A family of configurable monitor loudspeakers for high end home theater

(Photo above is a CM-7 in the Parts Express 0.5 cubic foot cabinet)

The CM-7 family of loudspeakers has been developed specifically to meet the needs of high performance home theater applications.

The most critical performance aspect for a home theater monitor is its ability to cleanly and concisely communicate the spoken voice while reproducing background information and audio "special effects" with the proper dynamics and weight. For the CM-7, frequency response has been deliberately tailored for spoken voice and movie soundtracks, and is incredibly accurate throughout the entire midrange.

"CM" stands for "Configurable Monitor", as the most difficult part of making a monitor loudspeaker work in a home theater system is to properly adjust loudspeaker placement, room modes and low frequency cut-off to mate properly with a subwoofer. The CM-7 has been designed with the optimum low frequency balance and resonance frequency, and an avenue to fine-tune the subwoofer-monitor transition region is provided via a user-configurable Scan-Speak "Scan Vent".

In addition, this loudspeaker is designed with a minimum resistance of a very high 6.70 Ohms, while still maintaining high sensitivity. This was done both to make the system easy for most amplifiers to drive, and to facilitate the construction of an MTM center channel with a minimum resistance higher than most.

- Placement optimized for minimum 24" from back wall.
- High Input Impedance
- High Sensitivity
- Magnetic Shielding
- Configurable Low-End Contour
- Optimized to mate with a subwoofer
- Excellent Articulation throughout the midrange

Unlike most loudspeaker systems, which are designed with existing drivers with their own set of compromises, here determining the system design goals first allowed these goals to determine the optimum woofer parameters.

First and foremost, knowing that this driver was destined to be used in a aperiodically damped loudspeaker enclosure, the driver was designed with a high DCR (6.7 Ohms) and a high Qts. (above 0.60).

Secondly, because the driver would not be required to reproduce any deep bass, targeting a fundamental resonance near 50Hz and a small Vas allows the driver to be used in a small enclosure without raising the system resonance above 80 Hz.

Thirdly, to get a high sensitivity and maximum articulation, a light cone material with minimum coating was required. A metal cone was originally considered for this driver as metal cones can be made very light and many individuals consider them the best in terms of detail and overall resolution. But this loudspeaker is being designed for very long listening session - in this case, two to three hours - typical movie length, and even the best metal cones tend to be "fatiguing" over long listening periods. So our choice is epoxy-impregnated woven Kevlar®, which is nearly as articulate as metal and provides a superior listening experience over extended periods.

To achieve the proper balance of sensitivity, low frequency resonance and Q, midrange dispersion and optimum high end roll off, we selected a 14mm cone with a rubber surround and coated cloth dust cap. The large cone diameter provides increased output, whilst the natural rising response of Kevlar® just before its high frequency roll off will mate perfectly with the tweeter. This driver is built on a massive, heavily ribbed 19mm cast aluminum frame and magnetically shielded motor, and issued as the North 19W-06S.

The Tweeter

The target 1.4kHz crossover frequency calls for a tweeter with high excursion, high power handling, and a low fundamental resonance. The 7.5" woofer frame defines a cabinet width minimum of 9". Therefore, we selected the magnetically shielded version of the North D25-06S.

The North D25-06S offers a frequency response optimized for cabinets from 8" to 11" in width, greatly simplifying crossover design. It also has very high sensitivity - 91dB- through most of its operating band. Because it is a very soft silk dome, it has a very sweet top end with that does not over-emphasize sibilance, critical for high end audio performance over long listening periods.

 Initial Measurements

Above are the anechoic curves of the North 19W-06Sand North D25-06S (shielded) in the original prototype cabinet. This is a 14 liter internal volume (0.5 cubic foot), with fascia dimensions 10 1/4" by 15 1/2". This measurement is taken at 0.7 meters to accurately measure the baffle diffraction step as well as fascia edge and driver diffraction effects. The woofer response is lowered 5dB for clarity.

The crucial diffraction effect to note for the tweeter curve is the broad 1dB "hump" created by baffle boundary reinforcement at about 1.8kHz. The North D25-06S was designed with a broad, -1.5dB dip centered at 2kHz to partially compensate for boundary reinforcement; otherwise, this "hump" would be 2.5dB and very difficult to control with a simple crossover.

The small wiggles in the heart of the tweeter response are caused by both edge diffraction and minor reflections from the woofer cone.

The woofer's response is sloped about 3dB per octave between the low frequency measurement limit and about 1kHz. This is a product of both the baffle diffraction step (or "6dB response step") and the natural rising response of the Kevlar® cone. A "perfect" driver would have a straight line rising 3dB per octave up to the turnover point; the 19W-06S misses this by only 0.25dB in the octave between 400 and 800 Hz.

 Crossover and Voicing Goals

Crossover design is the most critical aspect of loudspeaker system design. The crossover determines both the overall "voicing" of the loudspeaker, that is, its "character", and the quality of reproduction within that voicing.

All North Creek loudspeakers are voiced for maximum accuracy, particularly through the midrange where the ear is most sensitive. Any flaw in the midrange voicing and a loudspeaker will sound unnatural, seriously detracting from the illusion of recreating the actual event. Measurements and simulations are valuable only at the beginning of this process, as correctly balancing the midrange with low frequency performance and top end "sparkle" is beyond the realm of measurement equipment. For this reason, all North Creek loudspeakers are designed by ear, through listening, literally months and months of it.

We have always found the best sounding crossover to be a symmetric third order network, with the crossover frequency determined by the driver spacing and their natural roll-offs.

For the tweeter network, this is easily accomplished with a simple second order high pass filter with sensitivity compensation. The combination of the second order electrical roll off, the small "hump" around 2kHz, caused by baffle diffraction, the -1.5dB plateau between 1.5kHz and 3kHz designed into the North D25, and the tweeter's natural roll off, gives us a third order high pass.

For the woofer network, the blend of the drivers' natural rising response and the 6dB response step gives a natural 3dB per octave rise above 200Hz or so. Our network therefore uses a single series inductor to compensate for this rise. About 1.5kHz, the woofer goes into its first edge mode - the frequency where the wavelength of sound is equal to the cone circumference - which introduces a dip in its frequency response. At a factor of π/(2cosӨ) octaves above this frequency (where Ө is the cone flare angle) begins the typical "Kevlar® bumps" caused by radial modes. Many driver designers will add coatings to the cone body to damp these modes, and while this is effective it also increases the cone mass, decreasing sensitivity, which for this high-DCR system is an unacceptable compromise. Instead, we allowed the edge mode to determine our turnover frequency, and with the addition of a parallel capacitor-resistor combination, were able to achieve our target third order filter characteristic with almost textbook accuracy.

Crossover component values are:

Above are the frequency response and antiphase curves for the CM-7. These are taken with a calibrated microphone at 0.7 meters, with the mic 2.5 degrees horizontal and 0 degrees vertical at the midpoint of the loudspeaker fascia. System input is a MLS signal averaging 2.83 Volts. The splice to the close-mic woofer response accurately reflects the Qtc of 0.95 for the sealed box.

The curves are printed at 5dB per major division, twice the resolution of most published curves.

The North Creek CM-7's frequency response is + 2dB from 60 Hz to beyond 20kHz. More importantly, through the critical region from 230 Hz (just below middle "C") to 3kHz, the CM-7 measures an exceptional + 1dB. It is absolutely necessary that a loudspeaker be as flat as possible though this region to accurately reproduce the human voice.

The crossover frequency is 1300 Hz. This is low for a 2-way, but optimum for the woofer's natural frequency response characteristic and proper low end balance. The tweeter's electrical -3dB point is actually about 1900 Hz, twice the tweeter's fundamental resonance frequency, and the second order electrical slope prevents the tweeter from going into overexcursion.

The symmetric -28dB antiphase null is also centered at 1300 Hz. This is a good indicator that the drivers are in perfect addition throughout the transition region, and that there are no peaks in the power response or vertical off axis response.

The voltage plots above are measured at the drivers' terminals.

The tweeter voltage plot is a textbook second order high pass with a Q of 0.7 at 1900 Hz.

The woofer voltage plot is a progressive second order with an initial slope perfectly balancing the woofer's gentle rising response from 200 Hz to 1kHz, then changing to second order above the 6dB response step. The woofer's natural 6dB drop above its "edge" frequency brings about the desired third order low pass characteristic.

Above is the North Creek CM-7 input impedance with and without the impedance "twister" equalizer circuit.

