Here is a mathematical breakdown of the longest recorded reverb every recorded. A gunshot was recorded inside a fuel storage tank by Professor Cox in Scotland to shatter the record. The reverb lasted for 75 seconds.
The fuel storage facility shown to the right, and a mini photo tour are available at this BBC article.
‘I wanted to shout with astonishment, but I had to remain silent so as not to ruin the recording.’
–From Daily Post Interview
Here is a booming and thundersous sample of the sound that could become your cellphone text message ring. Hit play and continue reading.
“World Record for Longest Man-made Echo Shattered in Scotland”
Reads the headline from the Huffington Post. Our first modeling attempts considered the unique space as a box for sound propagation. Sound in air travels approximately 343 m/s. For a sound to exist for 75 seconds, means the sound bounced of the walls to travel 25km before it finally tired itself out. A gunshot has a source level of 140 dB. We needed a way to dissipate that energy through the walls and the air in order for the sound to “go away.” So after considering some approaches for ground absorption, air absorption, cylindrical, spherical… we suddenly realized…
Many media outlets led with the wrong headline. In his new book “Sonic Wonders”, he explains why the Guinness World Record incorrectly uses echo instead of reverb. The recording of 75 seconds shattered the old reverb record which was a wimpy 17 seconds. One point of clarification, sound could be heard for 112 seconds after the gunshot, but the broadband reverb time was 75 seconds. This blog post also explains the scientific method he used to set up and calibrate his microphones.
Putting the Classic Reverb Equation to the Test
Moving away from propagation… Will the classic equation for calculating reverb time in a room hold up?
(Not with broken online calculators)
RT60 = k (V/Sa)
RT60 represents the time it takes for the sound to go down 60 dB.
K=1.61 A constant to be used when working in meters.
V = Volume
Sa = Surface Area x Absorption Coefficient
Using the dimensions of the box
Len= 160m – “The length of two soccer pitches.”
Absorption = 0.01 For poured concrete.
Applying those values to the equation, and sure enough,
the reverb time is 75 seconds.
Bringing Home the Reverb
After the gunshot into the space, Professor Cox also recorded an impulse, which can be loaded into digital reverbs to recreate the effect. Here is a page to download the plug-in for the Ableton music making program.
Please visit the blog of Professor Cox its filled with great experiments. I just ordered his book, so my sincere apologies if any of this content is duplicated in his work. And there is an appointment in my calendar to go kayak under Echo Bridge this spring.