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| university:courses:alm1k:circuits1:alm-cir-11 [28 Oct 2018 02:54] – All new rewrite for new speaker from parts kit Doug Mercer | university:courses:alm1k:circuits1:alm-cir-11 [05 Apr 2023 18:44] (current) – [Procedure to use the ALICE Impedance Analyzer to measure speaker impedance:] Doug Mercer |
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| ======Activity: Measuring a Loudspeaker Impedance Profile====== | ======Activity: Measuring a Loudspeaker Impedance Profile, - ADALM1000====== |
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| =====Objective:===== | =====Objective:===== |
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| Knowing the resonate frequency and the minimum and maximum impedances are important when designing cross over filter networks for multiple driver speakers and the physical enclosure the speakers are mounted in. | Knowing the resonate frequency and the minimum and maximum impedances are important when designing cross over filter networks for multiple driver speakers and the physical enclosure the speakers are mounted in. |
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| | Dynamic loudspeakers are abysmally inefficient electro-mechanical conversion devices, as [[wp>Loudspeaker|Wikipedia]] substantiates: “Loudspeaker efficiency is defined as the sound power output divided by the electrical power input. Most loudspeakers are inefficient transducers; only about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to acoustic energy.” Yes, 1% is an abysmal efficiency as we try to “be more green”, but so far it is the best we have that sounds good (and each dynamic transducer only sounds good over a limited frequency range). |
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| ====Loudspeaker Impedance Model==== | ====Loudspeaker Impedance Model==== |
| • F<sub>S</sub> is the resonant frequency of a loudspeaker. The impedance of a loudspeaker is a maximum at F<sub>S</sub>. The resonant frequency is the point at which the total mass of the moving parts of the loudspeaker become balanced with the force of the speaker suspension when in motion. The resonant frequency information is important to prevent an enclosure from ringing. In general, the mass of the moving parts and the stiffness of the speaker suspension are the key elements that affect the resonant frequency. A vented enclosure (bass reflex) is tuned to F<sub>S</sub> so that the two work in unison. As a rule, a speaker with a lower F<sub>S</sub> is better for low-frequency reproduction than a speaker with a higher F<sub>S</sub>. | • F<sub>S</sub> is the resonant frequency of a loudspeaker. The impedance of a loudspeaker is a maximum at F<sub>S</sub>. The resonant frequency is the point at which the total mass of the moving parts of the loudspeaker become balanced with the force of the speaker suspension when in motion. The resonant frequency information is important to prevent an enclosure from ringing. In general, the mass of the moving parts and the stiffness of the speaker suspension are the key elements that affect the resonant frequency. A vented enclosure (bass reflex) is tuned to F<sub>S</sub> so that the two work in unison. As a rule, a speaker with a lower F<sub>S</sub> is better for low-frequency reproduction than a speaker with a higher F<sub>S</sub>. |
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| • R represents the mechanical resistance of a driver's suspension losses. | • R represents the mechanical resistance of a driver's suspension losses. Part of the “mechanical resistance” in the system is the resistance of the cone to moving through the air, which happens to be the mechanical process that produces the pressure variations that we perceive as ‘sound’. |
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| ====Materials:==== | ====Materials:==== |
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| Try setting channel A to a few different frequencies and see how the voltage across the speaker and the calculated Z changes. | Try setting channel A to a few different frequencies and see how the voltage across the speaker and the calculated Z changes. |
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| | {{ :university:courses:alm1k:circuits1:loudspeaker_imp_bb.png?300 |}} |
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| | <WRAP centeralign>Figure 4, Breadboard Connections</WRAP> |
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| ====Procedure to use the ALICE Bode Plotter:==== | ====Procedure to use the ALICE Bode Plotter:==== |
| {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen1.png?600 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen1.png?600 |}} |
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| <WRAP centeralign>Figure 4 Example frequency sweep</WRAP> | <WRAP centeralign>Figure 5 Example frequency sweep</WRAP> |
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| By saving the data as magnitude the signal generator amplitude (in volts rms) is saved to the file. You can calculate the magnitude of the speaker impedance Z is by dividing the voltage across the speaker V<sub>L</sub> by the current I<sub>L</sub>. I<sub>L</sub> is of course the voltage across the resistor divided by the resistance. | By saving the data as magnitude the signal generator amplitude (in volts rms) is saved to the file. You can calculate the magnitude of the speaker impedance Z is by dividing the voltage across the speaker V<sub>L</sub> by the current I<sub>L</sub>. I<sub>L</sub> is of course the voltage across the resistor divided by the resistance. |
| Subtracting the channel B voltage magnitude values from the channel A voltage magnitude values and dividing by the 50 Ω resistor allows you to calculate the current magnitude I<sub>L</sub>. The impedance Z will be the channel B voltage magnitude divided by the current magnitude I<sub>L.</sub> | Subtracting the channel B voltage magnitude values from the channel A voltage magnitude values and dividing by the 50 Ω resistor allows you to calculate the current magnitude I<sub>L</sub>. The impedance Z will be the channel B voltage magnitude divided by the current magnitude I<sub>L.</sub> |
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| You can now plot the calculated impedance Z vs Frequency. An example plot is shown in figure 5. Your speaker will probably look different than this. | You can now plot the calculated impedance Z vs Frequency. An example plot is shown in figure 6. Your speaker will probably look different than this. |
