Author Topic: Transducers- Guitar & Bass  (Read 13158 times)

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Offline CLD

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Re: Transducers- Guitar & Bass
« Reply #14 on: February 04, 2011, 08:09:49 am »
Glad to hear that, thanks! As I mentioned in my original post, I never questioned the tone (and still don't). Just surprised to see the speakers labeled that way.
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Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #15 on: February 04, 2011, 05:39:04 pm »
Bass vs guitar speakers. When I was at Sunn, I often used the same magnet assy for bass and guitar speakers of the same dia...at least in the "MI" line...as in musical instrument. Pole piece, top and bottom plate were usually the same for a given magnet size.
For guitar, I used a shorter voice coil winding ie: less gap overhang, higher efficiency, linear throw of maybe 1/4" and a harder cone material (Hawley 7DF or similar), stiffer surround and spider. I would try to get a frequency response with a dip at about 1K, and a peak at about 4K (think Celestion).  I would shoot for a fundamental resonance of around 50 to 55 hz. We would treat the surround with Moyan A400 (I think that's the #), which is kinda of clear rubbery stuff, which raises the resonance and keeps the surround from cracking.

For bass, I used larger coil wire, with more overhang, linear throw of 3/8" to 1/2". I used softer cone materials (don't remember the Hawley #s), and softer spider. I shot for a resonance of 38 to 42 HZ. I tried for a fairly smooth frequency response, without any intentional dips or peaks. We used a surround treatment (also a Moyan product) that lowered the resonance, and also prevented cracking. For most bass applications I liked to get an equivalent compliance (Vas) of about 4.5 cubic feet.

Not all of the above is true for the SPL speaker line...that was a different beast.
Engineer Sunn 1977-1983, Fender 1984-1990
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Offline Isaac

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Re: Transducers- Guitar & Bass
« Reply #16 on: February 05, 2011, 06:32:59 pm »
Rex, any info on the PA transducers? In my Sceptre cabinet, I have two 16 ohm guitar transducers and two 16 ohm PA transducers. The cones look the same, as do the surrounds, but I was wondering about any differences. I can get the exact model numbers, if that will help.
Isaac

Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #17 on: February 06, 2011, 07:42:34 pm »
Isaac. All I can tell you is what is "typically" different between PA and Guitar speakers.

Note: When I talk about guitar speakers, I am talking 6 string regular guitar, not bass guitar. Most of the following specifically addresses 12" speakers, but generally applies to other sizes.

Guitar speakers are often used in "open back" cabinets, which means there is virtually no "load" on the speaker: it is often operating in free space ie open air. The surround and spider have to be fairly stiff and limit the cone travel to prevent over-excursion on lower frequencies. Low E on a guitar is about 84Hz, so a guitar speaker isn't usually designed to go much below that...free air resonance of 65 to 75 Hz is OK. Also, guitar players on average do not like a "flat" frequency response, they seem to prefer a dip at about 1Khz...-3 to -6 db, and a similar peak at about 4khz. (especially those that play in overdrive mode) In descriptive terms, it reduces "honk", and adds "crunch". I credit Celestion with invented this, and it is partially accomplished by gluing the cone directly to the voice coil form, then the spider to the outside of the cone, which is the reverse of how most speakers are assembled. This couples the coil form resonance directly to the cone. (Ed Form, engineer at Celestion showed me this trick..thanks Ed. We later implemented it at Fender with Bob Gault of Eminence. Sorry Ed, Thanks Bob.) Also, cone break-up is often desired in a guitar speaker to add harmonics: thin cone using short fiber hard pulp, sometimes treated with shellac or similar. Guitar speakers are part of the sound creation process as opposed to the sound reinforcement process.

