AboutCleggsan Expertise Audio Design Engineer; systems, loudspeakers, cd/dvd players, etc. I am knowledgeable in all high end audio equipment and installations. MAKE SURE YOU GIVE A MAKE AND MODEL NUMBER AND CAR IF IT IS AN AUTO QUESTION.
Experience Have been associated with audio design, standards, testing and component systems for over 40 years.
Organizations Life Fellow Member of Audio Engineering Society
Life Senior Member of Institute of Electronic and Electrical Engineers
Publications Review technical manuscripts for IEEE Spectrum
Write occasionally for high-end audio publications
Education/Credentials BSEE/MSCS/MBA
Past/Present Clients Past Employee/advisor to several well known audio products manufacturers
Question Why does a speakers impedance vary with different frequencies,(i hear it does down as the frequency goes down)so why when I looked at one chart the speakers was: [20ohms@75hz] and [50ohms@30hz]?
Answer The speaker's voice coil has inductance due to it being a closely wound coil embedded in a magnetic field.
The reactance or impedance of an inductor is expressed as:
XsubL = 2 x pi x f x L = (2)(3.14)x frequency x inductance L.
Therefore, a pure inductor will double its impedance as the frequency rises an octave or frequency doubling.
The loudspeaker voice coil has some resistanc; usually just slightly less than the impedance rating. A typical 8 ohm speaker, for example, will have a dc resistance of about 6 to 7 ohms. The impedance curve of this typical speaker will show the impedance rising linearly once the frequency rises enough for the inductive effect to become much greater than the dc resistance, but at low frequencies the impedance will stay close to the resistive value until the speaker approaches its resonant frequency which then rises due to the acoustic properties of the cone under air pressure which cause mechanical loading on the cone system and makes the impedance rise. See:
