The Subwoofer Group Delay Thread

Discussion in 'Home Theater Projects' started by Rory Buszka, Dec 28, 2003.

  1. Rory Buszka

    Rory Buszka Supporting Actor

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    Ok, I am looking for some expert advice on this one:

    I am designing a vented enclosure for a Stryke AV12 woofer. I am wanting to get this design as "right" as possible before I build. Right now, the design is a 2 cubic foot enclosure vented to the exterior via three 3" flared ports. I have some basic knowledge of how group delay affects the sound of a subwoofer, but I still wonder how much group delay is acceptable for a "musical" vented sub. I was looking on the SVS web site to see if they had anything to say, and their FAQ page clarified to me that they do indeed pay attention to group delay when they design their subs. I realize that vented enclosures in general have higher group delay than a sealed enclosure, but I am really wondering what sort of group delay numbers I should be shooting for when I model the beast in WinISD. Right now, I am getting between 45ms and 55ms for my Group Delay numbers, and I am wondering how this compares to some of the subs you call "musical".
     
  2. Allen Ross

    Allen Ross Supporting Actor

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    OMG 45 is horrible, deff look at something less then 30 for HT, for music look for no more then 10.

    if you break a signal in the LF region down into fourier components and look at the phase plot of the speaker's transferfunction, and extrapulate the time delay curve, then you see these Forier comenents aren't produced at the same instance in the time domain, and thus: they don't superimpose properly to give you the same signal, with the same dynamics as the input.
     
  3. Michael R Price

    Michael R Price Screenwriter

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    I have no real opinion about this concept myself, since I don't have enough experience to know what group delay sounds like in the absence of distortion or frequency response anomalies. It's really debatable whether we can hear phase distortion up to a certain point. You may want to read this paper:
    http://www.adireaudio.com/tech_papers/sub_gd.htm

    You will probably want to ensure a reasonably low group delay (less than 20 ms maybe?) in the 30-60Hz range. This can be done by tuning a vented enclosure to a low frequency, under 20Hz, or using a sealed alignment. I personally would just calculate the port tuning so that with all 3 ports open you have a maximally flat response and high power handling (22Hz perhaps?) and by plugging one port you can get it slightly overdamped and probably more "musical" sounding (17Hz?). Adire recommends a small box tuned to 15Hz for the Tempest, as a compromise between music quality and output.
     
  4. GrahamT

    GrahamT Supporting Actor

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    Rory,
    I cant comment on the GD but as for that design goes, the Vb is very low and I am assuming you are going to tune to around 20 Hz. That would call for ports about 33" long. I tried the same thing with my Tempest and the performance dropped after 40 Hz. The sub will still take up a lot of room either way. Just something else to consider when designing your sub.
     
  5. Rory Buszka

    Rory Buszka Supporting Actor

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    Man, WinISD is freaking me out here. I can't seem to do anything with a vented AV12 without getting things like 65 and even 72ms group delay. Does someone have a proven design that I can look at and build?
     
  6. Michael R Price

    Michael R Price Screenwriter

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    Rory, have you tried using LspCAD?

    Edit - I realized the free version of LspCAD isn't able to model custom drivers. Give me a day or so and I'll model the AV12 vented alignments for you, I'll see what it says about group delay.
     
  7. Mark Hayenga

    Mark Hayenga Supporting Actor

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    Don't worry about group delay. Your room will almost certainly be doing much, much worse things.
     
  8. Andrew Steel

    Andrew Steel Extra

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    I have to second what Mark just said - the room acoustics at those frequencies will be much more significant unless you are in a very very well treated room.
    Has anyone wondered if passive radiator is worst, ported is next and sealed box is best - does this progression point towards dipoles being even better in trems of transient response or tightness as it is put in the adire article? It just seems like it gets worse as the order goes up.

    Andrew
     
  9. Mark Hayenga

    Mark Hayenga Supporting Actor

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    Group delay is just the negative derivative of the phase plot. Phase changes coincide with frequency response variations, and the more rapidly the frequency response changes the more phase shift you'll get. Higher order systems have more rapidly changing frequency responses, so you get bigger phase shifts where the response starts to roll off, and higher group delay.

