The following was originally a reply I posted in another thread also discussing HT speakers, but I think the bass extension portion of that discussion belongs here even more so, since that’s exactly what this thread is about:
Attempting to achieve reference SPL bass completely flat down to 20hz (vs e.g. 30hz or 35hz) is almost an unrealistic goal in most situations (except in a cost-is-no-object system, or really tiny rooms), when you consider you need something like 10 kilowatts and $10k of subwoofers to comfortably achieve 115db-120db @ 20hz at listening position in a large home theater room.
I want to point out that 115db-120db peak subwoofer output is
not my requirement. 115db SPL from LFE is the requirement to satisfy THX reference SPL specs! (The often quoted 105db reference SPL is for the main LCR speakers,
not LFE.) Also, this 115db requirement is for
LFE alone; if you set your LCR and other speakers to “small” on your AVR (so that their bass content goes to the subs), then you need to further increase the subwoofer peak capabilities to account for the extra bass content demands; hence, this is why I and the article linked above claim that home theater subwoofers must be capable of at least
115db SPL (and often up to
120db) in order to meet
official “THX reference” specs.
Now, when we are forced to choose a compromise (given how demanding 120db @ 20hz is in terms of cost, space, electrical power, etc.), do we prefer to severely reduce SPL capability, or do we prefer to somewhat reduce the depth of bass extension (to "only" that which high end theaters use)? That’s the key question of this thread.
To answer that question, imagine a double blind test comparing the two sound systems for a home cinema experience:
System A: LCR mains capable of 105db from 100hz to 20khz, and sub(s) capable of
105db down to 10hz.
System B: LCR mains capable of 105db from 100hz to 20khz, and sub(s) capable of
120db down to 30hz.
My claim: I bet you that in a double blind test consisting of many participants (spanning casual movie watchers, to film critics, to home theater audiophiles), the vast majority will overall prefer the experience produced by System B that prioritizes meeting reference level SPL first, even if bass extension "only" reaches down to 30hz (which seems to be the case with most real movie theaters, even extremely high end ones as mentioned by
@CDMC here). I claim this because
System A will compress during explosions and high dynamic range bass content, and will not be able to produce the same tactile sensation of impact that high SPL bass/midbass can (and as is required by the THX reference spec).
Even if your subwoofers can reproduce e.g. 115db at 40hz but not at 20hz, even 40hz sound effects may end up getting compressed anyway when simultaneous content at 20hz exceeds the subwoofer's capabilities (and causes the sub to start severely compressing its entire output)! The only exception I can imagine would be if there's a really intelligent DSP compression algorithm being used by the subwoofer, similar to how Buchardt's active speakers dynamically adjust the bass extension cut-off instead of compromising the linearity of all woofer frequencies. But I'm not sure if any subwoofers out there do this. This could also explain why some people report their HT system actually sounds better when intentionally rolling off the subwoofer's response below 30hz instead of 20hz or 10hz.
