Flown Subs, Ground Bounce, and People in 1/2 space

P

Phil Lewandowski

Sophomore
Jan 11, 2011
123
0
16
Cleveland, OH
Hey all,

Just wanted to start this off-shoot thread from the TEF system thread.


What I am pretty much interested is in what exactly can be done to figure out what specific effects (semi-hard data) the floor bounce of a flown subwoofer will have in an ideal environment.

I am thinking; say place a sub 25-50ft in the air outdoors, a person X distance away with ears 6ft off the ground, with nothing but flat hard surface between the person and under the sub. (No large objects nearby)

Trying to figure out if there is some sort of formula or way to figure out the effects of the floor bounce; to figure out what frequencies it would be effecting since the ear are not ground plane and more specifically what it will do to the overall output; since on other forums there has been debate that flying subs causes you to lose 'tons' of output. (They would say the theoretical ~6dB.)


Another thing Silas brought up was the idea of those lower frequencies not only being heard but being felt. I wonder how that would all fit in? For example, would the 'punch in the chest' realm of frequencies have to cancel out in the realm/level of ones body to not be felt. i.e. would cancel out lower than ear level to reduce the amount of 'kick?'
Thanks,
Phil
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Phil Lewandowski said:
I am thinking; say place a sub 25-50ft in the air outdoors, a person X distance away with ears 6ft off the ground, with nothing but flat hard surface between the person and under the sub. (No large objects nearby)

Trying to figure out if there is some sort of formula or way to figure out the effects of the floor bounce; to figure out what frequencies it would be effecting since the ear are not ground plane and more specifically what it will do to the overall output; since on other forums there has been debate that flying subs causes you to lose 'tons' of output. (They would say the theoretical ~6dB.)
Phil
Click to expand...

There should only be a single reflection off a single surface from a single source. Since the angle of reflection equals the angle of incidence it should be easy to figure out from ear height, speaker height. and distance from the stage, what the path length difference is in the only secondary path. There will some sort of comb filter caused by that reflection, but it may not be a big deal at sub frequencies.

For me, the improvement in coverage of flown subs outweighs any loss of 1/2 space gain present with ground stacks.

Mac
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

mackerr said:
There should only be a single reflection off a single surface from a single source. Since the angle of reflection equals the angle of incidence it should be easy to figure out from ear height, speaker height. and distance from the stage, what the path length difference is in the only secondary path. There will some sort of comb filter caused by that reflection, but it may not be a big deal at sub frequencies.

For me, the improvement in coverage of flown subs outweighs any loss of 1/2 space gain present with ground stacks.

Mac
Click to expand...


Thanks Mac, how do you go about figuring out this angle of incidence?


This is exactly what went through my mind; that there may only be one angle that would be imporant based on heights of speaker and ear/mic.


Thanks,
Phil
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Just to add to the discussion, there are a couple of other issues that make ground subs "louder" than flown subs.

First is the inverse square law. When the subs are on the floor they are closer to the listener (at least up close) so they are louder. Of course this is exactly why I prefer to fly the subs in a install situation. It provides a more even front to back coverage-because the people up front are not "plastered" by the subs. In the back of the room the distance between the floor and installed subs is less, so there is not as much difference there-but the floor will still be closer-so therefore louder-how much depends on the particular distances involved.

Of course in some situations this is preferable (plastering the people up front)and I do subs on the ground. It really depends on the particualr situation and the customers needs.

The other thing is the "tactile" sensation of the subs sitting on the floor. When you couple the vibration to the sound it is often more preferred. When they are flown, the direct coupling is essentially lost and the only vibrations are from the SPL itself. So you have to push the subs harder to get the same "feeling" as if they were on the floor. And people often mistake this "feeling" with pure SPL. We like to "feel" the bass.

That is why butt shakers work for home theatre-although I really don't care for the discontinunity of butt shakers alone.
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Phil Lewandowski said:
Thanks Mac, how do you go about figuring out this angle of incidence?
Click to expand...

