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Power handling and power compression in loudspeakers

Power handling and power compression in loudspeakers. Doug Button dougbutton@roadrunner.com. Ohm’s Pie Chart. Watts, Volts and dB. Decibels are a relative scale dB is a POWER RATIO Sound Pressure Level is an absolute scale expressed in Decibels relative to 0dB=20 μ P (RMS)

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Power handling and power compression in loudspeakers

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  1. Power handling and power compression in loudspeakers Doug Button dougbutton@roadrunner.com

  2. Ohm’s Pie Chart

  3. Watts, Volts and dB • Decibels are a relative scale • dB is a POWER RATIO • Sound Pressure Level is an absolute scale expressed in Decibels relative to 0dB=20μP(RMS) • Sound intensity is W/m2 • dB= 10 x Log(P1/P2) power ratio • dB= 20 x Log(E1/E2) voltage ratio • dB= 20 x Log(I1/I2) current ratio

  4. dB, Power, Volts and Amps • 3 dB is 2 times the power • 6 dB is 4 times the power • 10 dB is 10 times the power • 20 dB is 100 times the power • 6dB is 2 times the voltage • 6dB is 2 times the current • 20 dB is 10 times the voltage • 20 dB is 10 times the current

  5. Amplifier • Voltage multiplier • Input is in mV (ipod, cd player) • Output in Volts • Ratio is called gain. Typically in dB • Has maximum peak voltage (slightly less than supply rails) waveform ‘Clips’ • Has maximum RMS volts (3db less than peak) • Power rating is RMS volts into resistive load • Must be able to handling [lowest] impedance • Rating need not match speaker

  6. AC Volts (Root Mean Square) RMS Voltage = .707 Peak Voltage = 1 P to P = 2

  7. Random Voltage Peak V = 1 RMS V= .5 Crest factor=20*Log (Peak/RMS)= 6dB

  8. Impedance • DC Resistance (Re) • AC impedance • Nominal impedance Resonance = 50 Hz 8 ohms nominal Minimum impedance =7.5 Ohms DC Resistance= 6 Ohms Power handling is CALCULATED based on an RMS voltage into minimum Z

  9. Complex Load Impedance Current lags, 1st quad (inductive) Current leads, 4th quad (capacitive)

  10. Power Handling Specs • What voltage? • What impedance assumption? • Minimum impedance • Average impedance • Nominal impedance • Impedance under power? • RMS Power? Average Power? Real Power? Music Power? Peak Power? • Crest factor? • Sine is 3dB • Noise is 6dB or greater (often 12dB) • Music is 6dB or greater (as high as 25 to 30dB) • Amp power rating? • Sine wave at 1000 Hz X% THD into resistive load IEC, AES, EIA Power handling is CALCULATED based on an RMS voltage into minimum Z

  11. Credible ratings • IEC standard system power test: • Pink noise from 50Hz to 3250Hz slow roll off in HF more rapid at LF • 6dB crest factor • 100 hours • AES standard • One decade Pink noise • 2 hour duration • EIA 426A/B • B Based on power compression • A is like IEC 8 hours

  12. Heat Dissipation • Sets power handling • Dictates power compression • Limits Max SPL • DC resistance is linear with temperature DCR(warm)=DCR(room T)*(1+(∆T*TCR)) TCR= Thermal Coefficient of Resistivity=change in DCR/C TCR for Cu and Al ~ .004 ∆/C or 1/250 100% change in DCR (double)= ∆T of 250 C

  13. Thermal Model Analogy Voltage= temperature Current=power magnet coil P Q= real heat power ∆T2 ∆T1 R= oC/W Thermal Circuit Temperature rise vs. time in transducer

  14. Thermal Resistance DCR(warm)=DCR(room T)*(1+(∆T*TCR)) DCR(%change)=(∆T*TCR) or (∆T/250) ∆T=DCR(%change)* 250 30% change in DCR=0.3*250 = 75 Deg C Coil Temperature = 75+20 = 95 Deg C R=∆T/Q (Q=true power) Example: Q = 50 watts of power R=75/50= 1.5 deg C/watt

  15. Power Handling True Power max = Max ∆T / Rt Example (200 C)/(1.5 deg C/watt)= 133watts Min Z(full power)= Min Z (room T) + ∆ DCR Power(calc)=Power (true)*(Min Z(full power) /Min Z(room T))

  16. Power Handling Example Min Z(roomT) = 8 Ω DCR = 6 Ω Max T = 220 C Rt = 1.5 °C/W True power =200/1.5=133 watts Change in DCR = 200/250 x 6ohms=4.8 Ω Min Z(full) = 8+4.8 = 12.8 Ω Power(calculated) = 133 x (12.8/8) = 213 Watts V(rated P)=SQRT(Power(Calc)*min Z) = SQRT(213*8) = 41.3 V(RMS)

  17. Failure modes • Thermal, electrical power • Coil burns up, larger coils better! • Shorts out • Goes Open • Mechanical • Fatigue • Cone • Spider • Surround • Tinsels

  18. Time vs. Failure Analysis Data suggests a 2 to 1 power range for 2 to 300 hr

  19. Power rating • Good guide for what size amp to match with a speaker. • BUT, all it really tells you is how easily the speaker will break • A 200 watt speaker will break easier than a 400 watt speaker • Pay close attention to qualifiers such as peak, continuous, average, music, noise or RMS (misnomer)

  20. Power compression • Combination of reduced efficiency and less power delivered due to higher resistance • Rarely stated • Predictable from thermal model • DCR doubles at 525 F (270C) (approx 6dB compression in midband)

  21. Higher DCR reduces efficiency K x (BL)2 x (Sd)2 DCR x (Mms)2 Efficiency = Additionally: Higher impedance pulls less power

  22. Thiele-Small Parameters Higher DCR (Re) increases electrical Q Reduces damping

  23. Power compression

  24. Power compression • Impedance change with temperature

  25. Power compression • Power compression

  26. Power compression DCRhot=DCRcold*(1+(∆T*TCR)) DCR(%change)=(∆T*TCR) or (∆T/250) ∆T= DCR(%change)* 250 Power compression=20Log(1+ DCR(%change)) Power compression is 6dB when DCR doubles

  27. Power compression mismatch No compression Compressed tweeter

  28. Power compression mismatch No compression Compressed woofer

  29. Power compression matched Both compressed No compression

  30. Summary Power Handling • Power compression and power handling can be predicted based on a simple thermal model • Power ratings of speakers are not the true power (calculated). • Power rating of Amplifier and Power rating of speaker do not need to match, however matching them will yield the most possible output without damage • The amp simply needs to be able to handle the load (most amps can handle impedances down to 4 ohms) • Be very wary of power handling claims, check for qualifiers. Or misnomers (such as RMS power, Music power, Peak power, should be AVERAGE or CONTINUOUS).

  31. Summary Power Compression • Combination of reduced efficiency and less power delivered due to higher resistance • Power compression is never speced, but can be inferred from the power rating • Power compression changes the bass alignment • Power compression causes frequency response anomalies which are worst if components don’t compress equally

  32. Measurements http://www-classes.usc.edu/engr/ee-ep/ 499/423L/Power lecture April 2011/ Power rating and Power compression calculator 2011.xls Measure the DCR of your transducers

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