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.
Another specification of speakers is frequency response. One might
expect a nice smooth response, not what you see in 12.
There are several sound-related specifications to take away from
this chart.
First, the resonant frequency is the bump about 350 Hz. The
lower this number is the better the bass response.
Secondly, the Sound Pressure Level (SPL) rating is usually calculated
from the average of the output level at several frequencies around
1000 Hz. This speaker is rated at 82 dB. The larger this number, the
louder the speaker will be.
12: Frequency response (sound pressure level or SPL vs.
frequency) of a typical speaker used in model railroading
(28 x 35 mm – 8 ohms).
11: Impedance vs. frequency for a typical 8-ohm speaker,
showing a resonance at about 35 Hz.
12
11
MRH-Jan 2014
