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Do ground stacked subwoofers really give you 6dB for free?

By Nathan Lively

  • Any measurements made at ground plane will always be 6dB louder, despite subwoofer distance.
  • Flown subwoofers give you 6dB for free just like ground stacked as long as a maximum height is respected.

I have always thought that ground-stacked subs would produce 6dB more SPL because they are coupling with the ground. You’ve probably heard people say things like:

  • “Having subs next to a boundary gets you 6dB of additional sensitivity”
  • “The ground is like a mirror, doubling sub energy. “

I was just running a couple of tests for myself to confirm the reliability of this number, but I couldn’t. In fact, all of the models that I tested actually returned lower average level when the subwoofers were on the ground compared to flown in the air.

50ft Room

50ft room model
50ft room measurements

In the low range the flown sub dominates, but later the ground sub takes over.

150ft Room

150ft room model

I also thought that a corner placement would give you even more SPL, but I couldn’t prove that either.

Maybe the room gain from the walls is making the phenonenon harder to verify. Let’s try a bigger room.

300ft Room

300ft room model
300ft room measurements

Rats.

150ft – Outside

Surely if I turn off all of the walls, except for the floor, we’ll get that free 6dB.

150ft outside

Damn it. I want my free money back.

Why is this happening?

I have a two ideas:

  1. I put the mics in the wrong place.
  2. Flown subs get 6dB for free, too, baby!

The Mirror Effect

mirror effect

I had been taking ground plane measurements in the first models to remove the floor bounce, which made sense to me since I assumed an audience filled with people would have the same effect. There are two problems with this:

  1. Below 100 Hz human bodies have little absorption, which is mostly where our subwoofers live. Plus, it’s hard to predict exactly how the bodies will be distributed.
  2. If my measurement is coupled with the floor, it effectively shows half-space loading at any distance due to the mirror effect.

If the listener is located at the boundary he will hear a 6 dB louder direct signal than he would have heard if there was no floor regardless of where the subwoofer is located.

Comments On Half Space, David Gunness

If we zoom in on on the y-axis, it’s hard to tell which is which because any comb filtering is eliminated. No matter how far away the sub gets, measuring at the ground will show coupling.

ground plane only
Ground Sub
flown at ground plane
Flown Sub

If that’s the case then I should try measuring at head height.

Head Height

Let’s simplify the test by removing the walls and ceiling and using a single microphone position so that I can actually get this article done this year. I’ll move the mics up to head height, since that wasn’t working earlier.

Here’s the result from the same 300ft room, this time without walls, ceiling, or a 3-mic average.

Flown subs get 6dB for free, too, baby!

It is shown that a flown subwoofer [will] have a similar far-field efficiency to that of a ground-stacked subwoofer when a maximum subwoofer height is respected. This maximal height is linked to the venue depth via the DHER criterion, and depends on the listening height. For a standing audience, SPL efficiency is recovered for listening distances that are 5 times or more the subwoofer height. The audience benefits from a more homogeneous SPL distribution and an important SPL reduction close to the stage.

AES Convention Paper 10051, On the efficiency of flown vs. ground-stacked subwoofer configurations, Etienne Corteel, Hugo Coste-Dombre, Christophe Combet, Yoachim Horyn, and François Montignies

Pretty cool, right?

So if we want to recover SPL efficiency at ¾ audience depth, then the sub can be as high as 36.75ft and we get the added benefit of an improved front-to-back ratio.

5x rule for sub height

The real benefit of ground stacking has to with the fact that listeners’ ears are not typically on the floor, but four or five feet above it. If the speakers are elevated above the floor, 45 degrees above horizontal from the listeners perspective, the ground bounce will produce its first comb effect notch at about 80 Hz. If the elevation angle is 30 degrees, the first notch moves up to 113 Hz. If the subwoofers are on the floor, then propagation is parallel to the floor and there is no ground bounce. Hence, there is no comb effect.

Comments On Half Space, David Gunness

It’s interesting that our measurement position is less than 10º, putting the first dip from the comb filter at 297Hz, well out of the operational range of this subwoofer. If you wanted to create a null at 125Hz, you would measure at 72ft depth at 23.8º with the sub.

9.5º

The number isn’t black and white, of course. Even with the first null at 297Hz there is a 3dB drop at 140Hz.

floor bounce calculator
Merlijn Van Veen – Floor Bounce Calculator

So we can see that as we move closer to the sub, the difference in distance between direct and reflected sound will affect more of the operational range of the sub. At this time it is my understanding that with flown subwoofers we accept some amount of comb filtering in the front portion of the audience (anywhere before height of sub multiplied by five for standing head height) in exchange for improved front-to-back ratio, coupling with mains, and SPL efficiency in the rear portion of the audience.

What are your experiences on half-space loading? Comment below.

Loved this post? Try these:
  1. How Dave Gunness Brought Single-Driver Cardioid Subs To Market
  2. How far apart can I space my subwoofers?
  3. Do subwoofers need time alignment?

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Comments

  1. gerald says

    February 25, 2020 at 13:32

    Hi great article. But why are flown subs so rare to see?

    Reply
    • Nathan Lively says

      March 1, 2020 at 15:01

      Money+time. You need the rigging and the weight capacity and the time to it put, that I can think of right now.

      Reply
  2. Peter says

    February 26, 2020 at 15:10

    The X axis for the 150 foot room is different from the others, starting at 1 Hz.
    How come that response for the small room is smoother than for the bigger rooms?
    Where are the room nodes that there are supposed to be in a small room. The frequency response is as smooth for the small as for the responses outside. The bigger rooms have much more dips and peaks.

    Reply
    • Nathan Lively says

      March 5, 2020 at 14:13

      Hey Peter, thanks for your interest in the article.

      >The X axis for the 150 foot room is different from the others, starting at 1 Hz.
      Yes, the x-axis is different, but I’m not sure how that relates to 1 Hz.

      >How come that response for the small room is smoother than for the bigger rooms?
      I didn’t look into this very closely, but I did notice in the 150ft room that the comb filtering from multiple wall reflections were lining up around the same region, causing bigger dips in the response.

      >Where are the room nodes that there are supposed to be in a small room.
      I don’t know. I didn’t calculate them.

      Reply
      • Peter says

        March 6, 2020 at 04:25

        All the others go from 36 Hz to 124 Hz.
        The frequency response for the 150 ft room goes from 1Hz to 61 Hz. You can’t have this response for frequencies this low.

        The frequency response for the 50 ft room is just too smooth for an in room situation (no room modes). Look at the difference for the 150 ft room inside/outside. My guess is that the response for the 50 ft room is also ‘outside’

        Reply
        • Nathan Lively says

          March 9, 2020 at 14:08

          >The frequency response for the 150 ft room goes from 1Hz to 61 Hz.
          Ha! Thanks for catching that. Looks like the Y-axis was set to show row numbers instead of Hz. Fixed!

          >the 50 ft room is just too smooth
          Not necessarily. More walls = more reflections = more chaotic ripple. I would expect to see clearly defined comb filtering if there were a single boundary, like when we’re outside. Also keep in mind that you’re looking at the average of 3 mics. Here are all of the measurements solo.
          50ft room solo measurements

          And here’s the MAPP file if you want to play with it.

          Reply
  3. Christian Gehlen Candido says

    March 6, 2020 at 12:33

    Hi Nathan!!! I found a AES paper about it.
    On the efficiency of flown vs. ground-stacked subwoofer configurations.

    Reply
    • Nathan Lively says

      March 9, 2020 at 14:13

      Great! Is it the same one I referenced? Or something different?

      Reply

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