The lower bass frequencies on some recordings contain ambient information that gives spatial cues that let you percieve the size and volume of the recording venue.
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I can only expand on what has been said. The lowest note on a double bass is 42HZ and most speakers are rated to that frequency, however, a good subwoofer can put power (pressurize your room) in the bottom octave, This both gives dynamic life to the bass instruments, and it can reproduce the resonant frequencies of large halls. Your brain interprets these low sounds everyday in your environment and when they are missing from a recording it sounds less lifelike.
It also works when you run the main speakers full range as I do [and as REL recommends]. The English call the frequencies below the range of most instruments "Infra-bass" and that is what the REL is intended to reproduce. I start rolling my subs off at 23Hz but they still make a difference even on music you would not expect.
I thought its because the subwoofer is omnidirectional in the lower region and thus adds to and expands the soundstage that the conventional speakers, whose sonic signature is directional, can't possibly convey. So, together, you get both directional and omni-directional cues which gives a larger soundstage beyond the mere boundary of the listening room and somewhat takes you to the recording venue instead. Unless you have already a gargantuan full ranger, most speakers can use the help of a good sub or two in this department.
Roxy54 is on the right track. Notes at any particular frequency have both higher and lower harmonics. A system that isn't truly full range can't totally reproduce the lower harmonics. Adding a sub will allow reproduction of these harmonics which helps with the ambient info and spatial cues as mentioned by Roxy54. So, even music that doesn't seem to have much low frequency info can still be enhanced by the sub giving a better soundstage, imho. That's also the same reason why tweeters with excellent top end extension add to the spatial qualities of many recordings.
Rockyboy I experienced the same phenomenon you describe when I switched my Theta Casablanca to run a sub in 2 channel Stereo mode with my Usher Be 718s. Prior to that I was only running the sub for 5.1 home theater. Considering the fact that I already was crossing the loss pass signal to a phantom sub, I really wasnt expecting image and sound stage to improve. The best way to describe the change is to say it was like setting the Ushers free.
Stanwall, I am vey surprised to hear the subwoofer manufacturer recommends running the main speaker.
My understanding is there are tremendous benefits to be had by crossing over somewhere around or under 100Hz to a powered sub, even on quite good fairly 'full range' speaker.
Perhaps others can comment.
What Stanwal said is correct (I believe) and what you say is also correct (I believe). REL does recommend running their subs in paralel to the full range speaker and the speaker be ran full range. This is done without any additional crossovers. REL subs usually don't perform too well above 50 or 60 Hz, but are outstanding in the lower frequencies.
On the other hand, to get the benefits you mention you need an external crossover before the amp to let it only reproduce higher frequencies and the speaker do the same. Doing this you prevent the speaker go into low freqs where many speakers increase distortion a lot. So you get more headroom on the amp + less bass-related distortion from the speaker, and then let the sub handle all the low freqs. However, to do this you need a different kind of sub that you can crossover at 80 to 100 Hz...like a JL (I hear).
I hope this helps. Please note much of the above is not first hand experience but rather learnings from reading this forum!
Your observatons comport with my experience. I use KEF R104/2s with a Velodyne HGS-15. Crossover by the pre/pro is set at 80 Hz. The 104/2s sound fine full range by themselves, but I think they have a bit more transparency when relieved of LF duty by high passing. OTOH, when both the 104/2s and the sub cover the same mid-bass region transparency seems to suffer just a bit.
"I thought its because the subwoofer is omnidirectional in the lower region and thus adds to and expands the soundstage that the conventional speakers, whose sonic signature is directional, can't possibly convey."
Omni=directional speakers apply this same concept and resulting sonic effects to higher frequencies as well.
Thanks for your feedback. This is valuable info for me, as I ponder over the value of adding an external crossover before the amp. Would appreciate your thoughts on this.
I run B&W 804s, which are said to have a frequency response down to 38Hz -3dB and while B&W doesn't publish distortion figures I believe they probably increase distortion quite a bit at the lower frequencies...maybe under 50 Hz?. I also have a REL Storm III, which as noted doesn't like being crossed over much higher than 50 Hz...therefore I wonder if getting an external crossover, like a Bryston 10B, to let the B&Ws reproduce only freuqencies above say 50Hz is worth it. Unfortunately I live in a place where buying used and giving it a try is not easy...Any thoughts?
Sorry I'm way off topic!
If you can find an external crossover on Audiogon at a reasonable price, I'd give it a try. But if your B & Ws are down 3 dB at 38 Hz, they presumably start rolling off before that, and your REL could roll off before 50 Hz, so there may be little effective overlap. The actual LF frequency response of your B & Ws will depend on how you've located them in your room, particularly in regards to rear and side walls. I wouldn't rely on achieving the manufacturer's 38 Hz value in your home.