I used to be able to do it with math, but at this point someone whose last geometry and trig class was less than 40 years ago will have to show you how. I'd do it by trial and error with a CAD drawing.

Mac
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Phil Lewandowski said:
Thanks Mac, how do you go about figuring out this angle of incidence?


This is exactly what went through my mind; that there may only be one angle that would be imporant based on heights of speaker and ear/mic.


Thanks,
Phil
Click to expand...

Imagine two right triangles: one formed by the normal from the ground to the sub array, the ground and the particular vector we are interested in. The second by you, the ground and the reflected vector

[sub array]
 |     \
 |       \
 |         \
 |           \
 |             \
 |               \
 |                 \
 |                   \
 |                     \       ☺
 |                       \   /‒|‒
|____________  \/ _ᴧ_______[ASCii by chuck]


The ratio of the distance between your ear height divided by the distance to the point of incidence is equal to the ratio of the height of the sub array divided by the distance from the intersection of the normal to the ground plane to the point of incidence. Then once you have those distances figured out, you can use the Pythagorean theorem to determine the distance traveled by the reflected wave(find the hypotenuse of the two triangles and add them). You'd use only need pythagoras to determine the distance the direct wave traveled. This is of course assuming there is no refraction at the air ground boundary. You could get tangent involved and write an equation that relates the difference in distances. I'm estimating that if you did write that equation and relate that to the amount of summation and cancellation, for one particular frequency, the level over the distance would vary sinusoidally. (and some people say physics, diffeq and vector calculus is useless....)
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Phil Lewandowski said:
Trying to figure out if there is some sort of formula or way to figure out the effects of the floor bounce; to figure out what frequencies it would be effecting since the ear are not ground plane and more specifically what it will do to the overall output; since on other forums there has been debate that flying subs causes you to lose 'tons' of output. (They would say the theoretical ~6dB.)
Click to expand...
Read the Dave Gunness paper noted and some of Pat Brown's articles on the subject and they seem pretty clear that ground stacking does not typically provide the full theoretical effect of true half space loading and that the increase in level that may be obtained is due greatly to an increase in the indirect sound or as Dave says "...the fabled “coupling to the floor” is really “driving the reverberant field harder”."
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Thank you Charles for that.


I did actually read that 1/2 space paper from David Gunnes about 8-10 months ago and was a very helpful read, as was Pat Brown's paper that came out around a similiar time. I think it is time for a quick re-read to see what I missed the first 2 readthroughs.


Thanks,
Phil
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

The thought that really stuck out to me from the Gunness paper was the idea that the maximum coupling from the floor would actually be in the upward direction, not the forward direction.
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Jay Barracato said:
The thought that really stuck out to me from the Gunness paper was the idea that the maximum coupling from the floor would actually be in the upward direction, not the forward direction.
Click to expand...

IIRC, when the sub is directly on the ground, there is a very even ~6dB increase in the entire 180 degree half-sphere around it, since at the wavelengths we are dealing with I would think that even if the center of the driver were 8-12 inches off the ground it wouldn't have much of an effect. And when we started to move sub off the ground that is when it started to become uneven.

Also as you take the sub off the ground, directly above would seem to be where you would see the most destructive interference, since that would be the place where the time difference between the direct and reflected wave from directly below the sub would be greatest.


Take Care,
Phil
 
Last edited:
Re: Flown Subs, Ground Bounce, and People in 1/2 space

I wonder if most people want subs on the ground because their other option is not to fly them but to have them on a stage or riser that might be a couple feet off of the ground where the floor boundry could cause pretty serious cancelations.
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Brandon G Romanowski said:
I wonder if most people want subs on the ground because their other option is not to fly them but to have them on a stage or riser that might be a couple feet off of the ground where the floor boundry could cause pretty serious cancelations.
Click to expand...

