Conference napalm::commusic_v1

    
    	A speaker enclosure serves two basic purposes; to encase the
    relatively fragile speaker and to enhance it's operation. Even the
    totally closed cab enhances the operation of the speaker by placing
    a partition between the sound wave generated by the back of the
    cone and the front. By inspection, it can be seen that these waves
    will be 180 degrees out of phase and simply cancel if the partition
    were not in place.
    
    	Since sound takes time to travel, and since air has mass and
    compressability and other physics type relations exist, they can
    be put to use to further enhance the speakers operation. A tuned
    port can be introduced, if the physics involved point to a benefit
    in doing so. An example is size. A large enclosure has the possibility
    of enhancing a speakers bass response, since by it's sheer size
    it can be made to resonate acoustically to the speaker's bass output.
    A small enclosure, on the other hand, is limited in it's effect
    in using just the airmass/volume/compliance approach, which is why
    the so called "passive radiators" are employed. These "trick" the
    mechanics into operating as if the air had increased in density,
    for a given volume.
    
    	The horn type enclosures enhance the sound output from the front
    of the cone, by controlling the expansion rate carefully. Most horn's
    I've seen have venturis, similar to that seen in a gas engine
    carburator. (This has been done on the bass porting too...BIC venturi?) By
    pressurizing the sound in forcing it to compress through the venturi,
    it has a more uniform expansion and follows the horn contours better,
    I believe. By folding the horn, it's rather large geometry can be
    accomodated within rectangular enclosures of managable size. Most
    horns, unless *really* huge, only handle frequencies down to the
    upper most bass range, so rear of cone energy is not used and therefore
    the cab is sealed here. Altec VOTs are one exception, while EV sentry
    IV's are not.
    
    	Bose & co's latest design loads *both* the front and back of
    the speakers with a ported enclosure. Since both sides of the cone
    are enhanced operationally, these speakers are more efficient than
    previous designs that did one *or* the other.
    
    	Still more exotic designs attempt to integrate a planar radiation
    surface from driving it with an array of different voice coils,
    other's try to shrink the radiating surface to the mathematically
    ideal "point source" of sound. Reality keeps suggesting that the
    tried 'n true approach of using cone based drivers is best. It's
    kinda like automobile manufacturers trying to replace piston driven
    engines with turbines and rotarys, yeah - they work and it *can*
    be done, but the level of refinement will never approach that already
    invested in the more common solution.  
    
    	Joe Jas
    

RE < Note 1209.1 by ELESYS::JASNIEWSKI >

	Nice write-up, Joe.  Thanks.

	Let me take this a step further.  I want to start a long-term project
to build some bass speaker cabinets to go under some horns I aquired in a great
deal.  Normally, a project like this I'd not be interested in due to the amount
of time and energay it would require.  In most cases, it's better to just keep
an eye open on a good new or used deal.  BUT, since I only paid $50 on a set of
EV horns with the drivers and the tweeters (and I made an agreement with the
friend I bought them from to sell them back to him should I not need them), I 
don't think I could do better, even on the used market.

	So, I have the high end all set, somewhere about 800hz on up to 20Khz.
I now want to explore what is best in the low end.  I have been recommended a
ported design.  I'm willing to hear any other recommendations.

	What's the application?  I will be using them in a solo or duet vocalist
type show in small to medium sized halls (seldom over 200 people) with 
MIDI-based accompaniment (sometimes with guitar as well).  There'll be drums,
bass, some kind of piano, and any other synthesized instrument that seems
appropriate.  The instruments are usually secondary in priority to the vocals.

	So, what do y'all think?  Ported design sound o.k.?  Oh yeah, I have
speaker stands (if that makes any difference at this point).  If other designs
are recommended, can you give any pointers to plans available (i.e. published
kits, books, etc.)?  

	I have a VERY good workshop available (my father-in-law runs a 
wood-working business), so don't be afraid to bring up exotic designs.  The main
thing is; are detailed design parameters (for size, shape, angles,...) available
in some form?

