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Discussion Starter · #1 ·
Hello,


I plan on purchasing B&W dm303's all the way arround for my home theater as I have been reading nothing but good reviews (plus they fit my budget on an almost to good to be true level). These speakers are rated at 100 watts. Would 100 watts suffice for a room with the dimensions of 19 x 11 and a ceiling height of 7'? I will also be using a psb subsonic II to handle the base since the speakers will be run on small. Is there any formula, so to speak, for room size to power?


Thanks


Greg
 

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or SPL is what is generally considered with respect to filling up a room with sound. I to have wondered about this with respect to room volume (cu. ft.). I have not found a formula or rule of thumb. I always like to demo in home before I buy.


However, that said, you may want to consider this. The Dm303 is spec'd at 88 Db (spl) at 1 watt and 1 meter distance. At one hundred watts you bump that by 20 Db. (10 * log (100))


This gives you 108 Db at 1 meter distance. I think the fall off rate for direct radiators is 6 Db per doubling of the distance.


1 meter 108 Db

2 meter 102 Db

4 meter 96 Db

8 meter 90 Db


Some have measured the dm303 at 90-91 DB SPL. SO .. with your room size you get an idea of how this works.


Don't know how loud is loud for you? Radio Shack sells an SPL meter for cheap. Carry it around with you and go listen to speakers/music at different places. Soon you will have a feel for what your number is!


Things that make every thing I said hogwash have to do with room acoustics/modes and speaker placement.


I say get em for a weekend and demo them. Don't worry too much bout the numbers. If you like em, buy em.
 

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Discussion Starter · #3 ·
Thanks Jamin! That makes the most sense as my new room might have a few hills and valleys the old didn't ( the measurements are for the theater under construction ). My current room is 2/3rds the size of the new and the 75 watters I have in there are more than enough so I will definitely try and audition at home first.


Greg
 

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Guys,


While the presented SPL's are correct for a free field environment, things get a lot more complicated in a room. The sound waves are reflected at the room boundaries and form a reverberant sound field. To be able to handle this situation without resorting to very complex numerical methods, statistical methods are employed. Note that the statistical approach is not valid for low frequencies where the resonance patterns are deterministic rather than statistical. The in-room SPL vs distance formula may be expressed in the following way:


Lp = Lw + 10*log(gamma/(4*pi*d^2)+4/A)


"SPL = constant + free field term + room term"


where


Lp is sound pressure level

Lw is sound power level output from speaker(s)

gamma is directivity

d is distance from speaker

A is equivalent absorption area


So what does the tech-talk above really mean? For starters, speakers do not radiate equally in all directions and this is where directivity comes in. The directivity varies depending on driver size and configuration as well as baffle size. As a ground rule, gamma is usually 2 for large parts of the frequency range for most "normal" speakers.


The equivalent absorption area depends on how "damped" the room is, and will probably be around 10-40 m^2 for most listening rooms. For mid to high frequencies the absorption comes from fibrous materials in carpets, curtains, furniture etc - sound waves are forced to give up energy as friction when pressed through the materials. For low frequencies the absorption is primarily related to the properties of the room boundaries - flexible panels absorbing energy.


If we use the formula above to see how the SPL in the "normal" room varies with distance, we see that if we are more than some 1 m or 3 feet from the speaker, the SPL is more or less constant regardless of distance! Luckily for us, the human hearing system is able to separate the direct sound from the room sound to a large extent.
 

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And to answer the original question, assuming the room is more or less "normal" (A = 20 m^2), the maximum SPL (Lp) at the listening position will be:


Lw = 88 + 10*log(2*pi) + 10*log(100) + 10*log(2) = 119 dB


(sensitivity + correction term + amp power + 2 speakers)


Using the expression in the previous post:


Lp = Lw + 10*log(4/20) = 119 - 7 = 112 dB


Note that you do not need to know the speaker - listener distance as the the reverberant field dominates completely unless you sit closer than 1 m to the speakers.
 

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Seems like I am talking to myself here :).


For those of you with line source type speakers (Magnepans, Martin-Logans, Apogees etc) it might be interesting to know that the above mentioned factor gamma may be much more than 2, perhaps around 5-10 at 1 kHz and rising even higher with increasing frequency (a factor two per octave). This means that the relation of direct sound to reverberant (room) sound will be much higher. What does this mean in practice? Well, these speakers will perform much better than ordinary speakers in "live" rooms and are always more immune to room influence, even in well damped listening rooms.
 

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The monolog continues:


Most mainstream speakers are only capable of producing a reasonably clean output up to something like 100-105 dB in-room (some 10-20 W input power) even if they certainly can be forced to higher levels.


For a no compromise audio system a clean in-room peak SPL of 115 dB should be the ultimate goal (clean peaks of 115 dB sound surprisingly natural - loud but not overly so). In practice this system would have to be able to produce >120 dB (with some loss in SQ)!
 
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