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Old 19th February 2010, 03:38 PM   #1
mige0 is offline mige0  Austria
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Hi


this is in continuation of thoughts related to horn honk that started out at Earls thread but was not welcome there

Geddes on Waveguides
Geddes on Waveguides
Geddes on Waveguides

My own intention in investigating "horn honk" is to cut to the bones of that issue, I came across and would love to hear any thoughts related to this topic to possibly bring it down to sort of tech spec finally.

As for now we have :
- soongsc suspecting horn loading
- Jean-Michel suspecting GD
- and me suspecting IM and D-IM
to be the root cause of that strange sonic impact of horns at the lower end of their usable band width.



##########

Quote:
Originally Posted by Jmmlc View Post
Hello Michael,

IMHO the GD due to the crossover itself has not to be considered in that discussion because we yet see that we have solutions for the results of the summation of a low-pass and a high-pass crossover to have a (quasi) constant delay.

Then you try to compare GD in horns and in conventional loudspeakers. For conventional loudspeakers the load itself is at the origin of the GD non linearity. The closed enclosure operates as having a second order transfer function and a bass-reflex operates as having a 4th order tranfer function, with the corresponding phase non linearity). So there is theorically no main difference between horns and other loads. But in practice a high order load (as a BR) is rarely used to load a loudspeaker in the medium.


Best regards from Paris, France

Jean-Michel Le Cléac'h


I don't quite follow your thoughts, Jean-Michel – but to make sure we are on the same track, I run a few simus to show the overlay of the filter function of the horn at ~500Hz and an assumed XO HP filter at 1000Hz .

The simu below should reflect a typical situation of a horn crossed over ~ 1 octave above cut off pretty well :


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First trace in GREEN shows the behaviour of the horn alone (3rd order HP filter at 500Hz)

Second trace in BLUE shows the behaviour of the XO alone (3rd order HP filter at 1000Hz)

Third trace in RED shows the combined behaviour of the horn plus XO (3rd order XO-HP filter at 1000Hz on top of 3rd order horn-HP filter at 500Hz)



So – where or how could a GD correction be done any beneficial here - in passive XO?



Michael

Last edited by mige0; 19th February 2010 at 03:54 PM.
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Old 19th February 2010, 04:52 PM   #2
Jmmlc is offline Jmmlc  France
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Hello Michael,


There is 2 different things that I think you are mixing:

1) the use of a high pass crossover frequency Fh above a threshold frequency (defined as the frequency Ft above which the group delay of the uncompensated horn is small enough) .

2) the compensation of the group delay variation under that Ft.

Method 1) is quite similar but a bit more rigourous to the widely used method consisting in setting the crossover frequency Fh an octave above the acoustical cut-off frequency of the horn Fc.

My experience is that a 3rd order transfer function cannot modelize the behaviour of a horn around and below its acoustical cut-off (the GD compensation I used to mention was based on a 4th order transfer function and even it is only partially correct). So I doubt that your simulation is correct.

Also, near and below the acoustical cut-off of the horn, we simply cannot perform such simulation without considering the lower frequency loudspeaker. In the frequency interval inside which the contribution of one or the other of the 2 loudspeaker cannot be forgotten the position of the sound source will result of their mutual operation.

I gave an example of a voice seemingly coming from deep in the horn (near the throat) this is when the high-pass crossover frequency Fh is set to low near the acoustical cut-off Fc. Then the contribution of the horn dominate on the contribution of the LF loudspeaker (both on the frequency response and the group delay which one will show a peak between Fc and Fh).

Now, when we use a the high-pass crossover frequency above the threshold frequency Ft previously mentionned (and a quasioptimal crossover) then the group delay curve of the summation of the 2 loudspeakers shows far less more variation.

This is very clear on a CSD when we are performing the crossover/alignment settings of a 2 ways system (LF loudspeaker + a mid horn).

2) I never explained here until now, how I perform the group delay compensation.

Best regards from Paris, France

and have a good week-end!

Jean-Michel Le Cléac'h


Quote:
Originally Posted by mige0 View Post
Hi


I don't quite follow your thoughts, Jean-Michel – but to make sure we are on the same track, I run a few simus So – where or how could a GD correction be done any beneficial here - in passive XO?

