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Bass Management filter slopes - weird figures, and worrying implications...

Discussion in 'AV Receivers & Amplifiers' started by NicolasB, Nov 26, 2003.

  1. NicolasB

    NicolasB
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    I often read posts from people who seem to be under the impression that bass management is a "brick wall" filter. In other words, if the system is set up with "Small" speakers and the cross-over is set to 80Hz, they reckon that everything above 80Hz in the main channels is sent to the satellite speakers, and everything below 80Hz is sent to the subwoofer.

    This, of course, isn't true.

    Now, I've been working on some numbers for this, using my Bryston SP1.7 as an example; but I cannot believe that the figures I've got here are correct. Where am I going wrong...?


    The SP1.7's bass management's high-pass filter is "2nd order", which means 12dB per octave. In other words, for each octave below 80Hz that the signal is, the sound intensity is reduced by 12dB (correct?). Suppose we're playing a sound at reference level, 85 dB. Presumably this will be split evenly between the sub and satellite, so each will be producing 82dB for a total of 85.
    • An 80Hz signal sent to one of the satellite channels will therefore play at 82dB through the satellite speaker.
    • A 56.6Hz signal sent to one of the satellite channels will play at 76dB through the satellite speaker.
    • A 40Hz signal sent to one of the satellite channels will play at 70dB.
    • A 28Hz signal sent to one of the satellite channels will play at 64dB.
    • A 20Hz signal sent to one of the satellite channels will play at 58dB.
    The low-pass filter on the SP1.7 is 4th order (24dB per octave).
    • An 80Hz, 85dB signal sent to a satellite channel will play at 82dB through the subwoofer.
    • A 113Hz satellite-channel signal will play at 70dB through the sub.
    • A 160Hz satellite-channel signal will play at 58dB through the sub.
    • A 226Hz satellite-channel signal will play at 46dB through the sub.
    • A 320Hz satellite-channel signal will play at 34dB through the sub.
    Now, I can't believe that these figures are right! I might be able to swallow a subwoofer delivering 58dB at 160Hz. But what about dedicated satellite speakers? Can a speaker that is only rated down to 80Hz seriously be expected to produce a 70dB signal at 40Hz?

    Where am I going wrong?

    How do the 2nd-order high-pass, 4th-order low-pass filters of the SP1.7 compare to those of other processors?

    Even if other processors have 4th- or 5th-order high-pass filters, you're still going to end up pushing quite significant low frequency signals through satellite speakers. (4th order would be 70dB at 56.6Hz, or 58dB at 40Hz). Can dedicated satellite speakers really handle this kind of signal?

    Can subwoofers really cope with anything up to 66.5dB at 160Hz (assuming all 7 satellite channels are being driven at 80Hz, 85dB) on top of the LFE channel?

    And what are the implications for (for example) setting up a Behringer Feedback Destroyer to work with the subwoofer? Do we need to be doing frequency sweeps up to 300Hz or higher in order to fully control what the subwoofer is doing? And if so, can we actually get a room-curve reading at 300Hz without doing damage to the subwoofer in the process?

    And if the satellite speakers are only 6dB down at 56.6Hz, then surely they're going to be nearly as much of a problem as the subwoofer? (If your room imposes a +15dB peak at 56.6Hz, but this is "corrected" by a Behringer Feedback Destroyer, then your uncorrected satellites will be quite a few dB louder than your corrected subwoofer at that frequency!).
     
  2. kryten

    kryten
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    Bit busy so this is a quick reply. You've not taken into account the rolloff of the speaker itself, but you are mostly correct with the numbers.

    Smaller satelitte speakers won't manage to output the full signal they are passed at 40Hz, but they will be putting out something.

    Similarly the sub won't be as efficient at 160Hz, but its still active and producing output (unless it has its own filters).

    Now factor in the mid/tweeter crossover as well and you start to realise how difficult crossover design can be!!

    'Brick wall' filters have their own problems - I used a digital sony one at 72dB/octave for a while in the car and it sounded awful!
     
