The tech guy at ATC told me I could expect a "real world" LFB of about +4 dB. So, at least in this case, there's a slight difference between the projected boost and the mfr. tested boost. I'm sure this varies with the speaker under consideration and its crossover.
That's a surprising comment, coming from a speaker manufacturer.
The base boost doesn't really depend on any aspect of the speaker itself: it depends on the simple fact of soffit mounting. This is basic physics. Compare two identical speakers, one radiating freely into full space, and one radiating into half space. There will ALWAYS be a boost of 6dB for the one radiating into half space, since that half of the power that was going "backwards" is now forced to go "forwards". Doubling the power radiated forwards is an increase of 6 dB, regardless of the speaker, manufacturer, crossover, planet or galaxy in which this is done. A 100% increase in radiated power is a 6 dB boost, no doubt about it.
In fact, the boost could theoretically be GREATER than 6 dB if the speaker also happens to be getting a boost from other walls, or the floor, or the ceiling. For example, if you were to mount the speaker right in the "tri-corner" between two walls and the ceiling, you'd get a major boost. The only way that the boost could be LESS than 6 dB is if the original location of the speaker was such that it wasn't radiating into full space to start with. But if that were the case, then once again the boost would be identical for all speakers used in that location, regardless of brand, power, model or crossover. The soffit boost is independent of the speaker, amplifier, crossover, or anything else. It is an effect created by the soffit itself, and always gives a 6dB increase, as compared to the same speaker radiating into full space.
The only difference that the size/shape of the soffit makes, is that it changes the frequency at which the boost starts, and thus the frequency at which the roll-off should start. But it does not change the 6dB peak boost: that is a function of just soffit mounting, itself, nothing else.
Maybe he was talking about interaction with the rest of the room? In other words, a 4dB boost from soffit mounting AS COMPARED TO just having the speaker in a small room, close to walls? That would make sense, but if that's the case he explained it wrong: The roll-off you need is
still 6 dB as compared to full-space, but only 4dB as compared to being in a small room in proximity to walls.
The ATCs have no provision for switchable response, so I plan to test after the room is done and build a software filter in my Metric Halo D/As for the compensation.
Software EQ may or may not work well: It would have to be phase-linear for optimum performance.
Testing the room response is always a good idea, but you'd have to be very careful to differentiate between the modal response of the room, and the bass boost from the soffit. For any small room, it is possible to confuse the two issues, unless you use a good analysis program, such as REW. And of course, you cannot compensate for modal response issues by using EQ, since that is a time-domain problem, but you MUST compensate for the soffit bass boost, since that is purely a frequency and power issue.
The circuit that Eric mentions is pretty simple and easy to make, and should do the job well.
- Stuart -