Fastrac Cornwall horn pros and cons

[Chris A]

4 hours ago, tigerwoodKhorns said:

If I were building a set of tractrix horns I should make the sides straight

This sentence fragment alone is basically correct, if applied near the throat of the horn but not the mouth.  Flaring the mouth of the horn in both dimensions is important to control polars near the bottom of their frequency range and to get better low frequency extension.  A tractrix mouth flare is preferred over the last 1/3 of the horn's length. (Hint: it's easier to start at the mouth and work backward than designing the horn in the opposite direction.)

 

Look carefully at the K-510, K-402, and the horns on the KLF series, Forte II, Chorus II, Quartet, and THX, which are using something close to a 1.68:1 mouth ratio. The throat is a 1:1 ratio.  The taper from the throat to the mouth dimensions is basically a linear taper from 1:1 to 1.68:1.  Don't curve the throat entrance to the compression driver--just make it a short circular-to-square transition within about 1/4 inch axial length of the throat entrance.  Modeling clay works nice.

 

You will have to EQ this horn/driver combination, since it will be much closer to a constant coverage horn than the K-400.

 

Chris

[tigerwoodKhorns]

I don't think that I stated this correctly.  I just want to build a Tractrix horn from a spreadsheet. 

 

Using a spreadsheet like from Volvotrenter or ALK, I will build a set of horns with a tractrix flare with 2 flat sides and two curved sides (like the Martinelli horn that AL use to sell - same as the horn in the Edgar in the 1980 Speakerbuilder article).

 

2 Questions:

 

1.  Will I be better served putting the curved sides on the top and bottom and the flat sides on the sides (i.e. turn the horn sideways from the way that everyone normally does)? 

 

2.  Ignoring any cabinet limitations on size, what is the best height to width ratio?  The horns will be placed on top of the cabinet so I can go any size that I want. 

 

3.  Why not ask one more?  What fc should I use in the design (figuring 400 or 500 hz cutoff).  I know 1 octave below, but would say 240 Hz (larger horn) have any advantage?  Drivers will be JBL 2470 and K-55M

[Chris A]

1. Yes, but remember that the coverage of the horn in that dimension may be (will likely be) different, as in a smaller angle since I see almost every horn oriented with the curved sides horizontally.  Recommend adjusting the spreadsheet to regain about 90 degrees of coverage horizontally and 60 degrees vertically after turning the straight sides to the left and right sides. 

 

2. 1.68:1 is a good guess, width to height.  It may turn out to be different mouth dimension ratio to achieve 90 x 60 coverage, depending on the exact spreadsheet algorithms.  In fact, since the one side of the horn mouth flares and one doesn't in the type of tractrix horn that you describe, I would guess that the ratio of horizontal width to vertical height would be less than 1.68:1 if the tractrix flare is on the top and bottom sides.  What you're trying to do is to have a 90 to 60 ratio at start of the mouth flare.  In the case of the pure tractrix curve, that's effectively "X" (i.e., unknown) degrees, since it curves continuously from the throat with exactly with the same throat curve as an exponential expansion.  The idea is that you must be able to just see the compression driver throat while sighting down the line outside of the horn.  That's the off-axis angle where highest frequencies will begin to separate from the walls.

 

3. Yes, but size will begin to dominate.  In other words, you'll likely begin to get a K-402-sized horn...since I believe that's exactly what Roy did with the K-402 horn.  I believe that he designed the K-402 for more than one octave of polar control below the typical midrange driver high pass frequency.  The advantage is better polar control and much smoother polars all the way down to the high pass frequency without "waistbanding" of polars. (This is an interesting subject.)

 

Chris

[Marvel]

Erik Forker's spreadsheet is already set up to have flat sides with curved top and bottoms. That usually throws folks off when they use his calcs.

 

I think he also used Bruce Edgar's equations, at least when he started out. That's why he has all the Edgar info and documents on his website. Because of the flat sides, the throat appears to get narrower right after it starts. The actual area, however, continues the tractrix expansion. Al and I originally discussed this when he thought of having his horns molded intead of made out of wood. It would have required a two piece plug.

 

Bruce

[bliss53]

Just do it you will never look back.

 

[jorjen]

On 10/9/2016 at 3:10 PM, Honeybadger said:

Some people don't want to cut their Cornwalls.

Others feel that they will turn your Cornwalls into a Jorjen 3, and no longer Klipsch.

Klipsch die hards believe if it's not made by Klipsch it cant be any good.

 

I like Tractrix myself, I think it is an improvement over exponential horns.

LMAO! The Jorjen 3, never saw this until now.

[mopardave]

On 3/28/2017 at 5:59 PM, bliss53 said:

Just do it you will never look back.

 

Interesting. What horn, driver, cross over combo did you use?

[Mighty Favog]

36 minutes ago, mopardave said:

Interesting. What horn, driver, cross over combo did you use?

Those look like Dave Harris' Fastracs. Not sure if he's making them anymore.

 

That's a lot of magnets holding those on! I only needed them in each of the 4-corners and they did fine.

[jwc]

I think the fastrac lineup sounds excellent in a three way design keeping the tradition style passive crossover this forum is accustomed to.

 

There I said it.

[Deang]

The Fc is too high. There, I said it. 

[pzannucci]

11 hours ago, Deang said:

The Fc is too high. There, I said it. 

That has to be a fairly shallow horn.  What was the Fc?

[mopardave]

On 3/28/2017 at 5:59 PM, bliss53 said:

Just do it you will never look back.

 

I see most that use that mid horn and I assume its a k510, use it as a 2 way speaker. How does it sound as a 3 way?    Which one sounds better 2 or 3 way using that mid horn?

[Deang]

2 hours ago, pzannucci said:

That has to be a fairly shallow horn.  What was the Fc?

 

410

[Marvel]

The mouth is slightly truncated on the fastracs.

[Deang]

The point is - the Fc is too high for the stock networks, and I’ve often suspected the driver isn’t loading properly. 

[muel]

So what does too high FC sound like?  

[glens]

An unloaded driver would benefit from neither the "amplification" nor dispersion control of the horn, in this case.  How do you surmise that might sound?

[Chris A]

4 hours ago, muel said:

So what does too high FC sound like?  

A null  or drop-out in response around 400 Hz.  It's pretty important to have on-axis SPL response within ±3dB or so across the audible band.  In fact, some people think it's the most important single performance criterion for loudspeakers.  With passive crossovers, as Dean was saying, the entire network needs to be altered to accommodate a higher low frequency cutoff of the midrange horn/driver in each loudspeaker type (Klipschorn, La Scala, Belle, etc.).  That isn't trivial.

 

Chris

[muel]

Of course, I’m only measuring in my room but I never saw any drop there.  I’ll have to look again soon.  I could put stock AA’s back in to test.

[Chris A]

In-room measurements are always preferable (to conjecture)...

 

Chris