  3. lowrider

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    Yes, that is the common slope for bass management, as far as I know... :rolleyes:

    But the satellite´s response will compensate for the less steep filter...

    Also thats why you should equalize all speakers, like with TMREQ and KRELL amongst others... :devil:
     
  4. NicolasB

    NicolasB
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    Ah, but, but, but, isn't it the case that some dedicated satellite speakers can actually be damaged if you feed them a high-level, low-frequency signal? Is standard (2nd order) bass management actually going to be adequate protection in cases like this?

    This presumably also means that the response of a pseudo full-range speaker and the response of a dedicated satellite speaker, will be significantly different in the sub-80Hz range, even though both are set to "Small".

    Ugh. :mad:
     
  5. lovegroova

    lovegroova
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    Nic

    Please for godness sake, stop it, you'll drive yourself mad!

    Go and listen to some music and make sure you listen to the music and not the hifi. Relax and enjoy it! :smoke:

    Please take this post in the friendly manner in which it is intended.:smashin:
     
  6. lowrider

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    All true, so what, just harder work to configure the system properly... :suicide:
     
  7. Nic Rhodes

    Nic Rhodes
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    The THX bass management characteristics are 12 dB/octave high pass and 24 dB/octave low pass.

    Set the speaker settings are small and subwoofer on, then the bass management will attenuate low bass to the main speakers accordingly, as well as attenuate upper bass to the subwoofer.

    The high pass filter characteristic is 12 dB/octave, at 80 Hz, then a 40 Hz signal going to the speakers will be attenuated by 12 dB, but not removed completely. A 20 Hz signal going to the mains will be attenuated by 24 dB, but again, not removed completely. This is why you notice ‘some bass arriving at the mains, even though you have set them to "small". However, all is well and functioning normally (but not very good for small speakers!!).

    The 12 dB/octave high pass slope was chosen as it best integrates with the majority of home theater sealed-box speakers. These will have a 12 dB/octave roll-off below their design lower limit, combining to provide a perceived 24 dB/octave roll-off, and matching the 24 dB/octave low pass characteristic of the subwoofer output.

    This is how I understand the situation, you numbers seem fine
     
  8. Nic Rhodes

    Nic Rhodes
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    oh forgot to add bass manipulation (BFD?) really should be done on ALL channels like Lowrider says, this is why we need you mega power processing chips
     
  9. NicolasB

    NicolasB
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    IMO, the entire purpose of having a high-end system (and configuring it properly) is to enable the listener to do precisely that. The poorer the system or config are, the more the system corrupts or colours the sound and thus draws attention to itself; and so the harder it is to just listen to the sound rather than the system....
     
  10. alexs2

    alexs2
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    Nic....I also agree with the figures you've calculated as being broadly correct,from the processor's point of view.

    As kryten and others have said though,the roll-off of the speaker's response has to be factored in,as well as the contribution of any crossover components operating in that region.

    Again,to emphasize what kryten rightly said,"brick-wall" filters can add awful problems in terms of sound quality,with ringing and phase shifts added in.

    The theoretically ideal way of dealing with the entire processor/amp/speaker/sub/room interaction surely has to come down to a form of digital equalisation across the frequency band,as some manufacturers are beginning to offer,and hopefully the resolution of these will continue to improve as digital processing also improves in speed and cost.

    Then it comes down to those writing the algorithms doing a good job.....but that no doubt will be another matter!

    For the moment,having lived with 2,3,4 and 5 driver speakers,passive and active bi and tri-amped systems,with and without DSP,I still think the best way is to simply listen,experiment,and find the sound that suits you best.

    Some people like a bit of 2nd harmonic distortion chucked in,some don't....others like horns and SETs...and so on.

    To further muddy the waters,it should be remembered that not every processor's bass management is as good as the next's,and the distortion levels in some modes and at some frequency levels can be moderately high....in tests of course,and whether or not this is audible is debatable.
     
