low frequency impedance issues??

newbold said:

Looking at frequency response and speaker impedence curves, it seems that the lower the note I play the more impedance the speaker exerts.

How does one overcome this in a single speaker enclosure??

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The impedance rise at low frequencies is a good thing! It makes life easier (and longer) for the amplifier, which doesn't have to deliver as much current into a high impedance load so it doesn't run as hot as it would otherwise.

No need to "overcome" it; it's already been taken into account in the design stage, and is taken into account by whatever speaker design calculations or software you may want to use.

The very fact that you noticed the speaker's bass impedance peak and it alarmed you shows that your eyes and ears are open and you're THINKING about the technical aspects here. Good job!

What you see in your image is the free-air resonant peak. In a sealed box system, that impedance peak would be somewhat higher in frequency because the box will raise the speaker's resonant frequency.

In a vented box, you'd have twin impedance peaks with the valley between them centered on the port tuning frequency. Here's what the impedance curve of a vented system might look like:



Sometimes the twin peaks are the same height, and sometimes the other peak is taller. In this case, the relative heights of the impedance peaks indicates that the enclosure is tuned somewhat lower than optimum (in this case, about 27 Hz.). This curve is from a home audio speaker, not a bass cab.

newbold said:

How does one overcome this in a single speaker enclosure??

Click to expand...

One doesn't. It's supposed to do that.

The speaker impedance isn't an issue.... In an open back guitar cab, the impedance peak actually gives the amp the low boost that makes it sound good, and "live", instead of flat and trebly.

It's just a fact, not a problem.

If the system sounds good, don't sweat it.

The curve you see in the chart is the open air impedance.
It has little resemblance to the curve present in a vented box.

The impedance drops to its lowest point at the tuning frequency in a vented box. This comes very close to the DCR (DC resistance) of the voice coil, but never lower.

This is the frequency where the amp can draw the most power.

That impedance peak occurs at the resonant frequency of the speaker.
This is the frequency at which the speaker moves most freely.
Because it moves most freely at this frequency, this is the frequency
at which it needs less power -- so the high impedance at this frequency
is not a problem.

If you want to get a little more technical about it, the causal
relationship is actually the other way around. That is, the impedance is
high at that frequency is actually because the speaker moves most
easily at that frequency. It is this lack of physical resistance that
creates a higher reverse emf (voltage) which presents a higher
electrical resistance (impedance).

Bottom line: that impedance peak is not a problem.

bgavin said:

The impedance drops to its lowest point at the tuning frequency in a vented box. This comes very close to the DCR (DC resistance) of the voice coil, but never lower.

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Ah, just think if it did go lower. Playing the right note could let the cab put power back into the grid!