Questions Regarding Horn Design and Dimensions

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TN Allen
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Questions Regarding Horn Design and Dimensions

Post by TN Allen »

I have enjoyed several days reading through a few of the threads on your wonderful forum. I have learned quite a bit, and have a few questions, and thank anyone responding in advance. I have several phonographs, and various parts from others and have thought for a few years of building a large horn to explore how good non-electric 78RPM sound might be. But developing or finding suitable dimensions is a problem. I have in mind a horn of a 5-7' length X ~ 3' opening.

I've read the horn chapter in Modern Gramophones and Electrical Reproducers, and briefly considered the feasibility of building the 168" long 64 Cycle horn on page 100. It's been fun thinking the problems through, however, the work to set up to build and turn the large end is too much, given a smaller horn probably would do nearly as well, except in the very low frequencies, and I doubt there are many 78 RPM records capable of good 64Hz. frequencies sound.

The first question is, how are those of you building new horns designing these? Are you using the formulae in Modern Gramophones and Electrical Reproducers? Or perhaps, are you shortening the distances given in the page 100 table but maintaining the diameters?

The second question, are dimensions available for the E.M.G. horns? From what I've read so far, and heard on some of the YouTube files, it seems these are quite good. I suspect though that they may have been modified through experiment from the designer's original calculations to improve the sound before going into production. I've been reading The E.M.G. Story and while they relied on science in their designs, it does seem likely to me they also relied heavily on careful listening and intuition to produce their gramophones. Consequently, I assume reasonably accurate, given the effects of time on the paper horns, final dimensions would be critical to building a replica.

A third question, do the exponential horn designs consider ¼ wave effects. I base this upon experience building Transmission Line speaker cabinets, where the length of the signal is ¼ the full length of a desired low frequency response, rather than the full length.

Again, thank you to anyone who responds to my questions, or offers suggestions.

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Inigo
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Re: Questions Regarding Horn Design and Dimensions

Post by Inigo »

What I can say first is that, although there is not much bass in acoustic 78s (although some later Harmony records would give you a surprise) the exponential horn machines made by hmv and Victor, etc, produce a much more beautiful sound, rich and full, than the pre-1925 machines, as older Victrolas. Large wooden external horn gramophones give a better sound, but the exponential horns are a must. Even the small portables as the hmv101 or the Victrola 2-55 etc hill makes acoustics sound great. First of all, you get rid of that tubby sound and harsh mid treble range, which I find very unpleasant. Acoustics played on an orthophonic machine sound clean and full, and you'll hear things that are not well reproduced on the small conical horn machines. Seems as the sound of the record breathes in full, and they're much more pleasant to listen to. Some acoustic recordings are indeed very good, despite their limited frequency range.
Inigo

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Thank you for your observations. The first time I heard a Victrola Granada, I bought it on site, the sound was a revelation compared to earlier Victrolas I had heard. Shortly after that I found and bought a Credenza, even a more significant revelation. But now, I am interested in the EMG and EM Ginn machine sound. I doubt I'll have an opportunity to hear an original machine, other than via YouTube, consequently I am thinking of building and assembling something similar.

I know the dimensions and design are critical, and consequently I'll appreviate any assistance.

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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

That is "appreciate" rather than "appreviate". Typing on a phone certainly is a challenge.

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emgcr
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Re: Questions Regarding Horn Design and Dimensions

Post by emgcr »

TN Allen wrote: Sat Jul 10, 2021 10:44 am .................are dimensions available for the E.M.G. horns?
The straight answer is yes---for the larger examples. The details are available in page three of this post : viewtopic.php?f=11&t=15076&start=20

Although the horn template is specifically copying the profile of the EMG Xb Oversize horn, the flare is identical---but shorter---for the standard Mk Xb and Mk Xa. The three bell mouth diameters are 33 ½", 29 ½" and 28 ⅝" respectively. Thus the template can be used to reproduce exactly the original shape of these horns---or indeed any horn with desired final diameter lying on the same curve. The standard Xb 29 ½" horn was decided upon as being the maximum size that would coveniently fit through a single British standard door opening of similar width and had little to do with acoustic considerations per se.

It is important to note that the EMG sound does not just rely upon the horn---the whole carefully designed acoustic track through the bronze spigot, cast aluminium conduit, cast brass tonearm, swinging copper section attaching to the soundbox and the soundbox itself are all vitally important.

If you decide to build a horn I should be happy to make and send a paper copy of the profile. I have attached rough sketches below showing the bronze spigot and Oversize horn dimensions.
Attachments
EMG HORN SPIGOT..JPG
DSC03844.JPG

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

emgcr,

Thank you for the detailed information and links. I have looked at your extensive .pdf file regarding making new horns. The work is impressive! And what a project! Thank you also for the invitation to hear your horns. However, I live in Maine, nearer to New Hampshire, and do not travel. I'll try connecting a computer to an amplifier via Blue Tooth to have a better way to listen to your YouTube files. That will not be as suitable as listening on the spot, but will do for the present.

