Power handling and power compression in loudspeakers

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Power handling and power Power handling and power compression in compression in loudspeakers loudspeakers Doug Button Doug Button [email protected] [email protected]

description

Power handling and power compression in loudspeakers. Doug Button [email protected]. 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) - PowerPoint PPT Presentation

Transcript of Power handling and power compression in loudspeakers

Page 1: Power handling and power compression in loudspeakers

Power handling and power Power handling and power compression in loudspeakerscompression in loudspeakers

Doug ButtonDoug Button

[email protected]@roadrunner.com

Page 2: Power handling and power compression in loudspeakers

Ohm’s Pie ChartOhm’s Pie Chart

Page 3: Power handling and power compression in loudspeakers

Watts, Volts and dBWatts, Volts and dB

• Decibels are a relative scaleDecibels are a relative scale• dB is a POWER RATIOdB is a POWER RATIO• Sound Pressure Level is an absolute scale expressed Sound Pressure Level is an absolute scale expressed

in Decibels relative to 0dB=20in Decibels relative to 0dB=20μμPP(RMS)(RMS)

• Sound intensity is W/mSound intensity is W/m22

• dB= 10 x Log(PdB= 10 x Log(P11/P/P22) power ratio) power ratio

• dB= 20 x Log(EdB= 20 x Log(E11/E/E22) voltage ratio) voltage ratio

• dB= 20 x Log(IdB= 20 x Log(I11/I/I22) current ratio) current ratio

Page 4: Power handling and power compression in loudspeakers

dB, Power, Volts and AmpsdB, Power, Volts and Amps

• 3 dB is 2 times the power3 dB is 2 times the power• 6 dB is 4 times the power6 dB is 4 times the power• 10 dB is 10 times the power10 dB is 10 times the power• 20 dB is 100 times the power20 dB is 100 times the power

• 6dB is 2 times the voltage6dB is 2 times the voltage• 6dB is 2 times the current6dB is 2 times the current

• 20 dB is 10 times the voltage20 dB is 10 times the voltage• 20 dB is 10 times the current20 dB is 10 times the current

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AmplifierAmplifier• Voltage multiplierVoltage multiplier

– Input is in mV (ipod, cd player)Input is in mV (ipod, cd player)

– Output in VoltsOutput in Volts

• Ratio is called gain. Typically in dBRatio is called gain. Typically in dB• Has maximum peak voltage (slightly less than supply Has maximum peak voltage (slightly less than supply

rails) waveform ‘Clips’rails) waveform ‘Clips’• Has maximum RMS volts (3db less than peak)Has maximum RMS volts (3db less than peak)

– Power rating is RMS volts into resistive loadPower rating is RMS volts into resistive load

• Must be able to handling [lowest] impedanceMust be able to handling [lowest] impedance• Rating need not match speaker Rating need not match speaker

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AC VoltsAC VoltsVoltage vs Time

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Peak Voltage = 1Peak Voltage = 1 RMS Voltage = .707RMS Voltage = .707

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VnVRMS

221 ...

(Root Mean Square)(Root Mean Square)

P to PP to P= 2= 2

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Noise or music voltage wave from

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Peak V = 1Peak V = 1RMS V= .5RMS V= .5

Crest factor=20*Log (Peak/RMS)= 6dBCrest factor=20*Log (Peak/RMS)= 6dB

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ImpedanceImpedance• DC Resistance (Re)DC Resistance (Re)• AC impedanceAC impedance• Nominal impedanceNominal impedance

Impedance of a loudspeaker

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ms8 ohms nominal8 ohms nominal

DC Resistance= 6 OhmsDC Resistance= 6 Ohms

Resonance = 50 HzResonance = 50 Hz

Minimum impedance =7.5 OhmsMinimum impedance =7.5 Ohms

Power handling is CALCULATED based on an RMS voltage into minimum ZPower handling is CALCULATED based on an RMS voltage into minimum Z

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Complex Load ImpedanceComplex Load Impedancewoofer impedance

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Current leads, 4Current leads, 4thth quad (capacitive) quad (capacitive)

Current lags, 1st quad (inductive)Current lags, 1st quad (inductive)

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Power Handling SpecsPower Handling Specs• What voltage?What voltage?• What impedance assumption?What impedance assumption?

– Minimum impedanceMinimum impedance– Average impedanceAverage impedance– Nominal impedanceNominal impedance– Impedance under power?Impedance under power?

• RMS Power? Average Power? Real Power? Music Power? Peak RMS Power? Average Power? Real Power? Music Power? Peak Power?Power?

