The objective of this lab activity is to measure the impedance profile and the resonate frequency of a permanent magnet loudspeaker.
The chief electrical characteristic of a dynamic loudspeaker is its electrical impedance as a function of frequency. It can be visualized by plotting it as a graph, called the impedance curve.
The most common type of loudspeaker is an electro-mechanical transducer using a voice coil connected to a diaphragm or cone. The voice coil in moving coil loudspeakers is suspended in a magnetic field provided by a permanent magnet. As electric current flows through the voice coil, from an audio amplifier, the electro-magnetic field created by the current in the coil reacts against the permanent magnet's fixed field and moves the voice coil (also the cone). Alternating current will move the cone back and forth. The movement of the cove vibrates the air producing the sound.
The moving system of the loudspeaker, including the cone, cone suspension, spider and the voice coil, has a certain mass and compliance. This is most commonly modeled as a simple mass suspended by a spring that has a certain resonant frequency at which the system will vibrate most freely.
This frequency is known as the “free-space resonance” of the speaker and is designated by FS. At this frequency, since the voice coil is vibrating with the maximum peak-to-peak amplitude and velocity, the back-emf generated by coil motion in a magnetic field is also at its maximum. This causes the effective electrical impedance of the speaker to be at its maximum at FS, known as ZMAX. For frequencies just below resonance, the impedance rises rapidly as the frequency approaches FS and is inductive in nature. At resonance, the impedance is purely resistive and beyond it, as the impedance drops, it looks capacitive. The impedance reaches a minimum value, ZMIN, at some frequency where the behavior is mostly (but not perfectly) resistive over some range of frequencies. A speaker's rated or nominal impedance, ZNOM, is derived from this ZMIN value.
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.
Dynamic loudspeakers are abysmally inefficient electro-mechanical conversion devices, as 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).
To help understand the measurements we are about to make, a simplified electrical model of a loudspeaker is shown in figure 1.
Figure 1. Loudspeaker Impedance Model
The circuit in figure 1 has a dc resistance placed in series with a lossy parallel resonant circuit made up of L, R, and C, which models the dynamic impedance of the speaker over the frequency range of interest.
• Rdc is the dc resistance of the loudspeaker as measured with a DC ohm meter. The dc resistance is often referred to as the DCR in a speaker/subwoofer data sheet. The dc resistance measurement is usually less than the driver's nominal impedance ZNOM. Rdc is typically less than the specified loudspeaker impedance and the novice loudspeaker enthusiast may be fearful that the driver amplifier will be overloaded. However, because the inductance (L) of a speaker increases with an increase in frequency, it is unlikely that the driver amplifier actually sees the dc resistance as its load.
• L is the voice coil inductance usually measured in millihenries (mH). Typically, the industry standard is to measure the voice coil inductance at 1000 Hz. As frequencies increase above 0 Hz, there is a rise in impedance above the Rdc value. This is because the voice coil acts as an inductor. Consequently, the overall impedance of a loudspeaker is not a constant impedance, but can be represented as a dynamic profile that changes with input frequency as we will see when we make measurements. Maximum impedance, ZMAX, of the loudspeaker occurs at the resonant frequency, FS, of the loudspeaker.
• FS is the resonant frequency of a loudspeaker. The impedance of a loudspeaker is a maximum at FS. 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 FS so that the two work in unison. As a rule, a speaker with a lower FS is better for low-frequency reproduction than a speaker with a higher FS.
• 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’.
ADALM2000 Active Learning Module
1 - 100Ω Resistor (or any similar value)
1 - Loudspeaker, it is best if the speaker is one with a cone diameter larger than 4 inches such that is has a relatively low resonant frequency.
Build the circuit shown in figure 2, preferably using your solder-less breadboard. The loudspeaker can be in an enclosure or not.
Figure 2: Speaker measurement set up
Figure 3: Speaker measurement setup for VL and IL
In Scopy, start the Signal generator and generate a sine waveform with 8V peak-to-peak amplitude and 100 Hz frequency.
Start the Voltmeter and set both channels to AC (20Hz-800Hz). Using the Voltmeter tool we can calculate the speaker impedance Z at a single frequency by dividing the RMS voltage across the speaker (channel 1 RMS voltage) by the RMS current through the speaker, (channel 2 RMS current). The RMS current can be computed as the RMS voltage on channel 2 divided to the parallel equivalent resistance of R1 and R2. Try setting the signal generator to a few different frequencies and see how the voltage across the speaker and the calculated Z changes.
Figure 4: RMS voltage across the loudspeaker
You can plot the calculated impedance Z vs Frequency. The frequency of the signal generator is set in steps of 100 Hz and for each frequency you compute Z. The speaker impedance is small, approximately equal to the DC resistance in the linear region but is much higher at the resonance frequency FS. An example plot is shown in Figure 5. Your speaker will probably look different than this.
Figure 5: Example Plot of Calculated Impedance
Figure 6: Breadboard Connections for plotting the frequency response
In the Network analyzer tool you will do a logarithmic sweep. Set the start frequency to 100 Hz and the stop frequency to 1 kHz. Set the phase to vary from -30 to 30 degrees and the magnitude from 0 to 10 dB.
Figure 7: Frequency sweep of the loudspeaker circuit
Based on your measured data extract the L C and R for the speaker electrical model shown in figure 1 for the speaker you used. You can measure Rdc with a DC ohm meter if you have one available. Ignore LINPUT as it will be small compared to L. Enter these values in to a circuit simulation schematic of the model and generate a frequency response sweep from 10 Hz to 1 KHz and compare your model to the data you measured in the lab.
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