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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Subwoofer Forum => Topic started by: Ivan Beaver on March 22, 2017, 05:18:21 pm

Title: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 22, 2017, 05:18:21 pm
Here is a good example of what happens when you don't understand what is happening and just do it because somebody said it was "cool"

Take a look at the center sub

https://www.facebook.com/PAotD/photos/a.173817121282.132706.173813726282/10154271001041283/?type=3&theater

Title: Re: Importance of basic understanding & fundamentals
Post by: Tim McCulloch on March 22, 2017, 08:34:31 pm
Here is a good example of what happens when you don't understand what is happening and just do it because somebody said it was "cool"

Take a look at the center sub

https://www.facebook.com/PAotD/photos/a.173817121282.132706.173813726282/10154271001041283/?type=3&theater

The umbrella & table should be a little more to the stage left...

And I'm betting the directional sub setup was Not a Thing of Happiness.
Title: Re: Importance of basic understanding & fundamentals
Post by: Dave Garoutte on March 23, 2017, 01:05:59 pm
But subs have a 360 radiation pattern, right?
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 23, 2017, 06:52:18 pm
But subs have a 360 radiation pattern, right?
No.

It depends on the particular sub and the freq.

As with other loudspeakers, they get narrower as they go higher.

Yes, most are "generally" omni, but not always.

Some are VERY directional-without the use of electronics
Title: Re: Importance of basic understanding & fundamentals
Post by: Jeremy Young on March 23, 2017, 09:11:47 pm
Just so I'm clear, the issue with their setup is the lack of distance behind the subs to allow for the cardioid array to perform correctly?  Or did I just fall victim to the same lack of knowledge?
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 23, 2017, 09:24:10 pm
Just so I'm clear, the issue with their setup is the lack of distance behind the subs to allow for the cardioid array to perform correctly?  Or did I just fall victim to the same lack of knowledge?
That is the biggest problem.

Also the distance from the "radiating areas" of the front and rear subs are not far enough apart to provide cancelling down very low.

You physical distance-in both the front to rear distance and also be away from any boundaries that would reflect parts of the signal.

When you have reflections, you introduce all new signals that are out of phase (YES that is the correct use of the term-NOT polarity in this case) which will cancel at various freq, depending on the distance of arrival from the original signal and the reflected signal (plus any delay that might have been applied to the "directional" speaker.

Directional subs work best outside-free of reflections.  Indoors they need to be a good distance away from reflections.
Title: Re: Importance of basic understanding & fundamentals
Post by: Jay Barracato on March 24, 2017, 07:12:50 am
Just so I'm clear, the issue with their setup is the lack of distance behind the subs to allow for the cardioid array to perform correctly?  Or did I just fall victim to the same lack of knowledge?
Another way of thinking of this is if the front of the sub is three feet in front of the boundary, the reflection can be considered as another source three feet behind the boundary at a slightly lower level.

So in this case, out front, you have the direct signal from the two front facing subs, the reflection from the surface, the signal off the back of the back facing sub, plus it's reflection, all at slightly different times, smearing the signal.

Add in the low height of the boundary which will allow the air above it to move freely, which will create more smearing.

Sent from my XT1254 using Tapatalk

Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on March 24, 2017, 09:32:39 am


When you have reflections, you introduce all new signals that are out of phase (YES that is the correct use of the term-NOT polarity in this case) which will cancel at various freq, depending on the distance of arrival from the original signal and the reflected signal (plus any delay that might have been applied to the "directional" speaker.

Directional subs work best outside-free of reflections.  Indoors they need to be a good distance away from reflections.
If a reflected waveform is phase shifted, is that phase lead, or phase lag?  8)

JR
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 24, 2017, 01:33:54 pm
If a reflected waveform is phase shifted, is that phase lead, or phase lag?  8)

JR
Relative to the original signal (from the loudspeaker) the reflected signal would be lagging-due to the later arrival, by whatever the difference of arrival times is.
Title: Re: Importance of basic understanding & fundamentals
Post by: Jelmer de Jong on March 24, 2017, 02:33:34 pm
Here is a good example of what happens when you don't understand what is happening and just do it because somebody said it was "cool"

Take a look at the center sub

https://www.facebook.com/PAotD/photos/a.173817121282.132706.173813726282/10154271001041283/?type=3&theater
Lets assume someone out of your control decided the subs should be against the wall. Would you place them alle facing forward or alle facing backwards?
Title: Re: Importance of basic understanding & fundamentals
Post by: Luke Geis on March 24, 2017, 08:06:19 pm
I was more interested in the rather wacky trussing used to support the mains! While I am sure it works, I wouldn't entirely trust it.....