The fundamental woofer resonance frequency is 70 Hz with a Qtc of 0.95. This is a nice sounding sealed box alignment, with just a bit of punch to create the illusion of deeper bass. As a sealed system, the low frequency roll off is second order.

The second peak of 31 Ohms at 780 Hz is evidence of the low-Q nature of the crossover, and particularly the "progressive" nature of the woofer low pass progressive slope. For the amplifier-loudspeaker interface, a 31 Ohm peak in the heart of the midrange is difficult for some amplifiers to drive. The "twister" circuit consists of a series string of a 33uF capacitor, 12.1Ω resistor and 0.80mH fine gauge inductor, and the entire string is wired in parallel with the loudspeaker terminals. The "twister" is designed to have minimal impact on the low frequency tuning, and stabilized the impedance that the amplifier sees. System input impedance with the "twister" in place is 7.5 Ohms + 3 Ohms from 100 Hz to 20kHz, a simple load for any amplifier to drive.

Although it is not a simple matter to go from an MT to MTM loudspeaker using the same woofers, it is much less complicated than starting from scratch. The reason for this is that provided the voicing of the MT was done correctly, 1/3 octave pink noise measurements of both systems can be made with the tweeter position identical relative to the woofer and room position, and by comparing these measurement, one can get a substantial "head start" on the woofer frequency response shape.

It is not as simple as halving the inductors and resistors, and doubling the capacitors. An MTM interacts with the room a little differently both because in the near field it behaves a little like a line source, and in the lowest frequencies a phenomenon called "mutual coupling" makes the absolute SPL output of one woofer + one woofer = a little more than two woofers (although the f3 and Q remain the same). So the 1/3 octave pink noise measurement of the MT is a good starting target and with a finished reference system with identical low end tuning, the listening and final voicing goes much more quickly.

By fine-adjusting the MTM woofer low pass and massaging the tweeter attenuation, the optimum low pass filter was determined to be:

By keeping the shape of the tweeter frequency response but adjusting the attenuation, the tweeter network becomes

This network is the optimum whether the MTM is vertical for the Super Tower or horizontal for the center channel.

This is really a question of how far back one sits from the center channel. An horizontal MTM will exhibit a horizontal window were the frequency response varies by only + 2dB from the design axis. The third order network and low crossover is much less sensitive to horizontal lobing error than most topologies.

For the center channel, the + 2dB window is over about 18 degrees, meaning a two person seating area should be 9 feet or more distant from the center channel. Closer than this and the standard CM-7 MT configuration is preferred.

For many surround processors, low pass and high pass filters are part of the surround program, and one can use a standard, sealed CM-7 for the mains and all surround channels and allow the processor to determine the subwoofer transition frequency and response shape.

For systems where one is crossing passively from the subwoofer to the mains, adding a calibrated Scan-Vent and optimizing its Ql is just a matter of adding the correct calibrated felts. North Creek provides the calibrated Scan-Vent with these felts. Adding a Scan-Vent to the system changes the low frequency roll off to third order.

Above is input impedance Q-comparison of the CM-7. The upper curve is the standard, sealed box impedance, with a Qtc of about 0.95 and an impedance maximum of about 27 Ohms. The lower curve is with the Scan-Vent added and calibrated to a Ql of 3. The system Qtc is 0.7, which is optimum for passively mating to most subwoofers, and the low frequency roll off becomes third order. In addition, the impedance maximum is reduced to 18 Ohm, so a single large capacitor between the main amplifier and CM-7 will more accurately control the CM-7 through the stop band.

The preferred method to passively cross out the CM-7 is with a tiny capacitor between the pre-amp and power amp. We recommend a single 0.10uF Crescendo capacitor wired directly to the RCA center pin of the pre-amp-to-amplifier interconnect to accomplish this.

Subwoofer low pass slope should be either third order or forth order. Second order, commonly found in processors and many dedicated subwoofer amplifiers, is inadequate to cross out the sub quickly enough.

This system was designed in both full range, sealed box mode and with a first order high pass at 80 Hz and a calibrated Scan-Vent with Ql = 3. The subwoofers were dual down-firing Poseidons , with the built in 250 Watt amplifier and crossover set at 85 Hz. The Poseidon's crossover is third order, and matches seamlessly with the CM-7.

Of course this is the most important aspect of the CM-7's performance.

