Box Rise

[MisterGrubbs 5,000+ posts]

Okay, so "search" but I've been doing that for the last little bit, just recently a forum member more or less was trying to insult me and make me look stupid and made a suggestion with box rise in mind

What the hell is box rise, and what does it do?

All answers appreciated

Click to read more...

 

[SAG3 5,000+ posts]

i wasnt insulting you. back to topic.

 

[MisterGrubbs 5,000+ posts]

Irregardless, I still need to learn

Care to inform?

 

[Ali1 10+ year member]

it's something that affects your ohmload at the amp. If you take a DMM and check the ohms at the amp, you will find out that it won't be the number you wired it too. For instance, lets say you had 2 dual4ohms subs, and you wire it to 1ohm load. When you check the ohms at the amp, it will not say 1ohm. It will more than likely say 1.4-2.5ohms. So it's a b!tch because it rises your ohmload and affects how much power is the amp doing. But we all have it and there is absolutly nothing we can do about it.

 

[CBFryman2 5,000+ posts]

Box Rise...

someone is psycokenetic and can make their enclosure levitate perhaps?

If they ment Impeadance rise, it is just the fact that a Nominal impeadance rating free air is moot once you put it in an enclosure and apply voltage. Impeadance rise is affected by a lot of things. The stiffer the box the higher the pressure the bigger the impeadance spike. The more the box can flex (Bad for SQ and possible SPL assumeing the same level of power is put into the driver) the lower the impeadance rise will be. Impeadance rise can also come from a phenomenon called power compression. As the coils heat up they cause Re and inductace to rise making impeadance spike. Inductance is also a big contributor to impeadance rise, the higher the frequincy past a certian point the higher the impeadance rise will be. Impeadance is also highest at tuning in a ported enclosure and lowest at Fs (IIRC).

Impeadance rise is how people can run 1ohm nominal amps at 0.5ohm (for example) in competition with out frying the amp or sending it into protect. Impeadance rise is frequincy dependant so just because the impeadance rise is up to 2ohm at say 50Hz doesnt mean that is is up to 2ohm everywhere else. It could be 30ohm at 30Hz or it could be as low as Re (the DC resistance of the load).

And impedance rise can be a lot more than .4-1.5ohm like stated above. a 1ohm nominal impeadance climbing to ~3ohm is not uncommon...but neither is it climbing to 45+ ohm //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif.

 

[MisterGrubbs 5,000+ posts]

CB, you're saying that with every note played, impedence rise changes? Thus saying that every note more or less power is sent to the sub via the amp?

 

[mazdakid 5,000+ posts]

wow, could it really raise a sub amp up to 45ohms?

 

[squeak9798 5,000+ posts]

CB, you're saying that with every note played, impedence rise changes? Thus saying that every note more or less power is sent to the sub via the amp?

Impedance fluctuates with every note; yes. Impedance will fluctuate with frequency alone, not even taking into consideration all of the other factors like rise due to the enclosure, power, etc.

Ever take a look at an impedance graph?

http://www.madisound.com/pdf/peerless/830845.pdf

On the second page of that PDF is the impedance and FR graph. The black line (the "lowest" line throughout most of the graph) is the frequency-based impedance shift that will occur on that subwoofer.

Notice how it is NOT just a straight line. For example; at the resonant frequency of the subwoofer, the impedance spikes drastically (the reason for the huge spike down near 20hz).

Also, keep in mind that the variations in that graph relate solely to frequency, not hte other considerations like enclosure, power, etc.

Just for fun, here's another graph from a different subwoofer:

http://www.madisound.com/pdf/scanspeak/23w_4557t.pdf

Again, the "bottom most" line is the impedance. And, again, notice the huge spike at resonance, and that the line is not just a "straight line"....it's impedance shifts with frequency.

 

[squeak9798 5,000+ posts]

wow, could it really raise a sub amp up to 45ohms?

See my above post //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[MisterGrubbs 5,000+ posts]

So in other words, there is so much more shit ot learn that I had no clue existed

****IT

I thought I was getting somewhere //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

 

[CBFryman2 5,000+ posts]

wow, could it really raise a sub amp up to 45ohms?

yes, and beyond. Ive heard set ups where there where holes or dips in the bass for no particularly apparent reason. Could have been the cabbin, enclosure position, or the impeadance of the woofer had a huge spike in that frequincy band.

My Ava 18's ouput drops considetably after 50-60hz which is where inductance kicks in and makes my impeadance go through the roof. Before that it rose to almost 3ohm on a low power test, with powe compression I'd imagine it could be even higher.

 

[CBFryman2 5,000+ posts]

Squeak beat me...lol.

Good post.

 

[BassAddictJ 5,000+ posts]

So in other words, there is so much more shit ot learn that I had no clue existed
****IT

I thought I was getting somewhere //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

welcome to real car audio........ //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[MisterGrubbs 5,000+ posts]

Amen

So sooner or later I start learning the real stuff

Then I get a job at CC or somewhere and get some minor experience

Then get a job at a shop and get real experience? //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

 

[thch 10+ year member]

the spike at resonance makes sense by looking at the T/S specs. if you look closely, you'll notice that high Qms speakers have a narrow spike, while low Qms have a wide spike.

Qms is mechanical damping. basically how much energy can be bounced around at resonance. low Qms means energy is absorbed by something (mechanically). high Qms means energy moves from moving mass, to the suspension, and so on in a very efficient manner.

if you look at the SPL vs frequency, and Z vs frequency, you'll notice that the SPL level is still high, even with this lack of power. this means high efficiency. so why bother with ported boxes? why not just put a woofer in a sealed box, and get a 1kW @ 40ohm amp? well, look at excursion vs frequency -- since all sound is deliver by the cone, the speaker cannot really handel much power at that frequency even though it doesn't need much power.

a ported box has a similar point of high efficiency -- but it also has this point with a low impedance AND a cone excursion minimum. this means you have high efficiency and the ability to dump power to the speaker like no other. for SPL comps, this is great.

edit -- also, it is interesting to look at the impedance "phase" plot. impedance simply means something that impedes electrical flow. as it turns out, for AC, some elements will store energy then return it to the amp. the phase dictates how much energy is used vs bounced. notice that the impednace phase crosses 0 at Fs, where there is a peak impedance that has a width based upon Qms! neat eh?