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Old 23rd April 2004, 03:22 PM   #1
Rocky is offline Rocky  Norway
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Default Wide range horn confusion..

Looking at the Oris horn projects, some questions jump to my mind from statements I have read on horn design, horn bandwidth, and the "3-octave rule".. here are some quotes:

Quote:
Because horns have a limited bandwidth, they cannot do a "full" frequency range, but you can use a front horn to load a full range driver at its lower frequencies, making the unit wider range than just direct radiation alone would allow.
Quote:
In theory, to achieve maximum efficiency, horn dimension should be 1 wave-length long, and 1 wave-length in mouth circumference, at the lowest frequency. At 3 octaves higher, wavelengths are 1/10 of horn size, and too small for the horn to direct them. These small wave-lengths bounce around inside the horn chaotically. It is essential to rescale a smaller horn for the next 3 octaves.
The Oris horns utilizes fullrange drivers such as Lowther or AER, and is supposed to cover the frequency band from 150Hz and up, wich will be equal to a bandwidth of aprox. 7 octaves...

Now, a quote from the Oris horn website;

Quote:
The cabinet I developed for the Lowther was of course a horn, a front-horn with the throath in front of the driver and not like many other designs at the rear. Placed in front of the speaker makes the sound come from one spot, the interference from different sources (like with rear loaded horns) is not present and the sound-colour is the same over the complete frequency-range! An additional advantage is that the output at listening-position is minimal 6dB higher plus gaining more body in the low-mids!
The last statement in the quote inevitably suggests that the horn response not is configured to load only the lower rolloff frequency of the driver to extend bass response, but instead is used to load the entire band of 7 octaves..
How can this be?

ref:
Single Driver Website
Oris Horn Website
Lenard Audio

Click the image to open in full size.
The Oris Horn
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Old 23rd April 2004, 03:31 PM   #2
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Horns that are:

a) not folded
b) not using a compression driver (or not using a conventional driver with a compression chamber)

can achieve bandwidths greater than 3 octaves.

Regards

Charles
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Old 23rd April 2004, 04:52 PM   #3
Rocky is offline Rocky  Norway
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Thanks for clearing this up for me, Charles
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Old 26th April 2004, 02:03 PM   #4
Rocky is offline Rocky  Norway
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Some more questions related to the same issue;

Quote:
horn dimension should be 1 wave-length long, and 1 wave-length in mouth circumference, at the lowest frequency.
Does this apply to tractrix contours as well? My calculations points to a much shorter one.. actually <1/4 of the wavelength..

Does tractrix horns, paired with compression drivers, obey the 3-octave rule? I noticed Volvotreter's tractrix mid-horns operates from 650Hz to 10000Hz, being 4 octaves...

edit: V-treter's tactrix horn is actually made with cutoff @ 320Hz, giving a bandwidth of 5 octaves?

Anyone?
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Old 26th April 2004, 02:46 PM   #5
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Default Horns

Hi Rocky

Most people shorten the horn (and reduce mouth circumference) to 1/4 wavelength for convenience sake, however there is usually a price to pay in irregularities in response. It is possible to smooth some of these irregularities with programs like McBean's Horn Response, but generally, 1/2 wavelength will yield better results, full wavelength better yet. The last is easily possible only at high frequencies, naturally, due to the small wavelengths = small horn. Not so easy in the lower registers without a crane or several friends.

The contour or flare doesn't really gaurantee the horn will be properly loaded over wider bandwidth. Depends on the design and the driver in use. As the frequency goes up, the portion of the horn that remains useful retreats inward toward the apex, narrowing dispersion. Constant directivity horns may correct for this to some degree, but inevitably the off axis response diminishes.

3 octaves is a lot on a horn, but possible. A decade is even better, and nearing the optimum. Beyond a decade one wonders if the horn is serving any useful purpose in directivity.

Tim
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Old 26th April 2004, 03:40 PM   #6
Rocky is offline Rocky  Norway
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Thaks Tim.

With the bandwith limitations, I am not very keen on configuring the horn for any frequencies below the XO point.. say If I intend to cross at 500Hz, I configure my horn for 500Hz and not any lower to help it in the transition... How would this affect my desired 1st order crossovers...?

And regarding the tractrix shape... I use this formula to calculate it:
Click the image to open in full size.

How should I alter this shape to extend it's length to say 1/2 or 1/1 wavelength? Simply "stretching" it...?
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Old 26th April 2004, 04:37 PM   #7
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Default Horn Design

Rocky,

Most of the older math models will give you a ratio of throat to mouth size, length, plus the front and rear chamber dimensions.
The throat to mouth size combined with length will establish the flare for a given cut-off frequency.
It would be wize to allow for a slightly lower cut-off than intended for use, as the response ordinarily becomes irregular as you approach cut-off freq.

Have a look at the following:

http://www.users.bigpond.com/dmcbean/

This program will allow you to pick all manner of horns and compare the results. It takes some getting used to, but is fairly accurate. High frequency response shown in the model will probably be quite a bit less extended than on the actual horn.

Also, some horn math calculators:

http://melhuish.org/audio/tractrixcalc.html

Might have a look at the "projects" portion of this site.

