Servo Feedback Control of an Audio Subwoofer by Dalimar Vélez Advisor: Shawn Hunt

Overview Objectives Background and Concepts Problem Implementation Design Procedure Results Conclusion

Objectives Designing and building a Digital Servo-Feedback Control of an Audio Subwoofer.

Background and Concepts A subwoofer is a speaker who reproduces low frequencies (20 to 500 Hz). An accelerometer measures the acceleration of a system. (ex. ADXL150) Harmonics are integral multiple of the fundamental frequency.

Background and Concepts Filters: –Low pass –Adaptive filter structure for system identification –Adaptive filter structure for noise cancellation

Problem The subwoofer creates most of the harmonic distortion in a speaker. To improve this: –Measure subwoofer motion cone –Compare the output with the input –Correct the difference This is called a Servo Feedback Control

Implementation Design In order to reduce the noise in the subwoofer output is essential to know the distortion added by the system to the input signal. A system identification adaptive filter was developed to obtain the response of the subwoofer. This response is going to be used to modify the input of the subwoofer. These modifications will cancel the distortions, making the output of the subwoofer clearer.

Procedure Music is filtered by a low pass.

Procedure The accelerometer is connected directly to the cone, without signal amplification, to an Analog to Digital Converter (ADC) in the DSP board.

Procedure Develop algorithm for feedback loop.

Procedure Develop algorithm for frequency response and motion deviations correction.

Photos

Distortion produced by the cone: Results

The green graph represents the input of the system and the violet graph shows the output distortion created by the subwoofer Results

The adaptive filter identified the response of the subwoofer system. The algorithm reduced the harmonic distortion. The 180 Hz harmonic for this case was attenuated from -46.5dB to -65 dB.

Conclusion The distortion was caused by the physical properties of the subwoofer driver. The reduction of harmonics at the output of the subwoofer was accomplished thanks to the usage of the coefficients found by the adaptive filter. The hardware can be implemented in any other application independently of the driver used.

References [1] ±50g Single Axis Accelerometer with Analog Output [2] Harmonic – [3]Chassaing, Rulph. DSP Applications Using C and the TMS320C6x DSK. John Wiley & Sons, nc, NY 2002 [4]Antoniou, Andreas. Digital filters, analysis, design and applications, 2th ed. Mc Graw Hill, Inc,1993. [5]Doucet, Dougla., Fasani Rob. Hasenkamp, Pete r. and Senesky Matthew. Using an Accelerometer to Make an Active Subwoofer.

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