The Ultimate 2-Way System?

Bohlender-Graebener 75″ Ribbons + Titanic Subs
+ Active Digital Crossover + Digital EQ +
100% Integration with Room Boundaries…

= Speed, High Resolution, Extreme Transparency and
Visceral Bass Power All The Way Down to 20Hz

I love simplicity.

The most eccentric feature of my library, where I do most of my working and writing, is these ominous-looking ribbon speakers. It’s like a monstrous pair of electrostatic headphones. You can see the subwoofer in the corner.

Few things are more satisfying than taking an absolute minimum of “moving parts” and assembling them into a final creation that’s both powerful and elegant. This philosophy extends to everything I do.

Immediately after college, I worked at Jensen as an acoustical engineer. I designed OEM speakers for Honda, Mazda, Chrysler and Acura. Most of those designs were single full-range speakers, made on a penny-shaving budget.

I had to make a silk purse out of a sow’s ear and loved the challenge of covering the entire audible range with a $3-$5 driver.

If you have to cover the entire audible range with one small speaker, you have a nearly impossible challenge. But for years I dreamed of stretching the limits of 2-way design to the absolute max.

I had a hunch that by choosing two of the right drivers, I could create something that would rival the best and most expensive designs on the market.

After being out of the audio biz for 10 years the siren song to work on a really cool project was beckoning loud and clear. The goal: A simple, elegant, ultra-high performance 2-way system.

Inspired by my friend Tom Perazella (whose design also features the Bohlender-Graebener ribbons) this system strategically integrates high-performance drivers and room acoustics with Digital Signal Processing crossovers and EQ.

If you’re playing heavy metal, this system rocks. If you’re playing chamber music, it sounds like chamber music should sound: Intimate, transparent, soothing and stimulating at the same time.

All the virtues of electrostatics and dynamic drivers combined, with none of the drawbacks. (Sorry, electrostatics just don’t do justice to hard rock. This system does!)

The resolution is incredible. A friend of mine who used to work at a recording studio came over and told me this was the cleanest sound he’d ever heard in his 66 years. More than six octaves are seamlessly reproduced by a single, high-output, fast, linear, ultra-low mass driver.

With proper EQ the subs go all the way down to 20 Hz. You get body-pounding bass, bass that you can feel more than hear. Because of the strategic alignment between crossover points, room boundaries and speaker location, room resonances and boundary problems are effectively ‘factored out.’

I’ve always loved 2-way systems, the only problem is, with traditional approaches you never get the power and bandwidth you want. The Bohlender-Graebener RD75 Ribbon, however, with its 150 Hz – 20 KHz response and dynamic range, combined with digital crossovers and active amplification, changes all that.

The key concepts in this design are:

  • Set the crossover point so low the woofer can couple to the floor and both walls through its entire range – providing 9dB of extra output and no cancellation from wall reflections. (This 9dB is huge. It’s better than having twice as many woofers, especially in the mid-bass.)
  • Cover the entire mid and high frequency range with ONE driver for truly seamless reproduction of harmonics all through the range.
  • Use a line source for the mid- and high- frequency driver to obtain consistency throughout the room and eliminate floor and ceiling reflections.

    Frequency response and impulse response, a few inches away from the ribbon: Nearly flat from 20HZ to 20KHz. Note the impulse response and the lack of stored energy – very rare at any price. Also note: The subwoofer is 2 feet away from the microphone here, so everything below 200Hz includes the room. Otherwise the bass would be as flat as the mids and highs.

When I was 14 years old I read Roy Allison’s landmark paper on the effect of room boundaries on loudspeakers and discovered that the worst possible place to put a woofer is exactly where most high-end speakers put them – away from the walls and a foot or two off the floor. Absolutely terrible.

The best place to put a subwoofer is in the corner – IF you can deal with the time delay and the crossover issues. With Behringer’s digital crossovers I gained complete control over these things, so now the subwoofers drive the entire room with no cancellation, up to the crossover frequency of 200 Hz.

The subs easily rattle the walls and doors and you can feel the bass even at moderate volume levels.

