Common steps and sub-activities

Hearing large scale subwoofer speakers

A subwoofer is a loudspeaker capable of reproducing low pitched audio frequencies.

The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, but as we shall see, there are many notable exceptions which produce sound in the infrasound range.  They were originally designed to augment woofers that only covered audible and higher frequency ranges, but have taken on a life of their own, because the very deep bass notes they produce are loved by concert goers and home audio system listeners alike -  people love the ‘effects’ they produce! 

Quite!

The first subwoofers were developed in the 1960s to add bass response to home stereo systems. Subwoofers came into greater popular consciousness in the 1970s with the introduction of surround sound and sensurround in movies such as Earthquake, which produced loud low-frequency sounds through large subwoofers. When the compact disk appeared in the 1980s, their popularity grew, as music producers could add more low frequency content to recordings. Subwoofers were added to home cinema systems to match that being provided in cinemas.

They are now found in clubs, concert venues, theme parks, cinemas, PA systems, even in the stereo systems of custom car audio installations, which is a rather alarming thought, considering what they are capable of.

And gentle reader, the churches use them.

Subwoofers in films

There is quite a lot of apocryphal talk about the use of infrasound in films/movies. It seems to be believed that a number of studios have used infrasound to instill a sense of suspense or even fear in the audience.  There is a common belief, for example, that Alfred Hitchcock and Brian "Lustmord" Williams used it extensively.  The most recent addition to this list is the film ‘Paranormal Activity’. The only film to date that is known for a fact to have employed infrasound is the French film ‘Irréversible’. Some audience members found the film so disturbing that they had to exit the theatre.

Intensity

Subwoofers are not designed to be used at low intensity.  Most are designed to set your teeth rattling [fairly literally].  The number of subwoofers used in a concert, for example,  depends on a number of factors, including the size of the venue, whether it is indoors or outdoors, the amount of low-frequency content in the band's sound, the desired volume of the concert, and the design and construction of the enclosures,  but we all know that in general there are a large number. The 2009–2010 U2 360° Tour used 24 Clair Brothers BT-218 subwoofers  around the perimeter of the central circular stage, and 72 proprietary Clair Brothers cardioid S4 subwoofers placed underneath the ring-shaped "B" stage which encircles the central main stage.  A large dance club in contrast may have a row of four or five twin 18-inch subwoofer cabinets, or more.

Some example systems

The Abyss subwoofer can reproduce pitches from 18 Hz (which is about the pitch of the lowest rumbling notes on a huge pipe organ with 32-foot (9.8 m) bass pipes) to 120 Hz (±3 dB). But as we will see there is a lot of choice…...

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The Deepest. This subwoofer goes down to 16 Hz (anechoic response), creating sound so real you not only hear it - you actually feel it. Sure, claims of 16 Hz are made every day by manufacturers, but you'll only hear it if the sub is in a tiny closet-like room like the one they're measuring in. The EP600 is the first subwoofer to ever achieve it anechoically, in a room with no boundaries. You'll actually hear it all - guaranteed - and in most rooms, you'll even experience 15 Hz!

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The Matterhorn is a subwoofer model completed in March 2007 by Danley Sound Labs in Gainesville Georgia after a U.S. military request for a loudspeaker that could project infrasonic waves over a distance.

The Matterhorn was designed to reproduce a continuous sine wave from 15 to 20 Hz, and generate 94 dB at a distance of 250 meters (820 ft), and more than 140 dB for music playback measured at the horn mouth. It can generate a constant 15 Hz sine wave tone at 140 dB for 24 hours a day, seven days a week with extremely low harmonic distortion.

….Of the constant tone output capability, designer Tom Danley wrote that the "target 94 dB at 250 meters is not the essentially fictional 'burst' or 'peak SPL' nonsense in pro sound, or like the 'death burp' signal used in car sound contests."At the annual National Systems Contractors Association (NSCA) convention in March 2007, the Matterhorn was barred from making any loud demonstrations of its power because of concerns about damaging the building of the Orange County Convention Center.  Instead, using only a single 20 amp electrical circuit for safety, visitors were allowed to step inside the horn of the subwoofer for an "acoustic massage" as the fractionally powered Matterhorn reproduced low level 10–15 Hz waves.

… claimed to be the world's biggest subwoofer, this custom installation was manufactured in Italy by Royal Device and made of primarily of bricks, concrete and sound-deadening material consisting of two subwoofers embedded in the foundation of a listening room. The horn-loaded subwoofers each have a floor mouth that is 2.2 square meters (24 sq ft), and a horn length that is 9.5 meters (31 ft), in a cavity 1 meter (3 ft 3 in) under the floor of the listening room. Each subwoofer is driven by eight 18-inch subwoofer drivers with 100 millimeters (3.9 in) voice coils. The designers assert that the floor mouths of the horns are additionally loaded acoustically by a vertical wooden horn expansion and the room's ceiling to create a 10 Hz "full power" wave at the listening position.

A single 60-inch (1,500 mm) diameter subwoofer driver was designed by Richard Clark and David Navone with the help of Dr. Eugene Patronis of the Georgia Institute of Technology. The driver was intended to break sound pressure level records when mounted in a road vehicle, calculated to be able to achieve more than 180 dBSPL. It was built in 1997, driven by DC motors connected to a rotary crankshaft somewhat like in a piston engine. The cone diameter was 54 inches (1,400 mm) and was held in place with a 3-inch (76 mm) surround. With a 6-inch (150 mm) peak-to-peak stroke, it created a one-way air displacement of 6,871 cubic inches (112,600 cm3). It was capable of generating 5–20 Hz sine waves at various DC motor speeds—not as a response to audio signal—it could not play music. The driver was mounted in a stepvan owned by Tim Maynor but was too powerful for the amount of applied reinforcement and damaged the vehicle.The Concept Design 60-inch has not been shown in public after 1998.

A warning

Acoustic resonance is vibration induced by sound. This is high intensity sound so works in a different way to many of the low intensity mechanisms. It has the potential to make you ill.