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Double Bass Array (DBA) - The modern bass concept! - Page 7

post #181 of 321
Thread Starter 
Here is another measurement of my room gain. In opposite to the previous statements a DBA does not eliminate the room gain. Judge for yourself:

post #182 of 321
Are these measured results, with the DBA delay and phase inversion?

More detail, please....

JSS
post #183 of 321
Thread Starter 
Quote:
Originally Posted by maxmercy View Post

Are these measured results, with the DBA delay and phase inversion?

Yes. There were two experimental DBA's. One with one driver per array (1x1) and one with two (2x2). On both configurations the rear array was inverted and delayed by the time equivalent to the room length (6 m). All measurements were taken with the same subwoofers in closed boxes and in the same room. Amplitude responses were normalized to the same level to compare the gain under 1st-order room mode.
post #184 of 321
Do you have an theory/explanation as to why room gain continues to contribute?

This is very good news!

JSS
post #185 of 321
Quote:
Originally Posted by maxmercy View Post

Do you have an theory/explanation as to why room gain continues to contribute?

Room gain is indicative of the rising response as freq approached the zeroth mode at 0 Hz.

At 0 Hz wavelength is infinite so array effect that works on driver phase differences are not in effect, as all drivers are in phase everywhere in the room.
post #186 of 321
Quote:
Originally Posted by FoLLgoTT View Post

Yes. There were two experimental DBA's. One with one driver per array (1x1) and one with two (2x2). On both configurations the rear array was inverted and delayed by the time equivalent to the room length (6 m). All measurements were taken with the same subwoofers in closed boxes and in the same room. Amplitude responses were normalized to the same level to compare the gain under 1st-order room mode.

Hello FoLLgoTT

Very interesting and very good results :-) I struggle a bit understanding why you would invert the phase of the back subs.

My understanding is that the wavefront from the front sub is "launched" first and at the moment it reaches the back wall and is reflected, the back sub launches the same, but delayed and phase inverted, waveform. How ever I fail to see the logic in phase inverting the back sub under these circumstances, because the wave form from the -front- subs is being phase inverted when it reflects from the back wall. Maybe I am wrong about this, but would that not result in the phase inverted back-sub being -in phase- with the reflected wave, making the reflected sound even higher in amplitude.. ?

Of course I can see form your graphs that it works, but I completely fail to see how. Can you offer some insight in to this or point out my error?

http://www.acs.psu.edu/drussell/Demos/reflect/reflect.html

Dan
Edited by splotten - 9/30/13 at 12:52am
post #187 of 321
Thread Starter 
Quote:
Originally Posted by splotten View Post

Of course I can see form your graphs that it works, but I completely fail to see how. Can you offer some insight in to this or point out my error?

http://www.acs.psu.edu/drussell/Demos/reflect/reflect.html

Your link describes transversal waves. A longitudinal wave does not change polarity when getting reflected. The wave types just behave differently. smile.gif
post #188 of 321
"Do you have an theory/explanation as to why room gain continues to contribute?"

below the first mode, the room is completely pressurized (and depressurized) by the "wave" before the rear woofers begin to interact with it.

which brings us back to this again:

http://www.linkwitzlab.com/images/graphics/enclosure-spl.gif
post #189 of 321
Quote:
Originally Posted by FoLLgoTT View Post

Your link describes transversal waves. A longitudinal wave does not change polarity when getting reflected. The wave types just behave differently. smile.gif

Oh thats right. Had forgotten about longitudinal and transversal waves. In that case it makes complete sense.

I was thinking about how you could make an array that was cheaper than a DBA but would still work down into the low frequencies where the absorber in the SBA only works partially. I was thinking you could combine the absorber from the SBA with a single sub in the back like in the 1x1 DBA. That way the absorber would take care the higher frequencies and the single back sub would cancel out the reflected low frequencies. That way you might be able to get away with only 3 subs in stead of 4 and still retain the ability to control up to ~70Hz depending on room size. Also the back sub might not need be as powerful as the front subs because the absorber still partially dampens even the low frequencies. Any thoughts on that?
Edited by splotten - 9/30/13 at 3:26am
post #190 of 321
I wonder if anyone realizes the implications of these test results.....if you have a rectangular room, a few subs, and the ability to delay, you can cancel nearly all room mode problems.....

This is huge.

JSS
post #191 of 321
"I wonder if anyone realizes the implications of these test results.....if you have a rectangular room, a few subs, and the ability to delay, you can cancel nearly all room mode problems.....

This is huge."