• Crest factor? Crest factor? – Sine is 3dBSine is 3dB– Noise is 6dB or greater (often 12dB)Noise is 6dB or greater (often 12dB)– Music is 6dB or greater (as high as 25 to 30dB)Music is 6dB or greater (as high as 25 to 30dB)

• Amp power rating?Amp power rating?– Sine wave at 1000 Hz X% THD into resistive loadSine wave at 1000 Hz X% THD into resistive load

IEC, AES, EIA Power handling is IEC, AES, EIA Power handling is CALCULATED based on an RMS CALCULATED based on an RMS voltage into minimum Zvoltage into minimum Z

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Credible ratingsCredible ratings

• IEC standard system power test:IEC standard system power test:– Pink noise from 50Hz to 3250Hz Pink noise from 50Hz to 3250Hz

slow roll off in HF more rapid at LFslow roll off in HF more rapid at LF

– 6dB crest factor6dB crest factor

– 100 hours100 hours

• AES standardAES standard– One decade Pink noise One decade Pink noise

– 2 hour duration2 hour duration

• EIA 426A/BEIA 426A/B– B Based on power compressionB Based on power compression

– A is like IEC 8 hoursA is like IEC 8 hours

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Heat DissipationHeat Dissipation

• Sets power handlingSets power handling• Dictates power compressionDictates power compression• Limits Max SPLLimits Max SPL• DC resistance is linear with temperatureDC resistance is linear with temperature

DCRDCR(warm)=(warm)=DCRDCR(room T)*(room T)*(1+(∆T*TCR))(1+(∆T*TCR))TCR= Thermal Coefficient of Resistivity=change in DCR/CTCR= Thermal Coefficient of Resistivity=change in DCR/CTCR for Cu and Al ~ .004 ∆/C or 1/250TCR for Cu and Al ~ .004 ∆/C or 1/250

100% change in DCR (double)= ∆T of 250 C100% change in DCR (double)= ∆T of 250 C

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Thermal Model AnalogyThermal Model Analogy

Temperature rise vs. time in transducerTemperature rise vs. time in transducerThermal CircuitThermal Circuit

Q= real heat powerQ= real heat power

R= R= ooC/WC/W

Voltage= temperatureVoltage= temperatureCurrent=powerCurrent=power

PP ∆∆TT11 ∆∆TT22

coilcoil magnetmagnet

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Thermal ResistanceThermal Resistance

DCR(%change)=(∆T*TCR) or (∆T/250)DCR(%change)=(∆T*TCR) or (∆T/250)

∆∆T=DCR(%change)* 250T=DCR(%change)* 250

30% change in DCR=0.3*250 = 75 Deg C30% change in DCR=0.3*250 = 75 Deg C

Coil Temperature = 75+20 = 95 Deg CCoil Temperature = 75+20 = 95 Deg C

R=∆T/Q (Q=true power)R=∆T/Q (Q=true power)

Example: Q = 50 watts of powerExample: Q = 50 watts of powerR=75/50= 1.5 deg C/wattR=75/50= 1.5 deg C/watt

DCRDCR(warm)=(warm)=DCRDCR(room T)*(room T)*(1+(∆T*TCR))(1+(∆T*TCR))

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Power Handling Power Handling

True Power max = Max ∆T / RtTrue Power max = Max ∆T / Rt Example (200 C)/(1.5 deg C/watt)= 133wattsExample (200 C)/(1.5 deg C/watt)= 133watts

Min ZMin Z(full power)=(full power)= Min Z Min Z (room T) (room T) + ∆ DCR+ ∆ DCR

PowerPower(calc)(calc)=Power =Power (true)*(true)*(Min Z(Min Z(full power) (full power) /Min Z/Min Z(room T)(room T)))

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Power Handling ExamplePower Handling Example

Min ZMin Z(roomT) (roomT) = 8 = 8 ΩΩ DCR = 6 DCR = 6 ΩΩMax T = 220 C Rt = 1.5 °C/WMax T = 220 C Rt = 1.5 °C/W

True power =200/1.5=133 wattsTrue power =200/1.5=133 wattsChange in DCR = 200/250 x 6ohms=4.8 Change in DCR = 200/250 x 6ohms=4.8 ΩΩMin ZMin Z(full) (full) = 8+4.8 = 12.8 = 8+4.8 = 12.8 ΩΩ

PowerPower(calculated) (calculated) = 133 x (12.8/8) = 213 Watts= 133 x (12.8/8) = 213 Watts

VV(rated P)=(rated P)=SQRT(PowerSQRT(Power(Calc)*(Calc)*min Z) min Z) = = SQRT(213*8) = SQRT(213*8) = 41.3 V41.3 V(RMS)(RMS)

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Failure modesFailure modes

• Thermal, electrical powerThermal, electrical power– Coil burns up, larger coils better!Coil burns up, larger coils better!

