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Benefits of a Higher XO Frequency (LPF)

Discussion in 'Configuration & Setup' started by Zing, Oct 4, 2010.

  1. Zing

    Zing Retired Admin Famous

    What benefits are there to crossing over to the subwoofer at a higher frequency? I'll be glad to read the general benefits, though I think I know what most of them are. However, I'm inquiring about two specific aspects.

    First, if you're using a traditional 2-way speaker (1" soft dome and 7" Woofer) that's fairly inefficient (84dB rated sensitivity), would crossing to the sub at, say, 120Hz improve the sound quality of the midrange? Clarity? Definition?? And further, would characteristics such as imaging and soundstage be improved as well?

    Secondly, does this higher setting reduce the burden on the amplifier in such a way (albeit indirectly) that it essentially makes the speakers a bit more efficient (assuming the lower frequencies [<80Hz] are where the speaker is the most inefficient)?
  2. soundhound

    soundhound Well-Known Member

    Unless the subwoofer is located very close to your mains, crossing over at any frequency higher than about 80Hz will allow your ear to hear the sound of the subwoofer as a distinct source, which is not good. I cross my subs over at 100Hz (mostly to cure a room problem), but they are right next to my mains. Directing more low frequency material to the subs is always going to lessen the load on the amplifier which drives your mains, but once you get above 80Hz or so, the amount of this benefit is going to start to diminish. Its the very low frequencies which use the most power, and are most demanding of power supply current in your power amplifier.

    A speaker's efficiency is the same regardless of xover.
  3. Flint

    Flint "Do you know who I am?" Superstar

    1) For the fullrange bookshelf using a higher LFE crossover point will reduce the bass demands from the speaker and thus reduce the distortion with most music content at the same SPLs. Removing the bass also improves the dynamic range from the system at the same SPLs. The midrange would be cleaner, and the music would be slightly more defined.

    2) It doesn't necessarily reduce the "burden" on the amp to use a higher crossover point, but it does reduce the power demand on the amp as the deep bass can consume lots of power. There is no way to increase the efficiency of a speaker, so one watt will still translate to the same SPL, but the amp will have more headroom available for the midrange and treble if the bass is taken out of the signal.

    But those are only the benefits to the main speakers. You also have to take into account the impact on the bass of making such a move.

    Most commercial subwoofers are not engineered to sound good above 80Hz and it is very likely a 7" woofer will perform cleaner and tighter at 100Hz than one of those big, loose subwoofer drivers found in the most modern subwoofers. Also, the ability to "locate" the subwoofer is much easier as you raise the frequency range it operates in, and the room acoustics start getting goofy around 100Hz - the best placement for deep bass is typically in a corner while the best placement for the lower midrange is often better when the driver is placed away from the walls. There is more ability to get "stereo imaging" at higher frequencies, so the necessity for stereo subs is greater at higher frequencies, like 120Hz.

    In general, it is a bad idea to crossover a single LFE style sub above 80Hz.
  4. Zing

    Zing Retired Admin Famous

    My sub is indeed close to my mains, right behind the left speaker. And it was only after running True RTA that I noticed a smoother response after increasing XO point. It kept getting smoother and smoother as I incrementally went up to 120Hz. And that conflicts with Flint's statement.

    I don't understand it but my response had all sorts of issues when I crossed at 70, 80 and 90Hz. But at 100, 110 and 120, each of them was smoother than the previous one.

    Perhaps stranger still is the conflict with SVS' instructions concerning tuning and port plugging. As we all know, with each twist of the tuning knob, you're supposed to plug a port. Mine, for example, is natively tuned to 25Hz with all 3 ports open. If I turn the knob to 20Hz tune, I'm supposed to plug 1 port. And if I tune it to 16Hz, I should have 2 ports plugged. Again, using True RTA and following SVS' guidelines, it was a train wreck no matter how I had it tuned. The only thing that worked like it was supposed to is that each lower tuning decreased its output by a few dB. But then I started running sweeps for the various tunings without using any port plugs and the smoothest response of all was a plug-less 20Hz tune. Go figure.

    And yes, I realize I can't actually increase my speaker's efficiency with a crossover setting. Available headroom for other frequencies was what I meant.
  5. Flint

    Flint "Do you know who I am?" Superstar

    In most cases the quality of the sound from a standard subwoofer (like the PB12 Ultra) is not all the great when you start getting above 80Hz.
  6. soundhound

    soundhound Well-Known Member

    Ultimately smoother response is the reason I increased my subwoofers' crossover point to 100Hz. If your subs are located right next to your mains, there is no reason to not increase the crossover point as long as it doesn't introduce problems from the sub's upper range. If you are only using one sub though, having a higher than 80Hz crossover point is definitely going to screw up things - using two subs (preferably in stereo) would remedy that.

    This is the response of my system; The upper graph is with the use of a supertweeter, and the lower one is without.

  7. Alien

    Alien Active Member

    My sub locations' average output and response happened to be best directly between my mains, behind everything else. I discovered that my higher XO setting seems to also resolve acoustic issues caused by my mains placement...I put them pretty close to the side walls.

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