Listening evaluation of the CM-7 (without the subwoofer) began in September of 2002, and the fine points of the voicing were done with several superb recordings from Putumayo and Chesky…. although the Dead, Phish, Rusted Root and Widespread Panic also saw a lot of playing time.

 The first thing that strikes everyone who hears this loudspeaker is how incredibly open it is. This is attributed to both the natural Kevlar® signature, its remarkable flatness though the midband, the touch of "sparkle" that comes from the slight rise of the North D25-06S in the top octave.

Particularly, good recordings of female vocalists are reproduced so accurately and with such a sense of space and solidity of image that it is almost transfixing. The Putumayo CD "Women of Spirit" (PUTU137-2/M137-2) is a must for those who enjoy world music - excellent recordings of female vocalists and groups from all six continents. Even without the subwoofers, the CM-7's render virtually every recording on this CD with extraordinary accuracy and scale.

Male vocalists fare equally well. The Fairfield Four are reproduced with the correct balance of chesty resonance and tenor openness. Harry Connic Jr. and Ian Anderson have that "palpable presence". The Phish- and Widespread-guys sound like they're just having a good time.

In home theater applications, passively crossing out the CM-7 monitors and adding dual Poseidon subwoofers was unproblematic. The CM-7's resolving power means that spoken voice is articulated perfectly. Understanding dialogue even in the most complex passages is a testament to both the woven Kevlar® cone and the system's remarkably flat frequency response. The subwoofers add considerable weight to this loudspeaker system, but do not compromise its mid- and high-frequency performance in any way.

The CM-7's do have a placement constraint - a minimum of 24 inches from the back wall to the back of the speaker - means these cabinets need to be on stands or placed atop the subwoofers. In those environments where near-wall placement is required, the North Creek Okara II is a better choice.

Likewise, in the "standard" configuration (without the twister), the resolving power of the Kevlar® mid-woofer does not err on the side of "forgiving" - poor recordings and amplifiers that struggle with a dynamic load are revealed as such. But amps that can drive it, love it. This is attributed to the simplicity of the signal path. Adding the twister softens up the midrange considerably, and even SET's can easily drive the flat impedance without changing character.

For long term listening - and particularly with vocal and movie sound tracks, the CM-7's vanish into the background while doing a superb job of communicating the essence of the event.

We are offering the CM-7 in three versions:

Features matched pairs of North 19W-06S woofers and D25-06S tweeters. Woofer crossover is constructed with 16 AWG inductors and Mylar film capacitors, North resistors. Tweeter crossover features Zen bypassed by Harmony capacitors,

14 awg inductors, and North resistors. All components are hand matched to +1%. The crossover is fully assembled, star grounded, single wired. The cabinets are not included. Component list price $387 per pair.

Click for CM-7 Crossover Assembly Instructions and CM-7-C Crossover Assembly Instructions

Features matched pairs of North 19W-06S woofers and D25-06S tweeters.  Woofer crossover is constructed with 14 AWG inductors and Zen bypassed by Harmony polypropylene film capacitors, North resistors. Tweeter crossover features Zen polypropylene film bypassed by Crescendo bypass film-coil capacitors, 14 AWG inductors, and North resistors. All components are hand matched to +1%. The crossover is fully assembled, star grounded, bi-wired. The cabinets are not included. Component list price $484 per pair.

The photo above is a CM-7 in a Lee Taylor cabinet. Contact info@TaylorSpeakers.com directly for more info.

Click here for CM-7 Cabinet Drawings in Adobe .pdf format. The Parts Express 302-724 cabinet also works well with this kit, but one must coundersink the woofers for the grille to fit properly.

Click here for CM-7-C Cabinet Drawings and CM-7-C Upgraded Crossover Assembly Instructions.

The Madisound MD01 cabinet works perfectly with the CM-7 loudspeaker kit. Those using the Madisound cabinet should be sure to purchase the North Creek Bi-Wired Back Cup. Unfinished, pre-machined North Creek MDF/Plywood Fascias for the MD01 are $65 per pair.

The Parts Express 302-754 cabinet will work with the CM-7-C. Unfinished, pre-machined MDF/Plywood North Creek Fascias are available for the PE 302-754 for $65 each. Photos above are of the CM-7 in the PE 302 birch finish with North Creek fascias.

