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Importance of the Impedance Curve


DATS V2 from www.trueaudio.com


Bass Refelx Alignment
Impedance Curves are essential for the vote of Bass Reflex enclosures.

How to measure the speaker impedance

Speaker impedance is not a single value, instead, it changes with frequency. This means that you will need a graph to accurately tell the impedance of a driver. Also, since the enclosure will affect the properties of the speaker.

The BassRelex alignment is through the impendance minimum between the two impedance peaks defined and therefore accurately determined only with a impedance measurement.

The formular:

fb = 48.4 * D / Sqrt(V * (L + 0.8 * D))

  • fb = tuning frequency [ Hz ]
  • D = Port diameter [ mm ]
  • L = Portlength [ mm]
  • V = Enclosure volume [ liters ]

Depending on how the 2 peaks compare to each other, in terms of height,
we can consider 3 particular cases:

  1. The two peaks match in height.
    This means that the resonant frequency of the driver in free air (fs) matches the resonant frequency of the box (fb).
  2. The first peak is higher than the second means that fb > fs.
  3. The second peak is higher than the first peak means that fb < fs.

Depending on how reactive the speaker system is, the phase angle will take different values.
The phase angle will have 0 degrees at the resonant frequency and at the point where the impedance starts to rise due to voice coil inductance.

Phase angle characteristics:

  • In a purely resistive circuit, the current and voltage are in phase, resulting in a 0 phase angle.
  • In a capacitive circuit, the current will lead the voltage, resulting in a negative phase angle.
  • The current will lag behind the voltage, in an inductive circuit, resulting in a positive phase angle.


The measured Reflexport alignement is 30.0 Hz


The Reflexport frequency is measured at 0 degrees Phase.
  • D = 50 mm
  • L = 110 mm
  • V = 28 liters
fb = 48.4 * 50 / Sqrt(28 * (110 + 0.8 * 110)) = 37.34 Hz

The difference to the measured Portfrequency is 37.34 Hz


Calculate fb
Port diameter [Pd]  [ mm ]  
Port length [Pl]  [ mm ]  
Enclosure volume [Vb]  [ liters ]  



Port alignment frequency  (fb)  Hz




Enclosures Leaks
Impedance curves are helpful to find enclosures leaks or too little, too much or incorrectly found damping material arranged.


Improperly damping: Enclosure resonance at 220Hz


Too much or improperly arranged damping material is recognized by flattened impedancepeaks without rizing the impedance minimum inbetween


Leaks in vented systems make themselves felt through a less than optimal pronounced minimum between the impedance peaks.


Leaky enclosure

Here we have by not properly tightened the chassis screws a leaky enclosure produced.
The impedance peaks becomes flatter, the minimum between them increases.



Crossover Errors

A circuit error in the crossover is clearly show in the impedance curve of the loudspeaker.


Defect capacitor in the midrange section

A shorted capacitor in the impedance correction of the midrange driver.


Impedance measurements are an important systems tools for every loudspeaker builder

All measurement are done with ARTA




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