Also look at Till's posts concerning his horn fabrication here.

Hope all this will help, as a horn design is much more involved than a simple reflex enclosure. Not an easy task.

Tim
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Old 26th April 2004, 04:57 PM   #8
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Default Horns

Rocky,

You will need to esablish the throat ,mouth, and length, then figure cross-sectional areas through the horn at given lengths along the expansion.

Obviously, if the two sides are fixed at a given flare rate (no curvature), or are parallel surfaces with no flare, the design becomes easier.

Conical is a fixed flare angle with no curvature, and is easier to calculate and experiment with. Also, because of the fast flare rate there is usually less distortion in the throat.

Tractrix are commonly used for the midrange, and your 500 Hz cut-off will be around 70cm long full size.

Tim
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Old 27th April 2004, 11:51 PM   #9
Rocky is offline Rocky  Norway
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Thanks for the replys, Tim.

I must say I find the concept of horns especially interresting, but while a midrange horn can easily be fitted into my listening space, Bass horns are an entirely different matter. In fact out of the question for the time being. If I were to start on a horn system, I would cross from bass to mid at around 500Hz using a 2 inch compression driver, hopefully something like the TAD 4002 or a JBL 375 or something similar.. My concerns are two major ones;

1) 500Hz is well into the human voice frequency area. Taken into account that a huge bass horn is out of the question, I see no other alternate for the lower end than a coaxial lower extension. It frightens me to pair two potential very different sounding sources at this frequency.. Can anyone provide some feedback on this matter based on experience? As said, my midrange candidate being a 2 inch compression driver..

2) Crossovers.. My initial interrest for horn designs is founded in a newly aquired 300B SET delivering 9 sweet watts, well, at least 2-3 of the watts are very sweet.. My understanding is that SETs have "hate"-relationship to crossovers.. Now, Assuming the horn system is properly built (i'll get spherical horns milled to mathematical specs), and assuming the crossovers are well tuned and thought out, and the bass driver chosen with care; will a horn system outperform or underperform a simple well-thought full-range setup like for instance a Fertin field-coil dipole..? (as a companion for SET amps)

I should add that my goal is not very loud sound, but rather using as few watts as possible to get the sound I want.. For obvious reasons.. *distortion*
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Old 28th April 2004, 02:15 AM   #10
Ken L is offline Ken L  United States
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Default Re: Wide range horn confusion..

Quote:
Originally posted by Rocky
is configured to load only the lower rolloff frequency of the driver to extend bass response, but instead is used to load the entire band of 7 octaves......
How can this be?
I didn't follow the links, so I may be contradicted here _grin_ but the Oris and Azuras essentially are out of horn loading above 1500 hz. They depend on rising response of the driver in the upper bandwidth to offset the horn loading below that point. The Measurements I've seen of Azuras with DX4's drop off steeply above 10K. John K who posts as Kloss and has run several Oris configurations with two Oris and a Fostex tweeter is now configuring a three Oris horns per side with a super tweeter in the center.

I am using Azurahorns with DX4's and am now starting to work with a DDS ENG 90-1 horn (waveguide) and Radian 475PB compression driver with the intent of crossing at around 1250 hz. The Azura loads up to about 1500 hz, the eng 90-1 loads down to about 1000 hz.

If needed, I can add a horn loaded supertweeter on top for three way horn loaded system above 120 hz - at 120hz I am biamping with a digital crossover to a pair of NHT1259 sealed subs.

I would eventually like to have a 4-way fully horn loaded system, with bass horns- however, size, cost, complexity, WAF enter into the timing of that. The sealed NHT1259's pushed by a Crown amp sound so good that bass horns is my lowest priority at this point.

Will a 4 way horn setup underperform or outperform a well thought out full range or dipole setup?

At the risk of being contradicted here _grin_ while I personally lean to horns beating anything _bigger grin_ - I just don't think anyone can ever answer that with any certainty either way -

They're different, we not only have different priorities as listeners, but due to physiology we actually _hear_ things differently. I would easily believe that two seasoned listeners might hear both a really good open baffle setup and a really good horn setup while one listener preferred one and another listener preferred the other.

Every setup, no matter how _ultimate_ has tradeoffs. So it just depends on what tradeoffs an individual prefers over another different set of compromises.

FWIW, I don't think there is such a thing as a "full range driver" . I believe they would be better described as "wide band augmented" drivers. What bandwith(s) and how you augment is what the question is.

It usually takes individuals _years_ of tweaking, trial and effort, trying and discarding various efforts to get a horn system how they want it. I'm not aware of many that reach that point with horns that leave horns for open baffles. Nor am I aware of many that leave open baffles for horns. It does seem that open baffles proponents are growing, but they appear to be widely out numbered by horn enthusiasts.

What you really want to do is ensure that you have plenty of fun on the Journey - at times the destination becomes more and more elusive _big grin_

Regards

Ken L

PS the two best systems I have personally ever heard are low powered SET systems that have horns and crossovers - and both system owners were not still completely happy with the crossovers. But they were _magical_ and a true joy to listen to.

It's all about the _magic_ And _magic_ is elusive, poorly defined, and oh so hard to achieve.
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