please help me determine PONR for IB HT setup

[JinMTVT]

need to calculate the point of no returns
for how many 18" i will be using in my HT setup
( could be 15" ..not 100% on that yet )

they will occupy most of the front wall
in a "planar wave" type of array
( distributed equally over front wall area )

the room will be of 25' depth
by 20' width
by 8-9' height ( rear half is 8' for seats leveling )

How can we determine how many subs will yield the best $$$ VS performance for this particular setup ?


( please don't suggest other loudspeakers config, my choice is already made on to this one )

rear wave will be evacuated to garage room
( 26' by 26' )

i was planning for 16 individual 18" drivers
could also work with more 15" like the Dayton IB
over at parts express if $$$ and result are worth it

so would like a way to calculate response vs number of drivers and possibility!

thanks for your time!

[JinMTVT]

also ..
the number of drivers will have to be multiples of 4
for the planar wave system to work.

[bjorno]

...How can we determine how many subs will yield the best $$$ VS performance for this particular setup ?...

Nobody knows but you as it depend on your own requirements to start with, I guess if not complete or not known in detail, its difficult to answer but maybe this attempt to answer is adequate to start with:

Why not do most of the investigation works by your self? Make your choices and tell us to see if we can agree upon your driver/amp power demands/opinions of what is good performance for a HT set-up.
See 1(1) or go to: http://www.myhometheater.homestead.c...alculator.html

Added a 4 x driver DBA polar simulation for a room of 2.4 x 4 x 6 m where XO frequencies (LP= 12 dB/octave) is marked (black line) ensuring an acute (frontal, horizontal) wide enough initial planar (surge) wave coincident to both ears of any listener placed at the rear half of the room without any lateral velocity gradient occurring than caused by the main speakers (even at higher SPL when distortion often pops out from the subs randomly from the same but differently placed drivers) : 2(2).

b

1(2)

[bjorno]

2(2)

[JinMTVT]

woa!
nice answer !!!!

2 things come out of this mainly for me :

1- could you please explain all of this,
as this is seriously more advanced than my very very tiny limited knowledge now!!

2- i do not know exactly what i want/need as a result of this system, what i need right now is formulas or ratios ...stuff like that , that would permit me to determine
and play with different values to see what can be achieved and what would be the best possible compromise to choose on the drivers and number of drivers on this system ( sub bass system )


this system will have to match very efficient floor to ceiling line arrays ( again ..should be very efficient
as they will operate in nearfield and site not very far from the listening positions )

thanks again for your time!

[GM]

Originally posted by JinMTVT

they will occupy most of the front wall
in a "planar wave" type of array
( distributed equally over front wall area )

How can we determine how many subs will yield the best $$$ VS performance for this particular setup ?

Assuming an 80 Hz 4th order XO, then you ideally want the drivers < 1/4 WL apart at 160 Hz = ~13560"/4/160 = ~21.1875" c-t-c driver and to wall spacing. That's a lot of drivers and if 18"ers are used the wall will look like a giant's wine rack.

In reality though, unless you can damp the modes at the other end of the room this close spacing/large number of drivers will mostly be a waste of time/$$$/effort IMO compared to distributing around the room. Sure will be impressive though, like a large Bessel Array for cinema apps, but with a large, long bass-horn's acoustic efficiency, so I imagine the cones will barely move at 'voice of God' LFs.

[JinMTVT]

impressive is what i am after ...

[JinMTVT]

why do we need to space the drivers that close?

why the 1/4 WL formula ?

i would like to follow the " DBA" theory
( double bass array )
but minues the double ( rear reversed wave )

what would happen if i'd have only 16 18" drivers on the front wall of 20' by 9' high ?

they would be positionned according to the "mirror" image effect ...

[GM]

Because you said you wanted "a "planar wave" type of array
( distributed equally over front wall area )" and to do this requires enough polar overlap between drivers to cover the BW. If you XO lower and/or use a steeper slope, then you can use a wider spacing/fewer drivers. Or you can do it anyway and accept the collapse of the planar wavefront at some lower frequency. Never having built a large one I don't have a clue at what point it might be obviously audible depending on seating location.

[JinMTVT]

hi GM...
ok i understand what you mean globally,

but i seriously thought that the planar wave would work with a lot fewer % of the area of the wall covered by drivers ... i understand that it is frequency dependent

yet i have not sorted out what kind of XO i would like to work with ..since i do not have all the details worked
out ( this system is getting seriously tedious and complicated the further it grows... too many entricate details wich need assumptions right now to work with,
or require direct inputs of correct details to be able to determine and decide on components and details ...)

will an array of 18 or 15" sub permit me to go higher in the frequency of the crossovers ?
down to what hz is the bass required to come from the correct direction and situation ?

i do believe that up to a certain hz, the bass will start to be more directional and behave differently VS sub-bass systems ?

all this is seriously related, and i have yet to determine what kind of XO and driver sizing the line arrays will permit me as far as combing and driver spacing ..



Have you seen/read the paper on double bass array ?
have you seen/heard any examples of sub bass arrays
used to form mirror images and simulate a planar wave?

what i read and understood,
is that by correct placement of drviers on the front wall,
sub abss drivers are able to produce some kind of planar wave ( as distributed as it can be ...i do not have sufficient knowledge for that part ) wich will react in the room and travel through it as if it was confined in a horn/tube and behave as a near field @ -3db for doubling distance ...

what i wanted to work with is trying to eliminate the side and vertical room modes as much as possible without having to resort to bass traps, having the rear of the room as absorptive as possible for bass
( probably 2" of some wool or fiber + some back spacing
thus reducing a maximum the front/back modal )


What you are reffering to as distribution of the planar wave, doesn't sound radiate spherically at sub bass frequencies ? won't it all scatter and make some kind of big mess of a planar wave even if drivers are more sparce?
my mental image of it happening in 3d is of scaling spheres merging into each others at mid driver distance,
but i cannot think of what will happen afterwards?
will the waves continue to spread after colliding or will they merge and make some sort of field with higher and lower densities of pressure, constantly evolving and mixing until there is no more energy ?

i have felt once sub bass from a long port sitting right in front of it ...was big
and i can tell you that when the bass hits you,
it is something alot more than just sitting in a place where a spherical wave hits you partly while it grows to loose its power ...
am i dreaming or ?



bjorno:

thanks for the simulation and links
the room pressure distribution isn't supposed to be as your plots show, as i was told that with a planar wave type of config, the pressure moves evenly through the room as if it was going through a tunnel or a horn tube
so i am not sure what to do with your polar plots ..
though very instructive

Then, your html simulator doesn't show much info for different frequencies and would need to assume for only 3db of loss on distance ( if the theory of the planar wave behaving as nearfield is correct)
i believe that i will need to emulate the driver's response and calculate attenuation VS distance by myself, and then find a suitable SPL at the lower Hz i am looking for, the rest can be filtered out
that is IF the NO MODAL of the planar wave works correclty thus assuming no room gain for the sub bass frequencies ...

to be seen ...

please let me know if you guys have more info or think of something else that would help

could i use HornResp to calculate the SPL the system would achieve and then use the distance to calculate the loss ??