Crossover settings - avoiding a bass hole!

Discussion in 'Speakers & Subwoofers' started by Michael.M.C, Jan 17, 2004.

  1. Michael.M.C

    Michael.M.C Stunt Coordinator

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    Here is my situation. I have a new HK AVR325, a new Athena AS-P400 and various other speakers available for my setup.

    Among those are Pinnacle Alpines @ 45Hz - 22 kHz (+/- 3 dB), nice fronts but too large. I have some Bose double cubes (I am told these are effective from 200Hz up) and am considering a couple of additional Athena S.5 bookshelves which would be @ 80Hz-20KHz +/-3 dB.

    My main desire for the Bose is due to the size, mainly for the front left and right. (I do not have a Bose sub)

    The HK has a Triple Crossover Bass Managment System allowing me to set different crossover settings for each speaker group.

    So far, so good.

    BUT, when setting the HK for the sub, the instructions say to set it not lower than the HIGHEST of any of the main speakers. If I used some Bose speakers this would mean setting the sub frequency to 200Hz. However, it is my understanding the Athena sub is only effective to a little over 100Hz.

    Should I dump the Bose? If I use the Bose will I have a hole between the sub's 100Hz and the Bose 200Hz?

    Suggestions?

    Thanks!
     
  2. Edward J M

    Edward J M Cinematographer

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    Dump the Bose.

    To avoid localizing the subwoofer, try to keep the xo at 100 Hz or lower, preferably 80 Hz.

    If you use an 80 Hz xo with a 2nd order high pass bass management filter for the surround speakers, I would recommend using speakers with an F3 rating of around 60 Hz.

    Regards,

    Ed
     
  3. Michael.M.C

    Michael.M.C Stunt Coordinator

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    Edward, Thanks for the advise. I do not know what this statement means: "2nd order high pass bass management filter"

    Which speakers are you referring to with this sentence? "I would recommend using speakers with an F3 rating of around 60 Hz."

    If I get a couple of Athena S.5's they would be for the front and are 80Hz-20KHz +/-3 dB.

    I am considering moving the Pinncles to the rear. (or to the side if I go to 7.1)

    Thanks again!
     
  4. Edward J M

    Edward J M Cinematographer

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    The bass management circuit consists of high and low pass filters. High pass refers to the surround speakers you have set to small. Low pass refers to the subwoofer.

    If you select a crossover frequency of 80 Hz, the AVR will allow the surround speakers to play above 80 Hz, and the subwoofer to play below 80 Hz.

    To smooth the transition between the surrounds and the sub, the AVR will also allow the surround speakers to play below 80 Hz, albeit at a constantly diminishing rate of volume. The rate of volume reduction is known as the "filter rate".

    A 1st order filter is 6 dB/octave, 2nd order is 12 dB/octave, and so on. A typical high pass filter rate is 12 dB/octave. So if you select 80 Hz as your crossover frequency, an octave below that would be 40 Hz. The high pass filter would impose a 6 dB reduction in volume by 60 Hz, and a 12 dB reduction in volume by 40 Hz, as compared to the volume at 80 Hz.

    Conversely, the low pass filter rate for the subwoofer is usually steeper, typically 4th order (24 dB/octave). The reason the low pass filter rate is so steep is to prevent the subwoofer from playing much at all above the selected crossover point which would otherwise allow it to be easily localized and/or play above its design limits.

    The F3 rating of a speaker is the bass frequency at which the speaker volume has dropped 3 dB compared to the level at higher frequencies. So the F3 for the Athena S.5 would be 80 Hz. If you want to use the Athena, I'd recommend a crossover of 100 Hz.
     
  5. Michael.M.C

    Michael.M.C Stunt Coordinator

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    Ed,

    Thanks for explanation. I learn more each day.

    I am heading down to BB this afternoon. I may look at getting the full set of Athena Point 5's since I can take my center back to where I bought it, sell the bose on ebay and use the Pinnacles in a separate room for music until I decide whether to go 7.1.

    Just selling the bose will fetch me more than the set of Point 5's. That way I'll have a complete set of Athenas.

    I have to say Athena seems to be a good company. I had a few questions about the sub, emailed them a couple of times and each time they responded right away with advise and suggestions, and not just about their own product.

    Mike
     
  6. dave alan

    dave alan Second Unit

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    BM is based on a Linkwitz/Riley 24 dB/octave crossover (or, dual cascaded Butterworth filters).

    The original THX spec of a 12 dB/octave HP filter had sealed satellites in mind, which have a natural 2nd order drop off in the low end, which, when affected by a 2nd order HP filter, theoretically will then have a 24 dB/octave roll off that should match the 24 dB/octave LP filter that's applied to the subwoofer.

    Unfortunately, most of us use ported satellites, which roll of at a 24 dB/octave rate. Applying a 12 dB/octave HP filter results in a 36 dB/octave slope.

    In a perfect L/R crossover situation, both the HP and the LP filters cause the amplitude to be down 6 dB at the crossover point. The result is that they should sum to unity gain, and continue to sum to unity gain across the entire crossover region.

    This means that, instead, if you select 80 Hz, your ported sats are down 6 dB @ 80, 24 dB @ 60 and 42 dB @ 40 Hz, while your sub will be down 6 dB @ 80, 18 dB @ 120 and 30 dB @ 160 Hz.

    Room gain further skews the slopes, as the lower the sats go, the more room gain can lessen the slope, while the higher a sub goes, the less room gain affects the slope.

