Straight 40 Hz corner horn - only 1.4 m (4.6 ft) long!

[rick57]

I plugged in 50+ drivers into Volvotreter’s spreadsheet.
Two very good value & size options for a 40 Hz corner straight horn, apparently are: the Eminence Beta 15A only $75 (US Speakers) Vas of 335 litres, and P.Audio C15-300MB $110 (Loudspeakers Plus) Vas of 200 litres.
I suspect that that the P.Audio would have distortion. The Beta 15A needs a length of only 1.4 m (55 inches), the P.Audio 1.96 m (77 inches). Less space than a Lab12 would need; and long enough to fit most rooms.

I was planning on mounting the horns vertically, upside down in the corners, with the mouths facing the floor in the corner, and the mouth about half the mouth width off the floor (mouth calculates at 85 cm2 (34 inches2), so 43 cm (17”) off the floor).

Edgar in his 40 Hz Show Horn used a 45 degree plywood reflector as his last reflector before the mouth. I thought a very stiff (eg concrete pavers laid on a board) 45 degree reflector could couple the horn to the floor, after the mouth.

But I was just talking to a non horn retired acoustician, who suggested two problems.

- the horn (made of ply and/or bent plasterboard) might get excited by resonances from other audio sources in the room; and
- coupling a vertical horn into the corner, even with reflectors might not work.

Would it work? Has anyone done straight horns firing into a corner ??
Apparently Klipsch corner horns operating over a simple range used none-too-simple dampeners to avoid resonances. Would resonances be a problem??

How do you best couple a horn to a (concrete) floor?

Thanks

[rick57]

An altenative is to assume between 1/ 8th and 1/ 4 space, ie say 1/ 6th space, ie make the horn a little longer.


Now someone has pointed out to me offline, that the driver cone area is only about 84% of throat area – but the entire throat area must be driven to avoid undesirable effects.

I tried just a dozen 15 inch drivers in Volvotreter’s spreadsheet and 84% was easily the closest.
It must be almost a fluke to find a driver that suits the desired horn in other ways, and also the cone area is 100% of throat area.

So is there a technique (eg part of Hornresp?) to change the throat - cone match.
Or change the driver, eg lower the Fs by 5 Hz takes it to 98% - would a small mass added lower the Fs (without deleterious effects)?

I'm very curious as to the right way to pick a driver, or tune for an 84% fit.

Thanks

[rick57]

I’ve now input the 72 15” & 18” drivers listed by Parts Express in their "Pro Sound Drive Selection Guide", a handy extensive guide to the parameters needed by Leach horn math (with a bit o’ copy paste - transpose).

Only three drivers have a Volvotreter modelled throat area within 10% of cone area. Range of these: –2% to +6%.

* Am I right that any within that range should be close enough??

The Peavey 1801-8BW, and two Eminences - the Delta-Pro-15A and Delta-15LFA. Price range US$ 100-185.

The best choice appears to be the Delta-15LFA ($100).
Sd of 856 cm2 is 95% of AT throat area calculated for given driver of 897 cm2.
With 1/ 6th loading (between wall & corner) 40 Hz needs just 1.9 m (75”).

Driver “features”:
- Kapton coil former
- Polyamide-imide coated two-layer, 29 ga., copper voice coil
- Vented and extended core
- Pressed steel basket
- Paper cone
- Rolled cloth cone edge

Any comments on this driver or Eminence for this purpose are welcome . .

Thanks

[John Sheerin]

Do you specifically want to have a ~1:1 compression ratio?

[rick57]

Hi John

Hope you have time to answer a few questions . .

No, I had thought simply that a ~1:1 compression ratio was ideal.
Since your post, and searching I see that it’s just a minimum. I guess intended use (eg 100 dB v 120 dB) and driver construction would set a *maximum compression ratio; is there an ideal or *target, eg 100-200%?

