Watts required, by room size? - AVS Forum
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post #1 of 21 Old 01-27-2007, 11:38 AM - Thread Starter
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I have a living room that is 18'x18' with 11' ceilings. i'm wondering what size sub i should buy. How many watts do i need?? I'm not a full audiophile, and it is an apartment, so i don't need full power, but something of decent quality. Any recommendations?
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post #2 of 21 Old 01-27-2007, 11:55 AM
 
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What is your budget?
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post #3 of 21 Old 01-27-2007, 12:01 PM
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Quote:
Originally Posted by mga56grg View Post

I have a living room that is 18'x18' with 11' ceilings. i'm wondering what size sub i should buy. How many watts do i need?? I'm not a full audiophile, and it is an apartment, so i don't need full power, but something of decent quality. Any recommendations?

1) Do you listen to music or home theater more?
2) Do you like to listen at loud levels (although you are limited by the fact you are living in an apartment).
3) Do you plan on moving into a house anytime soon?

Life is good.
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post #4 of 21 Old 01-27-2007, 12:20 PM - Thread Starter
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Thanks for the quick responses - -
I can spend several hundred dollars (up to $450), and it will be used primarily for movies. I should be in a home in a year or so.
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post #5 of 21 Old 01-27-2007, 12:23 PM
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The question of how many watts is irrelevent when it's not in the context of the design of a sub. In other words, an efficient sub with 200 watts may very well play louder than an inefficient sub with 1000 watts.
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post #6 of 21 Old 01-27-2007, 12:44 PM - Thread Starter
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Quote:
Originally Posted by mojomike View Post

The question of how many watts is irrelevent when it's not in the context of the design of a sub. In other words, an efficient sub with 200 watts may very well play louder than an inefficient sub with 1000 watts.

on the informative/helpfulness scale, this = 0.
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post #7 of 21 Old 01-27-2007, 12:47 PM
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Quote:
Originally Posted by mga56grg View Post

Thanks for the quick responses - -
I can spend several hundred dollars (up to $450), and it will be used primarily for movies. I should be in a home in a year or so.

For HT, you wanted to get a ported subwoofer as long as size of the subwoofer enclosure is not a problem. SVS and HSU make great subwoofers.

The HSU STF-3 is selling for $479.

There is also the SVS PB10-NSD for $429.

There is also the Mirage S12.

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post #8 of 21 Old 01-27-2007, 12:50 PM
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mojomike is right. Don't worry about wattage or driver size. They are part of a package in which the designer balances many elements to achieve a balanced product. Taken out of context, they are meaningless.

Rather than specs, I'd recommend checking a manufacturer's reputation, then finding a model in their lineup that fits your needs. Around here, you'll find that SVS and Hsu are well thought of, although, of course, there are other good choices.

In that price range, the SVS PB-10 NSD is very popular as is the Hsu VTF-2 Mark II or it's less expensive sibling, the STF-2 (pretty close to the VTF for movies). If you can go a little bit higher, the new VTF-2 Mark III is a great value. All are very good quality subs.

I suggest you look over the SVS and Hsu web sites, as they are very informative. If you have questions about which model would fit you needs, email them with room size, budget and other pertinent info. They are very helpful.
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post #9 of 21 Old 01-27-2007, 12:52 PM
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Quote:
Originally Posted by mga56grg View Post

on the informative/helpfulness scale, this = 0.

You asked "How many watts do i need??", and Mojomike simply stated that the number of watts is irrelevant, and should not your primary factor when shopping for subwoofers.

To me, this is has an informative / helpfulness score of 10 / 10... No need to be rude, especially if somebody is trying to be helpful.

