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Old 3rd November 2014, 09:56 PM   #1
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Default I thought the appeal of horns was efficiency

I always thought that the appeal of horns was efficiency. I want to build a Frugel-horn mk3. And it turns out the mark audio 7.3 FH3 is very inefficient. I don't understand. Can someone explain why you would build an inefficient horn? Thanks.
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Old 3rd November 2014, 11:27 PM   #2
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When talking about a FR Horn we invariably are talking about a back loaded horn. The horn itself only functions at low frequencies and it does produce gain. This means that the efficiency of the system is the same as the driver as that is all that is opperating up above the horn cutoff (typically 250-350 Hz).

What does happen is that the LF gain means that the system may no longer need any BSC compensation... this means up to 6 dB more system sensitivity than a direct radiator where the MF/HF has to be shelved down with BSC.

Note: althou i recommend a minimum of 10W for the A7x, some people are very happy with using SE 2A3 driving them (3.5w). Much depends on how load you like to play and how big your room is.

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Old 3rd November 2014, 11:36 PM   #3
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A horn that is driven by the back of the driver is only trying to raise the bass up to the level of the driver's forward radiation (midrange and up). So a driver + horn of this kind isn't more efficient than the driver itself. (If it were, the bass would be too loud.)

If you want a high-efficiency system, then you probably want a front-loaded horn system (where the front of the driver is what loads the horn, not the back of the driver). This kind of horn will be more efficient across its whole bandwidth (compared to if there were no horn).

If you've seen a Klipschorn, that's a front-loaded 3-way. Even the woofer is firing "forward" at the horn's origin (though you can't see it, because of the complex way it's constructed).

Edit: Ah, crossposted.
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Old 3rd November 2014, 11:50 PM   #4
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Quote:
Originally Posted by rjbond3rd View Post
This kind of horn will be more efficient across its whole bandwidth (compared to if there were no horn).
The bandwidth of which is typically 3-4 octaves.

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Old 4th November 2014, 12:29 AM   #5
zacster is offline zacster  United States
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You can always use a Fostex FE126en driver, which is more efficient than the Alpair 7.3. 93db vs 85db. That's a pretty big difference, something I'm considering too. My PP 6B4G amp doesn't put out all that much power. It will play ridiculously loud with FE126e drivers, but won't with my big 2 ways.
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Old 4th November 2014, 12:31 AM   #6
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Quote:
Originally Posted by zacster View Post
My PP 6B4G amp doesn't put out all that much power.
The kind of amp we were using to drive the FH3 with A7 MOAP outside at the last VI diyFEST. Worked out just fine.

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Old 4th November 2014, 12:34 AM   #7
chrisb is offline chrisb  Canada
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It's interesting that you'd consider 85.8dB to be "very" inefficient - there are more than a few well respected commercial multiways with little more than a dB or so higher sensitivity than that. As Dave notes, there are more than a few folks content with using far less that 10W to drive the A7.3 in a variety of enclosures.

As to efficiency of horns - it's pretty much been summarized in the prior 3 posts.

What I happen to like about the FH3 (XL for larger drivers), is the effect of reflection of the rear radiated energy from back and side wall boundaries.
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Old 4th November 2014, 01:18 AM   #8
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There is a lot of enthusiasm for back loaded horns. What are the characterstics that you guys like?
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Old 4th November 2014, 01:56 AM   #9
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I like them for stuff that doesn't need a woofer.
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Old 4th November 2014, 03:28 AM   #10
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I am using a PE kit that I built 10 years ago that is an mtm with 5 1/4 aluminum woofers and and a silk dome tweeter that is not very efficient, but for acoustic and vocals it is very nice. Would it be similar to that? Or how might it be different?
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