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Old 16th March 2019, 08:10 PM   #151
Gnobuddy is offline Gnobuddy  Canada
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And from super-loud arrays of super-expensive 12" speakers, I'd like to go back to affordable 8" speakers, for low powered guitar amps of the sort discussed in this thread.

Okay, so stripped of marketing BS, a guitar speaker is basically a primitive speaker with a big floppy cone that beams treble like a searchlight, breaks up badly by 3 - 4 kHz, and has negligible sound output above maybe 5 - 6 kHz.

To make e-guitar sound good, our primitive speaker should preferably have a few other additional characteristics. The attached image shows the frequency response of a popular guitar speaker, along with some of its characteristics which I've noticed are frequently found in other guitar speaker frequency responses as well.

In the search for budget paging and public address speakers for e-guitar, then, we should look for at least some of these guitar-friendly characteristics. In particular, the slow and steady rise in SPL between roughly 300 Hz and 3 kHz, which is not a typical feature of paging speakers, but is ubiquitous in guitar-specific speakers.

Without this significant treble boost, guitars seem to sound dull and muffled, and valve amps sound less "valvey", probably because the treble boost brings up the level of the harmonics generated by the valve, effectively increasing the amount of low-order harmonic distortion arriving at the listener's ears.

Electric guitars make nasty harsh sounds at 6 kHz and higher, particularly when any distortion or overdrive is part of the sound. So the drastic treble cut above 3 - 4 kHz, produced by guitar speaker cone break-up, is also highly desirable. In conjunction with the slow treble boost from 300 Hz - 3 kHz, this built-in speaker EQ curve emphasises the low-order valve distortion some ears enjoy, and suppresses the high-order harshness they don't.

To sum up, our cheap P.A. speaker for e-guitar should, if possible, have at least some of the following characteristics:

1) A fundamental resonance not above 100 Hz, preferably between roughly 80 Hz and 100 Hz.

2) A poorly controlled fundamental resonance (high Qts, caused by a weak magnet or too-small sealed enclosure) if we want a little deep bass boost, like those famous 4x12 Marshall cabs ( Frequency Response of a Marshall 4x12 Cabinet ).

3) A slow and gradual rise in frequency response from around 300 Hz to around 3 - 4 kHz, if we want bright Fendery cleans.

4) An abrupt and drastic fall in high-frequency response above 3-4 kHz or so.

In my search, I found items 1) and 2) easy to find. Item 3) is considerably more difficult: a big treble boost like that does not sound good for actual paging and background music applications, so it's less likely to be found in speakers designed for those purposes.

Item 4) is also less likely to be found in a paging speaker, which is typically intended to be a "full range" speaker, i.e. to make at least some pretence of reproducing treble adequately for low-fi music applications.


-Gnobuddy
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Old 16th March 2019, 08:39 PM   #152
Gnobuddy is offline Gnobuddy  Canada
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A few years ago I found a $10 paging / public address speaker which turned out to have a 130 Hz fundamental resonance, and didn't have the built-in slow treble rise. With an equivalent treble boost built into the preamp, it sounded quite good for clean e-guitar, though. (Deep bass was a bit lacking, which might bother some, but it actually made the guitar "sit better in the mix" at jam sessions.)

Encouraged by that, I bought a second slightly more expensive full-range speaker, which turned out to be a bit of a dud. It wasn't awful, but was just a bit dull and characterless. Usable, but not thrilling. The problem seemed to be, once again, the lack of that steady rise in midrange response, from roughly 300 Hz to roughly 3 kHz.

Just recently, I found a budget speaker ( Misco JC8FD - Free Shipping Across Canada over $150 ) with the manufacturer's datasheet frequency response shown in the attached image. It seems to have almost all the characteristics I'm looking for for e-guitar use, except for one: there may be too much treble output above 5 kHz for distorted e-guitar.

If that does turn out to be an issue, I can think of a couple of ways to address that, though. One (suggested by J.M. Fahey in another thread) was to simply amputate the "whizzer cone" with an Exacto knife or curved nail-scissors. It would remain to be seen if this also removed the 3 kHz peak, which we do not want to lose.

Another possibility would be to add a small series inductor (I estimate around 200 uH - 300 uH) to roll off high treble. How well this works will depend on the guitar amp's output impedance - if it has a high output impedance, a series inductor won't do much.

A small parallel capacitor across the speaker terminals is another possibility, and could be used in conjunction with the small series inductor (basically making a simple crossover network of the sort normally used with woofers in Hi-Fi speaker systems.)

Finally, there is also the option of filtering out the high treble acoustically, for example, by using a sheet of open-cell urethane or similar foam between speaker and grille cloth. Simple, and easily tailored by varying the foam thickness.

