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Re: Virtual Bass Array

Posted: 20 Feb 2020 15:33
by Valentin Nicolaie
Hi Uli,

I have measured with VBA wav prefilter in LSR and the result, unsurprisingly, is exactly like when I integrate VBA as prefilters in Macro 0 after measuring with no prefilters. So far so good. Then, I started calculating the filters, as per your advise, with VBA prefilters added in Macro 0 but inactive. After Macro 1 I activated them. Then I generated the filters through the other Macros, and at the last step, I executed Macro 5 which returned a frequency response result where I see that the VBA was obviously applied twice - one time at measurement and second time at Macro 4-5. My question: is this only a visual thing? Like a side effect of using VBA prefilters at measurement but filters are correct?


P.S. I know I can test this by loading the correction filters in the LSR and remeasure, but I realized this situation only after I packed all gear and returned the room in the normal condition (chairs were removed etc)

Re: Virtual Bass Array

Posted: 21 Feb 2020 15:18
by UliBru
Hello Valentin,

the test convolution is the result of convolving the measurement with the filters.
As in your special case the measurement is already including the VBA filters and the correction again includes the VBA filters the test convolution result is visually wrong. This is caused by the measurement with VBA filters plus tweak with switching off/on the prefilters.
Whereas a new measurement with the correction filters applied would show the correct result. So please trust the correction filters.

Re: Virtual Bass Array

Posted: 23 Feb 2020 13:04
by Valentin Nicolaie
Many thanks for confirming to be only a visual side effect as I suspected! I've got great results integrating the VBA concept and together with a combination of shorter and longer FDWs and a target modeled after the actual speaker response measurement with a short FDW (3) I have now the system sounding the best ever. Probably beamforming technique may squeeze a little more out of it but I forgot the details in how to do it in the meantime as I did it only once about 4 years ago when there was the talk about it in the yahoo group. If I may suggest, as I know there was a instructions document, maybe you can add in Beamforming as a main topic in this forum too just like Virtual Bass Array here.

Re: Virtual Bass Array

Posted: 14 Nov 2021 17:07
by StabMe

I do understand that this forum is mainly dedicated to users of Acourate, but I thought that I might ask for an advice anyway.

So I think i understand the concept of VBA - configure the system so that it send a duplicate of the sound, delayd in time, reduced in gain and reversed in polarity, band passed only to bass frequencies, so that when the initial wave reflects from the back wall and hits the front wall it meets this reversed duplicate sound and those two cancel each other. Then there is a specific way of creating a filter for this to happen with the means of Acourate.

Now, I do have Acourate in my wish list but currently have no luxury of owning the licence. I use a 2 way system which is a multiple entry horn with two 15" Beyma drivers and a 2" Faital compression driver in each speaker. Those are located in the corners of a big room (6x10 meters). This is a full range system - works down to 25Hz with a bit of EQ. It is fully active - I use Reaper on my PC, a MOTU soundcard and a 4 channel amp.

Since Reaper is a DAW, i can use all sorts of signal processing. Will the concept work if i add a delayed, reversed in polarity and lowpassed signal along with main sound? Or is there some other secret sauce that i haven't digested yet, but is being done with Acourate?

My main objective is to reduce ringing at 25-80Hz range. Well, if it fixes some nulls - even better.


Re: Virtual Bass Array

Posted: 14 Nov 2021 17:13
by UliBru

there is no real secret sauce. You have to desing and apply a proper filter to create the desired cancellation wave at the right time.
I'm quite sure that Reaper can handle such a filter.

Re: Virtual Bass Array

Posted: 14 Nov 2021 17:16
by StabMe
Will try to do that with Reaper. Thanks!

Do you mind if I post my results here (or lack thereof) or should i start another thread?

Re: Virtual Bass Array

Posted: 14 Nov 2021 17:22
by UliBru
you can post here

Re: Virtual Bass Array

Posted: 26 Feb 2022 16:57
by Ecwl
I have to admit, I’ve gone back to this again because I always worry that my room length measurement is off, e.g. is it 3.33m or 3.30m or 3.36m
So I finally decided I was just going to measure the peak of that resonance frequency, e.g. 51.27Hz which would be 0.01950s = 936 samples.
Even though I was never off by more than 1Hz, psychologically, it seems that the more accurate VBA filter gives minimally more accurate bass transients. Could all be placebo. But I sleep better at night? Haha.

