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
Calculate f_{b}
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