The subs setup was definitely a lol moment.
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 25, 2017, 06:56:15 am
Lets assume someone out of your control decided the subs should be against the wall. Would you place them alle facing forward or alle facing backwards?
It depends on how high the subs are being used.

But "assuming" the input jacks would not be a problem (but they probably would be in most cases), the would be better (sonically) to face them towards the wall-but keep them maybe a foot away from it.

My big concern in most shows would be that either or both the cables/jacks would get stepped on or the subs would get shoved up against the wall.

By facing them into the wall, you have eliminated the reflection that would be coming off of the wall.

But since those subs are not very deep, this reflection would be higher in freq and may not cause an issue.

SO, once again, the freq of usage determines if it is a problem or not.

Without knowing the highest freq,, it is only guess work

Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on March 25, 2017, 04:27:43 pm
Relative to the original signal (from the loudspeaker) the reflected signal would be lagging-due to the later arrival, by whatever the difference of arrival times is.
The reflected signal is inverted polarity "and" time delayed.

Phase shift is a little messy for anything other than fixed sine waves.

Perhaps why I am not a speaker guy...  8) 

JR

PS: The polarity police are not happy.

Title: Re: Importance of basic understanding & fundamentals
Post by: Chris Grimshaw on March 26, 2017, 06:03:21 am
It depends on how high the subs are being used.

But "assuming" the input jacks would not be a problem (but they probably would be in most cases), the would be better (sonically) to face them towards the wall-but keep them maybe a foot away from it.


Huh, interesting.
I'm not sure I understand why it'd be sonically better to fire them at a 1' wide slot.

Sure, you might (might) get a little bit of bandpass effect that'll cut down a little on harmonics produced when you push the system, but chances are there'll also be an out-of-band peak. If it's far from the crossover, probaby not too much of an issue. If it's close to the crossover, the associated phase shift as you go over the peak might make integration with the tops more interesting.
The peak would also impose itself on the distortion profile, since it's an acoustic effect.

In short, I can see it sounding different. I'm not so sure it'll sound any better.

Chris
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 26, 2017, 07:46:45 am
Huh, interesting.
I'm not sure I understand why it'd be sonically better to fire them at a 1' wide slot.

Sure, you might (might) get a little bit of bandpass effect that'll cut down a little on harmonics produced when you push the system, but chances are there'll also be an out-of-band peak. If it's far from the crossover, probaby not too much of an issue. If it's close to the crossover, the associated phase shift as you go over the peak might make integration with the tops more interesting.
The peak would also impose itself on the distortion profile, since it's an acoustic effect.

In short, I can see it sounding different. I'm not so sure it'll sound any better.

Chris
Most subs are essentially "omni" at their lower freq.  Some more than others

When they are placed a couple of feet from a boundary, there will be reflected energy off of the boundary that will then "mix in" with the front radiation and cause combfiltering cancellations-due to the different time arrivals.

With the main energy going to the boundary, this "source" of energy will be at a single time-so the cancellations would be limited to the much lower energy radiating off of the rear of the cabinet.
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on March 26, 2017, 07:49:49 am
The reflected signal is inverted polarity "and" time delayed.

Phase shift is a little messy for anything other than fixed sine waves.

Perhaps why I am not a speaker guy...  8) 

JR

PS: The polarity police are not happy.
OK I guess I will have to take a warning ticket on that one.

The phase (due to the time of the signal path) would be "added to" the polarity shift of 180* of the reflection.