I have completed the E.M.G. Story, and continue to read the Modern Gramophones and Electrical Reproducers, and have begun to understand the development of the machines and some of the design considerations. Regarding the soundbox, I have at least one Orthophonic soundbox and tonearm in the Granada and probably another in the Credenza, though I need to uncover it to look. I thought one of these would be suitable to try a larger horn. I can order and replace gaskets for these if necessary, though these may have been replaced by the previous owners, as both appreciated and used the machines regularly.

I also appreciate the importance of the geometry of the horn, and the finish and fit of the various parts. These are foremost in my mind as I think through the possibilities and requirements for designing and building.

Thank you again.

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Inigo
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Re: Questions Regarding Horn Design and Dimensions

Post by Inigo »

As far as the most difficult part to build, at least for me, it's the tonearm, I always thought of starting the design of the horn at the base of the tonearm, using that dimension as the throat, as I would use one branded tonearm, such as that of the Credenza of whatever. Then I believe the conduit from the tonearm base to the horn base, plus the horn, would be entirely designed and handmade. I think the error cannot be too much...
Inigo

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Inigo,

You raise and interesting point. Looking at the table in the Modern Gramophones & Electrical Reproducers, the small end, throat(?), of the exponential horn is a .25" radius, or ½" diameter, which is close to the opening in one Victrola tonearm and soundbox connection I have. The other end of the Victrola tonearm, where it meets the horn in the cabinet, is 2.06" inside diameter, and the length of the sound path about 12". Looking again at the MG&ER table, the exponential horn develops a radius of 1.04", or a diameter of 2.08" at 48" along the length of the horn. So, the length the sound wave travels in the exponential horn is 4X the length traveled in the Victrola tonearm before reaching the same approximate diameter. That probably was impractical to manufacture at a reasonable cost around 1930, short of using part of a tuba, but might be possible presently, given sufficient resources and time, and a large space to set up a phonograph with a much smaller diameter and longer tonearm. On the other hand, perhaps there is not a significant disadvantage to the shorter tonearm transition from the soundbox diameter to the cabinet horn entrance.

Looking at photographs of the E.M.G. and Expert machines, the tonearms seems longer and of a significantly smaller diameter than the Victrola, and may be similar to the length and diameters in the table.

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Inigo
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Re: Questions Regarding Horn Design and Dimensions

Post by Inigo »

That's the point. If you design a true exponential tonearm for the same standard diameters, it will be very long. Watch the thin and long EMG tonearms... My approach is a shortcut, as commercial tonearms are much shorter to cover the same expansion from ½" to 2". I.E.= they are designed for a higher cutoff frequency, thus, expansion is faster.
The thing about EMG is that they followed (we believe) strictly the exponential expansion law calculated from the ½" at the soundbox to the 29"/33" at the mouth, full length bd no shortcuts, to a low freq cutoff. We don't know the value, but judging for the long path between these two diameters, it was lower than the commercial sound systems (VIctor, HMV, Columbia), for these had much shorter tonearms for the same expansion.
My guess, after those all lectures, is that you could compromise at the tonearm, then start with a true exponential low freq expansion from the tonearm base to the mouth of horn. You only shorten the tonearm part, but at the narrow end it seems that the imperfection is not too important. Thus, using a good commercial tonearm.
But this is my idea, and the one i'd bid for if I built one of these... But this is my particular idea, for it seems to me much more difficult to build a good exponential tonearm, so I'd use the longest one I could get, no matter the law at which it was constructed, given it is exponential, not conical.
But even this can be circumvented somehow. Wilson (and others) say that a straight tube at the beginning of the conduit would cause no harm, so maybe the tonearm could be made straight ½" diameter, and start the exponential expansion at the base only. To build a constant cross section tonearm is much easier (if any) than building one with a strict exponential flare... Using ½" commercial piping.
It is clear that what spoils the sound is a fast flare at the wide part of the horn, not so much at the narrow part.
There's a japanese colleague herein who has built his own EMG-esque machine, using an orthophonic Victrola tonearm.
viewtopic.php?p=274442#p274442
Inigo

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Inigo,
It would be interesting, no doubt challenging as well, to produce a tonearm tapered to the values in the MG&ER table, that mates to a horn maintaining the suggested exponential taper thoughout the sound path/transmission line. CAD, 3D printing and CNC milling machines make this reasonably feasible.

Had the E.M.G. and E.M Ginn companies had access to this technolgy, what might they have made? How might their machines have changed? And to be realistic, would there have been a significant improvement in the sound their machines produced? Of course this is a limited conjecture, as digital technology, available in one industry, would also have changed collateral industries. Which is to say the entire recording, manufacturing and playback systems would have changed. And we'd probably be about where we are today regarding recorded music, but 100 years sooner.

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