I'm working toward a bass treatment plan for my under-construction basement home theater. Notice I say working toward
a plan - that is not just to say that I don't have a plan at present, but also that I don't expect to have a plan until I have learned about what my room is actually doing. That's what I've come here for.
The theater is heavily built as a room within a room (more or less). The walls and ceiling are all fully independent from the other structures of the house and sheathed with 1/2" OSB and 2 layers of 5/8" drywall, all coated in Green Glue sound absorbing compound. The floor is poured concrete, as are the foundation walls which surround the screen end of the to a height of about half the wall (though the walls do not touch the concrete). It's about 22 feet long, 12 feet wide, and almost 9 feet tall. There is a 14" tall riser in the rear 40% (approx) of the room, not prepared for bass trapping (mostly filled with sand), and a 14" tall stage at the screen end of the room (completely filled with sand). There are soffits running down both sides of the room at the ceiling enclosing air ducts - nearly 1 foot tall by 1.5 feet wide. Here's an image of the front of the room as it stands now. You can see a lot more details by wading through the build thread in my forum signature.
Do I have a plan at all? Well, yes. Here it is:
- four subwoofers. 3 cubic foot sealed boxes with Dayton Audio UM 15-22 drivers (1 each). Each sub gets it's own channel of (2) EP4000 amps. Locations are pretty well locked into the corners.
- I don't have EQ and processing picked out, but I expect to need something - potentially something from MiniDSP.
- Passive treatment begins with a perforated helmholtz trap. 25 inches deep, 96 inches wide, and 40 inches tall. It doubles as a speaker stand for LCR. I have not yet finalized optimal perforation pattern or insulation sizing and spacing, but I expect strong absorption from about 70-80 Hz up to 180-200 Hz. The trap takes up 2/3 the width of the screen wall behind an acoustically transparent screen - almost 25% of the total area of the front wall of the room. This is not in place yet.
- LCR speakers are DIYSoundgroup Cheap Thrills, which may not be the strongest in low end output, but are still by most any reasonable standards very capable - sealed design with 15" pro woofers. F3 is near 80Hz.
So what's the question?
The first question I have is what are the axial modal resonances? What are their frequencies and bandwidths? I think I have enough data to answer that question for the length of the theater (not the vertical or lateral modes, only the longitudinal (? is that the right word? Axial length?).
I used a Behringer ECM8000 mic and external USB sound card with REW to collect data from a series of sweeps. All the settings here are default for REW: I didn't adjust sweep duration or windowing or anything like that. No smoothing has been applied. SPL is not calibrated, and no mic or soundcard calibration file was loaded. For these measurements, one sub was driven - no mains. The sub is in the front left corner of the room. The room is mostly empty - just speakers and a few boxes laying around - no furniture, no carpet, no exposed insulation. The mic placement is the only parameter changing from one sweep to the next,
and the mic placement only changes in one direction: it moves steadily from the left rear corner toward the front left corner, about 9-10 inches per sweep. The plots presented here represent movement of the mic just beyond 50% of the length of the room, always the same distance from the ceiling (not always form the floor - I extended the mic stand when I moved the stand from the riser to the floor), and always the same distance from the wall - about 5 inches. The overall highest output was the first measurement, nearest the corner. The output get steadily lower as I proceeded away from the corner.
Of particular interest to me while getting started is the set of mode markers I added via the EQ functions of REW. I set them based on what I was reading from the plots, not based on the physical dimensions of the room. The green marker at 52 Hz represents the first axial mode in the width (left-right) dimension. Since the output was very consistent in that range and peaked at 52 Hz, I adjusted the width setting in REW until that predicted resonance coincided with the peak - 52Hz. In the same way, I set the red marker at 57Hz for the secondary axial (length) mode. When the mic was in the corner, output at that frequency was a maximum. When the mic was near the center of the room, I was in or near a null at that frequency.
The things that trouble me are two: First, while it looks to me like I have set those two markers at the correct locations, based on performance in that range, outside that range I see very little further agreement between the modal markers and the actual peaks and nulls. Notice that if the secondary axial length mode is centered at 57Hz as I have placed it, the primary must have been around 28-29 Hz, but my measured peak is lower than that - about 26Hz. Second, and not plainly visible in this, is that the physical dimensions associated with the markers I have placed on this plot are smaller than my actual physical dimensions. I expected them to err in the other direction. I have come to expect (via reading, not via practice) that rooms actually resonate at frequencies lower than their dimensions would predict. Is this not true? Do online calculators like the one built into REW make assumptions about room contruction that my room violates? Have I misread the plots and set the markers wrong?
I'm in this for the long haul to fully understand what my room is doing in the hope of learning some things, like how effective can one bass trap be, and how much can I gain moving from one sub to several - how does phase effect that, and so on.
Please dive in with whatever you see, and I will keep posting as I collect and analyse more data. I expect over the next week or so to add data that should illuminate behavior in other directions - width and height. Once I have developed an understanding of the raw performance of the room in all three dimensions, I'll start adding other variables, like second, third and fourth subs, then the helmholtz trap, and eventually EQ. Once I am satisfied with the low end, I will move on to mains integration (absent persuasive advice to the contrary), and eventually to midrange and high frequency assessment and treatment - probably in other threads.
Thanks for any thoughts you have about this.