# Is a 1250 watt sub a little "overkill"



## Guest (Mar 10, 2007)

Hi: I am using a SVS 20-39 PC-Plus sub that has a 525 watt amp and I have the gain half way and the output is way more than I need at this setting, in my 12'Wx8'Hx24'D enclosed room. I notice that some other sub makers are using 1250 watt amps in their 10,12,15,18 inch models! I guess if you trying to fill a gym-sized room with bass, you would probably need a big amp. but I just don't see it for a normal listening room! My way of thinking has always been that " more is better", but not sure when it comes to subs! From using my sub, in my room,with ny gear, I think a 525 watt amp is just fine, as I can shake the walls with half gain. I am trying to learn why someone would need a 1250 amp in a sub, for home use!? Please learn me! Thanks, Huck:T


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## Darren (Apr 20, 2006)

Not at all. I have a 1,300 watt sub with 4 15" drivers in a 3,200 cubic foot room. It isn't overkill at all unless you configure the sub waaay hot


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## JCD (Apr 20, 2006)

There are a couple of reasons why I think you might see a 1250 watt amp. If you look at those subs that have a relatively small driver in a small box like the True Subwoofer by Sunfire, you need to make up for the relative size of the system by increasing the wattage of the amp. This sub uses a 2700watt amp.

Another reason why you might need a bigger amp is the relative sensitivity of the sub. If one sub has a sensitivity of say, 86, it would need an amp with a wattage of 1250 to be as loud as a sub with a sensitivity of 90 and a 525watt amp (you get about ~4db more with 1250amp over the 525watt amp).

I'd expect someone smarter than I in these things to chime in, but those are two reasons I came up with off the top of my head.

JCD


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## Guest (Mar 10, 2007)

Thanks for that! I never thought of the sensitivity issue, but sounds logical! I guess when manufacturers give a sensitivity figure for a subwoofer they use the standard ( at least for regular speakers)1 watt at 1 metre to give a db rating? Thanks, Huck


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## Wayne A. Pflughaupt (Apr 13, 2006)

JCD said:


> I'd expect someone smarter than I in these things to chime in, but those are two reasons I came up with off the top of my head.


No one’s chiming in because you nailed it! :T 

Bottom line, you can’t put a lot of stock in sub power ratings. All that really matters is its performance – extension, detail and SPL. A rough rule of thumb is that ported subs put out more SPL for the wattage than sealed subs.

A good case in point is the SVS SB12-Plus. It has more watts than the PB12-NSD. Since the former is small and sealed it needs the extra power to compete with the larger, ported sub.

Regards,
Wayne


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## Sonnie (Apr 11, 2006)

I certainly don't think 1250 watts is overkill by any means. I'm using 3000 watts on my subs in my 1800ft^3 room. Of course I'm not nearly using it all, but it gives me plenty of headroom for the most demanding needs.


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## mulveling (Feb 14, 2007)

Huck said:


> Hi: I am using a SVS 20-39 PC-Plus sub that has a 525 watt amp and I have the gain half way and the output is way more than I need at this setting, in my 12'Wx8'Hx24'D enclosed room. I notice that some other sub makers are using 1250 watt amps in their 10,12,15,18 inch models! I guess if you trying to fill a gym-sized room with bass, you would probably need a big amp. but I just don't see it for a normal listening room! My way of thinking has always been that " more is better", but not sure when it comes to subs! From using my sub, in my room,with ny gear, I think a 525 watt amp is just fine, as I can shake the walls with half gain. I am trying to learn why someone would need a 1250 amp in a sub, for home use!? Please learn me! Thanks, Huck:T


Also keep in mind your room has dimensions that are exact multiples of each other: 12' to 24' and 8' to 24'. Unless you've got serious room treatments, you'll probably have your room manifest some nasty peaks in the low end - this may make things sound "too powerful" at times, and with certain signals. If the peaks were tamed you'd get a more natural, even response that should integrate much better with the mids/highs.


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## Joe L. (Jan 23, 2007)

Hoffman's Iron Law states:
"You can have low end extension, high efficiency, or small enclosure size. Pick two, the third will be decided for you."

Due to the long wavelengths of the low frequencies reproduced by subwoofers, a subwoofer requires a large woofer, big cabinet, or massive amplifier. Hoffman's Iron Law, described by Henry Kloss in the mid-1950s and later turned into an exact mathematical formula by engineers Thiele and Small, governs the behavior of woofers. 

Essentially, it says that a woofer's efficiency is proportional to the volume of its cabinet and the *cube *of the lowest frequency it can produce before losing relative level (aka the cutoff frequency). 

Take, for example, a woofer whose response is flat down to 40 hertz in a 2-cubic-foot enclosure. To make its response flat down to 20 Hz, you must either increase the cabinet volume by eight times (to 16 cubic feet) or use eight times the amount of amplifier power to achieve the same listening volume.

Your SVS sub has an internal volume of roughly 4 cubic feet. If you wanted to have a sub with equal SPL and the same low frequency response (down to 20Hz) but were limited to a 2 cubic foot enclosure, you would need an amplifier 8 times your current amplifier (2^3=8) ... 525 x 8 = 4200 watts ... and you would need a driver capable of handling that power without it reaching physical excursion limits or its voice coil melting from the heat. I am not aware of any driver that can handle 4200 watts. In most, the max power before meltdown might be 1000 watts or so. In reality, the smaller, 2 cubic foot enclosure sub must either not play as loud, or as low... It can't.

See... pretty easy to see how other subs might need 1250 watts of power... They would only have to be slightly smaller in size than your sub (and want to achieve the same 20Hz response)

Personally, I have two 12.5 cubic foot enclosure subwoofers. Each is fed with about 750 watts of power. My large enclosures allow them to be fairly efficient AND play really low. The drivers I used can each easily handle the 750 watts of power without reaching their excursion limits. My combination of 1500 watts of power (total), 25 cubic feet of enclosure (total) and a pair of 18" drivers that can each move 6.5 Liters of air result in a frequency response down to 10Hz and below. My tradeoff for this was the enclosure size.

Your SVS sub is trades its larger size for low frequency response to 20Hz. That very respectable performance apparently needs an amplifier near 500 watts...

Aren't the laws of physics wonderful...

Joe L.


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## bobgpsr (Apr 20, 2006)

^^^ Superb first post! :clap:


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## Sonnie (Apr 11, 2006)

I'll second that one... :yes: 

Welcome to the Shack Joe L. ... You need to stick around... :T


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## Chrisbee (Apr 20, 2006)

I have never seen this described so eloquently or so clearly. Thankyou Joe.

Now I know why I have a modest 600 cu ft enclosure with 4 x 15" drivers driven by 900 watts RMS just to enjoy organ music at home. :nerd:


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## ISLAND1000 (May 2, 2007)

Huck, you're right in questioning high watts. Someone's gonna get hurt sooner or later and THEN we're gonna feel sorry.


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## ACGREEN (Feb 23, 2007)

No different then a car with 300+ HP. You rarely use all of the HP, but it is nice to have.


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## gsmollin (Apr 25, 2006)

I think the subwoofer power specifications are inflated. My Bob Carver subwoofer is rated at "1000 W RMS", but has a 4 A fuse on the 120 VAC power line. That gives a power rating of 480 VA off the power line, multiplied by the single-phase rectifier's power factor, and the efficiency of the amplifier delivers nowhere near 1000 W to the speaker. Perhaps 1000 W on a peak basis may be possible, but it is a short, non-repetitive peak. An organ pedal tone (for instance) would quickly overload this system, since they can be sustained for many seconds.


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