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| {{ :university:courses:alm1k:circuits1:alm-cir-laba11-fig5.png?600 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-laba11-fig5.png?600 |}} |
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| <WRAP centeralign>Figure 5, Example Plot of Calculated Impedance</WRAP> | <WRAP centeralign>Figure 6, Example Plot of Calculated Impedance</WRAP> |
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| The speaker impedance is small, approximately equal to the DC resistance in the linear region but is much higher at the resonance frequency F<sub>S</sub>. | The speaker impedance is small, approximately equal to the DC resistance in the linear region but is much higher at the resonance frequency F<sub>S</sub>. |
| ====Procedure to use the ALICE Impedance Analyzer to measure speaker impedance:==== | ====Procedure to use the ALICE Impedance Analyzer to measure speaker impedance:==== |
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| As shown in figure 6, Channel B again measures V<sub>L</sub> the voltage across the speaker. The impedance analyzer software uses the difference between the channel A voltage and channel B voltage as well as the relative phase between the channels to calculate the impedance based the value of the combined R<sub>1</sub>, R<sub>2</sub>. | As shown in figure 7, Channel B again measures V<sub>L</sub> the voltage across the speaker. The impedance analyzer software uses the difference between the channel A voltage and channel B voltage as well as the relative phase between the channels to calculate the impedance based the value of the combined R<sub>1</sub>, R<sub>2</sub>. |
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| {{ :university:courses:alm1k:circuits1:alm-cir-laba11-fig6.png?500 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-laba11-fig6.png?500 |}} |
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| <WRAP centeralign>Figure 6, Speaker impedance measurement setup</WRAP> | <WRAP centeralign>Figure 7, Speaker impedance measurement setup</WRAP> |
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| Open the ALICE Impedance Analyzer software tool. | Open the ALICE Impedance Analyzer software tool. |
| {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen2.png?600 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen2.png?600 |}} |
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| <WRAP centeralign>Figure 7, Impedance Measurement at frequency below resonance</WRAP> | <WRAP centeralign>Figure 8, Impedance Measurement at frequency below resonance</WRAP> |
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| Now set the frequency to the resonate value you obtained from the frequency sweep. You may want to fine adjust the value to find the exact point where the reactance is zero as shown in figure 7. | Now set the frequency to the resonate value you obtained from the frequency sweep. You may want to fine adjust the value to find the exact point where the reactance is zero as shown in figure 9. |
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| {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen3.png?600 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen3.png?600 |}} |
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| <WRAP centeralign>Figure 8. Impedance Measurement at resonate frequency</WRAP> | <WRAP centeralign>Figure 9. Impedance Measurement at resonate frequency</WRAP> |
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| This result should agree with the results from the frequency sweeps. The phase angle should be small and the series resistance is now larger at about 15 Ω. | This result should agree with the results from the frequency sweeps. The phase angle should be small and the series resistance is now larger at about 15 Ω. |
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| Now set the frequency to a point above the resonate frequency where the phase is near its negative peak as shown in figure 8, 500 Hz was used here. | Now set the frequency to a point above the resonate frequency where the phase is near its negative peak as shown in figure 10, 500 Hz was used here. |
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| {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen4.png?600 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-laba11-screen4.png?600 |}} |
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| <WRAP centeralign>Figure 9, Impedance Measurement at frequency above resonance</WRAP> | <WRAP centeralign>Figure 10, Impedance Measurement at frequency above resonance</WRAP> |
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| As can be now seen from the data, the phase angle should be negative. The series resistance of the speaker is still around 7 Ω but the reactance is capacitive. | As can be now seen from the data, the phase angle should be negative. The series resistance of the speaker is still around 7 Ω but the reactance is capacitive. |
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| Explain your results based on the parallel LC loudspeaker impedance model in figure 1. | Explain your results based on the parallel LC loudspeaker impedance model in figure 1. |
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| | **Resources:** |
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| | * Fritzing files: [[downgit>education_tools/tree/master/m1k/fritzing/loudspeaker_imp_bb |loudspeaker_imp_bb]] |
| | * LTSpice files: [[downgit>education_tools/tree/master/m1k/ltspice/loudspeaker_imp_ltspice | loudspeaker_imp_ltspice]] |
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| **For Further Reading:** | **For Further Reading:** |
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| [[http://www.analog.com/static/imported-files/application_notes/236037846AN_843.pdf|Measuring a Loudspeaker Impedance Profile Using the AD5933]]\\ | [[adi>static/imported-files/application_notes/236037846AN_843.pdf|Measuring a Loudspeaker Impedance Profile Using the AD5933]]\\ |
| [[http://en.wikipedia.org/wiki/Electrical_characteristics_of_dynamic_loudspeakers|Electrical characteristics of dynamic loudspeakers]]\\ | [[wp>Electrical_characteristics_of_dynamic_loudspeakers|Electrical characteristics of dynamic loudspeakers]]\\ |
| [[university:tools:m1k:alice:desk-top-users-guide|ALICE 1.2 DeskTop Software]] | [[university:tools:m1k:alice:desk-top-users-guide|ALICE 1.3 DeskTop Software]] |
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| **Return to ALM Lab Activity [[university:courses:alm1k:alm_circuits_lab_outline|Table of Contents]]** | **Return to ALM Lab Activity [[university:courses:alm1k:alm_circuits_lab_outline|Table of Contents]]** |
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