	Thanks,

	Dan

    
    	Dan,
    
    	Some considerations for your design;
    
    1. What's the bottom frequency, or, er, the -3db point of the cab?
    Do you want to see 20cps or would 40 do? Makes a big difference
    in the size of the enclosure.
    
    2. What are your cargo space restrictions, dimensionally? A cab
    design is no good if ya cant get it thru the door or into the van.
    
    3. What size drivers and how many? Do you prefer to use two 15"
    spkrs, one for each channel?
    
    
    	The design of your cab should balance the above factors to give
    what's best for you. Cargo dimension limitations will determine
    the biggest size, which will in turn determine the low freq cutoff
    point. The whole design revolves around the spkrs you plan to use.
    	The cliche' in design for PA these days is efficiency, which
    is why you see all the folded horns and such - even for the bass.
    A folded horn bass enclosure will be really huge - you may not be
    able to carry it or load it into the van!
    	Personally, I'm quite intrigued by the Bose double tuned cavity
    design, which apparently packs a lotta womp in a smaller volume
    enclosure. However, I sure that this design is unavailable for
    arbitrary speaker choices. You could always conduct a well thought-out
    experiment, which may yield a suitable enclosure on the second pass
    of building it, but I think you'd have to be really sold on this
    design to do so much work. The front of the speaker cone rings a
    cavity tuned to, say, 100 cycles while the back of the cone drives
    a larger resonant cavity tuned lower, say, 45 cycles. Each cavity
    is a simple tuned "ported" box, whose initial_guess dimensions would
    be easy to find or calculate. The larger cavity would probably use
    a ducted port, while the smaller definately would not. You'll lose
    some volume for the higher cavity out of the total enclosure volume,
    which will of course impact the low-end response, so you might be
    compromising here. *I* note that, the Peavey SP-X, X=1,2,3 series
    speakers are incredibly small for a 15" driver. By the size and
    the type of porting used, I'd guess these cant go lower than 50
    cycles anyway. Note the difference in sheer size between those cabs
    and VOT's with their *40* cycle cutoff. Apparently, the lowest note
    of the bass guitar does not need to be reinforced by modern small
    PA systems.
    
    	Joe Jas

RE < Note 1209.3 by ELESYS::JASNIEWSKI >

>    1. What's the bottom frequency, or, er, the -3db point of the cab?

	Funny thing, this.  I've always loved the sound of the SP-3's.  Yet
the specs in Peavey's own catalog say they only go down to 80 cycles.  
Apparently, It's not that important for my tastes to go too low...

>    2. What are your cargo space restrictions, dimensionally? A cab
>    design is no good if ya cant get it thru the door or into the van.

	Another glitche.  It appears I may be losing the access to a large van.
Now I'll be FORCED to build as small a cab as I can.  (Watch for a new sale note
on some larger equipment!)
	Of further relevance here is a somewhat practical, but mostly cosmetic
consideration.  My horns are about 27 1/2" wide.  Though that may do me in as
far as trying to fit a matching bass cab for them in whatever car I use to
transport my equipment, it'd be nice to have them match.  This is going under
the assumption that the bass cabs need to be placed up on the stands along with 
the horns.  If that's not important, then the size of the bass cabs is quite
open to flexibility.
    
>    3. What size drivers and how many? Do you prefer to use two 15"
>    spkrs, one for each channel?

	It'll be fifteens, for sure.  Right now I have a Pyle 15", and I should
be recieving in the mail today the detailed specifications of that driver from
the company.

	You see, if nothing else, I know I'll be able to fall back on the ported
design.  It will probably do me jus' fine.  I simply wanted to solicit opinions
on any other designs.
    
>    A folded horn bass enclosure will be really huge - you may not be
>    able to carry it or load it into the van!

	That's what I'm using now - an Acoustic bass cab with an 18" driver.
And that's the main reason why I want to replace it - it be big.

	That's interesting about the bose design.  I think for this time around,
though, that might be a little TOO complicated a design.

