput measured accordingly, Results showed that complete annhiliation of sound force was possible, if the gas bubbles were distributed according to x=lambda/4. These experiments were conducted at the Acoustic department of the Technical University of Charlottenburg / Berlin in 1938 (W. Kuhl).
-Meyer-Oberst 1952, translated with DeepL"The use of gas bubbles with uniform size and resonance frequency matching that of 15 kHz would be expected to result in highly effective sound energy destruction. However, bubbles tuned to 15 kHz in water have a diameter of approximately 0.4 mm, which is relatively large. The experiments conducted in the low tank yielded the following expectation:It is possible to manufacture sound-dampening materials in water for a frequency range of approximately 13 kHz in a way that involves creating an "injected" layer of fixed gas bubbles with a macroscopic diameter in the distance X/4 in front of a sound-absorbing boundary. The required density per unit area is very low. This approach provided a potential solution for the resonator. However, the technically unsuitable gas bubbles needed to be replaced with alternative, more suitable, oscillating structures with appropriate eigenfrequencies and losses. This led to the concept of using hollow spaces in compressible materials, which could be regarded as "frozen" gas bubbles.
Additionally, the X/4 water layer between the holes and the sound-absorbing cover presented a technical challenge. The concept of replacing the X/4 resonator with a closed parallel resonator with separate mass and suspension, which exhibits similar resonance characteristics to the open X/4 circuit, enabled the inclusion of the hull bulkhead as a mass within the oscillating system."
As the Eigenfrequency of the holes was 50 KHz, the small holes created no problems of their own because it was known that allied active Sonar gears used the frequency range of 9-17 KHz. Consequently, the requirement was formulated to create a surface coating for submarines, which reduces -as best as possible- sonar strength through a resonance body. The results of the chosen formula with air bubbles of two separate sizes inlaid in an artificial rubber matrix with two layers is shown here (again, from Meyer-Oberst 1952):
The target strength as measured by "REFLECTED SOUND STRENGTH" was greatly reduced as a consequence of this coating:
-Meyer-Oberst 1952, translated with DeepL"In the frequency range of 5 to 16 kHz, the reflection coefficient is plotted against the frequency. The temperature and pressure are 10°C and 15 atm, respectively. Figure 7 presents the curves for 3 to 18 mm steel bulkhead terminal boundary against air (...). It is evident that it is feasible to reduce the reflection coefficient over a broad frequency range, approximately an octave, to exceedingly low values for all encountered wall strengths. On average, the reflection strength in this frequency range was 5%. In some cases, it is even below this value, indicating that more than 99.75% of the incident sound energy is not reflected."
Prof. Meyer´s results lead to the development of anechoic coating for Uboats under the term "Alberich". Considerable effort was required by IG Farben to develop a produceable glue and rubber, which satisfied the operational requirements and reduced the source strength in the intended frequency range of 9-17 KHz. The average reflected sound strength was just below 10% the db of an untreated surface.
U67 was chosen to be tested. Therefore the Type IX Uboat was docked in Wilhelmshafen in mai and june 1941 and received full surface coating with these two-layer rubber mats. During the subsequent ride from Wilhelmshafen to Kiel, several of the rubber tiles fell off and approx. 25% of the surface area had to be fixed and re-glued. Subsequent sonar trials with Strahl off Apenrode and off Norway in a deep-water Lo Fjord were ambigous. It was difficult to exactly quantify the effect due to vastly varying sound conditions in these trials. However, it was determined that both, the self noise level of the submarine and the source strength under S-Gerät illumination was lower but the effect was larger for the latter. Yet, because loose tiles could generate recognizable hydrophone-effects, the Alberich coating of U-67 was removed in August, before the boat went to operational cruise.
Instead UD-4, ex dutch O-26, received Alberich coating in 1941 and was ready for trials by dec. 1941.
As with U-67, UD-4 underwent problems with loosening of tiles. The effect was traced down to surface cruise of these Uboats, so that wave-interaction caused an fatigue effect on the rubber tiles. An improved glue was devloped by IG-Farben in 1942 and U-470 (type VIIc) was covered by Alberich coating subsequently.
Apparently, the boat yielded much better results in adhesion of tiles. The coating was tested over three days by the UAS off Norway, which showed a reduction of source strength from 100% reflected sound strength of an uncoated boat illuminated by S-Mob device to just 17% reflected sound strength of the Alberich coated U-470 (Rössler 2006:133).
As an indication for an uncoated type VIIc, compare HMS GRAPH (ex U-570)´s proxy (optical) and actual acoustic measurements, which had a peak beam-on target strength of 25 db (NAVMAT P-9675):
These results of exceedingly dampened target strength of Alberich coated submarines (by GRAPHs example above, 17% of 25db max =4.25 db) triggered the coating of several more U-boats.
(to be continued...)
statistics: Posted by delcyros — 10:30 AM - Today — Replies 0 — Views 41