We’re always keen to offer advice and expertise on fan selection based on requirements and application so when Axair were approached to find a solution for a unique air movement project we were only too pleased to assist.
The call, from Chris Woolf, technical consultant at Rycote Microphone Windshields was typical of the type of enquiry that Axair Fans is adept at turning from a very basic outline into a detailed proposal. His initial enquiry was for “…a fan to give a steady velocity of 10m/s at a distance of 1 metre from a circular orifice, 300mm in diameter..” Chris was the first ever “Fellow of the Institute of Professional Sound” and in 2000 was the joint recipient of a Technical OSCAR for the development of the Modular Microphone Mounting system.
Further research determined that the fan was required for testing broadcast quality microphones. In the showbiz and broadcasting world, technology and electrical equipment is of paramount importance when it comes to performance and experience. When recording, the only desired sound source is the target speaker or activity so cutting back on environmental wind noises and preventing noise caused by handling and vibration was the control to be created. In terms of air movement the centrifugal force of wind currents can play havoc and cause some digital equipment a severe drop out of focus on the target sound source.
Given our extensive knowledge, we were approached with a working figure of 10 m/s at 1 metre and asked to specify the correct fan for the application and test involved. Chris needed a powerful, yet very quiet device to produce anything from a light breeze to, ideally, a full gale with as little mechanical noise as possible. Our Technical Manager, John Holden determined that we needed to achieve a discharge velocity of around 16m/s and could now calculate the air volume required (4072m³/h) and the Dynamic pressure at the discharge (154 Pascals). Experience suggested we required a further 100-150pa static pressure for inlet, internal ducting and sound attenuation giving a Total Pressure drop of around 300pa.
John selected the Rosenberg DRAE279-4 double inlet, forward curved centrifugal fan with a Sentera variable speed control, this fan has the benefit of relatively low inlet velocity, a “tight” scroll profile and is driven by a symmetrical, external rotor motor all of which contribute to low noise operation and a fan that delivers a clean, laminar airflow. The external rotor motor is ideal for simple but accurate and repeatable voltage speed control allowing Axair to specify our RE10-A electronic speed controller.
Chris then set about designing a housing, which could well be mistaken for a grandfather Dalek. At just shy of 2m high, more than 1m deep and nearly as wide, it was quite a monster. Having a difficulty finding anyone with previous experience of having built a wind machine, a carpenter friend, Alex Steele, was given several sheets of high grade plywood and told to take his courage in one hand, his ingenuity in the other, and a plunge-saw in a third.
The fan was housed in the base, surrounded by acoustic foam, and draws air in (quietly) from the rear. It blows out via a convoluted and baffled pathway, internally covered in sound absorbing foam, to exit at the 30 x 30cm front port. An optional magnetically attached restrictor can funnel the air through a 20 x 20cm port to give yet higher air speeds in a narrower jet.
The results are excellent. With the wide air stream anything up to a Moderate Breeze, Beaufort Force 5 (11m/s) is possible. With the narrower port in place it is possible to ramp up to a full Force 8 Gale. At normal testing speeds the noise of the machine is >48dBA, and even at maximum blast it is barely 10dB higher.
Chris quotes “With no pattern to follow a project of this sort can be difficult (and expensive) to bring to a satisfactory completion. Finding a company that is willing to take a real interest, and has good practical knowledge is a godsend.”
The wind machine (it’s called Alex after its maker) is already providing useful information for Rycote’s products, and will feature in a paper to be delivered at the Audio Engineering Society in Paris, June 2016. But it isn’t expected that the dalek will appear in person…
Well, it was a pleasure to help, we all look forward to even “quieter” microphones in the future; with help from Axair Fans.