Acoustic Noise Control for Fans

FIG. 1 is a simplified front view of a prior art muffin fan having mounting holes provided in the case thereof.

FIG. 2 is a partially cutaway side view of the fan of FIG. 1 showing typical prior art isolation mounting employed therewith.

FIG. 3 is a simplified drawing of an inertia base support for a large fan, or the like, as employed in the prior art for vibrational isolation.

FIG. 4 is a plan view of the acoustic isolation collar employed in the present invention.

FIG. 5 is a plan view of the preferred two part high inertia collar employed in the present invention.

FIG. 6-8 show how the components of FIGS. 4 and 5 could be adapted to be used with a round fan instead of with a rectangular fan.

FIG. 9 is a plan view of a fan assembled in combination with the mounting of the present invention.

FIG. 10A is a partially sectional side view on Section 10--10 of FIG. 9 with the assembly attached to a support structure over a cooling opening.

FIG. 10B is a view, similar to FIG. 10A of an alternative embodiment of acoustic isolation mounting in accordance with the present invention.

The present invention will be described hereinafter principally with reference to FIGS. 4, 5, 9 and 10. The major components of the present invention which permit a vast improvement over the prior art are the acoustic isolation collar 38 of FIG. 4 and the high inertia collar 40 of FIG. 5. The acoustic isolation collar 38 is molded of a damping/absorption/isolation material (hereinafter referred to as "DIAM") which preferably comprises an open celled polyurethane "acoustic foam" of a type well known to those skilled in the art which can be obtained from numerous sources.

As will be realized from the description hereinafter, the present inventionaccomplishes a multitude of functions, including (a) damping of the fan vibration by the DAIM; (b) containment and absorption of significant fraction of vibration-induced sound from the fan by the DAIM in conjunction with the high inertia collar; (c) vibration isolation of the fan from the supporting structure by the DAIM in conjunction with the high inertia collar; (d) inertial reduction of fan vibration by the high inertia collar at lower frequencies; (e) damping of any vibration of the high inertia collar by the DAIM; (f) secondary vibration isolation of the high inertia collar from the supporting structure by secondary isolators; (g) essentially air-tight sealing of the fan to its immediate support (i.e. the high inertial collar) by the DAIM; and (h) physical support of the fan, without any screws or other fasteners attaching to the fan, by the DAIM and the high inertia collar.

It should be noted that it is not necessary for all the foregoing functions to be accomplished for the present invention to provide substantial benefits over the prior art; but, each adds to the utility and value thereof.

The description of the tested embodiment contained hereinafter is of apparatus employed with a square muffin fan such as that labelled 10 and previously discussed with respect to FIGS. 1 and 2. As shown in FIG. 4, the present invention employs an acoustic isolation collar 38 of DIAM sized and shaped to fit circumferentially about the case 14 of the fan 10. Since the shape of the case 14 is regular, i.e. square, the inner shape of the collar 38 is also a square. If the case was irregular in shape, it would be preferred to mold the collar 38 to have an internal shape substantially identical to the exterior circumferential shape of the case. As shown in FIGS. 6 and 7, for example, if the fan 10' had a circular case 14', the acoustic isolation collar 38' would preferably be cylindrical in shape.

The preferred construction for the high inertia collar 40 is as shown in FIG. 5. Collar 40 is comprised of two identical steel half-collars 44 each having mating tabs 46 extending from the ends thereof. The tabs 46 have bores 48 therethrough through which bolts 50 can be passed to fasten the two half-collars together with nuts 52 as shown in FIG. 9. The sides of the two half-collars 44 are of length "d2-" which is slightly less than the length "d2" of the outer sides of the acoustic isolation collar 38 so that the acoustic isolation collar 38 is squeezed against the outside of the case 14 by the collar 40 as the two half-collars 44 are bolted together. Again, as shown in FIGS. 6-8, if the fan 10' was circular and the collar 38' was cylindrical, the half-collars 44' would be preferably semi-cylindrical in shape. It should be understood that the high inertia collar 40 is, as the name implies, constructed of heavy weight steel, iron, or the like so as to have substantial weight with respect to the weight of the fan 10. It is preferred that the high inertia collar 40 have a mass at least two to three times that of the fan 10. A light-weight metal or plastic band or strap would not have sufficient mass and, therefore, inertia to perform as required by the present invention.

