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:
f_{b} = 48.4 * D / Sqrt(V * (L + 0.8 *
D))
f_{b} = 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:
The two peaks match in height. This means that the resonant
frequency of the driver in free air (f_{s}) matches the
resonant frequency of the box (f_{b}).
The first peak is higher than the second means that
f_{b} > f_{s}.
The second peak is higher than the first peak means that
f_{b} < f_{s}.
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.
The difference to the measured Portfrequency is 7.34 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