The broader question of driver specs to look for ~ I found your post at AA a few months ago:
“For an 18" in a 30 hz m=.6 hyperbolic horn, look for an Fs around 80hz, Mms around 270g (alternatively Vas around 31.5L).
Qes would be around 0.6-0.8, BL around 30Tm.
This is for a 4:1 compression ratio, chamber behind the driver around 2500cm^3. That's just one example. Good luck finding a driver with those specs . .”

I was surprised that Fs is 1.67 octaves higher than Fc – could you say why so high? And if a driver with those specs is hard to find, for domestic (lower output) use, what is most important ~ less important?

Lastly I also saw your Horn deign guide recommends a horn’s acoustic length be half Fc, to operate in a “velocity-controlled manner”.
How is acoustic length measured or determined?
And what adverse happens outside the velocity-controlled range?

Help greatly appreciated

Regards

Richard

[rick57]

Anyone else able to shed any light on these questions?

Help appreciated . .

[rick57]

I’m guessing that by outside (below) the velocity-controlled range, you’re saying it becomes a direct radiator, so it loses efficiency.

Larger rear chamber lowers resonance. I’m guess this can be modelled in Hornresp?

Clarification on picking a good driver would still be appreciated.

[slowmotion]

Rick ,

have you tried simulating in Hornresp?

cheers

[rick57]

Hi Jan,

I’ve tried Hornresp a couple of times but didn’t really get the hang of it. I need to find a few uninterrupted hours.
I use Excel a lot at work so have learnt a bit from the 00s of values I input into Volvotreter’s spreadsheet.

If I knew the relevant formulas, I’d rather build a Excel model from scratch than learn Hornsresp.

Since my last post I found *two formulas for the low rolloff point of a driver in a horn:

(1) Fs * Qes (indirectly alluded to Olson)
I tried this on 80 odd 15-18” pro drivers, and got a range from 0 – 56 Hz! I was expecting maybe 25- 90 Hz. O hz!

(2) From a post of GM’s quoting a JBL tech sheet (from Keele's paper) with -3 dB frequency:
fLC = Qts * fS/ 2
(Using the Qes values I have, for Qts - with these sort of drivers usually pretty close) the results are of course half: 0- 28 Hz.


Tom D. says to use the formulas in Marshal Leach's AES paper. Anyone know what Leach said on this?

I wanted to go to Hornresp with a shortlist of about 5 drivers, but so far I’m not able to establish a shortlist.

Cheers

Rick

[John Sheerin]

Rick,
The recommended design you dug up was for a rear loaded horn, iirc. Thus the driver parameters would be different than for a front loaded horn (ie, a horn with a sealed rear chamber). The rear chamber provides additional stiffness in the form of the sealed volume of air. The rear loaded horn has no such additional stiffness, so to tune the system to the desired resonant frequency (which is in the middle of the bandwidth), you need a stiffer spider which for the same moving mass produces a higher resonant frequency of the driver. These driver parameters were derived from Leach's math using a very large rear chamber (which approaches the case of no rear chamber).

To answer some of your questions, most of the formulas you're probably using make the assumption that your horn is infinitely large. This is obviously not the case, so to get a true picture of what will happen when you shrink the horn, Hornresp is about as good as you can do. Programming Hornresp in Excel would be masochistic at best. Just figure out how to use it. Read the wiki here describing how to use it (search for my posts - I think that might have been related to one of the last posts I made).

Compression ratio in theory can be tied to the operating bandwidth you desire. Leach's math illustrates this. His AES paper is posted on his website for free. Read my website for more details, but I generally start with Leach's math and then go to Hornresp to tweak. Typical values are ~10:1 for HF compression drivers, 2:1 to 8:1 for low frequency horns. Most people say 4:1 is high, while I disagree - I have gone as high as 10:1, but with a very stiff cone. It just depends on your exact design.

Picking a good driver can be facilitated by using Leach's math. It is impossible to generalize besides reiterating Leach's math. I would say use something with shorting rings, though. That is very important (assuming they're designed correctly), but it also typically costs more and / or is not available.