The History Column: Acoustic Detection of Aircraft
This column describes something that wasn’t actually very successful. It concerns attempts in the 1920s and 1930s to detect aircraft by passive acoustic sensing.
Since WW1 it had been realised that the threat of attack by air was very real. In WW1 the UK had suffered over fifty bombing raids from German Zeppelin airships, and later Gotha bomber aircraft, on London, Great Yarmouth, Ramsgate and others, with a total of 1,392 civilian dead and 3,330 injured, and it was evident that there was no effective means of providing early warning against such raids. The earliest attempts used a network of observers, and acoustic techniques based on large horns or reflectors, but these only worked at all when conditions were optimal. The pictures below show some schemes of this kind.
In the UK, some large concrete mirrors were constructed, and several of these still exist in Kent in the south-east of England. A listener’s ear or a microphone were placed at the focal point to listen for the noise of incoming aircraft.
The problem was that it didn’t work very well. The British radar pioneer Sir Robert Watson-Watt mentions the technique in his autobiography in characteristically flowery language (managing to mix metaphors of Shakespeare and golf in the same paragraph), describing its operation as like
… Juliet’s dilemma of discrimination between nightingale and lark … Everything cooperating, the aircraft would probably be heard 8 miles away, could, infrequently be heard at sixteen miles, and the record for the course was about twenty-four – I do not know that the Big Ear ever detected a wholly unexpected aircraft approaching from the most favourable direction at this latter range. All this was, moreover, conditional on the visitor keeping to the fairway.
Even if it did work, a simple calculation shows that the time taken for the noise from an aircraft flying at 320 km/hr (200 mph), at a range of 32 km (20 miles) would take about 92 seconds to reach the sensor, during which time the aircraft would have travelled about 8.2 km (5.1 miles) – which is a substantial fraction of the target range.
On 10 November 1932 the British Prime Minister Stanley Baldwin in an address to the British Parliament said:
… I think it is well also for the man in the street to realise that there is no power on earth that can protect him from being bombed. Whatever people may tell him, the bomber will always get through.
The significance of this is that it showed very clearly that a better approach to air defence was needed, which paved the way to the development of radar.
Further information may be found in:
Watson-Watt, R., Three Steps to Victory, Odham’s Press, 1957.
Authored by Hugh Griffiths, University College London