Help with HF horn combing/lobing

[wchang]

So I prototyped a single Minimalist MEH (Minmeh) using Faital HF108/2x4" and passively filtered the response quite flat 160hz-12khz. Sounded A+ until I swayed my head laterally -- just a couple inches -- then it became noticeably dull/flat -- and a couple inches further -- lively sound came back -- and so on and on. Classic comb-filtering?

This was with a single speaker at 3m. To explore further, I disconnected the mids and listened from ~0.5m keeping one ear constant-30°-angled toward the throat for maximum acuity, as I adjusted head position laterally. Well, obvious combing or multi-petal lobing on test tones 4-12khz; 10khz sounded like pulse-pulse-pulse as I shifted. Then I flipped my body/neck/head 90° pretending the horn was narrow rather than wide, and moved accordingly (with some difficulty). Same result, qualitatively. Again, this is a single horn with just the HF108 playing.

I searched but could not find this issue mentioned. Help, please.

Minmeh HornFaital described here:

W

Post in thread 'Full Range Speaker Photo Gallery'

Friday at 12:36 PM

For some time now I've wanted to compare my reflector-coaxial PrimeRadiant against another quasi-coherent-point-source 2-way, the Multi-Entry-Horn. I can't really do the MEH justice, but here's a Minimalist MEH (Minmei) experiment done over three sessions, listening and tweaking the passive filters to tame back flat the horn-bloat FR -- a rather fool-hardy challenge. But it has been fun and the result extremely vivid, detailed, and dynamic -- if not "holographically deep" like my coherent speakers or a really good fullrange driver. I did more-or-less time-align the drivers but phase would...

• wchang

• 

W

Post in thread 'Full Range Speaker Photo Gallery'

Saturday at 6:21 AM

Surprisingly or not, with the Minmeh I can hear the comb-filtering pattern dave has often posted. From ~3m away slightly off-axis ~0.4m, ear angled for maximum acuity, the sweet spots are ~10cm apart shifting (head) horizontally, or ~20cm forward/back. At these nodes the music is more open and realistic, presumably due to very high frequency present; away sounds a (very) little duller, less micro-dynamic contrast. I first noticed this when, each time I thought it sounded A+, with a head-sway it no longer was.

How to reduce horn/mouth/throat diffraction combing? VHF should be throat...

• wchang

 

[weltersys]

So I prototyped a single Minimalist MEH (Minmeh) using Faital HF108/2x4" and passively filtered the response quite flat 160hz-12khz. Sounded A+ until I swayed my head laterally -- just a couple inches -- then it became noticeably dull/flat -- and a couple inches further -- lively sound came back -- and so on and on. Classic comb-filtering?

If this is what your MEH horn looks like:

Then the HF driver's output will diffract across the large flat square surface around it, the diffraction will reflect off the horn walls, the multiple arrival times from those reflections combined with the direct path from the HF will cause "classic comb-filtering" peaks and dips, constructive and destructive interference.

Art

 

[wchang]

Then the HF driver's output will diffract across the large flat square surface around it

Wow amazing Art! Resolved; getting ~35° horizontal, 20° vertical coverage of flat unchanging FR i.e. to 12khz that I could hear. Much appreciated.

 

[phase_accurate]

Very cool for palying around but still faaaar from optimal.

Regards

Charles

 

[wchang]

Very cool for palying around but still faaaar from optimal.

Thanks for the response -- learning its specific shortcomigs and potential improvements is my goal.

Assuredly non-optimal. With the mids pre-existing in-box the drivers could not fit any closer etc. "Optimal" in terms of max SPL or bottom extension in-horn weren't needed. Dispersion/directivity limited by cardboard etc. Actually this experiment exceeded my expectations in these regards given the (lack of) size. Very-very-HF atop my current hearing range was surely above normal, in the sense that it sounded like a harpsichord used to sound, when I was 2.5 decades younger tapping keys during concert intermissions etc. Perhaps that's not good? The top "demerit" in my book is likely going to be soundstage depth when completed in stereo (passively filtered). I don't know how "phase perfect" across bandwidth and angular coverage a 2-way MEH can be, even with DSP -- or for that matter, just a HF-only horn. I'm interested in this, but it now seems to me a veeery high bar to attain for a horn.

 

[weltersys]

I don't know how "phase perfect" across bandwidth and angular coverage a 2-way MEH can be, even with DSP -

With FIR DSP, the phase (and frequency) response can be made perfectly flat.
DSP won't have any effect on angular coverage other than making time alignment independent from physical location, and minimizing the overlap region where physical location of the horn entrance affects the coverage.

- or for that matter, just a HF-only horn. I'm interested in this, but it now seems to me a veeery high bar to attain for a horn.

You have set the bar veeery low- if you simply make your conical horn's throat match the HF driver's exit diameter, it's off axis response would improve tremendously.
As an example, the polar response of a 3.5" TC9 "full range driver" is consistent up to ~4kHz, the wavelength of the horn throat diameter, all the way out to 30 degrees off axis, the cardboard horn wall angle:

A simple first order passive filter could flatten it's HF response above 1.6kHz.

Conical horns have some limitations as far as polar response, for instance the Danley SH-50 MEH is a bit rough, but maintains a fairly constant 50 degree pattern from 400Hz to past 10kHz:

The B&C ME 64 horn using a co-ax HF driver directivity is more constant:

Anyway, having set the bar so low, improvements will be quite obvious.

Art