Fan noise is increasingly seen as a pollutant, so access to sophisticated acoustics measuring equipment, to people with training and extensive knowledge of acoustics and to an anechoic chamber is essential if sound continues to be a defining factor for designers.
A key challenge for a fan manufacturer hoping to analyse any noise element of a fan, is that quiet is a relative, subjective term. It is particularly important for anyone making products where a primary selling point is low noise.

The complications of decibels and the usual confusion about volume, power and logarithmic scales makes it tricky for typical customers to know by the numbers how it translates to the user experience. While loud noises are generally not that difficult to measure, as noise drops down towards inaudibility, the measurement challenges become great. Traditionally semi-anechoic chambers are used to perform precision and engineering-grade testing on a variety of devices ranging from handheld units to large axial fans. Typical measurements include product noise testing, sound source frequency response, and sound source directivity.

Sound absorption

Usually semi-anechoic chambers have a high performance wall panel system that provides the low-noise environment required to test low-noise products. A precision-grade free field environment is a product of the chamber's sound absorption system that contains foam or melamine wedges.

With fans, despite the existence of set standards for measuring noise and airflow, many manufacturers use microphone distances of 1m, 3ft or some other distance in free space (ie without any impedance on the fan load) to take a single point sound pressure level measurement. Others measure the same at 30cm or 50cm. Only a few take the time to do complete sound power measurements with the fan under some kind of load that at least approaches real life applications.

This means the manufacturers' specifications are often not directly comparable. It takes a trained engineer to calculate specifications obtained from contrasting techniques into comparable numbers. Even for them, it might be easier and certainly more accurate to take prospective fan samples and re-measure with one consistent technique.

Another tool in the armoury of the acoustic engineer is a reverberation chamber. These reflect sounds to produce a non-directional or diffuse sound field within the chamber. Unlike the sound pressure level produced by a device, the sound power level is a property of the device and is independent of the test environment. Reverberation chambers are also used to measure the sound absorption characteristics of materials or other items such as soft panels, screens or pieces of furniture (eg theatre seats, chairs, and sofas).

Today though, Computational Fluid Dynamics is increasingly used as an acoustic solver helping to predict noise emission. Fläkt Woods has worked on reducing noise levels from fans for many years. At the end of the '90s it had the CFD technology needed to discover which part of the fans produced the most noise.

As a rule, energy generated by the fan rotation is very much dependent on the diameter and the rotational speed of the fan, typically proportional to the fifth or sixth power of the linear velocity at the tip of the fan blades.

Generally, a large diameter fan at low rotational speed produces low frequency and comparatively low noise levels. The same volume flow rate can be achieved by increasing the rotational speed and reducing the diameter. In this case the frequency and the level of the noise is higher but the higher frequency noise can be more effectively attenuated by lining the duct with sound absorbing materials.

Fläkt Woods recently used computer simulations and 3D numerical modelling to derive the velocity and wake thickness on a series of tests for a range of large diameter fans. Because the circumferential distribution of axial velocity, the relative velocity can be processed using CFD at different span-wise locations from blade hub to tip, the outlet relative velocity contour can be calculated and the model can then be fed for a more accurate prediction of noise level.

Preliminary test

To differentiate the aerodynamic noise from the motor noise, a preliminary test of the motor was conducted to identify its spectral signature, so that an appropriate correction could be made in the noise measurements. An inlet bellmouth of aerodynamically optimum shape to provide uniform and un-separated flow suction was mounted on the inlet section of the fan. The test rig incorporated an acoustically treated length of duct to minimise extraneous noise. An airfoil louvre in the facility enabled the fan load and flow rate to be varied for the tests.

'Fan noise is a cause of environmental noise, as well as being a universal occupational concern,' says Paul Wenden, Fläkt Woods Engineering and Product management director. 'Increased running costs, often associated with conventional noise control, has led manufacturers to discover more about how noise is created, and thanks to improved testing methods, there has been a continuous process of change to fan designs, to take into account legislation and people's willingness to accept noise.'