+1, but harman showed that in their placement paper with subs located at 1/4 distances from walls. this just same thing turned sideways. i used harman research as basis to suggest such arrangement for mk's infinite baffle some time ago.

http://www.avsforum.com/t/1364279/finally-doing-what-is-needed/30#post_21059694
post #192 of 321
Thread Starter 
Quote:
Originally Posted by LTD02 View Post

+1, but harman showed that in their placement paper with subs located at 1/4 distances from walls. this just same thing turned sideways. i used harman research as basis to suggest such arrangement for mk's infinite baffle some time ago.

http://www.avsforum.com/t/1364279/finally-doing-what-is-needed/30#post_21059694

I simulated a true 1/4 configuration in this post. But it turned out to have more seat row variation than a DBA. Beside that the placement ist rather impractical. Or do you want to have "flying" subwoofers in your field of view? wink.gif
post #193 of 321
@FoLLgoTT, Mine is not a true double bass array in the sense that they are not quite 1/4 alone the wall. The actual ceiling and floor is much higher and lower than seen in the pic.

post #194 of 321
Quote:
Originally Posted by FoLLgoTT View Post

I simulated a true 1/4 configuration in this post. But it turned out to have more seat row variation than a DBA. Beside that the placement ist rather impractical. Or do you want to have "flying" subwoofers in your field of view? wink.gif

LTD02 was probably referring to a SBA at the floor.
post #195 of 321
Quote:
Originally Posted by FoLLgoTT View Post

Here is another measurement of my room gain. In opposite to the previous statements a DBA does not eliminate the room gain. Judge for yourself:


What are your measurement / window settings here?


Because you're below the modal region, it would be more accurate to do stepped sine wave measurements to quantify room gain. The math should show less because the phase difference between arrays is shrinking as you go lower in frequency and the polarity is opposite. Therefore I suggest you are seeing the shape of the window response acting on a sweep that is too short in duration (you can't fit a frequency whose period is longer than the test length....and it's a sweep so duration per frequency isn't the full length either).
post #196 of 321
Thread Starter 
@JapanDave
Very nice bass system! Is this one array or two?
Quote:
Originally Posted by MBentz View Post

What are your measurement / window settings here?

Sweep settings:

Bandwidth: 1 - 1000 Hz
Length: 256 k

Window:

Turkey 0.25 (both)
Left window: 125 ms
Winow ref time: 0 ms
Right window: 500 ms

This should be large enough for examining the room gain. Increasing the window size does not change much in the lower frequencies.
post #197 of 321
This is all foreign to me but I'm glad I stumbled upon this thread.
post #198 of 321
Quote:
Originally Posted by FoLLgoTT View Post

@JapanDave
Very nice bass system! Is this one array or two?
Sweep settings:
One at the front only unfortunately. I do have 4 subs at the back of the room, but I could not place them in the same alignment.
post #199 of 321
@JapanDave
Do you have any measurements after your rear IB was implemented? I didn't see anything current on your build threads.
post #200 of 321
Quote:
Originally Posted by LTD02 View Post

"I wonder if anyone realizes the implications of these test results.....if you have a rectangular room, a few subs, and the ability to delay, you can cancel nearly all room mode problems.....

This is huge."

+1, but harman showed that in their placement paper with subs located at 1/4 distances from walls. this just same thing turned sideways. i used harman research as basis to suggest such arrangement for mk's infinite baffle some time ago.

http://www.avsforum.com/t/1364279/finally-doing-what-is-needed/30#post_21059694

+1

Selective Mode Cancelation

use acoustic polarity, null points (placement)
and some damping (velocity and pressure)
controlled by time delay to fine tune summation (DSP)

results in well behave time and freq domain


Certainly nothing new
post #201 of 321
REW Beta 17 added a Room Simulator to simulate up to 4 subwoofers placed throughout the room. You can change the subs' location both vertically and horizontally and see what happens to room modes. Subs 3 & 4 point forward by default but can be rotated by using R or L keys. Controls for delay and gain for each sub will be included in the next release.

You can play around with a DBA vs front subwoofers and more surface absorption on the rear wall.
Quote:
Beta 17 changes (4th May 2013):
Added a simulator for rectangular rooms allowing up to 4 subs to be simulated at multiple listening positions

]
post #202 of 321
Thread Starter 
Quote:
Originally Posted by desertdome View Post

You can play around with a DBA vs front subwoofers and more surface absorption on the rear wall.

Yes, it is a nice feature and I used it often in the last months. But you are NOT able to simulate a true DBA, because it lacks the necessary delay and inversion of the signal. Right now you need a BEM simulation (like ABEC) to simulate a DBA.

