When you look into the throat .............

[Todd W. White]

When you say "cut off", do you mean the woofer frames are cut off (this was done by some people (read: IDIOTS) who put the larger-framed 515's in older cabinets, rather than make a new baffle board), or do you mean "Why is the opening for the woofer not totally round?" ???

[RacerXtreme]

Hi Todd,

no, I don't mean the woofer frames are cut off. I mean that when looking at the cabinet from the front of the speaker, you can't see the entire "round" outline of each woofer. The sides are slightly cut off from view by the throat or very inner most portion of the flare of the horn. The board that the speakers are mounted to is called the baffle board if I'm correct.

I know what's throwing you off. I've got my 210's laying on their side - NOT standing straight up and down. When I say "the sides" - it's technically the top and bottom of the horn flare if the cabinet is standing up like normal.

When you look into the throat of the horn, you cannot see the entire round outline of each woofer.

[bfish]

You've gotta' have a restriction, in order for compression to occur in a compression horn.

[GM]

Greets!

Don't know what you read, but the horn's expansion is truncated where the throat area (St) = the driver's effective piston area (Sd), so what is/isn't exposed is dependent on the shape chosen. The throat's area determines the horn's LF roll off, gain and to some extent its HF BW. With a compression ratio (CR) = 1:1, we get a so-so roll off point/gain and widest possible HF BW, which is best for a typical passive XO. The shape of the horn dictates its vertical/horizontal dispersion (polar response), so was optimized within the technical limitations of the day to blend the LF to the various HF horns over the widest BW possible while still keeping it relatively easy to make, ship, and gang together for arrays. IOW, it's a highly compromised design even by the standards of the day, but overall an excellent 'juggling' of conflicting requirements.

You do want to be careful though that the shrouded portion of the driver doesn't constitute an audible pathlength error in the intended BW or raise the CR too much. At 8" wide on a 15" driver, the 210's throat is vertically confining the BW below ~540 Hz, well below where the driver begins beaming, so with a 1:1 CR it's not an issue for <500 Hz XO points. On a similar note, if you look at early 825 cabs you'll notice that the high pass filter cutout is narrower than the horn throat's 8". This is to load it up to work with the 811/800 Hz XO, so later A7-8 using the A7-500 cabs aren't as optimized in the XO BW as the early units, though never having compared the two I don't know how audible it may be. 'Bean counters' at work, no doubt.

GM

[GM]

Greets!

Since these have a 1:1 CR, it's not a compression loaded design.

GM

[RacerXtreme]

If it's not a compression loaded design, then what is it?

[GM]

Greets!

With 'wings', it's just a ~72 Hz Fc truncated, reflex loaded horn. Laying on the floor with no 'wings' lowers it to ~60 Hz.

GM

[bfish]

Didn't read it, that's the answer I got from Bill at GPA when I posed the same question to him about a year ago.....maybe he was too busy, and blew me off with a glib answer. (wouldn't blame him).

I'm sure no math wiz, (obviously), but the 'masked' area referred to sure appears larger than the non-piston area of an Altec 15 (on a later 825), so I believed the answer I was given. While the Sd of a 515 is easy enough to find, what area did you come up with for the baffle opening? I've always thought the throat of these horns began immediately IN FRONT of the baffle, where the initial expansion occurs, which would be a larger area indeed.

Not tryin' to be smart, just tryin' to understand.

[RacerXtreme]

When you look into the throat of a 210 cabinet, why are the sides of the 515's cut off ?

I remember reading why but can't remember..... In other words, why don't you see the entire surface area of the woofer?

[GM]

Greets!

Looks are deceiving. I'm pretty math challenged myself, so drew it up full size way back when I had three of them. It's off a tiny amount, so not 100% accurate at 1:1, but nil acoustically.

You're right, many folks are confused by these type designs, mistakenly referring to the low pass filter (cutout) as the horn's throat, which in this case has an area > the driver's combined Sd. I guess I'm as much to blame as anybody since when folks ask what the CR means, I repeat the standard answer of it's the ratio of Sd:St, which is normally correct for a compression loaded horn since the low pass filter and horn throat has the same area.

If we use the cutout as 'Sd', then the CR would be ~1:1.13, indicating a more rapid pressure drop in the transition than there actually is since the baffle's 3/4" deep 'vent' combined with the air 'trapped' between it and the diaphragm damps it somewhat, making for a better acoustic match than just looking at it would imply.

GM