The crossover is a 12dB per octave (48dB is possible, but faster slopes sound worse because the filters ring), the sub and the satellites are in time alignment. The floor and walls are factored out of the subwoofer’s response, and the floor and ceiling are factored out of the ribbon’s response.

The digital EQ surgically manages standing waves and driver idiosyncrasies.

I tuned the system with pink noise and FFT with 1/12th octave precision and the response is smooth and level everywhere throughout the room. You can stand up, sit down, or even lay on the floor, and sound is coherent everywhere. As soon as your ear rises above 6 feet elevation, the high frequencies vanish. That’s because the ribbon emits a cylindrical wave that does not reach the ceiling.

The Bohlender-Graebener ribbons are finicky and have a number of quirks that have to be dealt with. It would be very hard to make these sound right with a passive crossover. However, the Behringer digital EQ provides almost limitless capacity to fine tune the system, even minor resonances which a passive crossover could never address.

You can clearly hear every layer of detail in the recording. Vocals are lush and full and transparent. Percussion has clarity and drive. Plucked strings are almost visceral, when they need to be.

For bass I chose the Dayton Titanic 12″ Subs, with an amazing 18mm Xmax.

These puppies can shove air an inch in each direction without distortion.

They’re in a small box and EQ makes up for the 40Hz rolloff and extends response flat to 20 Hz. For this space (12′ by 14′) they deliver as much pounding as I want, with very low distortion. Above 25 Hz, this system easily exceeds 110 dB anywhere in the room.

It would be possible to upgrade this to a 15″ subwoofer and get even more bass power. It would be possible to make the cabinet much larger too, and the system would suffer no penalties. After I achieve some new business goals I’ll make that mod :^>

I feel this system has been very successful in achieving its goals. This if course is only possible because of the digital signal processing capability of the Behringer DCX2496 and DEQ2496.

I made extensive use of the parametric EQ, compensating for the horn loading effect of the B-G aperture, several minor high frequency resonances, and a whole series of room modes between 40 Hz and 200 Hz. Such a level of tuning is not possible with off-the-shelf speakers, no matter how costly.

The result is extremely smooth response all the way across the spectrum, at the listening position, not just in a measurement chamber. All kinds of details that other speakers missed become easy to discern.

Even in a real room with reflections and standing waves, frequency response is even and impulse response is excellent. This was measured 1 meter in front of the speaker, in its normal position in my room. Corner placement of the sub greatly facilitates smooth response. I know of no other speaker that performs this well in both amplitude and phase in a real room. Notice that I haven’t romanticized these curves by smoothing them with my measurement software.

I even used the dynamic EQ capability in the Behringer crossover to build a dynamic loudness control. I generally hate loudness controls, they’re usually clunky and out of proportion. But this one boosts the low bass by 9dB at very low levels and as you turn the volume up the accent just gradually disappears.

It’s impossible to notice unless someone tells you it’s there – and of course you can hear the bass much more clearly when you’re playing the system at a whisper. It’s not tied to a volume control knob either, it operates automatically based on signal level.

Great for listening to quiet music while I work or write.

  • I’ve found that most of the time I do not like the way this changes the dynamics, so I usually bypass that function. But it’s always there if I need it.

The Case For A Low-Crossover-Frequency, 2-Way Design
Adapted from an article I wrote in Voice Coil magazine, VC Visits Tangband, 2004

You get more design options with wideband drive units. 90% of hi-fi speaker designs have a tweeter (usually a dome) and a crossover point somewhere between 1,500 and 6,000 Hz. This design is a time-honored tradition with plenty of advantages, including relatively inexpensive crossover components. But this is not the only way to design a 2-way speaker.

Using a wideband HF driver enables the designer to move the crossover frequency down a few octaves. This is a very practical way to solve several difficult design problems. A carefully designed ribbon or planar unit (or 2-3 inch dynamic driver) can operate effectively from 200 Hz to 20,000 Hz, allowing the designer to place the crossover frequency in the lower midrange instead of the upper midrange. This approach has a number of interesting advantages:

You can use a larger woofer in a 2-way system without adding a midrange driver. It’s fairly difficult to match a dome tweeter to a 12” woofer. The directivity and breakup modes of the woofer introduce a discontinuity that’s hard to overcome. But a 500 Hz crossover allows the woofer to operate in its piston band, and a larger woofer handles extended deep bass with less distortion than a smaller unit.

