Someone on Facebook was asking about the Danley BC subs, and I personally think that the concept is fairly straightforward, and it's come up on FB a few times so I thought I'd do a quick "brain dump" here.
On Facebook, I (sorta) joked that: "'Boundary Coupled' is just marketing speak for "the baffle is really big.'"
So let's elaborate on this a little bit:
First off, the primary reason that radiator has higher output on axis when it's on a horn or waveguide is because the waveguide takes the output from a radiator and constrains it into a narrower beam.
This is no different that now a light bulb can barely illuminate a room, but a ten watt diode laser can cut through wood:
A diode laser takes it's energy and focuses it in a point, a woofer in a baffle that is sufficiently small will radiate in all directions (spherical) and if you put a radiator onto a big flat baffle, the baffle itself will keep the energy from radiating backwards.
Next thing that we do is figure out "What is that frequency?"
Danley BC-218 measures five feed wide by five feet tall by 2.5 feet deep. That's a baffle face of 152cm x 152cm. The exit of the BC218 is NOT in the center of it's baffle, and that has an impact, but describing that effect is beyond the level of this discussion. (You can sim it in ABEC or VituixCad.)
This next thing is pretty important I think: Keep in mind that the radiation from the radiator has to wrap around the enclosure before it can radiate in 360 degrees. If we just looked at the front of the baffle, we could constrain radiation down to 224Hz. This is because the front of the baffle is 152x152cm and 224Hz is 152cm long.
BUT - once we include the sides of the enclosure, we're looking at radiation control down to 112Hz. That's because the additional thirty inches, on each side, increases the baffle size to 120 inches wide.
Now at his point you're probably wondering "what's the point of a subwoofer that "controls radiation" down to 112Hz?"
The answer to that, for the Danley BC Subs and Danley's B-DEAP subs that he made at Sound Physics Labs, is that they're really designed to be arrayed. That's why I didn't do a deep dive into the mouth location. The subs don't make a lot of sense when used in singles. Add a second BC-218 sub and you're controlling radiation down to 56Hz, easy.
The next step in this, is that once you place the sub on the floor, the floor itself increases the on-axis efficiency. This ALSO gets tricky, because the floor isn't a perfect mirror of the loudspeaker. But chances are good that you will be putting your speaker or subwoofer on a floor, so it stands to reason that you would want to leverage it to improve the performance of your speaker.
On a side note, if it's not obvious, you could avoid a lot of this by just cramming a subwoofer into a corner. But BC subs arguably are best outside or in a big room, where that huge baffle can constrain radiation. I think this is particularly important for festivals where you don't want the subwoofer to "bleed" into the next stage over, or the stage behind it.
I think it's also useful for clubs and the like, because it's controlled radiation reduces energy radiated into adjacent buildings.
On Facebook, I (sorta) joked that: "'Boundary Coupled' is just marketing speak for "the baffle is really big.'"
So let's elaborate on this a little bit:
First off, the primary reason that radiator has higher output on axis when it's on a horn or waveguide is because the waveguide takes the output from a radiator and constrains it into a narrower beam.
This is no different that now a light bulb can barely illuminate a room, but a ten watt diode laser can cut through wood:
A diode laser takes it's energy and focuses it in a point, a woofer in a baffle that is sufficiently small will radiate in all directions (spherical) and if you put a radiator onto a big flat baffle, the baffle itself will keep the energy from radiating backwards.
Next thing that we do is figure out "What is that frequency?"
Danley BC-218 measures five feed wide by five feet tall by 2.5 feet deep. That's a baffle face of 152cm x 152cm. The exit of the BC218 is NOT in the center of it's baffle, and that has an impact, but describing that effect is beyond the level of this discussion. (You can sim it in ABEC or VituixCad.)
This next thing is pretty important I think: Keep in mind that the radiation from the radiator has to wrap around the enclosure before it can radiate in 360 degrees. If we just looked at the front of the baffle, we could constrain radiation down to 224Hz. This is because the front of the baffle is 152x152cm and 224Hz is 152cm long.
BUT - once we include the sides of the enclosure, we're looking at radiation control down to 112Hz. That's because the additional thirty inches, on each side, increases the baffle size to 120 inches wide.