Turning now to FIGS. 9 and 10A, the present invention is shown being used to mount the fan 10 to a supporting structure 42 over a cooling opening 22 under acoustic noise controlling conditions. To effect the actual attaching of the fan/collar/collar assembly 10, 38, 40 to the supporting structure 42 in a preferred manner, mounting tabs 54 are attached to the half-collars 44 as shown in FIG. 9 by suitable fastening means such as isolation bolt/nuts 56 sold commercially as Wellnuts (TM). It is preferred to mount the fan/collar/collar assembly 10, 38, 40 spaced from supporting structure 42, preferably through an additional isolator so as to obtain the maximum noise abatement possible with the present invention.

Alternatively, as shown in FIG. 10B, acoustic isolation collar 38 may extend beyond the fan and collar 10, 40 in contact with supporting structure 42. In such case it is preferred that collar 38 extend at least one and preferably at least one and one-half times its thickness.

Having thus described the physical attributes and construction of the present invention, the operaton thereof and the benefits obtained thereby over the prior art as obtained from actual testing by the applicant will now be discussed briefly.

The present invention as hereinbefore described provides a superior reduction of mechanically (or electromechanically) caused noise compared to vibration isolation alone. This can be explained as follows: Since the actual source of vibration is more localized than the entire fan itself, it is desirable to confine the extent of fan vibration as much as possible to that local region. In the present invention, this is accomplished by damping the fan body or case. A simple test reveals that the DAIM does indeed damp the fan. If the fan alone is supported by a string and is struck sharply by a firmly held coin, the fan sounds metallic. This indicates that the natural frequencies have ben excited and that they persist, thus that damping is low. If the fan is placed in the collar/collar assembly 38, 40 of the present invention and the experiment repeated, the duller, more wooden sound is heard, indicating that the assembly provides damping. The effect of damping the fan with the DAIM is to reduce the amplitude and extent of mechanical vibration, and thus to reduce the amount of noise radiated directly from the fan. By preventing reverberant build-up of the structure-borne sound on the fan side of the isolator, the damping also makes the isolation more effective. It should be appreciated that even greater damping could be obtained by adhering dedicated damping material directly to the fan body as with adhesive, or the like. However, this approach increases the first cost associated with the special purpose damping material and may render impractical or impossible of the removal of the adhering material and reusing it on any replacement fans.

The effect of damping the massive high inertia collar with the DAIM is also to reduce the amplitude of mechanical vibration of that collar and thus to reduce the amount of noise radiated from that collar. Note that special purpose damping material could also be adhered to the high inertia collar to augment damping from the DAIM. This would incur the additional first cost mentioned above, but not the replacement costs associated with replacing special purpose damping applied to the fan.

The effect of surrounding the exterior surface of the fan with absorption nearly enclosed by a rigid massive surface is to contain and absorb sound that would otherwise be radiated from the perimeter of the fan. Generally, one need be concerned primarily here with higher frequency sound, corresponding to wavelengths in air shorter than, for example, ten times the fan thickness. For a fan 1.5 inches thick, this would correspond to a wavelength of 15 inches and frequencies about 900 Hz. Lower frequency sound will not radiate effectively from the isolated fan.

By similar reasoning, it generally is not critical to prevent the transmisson of lower frequency structure-borne sound to the surrounding massive high inertia collar (its dimensions and therefore its radiation efficiency being similar to the fan) provided the collar is isolated from larger expanses of sheet metal or other potential radiating surfaces as these larger surfaces would radiate the lower frequency sound efficiently.

The isolation of the low frequency structure-borne sound from the supporting structure, and containing it within the fan/collar assembly, where radiation efficiency is low, is accomplished by the secondary isolation where the present invention is attached to the supporting structure. In one tested embodiment, this isolation had a natural frequency of around 35 Hz and isolated frequencies above about 70 Hz.

The massive high inertia collar plays complementary roles at higher and lower frequencies. At higher frequencies, i.e. above twice the first resonance of the fan/collar/collar assembly, the fan vibrates freely, with little reactive force from the DAIM collar. The high inertia collar, however, provides enough mismatch in mechanical impedance that the motion of the DAIM is not transmitted to the high inertia collar. At lower frequencies, i.e. below half the first resonance of the fan/collar/collar assembly (half of 250 Hz in the design currently under commercial testing), the fan and high inertia collar are tightly coupled by the DAIM and appear to the force sources within the fan as a single mass, made several times greater than the fan itself by the addition of the massive collar. Assuming a constant force level, tripling the mass "seen" by the force source reduces vibration levels and noise level by about 10 dB.

It may also be noted and appreciated that the DAIM is the only support for the fan and that the fan is held in place to the high inertia collar by the DAIM without any screws or loose hardware attached to the fan.

As a final comment, in tests conducted by the Applicant herein, the total mounting structure of the present invention as hereinbefore described resulted in a total of 7.7 dB of airborne noise reduction.