RMS and Box Sizing

Systems

Guys, just need to clarify two things:

1) What is the relationship between RMS and Peak Power Handling, since Ive seen some set ups with low RMS subs being powered by higher output Amps anyway? (ie, JL's with 120W RMS powered by 500W Amps).


2) What are the consequences of using a box that was slightly smaller than recommended?

exalted512

most quality speakers can be over powered if its from a clean source...if its distorted power then they are likely to blow
a smaller box will not hurt sound quality that much, but it will limit the low end of the frequency range a little--it will also not be as loud as a bigger box
-Cody

MacLeod

Well, actually a smaller box will yield more db's than a larger one. However it will come at the expense of clarity and overall SQ.

As far as overpowering speakers, if you hook up a 120 watt sub to a 500 watt amp and play it at near full blast youll fry that sub. Now you take the 120 watt sub and use the 500 watt amp carefully and with a little common sense in the gain adjustments youll power the sub properly with a lot of headroom to spare.

exalted512

Originally posted by MacLeod
Well, actually a smaller box will yield more db's than a larger one. However it will come at the expense of clarity and overall SQ.

how so?
a smaller box will limit the amount of x-max, which in turn reduces the Dbs, also, a smaller box will increase motor strength which means it can control the sub better and will improve sound quality
-Cody

LittleCar_w/12s

I beleive Cody is right on this one, a smaller box is better for SQ, but makes your sub require more power to get the same SPL output. Because the displacement of a sub is designed to have a certain ratio to the enclosure, what you are doing is decreasing that ratio, which increases the air pressure, or SPL inside the box. This makes a more balanced driver, however it also makes it less efficient.
-Jerry

neomagus00

the efficiency of a slightly smaller box is indeed lower than a larger box, but should have no other adverse effect (except a minor effect on sq, perhaps better, perhaps worse). on the amp - the power output of any amp-driver system is limited by two things - the amp's power capability and the driver's. you have to ensure two things - first, that the amp isn't driven to clipping, and second, that the driver isn't pushed beyond its excursion limits (well, you also have to ensure that it's not pushed beyond its thermal power limit, but unless you're putting it in a crazy small sealed box, this shouldn't be an issue). since that 500 W (assuming RMS) amp is obviously more powerful than the 120W RMS driver, all you have to ensure is that the driver isn't pushed beyond its excursion limits (ahem, cody...)

exalted512

Originally posted by neomagus00
all you have to ensure is that the driver isn't pushed beyond its excursion limits (ahem, cody...)

i have no idea what youre talking about

Originally posted by LittleCar_w/12s
I beleive Cody is right on this one

im with him;)
-Cody

PoweredByDodge

a smaller box will infact increase relation of rest-position restoring vaccuum to X-axis movement of the subwoofer piston.

The higher the vaccuum, the "tighter" your music will sound (speaking for an average 'good quality' sub -- extremely high quality speakers can run infinite baffle and sound just as good - but nobody in here can afford that kind of equipment).

Anyway - as long as your enclosure isn't too small, then you wont choke off the motion of the speaker, you'll increase "virtual mechanical damping" (god i hate making up words) and you can dump a little more power into them.

MacLeod

Originally posted by PoweredByDodge
a smaller box will infact increase relation of rest-position restoring vaccuum to X-axis movement of the subwoofer piston.

Ummm...so does that mean it will be louder?

Its obvious that you guys are way more versed in the physics of this than I (no smartass tone intended) but here is what Ive based my statement on.

Orion specs for the P10 is .5 ft3 for SQ and .4 for SPL
MTX specs for the T6000 is .5 for SQ and .28 for SPL
Kicker specs for the L7 8 is .75 for SQ and .33 for SPL

Ive also experienced it first hand with my current MTX 8". I first built a box at .37 or so but tossed it because it left me no room under my seat. I then made a box at .29. The output was noticeably louder but was a little sloppier. Now I know there are a host of other reasons for this but my own common sense deducted it was because of the increase in enclosure size.

Ive also heard it said from time to time over the 15 years Ive been piddling with car audio that smaller is louder.

