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Old 22nd April 2004, 01:38 AM   #1
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Default will amp drive sub?

well i have a 250w sony xplod amp MS-502z i know that it is only 120w rms and i was thinkin of buying a 10 or 12 inch sub that is 150w rms would this amp drive this sub or would it barely beat at all?
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Old 22nd April 2004, 11:09 PM   #2
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Default power ratings are mostly nonsense

Unfortunately, power ratings are mostly nonsense. OK, that's a bit exaggerated for amps, but fairly true for speakers. You don't have to match the wattages at all.

Just follow this simple rule: if it sounds bad, TURN IT DOWN!

You will rarely fry anything following that rule. If the sub is not loud enough, then most likely you need more power, unless the woofers are very inexpensive. Even in the small cabin of a car, it can be surprising how much power you need. Car subs are usually designed to use small boxes, and small boxes are simply power hungry.

Is the amp 120W RMS x 2? Or is it a mono amp? What brand(s) of subs were you considering?
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Old 22nd April 2004, 11:41 PM   #3
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Is that the sony XM-502z? 120x1 @.1% THD 4 ohms.
That should be plenty of power for a 12" sub, if you want really loud, build a ported or bandpass box..
Remember that the box has a huge effect on the volume.
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Old 22nd April 2004, 11:50 PM   #4
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its a 120wrms x 1 and i was thinkin of buying a rockford fosgate punch z sub and buying a band pass box would this beat at all?
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Old 23rd April 2004, 06:09 AM   #5
Immo_G is offline Immo_G  Australia
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You've asked the same question in 3 threads btw.
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Old 23rd April 2004, 04:23 PM   #6
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Quote:
Originally posted by stevena2006
its a 120wrms x 1 and i was thinkin of buying a rockford fosgate punch z sub and buying a band pass box would this beat at all?

No, it wouldn't beat an Aura 1808 driven by a QSC Powerlight 6.0II (7000 watts RMS, you'll need a trailer behind your car to hold the 12-to-120V inverter).

More seriously, no of course it won't "beat all"; more power and a bigger woofer/box combo will pound harder. And 120W is not a lot of power for a subwoofer.

If you want to listen to jazz, classical, light rock, and aren't trying to vibrate your car, 120W + a sealed box woofer will be fine.

If you want to POUND, then you should have at least 300-500 watts, and at least a 12" if not 15" (I'm partial to Alpine and LOVE the Type X 15") Otherwise you'll just clip the amp and burn up something.

By the way, bandpass enclosures usually sound like dog poop as far as sound QUALITY is concerned. They are very resonant on purpose, to achieve high sound QUANTITY (power output per watt). By being so resonant, the sound does not start and stop with the signal from the amp. The woofer keeps on "ringing" and the sound is very blurry/boomy. Also, the high sensitivity is over a limited frequency range, so truly low bass like organ pipes is usually not good, and the midbass rolloff phase is complex meaning it is hard to match the sound to the satellite speakers. So use a bandpass just if you want to make a lot of boom-boom-boom and don't really care about clarity.
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Old 24th April 2004, 02:34 PM   #7
bknauss is offline bknauss  United States
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150W isn't going to beat at all. I've currently got about 300W on a 10" sub in a sealed box and it only really starts performing on rap and techno. I can't complain though since I mostly listen to rock and it blends wonderfully.

Keep in mind that 150W in a car is completely different from a home sub having 150W. The same sub was being used as a sub for the home theater in the same box, and I was feeding it about 75W with loud results.
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Old 27th April 2004, 07:35 AM   #8
tool49 is offline tool49  Canada
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Well it all depends on the sensitivity of the woofer... With a Cerwin-Vega HED 12inch (92db / 2.83V) or an Infinity 1230W 12inch (92db / 2.83V) you'll get more bass than you need with "only" 120WRMS and a sealed box.

I used to have two HED 12inch wired in parallel for 2ohm impedance with a 1x60WRMS at 4 ohm amp (which probably output around 100WRMS at 2 ohm) and I was hitting around 115db easily. I know 115 is not even in the SPL competition range, but it was easily loud enough to overload my full range speakers in the car (not to mention way bad for my ears).

I just checked the Rockford Fosgate Punch Z driver and it is rated at 90db / 2.83V. With 120W you can expect around 110db before clipping in a sealed box, which I think is way more than enough. Don't go for the newer Fosgate driver though. Since they're only rated at 86db / 2.83V sensitivity, with 120W, you'd run out of power at about 100db (half as loud as the Punch Z) which will sound low in the bass region on the highway when the tires are making a lot of noise in the cabin.

If your amp is 2ohm stable, you could also use a dual voice coil woofer with the same sensitivity to get approximately 3db extra SPL for free. (I could not find your amp details, but if it doesn't support 2ohm, it'll overheat and shutdown or fail when driven hard). Similarly, wiring a second woofer in parallel would give you a 2 ohm load assuming 4ohm woofers and double the air moving thus giving you a nice 5-6db spl extra. With that meager 120W amp and two Infinity 1230W, you'd get a good 116db in car! Be carefull not to push it too much as you'd be harming some fine woofers...

Hope this helps!
Sébastien
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Old 28th April 2004, 06:26 PM   #9
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Default Sensitivity is also a "meaningless" spec!

Sensitivity (90 dB/1 watt/1 meter) is another specificiation which is mostly meaningless for subwoofers.

For subs, this is usually calculated by a Thiele-Small parameter program, but it is the MIDRANGE sensitivity and has nothing to do with actual sensitivity at low frequencies. That has to do with the parameters of the driver (moving mass, magnet strength, suspension) and of the box (volume and type and tuning of box).

So unfortunately Sebastien, you cannot just say "The driver is 93 dB/1w/1m, and my amp is 120 watts = +21 dB, therefore I can play 114 dB." It simply doesn't work like that.

The actual sensitivity at 50 Hz might only be 80 dB or something, then you might get 80 + 21 = 101 dB, which is not very much as the ear is insensitive to low frequencies.

Of course, you get a sensitivity boost from the small space in the car, but you also "lose power" when all the bass frequencies are in the amp at once (the opposite of the bi-amping effect: in other words, you could get 101+ dB with a sine wave, but maybe not with music).

I'll still maintain that if you want to shove a woofer in a small box (which requires more power, to move the cone against the air pressure), 120W is not enough to "POUND." It's enough for musical listening probably, but not to attract attention and shake your body very much.
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Old 28th April 2004, 09:28 PM   #10
tool49 is offline tool49  Canada
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Hello head_unit, I'm far from trying to start a heated discussion here, but my values were measured in my small sedan (Acura 1.6EL) for the Cerwin Vega HED and the Infinity 1230W (not for the Fosgate though). The sensitivity was measured at 80Hz (the lowest accurate point for my splmeter). Now since the box size (both sealed) were chosen to give a 12db/octave cut under 70Hz and that the boost incured by my car is around 10db/octave starting at 65Hz, I assumed a nearly flat response within my car (slight slope down but nothing that can't be cured with a little equing). From my experience, with my music listening style, the pounding feeling comes from the kick drum located anywhere between 75Hz and 90Hz. It might vary under certain circumstances but, I still believe that with 120W you can achieve a decent rumble, given that you choose the right sub with the right box.

I never implied that these would shake your car (although it might anyway). If the goal is indeed to attract attention on a red light or to shake the whole car until the bolts come loose then, I agree, 120W isn't going cut it. If the objective was to obtain lower bass extension and decent sound levels at modest costs, then I say that yes it will probably do the job.

Hope this clarifies things!
Sébastien
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