Abstract:
Micro porous absorber structures that be tuned to attenuate noise at the blade pass frequency (BPF) of a cooling fan. The absorber may comprises a panel covered with micro-porous openings with an air gap under the panel. The size of the air gap may be adjusted dynamically to optimize noise attenuation for a fan operating at different speeds.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention are directed to quieter cooling systems and, more particularly, to sound absorption for fan cooling of computing devices and the like. 
       BACKGROUND INFORMATION 
       [0002]    Electronic components tend generate unwanted heat during use which should be channeled away from the device for proper operation. Heat sinks are often employed to radiate heat away from the device. Fans are also often used to more efficiently move heat away from the device to keep it cool and operating at a suitable temperature. 
         [0003]    The trend to ultra thin notebooks and high density blade servers may make it more difficult to provide sufficient cooling with conventional airflow cooling, due to the space constraints. Smaller fans may thus be used running at higher speeds to move more air; however, this tends to create more acoustic noise. 
         [0004]    Low acoustic noise may be important given end user preferences, eco labels, and emerging governmental procurement directives. The use of acoustic absorbers may allow for an increased airflow at the same noise level, but there is no room in the ultra thin systems for traditional, bulky acoustic absorbers such as foams. 
         [0005]    Absorbing materials such as foams may be applied, but they are too bulky and do not fit in ultra thin form factors. In addition, the open cell foams lead to airflow loss in the system. Finally, these materials are relatively expensive and hard to integrate. Therefore, acoustic absorbers are usually not used in most electronic devices. Radial blowers and cross flow blowers are available, but they are not optimized and do not contain an integrated noise control solutions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing and a better understanding of the present invention may become apparent from the following detailed description of arrangements and example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing arrangements and example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto. 
           [0007]      FIG. 1A  is a block diagram of an illustrative blower which may be used, for example, to cool a thin form factor computing device; 
           [0008]      FIG. 1B  is a block diagram of the blower having a top cover comprising the micro-porous panel according to one embodiment of the invention; 
           [0009]      FIG. 1C  is a cross sectional view of the blower taken along line A-A′ of  FIG. 1B   
           [0010]      FIG. 2  is a graph comparing sound absorption characteristics of a 32 mm foam absorber verses a 1.25 mm micro-porpus panel according on one embodiment; 
           [0011]      FIG. 3  is a diagram showing airflow through a cross flow blower; 
           [0012]      FIGS. 4A and 4B  are fan blade rotors having 20 blades and 27 blades, respectively; 
           [0013]      FIG. 5  is a graph showing pressure verses flow rate for a 20 blade rotor and a 27 blade rotor; 
           [0014]      FIG. 6  is a graph illustrating a noise spectrum for a cross-flow blower; and 
           [0015]      FIG. 6  is a graph showing noise reduction at various frequencies for a blower operating with the micro-porous panel absorber according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Described are integrated micro porous absorber structures that may be tuned to attenuate noise at the blade pass frequency (BPF) of a cooling fan. Thus, the absorber may be 25 times thinner than a regular foam type absorber. Using this approach, a 1.25 mm thin absorber was designed and integrated that absorbs more than 65% of the sound energy. A regular foam absorber would have to be 32 mm thick in order to achieve this level of absorption. The extremely thin design of the micro porous absorber allows delivering about 10% more cooling and a 6 dB noise reduction, or 25% more airflow at the same acoustic noise level. This enables to meet the upcoming stringent acoustic noise directives, and a 15 to 25% increase in (turbo mode/system) power for ultra thin notebook and blade server platforms. 
         [0017]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0018]    Referring now to  FIG. 1A , there is shown a blower which may be used for example to cool a thin form factor computing device. The blower may comprise a blower housing  100  and a fan  102 . This is of course a not-to-scale simplified diagram as a fan motor, vents, heat pipes, and any other items commonly found in blowers may also be present, though not shown. According to one embodiment of the invention, as shown in  FIG. 1B , a micro porous structure may be used to reduce noise associated with the blower. In this example, a micro porous blower cover  104  replaces the traditional blower cover. The micro-porous structure may be made out of, for example, perforated sheet metal, plastic or film, backed by an air layer. The material comprises a plurality of holes or perforations which may range in sizes and density. 
         [0019]      FIG. 1C  shows a cross-sectional view taken along line A-A′ from  FIG. 1B  showing the blower housing  100 , the fan  102  and the micro-porous cover  104 . The micro-porous structure cover  104  may be backed by a thin air layer  106 . The size of the air-layer  106  may be fixed or changed dynamically by an actuator  108  to tune the acoustic properties of the micro-porous cover  104 . For example, one or more actuators  108  may be used to raise and lower the cover  104  changing the size of the air gap  106  and thus dynamically adjusting the acoustic properties of the cover for optimization of noise reduction. The actuators may be for example piezoelectric actuators  108  which moves the cover up and down as illustrated by arrow  110 . 