Now Ive never been told this by an expert or physicist or anything Im just going off what Ive read and heard over the years and also based on my own personal experience so I could be wrong. It wouldnt be the first time. And no Cody, Im not wrong about JL.

neomagus00

yes, the rest position vacuum will be greater, but so will the rest position pressure when the driver is forced the other way... i thought about saying this before, but decided against it because i thought that it would have no net effect - that is, the air in the box pushes against the driver harder when it's compressed, and pulls it more when it's rarefied. net result... maybe more of a pressure difference (bigger pressure differential, bigger SPL), maybe not, i dunno

MacLeod

Originally posted by neomagus00
yes, the rest position vacuum will be greater, but so will the rest position pressure when the driver is forced the other way... i thought about saying this before, but decided against it because i thought that it would have no net effect - that is, the air in the box pushes against the driver harder when it's compressed, and pulls it more when it's rarefied. net result... maybe more of a pressure difference (bigger pressure differential, bigger SPL), maybe not, i dunno

Huh?!?!? You guys are making my eyes cross! LOL

So does this mean I was right or wrong??

Me not know words with many letters. Me know if loud or not loud.

Joelsbass

Bigger box = bigger volume @ lower power

when you go with a smaller box you do increase the pressure in both directions... there's more pressure pushing the sub back out and more pressure pulling it back in. This means that it also takes more force (aka power) to move the sub to its limit... lets say you wanted 20mm of excursion each way. it's going to take more power in a smaller sealed box to do this than in a larger sealed box because the ratio of the air displacement caused by the cone moving / the total volume of the box would be greater in a smaller box. basically the displacement of the moving cone is going to be the same in either box if you're going the full 20mm. however pressure would be greater in a smaller box because it has less internal air to work with. thereby requiring greater power to counteract this pressure... that's why Adire offers box size based power ratings for their subs... take the 10" brahma for instance... it takes 1000w of power to reach full excursion in a .5ft^3 box... 700w in a .87ft^3 box... and 7000w in a .11ft^3 box...



so in short.... Cody needs the us-amps USA2000 for his 2 10" brahma's not the 3000

neomagus00

yeah, i get that it takes more power to push a speaker to full excursion in a smaller box, the efficiency of the system is decreased. BUT, at full excursion, which is louder? the one with more air behind it because it can move easier? or the one with less air because it snaps back to rest easier?

mac - i dunno yet, i'm trying to find out

and no, just because it's cody, he needs the 3000 normal people, 2000, cody, 3000

Joelsbass

Originally posted by neomagus00
and no, just because it's cody, he needs the 3000 normal people, 2000, cody, 3000

yeah i guess you're right... cody is "special" and all :rolleyes:

exalted512

wow...i have my own genre now...
lets put it this way
10mm of excursion will not be as loud as 20mm of excursion
why?
because of displacement
what can cause a decrease in excursion at the same power levels?
a smaller box
-Cody

neomagus00

yes, but here we're assuming that enough power is available to push the driver to xmax regardless of box size

and you've always been in your own genre, you're just now realizing it

and to be more precise, since it matters in this argument, the 20 mm xmax is louder because the pressure differential between the peak of the wave and the trough of the wave is greater - the cone moves twice as far in the same amount of time, thus affecting the air pressure twice as much. we percieve this as more volume.

Joelsbass

Originally posted by neomagus00
yeah, i get that it takes more power to push a speaker to full excursion in a smaller box, the efficiency of the system is decreased. BUT, at full excursion, which is louder? the one with more air behind it because it can move easier? or the one with less air because it snaps back to rest easier?

at full excursion they should be equal. the difference comes from the power.

1000w to a sub in a .5ft^3 box will be quieter than 1000w to the same sub in a .75ft^3 box

exalted512

Originally posted by Joelsbass
at full excursion they should be equal. the difference comes from the power.

1000w to a sub in a .5ft^3 box will be quieter than 1000w to the same sub in a .75ft^3 box

exactly
-Cody

MacLeod

Well I dont have any fancy terms to defend my position but I do have this spec sheet from MTX. The .13 box puts out 108 db and the .63 puts out 105 all with 150 watts. Now I know these are trivial numbers but the sub still got louder the smaller the box.

As I said before the Orion P and Kicker L7 specs say the same: SQ go larger, SPL go smaller.

If Im wrong then isnt MTX, Orion and Kicker? Or am I missing something?

exalted512

look guys---theres actually a proven law that explains this
its called hoffman's iron law
it says out of all of these...you cant have all 3...you can get a little of two, a lot of one...but not three
1) Small Enclosure Size
2) Low Frequency Extension
3) High Efficiency
if the enclosure small you are taking away efficiency and low frequency extension
the higher the efficiency the louder the sub
the more low frequency extension the louder the sub
look it up
-Cody

MacLeod

So MTX, Orion and Kicker are wrong? The MTX T6000 is actually louder in the .6 than the .25? Then why is there a 3 db difference.