PA/sound reinforcement and bass speakers normally operate with a significant air load from the box: closed box, tuned port (bass reflex) or horn. The air load helps control movement at lower frequencies, so the suspension and spider can can be made less stiff, and have longer linear travel. Low E on a bass guitar is about 42 Hz, so it is typical for a PA/bass speaker to have a free air resonance around 38 to 45 hz. They are also usually designed to have a flat frequency response, with as few dips and peaks as possible. Cones are designed to have as little break-up as possible, thicker paper, longer and softer fiber....well-damped to reduce harmonics. The spider is typically glued to the voice coil form, then the cone to outside of the spider lip. This buffers coil form resonance from the cone. A PA speaker's job is to reinforce the sound that has already been created...simply make it louder without adding any characteristics of its own.

In the end, for guitar it all comes down to what style you play, and what you like. There are no rules: we designers try to do the right things wrong until enough guitar players like it.  Some old Chicago blues players used to take a perfectly good new JBL, EV or similar PA speaker and stab it a few times with a screwdriver in "just the right places" to get the sound they liked. Perfect !   Hope this helps.  
« Last Edit: February 07, 2011, 02:07:03 pm by Rex B »
Engineer Sunn 1977-1983, Fender 1984-1990
Model T Red Knob
200S Cabinet JBL D140Fs
Sunn SB 300 (Proto Coliseum 300)

Offline george

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Re: Transducers- Guitar & Bass
« Reply #18 on: February 07, 2011, 09:21:17 am »
Rex, you mentioned that Moyan A400 was used to raise the Fs for a guitar speaker and another edge damping material was used to lower Fs for a bass speaker. How's that work?

What type of venting did you use for driver motor design? I'm amazed that most speaker designers get this wrong and you can hear a driver "huffing" at low frequencies.

Great stuff. Thanks

Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #19 on: February 07, 2011, 01:43:51 pm »
George..good questions.

First...if anybody reading, especially speaker re-cone folks, has any corrections to what I write, please jump in...this is all from memory, and I'm sure I've dropped a brain cell or two in the last 30 or so years.

Surround function:It's a spring, it has a K factor...X amount of force moves it Y distance in the linear range. As it reaches the limits of its travel, it takes more force to move it Y distance ie: the nonlinear range. Non-linear=distortion. However, we want it to go highly non-linear at some point so the coil doesn't crash into the back plate or jump out of the gap. The spider does that same thing, with the additional important function of holding the coil in the center of the gap. (The surround also does this but to a lesser extent)  The surround and spider combine to determine the compliance, or spring factor of the speaker. There are many surround and spider configurations and materials to give the compliance and excursion desired. The compliance (spring factor) and the mass of the cone/coil assembly determine the free-air resonant frequency.  

Surround treatment.  There are many concoctions used for this, and for various functions. The simplest is in the case of a cloth surround when used in a reflex, closed or horn loaded configuration. It needs to be sealed with something to prevent air from blowing through it and leaking the air load designed into the enclosure. (Doesn't matter in open back cabs)  They can be treated at the manufacturer (such as Hawley) or we could do it ourselves.

In the case of integral, or paper surrounds, there is a catch 22. The same paper we want to be rigid on the cone has to be springy (compliant) at the surround. There is an additional problem of standing waves, which is where frequencies with a wavelength smaller that the cone diameter (above piston range) travel from the coil through the cone and surround, bounce off the speaker basket rim, and back down the cone in a self-perpetuating cycle. Surround treatment, or doping helps control this. It changes the speed of sound through the paper at the surround, usually slowing it down. By adding a surface treatment, such as rubbery A400 or similar, it adds to the thickness of the surround, increasing the K factor, or stiffening the spring and raising resonant frequency. It also adds loss, or damping (like a shock absorber) to absorb standing waves.  It also helps keep the paper from fatiguing and cracking. A400 is kind of like a diluted clear latex bathtub caulk.

To accomplish all of the above (damping etc), but lower the K factor and resonance frequency, another type of product is used (wish I could remember the product #), which soaks into the paper and softens it. You have probably seen this, because it remains sticky, and attracts dust. Think of it as a combination of WD40 and honey. It never fully dries.

Vented Pole Pieces. These are used to create airflow to cool the coil and gap. Yep...they can flutter. Ideally, the air flow is laminar...smooth and free of turbulence. At longer excursions ie lower frequencies, the air pocket contained under the dust cap gets compressed and decompressed to the point that it cannot flow smoothly through the vent. It becomes turbulent and tumbles, especially at the exit of the vent, where it abruptly changes pressure to whatever is outside the back of the speaker. Smack your palm on the end of a short pipe and you get the idea.

