posted
read the post that mr dank has started entitled "Impedance - Real and Imanginary" seems to be heading somewhat in this direction. and i believe that there was a post a while back about this. i ferget though =) impedance change, like. when playing music? or just when you stick the sub in? if you are talking about when u play music and stuff through it then, heat, current, voltage, size of the wave(and length), size of the box/port (spring effect), power being applied... lots of things.
------------------ I respect any sub that can make me pass out.
Dank and Audiophyle hit the nail right on the head.
The reactive portion of a speakers Z is dictated by certain aspects of the coil and the gap and the motion through the gap of the coil.
That is, as the coil moves through the magnetic gap, it acts as a voltage source. Because this voltage is always opposite in polarity to the input voltage, it acts to series-oppose the input voltage, creating a "real-time" resistance component that is the function of this "re-active" portion of the voltage.. Therefore, it is called "reactance", anti-source voltage or back EMF (electromotive force).
The coil motion is changes because the woofer excursion changes, right?
So, when you put the driver in a box, its excursion is radically different for the same power input, meaning the coil motion (and thus "reactance" portion of Z (impedance) ) is changed.
This change is based on the fact that the box changes the compliance of air on one side (vented, sealed) of or both sides of (bandpass) the driver.
This accounts for a changing resistance in boxes vs. free-air, AND different types of boxes.
ShadowStar
------------------ Still looking for that CHEAP Thunderdome :D
If the sub moves, use it. If the tweet's metal, lose it. If you car doesn't have Oz, I just can't excuse it.
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Ok, i follow what you're saying....covered that in Physics II and Circuits I last semester
so since the magnetic field and the current together create a reactive voltage depending on how fast the VC moves through the magnetic field (dependent on frequency, lower frequencies would move the cone slower...), the back-EMF would be greater at lower frequencies right?
now what i originally meant to ask, hehe, where is the impedance of a sub generally the highest in a ported box? If my reasoning above is correct (and i think my Physics book is sayin the same thing...could be wrong, not completely sober...) then the impedance at the tuning frequency would be...aw hell, i dunno....lowest?
Sheesh, thought i had something going there and lost it....
------------------ Team "Under Pressure"
wall w/three shocker15's and a cadence a7hc...in a tiny corolla
[This message has been edited by alaska (edited 01-23-2001).]
posted
Impedance is a function of frequency. You have to do an impedance plot. I use the woofer tester from parts express. works really good. Also, the tuning and impedance plot change big time once the box is in the car, and changes everytime you roll the window down or open a door.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
The imp. changes with the frequency. It also changes with encloser. When the cabinet "unloads" the woofer goes crazy. It sound terrible and give a high SPL. The imp. Is how much force that oppose the speaker to move along with the dc resistance. Some speakers of 4 ohm resistance can go up to about 80 ohms. These really help keep the load off the amp. A smaller speaker magnet general produce a smaller jump. This jump is the frequency of resonance. Yea that point SPL guys LOve. If you make a true one note wonder, you could power a dozen plus speakers in parallell with one amp. They would have to have a very sharp passive crossover though.
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Posts: 1147 | From: Rowlett, TX, USA | Registered: Jun 2000
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posted
A high SPL when the woofer "unloads"? Hmmm, haven't seen that effect before, i know when mine unloads (unloading being the where the acoustic spring within the enclosure is no longer strong enough to safely control the movement of the cone) it sounds like ass and doesn't hit for crap.
And why would you want to keep the load off of an amp? If it is designed to be stable at one ohm you'd want to run it as close to that impedance as possible so that you're getting full power out of it.
I'm confused, the jump you speak of, are you talking about the frequency response or the impedance curve of the sub? If you mean the impedance, i thought i was just told in another post here in Advanced Topics that the impedance was lowest at the tuning frequency...if you mean freq. response, someone please explain why the smaller magnet would produce a smaller spike. I'm sure there are subs out there with large magnets that don't have a massive spike in the response...
I've seen eight 15's run off of a single Hifonics amp and it was in no way a one note wonder (was a sealed enclosure), you can run multiple subs off of a single amp but the power will be evenly distributed between the subs (600W, 6 subs, 100W/sub), and i think i have heard that this is where the dampening factor can come into play (correct me if i'm wrong someone please).
------------------ Team "Under Pressure"
wall w/three shocker15's and a cadence a7hc...in a tiny corolla
posted
the magnet only controls the hump the higher Q which is caused by the bigger, better stronger magnet is in the impedence and not the freq response. Yes, I would want a big magnet too!
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Posts: 1147 | From: Rowlett, TX, USA | Registered: Jun 2000
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quote:Originally posted by oldtimer: The imp. changes with the frequency. It also changes with encloser. When the cabinet "unloads" the woofer goes crazy. It sound terrible and give a high SPL. The imp. Is how much force that oppose the speaker to move along with the dc resistance. Some speakers of 4 ohm resistance can go up to about 80 ohms. These really help keep the load off the amp. A smaller speaker magnet general produce a smaller jump. This jump is the frequency of resonance. Yea that point SPL guys LOve. If you make a true one note wonder, you could power a dozen plus speakers in parallell with one amp. They would have to have a very sharp passive crossover though.
Active X-over surely If you tune lower, are the impedance peaks less extreme? Just seems that low tuned boxes have very flat response curves. Also do teh box plotting programs assume linear input power or linear input voltage? Colin
quote:Originally posted by mr.dank: umm, The stronger the magnet, the lower the Q. The lower the Q, the more control the coil has on the cone.
Your right! I got that 100% backwards!
Response at resonance= 20 log Q
So a speaker of Q=2, the output at resonance is 16 times what it would be for a speaker of Q of 0.5 ! Thats 12db right off the top if my old mind still works right.
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Posts: 1147 | From: Rowlett, TX, USA | Registered: Jun 2000
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