Im no expert by ANY stretch of the imagination so I have to rely on those who are experts, like the engineers at the aforementioned companies. I would think they know what theyre talking about and wouldnt claim that a .25 enclosure would be louder than a .75 enclosure with the same wattage, if it werent.

Dont get me wrong, Im not saying youre wrong and Im right necessarily, Im just stating my conclusions from the evidence Ive seen. Maybe Im interpreting it wrong. Where is Geolemon when we need him?

neomagus00

Originally posted by exalted512
look guys---theres actually a proven law that explains this
its called hoffman's iron law
it says out of all of these...you cant have all 3...you can get a little of two, a lot of one...but not three
1) Small Enclosure Size
2) Low Frequency Extension
3) High Efficiency
if the enclosure small you are taking away efficiency and low frequency extension
the higher the efficiency the louder the sub
the more low frequency extension the louder the sub
look it up
-Cody

no - the higher the efficiency, the less power it takes to produce a given output, NOT necessarily the more output at xmax.

exalted512

im not understanding what youre saying neo...well i think i do...but run it by me again...i think youre misunderstanding what im saying
mac-
the difference could be is that they were louder at higher frequencies...say at 80 hz or so...maybe thats where theyre getting their numbers
-Cody

neomagus00

cody - you're almost completely right - if you can push a driver to full xmax in 1.0 ft^3 box with 100 Watts, then it'll take 200 Watts to do it in 0.5 ft^3. This is where you're right, and it'd be crazy to dispute this. BUT, given that they're both reaching xmax, they should both be giving out the same number of decibels (all other things being equal - this is where I'm confused, cause I'm wondering if all other things are equal).

my inquiry is a little more complex, cause i think that it involves the dynamics of the air suspension...

maybe... maybe it's a matter of perspective. okay, if you build a large box, it'll be able to take say 100 W to xmax at 20 Hz. Then, every note from 20-100 Hz is getting 100 W, and the point of a large box is that it allows the lower notes to play louder relative to the higher notes (louder than a small box, that is). If, however, one were to build a small enclosure that takes 200 W to achieve full xmax at 20 Hz (because the smaller enclosure controls the cone more, especially at lower frequencies), then every note gets 200 W, making your bass louder at the higher notes, but with a steeper roll-off to 20 Hz (where both boxes would, at full power, have the same output). So depending on how you look at it, everyone's right... follow?

exalted512

i never said that if you dont put more power to a smaller box that it wont be louder...if thats what you mean
i was talking to Joel the other day about how big ill be making my box when i put 3kw to each sub and its probably going to be about .5ft^3
for instance my box is going to be 1.8ft^3 when i run my subs off of the 550 watt RF amps i have, but when i have them running off of 3kw its going to be more like ~.5
then ill have a huge ported box that ill just put on top of the back seat when if i ever decide to go in a spl comp
the smaller box will probably sound better...why?
because it has a steeper slope...
you might say..."Well if it has a steeper slope then it would have worse SQ b/c its 'ideal' to have a straight line"
well...thats half right...it is 'ideal' to have a straight line...but due to the cabin gain effects of a cars interior...it ends up evening out
-Cody

Joelsbass

i thought we concluded that you wanted something closer to .35 or .4 cody? if i were you i'd run it past adire's tecs to find out what they say.....

exalted512

oo im going to...that is by no means my final conclusion
-Cody

MacLeod

Well according to that MTX spec sheet, the difference between a .13 and a .67 box are only 3 db so I would say that neither are really louder than the other at all. I mean, do you realize how small a .13 box is, and that a 3 db difference is barely audibel?!?

My conclusion.....in terms of SPL output, it doesnt really matter how big the box is. Within reason of course, we're not talking about a 4.5 box up against a .005.

Who's with me?

exalted512

JL sucks
-Cody

MacLeod

Adire sucks.

They suck so much they cant even get a real store to carry them. They have to sell out of their garage!

exalted512

JL sucks...even if you do buy their stuff and install it yourself your warranty goes out in 30 days:rolleyes:
-Cody