  11. Anodyne

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    This is what the Senior Audio Engineer at M&K told me:

    "Also some receivers' processors bass mgmt system incorrectly take the bass pickoff so that it's after the delay section which has yet two MORE problems: some brands delay/advance ONE channel, and some delay or advance ALL THE OTHER CHANNELS when any one is selected and adjusted. Therefore the bass below crossover can be phase shifted a full 90 to 180 degrees from where it belongs,; if it was shfted 180 degrees then if there was bass in that channel it would essentially disappear. so you would be hearing the sum of the 5 channels below 80 Hz with the subtraction of one of those channels; kind of like a low freq comb filter."

    Don't know whether this sheds any light on the matter, but interesting nevertheless.

    Will
     
  12. NicolasB

    NicolasB
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    Yet another reason to set all your speakers to Large and have done with it....

    (sigh)
     
  13. lovegroova

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    It seems to me that having spent thousands of pounds and endless hours on your system you still don't enjoy listening to music on it (e.g. selling your SP1.7 having only recently purchased it.)

    I'm just trying to save you from yourself :D

    PS. Alexs summed it up far more eloquently than I ever will.
     
  14. Anodyne

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    Or maybe try doing the bass management in the analog domain using one of M&K's LFE-4 contollers. I've often wondered if these would be an improvement over a pre-amp's built in processing.

    Will
     
  15. NicolasB

    NicolasB
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    I'm more interested in films than music, but yes, that's not an innaccurate assessment. Sadly my auditory reach exceeds my financial grasp.

    That would presumably fall foul of precisely the same problems the Senior Audio Engineer was talking about.
     
  16. alexs2

    alexs2
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    I'm lucky in that whilst my auditory reach,like yours,exceeds my financial grasp also(I definitely know what I'd LIKE to have!),the disparate group of components I now own works very well for me,and I also know I'd have to spend a LARGE amount of money to make it a lot better(as opposed to simply making it sound different,which I could do for anywhere between a few hundred and a few thousand pounds).

    Thus,in general,I'm pretty happy with the way it sounds,depending on the mood I'm in,and if I can really be bothered to warm up the Krells and wait for 30 minutes....or just dig out the CD player,a good DAC,and the headphone amp.....it's all totally subjective in the end.
     
  17. buns

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    what form are these filters? Are they software of hardware based? What about corssovers in our speakers..... do they have the same slopes? how about if using an active crossover?

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  18. lovegroova

    lovegroova
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    I'm most definitely with alexs2 on this one. I'd love to have £200k to spend on a system but I can't. (I'd love to have his Krell amps but I can't :D). I'll enjoy what I have and when I have the time, money and inclination to improve things, I will.
     
  19. NicolasB

    NicolasB
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    The problem is that most people have a minimum level of sound quality that they find acceptable. Drop below that, and they simply never actually listen to the system. Despite the fact that my current system (assuming I ever manage to buy a centre speaker and matching rears) has a list price of £16K plus cables, it hasn't yet crossed my minimum quality threshold. To make it cross it I'd have to spend several thousand more (much of it on acoustic treatments) which I simply don't have. :(

    Anyway, we're getting off the subject a bit, here....
     
  20. Nic Rhodes

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    welcome to real life, dreams exceeding expectations and a run of unfinished projects :)
     
  21. Jeff

    Jeff
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    I may be mad but I personally think you can have a poor quality sound that sounds good and perfectly enjoyable and high quality sound that just isn't. What ever price range you go for, try to make sure it sounds good and you can enjoy it. Spending more money doesn't necessarily make this easier.
     
  22. lowrider

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    Not really, it works very well with good monitors, for instance 805 Sig if you like, + two good subs...