Yes, both are happening-so it is not just the simple distance that is affecting the overall phase of the reflected signal with respect to the original signal
Title: Re: Importance of basic understanding & fundamentals
Post by: Tim Gurske on April 19, 2017, 11:38:40 pm
Hopefully in my lifetime we will get a good way to visualize sound waves, something better than Schlieren flow visualization or computer models. Like a FLIR camera for sound waves. Imagine being able to actually see what the sound is doing inside the venue. I know it would help me understand what is happening.
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on April 20, 2017, 07:08:17 am
Hopefully in my lifetime we will get a good way to visualize sound waves, something better than Schlieren flow visualization or computer models. Like a FLIR camera for sound waves. Imagine being able to actually see what the sound is doing inside the venue. I know it would help me understand what is happening.
Seeing it is one thing.

Knowing what you can and cannot do about it is quite another.

The more basic understanding you have, the better the chances of being able to resolve the issue.

But with many room, there is little you can do regarding the lower freq.

Due to the physical size of the room or where you can actually put speakers.
Title: Re: Importance of basic understanding & fundamentals
Post by: Tim McCulloch on April 20, 2017, 10:22:43 am
Hopefully in my lifetime we will get a good way to visualize sound waves, something better than Schlieren flow visualization or computer models. Like a FLIR camera for sound waves. Imagine being able to actually see what the sound is doing inside the venue. I know it would help me understand what is happening.

The problem in teaching or explaining sound is that it's invisible, but no amount of visualization is likely to create an "ah ha!" moment that can't be done already.
Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on April 20, 2017, 10:38:59 am
The problem in teaching or explaining sound is that it's invisible, but no amount of visualization is likely to create an "ah ha!" moment that can't be done already.
A classic visualization medium is the old pebble dropped into a mud puddle. While only in 2 dimensions.

JR 
Title: Re: Importance of basic understanding & fundamentals
Post by: Ed Hall on April 20, 2017, 11:04:59 am
The problem in teaching or explaining sound is that it's invisible, but no amount of visualization is likely to create an "ah ha!" moment that can't be done already.

The Ah-Ha! Moment for me happened in high school physics class. We were shown a movie (before video was everywhere) about sound waves. I understood the cyclic nature but always thought of a sign wave. The movie showed parallel lines moving closer together and further apart. Compression and rarefaction. Then it showed similar but using concentric circles. That was the moment! Since then I've seen sound in my head like the concentric circles, or partial circles-arcs.

Judging from the production quality, spliced film and cheap school I would assume the film was from the 60s. But it worked because the fundamentals don't change.
Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on April 20, 2017, 12:05:58 pm
another (very old) way to visibly demonstrate standing waves is the old sand on a flat metal plate that is vibrated by a speaker (or violin bow). The sand will aggregate over nodes and be pushed away from anti-nodes. Again only 2 dimensions but a square or rectangular metal plate is closer to a room shape than a mud puddle.

JR 
Title: Re: Importance of basic understanding & fundamentals
Post by: Chris Hindle on April 20, 2017, 12:50:24 pm
another (very old) way to visibly demonstrate standing waves is the old sand on a flat metal plate that is vibrated by a speaker (or violin bow). The sand will aggregate over nodes and be pushed away from anti-nodes. Again only 2 dimensions but a square or rectangular metal plate is closer to a room shape than a mud puddle.

JR
I dunno JR, I've been in some pretty ugly rooms.....
Title: Re: Importance of basic understanding & fundamentals
Post by: Tom Danley on May 13, 2017, 09:55:51 am
The Ah-Ha! Moment for me happened in high school physics class. We were shown a movie (before video was everywhere) about sound waves. I understood the cyclic nature but always thought of a sign wave. The movie showed parallel lines moving closer together and further apart. Compression and rarefaction. Then it showed similar but using concentric circles. That was the moment! Since then I've seen sound in my head like the concentric circles, or partial circles-arcs.

Judging from the production quality, spliced film and cheap school I would assume the film was from the 60s. But it worked because the fundamentals don't change.


