如何用SMARRT LIVE 测试阻抗曲线
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发布时间：2022-04-30 08:52
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时间：2022-06-20 04:39
1. AN INTRODUCTION TO LOUDSPEAKER IMPEDANCE
The term impedance is widely used in the professional audio instry, but frequently misunderstood and misapplied. Impedance is the total opposition to the flow of alternating current (AC current) in an electric circuit, and is a complex function of frequency as the ratio of voltage to current (Equation 1). The concept of impedance is analogous to resistance in direct-current (DC) circuits. While impedance includes resistance, it includes another element exclusive to AC circuits, reactance, which is e to the energy storage effects in AC circuits from components like inctors and capacitors,
engineering circles, impedance is thought of as a complex quantity, meaning it includes both real
(resistive) and imaginary (reactive) parts (Equation
2). It is this concept that accounts for the varying phase shift of impedance: current flows through resistive components in phasecomponents with a phase shift relative to the applied voltage. The impedance magnitude (Equation 3) contains the effects of both the resistive and reactive components, and indicates the total opposition to current in the circuit (ignoring phase). It is this magnitude function that is typically quoted in loudspeaker specifications, as it is the impedance magnitude that affects the total current required
from an amplifier when driving the loudspeaker. While the above general concept of impedance is universally used in many circuit I analysis tasks, the concept of load impedance or the input impedance of
a load (such as a loudspeaker) seen by a driving source (such as a power amplifier) is what we typically deal with when looking at loudspeaker characteristics (see Figure 1). Z
The electrical input impedance function of a real loudspeaker is defined by many factors, including electrical, mechanical, and
acoustical behavior. Electrically, the resistance and inctance of the
voice coil dominates, along with the presence of any passive crossover components. The mechanical mass, compliance (“springy-ness”), and Figure 1: resistance of the drivers form another component of the total impedance. Additionally, the acoustical impedance seen by the drivers
appears as part of the electrical impedance function, including the baffle loading effects, any loudspeaker ports, etc. All of these factors combine to create the impedance functions seen by measuring a typical loudspeaker.
Program in Architectural Acoustics, Rensselaer Polytechnic Institute, Troy, NY
Figure 2 shows the impedance magnitude-versus-frequency curve for a single low-frequency driver in both a sealed and ported enclosure. The strong dependence of impedance on frequency is easily seen. In the sealed example, the peak is created by the resonance between mechanical compliance and mass in the driver. The second peak appearing in the ported case is the acoustical tuning resonance of the vent. The rise in impedance at high frequencies is e to the inctance of the voice coil, while the minimum value of the graph is equal to the resistance of the voice coil. These characteristics are typical examples of measured data from real loudspeaker systems, providing vital information about the loudspeaker system for troubleshooting and design.
Figure 2: Input impedance of a single low-frequency driver The included bibliography lists several excellent in both a sealed and ported enclosure.
references for interpreting and applying this information.
2. IMPEDANCE MEASUREMENT TECHNIQUES
Using the concepts developed in Section 1, we can now investigate methods of measuring load impedance. Based on Figure 1, we can see that the load impedance function may be obtained directly if we are able to acquire signals representing both the voltage and the current into the load impedance over all frequencies of interest. Since computer sound cards respond to voltage signals, a signal proportional to the voltage across the load is easily acquired by simply feeding the load voltage directly to the sound card. However, other techniques must be used to acquire the current signal. The
current signal is most easily measured by inserting a shunt resistor in series with the load, creating a current shunt; the current in the load is then directly
I· proportional to the voltage across this shunt resistance
(see Figure 3). This is the method employed by most digital multimeters on the market to measure current. There are other methods of deriving the current signal, including the use of inctive current probes, etc., however, the shunt resistance method is the most Z practical technique for measuring loudspeaker impedance with SmaartLive.
As outlined above, if the voltage across the shunt Figure 3:Impedance measurement with a shunt resistor.
resistance is measured, the current may be derived.
However, if the source in Figure 3 is ground-referenced, the shunt resistor voltage is floating, so a differential amplifier must be used to directly measure this voltage. Using SmaartLive, the circuit required to use this differential (balanced) measurement technique is shown in Figure 4. The differential inputs may be provided using anything from a laboratory-grade differential amplifier to a balanced line-level input on a professional audio interface or mixer. However, for Smaart users desiring a simpler interface or lacking a differential input, a single-ended (unbalanced) measurement technique is possible, with SmaartLive calculating the exact load current internally. Both techniques have advantages and disadvantages, along with trade-offs associated with the selection of the value of the shunt resistor.
Figure 4: Smaart impedance measurement circuit with a
differential (balanced) input technique.
Table 1 compares the two measurement techniques, presenting the trade-offs associated with each. In general, for a laboratory-grade measurement solution, choose the differential method with a high-grade balanced-input preamplifier. If you desire a simple, practical solution, choose the single-ended method, being certain to adequately calibrate your measurement system appropriately. SmaartLive requires you to use a calibration resistor to calibrate the measurement configuration based on this reference resistance for maximum accuracy. This calibration resistance temporarily replaces the load impedance ring the calibration routine, which will be reviewed later.