As Phil L. just pointed out, raising the subs off the ground would cause the cancellation to be above the subs, not in front or behind. This cancellation does not affect any normal seating areas. I have done outdoor tests, raising a sub 1, 2, and 4 feet above the ground. At 10 meters in front of the sub position, the level differences were virtually nothing.

Large arrays of subs increase directivity, the proximity to the ground plane becomes less of a percentage of overall forward gain as the frontal area increases in size.

There is a rather large difference in the hemispherical radiation pattern of a single small LF source on the ground, the omnispherical radiation it has when flown a wavelength (or so) away from any surrounding surface, and the radiation pattern of a 10 meter long flown array, or a 3 x 3 meter ground stack.

Floor bounce from a large sub array is not a single vector angle, and is frequency dependent.
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Helge Dr. Bentsen said:
Measured some SB1000 last year, found no difference in a outdoor setup with two on the ground vs. two on a 1m tall platform, except in the extreme nearfield :)
Click to expand...

To put that ground reflection in perspective, imagine subs 2m off the ground, and a really tall listener with ears 2m off the ground, standing 10m away from the subs. The path length difference is going to be 2x(sqrt(5x5+2x2)-5)=0.4m. A half wave of 40cm puts you up around 400Hz for the first full cancellation. There will still be summation below 200Hz. As the listener gets closer to the speakers the first cancellation frequency will get lower, but if the heights of the sub and listener are lower the frequency will go up. I don't think cancellation due to ground reflection from subs that are not right on the ground is a real issue. In a crowd, absorption by bodies standing in front of the subs is probably a bigger issue.

Mac
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

mackerr said:
To put that ground reflection in perspective, imagine subs 2m off the ground, and a really tall listener with ears 2m off the ground, standing 10m away from the subs. The path length difference is going to be 2x(sqrt(5x5+2x2)-5)=0.4m. A half wave of 40cm puts you up around 400Hz for the first full cancellation. There will still be summation below 200Hz. As the listener gets closer to the speakers the first cancellation frequency will get lower, but if the heights of the sub and listener are lower the frequency will go up. I don't think cancellation due to ground reflection from subs that are not right on the ground is a real issue. In a crowd, absorption by bodies standing in front of the subs is probably a bigger issue.

Mac
Click to expand...
I agree completely.

In the olden days using a 200 Hz low/low mid crossover point, it was obvious that the bodies absorbed the upper bass frequencies, so we always elevated the bass cabinets on 22.5, 30, or 45 inch high road cases so at least the upper portion of the bass cabinets were above head height. In venues that did not have enough ceiling height to put the bass up, the sound (and front rows of people) suffered.

The extra frontal area of the road cases actually increased the LF level out front, but the primary reason we did it was to avoid the loss of upper bass the punters would cause, a passive “punch” filter that left nothing but mud.

Even as low as 80 Hz a single 18” has some directivity, a mass of bodies is an effective filter down pretty low.
One of many reasons the rising upper response of many subs may actually preferable to a flat response.

On any indoor gig, floor, ceiling and side wall LF bounce reflections are a fact of life. In the big musical picture, those reflections and any local attendant LF notch cancellations rate pretty low.

Art Welter
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Thank you Bennett that helps a lot. It does look like at that height, that there is a cancellation that is showing up about 5-6ft off the ground above about 70hz.

I should try and mess around with MAPP again and see what happens as you then lower that sub to say 10-20ft off the ground.

Thanks,
Phil
 
Re: Flown Subs, Ground Bounce, and People in 1/2 space

Jay Barracato said:
The thought that really stuck out to me from the Gunness paper was the idea that the maximum coupling from the floor would actually be in the upward direction, not the forward direction.
Click to expand...

This may be why ground stacked subs actually work better in closed arenas rather than flown subs for coverage of the upper levels. Dave Rat did a good blog post about that.
 
Top Bottom