Now at his point you're probably wondering "what's the point of a subwoofer that "controls radiation" down to 112Hz?"
The answer to that, for the Danley BC Subs and Danley's B-DEAP subs that he made at Sound Physics Labs, is that they're really designed to be arrayed. That's why I didn't do a deep dive into the mouth location. The subs don't make a lot of sense when used in singles. Add a second BC-218 sub and you're controlling radiation down to 56Hz, easy.
The next step in this, is that once you place the sub on the floor, the floor itself increases the on-axis efficiency. This ALSO gets tricky, because the floor isn't a perfect mirror of the loudspeaker. But chances are good that you will be putting your speaker or subwoofer on a floor, so it stands to reason that you would want to leverage it to improve the performance of your speaker.
On a side note, if it's not obvious, you could avoid a lot of this by just cramming a subwoofer into a corner. But BC subs arguably are best outside or in a big room, where that huge baffle can constrain radiation. I think this is particularly important for festivals where you don't want the subwoofer to "bleed" into the next stage over, or the stage behind it.
I think it's also useful for clubs and the like, because it's controlled radiation reduces energy radiated into adjacent buildings.
My original thread on this, from 9.5 years ago: https://www.diyaudio.com/community/...t-reverse-engineering-a-danley-design.262490/
A few other observations:
Possibly one of the reasons that these boxes are a bit of a mystery to people is that they're not easily simulated. A BC218 is very unlikely to be mounted up in the flair, so there's a "base" assumption that we can use the floor as an acoustic mirror.
As Art observed in the thread linked in post two, the orientation of the subwoofer box makes a difference here. The orientation of the box changes the area of the mouth and that changes the acoustic impedance. Martin King describes this in one of his threads, and I think I made one about it, around four years ago. Martin calls it a "mass loaded transmission line" but the concept also applies to tapped horns and front loaded horns.
Taken to the extreme, someone who was renting these might even have a sub box design that added barn doors are perhaps had an adjustable mouth. Adjusting the mouth size by tipping the box over is brilliant, because simply masking the mouth off with plywood would be prone to rattling.
Possibly one of the reasons that these boxes are a bit of a mystery to people is that they're not easily simulated. A BC218 is very unlikely to be mounted up in the flair, so there's a "base" assumption that we can use the floor as an acoustic mirror.
As Art observed in the thread linked in post two, the orientation of the subwoofer box makes a difference here. The orientation of the box changes the area of the mouth and that changes the acoustic impedance. Martin King describes this in one of his threads, and I think I made one about it, around four years ago. Martin calls it a "mass loaded transmission line" but the concept also applies to tapped horns and front loaded horns.
Taken to the extreme, someone who was renting these might even have a sub box design that added barn doors are perhaps had an adjustable mouth. Adjusting the mouth size by tipping the box over is brilliant, because simply masking the mouth off with plywood would be prone to rattling.
Tom Danley's "Bdeap" (Boundary Dependent External Air Path) shows boundary dependent low frequency extension when a single cabinet front face is placed various distances from a wall corner junction.
https://www.freepatentsonline.com/20040238268.pdf
The BC (Boundary Coupled) front loaded horns (FLH) may be arrayed or used singly and still have considerable pattern control well below the wavelength of the baffle and cabinet sides, 112Hz by your estimation.
Increased on-axis forward gain up to +4dB from the cabinet frontal boundary below 70Hz is evident in the "BC18 flat" compared to the "BC218 half moon" vertical orientation:
A +6dB gain when doubling equally driven low frequency cabinets is standard, gain above that level is boundary dependent forward gain.
The BC218 relative lower frequency forward gain is actually more in the single cabinet vertical "half moon" orientation than the two-cabinet "end to end" array.
The BC412 and BC415 use single center mouth locations, the BC215 and BC218 dual side mouth locations offer more options for coupling between themselves and external boundaries.
Art
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I love reading discussions like this, thanks to you two in particular, for a straightforward and no-BS practical breakdown of technical concepts.
I for one would love to figure these things out.
Looking at these and hearing them a couple of times at INFOCOMM and hearing them outdoors a few times I think they sound quite a bit better than tapped horns.
I have THMINI’s and had TH115’s and have spent a lot of time with them.
I am looking forward seeing / making some progress on the BC discovery.
