Originally Posted by Dawn Gordon
Yes and no. You'll notice that Triad also factors the surrounds into the mix. Triad dealer training was very specific about cubic feet and which speakers to use.
Craig, technically, you are correct. But remember each room is going to be different. For example, let's say you have a room that's 22' long x 16' wide x 14 feet high. The front seats are at 14 feet from the LCRs, the back of the room houses a pool table. Now, if you base the room on the front speaker distance you should go with Bronze LCRs, Bronze Surrounds and a bunch of subs. But quite honestly, Bronze speakers don't belong in a room that size. There's a reason Triad is insistent on cubic feet.
Obviously, I've never taken the Triad dealer training, since I'm not a dealer, just an A/V enthusiast. However, if Triad uses room volume to specify speaker size, that doesn't make sense to me. While admittedly, the larger the volume of the room, the greater the opportunity to lengthen the listening distance. But this is only an indirect measure of the speaker's SPL requirements, and the seating distance from speaker to listener is a much more direct measure. In the example you gave above, the issue is more about the 14' listening distance than about the total cubic volume of the room. Move the seating forward to 10 ft. and the Bronze speakers would be fine even in that large a space. If you actually require a 14ft. listening distance, step up to the Silvers. The volume of the space behind the LP, which is where the additional cubic footage would be, doesn't add or subtract substantially to the room reinforcement.
The inverse square law determines the volume loss vs. distance, where SPL drops off by 6 dB per doubling of distance. https://en.wikipedia.org/wiki/Inverse-square_law
In other words, if you move from 1' to 2' you lose 6 dB, 2' to 4', you lose another 6 dB, from 4' to 8' another 6 dB, etc.
Another good explanation here: http://hyperphysics.phy-astr.gsu.edu...ic/invsqs.html
(That page also has a calculator that will calculate the volume loss per distance.)
In other words, by the Inverse Square Law, the closer you sit to the speakers, the louder the SPL will be. The reverse is also true: the further you sit from the speakers, the more speaker output capability you need to reach any given maximum SPL.
However, the Inverse Square Law is based on no boundaries providing reinforcement of the sound. In a room, obviously this is not the case, and the walls, floor and ceiling provide "room reinforcement." Different room environments will offer differing levels of "room reinforcement." The amount of absorption vs. reflection of the boundaries will influence the room reinforcement. A highly reflective room will be louder than a room with lots of soft furnishings, rugs and drapes, not to mention acoustic treatments. The distances of the speakers to the walls will also impact the SPL's of the reflections, as will the distance from the walls to the listening position. (By the same Inverse Square Law, the SPL at the wall surface will be less the further the speaker is from the wall, and the SPL of the reflected sound will decrease even more the further the listener sits from the wall.) Even the wall construction will impact the absorption by the walls. Concrete block or poured walls will be more reflective than drywall and stud walls. All these things will factor into the speaker size selection... and none of them show up in the Speaker Size Selection guidelines. So, the closer you sit to the speakers and the walls, and the more reflective the walls, the more direct and indirect SPL you hear.
The volume behind the listening position only provides very slight reinforcement because the reflections from behind the listener come from even lower SPL reflections which then drop even more as they travel back to the listener. By the time they arrive at the listener, they are low enough in SPL that they don't add appreciably to the SPL at the listening position. Therefore, the listening distance to the speaker is the primary determinant, followed by the proximity to the room boundaries. The cubic volume of the room is only a very "indirect" measure of the the maximum SPL requirements for a speaker.
In this example, compare the length of the green direct lines to the total length of each of the yellow ray traces. By the inverse square law, those MUCH longer yellow lines will be at greatly reduced in SPL compared to the direct sound represented by the green lines. You can also easily see how much shorter the distance is for the "early" reflections than for the "late" reflections that go behind the listener and travel a much greater distance. The early reflections will arrive at the listening position with much higher SPL than the later reflections. All of the extra distance the late reflections need to travel will cause those room reinforcements to be diminished in SPL by the time they arrive at the listening position. Increasing the volume behind the listener will decrease the impact of those later reflections, but they are not a significant determinant of the SPL at the LP anyway.
Bottom line, the SPL at the listening position will be determined primarily by the 1) the distance from the direct sound of the speaker, and 2) proximity of the speakers to the boundary walls that impact the earliest and highest level reflections. The SPL added by the late reflections may be a minor factor, but significantly diminished relative to the first two. Moving the listening position further away from the speakers will increase the speaker/amplifier requirements requirements significantly. The 6 dB drop that accompanies a doubling of distance, doubles the required driver excursion AND doubles the amplifier power required to drive the doubled excursion.
I use this SPL calculator: https://myhometheater.homestead.com/splcalculator.html
It factors in speaker seating distance, distance to boundary walls, speaker sensitivity, amplifier power, and number of speakers. It does NOT factor in the cubic volume of the room. It's not perfect and I only use it as a starting point, but it makes more sense to me than using cubic volume to determine maximum speaker SPL capability. (In addition, I always factor in 6 dB of headroom over the maximum required SPL level. No system I've helped design has ever suffered from "not enough speaker for the room."
Triad speakers make this easier due to their generally high sensitivity and strong power handling, and their ability to retain their sound quality at higher SPL's. The fact that they're all designed for use with subwoofers, and have specifically limited LF extension, is also a benefit.
Subwoofers are a completely different story, and there, room volume becomes the primary determinant. But that's a whole different discussion.