         [0020]    The micro porous panel absorber or cover  104  comprises a thin sheet with micro perforations backed by a thin air layer  106  and may be fabricated using standard materials and high volume manufacturing (HVM) processes. The characteristics of the absorber, such as pore diameter, pore density, sheet thickness and air layer thickness determine the absorption behavior. The acoustic absorption curve typically shows a peak in a certain frequency range, and the location and the width of this peak can be controlled with these parameters. This can be done in a passive sense, e.g. a static mechanical design, but more complex solutions are also possible as mentioned above where the air layer  106  is dynamically adjusted to shift the sound absorption in case of large fan speed variations. This allows the absorber solution to be tailored to absorb the noise generated by the fan or other noise sources. 
         [0021]    The new micro porous panel absorber  104  allows ultra thin designs that are more than 25 times thinner than existing foam type solutions. Therefore it can be integrated into blowers, axial fans, notebooks, CE devices, desktops, servers or mobile internet devices. It also does not require new materials, and as opposed to for example open cell foams, does not have a negative impact on the airflow because the pore diameter is so small. 
         [0022]      FIG. 2  shows a graph comparing foam absorbers to the micro porous absorber of the present invention. A value of zero means that no energy is absorbed, while a value of 1 means that all energy is absorbed. The graph shows a comparison with an open cell foam material of 32 mm thick. The results show that the micro porous panel absorber has approximately the same absorption, but is only 1.25 mm thick as compared to the 32 mm thick foam. The peak at which the maximum absorption occurs and the width of this peak can be controlled by adjusting the values for the pore diameter, the pore density, the sheet thickness and the air layer thickness. Thus, the micro porous panel absorber allows ultra thin designs that can be integrated into blowers or notebooks. 
         [0023]    Referring to  FIG. 3 , embodiments of the invention also include optimizing fan design to be used in conjunction with the micro porous absorbers. Current and future notebook cooling solutions use a regular radial blower or cross flow blower to deliver airflow to the system. As shown, a radial cross-flow blower  301  has an air intake  300  through the top and/or bottom, and an air outlet on the side. In the center of the blower a blade rotor  302  rotates to draw air in the inlet  300  and push it out of the outlet  304 . The cross flow blower  301  uses a side-in side-out concept, which is especially suited for ultra thin form factors where there is very little space for the top or bottom clearances required for a regular blower. Embodiments of the invention disclose an optimized integrated design, comprising an optimized rotor design, coupled with an integrated noise control solution using a micro porous panel absorber. 
         [0024]    Numerical simulations and experiments have shown that an optimal blade number exists for a given rotor.  FIG. 4A  shows a 20-blade rotor and  FIG. 4B  shows a 27-blade rotor, as examples.  FIG. 5  is a graph showing air pressure verses flow rate for both the 20-blade and 27 blade rotors. The blade pass frequency is determined by the rotational speed of the fan and the number of blades. Optimizing the rotor blade number may lead to an airflow increase of about 10%. The fan rotation will result in an acoustic noise spectrum containing distinct peaks at the blade pass frequency and the higher harmonics. This was experimentally confirmed by comparing a standard 20 blade rotor to a  27  blade rotor for the cross flow blower. The simulations show that the optimal blade number for these types of rotors is approximately 25. 
         [0025]      FIG. 6  is a graph illustrating the fan noise spectrum of a cross flow blower. The blade pass frequency at which most noise occurs is determined by the rotational speed of the fan, RPM, and the number of blades, N: 
         [0000]        f   BPF =(RPM* N )/60 
         [0026]    The micro porous panel absorber design is now designed and integrated to work exactly at the main blade pass frequency. 
         [0027]    An example of such an integration is shown in  FIGS. 1B and 1C  discussed above where the top cover of the blower is replaced by a micro perforated panel, backed by an air layer. The noise reduction performance of this sample was measured experimentally. The results are given in  FIG. 7  which shows various noise reductions at different frequencies. The graph shows that there is very significant absorption of up to 6 dB in the ⅓ octave bands for which the absorber was designed. Interestingly, it also provides significant noise reduction at the second harmonic of this frequency. The absorber could also be integrated into the side walls of the blower or system to allow the thinnest system possible. 
         [0028]    The application of the integrated and tuned micro porous absorbers is not limited to cooling fans. Other applications involve integration into chassis, casing or enclosure designs, heat exchanger designs using perforated fins with the separating air layers as micro porous absorbers. Also, an active concept may be used where the air layer thickness is adjusted automatically using an actuator mechanism. The rotor speed and the acoustic noise levels can be sensed and made available to the system to allow such an active absorber design. 
         [0029]    The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
         [0030]    These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.