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[Ivan Beaver]

I had a few minutes and wanted to do a little test-for which there has been lots of "discussion" on.

Basically whole space vs half space of subs and what happens to the levels and freq response.

Here is my setup.  I placed a mic 100' away.  To get far enough away so that inverse square law would only affect it a little bit.

I used two different designs of subs (a tapped horn and a "basically" normal bass horn).  I have no reason to believe a normal ported sub would be any different.

No xover was used and just 1 eq filter (quickly applied) to flatten out the response to be easier to look at.  The eq filter is NOT the same on both cabinets.  In both cases it was a CUT only, and no boost was applied.

I measured the cabinet on the ground-then raised it up so the bottom was 15.5' off the ground and measured again.

 That was as high as my forklift would go.  Sorry it was not more.  But you should get the idea.

I did adjust the levels between the different cabinets, but NOT between the measurements of each cabinet.

As you can see, the biggest thing that changed was the HIGHER FREQ-NOT the low freq.  This is because the mic is not on axis with the sub and the higher freq are starting to become a little directional.

So this PROVES (at least to me) that the whole idea of "getting more bass" when the subs are on the ground OR simply that you can "add a magical 6dB" to a whole space measurement is simply false.

Since the mic (or person listening) is already in half space, you get the loading.  But no additional loading when both are in half space.

Yes the curves are not exactly on top of each other, but remember that the distance to the mic is further when the cabinet was in the air.

In both cases the PURPLE trace is the ground and the GREEN is in the air.

[Art Welter]

I measured the cabinet on the ground-then raised it up so the bottom was 15.5' off the ground and measured again.

So this PROVES (at least to me) that the whole idea of "getting more bass" when the subs are on the ground OR simply that you can "add a magical 6dB" to a whole space measurement is simply false.

Ivan,

What was the distance of the subs and mic to any buildings?

Art

[Ivan Beaver]

Ivan,

What was the distance of the subs and mic to any buildings?

Art

I just did this in the back parking lot-so not completely free of reflections.

Moving everything to our "reflection free" area would require A LOT more time and effort than I had.  The forklift doesn't move to easy

Behind the sub is the building.  It is about 60-70' behind and is basically about 24' high.  It has "lossy" walls.

The building was about at a 45* angle to the rear of the sub

Behind the mic the nearest "large object" is probably 400' or more.  There are some trees and a creek between the mic and the next building

The mic was laying on grass. 

[Mark Wilkinson]

Interesting...  but is 15ft high enough to consider it a move from half to full space?  Particularly given the long wavelengths involved?

I'm thinking that 15ft over 100ft makes for more of a long flat acoustic mirror, than a change of space.
And that floor bounce summation at all freqs will range from minimally destructive at the longest wavelengths to greater destruction at shortest...like what seems to be going on with the plots..

[Art Welter]

Behind the sub is the building.  It is about 60-70' behind and is basically about 24' high.  It has "lossy" walls.

The building was about at a 45* angle to the rear of the sub

Behind the mic the nearest "large object" is probably 400' or more.  There are some trees and a creek between the mic and the next building

The mic was laying on grass.

Thanks, the distance to boundary clarifications are important to determine whether there was actually a half space to free space change.

For what it's worth, I have done a similar test on a smaller scale, same results. That said, if the test was done at 15.5 feet with the mic on the ground, then the cabinet and mic lifted 15.5 feet high, the second measurement would show a loss in bass response compared to the first.

[Ivan Beaver]

Thanks, the distance to boundary clarifications are important to determine whether there was actually a half space to free space change.

For what it's worth, I have done a similar test on a smaller scale, same results. That said, if the test was done at 15.5 feet with the mic on the ground, then the cabinet and mic lifted 15.5 feet high, the second measurement would show a loss in bass response compared to the first.

When I get a chance I can redo the test.  I have a 17' tall mic stand that I could use.

Don't expect anything this week however.

My whole "thinking" was that in almost no case are the people and the cabinet in whole space-so that spec doesn't really tell anything useful.

[Ivan Beaver]

Interesting...  but is 15ft high enough to consider it a move from half to full space?  Particularly given the long wavelengths involved?

I'm thinking that 15ft over 100ft makes for more of a long flat acoustic mirror, than a change of space.
And that floor bounce summation at all freqs will range from minimally destructive at the longest wavelengths to greater destruction at shortest...like what seems to be going on with the plots..

As I said-my forklift only went that high.  To go higher would require renting in a larger lift.

Since there was not enough of any change to be concerned with, I don't see any reason to spend the money on a larger lift.

And in many cases (except the larger shows) most subs are not much higher than 15' anyway.  And that was to the BOTTOM of the cabinet.

The 100' distance was choosen as a "typical" FOH position (yes it varies), but was far enough away to get an idea of what was happening.

There is NO floor bounce (at least at those freq), since the mic was lying on the ground.

I don't think there is any "destructive interference" going on at the higher freq (meaning 100-200Hz).  I am pretty sure those differences are because the mic is not on axis with the loudspeaker and those freq are starting to narrow a bit.  I could be wrong however.

Either way-there is not enough of a difference to be concerned with.

[duane massey]

Very interesting. Thanks, Ivan.

[Mike Pyle]

As you can see, the biggest thing that changed was the HIGHER FREQ-NOT the low freq.  This is because the mic is not on axis with the sub and the higher freq are starting to become a little directional.

And yet, anyone can hear the difference in bass response between a monitor on a stand versus one on the floor. Why do you think that is?

[Ivan Beaver]

And yet, anyone can hear the difference in bass response between a monitor on a stand versus one on the floor. Why do you think that is?

Part of the reason is the position of the ears-the floor (or other reflective surface) and the speaker.

When measuring subs, the mic is on the ground (to avoid reflections).

Reflections can/will cause all sorts of deviations in the response that are totally position dependent.

So when measuring any loudspeaker, you need to reduce/eliminate the reflections as much as possible in order to get any sort of useful measurement of the device.

The idea of measurement is to measure the device itself-without any outside influences.

At least in the way the device will be used.

For example-I believe that floor wedges should be measured on the floor (since that is their intended position).  Typical full range speakers-no.  You are more concerned with the "non reflective" performance.

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