Last edited by Jmmlc; 19th February 2010 at 04:55 PM.
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Old 19th February 2010, 05:28 PM   #3
mige0 is offline mige0  Austria
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Ok thanks - there is quite a lot to sort out as it seems


My starting point (to have a basis) was the group delay plot you calculated from the measurements of my contour


Click the image to open in full size.

Then I calculated a simple 3rd order Butterworth at 500Hz which – too me – seemed to fit nicely regarding slope and value of group delay:

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- I wasn't after precision – just to get a handle on the subject...


Now – for me the question is *if* I model a filter that (more or less) mimcis the same behaviour using a direct radiator – would that one sound the same honky in the lower department?
(I admit I haven't tried, but doubt it would come out that way - but anyway).

Following further your approach I understand that you are after treating the horn's group delay and leave alone the group delay generated by the XO - where you apply some special tricks anyway.

Would you say that we are - so far - on the same track now ?

If so – I think what I did with PC response shaping here

Beyond the Ariel

is basically where you aim for too – no?



Would leave the question if we both have successfully caught "horn honk" already ???


Michael

Last edited by mige0; 19th February 2010 at 05:37 PM.
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Old 19th February 2010, 06:07 PM   #4
Jmmlc is offline Jmmlc  France
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Hello Michael,

IMHO the group delay rise of a horn when the frequency decrease toward its acoustical cut-off is only a part of what classically people name "horn sound".

For "honkyness" more important seems to be what I call "tuned pipe effect" which is due to multiple reflections of waves from a truncated mouth to the diaphragm.

But yes, if a loudspeaker + load and/or crossover mimics the illustrated behaviour, the part of the horn sound only related to the group delay variation will be similar. (That's why people for which phase distortion is audible dislike 4th, 6th, 8th order Linkwitz-Riley LP + HP crossovers.

I canot see in your linked measurements the group delay curves or the phase curves...

You probably know better than me that inside the frequency ranges inside which a loudpeaker possess a min phase behaviour linearizing the frequency response curve will also linearize the phase. (IMHO it is not usable to compensate the rise of the group delay of the horn near its cut-off).

But phase equalization may also be used that has no effect on the frequency response curve but only on the phase.

Also with digital technics as FIR filters or with pulse reflection technics we can also linearize both the frequency response and the phase...

My solution is different (I'll explain later more precisely if needed) as it is based on the subtraction of 2 channels to obtain a kind of filter for which the delay will be lower at low frequency than at high frequency (which is the inverse of the normal behaviour of polynomial filters). The output of the first channel delivers the input signal delayed (a special delay is applied to the signal to be filtered by the High Pass crossover) and the second channel delivers theinput signals having been filtered through a 4th order low-pass Linkwitz-Riley having an Fc one octave over the acoustical cut-off of the horn. The two channels are then summed at their output by a simple 2 resistors summator. (one channel inverted to perform the mention subtraction...). Well, this is surely not very clear, and it is time for me to go in week-end

Best regards from Paris, France

Jean-Michel Le Cléac'h






Quote:
Originally Posted by mige0 View Post

My starting point (to have a basis) was the group delay plot you calculated for my contour


Then I calculated a simple 3rd order Butterworth at 500Hz which – too me – seemed to fit nicely regarding slope and value of group delay:


Now – for me the question is *if* I model a filter that (more or less) mimics the same behaviour using a direct radiator – would that one sound the same honky in the lower department?
(I admit I haven't tried, but doubt it would come out that way - but anyway).

Following further your approach I understand that you are after treating the horn's group delay and leave alone the group delay generated by the XO - where you apply some special tricks anyway.

Would you say that we are - so far - on the same track now ?

If so – I think what I did with PC response shaping here

Beyond the Ariel

is basically where you aim for too – no?

Last edited by Jmmlc; 19th February 2010 at 06:12 PM.
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Old 19th February 2010, 06:31 PM   #5
mige0 is offline mige0  Austria
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Quote:
Originally Posted by Jmmlc View Post
I gave an example of a voice seemingly coming from deep in the horn (near the throat) this is when the high-pass crossover frequency Fh is set to low near the acoustical cut-off Fc. Then the contribution of the horn dominate on the contribution of the LF loudspeaker (both on the frequency response and the group delay which one will show a peak between Fc and Fh).
This is clear until the point " the contribution of the horn (will) dominate " .

