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Aaron Gipp, Victor Salov, Udara Cabraal Introduction Overpowering volume of drums in small venues Methods of controlling volume already in place Drum shields Dampening Pads Electronic Drums Pitfalls of current methods Objective Create more pleasant listening experience Utilize Active Noise Cancellation for volume control Attenuation of up to 6 dB in three directions Minimal distortion between “dry” and “wet” signal Scope Single 12” Tom instead of entire drum kit Complexity of 3D spherical waves Apply theory to one drum and extrapolate to full kit Attenuation in single directions Only one speaker for cancellation “Nodal lines” Cancellation at single point if too complex Original Design Low-Pass and High-Pass Filters Eliminate sounds not associated with drum Bessel Filter design for exceptional linear phase Eighth-order Original Design Images taken from: http://www.dspguide.com/ch3/4.htm Original Design Pre-Amplifier Op-amp circuit for preliminary amplification Vout = Vin(1+Rf/Rg) Op-amp: TI LM741CN Rf = 33.44 kΩ Rg = 1.989 kΩ Vout = 17.812Vin Image taken from: http://en.wikipedia.org/wiki/File:Operational_amplifier_noninverting.svg Original Design Inverter Op-amp circuit used to invert polarity of signal Resistor values nearly identical for unity gain Theoretical 180° phase shift for all frequencies without time delay Original Design Inverter Design Vout = Vin(-Rf/Rin) Op-amp: TI LM741CN Rf = 38.75 kΩ Rin = 39.14 kΩ Vout = Vin(-0.99) Image taken from: http://en.wikipedia.org/wiki/File:OpAmp_Inverting_Amplifier.svg Original Design Final Amplifier Op-amp circuit used to control gain of inverted signal Almost identical to pre-amplifier Vout = Vin(1+Rf/Rg) Op-amp: TI LM741CN Rf = RV6NAYSD503A-P Clarostat 50kΩ single-turn ½ Watt potentiometer Rg = 1.476kΩ Vout ranges from 1.000Vin to 34.198Vin Op Amp Golden Rules The op-amp has infinite open-loop gain. The input impedance of the +/− inputs is infinite. (The inputs are ideal voltmeters). The output impedance is zero. (The output is an ideal voltage source.) No current flows into the +/− inputs of the op amp. In a circuit with negative feedback, the output of the op amp will try to adjust its output so that the voltage difference between the + and − inputs is zero (V+ = V−). Original Design Additional Parts Omnidirectional MIC48 Multimedia Computer (electret) Microphone 50Hz-16kHz frequency range 665-AS05008PR2R PUI Audio 2” 8ohm .4W speakers 550Hz-4.5kHz frequency range Drum-Striking Apparatus Consistency in measurements Same height, same location on drum Original Design Inherit drum signal with microphone Filter sound Invert Amplify Project back at source for cancellation “Dry” and “wet” signals propagate in opposite directions Circuitry Testing Procedures Take measurements of the input vs. output V. Check for phase shifting on the inverter. Vary input voltages and frequencies for inputs and check for gain. Measure signal attenuation if it occurred. Phase-Shift Testing Used 39K resistors *2, 1.5K resistor *1 Tests for Original Design Tested Pre-Amplifier for similar frequencies Tested for unusual amplification or phase shift CHAN2 -0.002 -0.001 Seconds 0 0.001 0.002 1 kHz, 0.2Vpp 0.003 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 -0.0004 Volts CHAN1 Volts 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 -0.003 CHAN1 CHAN2 -0.0002 0 Seconds 0.0002 550 Hz, 0.2Vpp 0.0004 Tests for Original Design Tested Pre-Amplifier with Inverter for similar 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 -0.0015 -0.001-0.0005 2.5 2 1.5 1 CHAN1 CHAN2 Volts Volts frequencies Tested for unusual amplification or phase shift 0.5 0 CHAN1 -0.5 CHAN2 -1 -1.5 0 0.0005 0.001 0.0015 Seconds 1 kHz, 0.2Vpp -2 -0.0004 -0.0002 0 0.0002 0.0004 Seconds 550 Hz, 0.2Vpp Tests for Original Design Tested final amplifier for similar frequencies As potentiometer varies, we progress from unity gain to a gain of ~35 0.15 6 0.1 4 2 0 -0.05 CHAN1 -0.1 CHAN2 0 CHAN1 -2 CHAN2 -4 -0.15 -0.2 -0.002 Volts Volts 0.05 -0.001 0 0.001 0.002 Seconds 1 kHz, 0.28Vpp min gain -6 -0.002 -0.001 0 0.001 0.002 Seconds 1 kHz, 0.28Vpp max gain Tests for Original Design Tested all three components for similar frequencies Clipping for max gain from exceeding supply voltage Unsmooth due possibly to high frequency DC offset due to input offset voltage of op-amps 8 6 CHAN1 CHAN2 Volts 4 Volts 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.002 2 CHAN1 0 CHAN2 -2 -0.001 0 0.001 0.002 Seconds 1 kHz, min gain -4 -0.002 -0.001 0 Seconds 0.001 0.002 1 kHz, max gain Drum Sound Experiment Created using device for consistency. Drum Sound Experiment Created using device for consistency. Replicated multiple times to check consistency. Drum Sound Experiment Created using device for consistency. Replicated multiple times to check consistency. Measured using microphone and oscilloscope. Drum Sound Experiment Created using device for consistency. Replicated multiple times to check consistency. Measured using microphone and oscilloscope. On average we saw this in Voltage vs. Time plots: Successes Inverter