Life is good.
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post #10 of 21 Old 01-27-2007, 01:01 PM - Thread Starter
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thanks guys. i'll narrow down and research these
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post #11 of 21 Old 01-28-2007, 12:15 AM
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Quote:
Originally Posted by mga56grg View Post

on the informative/helpfulness scale, this = 0.

rudeness rears its ugly head.
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post #12 of 21 Old 01-28-2007, 11:07 AM
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The design of audio subwoofers has, for many years, been predicated on a theory known as "Hoffman's Iron Law" which provides:

Loudspeaker effiency = volume of enclosure / ( cube of cutoff_frequency)

where cutoff_frequency(F3) is the desired low frequency cutoff or limit for the subwoofer

Unfortunately, if one wishes to reduce the low frequency cutoff of a subwoofer from, for example, 50 Hz to 18 Hz while retaining the same efficiency, the volume of the enclosure must be significantly increased. Or, if one wishes to decrease box volume from, for example, 1 ft3 to 0.4 ft3 and, at the same time, decrease the low frequency cutoff from, for example, 50 Hz to 18 Hz, efficiency drops by a factor of approximately 53.

Consequently, a woofer designer finds that where a 50 watt or 100 watt amplifier might have operated a 1 ft3 woofer at a 50 Hz low frequency cutoff, a 0.4 ft3 box at 18 Hz low frequency cutoff will require an amplifier that is approximately 53 times larger than conventional... in other words, an amplifier somewhere between 2500 watts and 5300 watts. They would also require an subwoofer driver able to handle this amount of power without burning up and without running out of linear excursion (don't want the voicecoil bottoming out or being launched into the room) This explains why you will not find any small subwoofers that can play very deep with any respectible sound-pressure-level. Basic physics get in the way.

Today, there are many DVDs with sound-tracks with low-frequency effects down to 10Hz and lower. Many commercial subwoofers have low frequency cutoffs somewhere 50 and 100 Hertz and amplifiers around 100 to 200 watts, these subs don't even come close to reproducing the low bass on these sound-tracks, but might be fine for music where low-frequency bass seldom goes below 50Hz. Other subs sold for use in a home theater may have a response down to 20 Hz. These are usually a bit larger, with larger amaplifiers, trading larger size and efficiency for lower frequency response.

There are many commercial subwoofers that employ amplifiers that are capable of over 1000 watts. The "Sunfire" brand is one example. They use a powerful 2500 watt amplifier in a small enclosure to produce good quality low frequency bass. They trraded efficiency for small enclosure size.

Other brands of subwoofers have much larger enclosures and much smaller amplifiers (Most subwoofer "plate-amplifiers" produce from 75 to 250 watts)

So...

Hoffman's Iron Law says that the efficiency of a woofer is proportional to cabinet volume multiplied by the cube of the F3. In other words, even thouogh you want a small box, high efficiency and deep bass, you only get to pick 2... the third is decided by physics. (Dispite what the marketing departments want you to believe)

If you want a small box, you can have deep bass or high efficiency, not both. Likewise, if you want deep bass, you get a small box or high efficiency, not both. Last, if you want high efficiency, you get deep bass or a small box, but not both.

There is no way to cheat this law until you find a way around the laws of physics, so it's something that you need to be aware of when evaluating your needs, and also when evaluating manufacturer's sometimes outlandish claims.

How many "watts" do you need??? I'll say you probably need somoewhere between 50 and 50,000 watts.

Joe L.
(I have two 18" Ascendent Avalanche drivers, each in a sealed 12.5 cubic foot enclosure, each powered by a 500 watt amplifier, in a room roughly 20x14x8 feet.
As a result, my low frequency cutoff is somewhere near 10Hz. Note... these are LARGE enclosures compared to most subwoofers, but I traded size for extended low frequency response... and I needed 500 watts per driver)
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post #13 of 21 Old 01-28-2007, 05:13 PM
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ok soooo.... if we were to know the cutoff frequency, the watts and the size of the enclosure, would there be a way of finding out how well it would work in a particular size room?
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post #14 of 21 Old 01-28-2007, 06:37 PM
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Quote:
Originally Posted by Jotsy View Post

ok soooo.... if we were to know the cutoff frequency, the watts and the size of the enclosure, would there be a way of finding out how well it would work in a particular size room?

Close... you would need to know the T/S parameters of the driver (those determine how much the cone will move when driven by a given amount of power at a given frequency) and the size of the enclosure, and how it is constructed. Sealed vs. ported. If ported, we need to know the frequency the enclosure is tuned to.. (roughly put, its cutoff frequency)

Then, we can fairly accurately predict performance. The T/S parameters will allow a program to determine combined efficiency and frequency response in a given enclosure volume and tuning.

