06 Aug 2018

TRW-17 Subwoofer

This isn't a desk fan, it's an infrasonic transducer used to simulate earthquakes, explosions and rocket launches

SPECIFICATIONS

  • Amplifier Requirement: 150 watts @ 8 ohms
  • Impedance: 8 ohms 0Hz : 0Hz
  • Frequency Response: 1Hz : 30Hz ± 4dB
  • Suggested Crossover: 20Hz @ 18dB/octave
  • Sensitivity 94dB 1 watt 1 meter @10Hz
  • Maximum Acoustic Output: >115dB between 1 and 20Hz.
  • Distortion: Typically 3% or less between 1 and 20Hz @90dB
  • Warranty : 3 years parts and labor

image courtesy www.eminent-tech.com

That’s a subwoofer?

I have nobody to blame but myself, but it has been a while since I’ve seen something in the audio world that gave me the reaction I had when I came across this little guy. The TRW-17, or Thigpen Rotary Woofer is a subwoofer that claims to be able to produce sound pressure levels of 120dB at 1 Hz. Not a typo: this speaker is supposed to be able to shake your house at one revolution per second. I should clarify that this doesn’t mean that the fan blades would be spinning once per second to do this. In fact, the way this device works is a little stranger.

How does it work?


An acoustic wave should be thought of as the transmission of energy by a pressure wave. A traditional diaphram-based speaker produces this pressure wave by traveling back and forth in its surround. As it pushes out of its enclosure, the air immediately in front of the cone is compressed. As the cone is pulled into the volume of the enclosure, the air rarefacts. The pressure oscillation is centered on the ambient air pressure of the room; while at rest, the cone is not exerting any force on the air surrounding it.

This works exceedingly well for the majority of the frequency range of human hearing. In order to produce lower frequencies at consistent amplitudes, larger drivers are used. In compromise, higher frequencies are split off and sent to drivers with smaller, more rigid cones. This is the 2-way or 3-way speaker design we all know and love. The problem with pushing air around in this way is that the lower range of your possible reproduction spectrum is limited by the geometry of your subwoofer cone. The surface area and excursion of the driver together determine how far down you can go.

This “rotary subwoofer” works by establishing positive pressure in the listening environment and then modulating that pressure. You are essentially “inflating” your room, and rapidly (well, not really, I can clap faster than 1 Hz) oscillating how fast you are inflating the room. The device outputs a wave with a DC offset, the equivalent of a traditional speaker cone constantly moving toward you and vibrating as it does so.

The manufacturer of this device compares the physics involved to the phenomenon of holding your hand out the window of a moving vs. unmoving car. Once the car is going fast enough, you can strongly influence the magnitude and direction of the force acting on your hand by changing your hand’s shape very slightly, whereas trying the same thing while the car is stopped does nothing.

What is it used for?

I got in touch with the creator, Bruce Thigpen. He says that most TRW-17 units are sold to an even split between home theater enthusiasts and researchers, the latter of which includes Boeing and Oak Ridge National Laboratory. He also mentions the device has found a niche market in theme parks. Applications I was able to find on the internet include simulations of orbital rocket launches, earthquakes, the reproduction of the sound of a helicoptor, and as part of the sound system of a church.

This paper (.pdf), published by the Infrasound Laboratory at the University of Hawaii at Manoa shows the results of an experiment in which a TRW-17 was mounted in a 17 foot U-Haul truck and recorded by an infrasonic array 3.8 kilometers away, acheiving a signal-to-noise ratio of up to 15 dB!

Where can I hear one?

Unfortunately, the majority of these subwoofers are in private residences or research facilities, but here are a few places you can find the TRW-17:

  • The Polynesian Cultural Center in Hawaii
  • The Evergreen Aviation Museum [Titan II Missile exhibit] in Oregon
  • The Middle Collegiate Church in Manhattan

rotary sub render

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