I think this speaker looks like a very promising candidate, so I'm definitely going to get at least one of them to experiment with.

Incidentally, the speaker brand (MISCO) seems to be derived from "Minneapolis Speaker Company", so those of you in the USA can probably find this speaker in America at much lower cost than I'd have to pay here in BC.


-Gnobuddy
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Old 16th March 2019, 11:27 PM   #153
klingo is offline klingo
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Hi, nice FR.

Regarding the foam trick, i recently try 1/2" foam between grill-cloth and baffle board with a 2.5/3" hole centered with the cone (the donut beam blocker?). It is supposed to widen the hf dispersion and to flatten on axis treble response. people have used 3/4 to 1" foam for that.
In my case, an old blues jr/stock speaker with 18w lite II inside, it works like a charm, a bit wider sweet spot, nicer sounding and no grill rattle. So i did it to my 5F2/emi1058 speaker with the same results.
8" speakers are less beamy than 12" but foam trick could do well.
I don't know why but conjonctive or zobel network never did it for me.
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Old 17th March 2019, 12:27 AM   #154
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by klingo View Post
1/2" foam between grill-cloth and baffle board with a 2.5/3" hole centered with the cone (the donut beam blocker?)
Thanks for your feedback!

I read about the foam donut trick on some other forum, where I think it was called the "Mitchell Doughnut". As you say, the purpose is to improve treble dispersion.

The "Mitchell Doughnut" works because that open-cell foam absorbs treble better than it absorbs bass. Cut into a doughnut, it reduces treble radiated by most of the cone, allowing only treble from the central 3" region to come through. In effect, the big 12" speaker turns into a 3" tweeter at high frequencies. A very clever idea!

With the little 8" Misco speaker, the problem I'm concerned with is something else: the treble response doesn't fall as sharply above the 3 kHz peak, compared to "real" guitar speakers. That might sound harsh, particularly for overdriven or distorted guitar sounds.

So what happens if we cover the entire Misco speaker with foam, without cutting the 3" hole in the middle? It should "turn down the treble", and we can control how much by choosing the thickness of the foam. I think this might be one way to tame excessive high frequency harshness, if that turns out to be a problem.


-Gnobuddy
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Old 20th March 2019, 01:35 AM   #155
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Gnobuddy View Post
Unfortunately, it turns out that approach has many sonic limitations. In the world of good-quality audio reproduction (Hi-Fi, for lack of a better term), the speaker quickly evolved into something different, and capable of producing much more accurate sound, at the expense of sheer efficiency. Surrounds and spiders got softer. Cones got smaller, stiffer and heavier. "Whizzer" cones were added to the centre to extend treble response.
In an
SOS interview with Charlie Watkins he makes the following comment describing the first PA he designed in the late '60s:
Quote:
Originally Posted by Charlie Watkins
“Then I got in touch with Goodmans. They had a speaker called the Axiom 301, with a concentric cone. All the other things that they were offering to the likes of me were general-purpose with horrible stiff cones. That was what made the difference; for vocals you needed gentle cones that, when you touched them, they moved easily, not like the horrible stiff things they were using for guitar speakers.
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Old 20th March 2019, 03:03 AM   #156
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by thoglette View Post
...interview with Charlie Watkins...
Thank your for that link. I found it fascinating reading. I knew a little about WEM the sound-reinforcement company, and I'd heard about the Watkins Dominator, but that was all I knew. I had no idea who Charlie Watkins was, or just how much crucial technology he'd contributed to the popular music we all grew up with.

What an amazing man, and what a life story!


-Gnobuddy
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Old 20th March 2019, 08:50 AM   #157
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Gnobuddy View Post
T It should "turn down the treble", and we can control how much by choosing the thickness of the foam. I think this might be one way to tame excessive high frequency harshness, if that turns out to be a problem.
You can try my other hack: mount the driver back-to-front.

It is serious ugly but significantly cuts treble from a wizzer equipped cone.

Can't remember the theory (my brain is suffering a bad case of Lord Lucas today) but solid patterns in front of the cone also work as filters. You could probably mount the baffle halfway back and use a carefully designed front baffle to tune the response the way you want. But cutting the wizzer off might be easier
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Old 20th March 2019, 03:48 PM   #158
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by thoglette View Post
You can try my other hack: mount the driver back-to-front.
Thanks, that's one more idea to add to the list of things to try.

I recently found out that one of the studio tricks to tame a harsh electric guitar sound is to mic the rear of the speaker cab, and not the front.

I've tried two different budget-priced P.A. speakers in my guitar amps so far, both with whizzer cones. One of them actually sounds dull, even with the whizzer. Probably because it's missing that built-in treble boost from 300 Hz to 3 kHz.


-Gnobuddy
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