there is generally a saddle shape in the impedance curve where the low point between peaks is usually the port frequency.
6 responses Add your response
Yes, if you mean a speaker driver. A driver's impedance, by definition, is a change in resistance at different frequencies at constant voltage. While impedance depends on electrical factors, the driver's surrounding also affects the impedance.
A driver in free air will be at its highest impedance at its resonance frequency. When it is in an enclosure, the resonance frequency shifts to a different value, which depends on the enclosure size. The new rf will have a different impedance, which is much lower than in free air.
A ported type enclosure will cause the driver to have its lowest impedance at resonance - whereas in a sealed box the same driver will have its highest impedance.
There are HUGE differences in cabinet design vs impedance at resonance. Much of this has to do with the electrical characteristics of the drivers selected, the alignment of the box and the type and density of materials used to damp the internal cavity of the box.
I discussed this in a very long and heated thread pertaining to sealed vs ported designs on the Speaker Asylum with Bobby Palkovic of Merlin and Dan Wiggins of Adire Audio.
There is a wealth of information contained in this thread that you won't find anywhere else in print form, so take your time when reading it. I have had people email me and tell me that they learned more about bass / speaker performance / amplifier compatibility from that specific thread ( and a few others that were taking place at the same time ) than anything that they've ever read before. This is probably because i contradict and "explain away" what is a phenomenally common audio myth. That is, most people ( even speaker designers & EE's ) equate a higher impedance speaker or a speaker with a bigger impedance peak at resonance with more control due to an improved "damping factor". Damping factor is the ratio of the speaker's impedance to the output impedance of the amplifier it is connected to. Since a higher damping factor is what most people have read is "good" in most textbooks and audio articles, that is what they believe. This is completely WRONG in some situations i.e. "backwards thinking" and i explain why in this thread. The points that i bring up in this thread are highly controversial, but are based on logic and basic electronics.
If you have ported or passive radiator based speakers, be prepared to have your toes stepped on in a major fashion. You won't like what you read. The more that you spent on them, the less you'll like it. Facts and reality can be a real kick in the teeth sometimes. Sean
PS... I tried to keep things ASAP ( as simple as possible ) on this thread for those that wanted to follow along with what is a complex subject. This gives one the ability to learn as much as they can while having fun and watching the fur fly at the same time : )
Thanks for all the responses.
Thinkat and Bombaywalla, you guys confirm what I observed, so now the question is: why does it do that?
Piezo, If what you say is true, then what could account for the correlation between impedance and port tuning?
Gs5556, is this to say there is a relationship between physical impedance and electrical impedance?
Sean, I'm still reading - thanks,