Open Baffle With a Cancelling Driver, Anyone?

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I like the open and uncolored sound of open baffles, but they are simply too large (or requires too much power and equalization), so I came up with such an idea, that is to add another driver enclosed in a box that is relatively large for the driver (the driver is wired in phase as the primary driver), and firing up to cancel the rear radiating wave of the primary driver.

I am not expecting the same level of performance of a real OB, but at least an improvement over original closed box.

The most likely candidate for the driver will be the Tangband W3 1364SA.

Can anyone give me some advice?:D
 

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Hi,

I like the open and uncolored sound of open baffles, but they are simply too large (or requires too much power and equalization), so I came up with such an idea, that is to add another driver enclosed in a box that is relatively large for the driver (the driver is wired in phase as the primary driver), and firing up to cancel the rear radiating wave of the primary driver.

If both drivers are correctly equalised to have the same frequency response you will have made a cardiode system. This is probably not what you had in mind, but it is a good idea... I have been exploring such systems before but with much larger drivers and DSP to do the EQ.

Ciao T
 
You need to low-pass the cancelling driver (a single inductor will be fine for this).
I agree that sealed is the way to go for this.

You'll need minimal eq with this kind of set-up. A design like this took a 3" driver down to 60Hz. It couldn't go loud at that, but that's where it went to. Sounded lovely. Just limited SPL (small driver, what's to be expected?)

Chris
 
interesting idea....not new i guess, from what others have said, but interesting nevertheless.

With a 90 degree angle of incidence you arent going to get anywhere near total cancellation though are you? I would be tempted to try 2 OBS back to back, with the rear one (the canceller) loaded into a closed box, (or even aperiodic TL) as described by you. That way you could play with their separation distance, to see what effect that would have on FR or SQ.

Obviously for the best results, ALL drivers would need to be the same
 
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interesting idea....not new i guess, from what others have said, but interesting nevertheless.

With a 90 degree angle of incidence you arent going to get anywhere near total cancellation though are you? I would be tempted to try 2 OBS back to back, with the rear one (the canceller) loaded into a closed box, (or even aperiodic TL) as described by you. That way you could play with their separation distance, to see what effect that would have on FR or SQ.

Obviously for the best results, ALL drivers would need to be the same

By putting two OBs together, it would be something like a push pull (isobaric loading?) design, which should work well (but I'm not sure if it is art deco friendly)

You're right, I agree that identical drivers will perform better (more consistent between the two drivers)
 
Hi borispm,
I made boxes two months ago and have 4 Vifa P21 as a crude test of this concept, but have lack of time for hobbies. Maybe it could be done this week, just maybe...

Unfortunatly, I have no means of measurments, so take it for what it is, purely subjective.

Peter

As long as someone can prove this idea work, I'll be happy to how it will sound like. Try it if you have time, Peter, I want to know what you think about it after finishing your own prototype.
 
interesting idea....not new i guess, from what others have said, but interesting nevertheless.

With a 90 degree angle of incidence you arent going to get anywhere near total cancellation though are you? I would be tempted to try 2 OBS back to back, with the rear one (the canceller) loaded into a closed box, (or even aperiodic TL) as described by you. That way you could play with their separation distance, to see what effect that would have on FR or SQ.

Obviously for the best results, ALL drivers would need to be the same

I'm curious - why do you say you won't get near 100% cancellation?
I did try the speakers one behind the other, but the lovely open OB sound just doesn't work when you have a near-field reflector. The bass might be improved (though I doubt it...), but the strange reflections would put me off.

Chris
 
Something like the Tekton OB, but I'm not so sure about the bass reflex design of the cancellation driver, because it DOES add resonance (and more box-sound) to the cancellation driver than a seal box will.

IMO that "Tekton" design using a BR cabinet is not a bad idea at all.

At the very low end the contribution of the small OB will be close to
nothing compared to the contribution of the boxed woofer due to
baffle diffraction.

So the decision is mainly if you want a closed or a BR box at the low end.

