We have successfully produced infrasound, as a duplicate of that produced by Industrial Wind Turbines. We have been able to produce this Infrasound inside a research chamber, capable of accommodating a human test subject. It is our vision that this project will permit others, with appropriate medical training and ethical oversight, to research human thresholds and the effects of this infrasound on humans. Our role has focused on producing the tools, systems, and hardware required, to permit this research to go forward. This paper describes the evolution of our project from the original vision, through the construction of proof of concept prototypes, a series of improved models and their associated accessories /operating systems, to the final test chamber as it stands now ready to deploy. Also included are the mathematical and computational data supporting our claim that infrasound conditions inside the chamber can be made to duplicate those from actual Industrial wind turbines at approved setback distances.
Infrasound refers to low frequency sound (pressure changes in air), that are below the range of human hearing. Human hearing is typically defined as from 20Hz to 20,000 Hz (Hertz, also known as cycles per second).
Infrasound waves are the same variations of air pressure relative to atmospheric but occur at much lower frequency levels (from 0.1hz to 20 Hz) and are below the audible level.
Audible sound is in the range of 20 Hz to 20,000 Hz. Hz refers to the number of cycles per second Decibel is the unit of measurement for sound. Decibels are a logarithmic scale. Adding decibels has the effect of multiplication of pressure levels (Pascals). For example:
Adding 20 db corresponds to multiplication of pressure level (Pa) by 10 Adding 40 db corresponds to multiplication of pressure level (Pa) by 100 Adding 60 db corresponds to multiplication of pressure level (Pa) by 1000
Traditional methods have involved either a sealed chamber or a modulated input.
The simplest method to generate infra sound is to use a sealed chamber. Infra sound is generated by sealed speaker cabinets(s), placed inside the chamber. As the speaker cone moves outward, the total occupied volume (speaker enclosure + diaphragm) increases slightly.
As the speaker cone moves inward, the total volume decreases. Since the volume of the chamber is fixed, the chamber pressure will decrease inversely with the increasing speaker volume. We note that for a large chamber volume, multiple speakers would be required, since each speaker has only limited displacement (typically 5 to 10mm for commercially available sub woofers). Furthermore, this type of chamber requires a perfect seal, which is difficult to achieve. Finally the sealed chamber must have no ventilation, so human experiments must be limited and carefully monitored.
A device of this type for Wind turbine simulation was described by Walker and Celano (2015) A related idea is for the speakers to occupy a part of the chamber surface, such as the wall or ceiling. A similar principle applies, as the movement of speakers increases or decreases the volume of the chamber. This type of device was used by Nussbaum and Reinis (1985) and most recently by Cooper et al (2017).
Finally, a variant of this method was used to provide infra sound stimulus directly to the ear, via a tube and a sealed ear piece, while performing simultaneous fMRI brain scanning. (Weichenberger et al, 2017). Note that in this case, only the ear was stimulated, so vestibular or whole body effects cannot be evaluated.
A second approach to generating infra sound in an area is to blow air into (or suck air out of) a chamber, room, or other enclosure. As air blows into the chamber, the pressure increases and sucking air out reduces the pressure. To generate infra sound air is repeatedly pumped in and out of the chamber (or room) at the desired frequency. Since these devices pump air, the listening chamber need not be completely sealed. As long as the air flow of the fan is sufficiently strong, it can overcome the leaks in the room/chamber, and very high levels of infra sound can be generated. This allows use in a wide variety of spaces, including listening rooms, performance spaces, etc.
In this technology, a “rotary woofer” or “woofer fan”, uses a fan with a variable blade pitch (Park et al, 2009).
In the forward position the blades generate positive flow, while in the backward position they generate negative flow. A neutral position generates no flow.
The blades are moved by a “voice coil”, an electromagnetic system, just like a regular loud speaker. To generate low frequency sound the fans are tilted in and out at the desired frequency. The woofer fan is placed in a sound isolated chamber, outside the listening room, or may be incorporated into a ventilation system. Most recently, Kevin Dooley Inc. have produced a miniature version of this device, using a very high speed fan (10000 rpm) in a small (approx. 6" diameter) enclosure (Dooley, 2015).
In October 2015 funds were approved by University of Waterloo Office of Research for the purchase of a woofer fan type device from Kevin Dooley Inc.
When we were unable to complete this purchase, we considered construction of our own woofer fan, but quickly decided against this, due to the complexity of construction.
We then began working on a new approach.
Our system uses a separate constant air source and modulates the output.
We use a fan with a fixed air flow that pressurizes a chamber to some (small amount) above atmosphere.
The inside chamber pressure is modulated by an exit valve responding to specific time and frequency commands as further outlined.
This paper describes the technical aspects of reproducing infrasound in a chamber, the associated methods and equipment needed, and the evolution from a proof of concept prototype to a full size chamber capable of accommodating a human test subject.
We determined that actual human testing and data collection would be left to
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