Acoustical Anachronism

Jello happy reader,

A recent posting in Gizmodo, carrying an inane headline, incited me to post. Now, I don’t expect insightful journalism either here at The Bitstream or from Gizmodo but, the piece was about a practical application of psychoacoustics and basic physics, so I decided to dive into this subject a bit as the fundamental tech involved is more widespread than one may think…

First off, several readers provided substantive comments, particularly “fugspit” and “Haughey6,” who mentioned the fixed portion of Britain’s acoustical early warning grid erected in the 1920’s as the military got hip to the use of aviation in wartime.


Listening Ear at Denge, UK

“Listening Ear” at Denge, UK


As local historian Richard Scarth explained in a 2004 BBC interview discussing the Dungeness listening ears seen above, “…they’re actually acoustic mirrors, made to catch the sound of approaching aircraft, and were a primitive early warning system between the wars but quickly rendered obsolete by radar. Trained listeners huddled in bunkers at the mirrors’ feet, using a kind of stethoscope to pin-point distant noises from the sky.” The Brits also had mobile versions which were brought into play early in WWII when a fixed installation got bombed.

In the Gizmodo article, the author states that “Sound ranging-which uses sound as a way to locate artillery pieces-started during World War I and continued into World War II, even with the advent of the radar.” Turns out that the tactical use of acoustical ranging is alive and well today, particularly in urban settings. Law enforcement and the military use modern, automatic versions to quickly locate small arms fire, such as gang shootouts, domestic homicides and snipers, so subsequent action can be taken. Triangulation is the key…

QinetiQ NARadiance TechnologiesAAI TextronM2 Technologies and ShotSpotter are all vendors of this technology. The venerable tech consultancy BBN, widely known for their acoustical expertise, has developed Boomerang, a mobile version for use by the military. The detector array for BBN Tech’s Boomerang mobile shooter detection system is shown below.


detector array for BBN Tech’s Boomerang mobile shooter detection system


Talking about mobile military, iRobot, manufacturers of the Roomba line of housebots, have incorporated BioMimetic Systems‘ ADF (Acoustic Direction Finding) technology into their PackBot line for soldiers in the field.


iRobot’s PackBot 500 with with RedOwl Sniper Detection Kit

iRobot’s PackBot 500 with with RedOwl Sniper Detection Kit


So, what do these modern systems and the gargantuan ear trumpets in the Gizmodo article have in common? An array of detectors, separated in space so as to accentuate arrival times.

Nowadays, phased arrays are more often used, at centimeter wavelengths, as electronically steerable radar transducers. The RADAR array below is a giant version of the radome’s seen on boats, planes and at airports, except it uses a fixed array of transducers to steer a synthesized beam.


A PAVE PAWS installation in Alaska

A PAVE PAWS installation in Alaska


Speaking of which, a common computer touchscreen display technology uses a high frequency version of that same acoustic triangulation concept to determine X–Y finger placement.


Tyco’s Elo IntelliTouch SAW touchscreen overlay

Tyco’s Elo IntelliTouch SAW touchscreen overlay


When transducers are used to strictly to transmit at audio frequencies rather than receive, you may end up with the phased array “soundbars” that are becoming increasingly common in home theater installations in place of discreet LCR speakers. These products synthesize a surround sound field from what visually appears to be a simple line source.

To bring it all back to the start of this posting, may I remind you all of Michael Gerzon’s Ambisonics technology, where a phased array captures multiple diverse amplitudes and arrival times at one coincident point in space. When processed and played back correctly, our brain “decodes” that auditory information into a highly credible sound field. As with the acoustical arrays of old, our human auditory system ain’t too shabby when it comes to processing and localizing incoming information.


Nick Mariette’s DIY Soundfield array

Nick Mariette’s DIY Soundfield array captures multi–phase & amplitude information


Well, that’s all for now. If you look carefully, you’ll see that technology is a slow march of refinement, punctuated by random bursts of discovery, kinda’ like evolution. Happy birthday, John Jonah and Chas. Darwin and, everyone continue to geek!

One Response to “Acoustical Anachronism”

  1. BTW, BBN mentioned above also developed the first (network) router, dubbed the IMP or Interface Message Processor, for linking together end points on the ARPAnet, the world’s first practical packet switching network and predecessor of today’s Internet. The IMP supported a blazing 50 kbps data rate between nodes!

    For more info, visit: