Power handling and power compression in loudspeakers
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Transcript of 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
Ohm’s Pie ChartOhm’s Pie Chart
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
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
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
AC VoltsAC VoltsVoltage vs Time
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-1
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Time
Vo
lta
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Peak Voltage = 1Peak Voltage = 1 RMS Voltage = .707RMS Voltage = .707
n
VnVRMS
221 ...
(Root Mean Square)(Root Mean Square)
P to PP to P= 2= 2
Noise or music voltage wave from
-1.5
-1
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Time
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ltag
eRandom VoltageRandom Voltage
Peak V = 1Peak V = 1RMS V= .5RMS V= .5
Crest factor=20*Log (Peak/RMS)= 6dBCrest factor=20*Log (Peak/RMS)= 6dB
ImpedanceImpedance• DC Resistance (Re)DC Resistance (Re)• AC impedanceAC impedance• Nominal impedanceNominal impedance
Impedance of a loudspeaker
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100
1 100 10000
Frequency
Oh
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
Complex Load ImpedanceComplex Load Impedancewoofer impedance
0.00
10.00
20.00
30.00
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10.00 1000.00
freq
Oh
ms
Woofer phase
-90.00
-60.00
-30.00
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30.00
60.00
90.00
10.00 1000.00
Freq
Ph
as
e a
ng
le
Current leads, 4Current leads, 4thth quad (capacitive) quad (capacitive)
Current lags, 1st quad (inductive)Current lags, 1st quad (inductive)
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
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
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
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
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))
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)))
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)
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
Time vs. Failure AnalysisTime vs. Failure Analysis
VGC Transducers
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1000
500 600 700 800 900 1000 1100 1200 1300
Watts
Hou
rs u
ntil
failu
re
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
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)
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)
Higher DCR reduces efficiencyHigher DCR reduces efficiency
K x (BL)2 x (Sd)2
DCR x (Mms)2Efficiency =
Additionally: Higher impedance pulls less power
CmsFsLBQes
222
Re
Thiele-Small ParametersThiele-Small Parameters
MmsCmsFs
1
2
1
Higher DCR (Re) increases electrical Q Reduces damping
Power compressionPower compression
100
105
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1 10 100
So
un
d p
ress
ure
leve
l @ 1
m
Input power, Watts.
Output vs Input power
Real
Ideal
• Impedance change with temperatureImpedance change with temperature
Impedance Magnitude vs. Freq, as Temperature changes
1
10
100
10 100 1000
Frequency (Hz)
Imp
edan
ce M
agn
itu
de
(Oh
ms)
Z 20deg C
Z 70 deg C
Z 120 deg C
Z 170 deg C
Z 220 deg C
Z 270 deg C
Power compressionPower compression
• Power compressionPower compression
On Axis SPL vs. Freq showing compression vs temperture and frequency
60
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70
75
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90
95
100
10 100 1000
Frequency (Hz)
Mag
nit
ud
e S
PL
(d
B) SPL 20deg C
SPL 70 deg C
SPL 120 deg C
SPL 170 deg C
SPL 220 deg C
SPL 270 deg C
Power compressionPower compression
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
Power compression mismatchPower compression mismatch
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freq
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freq
No compressionNo compression Compressed tweeterCompressed tweeter60
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10 1000 100000
Frerquency
Power compression mismatchPower compression mismatch
0
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freq
No compressionNo compression Compressed wooferCompressed woofer
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freq
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Power compression matchedPower compression matched
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freq
No compressionNo compression60
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0
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freq
Both compressedBoth compressed
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).
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
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