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Number of speakers affect on tone?


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11/25/1997 2:34 PM
SProuty
Number of speakers affect on tone?
In my on going quest to get a little more clean head-room out of my Vibro-King I discovered, much to my amazement, that only two of the three speakers were functioning ( i.e. no sound at all and an open-circuit resistance value when measured with my DMM). I am fairly certain that the amp has been this way since I bought it (about six months ago). The speaker is being replaced under warranty but it's going to to take a few weeks to get to me. So my question is: will the additional speaker help, hurt or have no effect on my ability to get more clean volume? Any comments would be appreciated (except for those wondering how I could be such an idiot and not notice this until now).  
 
Thanks,  
 
Stephen Prouty
 
11/25/1997 4:55 PM
JC

I'm not as experienced or knowledgeable as some of the contributors to this page, but here goes what I know.  
 
The 3 10" speakers are probably wired in parallel. If they're 8 ohms each, that puts the nominal impedance somewhere just over 2 ohms. (3 16-ohm speakers = 4 ohm load) Whatever the nominal impedance, your amp is idealized for that load, and will put out all of its 60 watts at that load.  
 
Let's say they're 8 ohms. Disconnecting one of these speakers would INCREASE the total load from 2 to to 4 ohms, thereby reducing the total output of your amp by about 30% or so. Ergo, it's cutting into your clean headroom because everything's working harder. It might also scoop your mids a little bit. I would imagine it would shorten the life of your output tubes as well.  
 
(Disclaimer: these figures are pathetically approximate; I welcome any more accurate corrections by someone in the know!!! Also, if they're not wired in parallel, then none of this is true.)  
 
Plus, the extra 300 or so square inches of speaker surface area are going to handle your bass frequencies much better than two speakers alone. It's a better deal all the way around for you.  
 
JC  
8-)>
 
11/25/1997 5:58 PM
David Axt

I read somewhere that two 12" speakers can reproduce lower frequencies better than on 15" speaker. The theory is that the two 12" have greater surface area than one 15" speaker.  
 
Personally I find this concept hard to believe. I would believe that impedance being equal that 2 speakers are louder than 1 because they push more air.....but only at certain frequencies.  
 
What do others think?
 
11/28/1997 9:21 AM
JC

I'm not sure I understand what you're asking.  
 
Two twelves are louder that one fifteen because of their increased surface area, so they can push more air and create more bass. From what I understand, bass frequencies consume much more power than highs.  
 
Could you clarify what you were asking?
 
11/28/1997 11:12 AM
Charles

Hi Jason,  
 
>> Two twelves are louder that one fifteen because of their increased surface area, so they can push more air and create more bass. From what I understand, bass frequencies consume much more power than highs. <<  
 
This might be a place where the practical, and the theoretical don't jive. In my humble experience the single 15 usually has a better bottom than the 2x12 combo. I did some fast calculation and I see that 2x12 is about 225 sq in. vs 175 sq in. for the single 15. Thats a difference, but I'm not sure if it's a big enough one to matter. At low freqs, it seems like there must be some phase cancellation in the pair of speakers no matter how well matched everything is. I don't mean because of the cabinetry, but because the two speakers are just going to react a little differently. Maybe it's because the 15 lacks a certain top end. Maybe its all of the above and more (or none of the above). But, my ears say that the single 15 usually has a tighter and smoother low end response.  
 
All clarification - Welcome!!
 
11/28/1997 12:09 PM
JC

Yep, I completely goofed on my calculations, that was pretty dumb.  
 
I've often wondered about the phase cancellation effect myself. No doubt, a single-speaker combo sounds better to my ears when you can get away with it. Sometimes one speaker won't cut it though. I was under the impression that higher frequencies are subject to more audible phase cancellation (to the human ear anyway) than lower frequencies. Maybe that's not true either.  
 
I had a different "bass" experience: back when I used to play a Twin, I used 2 12" JBL's, but I wanted tighter bass. I cut a new speakerboard for the amp and tried a 4-ohm 15" JBL D-140, and then a 15" Black Widow, and then a 15" Altec. After wasting a lot of money, I decided that the twelves together actually had better bass and much better highs to my ears than the best of these 15" speakers. (Then again, perhaps this is because the D was engineered to be used in a Showman sealed cabinet, not an open enclosure.)  
 
One final thing I've noticed that I wish someone would explain to me: the smaller the speaker's diameter, the less "directional" it seems to be. Using a 15" speaker in my Twin, I had to put the amp in a chair and tilt it to hear it clearly. If I stood it upright on the floor and set a tone that sounded good and full from my position on stage, the audience was getting blasted with screaming obnoxious highs. If I set my tone standing out in the audience, it sounded like there was a blanket over the amp when I got back on stage.  
 
Even with the amp tilted, the more I moved from side to side, the less highs I could hear. Two 12's in the Twin weren't nearly as bad about this, and the single 12" I currently use with my Bassman head isn't as bad either. But, by stark contrast, I was amazed when I borrowed a friend's 2-10" Music Man (with EVM speakers), and found that I could set the amp upright, on the floor, and I could move around a lot without losing highs.  
 
Why is this?
 
11/29/1997 9:46 AM
Charles

>> I was under the impression that higher frequencies are subject to more audible phase cancellation (to the human ear anyway) than lower frequencies. Maybe that's not true either. <<  
 
I dunno, put two speakers out of phase and the bottom is lost. They sound weak, thin, trebly. I think what we're taking about is fundamental cancellation, with harmonics getting thu.  
 
I've not noticed the directional thing. I'll have to pay some attention and see if I can tell the difference too.
 

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