Non-euclidian maybe...?

Got that from a website:
"THREE TO ONE RULE
In order to get maximum gain (level) out of a PA system, microphones which are picking up the same sound source (e.g. a chorus on a large stage) should be three times further from each other than from the sound source. This minimises COMB FILTERING."

One may become crazy trying...

I think they mean at least three times further which is not that hard given the mics are usually within a couple of inches of the source.

Cheers

Ian

sweetwater sez said:

When using two microphones to record a source, normally you will get the best results by placing the second mic three times the distance from the first mic that the first mic is from the source. This is known as the “3:1 Rule of Microphone Placement.”

An example: If the first mic is 1 foot from a source, the second mic should be placed 3 feet from the first mic.

While many engineers believe that adhering to the 3:1 Rule will minimize phase cancellation, technically this is not true. The 3:1 Rule works because the level of the signal entering the second mic (the one farther away) is reduced in level compared to the signal entering the first mic; tripling the distance substantially reduces the relative level of the signal in the two mics. This reduces the effects of phase cancellation, since the most cancellation will occur when the two mic signal levels are equal. In fact, if you crank up the gain on the second, more distant mic, you will still hear phase cancellation when it is added to the first mic.

Keep in mind that rules are meant to be broken; you may prefer the sound created by ignoring the 3:1 Rule — experiment and let your ears be your guide!

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YMMV

general advice for beginners.

JR

JohnRoberts said:

When using two microphones to record a source, normally you will get the best results by placing the second mic three times the distance from the first mic that the first mic is from the source. This is known as the “3:1 Rule of Microphone Placement.”

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Indeed, that makes sense compared to the quote "microphones which are picking up the same sound source... should be three times further from each other than from the sound source.", which is a geometric impossibility.
How a sensible recommendation becomes a headbanger, just for neglecting a few words.

Unfortunately most websites and marketing pitches are not written by engineers but by pseudo salesman whose grasp of physics is tenuous at best.

I recall one AES show where I had to follow a new wet behind the ears sales manager around who learned all the buzz words, but had no clue of what they meant. I kept the damage in check when I could, but I couldn't follow him around the entire show.  : (he was a great salesman but had "peter principled" above his level of competence.)

JR

JohnRoberts said:

Unfortunately most websites and marketing pitches are not written by engineers but by pseudo salesman whose grasp of physics is tenuous at best.

I recall one AES show where I had to follow a new wet behind the ears sales manager around who learned all the buzz words, but had no clue of what they meant. I kept the damage in check when I could, but I couldn't follow him around the entire show.  : (he was a great salesman but had "peter principled" above his level of competence.)

JR

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Even more so today, with the internet. I have exactly the same issues with the sales guys. When they're on the road, they get so much BS from the competitors reps that they start using the same unfounded arguments.

abbey road d enfer said:

Even more so today, with the internet. I have exactly the same issues with the sales guys. When they're on the road, they get so much BS from the competitors reps that they start using the same unfounded arguments.

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I spent most of my time with reps debunking BS sales pitches proffered by competitors. Peavey was an easy target because they did relatively little advertising compared to most competitors.

My classic example was a mixer company (start-up at the time) who spent $1M a month advertising 1 SKU.. Peavey didn't spend that much advertising a thousand SKUs. The dealers were overwhelmed by a parade of sheeple presold on that one mixer. FF a year or two later when they had expanded to 3 mixers or so... A story shared by one rep was walking into a dealer of his that sold both Peavey and this mixer start -up... the dealer was demoing the mixer with Peavey amp and Peavey speakers. The dealer had the gain structure AFU clipping the sh__ out of the mixer (the mixer did not detect for clipping at enough points in each channel, so it was possible to overload the front end without indication of clipping.) The dealer was telling customers that the XYZ mixer was too powerful and blowing away the Peavey amp that couldn't handle it.  : :.  My rep walked over to the demo and cleaned up the faulty mixer gain structure, but the dealer and customers were still drunk on the marketing kool aid. 

JR

Not exactly the same, but anyway related. I'm facing a barrage of SE's that say my systems are not powerful enough compared to other systems. They want to ride the meters on their digital desks at -18 peak and get the full blast from the speakers. Some say it's because we run the amps at 26dB gain when most of the competitors run them at 32 or more. I told them to add 6dB at the front (the processor allows 20), and suddenly they're happy.
I understand that running the digital chain at -18dBfs makes sense if there is analog processing further down, but it seems there is a religion on this issue; I wonder if they're worried about wearing out the meters, or the MSB's 
Reminds me of my beginnings when the typical sound system was a Martin rig and H+H amplifiers. The sensitivity of the H+H amps was about 300mV; SE's were amazed how loud the system sounded with the master fader (and meters) at -10. I had to show them they could not really go much beyond...

abbey road d enfer said:

Not exactly the same, but anyway related. I'm facing a barrage of SE's that say my systems are not powerful enough compared to other systems. They want to ride the meters on their digital desks at -18 peak and get the full blast from the speakers. Some say it's because we run the amps at 26dB gain when most of the competitors run them at 32 or more. I told them to add 6dB at the front (the processor allows 20), and suddenly they're happy.
I understand that running the digital chain at -18dBfs makes sense if there is analog processing further down, but it seems there is a religion on this issue; I wonder if they're worried about wearing out the meters, or the MSB's 
Reminds me of my beginnings when the typical sound system was a Martin rig and H+H amplifiers. The sensitivity of the H+H amps was about 300mV; SE's were amazed how loud the system sounded with the master fader (and meters) at -10. I had to show them they could not really go much beyond...

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In mass market products there is a surprising amount of engineering that goes into user ergonomics that affect perception.