    I think dipoles still terminate at 12dB/octave (it should since you've still got a basic mass-spring-damper 2nd order system), so theoretically it's not a lot better than a sealed box (a low Q dipole will be better than a midQ sealed box, but a highQ dipole will be worse than a midQ sealed box). I think the main benefit of dipoles is that the side lobe cancellation helps to prevent room mode excitation.

    I think I'm more or less correct, but someone call bullshit on me if I'm wrong.
     
  10. Greg Monfort

    Greg Monfort Supporting Actor

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    >I am designing a vented enclosure for a Stryke AV12 woofer. I am wanting to get this design as "right" as possible before I build. Right now, the design is a 2 cubic foot enclosure vented to the exterior via three 3" flared ports.

    ====

    OK, then you're off on the wrong foot since the cab's way too small if the specs I have are reasonably accurate, with too many vents (adds too much acoustic resistance), unless of course you don't plan to drive with it with more than a couple of watts.

    Being an HT forum, I doubt this is the case, so a compromise between a cab optimized for late night and/or SAF listening and WFO needs to be found. You don't mention what amp you're using, but I recommend it at least equals the driver's rating, and preferably 1.414-2x it as you never want to clip the amp.

    Ideally you need to find out how much power the driver can heat sink before it begins compressing (Qes/Qts goes up, ergo so goes cab Vb) and design the system around this value to keep it from going non-linear when the action picks up.

    ====

    >I have some basic knowledge of how group delay affects the sound of a subwoofer, but I still wonder how much group delay is acceptable for a "musical" vented sub.

    ====

    This is so person/system/room dependent that I can only comment on what works for me in my room.

    ====

    >I was looking on the SVS web site to see if they had anything to say, and their FAQ page clarified to me that they do indeed pay attention to group delay when they design their subs. I realize that vented enclosures in general have higher group delay than a sealed enclosure, but I am really wondering what sort of group delay numbers I should be shooting for when I model the beast in WinISD. Right now, I am getting between 45ms and 55ms for my Group Delay numbers, and I am wondering how this compares to some of the subs you call "musical".

    ====

    My rule-of-thumb WRT sub duty is to tune low enough that it's decayed to inaudibility by 60Hz which means a tuning point at/below the lowest signal the system is likely to see. 16-18Hz works well for both HT and organ music, with
     
  11. Michael R Price

    Michael R Price Screenwriter

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    Well, the alignment you described is unlikely given the port length needed. I think you can get good results using PRs in 2 cubic feet, however, using ports looks to be practical starting around... 4 cubic feet? And it doesn't look like group delay will be a problem as long as the tuning point is below 25Hz or so. I think that's less of an issue than room placement, for example.
     
  12. Rory Buszka

    Rory Buszka Supporting Actor

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    I am now going with a pair of 15" passive radiators instead of the three flared ports. The box is almost invariably going to be too big.

    Mr. Price: Thank you for your offer. Please model an enclosure that is 14"x18"x19" (internally) if you would, with the 15" PRs. I'm not sure if LspCAD will calculate masses of PRs but just the length of the corresponding 14" ports will suffice. I can calculate the mass of the PRs on my own.
     
  13. Michael R Price

    Michael R Price Screenwriter

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    Rory, no problem! I posted a picture with two screenshots from LspCAD, the top half shows the simulation at 2.83v (2 watts) input and in the bottom half it's 500 watts. Your box might come out a few liters more or less than 70, but the result shouldn't be much different.

    http://home.comcast.net/~jdprice1/pictures/av15pr.gif

    The 1400 gram PRs from Stryke are about right for maximally flat anechoic response (tuning of 23.6 Hz). It might be a good idea to move the tuning down a little bit especially if you have a small room. I think anywhere from 1400-2000 grams on each PR would work well... 2kg PRs corresponds to a tuning of 19 Hz.


    That's really funny... I don't think anyone has ever called me "Mr. Price" before... I'm even younger than you! [​IMG]
     

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