27
Finding the Tuning Frequency of a Ported
Subwoofer Box System (f
B
)
The f
B
is the resonant frequency of the ported subwoofer
enclosure. Measuring this will give you some idea of the
usable bandwidth of the speaker and also if the target tuning
frequency when constructing the enclosure was met.
Generally a infrasonic filter (subsonic filter) should be used
on ported subwoofer enclosures. The frequency of said filter
should be set just below the f
B
frequency.
1. To measure f
B
, make sure the room or area around the
subwoofer is quiet and free from moving air.
2. Connect the test leads to the speaker system, if it has
multiple voice coils they all need to be connected. A
parallel connection will work fine.
Make sure there is
no amplifier connected or damage to the IM-SG
could occur!
3. Turn on the IM-SG by holding the
On
button for 1
second.
4. After the unit is operating, turn the frequency down as far
as it goes. Then slowly raise the frequency until a
first
peak in impedance is found. If you go too far and pass
the peak, just back down the frequency until the peak in
impedance is found. Let us call this frequency peak
f
L
.
5. Continue increasing the frequency slowly until a
second
peak in the impedance is found. Let us call this
frequency peak
f
H
. Ported enclosures have two
impedance peaks, see Figure 5.
6. Now slowly decrease the frequency, you are looking for
the
minimum
impedance that is between
f
L
and
f
H
.
7. The frequency at which this minimum impedance occurs
between
f
L
and
f
H
is called
f
B
. This is the tuning
frequency of the ported subwoofer enclosure.
Summary of Contents for SMD IM-SG
Page 1: ...IM SG Impedance Meter Signal Generator Owner s Manual ...
Page 20: ...20 Figure 2 Impedance curve of a typical 4 ohm 1 tweeter Notice the fS at 1 59 kHz ...
Page 22: ...22 Figure 3 fC of a single 4 ohm 15 inch subwoofer in a 2 cubic feet sealed box ...
Page 44: ...44 Notes ...
Page 45: ...45 Notes ...