I recall one next generation upgrade of a high volume powered mixer. The channels used simple one knob gain/mix level controls. My senior mixer engineer improved the circuit design to deliver better fader kill when full down, but the new circuit delivered about 2 dB less voltage gain at 12 o'clock...  After about 6 months of the new generation version being in the stores, I got tired of hearing so many reports that the new version wasn't as powerful as the former version.  I knew this wasn't possible because the new version used the exact same power amp module. No matter how many times I told them to just turn up the gain the complaints didn't stop.

Since the customer (dealer) is always right I had my senior engineer tool up a new custom pot with the exact same gain at 12 o'clock as the former version. Magically the low power complaints stopped over night (while it took a several months to get the new pot transition accomplished.)

JR

"...to get maximum gain (level) out of a PA .... This minimises COMB FILTERING."

Uh, what is the problem this addresses? Gain? Or Comb?

Maximum gain on a large choir leads to one mike per mouth. Taking mouth centers at 24", this leads to <8" mouth-mike, a reasonable value for modern pop-filtered vocal mikes. But it is an impossible mixing problem. (Ideally you set all gains the same and all singers hold a specified distance-- won't happen.)

If the number of mikes is limited, the 3:1 rule will leave parts of the choir under-miked. About a third of the mouths will be significantly down in level. It may give good gain but poor balance. Important voices need to be shuffled closer to a mike.

In non-Rock chorus, I always did OK by staging the choir in an arc and using an XY pair at the focus. This also avoids end-mikes being close to typical speaker locations.

JohnRoberts said:

In mass market products there is a surprising amount of engineering that goes into user ergonomics that affect perception.

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I'd say that is at least as important as having the device perform the function it was intended for. Maybe more important than ultimate performance. If users don't get what they want or expect with ease, the raw electronic design doesn't mean much. Not everyone agrees. I hate products with a zillion knob that can do 'everything'.  I know I won't use anything if I don't like the ergonomics no matter how good it sounds.

PRR said:

"...to get maximum gain (level) out of a PA .... This minimises COMB FILTERING."

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IMO it optimizes comb filtering only; by making the level of the distant mike 10dB lower, destructive interference is only about 1dB (assuming equal gain/sensitivity), but you are right, it does not necessarily result in good balance.
In fact, this rule can be applied to a small group, but the example is just the most unappropriate.

The whole concept is more for recording than live sound.  I can’t imagine mic’ing anything with a close mic  and a distant mic for ‘sound’ in a live situation. Seems like asking for trouble in a lot of ways.

Gold said:

I'd say that is at least as important as having the device perform the function it was intended for. Maybe more important than ultimate performance. If users don't get what they want or expect with ease, the raw electronic design doesn't mean much. Not everyone agrees. I hate products with a zillion knob that can do 'everything'.  I know I won't use anything if I don't like the ergonomics no matter how good it sounds.

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My gripe is that some designers deliberately fool customers by putting excessive gain in order to wow them. It is not good because there are adverse effects, typically lack of headroom. It's a stupid game, because the next stupider one will add 6 dB gain and become the new fad, till another stupidest adds another 6.
You would be surprized how many people don't understand the joke "these go to eleven" and take it seriously.

PRR said:

... Ideally you set all gains the same and all singers hold a specified distance-- won't happen.)

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What if we used some old technology but expanded it a bit?  Start with something like this:

Gold said:

The whole concept is more for recording than live sound.  I can’t imagine mic’ing anything with a close mic  and a distant mic for ‘sound’ in a live situation. Seems like asking for trouble in a lot of ways.

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Not to feed the hypothetical but not that usual to put two mics on a guitar amp.

JR

abbey road d enfer said:

My gripe is that some designers deliberately fool customers by putting excessive gain in order to wow them. It is not good because there are adverse effects, typically lack of headroom. It's a stupid game, because the next stupider one will add 6 dB gain and become the new fad, till another stupidest adds another 6.
You would be surprized how many people don't understand the joke "these go to eleven" and take it seriously.

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Well that’s a design decision. I agree it’s not a good one. Lots of people like sizzle. I’d rather have customers that like steak. I think in the long run designing for the customers you want is the best bet. You can’t be both McDonalds and a Michelin restaurant.

JohnRoberts said:

Not to feed the hypothetical but not that usual to put two mics on a guitar amp.

JR

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I don’t think I’ve ever seen close and distant micing on anything in a live situation. In recording it’s common.

Maybe a microphone array and spot mics for a large ensemble? If the ensemble is big enough to accommodate spot mics then the rule would take care of itself.

abbey road d enfer said:

My gripe is that some designers deliberately fool customers by putting excessive gain in order to wow them. It is not good because there are adverse effects, typically lack of headroom. It's a stupid game, because the next stupider one will add 6 dB gain and become the new fad, till another stupidest adds another 6.
You would be surprized how many people don't understand the joke "these go to eleven" and take it seriously.

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I did suggest that we label the gain trim on a vacuum tube mic preamp going up to 11... as an homage to "spinal tap"...
===

I have seen low information consumers prefer linear taper over audio taper pots because of significantly more output at 12 oclock. (The customer is always right even when a flaming idiot.) :

JR

@Gold... indeed you don't distant mic backline in live situations with significant stage wash (intentionally). BUT when people have too many inputs available they sometimes use them, and inadvertent distant miking of other gear on stage happens.  Some people put guitar amps inside soundproof boxes back stage to manage stage wash. A million ways to use mics poorly.

Gold said:

The whole concept is more for recording than live sound.  I can’t imagine mic’ing anything with a close mic  and a distant mic for ‘sound’ in a live situation.

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AB/Spaced Pair for overheads?