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speaker build, should drivers be off center


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stevdart 
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Posted: February 18, 2006 at 5:56 AM / IP Logged  
Rod Elliott, in article on passive filters says this: 
  • A really well designed crossover is of no use if the box is not designed correctly, is inadequately braced, or has drivers mounted equidistant from two or more edges - these cause high frequency refractions that "smear" the stereo image.

    It seems to me he is saying that if the drivers are mounted the same distance from both the left and right edges of the front baffle, the sound will suffer.  Now, when I look at commercial speakers and even the kits like this Exodus, I can see that drivers are alligned down the center in most(?) cases.

    Is this a real problem, and does it affect the mids as well as the tweeter?  Should it be enough of a concern to design an array of drivers off-center?  I'm referring to a M-T-M-W three-way home speaker, where the MTM will mostly fill up the narrow topmost part of the cabinet.  I don't know whether to allign them down the center or whether I should design it so that one side is a little wider.  I'm afraid the end result might be that it looks like I measured wrong before I cut the holes if there's not an equal distance to each side.

    What's your opinion on this?

  • Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
    sedate 
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    Posted: February 18, 2006 at 7:04 AM / IP Logged  
    Uhh.
    If the sentence "these cause high frequency refractions that "smear" the stereo image" wasn't couched in such a techo-literate paper, I'd beat that statement like a pinata.
    The concept I guess he's conveying sounds *kinda* like standing wave BS that floats around car stereos... except he makes a wierd choice in the word 'refraction'... which means nothing that I can relate to an operational tweeter or mid in a cabinet:
    deflection from a straight path undergone by a light ray or energy wave in passing obliquely from one medium (as air) into another (as glass) in which its velocity is different
    Perhaps this guy is implying that the High frequency sounds eminating from the tweeter can hit the wood, cause vibration, and then the wood can vibrate the air, which then, *ahem* would "' smear' the stereo image" ...
    Which of course is something you'd need a shotgun-mic to pic up and not something that would be audible in anyway or other wise 'smear' the sound.. let alone why that would only happen when drivers are equidistant to the sides of the cabinet.
    For that matter, what is 'smearing' the stereo image? A stereo 'image' is the auditory sense of being placed in an orchestra, with proper right/left seperation and vocals in front and what not... I couldn't concieve, especially given the total lack of explanation here, that the placement of drivers INSIDE a cabinet could possible affect the 'stereo image' of the finished product.
    Lets be realistic here, xovers, power applied, distance between speakers.. shure, all would greatly affect the stereo image, but proper driver alightment? Equidistant from two edges? OH COME ON?! Like EVERY OTHER SPEAKER CABINET I'VE EVER EVEN SEEN?
    stevdart, I think you should make your new speakers look pretty and not worry about unexplained hogwash that clearly contridicts vast tracks of experience..
    "I'm finished!" - Daniel Plainview
    stevdart 
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    Posted: February 18, 2006 at 7:50 AM / IP Logged  

    I thought maybe he was referring to a time factor, when sound waves roll off the baffle and around the sides of the cabinet, on the outside.  I didn't consider that he might have meant the inside of the enclosure.  But I think you're right about the aesthetics and all the other speakers that are produced.  And I don't doubt these speakers will have plenty of little irregularities anyway.  ;)  concept sketch with 1/2" added to one side...I'd rather have the red stripe running through the center - meaning that the drivers are centered - at the top and just off center at the wider bottom part.  My original idea was to have that odd off-center look at the bottom anyway.

    Z66_withtv.jpg

    kfr01:  You wrote some very good reviews.  You have found your calling.  With your big kit build, how did you bevel the edges?  Table saw cut and sanding?

    Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
    kfr01 
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    Posted: February 18, 2006 at 8:31 AM / IP Logged  
    stevdart wrote:

    I thought maybe he was referring to a time factor, when sound waves roll off the baffle and around the sides of the cabinet, on the outside.  I didn't consider that he might have meant the inside of the enclosure.  But I think you're right about the aesthetics and all the other speakers that are produced.  And I don't doubt these speakers will have plenty of little irregularities anyway.  ;)  concept sketch with 1/2" added to one side...I'd rather have the red stripe running through the center - meaning that the drivers are centered - at the top and just off center at the wider bottom part.  My original idea was to have that odd off-center look at the bottom anyway.

    Z66_withtv.jpg

    kfr01:  You wrote some very good reviews.  You have found your calling.  With your big kit build, how did you bevel the edges?  Table saw cut and sanding?

    Stevdart:

    On the road right now, but I can look up the answer to your off-center question in the loudspeaker cookbook by vance dickason when I get home on Sunday.  It actually has a relatively decent section explaining edge diffraction.  Stay tuned.

    Kevin and Dan W.  of Exodus/Adire said they didn't offset the tweeter because it didn't make any practical difference with regards to final frequency response. Ideed, they tried it both ways, see the prototype here: http://www.diycable.com/main/product_info.php?cPath=24_92&products_id=539

    I guess if I was starting from scratch and making specific left/right speakers, I would offset the tweeter.  The theory in Dickason's book seems to be good.

    Beveled edges?  Cheapo ryobi router and the right 45 degree chamfer bit. 

    Cheapest I found at the size I wanted:

    http://www.routerbits.com/cgi-routerbits/sr.cgi?1140272577_5539+33

    #2306.  It worked relatively well, but I'd invest in a large roundover before buying another chamfer.  A large roundover should reduce diffraction effects even more (in theory) and provide an easier edge to wrap veneer around.

    Cheers!   Meeting w/ bank + 10 home showings yesterday.   20 home showings today.  Ahhhh! 

    New Project: 2003 Pathfinder
    stevdart 
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    Posted: February 18, 2006 at 9:02 AM / IP Logged  
    Good info, thanks for the router tips.  If it's just the tweeter and not the mids, I'd like to find out if just about a quarter of an inch difference is enough to be considered offset.  Probably like 1" on one side and 1 1/4" on the other.  Then I could just align all the drivers to the same vertical line along the inside edge and that would result in a slight tweeter offset, and the mids would be centered.  I won't be testing using a microphone so I'm just working with theory.
    Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
    sedate 
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    Posted: February 18, 2006 at 9:45 AM / IP Logged  
    kfr01 wrote:
    It actually has a relatively decent section explaining edge diffraction. Stay tuned.
    *I can't wait.*
    "I'm finished!" - Daniel Plainview
    DYohn 
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    Joined: April 22, 2003
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    Posted: February 18, 2006 at 10:02 AM / IP Logged  

    The theory of offsetting drivers (especially high-frequency drivers) is to minimize edge diffraction effects.  Edge diffraction is a type of distortion caused by the high frequency sound waves moving at a different angle when they reach the edges of the enclosure.  It can be measured as a "ringing" or harmonic of the wavelength equal to the width of the baffle.  If the sound source is centered in the baffle, the ringing is more pronounced since it emanates from two sources (left and right edge) at the same time.  In severe cases it creates a "sound of the cabinet," a constant tone that never changes like a resonance, and can be very annoying.  By offsetting the tweeter two different edge tones are created, each at much lower SPL which is easier to get lost in the overall sound.

    The same effect in woofers is called baffle step and creates a reduction in bass frequencies proportional to the wavelength of the baffle's width.  Reducing edge diffraction effects is the main reason for beveled or rounded cabinet edges, and for the strange shapes of some higher-end speaker systems.