 Hi Ed
A  clear mental image of how sound behaves in a given circumstance can be very useful.  I love this stuff, maybe I can add some more to it.
The picture of bunched up and stretched apart lines representing local areas of increased and decreased pressure is a good one but there is another part which can be added to make a more complete image.
Take radio waves as an analogue, one can pick up radio signals two ways, one you use a ¼ wave length or other antenna which picks up the Voltage component of the radio wave OR one can make a loop coil antenna which uses the magnetic portion of the wave to induce a voltage in the loop.  The voltage and magnetic potentials are both since waves BUT they are shifted 90 degrees with respect to each other, in quadrature and where the magnetic potential is greatest, the Voltage potential is Zero and vis versa..
With sound there is a similarity although entirely different mechanisms.  With air, we have about 80 miles of it piled up above us and down here, the “weight” of all that is about 15 pounds per square inch. Thankfully we know nothing else and are sensitive to VERY small changes in pressure within a certain rates of change.  If you represented the pressure of a sound wave as a sine wave, going both above and below ambient air pressure, one has the density part of the sound wave, the pressure and THAT is what we hear, changes in pressure.
Part B is that shifted 90 degrees to that pressure sine curve is the Velocity one see if they followed a given air molecule.  In the animations, one can see the motion is zero when the lines or particles are both closest and farthest apart.   So sound is really two things in quadrature, potential and kinetic energy.
The velocity part of sound “we” don’t think that much about unless you design horns scientifically although there are examples of “velocity devices” such as a velocity microphone.  Where a “normal” condenser or dynamic mic is a pressure transducer, a velocity mic has a VERY light casually suspended conductor in an magnetic field. It is so light, it is swept along by the air molecules and the voltage it generates tied more to velocity and not pressure.
I could ramble on about sound but I won’t, instead for those still reading, I will pose a thought experiment or question.
Starting at DC and going up in frequency we call that part of the electromagnetic radio “radio waves” and all of these are the same in that they are that voltage and magnetic potential in quadrature arrangement.  As you go higher and higher up in frequency and shorter and shorter in wavelength, high MHz / low GHZ, radio waves travel through copper tubes and plumbing, at high GHz radio components look a lot like optical components but “somewhere” we cross into light.   
With light, we talk photons and the odd fact that experiments show light behaves like both a particle and a wave.   Is the fact that light demonstrates itself to be a wave evidence that it is also a voltage and magnetic potential in quadrature but that “we” are unable to resolve the difference at that scale and use the term photon instead to describe a short (perhaps minimum quantity that can be radiated) envelope of em energy.
Ok, so while am cleaning out the accumulated “spinach” and fire ant mulch under the deck and sharpening the blades on the lawn tractor, give that some thought.  Grass grows fast in the spring in Georgia!
Best,
Tom
Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on May 13, 2017, 11:19:25 am
Grass and fire ants grow fast in MS too....  lots of symmetry to be found if we look under the right rocks...

JR
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on May 13, 2017, 11:30:40 am
Grass and fire ants grow fast in MS too....  lots of symmetry to be found if we look under the right rocks...

JR
But up at Toms place-sometimes the right rock can have gold (literally) under it. :)

There is a theory that the ancient Myan city of gold was actually up near Dahlongia Ga.  They have found Myan ruins in the area, and the first gold rush in the US was there.
Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on May 13, 2017, 02:04:29 pm
But up at Toms place-sometimes the right rock can have gold (literally) under it. :)

There is a theory that the ancient Myan city of gold was actually up near Dahlongia Ga.  They have found Myan ruins in the area, and the first gold rush in the US was there.

Are you sure his name isn't Tom Sawyer and that is a ruse to get gold crazed prospectors to pick up his rocks?

JR
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on May 13, 2017, 03:03:17 pm
Are you sure his name isn't Tom Sawyer and that is a ruse to get gold crazed prospectors to pick up his rocks?

JR
Actually he is playing around with an idea to use sound to extract the gold dust in decent quantities.

There is lots of dust around, it could all add up ;)
Title: Re: Importance of basic understanding & fundamentals
Post by: John Roberts {JR} on May 13, 2017, 03:25:48 pm
Actually he is playing around with an idea to use sound to extract the gold dust in decent quantities.

There is lots of dust around, it could all add up ;)
Big dog miners used water to separate it out, as the heavier gold would precipitate out... Sound might work but seems like a mess... and loud.

JR
Title: Re: Importance of basic understanding & fundamentals
Post by: Ivan Beaver on May 13, 2017, 03:30:30 pm
Big dog miners used water to separate it out, as the heavier gold would precipitate out... Sound might work but seems like a mess... and loud.

JR
And fun