Though a SBA with absorbing back wall can be simulated with REW.
post #203 of 321
Quote:
Originally Posted by FoLLgoTT View Post

Yes, it is a nice feature and I used it often in the last months. But you are NOT able to simulate a true DBA, because it lacks the necessary delay and inversion of the signal. Right now you need a BEM simulation (like ABEC) to simulate a DBA.

Though a SBA with absorbing back wall can be simulated with REW.

I just realized that you are the one that asked John for delay and gains. cool.gif He said he had already added them back on June 11th and the next build would be released soon. Now it is already October.

I just checked the REW Beta downloads folder and 14 people are viewing it. Did a bunch of people reading this thread just head over to get the latest REW? biggrin.gif
post #204 of 321
Quote:
Originally Posted by desertdome View Post

I just checked the REW Beta downloads folder and 14 people are viewing it. Did a bunch of people reading this thread just head over to get the latest REW? biggrin.gif

Probably. I will when I get home. biggrin.gif
post #205 of 321
Quote:
Originally Posted by Guygasm View Post

@JapanDave
Do you have any measurements after your rear IB was implemented? I didn't see anything current on your build threads.
No, I really do have to try and motivate myself to update my thread. Lately, work has been kicking my a$$ and I have been enjoying the theater too much. I have promised everyone more pics of the theater for a long time now. Will try and get onto it.
post #206 of 321
Hey, no pressure, just curious. Enjoy it. It is a beautiful thing.
post #207 of 321
I have been thinking about this since I last posted here and have a few questions.
Does the woofer size mean more than the lowest output levels? i.e. would 15" woofers capable of 25hz be better than 12" or even 10" capable of 20hz?
and....
how do you control the rear array seperate of the front? A link for this would be appreciated.
and...
do they need to be in a single large box at each end of the room or can they be open to the room/have no sides or back?
post #208 of 321
Quote:
Originally Posted by brwsaw View Post

I have been thinking about this since I last posted here and have a few questions.
Does the woofer size mean more than the lowest output levels? i.e. would 15" woofers capable of 25hz be better than 12" or even 10" capable of 20hz?
and....
SPL st a given frequency is determined by volume displacement, ie the area of the cone (Sd) multiplied by the excursion (Xmax), or bore and stroke if you will.

If a 10, 12 and 15" driver had the same Xmax then the 15 would be capable of more SPL than the 12, and the 12 more than the 10 at all frequencies in the operating passband. Larger drivers have the possibility of more excursion because they can accommodate larger surrounds, but it depends upon what decisions the manufacturer made with component parts whether it does or not.

If you have a sub (system) capable of a certain SPL at say 20Hz, then to get the same SPL at 10Hz would require 4x the volume displacement.
post #209 of 321
I need to read up on this some more.
I really like the idea of having a wall of woofers at each end of the room and none on the floor.
post #210 of 321
Hey.
I'm about to rebuild my HT with a false wall and and a AT screen, and I think the SBA with a absorbing back wall looks like a very good solution for the bass issues that's been troubling me for the last six years. I've used some different sub setup over the years, and spent a lot of time crawling around doing measurement with XTZ RA and omnimic. My current sub setup is 4 Rythmik F12 EQed first with a JBL BassQ and than together with the rest of the speakers with ARC in my processor. The bass is OK in the sweetspot, but it's still room for improvement.
My room is 3.3m x 5.6m x 2.4m and I've one row of 4 seats about 3.7m from the front wall.
The simulations in REW looks very promising, and really like the thought of similar bass response for all the seats and that it doesn't change depending on the height. I guess it's possible to EQ any small peaks or dips between 30 and 60hz for the axial length modes that it's not fully taken care of by the absorbing back wall.

But now for the questions smile.gif
1. On the front wall I've a box for the ventilation( 20x20cm and stick out about 50cm into the room), will this affect the bass array?
2. The frame for the screen and some of the support for the false wall will be in front of some of the subs. Will this affect the bass array?
3. To be able to support the weight of the subs and get them into the right height, I've been looking into using some adjustable workshop shelf(I've no idea if this is the correct English word) but only using the needed shelfs. Will this have any impact on the bass array. i.e will it reduce the mirror effect from the ceiling and floor so the axial height modes will still be present?
4. Is it any other advantage of using a denser array than 2x2 than to increase the upper frequency for the array? Or to ask the questions in a slightly different way; Is it better to use more cheaper and smaller drivers than 4 killer woofers? I will go DIY for the subs if I decide to go the SBA with absorbing back wall route.
5. Is there any other benefits than to save space to use low profile cabinets like FoLLgoTT have used?
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