Solutions: One is to simply bore a bigger hole in the pole piece that can accommodate more air. However, at some point, you remove enough metal that you reduce the amount of flux the pole piece can deliver to the gap. The efficiency of the speaker drops and you are paying for magnet you aren't using. Another is to vent the dust cap or use a porous dust cap that can't build as much pressure under it. This can also be somewhat self-defeating in that it it creates a leak from the cabinet enclosure through the vent to the outside, plus reduces air flow through the vent, which was the idea in the first place. Sometimes you will see this in a speaker with a rigid dust cap with a hole in the center covered by cloth. Another elegant solution which I first saw on Hartley Peavy's Black Widow, was to taper the back of the vent so the transition to the outside was gradual..kind of like a little horn.  I'm not sure if Peavy invented it, but it was a great idea and as far as I know, it worked. In my case on the SPL speakers, we had a big enough pole piece and a huge magnet wallop (96 oz). We simply increased the size of the vent until the flux dropped a bit in the gap, then backed off to the previous size vent. It worked and didn't flutter.

Hope this helps...Thanks...Rex
« Last Edit: February 07, 2011, 02:09:54 pm by Rex B »
Engineer Sunn 1977-1983, Fender 1984-1990
Model T Red Knob
200S Cabinet JBL D140Fs
Sunn SB 300 (Proto Coliseum 300)

Offline george

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Re: Transducers- Guitar & Bass
« Reply #20 on: February 07, 2011, 08:38:45 pm »
Thanks Rex,

   I worked for a guy who did many of the Celestion MI speaker designs in the 50's and 60's. He was a malignant narcissist but remembered much about those days.

Venting the former of the VC also used to vent the motor.

I'm not sure if you are familiar with Klippel ? http://www.klippel.de/  Great measurement systems if you have the $$.

Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #21 on: February 08, 2011, 01:20:35 pm »
George.  Klippel. Wasn't familiar, but am now. Wow, very cool. We had to jump through alot of hoops and calculations in my day to get a fraction of the info the Klippel gear provides. NICE!

Yep, venting gets the heat out of the entire motor assembly. Heat = death at some point. My definition of power handling was 12 hr, pink noise, 20 hz to 20khz, 60 degree F ambient, RMS measurement. Ran the test overnight. I would keep raising the power each night until the coil wire came unglued. Rating would be 50% of the highest no-fail power. Any failure other than coil wire coming off the form would be cause for redesign. I am damn lucky I didn't burn Sunn down. We built our own amplifier to test the SPL stuff because we didn't find a commercially available power amp at the time that could get them to fail. They either didn't have the headroom, or would burn up before the speaker would. Fun times.   
Engineer Sunn 1977-1983, Fender 1984-1990
Model T Red Knob
200S Cabinet JBL D140Fs
Sunn SB 300 (Proto Coliseum 300)

Offline george

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Re: Transducers- Guitar & Bass
« Reply #22 on: February 08, 2011, 09:56:14 pm »
Interesting how the "industry" hasn't formalized on a process/test procedure for max. power handling specification??  Too much of a marketing opportunity  :wink:

I'm also amused when a tech/musician starts blaming the driver for a problem with the amplifier.

Rex, when did you work at Sunn?

Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #23 on: February 15, 2011, 11:08:16 am »
George. I joined Sunn in 1977 as Electro-Acoustic Engineer. Left Sunn in 1984 to work for Fender in Fullerton, then Fender bought Sunn in 1985. Because I had worked at Sunn, they moved me back to Oregon to help reorganize Sunn and I was right back where I started. Left Fender/Sunn in 1990 to do circuit board work.....mostly for Fender, JBL and Mackie.
Engineer Sunn 1977-1983, Fender 1984-1990
Model T Red Knob
200S Cabinet JBL D140Fs
Sunn SB 300 (Proto Coliseum 300)