>    compromising here. *I* note that, the Peavey SP-X, X=1,2,3 series
>    speakers are incredibly small for a 15" driver. By the size and
>    the type of porting used, I'd guess these cant go lower than 50
>    cycles anyway. 

	As I said earlier - SP3's 80hz - 16Khz
		Sp2's and Sp1's - 60hz - 16Khz

	Dan

    
    	Dan, you might try the "JBL scoop" enclosure. It's about the same
    size as a 4 drawer filing cabinet, and takes a 15...I see that space
    is of paramount importance, now...(I also see I was within an octave,
    (at least) in my guess for the low end rolloff frequency of the SP-3s.
    With such a small cab and those funky slotted reflex ports on either
    side of the horn...)
    	Ray T's TOA cabs seemed to work pretty nice...heck, it's been
    so long, I cant remember if they had 15s for the bass driver or
    not. I do have a "critical dimension" drawing for those, though.
    	The JBL scoop is a fairly popular enclosure and I'm sure plans
    are available readily. (I once sought employment as a sales rep for
    a small outfit which built this enclosure and others. That was part
    time and in Buffalo N.Y.) You might want to seek out what these
    look like, probably from an old JBL catalogue, if you're not familiar
    with them already.                 
    
    	Joe
    

RE < Note 1209.5 by ELESYS::JASNIEWSKI >

	I have seen scoops and am interested.  I would be interested in any
design plans or measurements that you might have available.  If you don't
have them, do you know where they can be had?

	Any solid 'down-on-paper' designs or dimensions would be of great help
in desiding on what to build.

	Thanks,

	Dan
    

    
    	Dan,
    
    	Unfortunately, I dont have plans for the scoops...anybody out
    there know of a place to find these?? Might try the guys at NEST.
    
    	Joe
    

RE < Note 1209.7 by ELESYS::JASNIEWSKI >

> Might try the guys at NEST.

	I only go there when I have to.  While they have good prices, they
are very pushy toward their own views of sound equipment and have made me feel
REAL dumb for listening to anyone else.  I suggest to ANYONE who goes there to
know what you want beforehand and insist on it once you get there.  They've got
the best prices I've been able to find, though.  And you can buy refurbished
items, too, for a great discount.
    
	Dan    

    Re-.6
    I have a pair of JBL scoop copies loaded w/Altec 15's I'd like to
    trade for Peavey FH-1's or maybe sell. These cabs are a little rough
    on the outside, but are very tight. I painted them last year. I've
    got the Altec 511B horns w/808-8A drivers also. BTW, the scoops
    are 2'W x 2'D 'x 4'H.
    
    Neal  DTN 291-7285    
    
    

	Well, after a long time of fussing and delaying the building project,
I finally purchased a pair of used thiele-type bass cabs with EV woofers.
Funny thing, I had gotten some papers detailing a nice compact design sent to me
from another noter which looked to be about the easiest to build and the best
compromise (size for efficiency).  Still I could not find time to get down to
my father-in-law's shop to build them.  Last weekend I answered an ad for some
small thiele cabs and what to my wondering eyes did appear?  The very same
design of cab that I was intending to build!

	Now...  Can anyone give me a lead on where to find the material for 
building a suitable heavy grill for the front of these babies?  I don't really 
know where to begin looking.

	Dan

    
    There's two ways to go, you can use a grille over the driver or a
    grille over the whole front baffle. Speaker grilles are cheap, about
    $12 for 15" variety. The grille over the front approach would have to
    be custom made. There are also square grilles that cover more than 
    the speaker itself.
    
    CdH
    

    Get some diamond-wire fencing, this is wire mesh that is soldered
    every 1/4". Make a frame using 3/8" dowel rod inserted into corner
    pieces. Spray it black. Bend the wire cloth over the doweling, wrap
    each edge with wire and spray the whole thing black. Then you can
    mount your new dynamite-proof grill to the box face with Tee-nuts.
    
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