Originally Posted by
RMC
Hi Ian, (And Robert)
I couldn't pass on this one (your post # 62). With all due respect, I don't know where you are getting some of that info, but mine is identified below.
"Before getting too carried away with a flat response attempt to consider the real impact of room gain and baffle step." Agreed. If a sub was placed in the middle of a room (away from boundaries) and on a pedestal, base or stand, its unlikely one would get the 6 db room gain from boundary (since not real half-space).
"Room gain is un predictable but normally courts below 150 Hz and I'd dependant on the woofer location on the baffle and distance from room boundary ie the wall floor junction." More or less agreed because room gain is somewhat predictable. Electro-Voice states, in the System Positioning section, that "... the audible location of a subwoofer operating at a sufficiently low crossover frequency (below about 150 hz) will not be particularly evident." (Data sheet, PI218L, Low-Frequency Sound reinforcement System, 1994, P.3) I understand from this that 150 hz, you also mentioned, is the point where bass frequencies start to get omnidirectional, and therefore more difficult to locate, plus can benefit from room boundary placement gain instead of "wrapping around" the box as "diffraction loss"...
"Proximity to the floor adds 6 Db room gain, proximity to floor and wall adds 12 Db room gain and proximity to a room corner/ floor adds up to 18 Db of room gain." Two of these numbers appear to be wrong. First, floor OR wall placement does provide half-space (2 Pi) environment that leads to 6 db room gain, but one has to mention compared to full-space (4 Pi, or free space) environment. Again, from the same E-V data sheet mentioned above, in the same System Positioning section on P.3: "Floor location provides the acoustic half-space environment associated with the 6.1% system efficiency noted in the Specifications sections." Moreover adds: "Location at a floor-wall junction (acoustic quarter space) doubles efficiency (a 3-db increase in acoustic power level)..." That is compared to floor location only. Then further adds: "Corner placement (acoustic eighth space) doubles efficiency again..." Which I understand means another 3 db increase compared to floor-wall location. So, if I count correctly, it would be 6 db gain from full-space to half-space, then 3 db gain from half-space to quarter space, and finally another 3 db gain from quarter space to eighth space, for a total of 12 db of possible room gain from speaker placement. Not exactly the same numbers as yours.
"Most bass reflex box simulator assume the baffle is suspended in free space." I think this statement is questionable. John Eargle (JBL) writes "The data calculated from the thiele/Small parameters refers only to the LF performance of the system ... Further, the analysis assumes that the loudspeakers will be locating adjacent to a single reflective boundary, such as the ground, or a wall." (Handbook of Sound System Design, ELAR, 1989, P. 106). And in the Low-Frequency Systems and Enclosures chapter, section Thiele-Small parameters, of his "Loudspeaker Handbook", Chapman & Hall, 1997, P. 58, John Eargle makes it even more clear, if it wasn't enough: "Modeling of the system's response functions assumes that the enclosure is mounted in a large wall (a so called 2 Pi, or half-space, boundary condition)." My understanding is that any software using T/S model should therefore assume 2 Pi boundary condition of use, not free space.
"One way of evaluating your box response with the above effects is to try and measure your box outside on the drive way using a test set from the likes of Parts Express at 2 metres with the box on the surface of your drive well clear of any structures" "Then start to move the box closer to a rear wall. You will notice the bass response change. By locating the box at a distance to the rear wall equivalent to your listening room you will have some idea on the impact of room gain" Bass wise, to me this is half-space front wave radiation, as inside a room, however with no reflections from walls and/or ceiling since there are none in that setup... Moving closer to a rear wall as you say, leads to quarter space radiation, as inside a room, however again with no wall and ceiling reflections in your setup. You can also get "some idea on the impact of room gain" by doing the same inside, but with reflections...
"But the point here is don't look at your box response simulation in isolation. Some experimenting will yield a much better sound in your room." Mostly agreed. Speaker placement experimentation MAY lead to better sounding, not automatically much better, since there are always constraints in a room.
"if you intend to use a Db drive rack much of this can be compensated with parametric equalisation." In my post # 59 I did suggest parametric EQ relief but on the driver's mid-bass rising response: "Adjustable Parametric EQ on the "agressive or offensive" mid-bass level is another interesting alternative..." Whereas your suggestion seems to be for the low end. Regards,
Richard