Insights
The Repel-Attract Driver (RAD): A Technical Deep Dive
RAD draws on more than 30 years of electromechanical research. It is protected by 9 issued US patents, with more than 10 pending.
By Brane Audio6 min read
The Road to RAD
While there has been massive innovation in audio over the past century, from solid state amplifiers, high energy batteries, digital signal processors, and voice control, there has been virtually no innovation on the basic audio driver. The Repel-Attract Driver (RAD) is Brane Audio's proprietary speaker driver technology and it is considered the most significant advancement in audio driver technology in over a century.
RAD is the first commercial driver to overcome Hofmann's Iron Law: the rule that states a speaker can be small, efficient, or produce deep bass, but never all three at once. RAD breaks the Iron Law through an array of permanent magnets that cancel the internal air pressure forces inside an enclosure. The first proof of concept is Brane Audio’s flagship product Brane X, a 7.7 pound speaker that gets down to 27.1 Hz and has up to 12-hours of battery. This bass output, size and efficiency was once thought to be impossible.
RAD draws on more than 30 years of electromechanical research. It is protected by 9 issued US patents, with more than 10 pending.
Below is a deeper technical dive on our groundbreaking technology.
The problem: Hofmann's Iron Law
Hofmann’s Iron Law has governed speaker engineering for a century. The longstanding audio law states that in order to produce deep bass, a speaker must either be large or draw a tremendous amount of electrical power. In other words, no speaker on the market can be simultaneously small, efficient, and produce deep bass, until Brane Audio.
The Primary Cause = Air Pressure
The root cause of the Iron Law is the internal air pressure variation within a speaker cabinet. Deep notes require large excursions of a speaker cone/panel as it moves in and out of its enclosure. These excursions create large variations in internal air pressure that must be overcome with electrical power routed to a voice coil actuator. If the speaker enclosure is small, the required amount of electrical power is large. To reduce electrical power, the speaker enclosure size must increase to lower internal pressure variations. This power and size tradeoff has limited the performance of speakers for decades.
How RAD Works: the Magnetic Negative Spring
RAD combats the air pressure by using an array of permanent magnets that perfectly cancels out the internal air force pressure.
RAD deploys a novel “magnetic negative spring” (MNS). Unlike a conventional spring with a force that is oppositely directed to its direction of motion, an MNS must produce a force in the same direction of motion. This force must rise linearly with increasing distance.
Picture a baseball balanced exactly on the ridge of a roof. At the peak it stays put. Nudge it slightly and it doesn’t come back. It rolls further away, and faster the further it goes. That’s an MNS. Once the panel moves off center, the force pushes it further in the same direction, and that force grows with distance.
The RAD needs a four-axis passive magnetic bearing to keep the magnets centered radially, along with an active Lorentz force and axial magnetic bearing to keep the magnets axially centered when music is not playing.
The net force is about tenfold smaller than it would be for a conventional speaker.
Asymmetrical Force Compensation
Because air resists compression more than expansion, a symmetrical negative spring would leave a residual imbalance. RAD shapes its magnetic forces asymmetrically to match the air's behavior on each side of the stroke. Matching the curve, not just the slope at center, is one way that RAD holds its advantage across the full excursion.
Real-Time Position Sensing
RAD tracks the voice coil position at all times with a dedicated high-excursion position sensor. That data feeds back to the amplifier. If the coil is off center at a zero crossing of the music signal, the amplifier adds a small corrective force to recenter it. When music is not playing, the active axial bearing holds the magnets centered.
The Launch System
When a RAD speaker is off, the negative spring pulls the panel to one end of its travel and it rests there. Pulling it back to center with the amplifier alone could take up to 1,000 W, far more than a portable speaker can supply.
Instead, a small air pump handles startup. It adjusts the pressure inside the enclosure to move the panel to center, then hands control to the position sensing system. The pump stays on standby during playback to recenter the panel in rare events, such as operation at high altitude where the air is thinner.
Earlier negative spring designs ran an air pump continuously, which added noise and slow response. RAD uses it only at launch and as a backstop.