Re: Virtual Bass Array

Posted: 21 Apr 2022 16:41
by dbr
Wow, great application of Acourate! I have a few questions to Uli:

1. a) You state that "with a DBA system the soundwave will travel only one time through the room." In an article from I've read by Nielsen and Celestinos, called "Low Frequency Sound Control in Rectangular Rooms using CABS (Controlled Acoustic Bass System) will also reduce sound transmission to neighbouring rooms" (2011) their experimentation with a DBA consisting of two subwoofers at the front and two at the back, with the inverted time compensated sound wave, the sound wave is completely cancelled and the resulting longitudinal echo from the back wall is removed.

Their experimentation resulted in a reduction of cumulative spectral decay between 10-100 Hz from 500ms to 100 ms . That is, the result was a very "dry" room in terms of bass. Assuming the same dual front subwoofer set up, placed in identical locations in an identical room as that used by N&C in their testing, would the VBA have the same effect of cumulative spectral decay to this extent? That is could I expect such a quick spectral decay with the VBA? Or would it result in spectral decay remaining in the frequencies not compensated for?

1. b) With the VBA the back wall reflection still takes place and then it is compensated for by sending out the inverted adjusted compensation of the reflection. So the original signal passes one time, reflects and then it gets eliminated in a series of compensating pulses. These are adjusted at the listening location, so the sound wave passes two times behind the listener, but never passes the listener twice with the VBA, correct?

2. With the DBA the neighbouring room enjoyed a 10 dB reduction and the resonance frequencies, but also experienced increased decibels at the anti-resonance frequencies. I assume that neighbours will still hear the sound and no such benefit will result from a VBA, since the back wall still receives the full signal once and it bounces off that wall, sending some of the signal through. Is this correctly understood?

3. In your text above you state "It is also possible to send the music signal through a delay, to invert it and to add it to the original music signal. So no FIR filter is required, a stupid DSP may do it. The disadvantages of such a solution will be discussed later." The DBA used by Nielsen and Celestinos uses precisely such a more simple DSP solution. No FIR filter is applied. What are the disadvantages of such a simple DSP that you referred to compared to this more elaborate FIR-filter (apart from the cost of additional speakers on the back wall and extra amplification)?

4. The setup used by N&C requires mono bass, does the VBA also require mono bass?

5. You mention that "...the echo cancellation will also work in case of a distance between front speaker and front wall." Could this VBA then be used if the bass is produced by dipoles?

6. The reason I got interested in DBA's is the potential to lift dips caused by the rear wall reflection. In your example there is a dip at 45-60 Hz, which looks to be compensated for by 5 dB or so. Some of that dip still remains. Could that be further compensated for?

This looks like a very appealing proposition.

Kind regards,


Re: Virtual Bass Array

Posted: 22 Apr 2022 16:43
by UliBru
Hello David,

I see more or less 4 categories of listening setups.
1. Best case: no back wall. The sound is passing the ears and then it disappers in nirwana
2. Very good case: a DBA cancels the waves and thus avoids standing waves. IMO a very good symmetric setup is required, otherwise the waves from the rearside do not match to the waves from the frontside.
3. Poor man's approach: a VBA does not need the extra DBA hardware. It is a compromise. Of course the sound will travel at least two times through the room
4. Standard case: low frequency waves will create standing waves with peaks and dips in the frequency response

So the spectral decay will of course take an increasing time from 1. to 4.
Your ideas and assumptions about a neighbour in a next room are ok, here a cancellation by a DBA promises the best result whereas 1. even is the worst case if your neighbour sits behind your room ;)

A mono bass with a VBA has advantages. But please do not forget that a non-symmetric room or furniture may cause big problems too. There is even some logical conflicht with a VBA (and DBA too): a low frequency will radiate from a bass driver as spherical wave with increasing radius. It will usually also get reflected at sidewalls, floor and ceiling before it reaches the bass driver playing the delayed and inverted bass signal. So there is a mismatch of wave dimensions.

For dipoles the VBA logic also works.

As a VBA is less effective compared to a DBA you cannot expect the same quality of dip correction. Physics is phaysics.