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[Tim McCulloch]

Jeff Wheeler wrote on Thu, 13 August 2009 22:06

Tim McCulloch wrote on Thu, 13 August 2009 20:22 One more thing... acoustic waves, once they leave their point of origin, never combine. They exist as discreet events that are combined at the listener's eardrum. Something to think about. I don't understand.  If I throw a rock into a wavy pond, the new waves made by the rock will be stretched and compressed across the waves on the pond.  The air works the same way, doesn't it; and is that not "combining?"  Maybe I am confused about the terminology here?

That's precisely it.  They move each other physically, but they never COMBINE to form a new, unique wave.

[Bob Leonard]

All the speaker theory aside I've been running a dual system for over 15 years. The additional cost will depend on your goal. If your goal is to remove vocals from the backline, as was mine, then you'll need speakers, amps, EQ, etc. to support the vocals in addition to what you may want to support the backline. I DO NOT run subs for vocal use because they just aren't needed if you use the right cabinets, which for me are currently SRX722's.

The benefits far out weigh the additional costs. Running separately I am able to tune the vocal half of the system for voice with no compromise made for the backline, and tune the backline half of the system with no compromise to the vocals. I have found that I can be more creative with EQ, compression, effects and levels. Vocals benefit from the ability to provide more clarity and higher output levels.

Now a quick word on higher GBF with the vocal side. This has all been discussed so I'll be quick. I'm able to acheive higher output levels with better GBF because there are far fewer open mics effecting the input/output relationship of the vocal half of the system. By running a dual system you have effectively removed all of the additional sources which can cause or contribute to feedback in that half of the system. Guitar amps, drums, etc. have no effect other than wash which is minimal and can be easily controlled.

There is no combing effect between the two systems and I can now use dissimilar speakers side by side, in my case 722's and 725's adjusting the entire backline and all vocal levels independently with a pair of faders. Three actually, BUS 1/2 for the backline, and the master for vocals. And in this case, size doesn't matter. Any system in any room will sound better.

[E. Lee Dickinson]

Jeff Wheeler wrote on Thu, 13 August 2009 23:06

E. Lee Dickinson wrote on Thu, 13 August 2009 21:40 The electrical signal is fine.. a low frequency tone and a high frequency tone. But because of the physical movement of the speaker, the high frequency tone now has a pitch shift added to it. I understand that part now.  I don't understand how this would not happen electrically.

The Doppler effect is a physical phenomenon that has to do with the movement of the sound source. As an object comes towards you, the waves are compressed leading to higher pitch. As the object moves away from you, the waves are elongated. The transducer is creating the wave at a fixed frequency, but its movement relative to you affects the wavelength.

It has nothing to do with wave interactions.

[Jack Arnott]

Art Welter wrote on Thu, 13 August 2009 15:51

Jeff, (Doppler distortion) caused by lower frequencies modulating the higher frequencies. Art Welter

Hello Art,
First off, this is a good post, but I am in disagreement with this one portion.

I understand that you know more about speakers than me, but this has always seemed counter intuitive to me.

Basic physics classes taught us that when you add two waves together, IE a 100hz wave, and a 1khz wave, that you just add, or subtract, depending on the position of the curve, the two curves to get the final curve, which is a single complex curve, but the ear hears it as two sine waves. It is no different whether it is electronic, or physical, or on paper, the two sine waves are still as original, or as separate.

What will happen is the lower end of the spectrum will run out of poop, or clip, or have power compression, that will affect the high end. But, in a fixed system where the speaker box is not moving, there is no such thing as doppler distortion.

The best response to this was made by Tom Danley. Someone in the early aughts doubted the single horn of the td-1 and brought up this very point. He discounted it firmly, soundly, and thoughtfully. Much better than I ever could. Said that it would not happen until 147db when the air in the horn started to physically distort. And since the signal does not physically couple until the throat of the horn, there is no possibility of it distorting the signal from the driver, at the mouth of the horn.

So the same applies to a full system. The signals will couple whether they are in a single point source, or next to each other. The only time you will get doppler is if it is moving, IE on top of a bus, ambulance, etc, or swinging in the wind, not stationary from the original source.

Take that wikipedia.

Regards, Jack

[Andy Peters]

Jeff Wheeler wrote on Thu, 13 August 2009 20:06

E. Lee Dickinson wrote on Thu, 13 August 2009 21:40 The electrical signal is fine.. a low frequency tone and a high frequency tone. But because of the physical movement of the speaker, the high frequency tone now has a pitch shift added to it. I understand that part now.  I don't understand how this would not happen electrically.

Mixing the signals electrically doesn't have the Doppler component. Consider a 50 Hz tone and a 1234 Hz tone. Trigger the tone generator so they start at the same time.  Neither signal affects the other. You can see this on a spectrum analyzer. Assuming levels are such that you don't distort, you'll see two distinct, stable tones.

