Isobaric Driver Arrangements

[Metatron]

Has anybody here built various isobaric speakers?

I'm looking for information that's a bit more in-depth than the typical information. Specifically, nearly all configurations I see use the clamshell or back-to-back driver configuration. I'm looking for if anything special applies if the drivers are further apart, or if a driver at each end of a box tube wired out of phase can essentially be folded, much like DTQWT speakers are a folded horn.

What I've been looking to do is get particularly deep bass from a very small enclosure. I tried the Dutch & Dutch 8C and these have two 8" drivers in the back for boundary loading. I've looked around trying to find drivers that will get down to -3db @ ~30Hz (like the D&D 8C), and do so in approximately the size I suspect the bass portion of the D&D 8C uses, based on the dimensions given on their website for the overall speaker. However, I cannot find any configuration that would permit as small a volume as they use.

The only thing I know that could allow extra output from their drivers is the active amplification and DSP they apply, which would require the drivers to be high excursion.

Another way, which I thought may or may not work is to isobarically load the sealed enclosure. Since there are two visible drivers, I was wondering if they are wired out of phase so the internal air pressure remains constant and represent the aforementioned folded box tube - but then I was thinking the output might just cancel (?)**. If they are configured out of phase to keep internal pressure constant, then they meet the definition of isobaric, but I am not familiar with anybody or any article examining more elaborate configurations or 'folding' a box.

Any pointers/help appreciated.

**I can confirm that adjacent drivers in a folded box will cancel if out of phase, but still unsure if there could be hidden drivers on an internal partition within the enclosure of the D&D 8C. Also unsure about anything special if driver arrangement has some significant distance between clamshell or back-to-back isobaric arrangements. I assume that while common designs have the drivers touching, it is not actually necessary so long as their motion is synchronized to maintain equal pressure in the cabinet.

 

[bobovox]

Not isobaric, but my attempt to replicate the performance:

D&D claim 106dB @1m from 35Hz; by my calculations, with two drivers of 8" this requires Xmax of 14mm.

http://www.loudspeakerdatabase.com/Dayton/UM8-22 appears to fit the bill.

2 x Dayton Audio  UM8-22 in a 30l enclosure (not isobaric) gives f_b=50Hz with Q_TC = 0.90.

Apply Linkwitz transform in DSP to give Q_TC=0.71 & f_b = 25Hz.

__

I now wait to be educated as to what I've done wrong.

 

[parcelmonkey]

Couldn't you just speak with the chaps at Neat? They're helpful & friendly, surely they would at least put you on the right path.

 

[Metatron]

Not isobaric, but my attempt to replicate the performance:

D&D claim 106dB @1m from 35Hz; by my calculations, with two drivers of 8" this requires Xmax of 14mm.

http://www.loudspeakerdatabase.com/Dayton/UM8-22 appears to fit the bill.

2 x Dayton Audio  UM8-22 in a 30l enclosure (not isobaric) gives f_b=50Hz with Q_TC = 0.90.

Apply Linkwitz transform in DSP to give Q_TC=0.71 & f_b = 25Hz.

__

I now wait to be educated as to what I've done wrong.

Thanks for this, I will look into a Sim with the software I am using. I do hope group delay for that much low output is not high.

I think you're educating me. 

EDIT: That you've got deep bass in a smallish enclosure without isobaric loading is impressive if a proper sim shows it to be accurate.  :notworthy:

EDIT2: What sim software did/do you use?

EDIT3: Thus far, was planning on an all SB Acoustics affair with higher efficiency than the Dayton drivers. I was planning on using this driver which is probably a bit better in a ported box than a sealed one, although good enough for either: https://www.falconacoustics.co.uk/downloads/SBA/8in SB23MFCL45-4.pdf

 

[bobovox]

Thanks for this, I will look into a Sim with the software I am using. I do hope group delay for that much low output is not high.

I think you're educating me. 

EDIT: That you've got deep bass in a smallish enclosure without isobaric loading is impressive if a proper sim shows it to be accurate.  :notworthy:

EDIT2: What sim software did/do you use?

EDIT3: Thus far, was planning on an all SB Acoustics affair with higher efficiency than the Dayton drivers. I was planning on using this driver which is probably a bit better in a ported box than a sealed one, although good enough for either: https://www.falconacoustics.co.uk/downloads/SBA/8in SB23MFCL45-4.pdf

I'm just using the basic equations relating driver f_s, Q_TS V_as with box volume Vc to give system Q_TC & f_c & f_3, put in a simple spreadsheet.

The Linkwitz transform is extending the bass response by an octave. There is a spreadsheet on the MiniDSP website that will calculate the biquad coefficients and visualise the original and target roll-offs and the transfer function. The LT is adding around 6dB at 35Hz so amplifier power requirement is quadrupled. High power class D amps are  relatively cheap these days but we need to take care with the power handling of the drive units.

SBA are in the habit of quoting peak to peak Xmax - most manufacturers quote the amplitude (+/-) value. With +/-12mm and two drivers you get around 103dB at 1m.

 

[Metatron]

I'm just using the basic equations relating driver f_s, Q_TS V_as with box volume Vc to give system Q_TC & f_c & f_3, put in a simple spreadsheet.

The Linkwitz transform is extending the bass response by an octave. There is a spreadsheet on the MiniDSP website that will calculate the biquad coefficients and visualise the original and target roll-offs and the transfer function. The LT is adding around 6dB at 35Hz so amplifier power requirement is quadrupled. High power class D amps are  relatively cheap these days but we need to take care with the power handling of the drive units.

SBA are in the habit of quoting peak to peak Xmax - most manufacturers quote the amplitude (+/-) value. With +/-12mm and two drivers you get around 103dB at 1m.

I will check the spreadsheet out and miniDSP. I downloaded WinISD, but it renders painfully small on my 4k screen, so I tried playing with the Windows compatibility properties for it, but it won't play ball and scale legibly.

Outside of that I have Vituix, which I need the FRD and and similar files I can get at Parts-Express, but not for the SBA drivers or many others that Parts-Express don't sell.

I don't care about matching D&D's 106 db@1m output, I care more for sound quality and minimizing group delay, which often is worse the deeper we make the bass response. At most I'd tolerate 12ms group delay below 50Hz, ideally 6ms or less. Noted the p-p excursion on the SBA datasheets. 

Thanks for the pointers.