    Even better if you have all speakers and subs at aproximately the same distance and then a bit of EQ for all channels... :smashin:
     
  23. buns

    buns
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    What are the main channels encoded to actually? I know that the sub is encoded right up to 120Hz but thats it...... so i def see an arguement to run right up to 120Hz with the sub

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  24. NicolasB

    NicolasB
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    Yeah, but there's the thing I was talking about just now.... Sig 805s are "pseudo full-range". The bass response of the speaker doesn't tail off noticeably until you're below 50Hz, and even at 40Hz is only -6dB.

    That means that the "satellites producing 76dB at 56.6Hz" condition applies. Hence, need for EQ on satellite channels as well as subwoofer, and all the rest of it, even if they are set to small. And that's before you start to consider bass-management-induced distortion, phase issues, and so on.

    I must say I'm increasingly forming the opinion that, unless your speakers are dedicated satellite speakers (i.e. not intended to be used at all except with a subwoofer, and with a built-in bass roll-off that starts at 80Hz) you may well be better off leaving them set to Large.

    An interesting (related) question:

    We all know that, in theory, the LFE channel of a movie is 0-120Hz. But in practice it's pretty unusual for a real-world film soundtrack to include anything in the 80-120Hz range. Similarly, the theoretical range for the surround channels is (depending on who you ask) 20Hz-20kHz, or even 0-20kHz. But how often in the real world do you get information in the main 5 channels that is below, say, 40Hz?

    If the answer to that is "hardly ever" then that's even more of a reason to set all your speakers to "Large", even if they only go down to 40Hz or so.

    (I wonder if there's a difference between fronts and rears, here, too - are rear channels less likely to contain low-frequency information than front channels? Of course there would still be a better case for setting your speakers to "Small" for stereo metrial than there is for material with an LFE channel.)
     
  25. lowrider

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    Right, so if you xover at 50 or 60 hz the roll-off will compensate the different slopes, just like it is suposed to...

    I dont think that distortion is an issue at low frequencies, at least with fair quality equipment, and phase is not an issue with all speakers at the same distance...

    Of course it would be ideal all speakers large, 20hz - 20khz, but then you would have to have an huge treated room, or a lot of EQ...

    The surround channels, as long as timbre is well matched are not an issue, regardeless of loosing some bass, on the other hand, as they usually are close to walls, bass will be reinforced anyway...
     
  26. chrisgeary

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    buns, i think the main channels (as may potentially be the case for the centre and rear) can be encoded full range. someone told me once that armageddon has LFE in the front L+R channels as well as the .1 channel. whether movie soundtracks only use LFE for sub 120hz or not is another question. anyone know?

    Edit: duh! just re-read my post - i've restated your question buns! my point was to add the armageddon statement. :smashin:
     
  27. NicolasB

    NicolasB
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    Yeah, but with most hardware you can't do that, because then you lose the stuff in the LFE channel above 50/60Hz.

    Depends how much distortion you're talking about. For cheaper equipment I've heard figures of 20% being bandied about....

    How many listening rooms actually permit that?
     
  28. buns

    buns
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    so in fact we should be listening at full range and sub 120 because that is the way the track is encoded....... is doing anything else not rather akin to cutting the sides off a scope picture and zooming it to fit your 16:9 display?

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  29. lowrider

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    I am just showing you a feasible alternative, but you always need proper gear and room for near perfection... :devil:

    Anyway, I enjoyed very much my AV while I "just" had a receiver, still with a proper room and setup, I´d say... :laugh:
     
  30. NicolasB

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    Well, that's certainly a valid argument.

    Sub/sat systems have some potential advantages too: you can get better treble performance per £ if you can just forget about the bass performance; and if the bass really is only coming from the sub, it's likely to be easier to control in terms of room effects.

    Obviously a lot of it is going to depend on what the actual content of the main 5/6 channels in 5.1/6.1 recordings is in practice, as opposed to in theory. If your recording does actually have stuff going on at 20Hz in the main channels, then very few main speakers are going to do a good job of it without bass management....

    I think perhaps one conclusion we can draw from this thread is that bass management is likely to be substantially less useful for "bookshelf" and "stand mount" speakers than it is for dedicated satellite speakers.
     

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