Since a BR design with an appropriate driver and alignment can do
a better compromise between size, max spl and lower frequency limit,
it is still attractive IMO.

When the path length of the OB driver to the edge of the baffle
- dipole path length - comes into the range of a quarter wavelength,
the OB contribution will be significant and the whole thing will
morph into a cardioid.

When again the distance between the rear of the OB driver and
the "cancellation driver" becomes comparable to wavelength,
the cancellation will not work properly anymore and get frequency
dependent too.

So low pass filtering might be a good idea (maybe 1st Order with
compensation of voice coil inductance).

Then you will have a dipole from upper bass/lower midrange
range upwards.

To maintain cardioid like radiation in the midrange one could try to
damp the rear radiation in the midrange with some felt or
stuffing material held by a very open structure (a net e.g.).

The trick in that system will be to find the baffle size in a right
way, to integrate with the box seamlessly from mid- to upper
bass. Even the low pass filter will need some experimentation.

It is nevertheless an interesting concept IMO.

Giving the OB driver its own amp and a 1st order line level
highpass filter with fc in the range of its resonance or a bit
above would reduce excursion of the OB driver
in the deep bass and increase dynamic headroom of the
whole thing without affecting sonic impression very much -
despite of less distortion.

It is an OB with boxed subwoofer, serving as cancellation driver
from midbass on.

Regards
 
IMO that "Tekton" design using a BR cabinet is not a bad idea at all.

At the very low end the contribution of the small OB will be close to
nothing compared to the contribution of the boxed woofer due to
baffle diffraction.

So the decision is mainly if you want a closed or a BR box at the low end.

Since a BR design with an appropriate driver and alignment can do
a better compromise between size, max spl and lower frequency limit,
it is still attractive IMO.


Regards

I thought Tekton had the bass reflex box because the Fostex drivers he uses do not have much bass in a closed box (from winISD it rolls off at around 160Hz!)
But if I were Tekton, I would have made it a BR as well to get more bass from those Fostex drivers.

Thank you for your suggestion, Line Array. I will try to investigate more about the distance between the cancellation driver and the OB driver.

BTW, can I omit the low pass if I were to mount the cancellation driver in an inverted manner (with the magnet facing the outside?):rolleyes:

I thought the rear radiating part radiates up to mids only. :D
Please let me know if I were wrong about that. :p
 
It would appear that the design has been mis-understood.
The rear-mounted speaker has only one function - to cancel the rear-wave of the OB driver at low frequencies, thus giving cardiod bass radiation.

Porting it means you get a dip in the excursion-frequency graph, so it's not cancelling the rearwave at that frequency. However, the port will be radiating that frequency, so you'll get monopole bass (not cardiod) at that frequency.
It gets worse when you go below the port tuning, because then the rear-mounted driver has little output (it's cancelling itself out due to being unloaded), so you'll get little bass from the OB driver, which now has to cope on it's own.

Seems to me like a bigger-than-average sealed box would be the way to go, in order to...
- keep the excursion-frequency graphs looking similar
thus
maintaining effective cancellation all the way down to a theoretical 0Hz.

Chris
 
It would appear that the design has been mis-understood.
The rear-mounted speaker has only one function - to cancel the rear-wave of the OB driver at low frequencies, thus giving cardiod bass radiation.
...

Hi Chris,

i do not think there is a misunderstanding from my side.

This design - like every one - lives from dimensioning.

If You want the "puristic appoach" with similar excursion vs. frequency
from OB and cancellation driver, the "big closed box" with
Vb>>Vas is the way to go, agreed.

But it you want a real cardioid down to fs of driver, say 30Hz,
the first thing you need is an OB design that goes that low.

Since the cardioid can be seen as superposition of dipole and monopole,
it is clear from dimensioning of the "Tekton" design, that at low frequencies
the dipole contribution is very small due to the toy baffle size.

In the bass the "Tekton" OB is a sonic dwarf and the box is a giant in
comparation.