worked (phase shift of 180°) for all frequencies with minor amplification No noticeable phase shift for pre-amplifer Incorrect amplification in inverter/pre-amplifier compensated by final amplifier Drum-striking apparatus able to produce consistent measurements Challenges Unsmooth signals for three-component system Phase shift for final amplifier at max gain and high frequencies Ex: 1 kHz => 2° 5 kHz => 13° 10 kHz => 30° Op-amps unable to source sufficient current to power speaker Max voltage without distortion = 0.2V => 0.005W Filters’ original purpose deemed unnecessary Original microphone not reliable New Design/Replacements Shure SM57 and Shure SM48 dynamic microphones replace electret microphone Increased reliability and sensitivity Mixer to replace pre-amplifier Less distortion (smoother signals) at high frequencies Behringer Eurolive B215A (400W), Marshall MG30DFX (30W) powered amplifier/speaker combinations to replace old speakers More “head room” Broader frequency band Lower distortion (1% vs 5%) Tests for New Design Verified that old speakers would not be able to support voltage above ~0.2V Ch1 = input to final amplifier, Ch2 = input to speaker 0.6 0.4 Volts 0.2 0 CHAN1 CHAN2 -0.2 -0.4 -0.6 -0.0015 -0.001 -0.0005 0 Seconds 0.0005 0.001 0.0015 Tests for New Design Re-tested inverter circuit for new prominent frequencies of drum (up to 500Hz) -0.01 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 CHAN1 Volts Volts Later learned frequency content up to about 6kHz CHAN2 -0.005 0 0.005 Seconds 162Hz, 4Vpp 0.01 -0.01 10 8 6 4 2 0 -2 -4 -6 -8 -10 CHAN1 CHAN2 -0.005 0 0.005 Seconds 233Hz, 15Vpp 0.01 Tests for New Design Verified microphone could inherit signal and interpret frequency, and inverter could invert signal 100Hz 1kHz Tests for New System Tested for phase response of system and air Distance between microphone and speaker = 0.0m Setup: FG => Marshall => SM57 => mixer Ch1 of scope measured output of FG Ch2 of scope measured output of mixer (Phase difference)/(frequency difference) constant in linear phase system Tests for New System D=0.0m t=time delay (approximately) in ms frequency 1 2 3 4 5 6 7 8 9 10 Avg Slp t 100Hz 78 81 78 77 77 81 80 78 79 80 79 .86 2.4 200Hz 163 161 167 164 167 166 165 164 166 165 165 .21 .57 300Hz 184 188 185 185 186 185 186 184 185 185 185 .31 .86 400Hz 215 217 218 216 216 218 214 215 216 216 216 .17 .47 500Hz 234 232 235 233 231 233 233 233 232 233 233 .19 .54 Tests For New System Similar test for D=10cm Phase delay due to air = Phase (D=10cm) – Phase (D=0cm) Expected phase calculated for vsound in air = 343.2 m/s Frequency Average Phase delay due to air Expected phase 100Hz 2.3 283.4 10.49 200Hz 150.3 345.5 20.98 300Hz 170.3 345 31.47 400Hz 232.9 16.8 41.96 500Hz 264.8 31.9 52.45 Tests for New System Similar test for D=20cm Phase delay due to air = Phase (D=20cm) – Phase (D=0cm) Frequency Average Phase delay due to air Expected phase 100Hz 348.5 269.6 20.98 200Hz 165.1 0.3 41.96 300Hz 182.6 357.3 62.94 400Hz 262.5 46.4 83.92 500Hz 317.7 84.8 104.90 Tests for New System Tested for cancellation of a single frequency by pointing two speakers at each other Setup: FG => Marshall => SM57 => mixer => inverter => Behringer Measured SPL with Sound Level Meter Cancellation at certain points called “nodes” Discovered this orientation would create standing waves Pointed both signals in same direction from then on Tests for New System Took measurements of dry drum signal from ten feet away with Sound Level Meter 30 samples, average = 76.24, standard deviation = 4.23 Deemed SPL meter somewhat unreliable, took measurements with scope Tests for New System Based on system/air non-linear phase response, we can only cancel narrow band of frequencies Re-sampled drum to select peak frequencies above 0dB (193.5, 196, 212, 230, 236 Hz) Frequency Average Distance (m) 193.5Hz 328 1.615 196Hz 327.1 1.602 212Hz 348.8 1.556 230Hz 207.9 0.836 236Hz 283.1 1.14 Tests for New System Varied distance between microphone and speaker between aforementioned values (1-2m separation) and tested with varying volumes Voltage Signal and FFT plot of drum by itself; 8.83Vpp for 1s, 2V/div; 500Hz span, -25dBoffset, 5dB/div Tests for New Design Original Signal D=1.4m with arbitrary amplitude gain New Design Successes Most of frequency content above 500Hz far below 0dB, so cancelling narrow band might have worked System can inherit sound, interpret frequency, and invert signal Learned both “dry” and “wet” signals must travel in same direction New Design Challenges Non-linear phase response associated with electronics (different frequencies delayed by different amounts) Air being a dispersive medium (different frequencies will travel faster than others) No notable attenuation; peak-to-peak voltage always rose, average must be computed “by sight” Recommendations/Future Work Using DSP, create filter bank for all prominent frequencies Measure system phase response for all prominent frequencies Implement linear-phase filter for each frequency, delaying each to match frequency with longest phase delay Compensate for system non-linearity and air dispersion