    Add an additional change in slope if the sub is corner loaded.

    Usually, this will cause a dip @ crossover and a hump around 90 Hz.

    Confused yet?

    The best thing to do, generally, is to graph your sats with the sub off, then graph your sub with the sats off and, finally, do a graph with both on, all on the same graph.

    This usually will explain a 'hole' or a 'hump' at and around the crossover point.

    Selectable slopes, adjustable phase and sub placement will allow for a fairly flat response across the crossover region in most rooms.

    Most people seem to be depending on EQ (BFD) to correct these scenarios, though the truth is that EQ has never been able to correct for a poor crossover configuration or room induced problems (and, let's face it...feedback ain't a big HT problem). Parametric EQ should be used as a 'season to taste' tool, IMO. Unfortunately, selectable slopes and adjustable phase are not available in most MC audio setups, leaving the BFD as the cheap fix of choice.

    Sorry for the ramble...I just wanted to clarify that the HP filter in your AVR will cause the amplitude to be down 6 dB at the filter point, not 0 dB.

    BTW, definitely dump the Bose.
     
  7. Edward J M

    Edward J M Cinematographer

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    Great post, Dave, especially about the THX surround speakers being acoustic suspension with a natural 2nd order roll-off. I considered throwing that into my original post, but didn't want to confuse the issue and you explained it perfectly. Agreed, most of us use vented surrounds and the surround/sub roll-offs are not symmetrical - a problem some have pointed out and deal with by using external BM with adjustable filter rates.

    Anyway, I didn't say (and hope I didn't imply) that the amplitude would be 0 dB at the filter frequency. I said the level (for a 2nd order filter) would be -6 dB 1/2 octave lower, and -12 dB 1 octave lower, relative to the level at the filter frequency. I actually never specified the level at the filter point.

    I agree, the filter point is the frequency where the response of the filter falls to some level below that of the unfiltered ("passed") frequencies - generally -6 dB.

    This is a pretty good link for general filter and crossover information.

    http://www.eaw.com/support/processors/faq.shtml#1
     
  8. dave alan

    dave alan Second Unit

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    Sorry to have misread your post, Edward.

    Also, as usual, a great link...Thanx!
     
  9. Philip>L

    Philip>L Stunt Coordinator

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    According to this link (posted in this forum's FAQ)regarding Bose cubes:

    www_intellexual_net/boseframes.html

    The frequency response of the satellite cubes has been measured at:

    280Hz to 13.3kHz @+/- 10dB

    So you're going to have a BIG, BIG hole if you try to use these with a 100Hz crossover.
     
  10. Michael.M.C

    Michael.M.C Stunt Coordinator

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    Philip, Et al,

    That's a bigger hole than even I had imagined. [​IMG]

    I am definately NOT going with bose. I am trying to decide what set of relatively small speakers to buy (another topic in this forum).

    Thanks,

    Mike
     
  11. MuneebM

    MuneebM Supporting Actor

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    On the note of crossover settings to avoid bass holes: I currently have an HSU STF-2 subwoofer. Its an awesome sub, but I can't really feel the rumbles while seated on my couch, 10 feet away from the sub, mainly because I have a carpeted concrete floor in my basement. The same sub really shakes the room at my friend's house, which is on the 2nd floor. All this to say that I have to run the HSU's gain pretty high, 3/4 or more. When I run the crossover on the HSU at 70 Hz, at this high gain, I can sometimes hear "port distortion" if there is a loud explosion (especially when the wall explodes during the battle at Helm's Deep in LOTR 2). I have to run my HSU at 70 Hz because my HTIB front speakers have a frequency response of 70Hz and up.

    I don't want to run my HSU sub's crossover so high, so my solution was to get myself a new set of speakers. I ordered a pair of JBL S310II floorstanding beasts to resolve this, and their frequency response is 37 Hz and up. Once I get these speakers, should I run my HSU at 37 Hz and lower, OR should there be some form of intersection to avoid any hole at all? The HSU performs incredibly well below 40 Hz even at high gain. Any suggestions? What do people normally run their sub's crossover at when they have large floorstanding front speakers with so much bottom end?
     
  12. Edward J M

    Edward J M Cinematographer

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    The port noise and/or harmonic distortion you are hearing won't be solved by lowering the xo of the sub. You are simply pushing the STF-2 too hard in the 25-40 Hz region for the room/floor/distance and exceeding the limits of the sub, and it is chugging/distorting, etc. After all, it is only a 10" woofer with an 8 mm xMax and it has limitations.

    How are you managing your bass? You should be doing it with the AVR or pre/pro and setting the STF-2 crossover to OUT. All speakers set to small, the sub set to on, and an 80 Hz (or maybe 100 Hz) xo is about right for your current HTIB speakers.

    I recommend against a speaker level connection to the STF-2 (i.e., using the STF-2's fixed high pass and variable low pass filter) for a number of reasons, but the primary one is that it sucks too much AVR amp power that would otherwise be reserved for your surround speakers.

    You'd probably be best served by adding a second STF-2 and stacking them than by ordering larger front towers. Be aware that the distribution of bass varies greatly both within a DVD and between DVDs. Some DVDs make heavy and consistent use of the LFE channel, and others don't. And some DVDs have full power, full range bass in all channels and some don't. So the benefit of using larger mains will not always be realized, depending on the DVD and depending on how you configure your BM circuit in your AVR.

    Regards,

    Ed
     

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