Barry.
Looking at these and hearing them a couple of times at INFOCOMM and hearing them outdoors a few times I think they sound quite a bit better than tapped horns.
I have THMINI’s and had TH115’s and have spent a lot of time with them.
I am looking forward seeing / making some progress on the BC discovery.
Barry.
None of the DSL (Danley Sound Labs) loudspeakers use the old Intersonics SDL (Servo Drive Loudspeaker) servo motor technology.
The SDL BT-7 had just over 1" peak to peak excursion, an Xmax of ~12mm.
The linear excursion potential of standard moving coil transducers surpassed the rotary servo motor to belt-drive system last century.
The drivers used in the DSL BC don't look as interesting, but have considerably more displacement capability.
A manufacturer can always choose to simply prove the function of their design by providing acoustic measurements.
Making a really large, cool looking sub, inventing a marketing term like "Boundary Coupled", leaving the actual facts and data completely off the table and instead creating a mystique around your product will probably market & sell better though.
Additional note: when assessing loudspeakers / loudspeaker arrays of this size, always take special care to determine if you're actually in the far field of the system. The near field can easily extend to several meters, and the behavior of the speaker (array) will be significantly different within.
Making a really large, cool looking sub, inventing a marketing term like "Boundary Coupled", leaving the actual facts and data completely off the table and instead creating a mystique around your product will probably market & sell better though.
Additional note: when assessing loudspeakers / loudspeaker arrays of this size, always take special care to determine if you're actually in the far field of the system. The near field can easily extend to several meters, and the behavior of the speaker (array) will be significantly different within.
Although DSL does invent plenty of "marketing terms", they do measure all their subs outdoors at 10 meters with 28.3 volts, and usually also provide the impedance as well as the frequency response charts.
The BC218 frequency response chart in post #4 sensitivity must be weighed against it's minimum impedance of 3 ohms (2x6ohm) at 90 Hz, but at least it is right there on the impedance chart:
Wish more did, it's difficult to compare sub specs between different manufacturers.
The BC218 frequency response chart in post #4 sensitivity must be weighed against it's minimum impedance of 3 ohms (2x6ohm) at 90 Hz, but at least it is right there on the impedance chart:
As DSL does.
Wish more did, it's difficult to compare sub specs between different manufacturers.
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Nobody produces these drivers like "old Intersonics SDL (Servo Drive Loudspeaker)" any more?
Practically it could do more cone travel than classical drivers and there are no thermal or thiele small restrictions.
It should be easier to get undistorted sound?
Practically it could do more cone travel than classical drivers and there are no thermal or thiele small restrictions.
It should be easier to get undistorted sound?
Not commercially.
All motors have thermal limits (restrictions).
The TS parameters could be measured, but practically the stiff, heavy cones mass caused the belts to shred quickly if run too high in frequency.
The stepper motors Intersonics used would reach their thermal limits at ~1" peak to peak for two 15" cones, and after many initial burnouts, used fan cooling.
They also were a tough load for most amplifiers, later versions also incorporated a transformer.
There was nothing "easy" about the "servo drive loudspeaker" 😉
At the time they were developed, there were few or no drivers designed specifically for high LF excursion in horn loaded applications.
Their excursion potential was about double of most drivers of the time, so they could go around +6dB louder at similar distortion levels.
The stepper motors the SDL used went out of production, suitable replacements were more expensive.
It didn't take long (in historical terms) for conventional drivers to surpass the undistorted output potential of the SDL at lower cost.
Thank you for information.
Nobody builds (bass) loudspeakers with a moving magnet (at the driver, maybe small long neodym) and a fixed voice coil. The voice coil could be perfectly cooled like this.
I can only imagine that the small magnet would not be strong enough or if strong enough would be too heavy.
Nobody builds (bass) loudspeakers with a moving magnet (at the driver, maybe small long neodym) and a fixed voice coil. The voice coil could be perfectly cooled like this.
I can only imagine that the small magnet would not be strong enough or if strong enough would be too heavy.
The Powersoft M-Force driver and amp system has a moving magnet with fixed coil
https://www.powersoft.com/en/products/transducers/m-system/m-force-301p02/
https://www.powersoft.com/en/products/transducers/m-system/m-force-301p02/