Intl. Notebook | Apr 19 2016 |
This curious photo shows a bit of pulp-era technology—the acoustic mirror or acoustic locator. It was used to detect the approach of aircraft. The examples above and below are from Japan, Finland, Russia, Sweden, and other countries, and date from the 1930s to the eve of World War II, when they were replaced by radar systems. Apparently, these worked quite well, picking up engine noise from miles away. But as aircraft became speedier the effective range of the devices decreased—enemy planes would reach the site where a mirror was located within a minute or two of being detected. At such speeds, a spotter with a good pair of binoculars and decent visibility would see the planes the same time a mirror heard them. But before airspeeds increased these were the surest way to detect an oncoming aerial attack or reconnaissance flight, particularly at night. In addition to portable mirrors, some countries used cast concrete to construct massive versions that had a twenty-mile range. Great Britain built the last set of these in 1943 when fears surfaced that the Germans had developed a means to jam radar. Built to endure weather and time, some survive and have become tourist attractions. We have ten more crazy acoustic mirror designs below for your enjoyment.