I bet, if you apply a 40Hz tone to the system and then switch on and
of the OB driver it will be very hard to hear or measure any difference:

Moving the system half a meter in the room will most probably
produce a bigger differency in frequency responce at deep bass.

As i understand the design, it is aimed to have a small baffle
but nevertheless an "OB" like directed sound from midbass upwards,
without loosing low bass.

The price: Mainly monopole radiation in the deep bass, because small
dipole contribution. BR loading does not really affect the design,
because the BR tuning frequency will be typically in a frequency
range, where the OB has already rolled off maybe about 6-12 db
dependent on size.

To be "puristic" you will have to upsize the OB component, but then
the design looses its charme, which comes from being tiny IMO.

Just my 2ct ...
 
...

Thank you for your suggestion, Line Array. I will try to
investigate more about the distance between the cancellation driver
and the OB driver.

BTW, can I omit the low pass if I were to mount the cancellation
driver in an inverted manner (with the magnet facing the outside?):rolleyes:

I thought the rear radiating part radiates up to mids only. :D
Please let me know if I were wrong about that. :p


The difference in front and rear radiation depends on form
of spider and basket. Normally in small to mid sized drivers
with open basket, there is little difference below 1Khz.

The low pass corner frequency will be somewhere in the "hundrets"
IMO, so turning the driver inside out will nor really help.

The "OB rear side driver" and the "cancellation driver" will form
an additional "unwanted dipole" when their distance will be about
1/4 wavelength or larger (340Hz is approx 1m Wavelength).

The effect starts below that frequency and will increase from there
with rising frequency. That dipole with vertical radiation axis
(when assuming "Tekton" like configuration) will disturb the radiation
pattern and lower the intended cancellation effect.

The farther apart the drivers are, the lower the corner frequency
of the low pass should be. A small distance would be desirable.


To avoid "box coloration", which is one of the main goals of the
design as i - and i guess you too - understand it, it is favourable
to have the lowest box resonances due to standing waves well above
the corner frequency of the "cancellation driver". This is an additional
argument for the lowpass filter: IMO it can be a very important
component in the design for avoiding "box coloration".

To have the box compact, rigid built and well damped is desirable.
In fact its function is subwoofer like with an additional purpose
of rear cancellation.

A rear absorber on the OB could be designed to substitute
the cancellation effect from say 300 Hz upwards, so the design
could be a cardioid even in the midrange.

For uniform horizontal radiation pattern of a dipole baffle,
the baffle width should not exceed about 2.2 X diameter of the
driver to avoid side lobes.


You will find information on dipol path length and baffle size
issues on e.g.


Music and Design
Linkwitz Lab - Loudspeaker Design
Dipolplus


A commonly used tool for baffle diffraction estimation is "The Edge"
from Tolvan Data. It does only estimate baffle diffraction, not the
additional rolloff due to the driver parameters, but you can switch
between "open baffle" and an idealized closed box of same front
baffle size e.g. to play around with the effects i mentioned.

Cheers
 
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Just an idea:

Maybe an open "absorber chamber" like a net or a mesh could cover
both drivers, mounted with magnets facing outside, baskets
protecting cones them from touching the absorber material.

That "absorber chamber" could look very cool if encased in a fabric
or metal mesh illuminated from inside, magic !

Outside a small warning sign:

CAUTION ACOUSTIC CANCELLATION ZONE !

:D
 
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I see the problem here.
You seem to be visualising the system as an OB with bass helper.

Instead, try to see that it's more like a way of creating an infinite baffle system. If the rear-wave is completely cancelled, the result will be exactly monopole radiation.

The width of the baffle is irrelevant to the low frequencies in this design. On a standard OB, it would determine when the rear-wave begins to cancel the front-wave (the one we need). The box behind it means there is no rear-wave. It's turned into pressure inside the box. However, because the rear-wave from the OB speaker has met no resistance in being cancelled, it sees an infinite baffle.


I would agree that some mesh, illuminated (blue LEDs, anyone? or better still, standard filament bulbs in parallel, light up when the bass is being cancelled.) would look extremely cool.

Chris
 
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