Now just think about the travel of the average woofer -- assume it moves a couple of inches total. The 50 Hz tone will make the woofer move in a way that's obvious to the naked eye. Now put the 1234 Hz tone on top of it. The starting point of the 1234 Hz tone depends on the position of the cone, which is largely determined by the 50 Hz tone's phase (ranging from 0 to 360 degrees).  You can fit many full cycles of the higher-frequency tone in one cycle of the lower-frequency tone, and at any given time your 1234 Hz tone's cycle may start when the woofer cone is fully extended out, or fully moved backwards.

So this is a very subtle effect, and you might think, "well, don't crossovers mitigate this?" And the answer is mostly yes; certainly a 6 kHz tone shouldn't be coming out of the subs at any detectable level. But there are many systems with a 15" woofer and a 1" or so exit compression driver, and that 15 is expected to handle 50 Hz and up to 2 kHz. Basically you have other, much more significant, problems if you're doing that (15s that high are beamy, cheap 1"-throat horns don't go very low, etc).

-a

[Toby Mills]

At the end of the day, I believe you are really only going to gain a significant benefit at the very high end of the market.

If you are talking about a $1000 powered box and running a dual PA, then I'd sooner spend $2000 on one higher quality box rather than running 2 x $1000 boxes as a dual PA.

It goes up from there as well, if you thinking of running a dual PA on a $2000, I'd spend the money on one $4000 box and I reckon you would get more 'quality for money'.

A K1 system is probably worth twice as much as a dual V-DOSC and would sound better than a dual V-DOSC.

I personally don't believe there is any advantage in running a dual PA until you have run out of other options for improving the sound. Its kind of like the last cherry on the top of a very expensive cake when you have already optimized every other aspect of a system.

[Dick Rees]

Toby....

I certainly respect your opinion but personally find that simply adding a vox only speaker/side solves a lot of problems when attempting to get maximum clarity and efficiency in a system without taking the quantum leap to a much larger system.

I think of my speaker system as the canvas upon which I paint the sonic picture.  A dual PA approach simply gives me a bigger canvas without much additional expense and effort and allows more definition to the sonic "colors".

I am a small operator, but the laws of physics should not differentiate between my system and a larger one......

[Jeff Wheeler]

Toby Mills wrote on Fri, 14 August 2009 00:25

If you are talking about a $1000 powered box and running a dual PA, then I'd sooner spend $2000 on one higher quality box rather than running 2 x $1000 boxes as a dual PA.

This may or may not be true when you are always running one rig only, but if you have two sets of $1000 boxes for doing multiple gigs/day and don't happen to be using them all on a particular day, perhaps there is reason enough to bring them all out to one gig.

I unfortunately don't own multiple sets of $1000 boxes (my nicest mains are a pair of MRX525) but I do have two sets of extra 2x15s and more wedges than I usually need, so I am in a good spot to experiment.  If I can get more clarity out of two sets of cheaper boxes than going up to 722s, for example, I will gladly do that and have a more flexible inventory.

[Kristian Johnsen]

Jeff Wheeler wrote on Fri, 14 August 2009 05:06

E. Lee Dickinson wrote on Thu, 13 August 2009 21:40 The electrical signal is fine.. a low frequency tone and a high frequency tone. But because of the physical movement of the speaker, the high frequency tone now has a pitch shift added to it. I understand that part now.  I don't understand how this would not happen electrically. Tim McCulloch wrote on Thu, 13 August 2009 20:22

If one were to look at the output of the mixer summing the two sine wave singnals on an oscilloscope (a device that draws up a visual representation of the signals you are measuring) you'd just see two curves on top of each other.  When the two signals are both reproduced by the same speaker, the cone produces the high tone while it is moving back and forth at the frequency of the low tone.  In really simple terms, the high tone is produced while what's producing it is moving back and forth.  This would be like playing just the high tone and moving the entire speaker box back and forth on a paint shaker, like someone else mentioned.

[Mike Christy]

Tim McCulloch wrote on Thu, 13 August 2009 23:10

Jeff Wheeler wrote on Thu, 13 August 2009 22:06 Tim McCulloch wrote on Thu, 13 August 2009 20:22 One more thing... acoustic waves, once they leave their point of origin, never combine. They exist as discreet events that are combined at the listener's eardrum. Something to think about. I don't understand.  If I throw a rock into a wavy pond, the new waves made by the rock will be stretched and compressed across the waves on the pond.  The air works the same way, doesn't it; and is that not "combining?"  Maybe I am confused about the terminology here? That's precisely it.  They move each other physically, but they never COMBINE to form a new, unique wave.

Actually, the water never moves, it is stationary, it is the wave that moves.