    Now while it can sometimes be detected and often times measured, is it important?  Most people cannot hear it as most times the SPL of the HF edge diffraction is low compared to the main signal.  Also, depending on the width of the baffle, the wavelength may be outside any critical musical areas or outside hiuman hearing (and unimportant) or right in the middle of the main information area (and completely covered up.)  In most cases, centering the drivers will not cause sufficient HF edge diffraction to be worth making them offset.  Bass baffle step diffraction, on the other hand, can cause a system to sound like it has too much mid range or weak or soft bass response, and often must be compensated for.  Offsetting the woofer can help reduce this effect, but most times you have to compensate by including a BSC (baffle step correction) filter in your crossover.  Or by increasing the low frequency efficiency (like using two woofers) to overcome it.  Or by making the baffle wider.

    There is a more practical reason for offsetting tweeters in home audio speakers.  Since high-frequency sound is directional, it is sometimes useful to move the left tweeter further left and the right tweeter further right to help widen the sound field.  But again, while the effect may be measurable, it is not always significant.

    Some people just like the look, of course.  This is also a legitimate reason for offsetting drivers.  I fall into this camp.  speaker build, should drivers be off center -- posted image.  Bottom line: center your drivers unless you can measure or hear any cabinet ringing caused by edge diffraction.  Then try rounding the edges first to eliminate it.  Unless you like the way offsetting looks, then go for it.

    As far as time alignment, that is a completely different thing and is corrected by aligning the acoustic centers from front to back rather than from side to side.  There are many papers written on this...

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    stevdart 
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    Posted: February 19, 2006 at 2:10 AM / IP Logged  

    Thanks for the explanation, DYohn.  You make these things so much easier to understand when you tell it.  speaker build, should drivers be off center -- posted image.

    I looked at Barry's Handy Freq-to-Wavelength chart to get an idea of how center-aligned MTM could cause problems.  This baffle, at 7" wide, looks like it might be a bit narrow to work with unless I take some precautions against these possible problems.  It looks like 7" wavelength is about 1900 Hz, half of 7" would be about 3900 Hz.  Multiples of 7" would hit at about 950 and 650 Hz.  The mids are crossed at 500 Hz but still outputting well below that, so I could see how their output might be affected a couple more places down the scale.

    I will round over the edges for sure, and I think I should go ahead and do the offset and push the tweeter out to the outer edge, the mids toward the inner.  Doing that, there wouldn't be a whole lot of difference in the width of the baffle sides for either driver, but I'd rather take reasonable precautions since I'm scientific only to a degree in this hobby (testing is crude, for one)  ;).  My present MTM center channel is 7" wide and there was plenty of room for the offset that I used on it...and it sounds really good.  I'll copy that look on these mains and keep the driver alignment as it is on the center channel, just doing a rebuild of the cabinet as planned anyway.

    Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
    DYohn 
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    Posted: February 19, 2006 at 11:25 AM / IP Logged  
    Steve, one trick I've used to minimize the offset look is to align one edge of the tweeter mounting plate with the edge of the mid or woofer frame.  This way the drivers line up along one side, but the acoustic centers are offset.  It works especially well with an MTM center channel as the "tops" of all the drivers are aligned, which offsets the tweeter towards the top of the cabinet.  (BTW, edge diffraction ringing is often heard as the 3rd harmonic of the baffle width wavelength if you are trying to detect it.)
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    stevdart 
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    Posted: February 19, 2006 at 12:21 PM / IP Logged  

    Gotcha.  I wonder if Barry has a handy harmonics table ;) ?  The tweeter is about 4" total diam while the woofers width is 5", so they may be a bit too close in diameter to align like that.  centerchannel.jpg is the way I offset them in my current piece that so desperately needs a facelift (if I'd built this cab in 8th grade shop class I probably would have got my usual 'D' ;)  But, in this case, would that little bit of a center offset created by aligning the upper edge of these drivers as you described amount to enough baffle difference to make a change? 

    (BTW, the offset look doesn't bother me and I'm using a paint scheme to throw the eye off anyway, with the bright red stripe aligned with the mids toward the innermost part of the cabinet.  My wife wants red and if that's all it takes to get the coveted SAF them I'm all for it!)

    I'll use the offset variation in this current center channel config as the working model.

    Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
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