• Shorts outShorts out• Goes OpenGoes Open

• MechanicalMechanical– FatigueFatigue

• ConeCone• SpiderSpider• SurroundSurround• TinselsTinsels

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Time vs. Failure AnalysisTime vs. Failure Analysis

VGC Transducers

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1996 1.5% warranty rate1992 3.4% warranty rate1990 5% warranty rate

Data suggests a 2 to 1 power range for 2 to 300 hrData suggests a 2 to 1 power range for 2 to 300 hr

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Power ratingPower rating

• Good guide for what size amp to match with a speaker.Good guide for what size amp to match with a speaker.• BUT, all it really tells you is how easily the speaker will BUT, all it really tells you is how easily the speaker will

breakbreak

• A 200 watt speaker will break easier than A 200 watt speaker will break easier than a 400 watt speakera 400 watt speaker

• Pay close attention to qualifiers such as peak, Pay close attention to qualifiers such as peak, continuous, average, music, noise or RMS (misnomer)continuous, average, music, noise or RMS (misnomer)

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Power compressionPower compression

• Combination of reduced efficiency and Combination of reduced efficiency and less power delivered due to higher less power delivered due to higher resistanceresistance

• Rarely stated Rarely stated • Predictable from thermal modelPredictable from thermal model• DCR doubles at 525 F (270C)DCR doubles at 525 F (270C)

(approx 6dB compression in midband)(approx 6dB compression in midband)

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Higher DCR reduces efficiencyHigher DCR reduces efficiency

K x (BL)2 x (Sd)2

DCR x (Mms)2Efficiency =

Additionally: Higher impedance pulls less power

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CmsFsLBQes

222

Re

Thiele-Small ParametersThiele-Small Parameters

MmsCmsFs

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Higher DCR (Re) increases electrical Q Reduces damping

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Power compressionPower compression

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Input power, Watts.

Output vs Input power

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• Impedance change with temperatureImpedance change with temperature

Impedance Magnitude vs. Freq, as Temperature changes

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Power compressionPower compression

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• Power compressionPower compression

On Axis SPL vs. Freq showing compression vs temperture and frequency

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Power compressionPower compression

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Power compression Power compression

DCRDCR(%change)(%change)=(=(∆T*TCR) or (∆T/250)∆T*TCR) or (∆T/250)∆∆T= DCRT= DCR(%change)*(%change)* 250 250

DCRDCRhothot=DCR=DCRcoldcold*(1+(∆T*TCR))*(1+(∆T*TCR))

Power compression=20Log(1+ DCRPower compression=20Log(1+ DCR(%change)(%change)))

Power compression is 6dB when DCR doublesPower compression is 6dB when DCR doubles

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Power compression mismatchPower compression mismatch

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Power compression mismatchPower compression mismatch

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Power compression matchedPower compression matched

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Summary Power HandlingSummary Power Handling• Power compression and power handling can be Power compression and power handling can be

predicted based on a simple thermal modelpredicted based on a simple thermal model• Power ratings of speakers are not the true power Power ratings of speakers are not the true power

(calculated).(calculated).• Power rating of Amplifier and Power rating of speaker Power rating of Amplifier and Power rating of speaker

do not need to match, however matching them will yield do not need to match, however matching them will yield the most possible output without damagethe most possible output without damage

• The amp simply needs to be able to handle the load The amp simply needs to be able to handle the load (most amps can handle impedances down to 4 ohms)(most amps can handle impedances down to 4 ohms)

• Be very wary of power handling claims, check for Be very wary of power handling claims, check for qualifiers. Or misnomers (such as RMS power, Music qualifiers. Or misnomers (such as RMS power, Music power, Peak power, should be AVERAGE or power, Peak power, should be AVERAGE or CONTINUOUS).CONTINUOUS).

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Summary Power CompressionSummary Power Compression

• Combination of reduced efficiency and less power Combination of reduced efficiency and less power delivered due to higher resistancedelivered due to higher resistance

• Power compression is never speced, but can be Power compression is never speced, but can be inferred from the power ratinginferred from the power rating

• Power compression changes the bass alignmentPower compression changes the bass alignment• Power compression causes frequency response Power compression causes frequency response

anomalies which are worst if components don’t anomalies which are worst if components don’t compress equallycompress equally

Page 32: Power handling and power compression in loudspeakers

MeasurementsMeasurements

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