The mechanical construction of the driver combined with its electrical parameters (T/S parameters) will dictate how much power will be required to cause the cone of the driver to move to its excursion limits. (mechanical limits) at a given frequency. The amount of power required to get to those limits varies with frequency and the enclosure the driver is mounted in. The size of the driver and the amount of air it can move within its linear excursion limits will determine how loud it will play in a given room.

Knowing the "watts" (as you stated) tells us nothing unless we know the efficiency of the driver/enclosure combination, its tuning, and the frequency of a given signal.

I guess what we are saying is that the amplifier needed (and its wattage) is the LAST thing you determine after you design a driver/enclosure combination. Basically you choose an amplifier that will not cause the cone of the driver to go beyond its excursion limits. If the cone is moving just under its limits then it is moving as much air as it can, and playing as loud as it can. If you need louder bass, then you need either a bigger driver, or a driver with a larger XMax (excursion limit), or multiple drivers. The end result is the same, you can move more air and generate higher sound-pressure-levels.

If you are shopping for a sub, choose your desired cutoff frequency (18-20 Hz is a reasonable limit for many people) and then choose how loud you wish to play the sub. Once you know those you can ask the manufacturers if their sub can produce that level at your seating position in your size room. As I said, it is all about moving air and small drivers will not be able to move as much air as larger drivers. However much power it will take to move a volume of air with a driver does not matter. As I said, it might be 50 watts, 100 watts, 500 watts, or much much more.

For you size room I would not go below a 12" driver, although a high-excursion 10" might do if you do not want to go very loud, or very deep.

I think I read somewhere that every time you go down an octive you need to move 4 times as much air to generate the same SPL. It really comes down to physics and the ability to move air at the desired frequencies, not marketing, or wattage.

Joe L.
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post #15 of 21 Old 01-29-2007, 09:01 AM
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Quote:
Originally Posted by mga56grg View Post

on the informative/helpfulness scale, this = 0.

Wow... welcome to the forum.
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post #16 of 21 Old 01-29-2007, 09:10 AM
 
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This is a very useful tool.

SPL db calculator
http://www.stageaccompany.com/en/support/splcalc.php
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post #17 of 21 Old 01-29-2007, 09:13 AM
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Yeah, basically the idea that watts = performance is a marketing gimmick. Kind of like how peak horsepower sells cars.
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post #18 of 21 Old 01-29-2007, 09:24 AM
 
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Kinder, never over rev that engine or it will cow up! Never under power the sound system and don't overload the input beyond 0db!

Once you reach 0db there's no other place to go!
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post #19 of 21 Old 01-29-2007, 10:17 AM
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SVS PB10-NSD for $429

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post #20 of 21 Old 01-29-2007, 10:24 AM
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Quote:
Originally Posted by mga56grg View Post

I have a living room that is 18'x18' with 11' ceilings. i'm wondering what size sub i should buy. How many watts do i need?? I'm not a full audiophile, and it is an apartment, so i don't need full power, but something of decent quality. Any recommendations?


Around 3500cu ft,what size sub. Size is not an automatic indicator of performance.
Wattage is also often used to seel products,the real performance ius measured in clean output down deep,not watts. Acoustic watts and electrical watts are not the same.

You can have a sub with a 1KW ampand another with "only" 100W,the one with the 0.1KW rating besting the 1KW paper monster in acoustic output.

This said...

HSU VTF2
SVS PB10

Are good starters,low cost and great performance.

Ask yourself mortal , do you have as much displacement as me ? The answer is no unless you have a Windmere fan sub.
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post #21 of 21 Old 01-29-2007, 10:55 AM
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Lets not forget the TOTAL output from all speakers. Systems with large sats, that have, lets say, 8" mid-bass drivers will contribute quite a bit to the boom effects.
With that thought in mind, a good rule of thumb is about 1W/cuft, or close to it.
My room is 3100cuft and I've got 2880W total(1280w from the 7 sats, plus 1600w from 2 subs). That's almost the 1:1 ratio.
The subs are 4 ohm units that are rated @ 800w and can take 1200w. They're not as efficient as my 12", 8 ohm sub, but they move a lot more air, and will shake the room.
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