The ear is most sensitive at the exact place most designers put a crossover point.The oft-referenced Fletcher-Munson curves show that we hear best in the 1000 Hz – 5000 Hz band. Any anomalies in the crossover range become blazingly obvious. Moving the

Fletcher-Munson Curves

The famous Fletcher-Munson curves show that the ear is most sensitive at 3KHz – precisely where most speaker designers put their crossover point. Big mistake!

crossover out of that range reduces the severity of the problem.

This give you much more flexibility with respect to room boundaries. Most commercially available speaker designs blatantly attempt to ignore the room, but experienced designers know that room acoustics are as inevitable as death and taxes.

Roy Allison’s papers [1] on room boundaries back in the 1970’s showed that at low frequencies, walls reinforce bass response. But especially from 100-500 Hz, dramatic cancellations occur, wreaking havoc with real-world response in the room.

Allison’s solution, also used by Boston Acoustics, Acoustic Research, NHT and others, was a 3-way system that cooperates with room boundaries. He put the woofer very close to room boundaries, and placed mid- and high-frequency drivers far away.

This is very effective, and it results in far more accurate response in the room. But this is totally useless if you have a 2-way speaker with 3000 Hz crossover frequency. If the woofer has to cover that entire range, cancellations from room boundaries and “floor bounce” are inevitable, no matter how close or far your woofer is from the walls and floor.

On the other hand, if the crossover frequency is well below 1000 Hz, you can effectively couple the woofer to the floor, the rear wall and maybe even the corner. Now you deal with room boundaries much more effectively. By adding a wideband HF driver, you can achieve all of Allison’s goals in a simple 2-way system:

  • Lower THD and IM distortion. If a woofer must reproduce low bass at high SPL’s, it will require long excursions and their associated distortions. All frequencies that it covers will be affected. A low crossover frequency keeps these distortions out of the midrange.
  • More flexibility in cabinet design. To avoid lobing problems at the crossover frequency, you have to locate drivers within one wavelength of each other. At 3000 Hz, the driver centers have to be 4 inches apart – often difficult to achieve. At 300 Hz, you have 40 inches to work with. The woofer can be placed near the floor or corner (and will be tightly coupled to it) and the HF unit can be much further from the floor, and de-coupled from it.
  • Modest directivity at high frequencies. Sometimes a bit of directivity is desirable. If all listeners cannot be seated in the “sweet spot,” the speakers can be toed in. Listeners are off-axis from the speaker which are in closer proximity, and on-axis with the speaker that is further away. This maintains more stable stereo imaging throughout the room.
  • Line arrays. Wideband drivers can be placed very close to each other and used in multiples for higher output and more controlled directivity. The Bohlender-Graebener ribbon is a pure 75 inch line source.
  • Greater Sensitivity, deeper bass extension, or a smaller cabinet. According to Roy Allison, each boundary that the woofer is coupled to adds 3dB to the power response of the system. If the woofer is coupled to nearby room boundaries over its entire operating range, you can buy yourself 3, 6 or even 9 dB more sensitivity. Or you can trade that sensitivity gain for bass extension or a smaller cabinet, by choosing a woofer with different Thiele/Small parameters. That’s a very significant gain!

Disadvantages? In speaker design, there’s no such thing as an advantage that doesn’t come with a disadvantage. A 2-way system with passive crossovers will typically have more expensive crossover components, and unless arrays are used, will probably also have power handling limitations.

These disadvantages are offset by not having to purchase a midrange driver, the inherent simplicity of a 2-way crossover, and reduced distortion and better cooperation with the room. And with Behringer’s digital crossovers, there is infinite flexibility and a low crossover point is no more expensive than a high one.

Personally I believe if you have to have a crossover, the best place to put it is between 150Hz and 500Hz. If you’re going to divide the audible spectrum into two parts, it makes the most sense to separate the bass from the midrange and treble.