Offline tinnitus

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Re: Transducers- Guitar & Bass
« Reply #24 on: February 20, 2011, 11:01:51 pm »
Rex. Great reading. Fascinating stuff. Do you have any suggestions for preserving the integrity of the SPL products in my (now) "vintage" collection. 81xx, 86xx, 83xx, and 88xx 's.
The cabinets are doing well, having quit the gigging some years ago.
Urban legend has told me several things about speakers:
1) magnets fade over the years
2) magnets get "sweeter" over the years
3) spiders go limp over the years
4) the cones "dry out" and disintegrate
is this all crap or what do I really need to worry and plan for 'over the years'?
Well I don't know much. It all still sounds great to me. I appreciate seeing something on the forum besides guitar stuff, i.e. the Biamp section. Was Conrad ever involved with the sound reinforcement product lines at Sunn?- tinnitus (WHAT?)

Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #25 on: February 22, 2011, 11:21:13 am »
Tinnitus. 

The urban legends you listed, in general, have some truth to them.

Magnet fade / Sweetening.  Magnets do lose some of their strength over time....everything will continue to move toward its lowest energy state over time. However, it it usually pretty slow, and personally, I don't consider it much of a factor. Getting "sweeter".....um, not sure that that means, it's a subjective term. If the magnet has lost some of its strength....which would mean the flux density in the gap has dropped, and the "Q" will rise, (ie: less electrical damping) and the speaker will tend to resonate a bit more freely. It won't sound as "tight" as it did. To understand "Q", imagine you send a signal to a speaker that tells it to move forward 1/4", then come back to center and stop. The lower the Q, the quicker it will stop at center. The higher the Q, it will overshoot past center, rebound back toward center, and overshoot again the other direction until it finally settles out. In general, the larger the magnet, the lower the Q.  The sound does change as Q changes, but some people prefer a looser higher-Q sound as having more "character".  Higher Q also in general means lower efficiency and less power handing, so there is a tradeoff.

Spiders. Yes, all springs (in this case, spiders and surrounds) weaken over time. (like springs on an old car) You can also think of this as getting "broken in". I'm not sure about other designers, but I counted on this, and designed the spider and surround to be a bit tighter than I intended, so over time, it would settle to where I wanted it to be. Hours of use, temperature, humidity etc affect this.

Cones "dry out". Yes, especially untreated paper cones. They can and usually do become more brittle. The thinner and lighter the cone, the quicker this will lead to failure. Cigarette smoke and bright sunlight are NOT good for paper cones, they weaken the fibers, and break down the binders that hold the fibers together,  causing them to disintegrate.  I have applied a thin coating of spray lacquer to old brittle cones to help them hold together a while longer, and to protect them from the elements.

I believe Conrad was involved in Sunn in the early PA stages. I think he and Dick McLoud (who also went to Biamp) were primary designers. That's just a guess, I came in not long after they left.
Engineer Sunn 1977-1983, Fender 1984-1990
Model T Red Knob
200S Cabinet JBL D140Fs
Sunn SB 300 (Proto Coliseum 300)

Offline Isaac

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Re: Transducers- Guitar & Bass
« Reply #26 on: February 24, 2011, 09:07:33 am »
On a related note, though not applicable to Sunn Transducers, the most common failure due to aging is that foam surrounds will fall apart with age. This is called foam rot. Of course, Sunn Transducers didn't have foam surrounds, but I thought I'd throw this in, anyway.
Isaac

Offline Rex B

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Re: Transducers- Guitar & Bass
« Reply #27 on: March 15, 2011, 12:02:54 pm »
Actually, I DID use a foam surround on one product: The SB160 bass combo (I think that was the model number) , which was a bass amp version of the Alpha Guitar amp.. It was a tiny box, and I needed to get the free air resonance as low as I could to get any bass response at all. I remember treating it with A400 on some of the protos to prevent foam rot, but not sure if we implemented this in production. 
Engineer Sunn 1977-1983, Fender 1984-1990
Model T Red Knob
200S Cabinet JBL D140Fs
Sunn SB 300 (Proto Coliseum 300)