Horn Question

**Mustang Marvin** · April 7th, 2011, 05:06 PM

This question has probably been asked before, but I must have missed the answer.
When installing a horn on top of an enclosure, do you position the voice coils in alignment or as the Altec documentation states, align the cone and the horn opening?
Thanks.

**bfish** · April 7th, 2011, 06:25 PM

For precision;

(Re)wire the HF driver backwards. (Leave the XO in place, everything but the HF wired normally).
Play a test tone the same frequency as the crossover point.
Measure (software/mic, RatShack dB meter, or ears if that's all ya got) a ~meter away (not critical exactly how far). Height halfway between the drivers vertically;
Have a helper slide the HF horn forwards and back, while you monitor the sound level for the lowest reading.
Leave the horn where the lowest reading was observed and rewire the HF back to normal.

(Why not just look for the highest output without the rewire? Because cancellation is more powerful than addition, and therefore more obvious).

Added
Then have your helper fetch a beverage while you cue some tunes.

**rontec** · April 7th, 2011, 11:44 PM

Very helpful, thanks bfish. I'll try this with my 828 cabs with top mounted 808's
and 421's. And the beverage idea is good too!

**Mustang Marvin** · April 8th, 2011, 10:04 AM

Actually the beverage comes first!
Maybe thats my problem...:laffingassoff:

**bfish** · April 8th, 2011, 06:16 PM

Originally Posted by rontec

Very helpful, thanks bfish. I'll try this with my 828 cabs with top mounted 808's
and 421's. And the beverage idea is good too!

You're welome. I should add that the higher the XO frequency, the smaller the sweet spot becomes.

**Mustang Marvin** · April 9th, 2011, 10:17 AM

Using an "L" pad the balance in volume can be achieved. I was more concerned with the timing. I have heard different opinions regarding the positioning of the horn for timing reasons.

1, Align voice coil with diaphram.
2, Align horn flare opening with cone.

The Altec documentation I have suggests #2.
I have also heard #1 from a reliable sorce.
Opinions???

**whitebroncoii** · April 12th, 2011, 10:24 AM

Mustang Marvin and Altec Best: Please re-read bfish's response. He describes an excellent method for aligning a two-way loudspeaker. The acoustic center (source of the sound) on a woofer is not the voice-coil or the edge of the frame, and the acoustic center of a HF horn/driver is not the voice-coil or the edge of the horn. bfish's method is described in old Altec literature for theater system set-up . . .

Another way to align a two-way loudspeaker is to measure the time difference using an analyzer. A passive system is then physically aligned (fore/aft position of the source). You can adjust the alignment in a by-amp system using digital delay. Delay the closer acoustic center to the distant acoustic center. These methods require special equipment (expensive), highlighting why bfish has described an excellent method for most of us to use.

An AES white paper that I assisted on offers this;
The closer the acoustic center is to aligned, the more phase damage, until they are perfectly aligned and in-phase. The converse is that the farther out of alignment the less damage occurs. Aligning the mouth of the horn with the front edge of the woofer can sound better than lining up the voice-coils!

**bfish** · April 12th, 2011, 05:55 PM

Thanks G!

I tried to be simplistic, that's usually pretty easy for a simpleton like me...

**Steve Schell** · April 13th, 2011, 04:42 PM

As I understand this situation (may be a bit faulty here and there) the "acoustic center" or virtual source of sound can be considered to be at the plane of the voice coil on a high frequency compression driver due to the very light moving mass. On a woofer with many times the moving mass the acoustic center is somewhat behind the plane of the voice coil due to inertia slowing the response time. This helps somewhat on a coaxial driver like a 604, at least partially compensating for the distance between low and high frequency voice coils.

Bfish's method of reversing polarity of one driver is a good one, as it creates a cancellation at certain points in space due to one driver moving out while the other is moving in. At points in space where the distance between acoustic centers of the two drivers are equal the cancellation is most complete and the null is greatest. Assuming vertical alignment of high and low frequency drivers, one can visualize how the locations of greatest cancellation would form a straight line (or beam) whose angle vertically would vary as one driver is moved closer or farther from the listening position while the other is held constant. Find that point where the beam coincides with the listening position and the alignment is optimal.

There is more to this Humpty Dumpty job of putting the audio signal back together, at least when it comes to creating a playback which is most transparent to the source, preserving localization (imaging) information in a stereo recording. Second order passive crossovers require the high and low frequency drivers to be connected in opposite polarity. This preserves even power response of the system through the crossover region, though imaging flies out the window. Audio waveforms are highly transient and asymmetric, and a system with drivers connected in opposite polarity cannot reconstruct such waveforms. First and third order networks permit the drivers to be connected in the same polarity. First order networks can sound terrific though they offer very little protection from low frequencies for the high frequency driver. Third order networks offer great protection, though at the cost of a high parts count and possible sonic effects due to the signal having to fight its way through all those parts. No free lunch!