Features matched quads of North D25-06S woofers, matched pair of North D25-06S tweeters. All inductors are 12 AWG. Capacitors are Cascade-Bypassed Zen metallized film and Crescendo film-foil. All North power resistors. All components are hand matched to 1%. Crossovers are fully assembled, star-grounded, bi-wired. Package includes driver, crossovers, gaskets, all required mounting hardware, stuffing, spikes, etc…. everything except the wood. Component list price $710.20 per pair.

Photo at right is a pair of MTM Super Tower's at Lee Taylor's shop in Philmont. For more info, contact Lee at www.TaylorSpeakers.com.

Click for Super Tower Cabinet Drawings and Tower Crossover Assembly Instructions

The Madisound MD03 cabinet is compatible with the CM-77. Fascias are not available at this time. Please ask.

Impedance equalization "twister" …$30 each assembled, $19 each unassembled.

CM-7 Drivers: (for those who already have the crossover components)

Cabinet: cabinet volume required is 11 to 17 liters per woofer, acoustic suspension. Minimum baffle width is 9". Cabinets are available directly from LeeWTaylor@verizon.net. Drawings are available on this CM-7 Cabinet Drawings link.

The Madisound MD01 also works perfectly with the CM-7.

The Parts Express 302-754 cabinet will work with the CM-7-C. Unfinished, pre-machined MDF/Plywood North Creek Fascias are availalbe for the PE 302-754 for $65 each.

The Madisound MD03 cabinet is compatible with the CM-77. Fascias are not available at this time. Please ask.

  Designer's Commentary

This was a very interesting project as I designed not only the cabinet and crossover, but the actual drivers as well.

Interesting also because my reference home theater equipment - North Creek Vision Signature mains and Vision Signature Center channel, Okara II surrounds, dual Poseidon subs - use paper or poly cone woofers, while my reference 2-channel Rhythm Signatures use paper-carbon fiber cones. So I have become accustomed to loudspeakers that are relatively forgiving in the midrange - compared to Kevlar®, anyway. I had not worked with Kevlar® since 1997. It took me quite a while to get used to all of that resolving power again. Once I became accustomed to it, I found that I rather enjoyed the level of detail the 19W-06S could convey.

The voicing was really touchy on this loudspeaker. While I generally keep inductors in the lab in 10% steps (from 0.12mH up to 5.0mH), with the CM-7 I had to go to 5% steps between 3mH and 5mH to determine that "magical" value that perfectly balances the low frequency weight and lower midrange warmth - which took months of serious listening. 4.25mH seems high at first for a 2-way crossover, but this is a high DCR system with a low crossover frequency. Were the 19W-06S a 5.6 Ohm DCR like most other "8-Ohm" drivers, the coil would be scaled to 3.3mH and if a 1.8kHz crossover frequency were targeted, scaled to around 2.25mH, which is not unusual.

I was also pleasantly surprised that the North D25-06S could take a crossover frequency this low without being overdriven, and it manages it quite well. I designed the tweeter with 1mm of throw, but never expected use it all.

Designing the 16 AWG inductor suitable for the standard version of this loudspeaker required tooling a geometrically optimized former to wind the 4.25mH inductor with the absolute minimum DCR. This was accomplished with the "inductor" sub-program in Calsod. The 16 gauge is about 85% as good as the 14 gauge for this application - 16 gauge adds a touch more weight in the bass (as one would expect from the higher Qtc), and looses a bit of resolving power in the mid-bass and lower midrange. However, for the 0.56mH tweeter shunt inductor, the 16 gauge was nowhere near as good as the 14 gauge, and for this reason we are offering the CM-7 only with the 14 gauge inductor in the tweeter circuit (12 gauge in the Super Towers).

The twister - some amps need it, some do not. Our reference Classe Audio sounded better with the twister out of the circuit - it also doubles power when the impedance is halved and this is probably a good indication why it could care less about 31 Ohms. Same with the PS Audio 200W powerhouse, which could drive a nail if required to do so. The Red Rose Music Rosette required the twister to sound its best - the midrange was softened up considerably compared to the system without the twister… I would almost call it "forgiving". In the home theater, the B&K main amp was 50/50 - the image was more stable and soundstage larger with the twister in - it seemed to "gel", for lack of a better term, and again the midrange was considerably softer. The 35 Watt Adcoms driving the CM-7 surrounds preferred the twister in. With the 3-Watt SET's, the twister was essential. Low power tube amps driving a dynamic load through an output transformer will always have a difficult time, but with the "twisted" flat impedance, the SET's were very comfortable. It is because of all of this variability that we have made this circuit an option.