It gives you the most flexible use of room boundaries, and it separates long-excursion bass reproduction from the rest of the band. With the right HF drivers, the performance is high indeed.

A 6 foot ribbon has high power handling and delivers a cylindrical wave that projects farther than a point source. A point source drops 6 dB every time you double distance; this ribbon only drops 3dB. When you walk up to it, it doesn’t get loud and shrill. You can put your ear next to the ribbon even when it’s playing loud and it doesn’t hurt. When you walk across the room to one corner you can hear the other speaker clearly.

The Case for Phase Linearity & Waveform Fidelity

Mainstream audio manufacturers pay little attention to phase lags in speaker design. Most speakers have real-world (acoustic) crossover slopes of 18-24dB per octave, many designs invert phase one or multiple times. Multiple 360 degree rotations are quite common. Fewer than 2% of speaker designs even attempt linear phase. Many fail. Most audiophiles have been told that phase and polarity are inaudible so they say it doesn’t matter anyway.

+/- 30 degrees phase response from 50-15KHz means complex waveforms are reproduced intact. Seldom do most speaker designers even attempt this level of precision.

I disagree, and so did an engineer I greatly admire, the late John Dunlavy. Dunlavy’s designs from his company Duntech are legendary, some of the most highly regarded high-accuracy speakers in existence.

I remember reading about his flagship speaker, the Sovereign, in Audio magazine in 1986. I was a senior in high school. He talked about his speaker being an analogue of an antenna array and this inspired me to pursue Electrical Engineering in college.

Dunlavy explained in an interview that the #1 priority in his designs was to get timing and the waveforms right first, then aim for flat frequency response later. (It’s physically impossible to do this in reverse order.) He said if you want a speaker to sound like a real symphony, you must achieve waveform fidelity.

Waveform fidelity is rarely even talked about, and few speakers can reproduce square waves with any accuracy.

This was important to me. It’s one of the reasons I chose a 2-way design. This has both subs and satellites in the same phase response, with 2nd order Bessel filters and a great deal of attention given to proper time alignment between low and high frequencies.

It’s not generally considered possible to achieve phase linearity without using a 6dB crossover, but it’s possible with 12dB crossovers and careful time alignment.

The phase response is +/- 30 degrees from 50Hz to 15KHz and impulse response is excellent. The ringing amplitude is less than 25% of the main pulse. This makes for clean, clear percussion and snare drums that sound like real snare drums, cymbals that really sound like a wooden stick with nylon tips striking metal.

To my knowledge, this is the only speaker system in the world that (1) fully integrates floor, both walls & entire room into the design and (2) achieves linear phase response across the entire audible spectrum, with no polarity reversals or phase rotations.

This is a unique design. A few speakers can do this in a sterile, theoretical sound lab, but no speaker I’m aware of achieves all these goals at the same time in a real listening room. You experience high output, authoritative bass and fast, clean impulse response at the listening position.

Each subwoofer is driven by its own Crown DC300 amp in bridged mono, roughly 500 watts per unit. The satellites are powered by a Hafler DH500, conservatively 250 watts per channel. We’ve got plenty o’ power here. The satellites sound really sweet when driven by tubes.

This can do justice to any kind of music, from Yo Yo Ma to Nine Inch Nails, with exquisite detail. People who come to visit always ask about them and enjoy the audition. It delivers bass notes carved in granite, spectacular sonic images and lush vocals. Best of all, it’s the only one of its kind in the whole world, and it’s in my library :^>

Perry Marshall

P.S.: The audiophile magazines I read when I was a teenager taught me how to write. The scientific papers I read by people like Roy Allison – and my theological education – taught me how to think. Speaker Builder (now Audio Express) taught me how to build stuff. I’m eternally grateful for all.

P.P.S: 2015 update – I’ve replaced the Behringer crossovers and EQs with MiniDSP 2x8s which sound considerably better.


Transmission Line Speaker Design

[1] Roy F. Allison, “The Influence of Room boundaries on Loudspeaker Output, JAES, June 1974