The Result: Brane X
Brane X is the first portable speaker in its size class with an internal subwoofer, and it exists because of RAD. These numbers map directly to the Iron Law. A conventional design would need a much larger cabinet or a much larger battery to reach 27.1 Hz at this output. RAD removes most of the air spring force, so the amplifier and battery can stay small.
Why earlier attempts failed
No law of physics rules out a negative spring woofer. There have been multiple attempts in speakers as early as the 1950s, and the issue has always been control system stability. Most earlier designs tried to run the negative spring as a passive system, without a dedicated amplifier, signal processing, or real-time position sensing. Every fix for one instability tended to create another.
RAD2: Solving Inertia
RAD2 is the next generation of the Repel-Attract Driver. On top of the core air force cancellation, RAD2 minimizes the power required to overcome the inertia of the moving sound panel by dynamically adjusting the driver’s resonant frequency as a function of the music it is reproducing. If a pop song has a repetitive 25 Hz note, RAD2 will adjust its resonant frequency to 25 Hz to minimize power draw.
Scaling RAD
RAD is not tied to one product size. Our research shows the technology can scale up to concert-size speakers and down to earbuds while keeping an order-of-magnitude advantage in both size and power consumption.
A speaker that’s a fraction of the size needs less material to build and weighs less to ship. Across the billions of speakers in phones, cars, homes, and venues, those savings add up to a meaningful drop in the energy and materials audio requires.
Backed by 9 Issued Patents, with 10+ More Pending
RAD and RAD2 are protected by the following issued US patents.
Independent Review and Recommended Reading
In its November 2024 issue, Voice Coil magazine ran a patent review by James Croft of Croft Acoustical covering two RAD patents. Croft has reviewed more than two dozen negative spring woofer patents over three decades for the publication.
According to the audioXpress summary of that issue, Croft found that RAD, with its high-excursion position sensor, can claim a unique breakthrough on the negative spring concept. He also notes the concept's potential to deliver bass at least one to two octaves below Iron Law limits.
Read the issue summary: Reinforced Loudspeaker Performance in Voice Coil November 2024 (audioXpress).
Frequently Asked Questions
What is the Repel-Attract Driver (RAD)?
RAD is Brane Audio's proprietary speaker driver technology. It uses a magnetic negative spring to cancel the air pressure forces inside an enclosure, letting compact, battery-powered speakers produce deep sub-bass.
What is Hofmann's Iron Law?
Hofmann's Iron Law states that a loudspeaker must trade off low-frequency extension, enclosure size, and efficiency. Deep bass has traditionally required either a large cabinet or high power.
What is a magnetic negative spring?
A magnetic negative spring produces a force in the same direction as displacement, rising linearly with distance. In RAD, it offsets the air spring inside the enclosure, cutting the net force on the moving panel by about 10x.
Is the negative spring woofer a new idea?
No. The first patents date to 1955, and more than two dozen followed. RAD is the first to reach the market as a commercial product, thanks to active position sensing, magnetic bearings, a dedicated control system, and several other innovations.
How does RAD keep the speaker cone centered?
A four-axis passive magnetic bearing keeps the magnets centered radially. A high-excursion position sensor coupled with an active Lorentz force and active axial magnetic bearing keeps them centered axially.
What happens when a RAD speaker turns on?
When the speaker is off, the panel rests at one end of its travel. At startup, a small air pump moves it to center, then active position sensing takes over.
How much power does RAD save?
Brane X uses about 1/10th the power of a conventional speaker in the subwoofer range, while delivering a 10x increase in sub-bass.
What is the difference between RAD and RAD2?
RAD cancels air pressure forces. RAD2 also cancels inertia by tuning the driver's resonant frequency to the music in real time, delivering 30x deep-bass output per unit of size and power.
Which products use RAD?
Brane X runs on RAD. Future products will run on RAD2.
Is the Brane X's RAD technology patented?
Yes, Brane X and its Repel-Attract Driver technology are protected by multiple issued and pending patents.
Can RAD scale to other product sizes?
Yes. Our research shows RAD can scale from earbuds to concert-size speakers while keeping an order-of-magnitude advantage in size and power consumption.
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