Home » Audio » Speaker » Horn Depth & Mouth Diffraction
Horn Depth & Mouth Diffraction [message #19162] Sun, 11 March 2007 13:03 Go to next message
Cuppa Joe is currently offline  Cuppa Joe
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Registered: May 2009
Viscount
When we discussed mouth diffraction in the past, it was under the assumption that the horn in question had an axial depth of at least 1/4 WL of the cutoff frequency.

On another forum, a designer/manufacturer informed a poster that a conical horn's axial depth needs to be at least a FULL WL at cutoff in order to prevent any mouth diffraction. For a 160Hz midbass horn (for instance), that would mean an axial depth of approx. 7ft!

Is he correct, or just over-cautious? How significant is this "spillage" at 1/4 WL, if such is the case? One of my 3 designs could be in the toilet....

Re: Horn Depth & Mouth Diffraction [message #19163 is a reply to message #19162] Sun, 11 March 2007 13:15 Go to previous messageGo to next message
Cuppa Joe is currently offline  Cuppa Joe
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Of course, the above was contingent upon the stipulation that the horn mouth perimeter was square and equalled the WL.

Re: Horn Depth & Mouth Diffraction [message #19164 is a reply to message #19163] Mon, 12 March 2007 08:18 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Registered: January 2001
Illuminati (33rd Degree)

I don't think the reference to horn length had anything to do with mouth diffraction. I think it was the reactance due to reflection from the mouth transition that he was referring to, which is a different matter. Diffraction is strictly a matter of aperature size verses wavelength, and can occur in a slit having no path length.


Re: Horn Depth & Mouth Diffraction [message #19165 is a reply to message #19162] Mon, 12 March 2007 11:49 Go to previous messageGo to next message
Bill Wassilak is currently offline  Bill Wassilak
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He's just over cautious, that would apply if you were using a horn in total free space, like up on a utility pole or something.

Re: Horn Depth & Mouth Diffraction [message #19166 is a reply to message #19164] Mon, 12 March 2007 18:47 Go to previous messageGo to next message
DMoore is currently offline  DMoore
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Location: Seattle
Baron
I agree - horn length is a mouth reflection/reactance issue.

Re: Horn Depth & Mouth Diffraction [message #19168 is a reply to message #19166] Mon, 12 March 2007 20:23 Go to previous messageGo to next message
Cuppa Joe is currently offline  Cuppa Joe
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So that you guys don't have to go by my interpretation, here are excerpts from the original quote (with spelling corrections):

"Once the wavelength is longer than the horn, diffraction comes into play. The larger frequencies will reach the mouth and then go sideways."

"Even with a large mid flare you only end up controlling the top half of the range."

"...if your low cutoff in the mid is 150Hz, then the length of your horn for total control would have to be 2.29 metres long minimum."

My concern, of course, is significant comb filtering in an arrayable multi-box system, whether vertical or (especially) horizontal. In an arced horizontal system, the diffractive spillage would also spell noticeable multiple arrival times. Yes, I know, the arced cluster arrays of the past are acoustically taboo, however my aim is toward a single-tier array of only 3 to 5 traps. The fact that several people in the past have told me that it can't ever sound good just makes me all the more stubborn!!

Re: Horn Depth & Mouth Diffraction [message #19169 is a reply to message #19168] Tue, 13 March 2007 09:55 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Registered: January 2001
Illuminati (33rd Degree)

Path length sets the position of standing wave nodes, but it is not a determiner of diffraction. Diffraction is set by aperture size and wavelength. The size of the opening and the wavelengths that pass through determine the amount of pattern spreading from diffraction.

If your main concern is comb filtering, you might consider that at 150Hz, wavelength is 7.5 feet. Sounds sources can be nearly 2 feet apart and still sum as a single sound source.
Re: Horn Depth & Mouth Diffraction [message #19170 is a reply to message #19169] Tue, 13 March 2007 21:14 Go to previous messageGo to next message
Cuppa Joe is currently offline  Cuppa Joe
Messages: 103
Registered: May 2009
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So Bill is right, this manufacturer is just being overly cautious. (And I thought MY shorts were a little tight....) He might also have some confusion about how large the mouth needs to be to avoid diffraction. Thanks guys, back to feeling better!

Re: Horn Depth & Mouth Diffraction [message #19171 is a reply to message #19170] Wed, 14 March 2007 13:09 Go to previous messageGo to next message
DMoore is currently offline  DMoore
Messages: 58
Registered: May 2009
Location: Seattle
Baron
The issues of comb-filtering which I have experienced with folded bass horns occurs when the frequencies (bass and midrange) involved overlap too much, that is, the bass horn is putting out too much SPL at upper bass frequencies which overlap the midrange output. This is dependent on the design of the bass horn, the chosen crossover point, and the particular upper frequency corner of the driver in it.

This can be avoided by using a steeper slope on the crossover network or employing a woofer with a lower mass roll-off, etc.

Usually, a folded horn will tend to knock-off the upper frequency range because of the folds cancelling out the smaller wavelengths first, but not always, especially if the folds are designed to maintain phase integrity.

A reasonably straight (or one employing few folds or less-drastic folds, for instance) long bass horn will usually be more likely to create comb-filtering problems generally due to its inherent ability to pass frequencies with wavelengths that physically fit inside or across its channels and mouth.

The longer the pathway, the more phase-delay will occur between the midrange horn and the bass horn, same relative frequency. The difference in phase and/or time associated with the disparate sound paths from the two (or more) horn mouths will result in comb-filtering along the way to the listening position.

Those smaller wavelengths also will tend to "beam" as they fit inside of the mouth size/flare and no longer follow the flare rate seen at the mouth as they get smaller (per Olson).

But let me tell you from experiencing it firsthand, you will definitely hear comb-filtering as recognisable distortion if it is there at all! It's pretty horrible sounding - right up there with a bad diaphragm!

DM


Path length delay [message #19172 is a reply to message #19171] Wed, 14 March 2007 14:27 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18783
Registered: January 2001
Illuminati (33rd Degree)

You're right about the delay caused by path length. If the subwoofers are basshorns, it's best to set the top boxes on a delay to compensate.


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