Sure - I see horn and XO as a combined filter and its easy to understand that it might not be exactly optimal this way from the stand point of smooth merging low and high.
The difference between the BLUE traces (for the XO-HP only) and the RED one (For XO-HP plus horn-HP) in the former posting is an obviously increased GD at the XO frequency.

But to what do you refer as to be "dominating" ?
I mean – if we were *merely* after diminishing GD at and below XO then a valid approach would be to rise the cut off frequency of the horn like below – no ?


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again GREEN trace shows the behaviour of the horn alone - 3rd order HP filter at 1000Hz now

RED trace shows the combined behaviour of the horn plus XO (2rd order XO-HP filter at 1000Hz on top of 3rd order horn-HP filter at 1000Hz)

Obviously that's not the solution to treat horn honk – though comparing RED traces (total) GD at XO is ~ the same as before and GD below XO is ~ cut half...

This makes me think about the additional sonic factor that enters the picture regarding horn honk...


Michael

Last edited by mige0; 19th February 2010 at 06:38 PM.
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Old 19th February 2010, 09:37 PM   #6
mige0 is offline mige0  Austria
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Quote:
Originally Posted by Jmmlc View Post
IMHO the group delay rise of a horn when the frequency decrease toward its acoustical cut-off is only a part of what classically people name "horn sound".

For "honkyness" more important seems to be what I call "tuned pipe effect" which is due to multiple reflections of waves from a truncated mouth to the diaphragm.

But yes, if a loudspeaker + load and/or crossover mimics the illustrated behaviour, the part of the horn sound only related to the group delay variation will be similar. (That's why people for which phase distortion is audible dislike 4th, 6th, 8th order Linkwitz-Riley LP + HP crossovers.
Thanks - splitting the issue this way makes a lot of sense to me – have a good weekend


I'd love to hear more about your GD treatment - possibly some more plots to share ?


Michael
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Old 19th February 2010, 10:19 PM   #7
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Quote:
For "honkyness" more important seems to be what I call "tuned pipe effect" which is due to multiple reflections of waves from a truncated mouth to the diaphragm.
Yup. Everyone should try Jack Bouska's 'towel mod' experiment. Roll up a magazine into a horn and speak through it. Bad honk. Then roll up a towel and wrap it around the mouth of the horn. Much less honk. He even backs it up with measurements showing the period of the modulation is proportional to the horn length.
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Old 19th February 2010, 11:12 PM   #8
mige0 is offline mige0  Austria
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I have not the slightest doubt that the towel trick works like a charm....

.... a sharp ended conical is the worst one can possibly think of in terms of diffraction alignment - most anything might work wonders here

the LeCleach contour is the most ingenious and thoughtful approach on the other hand to deal with the transition into free space I can think of
(very little to improve upon IMO - despite maybe a modified approach to "finalize" the contour)

I'm very curious if its possible to get together and sort out the complete horn honk mix
As for the "newspaper horn" we might have to add "flexing of the contour" ?!


Michael

Last edited by mige0; 19th February 2010 at 11:25 PM.
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Old 20th February 2010, 12:27 AM   #9
Pano is offline Pano  United States
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The towel trick works for sure. See my old thread here:
Horn Honk and Towels

Getting rid of the abrupt edge seems to be the key. As in Jean-Michel's profile.

I've even experimented with fleece around the edges of horn tweeters and my big multi-cell horns. There is a difference.
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Old 20th February 2010, 12:49 AM   #10
john k... is offline john k...  United States
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Quote:
Originally Posted by Jmmlc View Post
Hello Michael,



My solution is different (I'll explain later more precisely if needed) as it is based on the subtraction of 2 channels to obtain a kind of filter for which the delay will be lower at low frequency than at high frequency (which is the inverse of the normal behaviour of polynomial filters). The output of the first channel delivers the input signal delayed (a special delay is applied to the signal to be filtered by the High Pass crossover) and the second channel delivers theinput signals having been filtered through a 4th order low-pass Linkwitz-Riley having an Fc one octave over the acoustical cut-off of the horn. The two channels are then summed at their output by a simple 2 resistors summator. (one channel inverted to perform the mention subtraction...). Well, this is surely not very clear, and it is time for me to go in week-end

Best regards from Paris, France

Jean-Michel Le Cléac'h

This souind like the subtractive-delay approach first presented by Vanderkoov and Lipshtiz, no?
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