Biwiring eliminates the ground loop caused by the speaker cable resistance improves clarity at every level. One can also bi-wire the surround loudspeakers (by installing 4-conductor architectural cable). It is optional for the surround version of this speaker, but strongly suggested for the mains and center channel.

The Oak cabinets are 11 liter AMS boxes available from MCM electronics (MCM # 80-375). Woodstyle also makes an 11.6 liter cabinet (WS-802) available from Madisound that works fine. The new 0.5 cu. ft. finished cabinet from Parts Express (#302-724) will also work, and is by far the most attractive of the three, but the woofer frame needs to be fully recessed for the grille frame to sit flush. The Madisound MD01 is a better choice for the CM-7 as the fascia is a bit wider. For the Center Channel, the Parts Express #302-754 cabinet works perfectly and one can also build a stand-mounted version of the CM-77 Super Tower in this cabinet and get excellent results. The Madisound MD03 works perfectly with the CM77 Super Tower. All of these 3/4" cabinets should be modified with the addition of a ladder brace or a circumference brace; the stock 3/4" particle board cabinets buzz.

Among our design goals for this project was to take the complicated process of loudspeaker design and proceed with it straightforwardly. From a designer's point of view, these loudspeakers have a bit of everything. The tweeter was designed with a dip to match the baffle width hump; the tweeter crossover is straight forward (electrical second order plus an L-pad to achieve a third order acoustic slope); the physical properties of the woofer cone material determine the low pass crossover frequency while the acoustic effects determine its shape; low frequency performance is configurable (via the Scan-Vent), and the impedance is configurable (via the twister). The cabinet is as simple as it gets.

The CM-7 family is truly exception for such inexpensive loudspeakers. We are now offering them with the crossovers unassembled along with detailed assembly instructions as they also provide an excellent avenue for the beginning speaker builder (or audiophile-on-a-budget) to build some excellent loudspeakers while learning and developing the mechanics of speaker building. We are also offering this project to our local high schools. Other high schools or trade schools interested in the CM-7's should contact North Creek directly via Info@NorthCreekMusic.com.

-George Short, President

North Creek Music Systems

March 2003

Builders' Comments

Hi George,

I just wanted to tell you how happy I am with the
CM-7-C I just completed. Is there a 'break in'
required for it?? It sounds great no matter how loud I
turn up my Onkyo amp. It certainly out performs the
******** set I built a few years ago.

In the attached picture you can see I have it atop my
bigscreen TV. The speaker back is about 18" from the
wall. I am thinking about how I can change a few
things in my living/listening room so I can be sure to
get the same or more distance from the CM-7 super
towers I plan on ordering very soon!

Thank you,

Andrew

 Hey George,

As per my previous E-mail, I think I found the distortion problem. I started changing drivers around trying to isolate the problem, and evidently there was a loose connection, because the noise is gone. Thank goodness!

Straight out of the box these speakers sound wonderful. I wonder how good they will sound once they burn in?

Using my Radio Shack spl meter, set at listening position, I'm getting a fairly flat response down to 31.5hz. The point being, I don't think you even need a sub to listen to music. Are my ears, and the spl meter, playing tricks on me, or are the speakers actually producing deep bass that loud?

....Anyway George, I'm very happy with the speakers. Usually new electronics sound bright and harsh at first, but not these. Right now they are setting on cinder blocks and the bass is very tight. Can't wait for the stands.

Thanks,

Hugh

Hi George,

I curently have the CM-7 w/upgraded crossovers and am very pleased with the level of performance given their modest cost. Cabinets are overbuilt, even by your specs and recommendations. I am firmly convinced that you are on the right track with your emphasis on solidity here. They weigh 33 lbs apiece!

My next upgrade is to be to the Borealis Unlimited. We have had several e-mail chats on this and I am convinced that this is where I want to go.

-Rob C.

Copyright: This publication is copyright © 2003, 2004, 2005 and 2006 by North Creek Music Systems. All rights are reserved.