posted
Hey alaska, I don't work for any companies, I build amps from scrap and scratch. Mostly monoblocks for home theater subs. My specialties include box design, amp design and crossover design.
So, let me get this straight. Say I do some tests in the garage, i find that with a reference level of 90 dB at 100 Hz, the highest spl I get is 110 dB at 45 Hz. Now, I have determined the tuning of the box at 35 Hz. Finally, I decide to find the imedance of the sub in-box at all frequencies. To my amazement, the highest impedance is 45 Hz and happens to be well over 20 ohms. WTF??? My loudest frequency is showing a 20 ohm load to the amp, damn. Then I find that at tuning, 35 Hz, the impedance is just under 2 ohm. When I go to my local comp, I play a 35 Hz burp as loud as possible and I get a whooping 120 dB's and I blow both my subs.
electrically speaking, tuning is the best freqeuncy to play since the impedance is low. Well, it turns out that cone motion is low and port output is at max. Burping at this frequency will probably overheat the coils(even if just for a minute). Yet burping at tuning will often yield a lower score than burping above it. Why do you want to play the frequency with the lowest impedance if you already know the loudest to start with. If anything, switch amps to something that puts out max power at 4 ohm to take advantga of the power.
All that just to say, once the box is built and you have no intentions to change it, you're screwed. One last time, you cannot figure out your loudest frequency from in-box impedance, power output and some good graphs. Especially in-car.
Get a meter, run your amp just before clipping and run through the frequencies. This is the only trustworthy way to get the highest output and win comps.
The main reason I'm against this thread is because noone has given tested and true t/s specs for the woofer in question(especially both ShockerII 15's in a common chamber reflex). If this were the case, I could have given you an in-box impedance graph which would be accurate but not exact. Even then, it serves no purpose exept to give you more information and help you understand the box function in more detail.
Besides, good luck and don't give up the pursuit of knowledge.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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posted
emu... after reading what you wrote i'm not sure if you and alaska are on the same wavelength.. it seems your both saying the same thing.. but differntly, I don't think he wants to play the lowest impedance freq at the comp I think he wants to find out what his peak freq impedance is.. (alaska.. if your not doing that, you should be) from my point of view you want to match your amp to the ACTUAL impedance of the driver at the freq you burp.. IE if you have a dual 2 ohm sub and an amp that works at 4 ohms.. well for daily driving your gonna series the coils.. BUT in the box in the car, at your peak freq your impedance might be 4 ohm or higher with the coils in PARALLEL, so you'd want to parallel them and run it at a nominal imp of 1 ohm on your 4 ohm amp (burping may not work on all amps, but ramping will on pretty much any..) so instead of having 16 ohms (pair of 8ohm at peak freq series'd coils) you'd have 4 ohms.. that's a power factor of 4 which may give you 6db.. so in my opinion this is one of the most useful (and cheapest!!!!!) things you can try and do to get louder.. my next subs will be quad 1 ohm coils, so ideally I'll have a fair bit of impedance rise to give me a useable load on the amp for a burp at my peak freq, instead of my situation right now where I'm running 4 dei 600ds each pair bridged into "around" 6-7 ohms, that is NOWHERE near the max power I could be getting out of them.. (the following numbers are estimates) 2 ohm bridged 1200watts @ 12v 4 ohm bridged 800watts @ 12v 6 ohm bridged 650watts @ 12v that gives me 1300watts total on my DD, while this is a bunch of power, it may be possible to double the output if the load were lower, and beefing up the electrical system so the amps see 14+v instead of 12 may increase that too.. I could see 4-6db with some small changes...but that's just theory.. but without theory and ideas how are we gonna get louder?? ok.. I've rambled on enough, I hope to be putting these theories into practice soon, so I'll tell you if it works...
-------------------- Beyond Audio Inhuman 12 (x2) -- US Amps 2000x -- Pioneer DEH-P9300 -- MB Quart components -- Orion xtr 500.4 -- RW Audio LD6.24MS"burp" -- 151.3 & climbing -- Member of Team Hertz Canada -- robin@rwaudio.com Posts: 735 | From: Calgary AB | Registered: Oct 1999
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emu...i think RW's right, we're sayin the same thing just differently. and yes RW i'm testing at the frequency i peak at.
i'm not actually plotting the entire impedance range of my sub, just at the loudest frequency. because i'm running my amp at 2 ohms when it does max power at 1 ohm, and the fact that subwoofer impedance rises when put in a box, i figured that the actual impedance my amp was seeing was higher than what it puts out max power to.
now, if the impedance rise is high enough, i can rewire my subs from a 2 ohm nominal impedance to a .5 ohm nominal impedance. why? because the in-box impedance of my subs at that note is high enough (at least .5 ohms higher) so that when the amp is burped it sees at minimum of 1 ohm (.5 nominal + .5 actual = 1ohm) which is within specified operating parameters for that amp. also, since the impedance is lowered i will gain more power which can help with SPL levels, especially those at lower levels. i mean, an extra 500W added to 5000W isn't gonna make much of a difference. but 500W added to 1500W can... understand what i'm trying to say? i could reduce impedance by increasing port area but i don't really have the room to do it so i'm just gonna reconfigure my subs to a .5 ohm load for competition use only.
are we on the same wavelength now?
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
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Elemental Designs
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posted
well i already said this...but..there is a very competitive ss3-4 competiter who has figured his enclosureand woofer set up to rise to 5 ohms when wired to 1ohm,when he hit his freq..so he is runnin 4 ohm amps..so he is getttin almost full power outof them..so..check this..if he had the amp wired at the recamended 4ohm..the when he hit his freq..he would havelikea 9ohm load at his feq..so he would only bve gettin half power...maybe this explanation will help??? i dunno
-------------------- Alan Hall Broken Silence CC 2007 usaci mod 1001-2000 champion 163.6 2003 usaci mod151-300 legal record holder 4th 2002 usaci finals mod 0-150 6th 2003 usaci finals mod 151-300 4th 2004 usaci finals prostock 1001-5000 4th 2005 usac finals mod 301-900 Posts: 667 | From: Troy IL | Registered: Apr 2000
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posted
posted by alaska: "i'm testing at the frequency i peak at."
damnit, all that bickering for nothing. It's probably because I don't re-read the old posts. Anyways, keep it up, you're on the right track.
Two ways of testing, use a resistor in series with approximately the same resistance, about 10 ohms. You can compare voltages to figure out impedance. using the formula.... speakerimpedance = v(across speaker) * 10 ohms (test resisotr) / v(across resistor).
or using a 1000 ohm resistor in parallel, you can find this setup almost everywhere.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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posted
hehehe, i agree, all that arguing for nothing...when i argue i at least like it to be worthwhile lol
how much power should those resistors be able to handle to allow me an accurate result? and where's a good place to find em...partsexpress.com?
thx
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist.
posted
Just a note, Ive heard that adding a resistor in series changes the electrical Q of the speaker and thus the response...
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
In fact it will, but not enough to screw up the readings.
You can use a 1W power resistor, just test at a reasonable volume(read, if the resistor is burning hot, turn it down). You can pick them up at pretty much any electrical store.
I use both methods for testing. For quick tests, i use the resistor in series. If I'm taking my time, it's a 1000 ohm resistor in parallel. This is the more common setup you'll see but longer to setup and it assumes the amp output is linear and constant amplitude.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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