DCC Impulses Column - 9
From Mr. DCC’s workbench
– Speaker specifications
In this column, I’ve discussed speakers as if they had constant im-
pedance, for example 8 ohms. They don’t. Figure 11 shows a plot
of impedance vs. frequency for a typical 8-ohm speaker. While this
speaker would be too large to fit in our locomotives, the concept is
what is important. How do I know that this speaker is too large? It
has a resonant frequency in the 35 Hz range. Since one of the ma-
jor contributors to this resonance is the size of the cone, and speak-
ers that fit in locos resonate from 100 to 400 Hz, this speaker is too
large. It’s just a matter of physics.
What is important to see is that this “8-ohm” speaker ranges from
about 6-ohms to 50-ohms. So the decoder will not be seeing exact-
ly 8 ohms at more than a few frequencies. It will be close, though.
If you measure this speaker with a DC ohmmeter, you will probably
get a reading around 5 or 6 ohms.
11: Impedance vs. frequency for a typical 8-ohm speaker,
showing a resonance at about 35 Hz.
11
MRH-Jan 2014
