Abstract:
In one embodiment to reduce unwanted acoustic fan noise, the control signal for a computer system cooling fan is modulated so that the acoustic noise power spectral density of the fan has a bandwidth greater than when the control signal is constant.

Description:
FIELD 
     Embodiments of the present invention relate to fan speed control, and more specifically, to controlling the fan so that it is perceived as quieter. 
     BACKGROUND 
     Illustrated in FIG. 1 is a portion of a computer system comprising central processing unit (CPU)  102 , chipset  104 , graphics card  106 , and memory  108 . Chipset  104  allows for communication between CPU  102 , memory  108 , and graphics card  106 , as well as other peripheral components (not shown) connected to system bus  110 . One or more of these system components may generate an appreciable amount of heat, so that fan  112  is employed to cool part or all of the computer system. Usually, CPU  102  generates the most heat, and for some computer systems, more than one fan may be employed. 
     Customers of computer systems, both in the home and in business, often prefer computer systems with quiet fans. As a result, many original equipment manufacturers are sensitive to the acoustic noise generated by fans. One approach to reducing fan noise is to adjust the fan speed according to measured temperature, as shown in FIG.  1 . Heat sensor  112  provides to fan voltage controller  114  a signal indicative of temperature. Heat sensor  112  may be placed near CPU  102 , for example, and may be integrated with CPU  102 . Fan voltage controller  114  provides a fan control voltage to fan  112  so as to adjust the speed of fan  112 . When heat sensor  112  indicates that maximum cooling is needed, the fan control voltage is increased to its maximum nominal value so that fan  112  circulates sufficient air flow for cooling. When maximum cooling is not required, fan voltage controller  114  lowers the fan control voltage supplied to fan  112  so that the speed of fan  112  is reduced, along with the accompanying acoustic noise. Fan voltage controller  114  may even cause fan  112  to stand idle. 
     The combination of sensor  112 , fan voltage controller  114 , and fan  112  comprises a closed feedback loop so that the computer system of FIG. 1 is sufficiently cooled, so that the acoustic noise generated by fan  112  is reduced when CPU  102  is not operating at its maximum workload. However, the acoustic noise reduction scheme of FIG. 1 does not provide a reduction in acoustic noise when CPU  102  is operating at its maximum workload. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art computer system with a closed loop feedback for fan speed control. 
     FIG. 2 is an embodiment of the present invention. 
     FIG. 3A illustrates a fan control voltage according to an embodiment of the present invention; and FIG. 3B illustrates a noise power spectral density for constant fan control voltage and a noise power spectral density for the modulated fan control voltage illustrated in FIG.  3 A. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention provide a time varying fan control voltage so as to spread the acoustic noise power spectral density generated by the fan over a larger bandwidth than when compared to the case in which the fan control voltage is constant over appreciable periods of time. FIG. 2 provides an embodiment. Signal generator  202  provides a signal, which is filtered by a highpass filter comprising resistor  204  and capacitor  206 , so as to provide a modulating signal X(t) to modulator  208 . Modulator  208  modulates a supply voltage V CC  in response to the modulating signal X(t) to provide a modulated fan control voltage, denoted as V FAN (t). 
     An example of V FAN (t) is shown in FIG. 3A, where for simplicity only one period of V FAN (t) is shown, with a period denoted by T. In this example, V FAN (t) is a triangular waveform, but many other types of waveforms may be employed. In the embodiment of FIG. 3A, the maximum of V FAN (t) is simply denoted by V FAN , and the minimum is denoted as (1−δ)V FAN , where δ is a positive number less than one. For one embodiment, a 3 dB (50%) reduction in perceived acoustic noise power has been measured for a modulation frequency of 10 Hz (T=0.1 sec) and an amplitude modulation of 5% (δ=0.05). This 3 dB reduction in measured acoustic noise power is due to the spreading of the noise power spectral density into frequencies not readily perceived as noise by a human listener. 
     In other embodiments, the modulated fan control voltage waveform may be obtained from a pseudo random sequence, or a random sequence. For some embodiments, signal generator  202  may be realized by a functional unit within chipset  110 . Modulator  208  may be realized in a number of ways. For example, if X(t) is a periodic signal, then modulator  208  may be a simple summer or multiplier so that X(t) is added to or multiplied by V CC  to provide the modulated fan control voltage V FAN (t). 
     FIG. 3B provides an example of spreading the acoustic noise power spectral density by the modulated voltage waveform of FIG.  3 A. Curve  302  is the acoustic noise power spectral density for a constant fan control voltage V FAN (t)=V FAN . Curve  302  is seen to be relatively sharply peaked about some nominal acoustic frequency, denoted as f NOM . Curve  304  is the acoustic noise power spectral density for the modulated fan control voltage V FAN (t) of FIG.  3 A. The bandwidth of curve  304  is denoted as Δ F , which is seen to be larger than the bandwidth of curve  302 . To first order, Δ F  may be approximated as Δ F ˜αδf NOM , where α is a positive scalar. However, for many systems, the relationship between bandwidth spreading and modulation factor δ may be more complicated. The maximum (peak) of curve  304  is less than the maximum of curve  302 , where the difference is denoted as Δ N . 
     Noise having the power spectral density of curve  304  may be perceived by a human observer as less noisy than a noise source having the power spectral density of curve  302 . By modulating the fan voltage, the frequency content of the acoustic noise power spectral density is spread into frequencies that may be perceived as less noisy by a human listener. Even if voltage modulation does not change the total noise power (area under the spectral density curve), the peak of the power spectral density will decrease just to maintain the same area under the curves. In general, there may be a decrease in total noise power due to voltage modulation, which contributes to a lower peak power spectral density, but the majority of the decrease in peak power spectral density is due to spreading of the power spectral density. Consequently, it is observed in the described embodiments that fan voltage modulation reduces the noise power spectral density for frequencies considered “noisy” by an observer. 
     Furthermore, it may not be necessary for the peak of the power spectral density for a modulated control voltage to be less than the peak of the power spectral density for a constant control voltage, provided the spectral peak for a modulated control voltage occurs at a sufficiently lower frequency than f NOM . This is due to human perception, where a lower frequency noise may be perceived as less annoying than a higher frequency noise. However, for the embodiment represented by FIGS. 3A and 3B, the power spectral density for the modulated fan is everywhere lower than that for the constant control voltage case. 
     Various modifications may be made to the disclosed embodiments without departing from the scope of the invention as claimed below. For example, if fan speed is responsive to supply current, then supply current to the fan may be modulated so that the noise power spectral density has the desired characteristics as described above for the embodiments with modulated control voltage.