511b => 511e-ish

[89-300ce]

Just some blue sky thinking.

I plan on using 511b's with a model 19 crossover. I estimate that I have 6" of additional delay on the horn compared to the 811b's. I'm under the impression that Altec used such a long throat to keep dispersion to a minimum and focus most of what HF they could produce on axis.

If I'm now using BMS drivers could I now shorten the horn and thereby improve _my_ time alignment and dispersion?

Jorg

[Zilch]

511E is a 1.4" entry, no? Your 4552 BMS are 1", and fit the standard 511B.

The best way to deal with the time alignment issue, if you perceive there is one, is to delay the woofer.

[89-300ce]

Yes, I would have to use the 511b flange and make a more rapid transition to the remaining horn i.d. I have access to a machine shop and aluminum welders.

It's really just a thought though. Likely not worth the effort and it would butcher the horns.

Still, I wonder if a shorter throat would improve dispersion/off axis performance.

Jorg

[Earl K]

Hi Jorg,

- Time Aligning™ 2 drivers is not as simplistic as just aligning the voice-coils of the two drivers. ( caveat; if one is dealing with an all cone 4 way design this technique can actually work quite well ).
- If you apply this philosophy ( of simply aligning the voicecoils ) to a two-way system ( which includes a horn driver and a baffle mounted woofer ) you'll eventually encounter a bunch of paradoxes ( about signal alignment / & just what areas of the overall response band-width are the most important to get into alignment ).

- An example follows ; pay attention to the second paragraph in the following cut-sheet :

http://www.lansingheritage.org/images/altec/specs/pro-systems/9846-8a/page02.jpg

- Look at the front of this system and now imagine just how far apart the voice-coils actually are. See the baffle ? That woofer was moved forward to create this type of alignment .

http://www.lansingheritage.org/images/altec/specs/pro-systems/9846-8a/page01.jpg

- So what gives ?

- In this speaker design Altec decided to align the "Acoustic Centres" of the two driver types ( at crossover ) / rather than the pistonic pass band response ( say, 150 hz compared to 2000hz ) .
- The acoustic centre is the apparent spatial "point of origin" for any one particular frequency, as reproduced by the driver . The "apparent point of origin" shifts ( in real space ) per frequency .

- Typically in a large woofer , once the frequencies of interest go above the drivers ability to respond in a "pistonic manner" ( around 300 hz for a 15" woofer ), the frequencies are reproduced with ever increasing latency ( ie; delay ). This will have the upper frequencies appear to originate from [u]behind the actual physical location of the voice-coil [u/].

(i) A woofer with a heavy cone, such as this Altec 411a has a frequency dependant delay induced by its' heavy cone that places the crossover frequencies some distance behind the woofers' voice-coil.
- The amount of time shift is linked to cone weight, BL, Le ( & ???, I don't know all of the factors and I don't have a formula to predict the location ).
- Where are the relative acoustic centres ( at 800 hz ) of the 2235H or a K145 ? I don't know, I only know to expect that they will be in different locations relative to each other.
- FWIW, at 800hz , my ME150H woofs ( using a heavily aquaplassed cone ) appear to orginate the wave impulse at least 8 inches behind the voice coil of the woofer ( included here is the effect of the elctronic crossover ) . I should really be delaying my horn circuit ( but I can't ) so I choose to add extra delay to the woofer circuit ( throwing impulse alignment out the window ) .

(ii) Where does this info leave you ? Unfortunately, in a bit of a tough spot, because to my knowledge no one ( for your benefit ) in the CLIO CLUB ( or others) have a K145 to measure the acoustic centre of your chosen crossover point . It would be handy to know that a K145 ( at 500 hz ) orginates an electric pulse at something like 3 or 4" behind the voice-coil ( just a wild guess) and at 800 hz maybe6" behind ( these are just wild-asse guesses not to be used ) . This knowledge would be nice to have but it just isn't published so the DIYer is at a real disadvantage.

- ( OTOH, ) You could to choose to align the pistonic passband response of the individual drivers / by aligning the voice coils / just be aware that in the crossover region there may be some astigmatic sort of time error ( the severity , dependant on the woofer used ) . The passband response outside of this area will be aligned .
- Buying an inexpensive stereo two-way crossover with a delay adjustment ( for at least the woofer section ) is a good way to start playing around with correcting delay offsets ( physical & those induced by passive components ) .
- Different alignments ( pass-band vs acoustic centres ) do offer different presentations of the music. I prefer the music presentation that results from aligning the acoustic centres / while I see others seem to prefer other approaches .

<> Earl K

[89-300ce]

Hello Earl,

I had assumed that when using the Altec 19 crossover with the 511b horn I should try to place the horn driver in the same location as on the 811b, which by my estimation is 6" shorter flange to flange than the 511b. Therefore I should project the horn driver 6" forward or move the woofer 6" back. Now it seems my woofer itself will throw me a curve.

I had previously tried to figure out the 9846 cabinet you had shown me in the "Large 2-way" thread, but couldn't make sense of it. With my line of reasoning the woofer should have been recessed and the horn projected. I assumed the lower crossover point and different crossover components where the cause. I didn't see the woofers as a factor.


Perhaps my best option now is to place the k145's into a friends 5.5ft boxes with model 19 crossovers, mount the horns on a sheet of plywood with a vertical offset between the transducer and horn identical to the model 19, and then try to find the best horn location by ear to see where that leaves me.


Jorg

[Earl K]

Perhaps my best option now is to place the k145's into a friends 5.5ft boxes with model 19 crossovers, mount the horns on a sheet of plywood with a vertical offset between the transducer and horn identical to the model 19, and then try to find the best horn location by ear to see where that leaves me.

- Yes, I'd say that at this point in your DIY project, keeping the horn outside of a 5.5 cu' test box, is the best way to go .
- That way you can slide the horn backwards & forwards looking for the best signal summing in the crossover region .


< . :)