The past generation of large concert loudspeakers has been dominated by line arrays. While line arrays maybe well-suited to arenas, outdoor music festivals and stadium applications, for more modest-sized venues, point-source and trapezoidal-type loudspeakers are still quite viable. I have a personal preference for large-format, point-source P.A. loudspeaker systems (with multi-woofers arrayed for directional bass) for my smaller venue P.A. system design projects, for several reasons:
- The reduction of low-frequency energy on-stage improves gain-before-feedback.
- The reduction of excessive low-frequency energy on-stage makes it easier for musicians to hear their monitor mix.
- One-box array designs need much less vertical space, than a typical line-array.
- One-box array designs are pre-arrayed internally, so only one box needs to be rigged and wired, greatly speeding the time for installation, reducing product and installation labor costs.
- The co-axial/MTM speaker driver configuration provides a much smoother polar plot (coverage pattern) as compared to J-shaped line-arrays.
- The point-source driver configuration provides a symmetrical acoustical radiation pattern that avoids the time smear that can be an issue with large curved line-arrays.
A Little History
In 1945, the Altec Lansing 604 Duplex (coaxial) speaker was introduced, and it soon became the recording industry standard for coaxial studio monitors in the U.S. (See sidebar, page 33). Soon after, Tannoy’s Dual Concentrics dominated Europe from the 1950s onward. And lately, small point-source main loudspeakers and stage monitors using coaxial drivers are on the rise, and are now made by several leading pro-sound loudspeaker manufacturers.
Coaxial loudspeakers enable sound from two (high and low-frequency) drivers to come from one source. This characteristic allows a wider field of listening to a synchronized summation of loudspeaker drivers than loudspeaker enclosures containing separated drivers, as the pattern of response from a coaxial is symmetric around the axis of the loudspeaker.
Let’s look into some of the design benefits of point-source and coaxial loudspeaker systems. Speaker lobing is a phenomenon that happens when two or more speaker drivers that play the same frequency, are located a certain distance apart from each other.
This is almost always the case in multi-way speakers. Let’s say you have a 2-way system with a mid-bass driver and a tweeter. These drivers will play the same frequency at the XO (cross-over) region. They are not coaxial drivers, so they have some distance between them. Based on the driver size, the XO frequency and the space between the drivers, speaker lobing will happen at the crossover point. The XO slope can also tilt dispersion/lobes.
Fig. 1: A conventional 2-way speaker design
Fig. 1 (at right) shows a polar plot of a typical 2-way system. The XO frequency is 3k Hz, so the distance between the driver centers is 11.4 cm (wavelength of 3k Hz). The listener is located on-axis. You can see the multiple lobes in the polar plot. That’s the origin of the term “speaker lobing.” On the vertical axis, there are areas of high and low sound levels. So, if you move yourself off-center, you will have a drop in SPL at some frequencies.
Fig. 2: An MTM 2-way speaker design
Fig. 2 (page 32) shows that a better solution to minimize speaker lobing is to use an MTM (Mid-Tweeter-Mid) speaker/driver configuration. The MTM speaker design makes a symmetrical acoustical radiation pattern with far less severe lobing, with more uniform polar plots, thus providing more uniform sound coverage. And coaxial driver configurations are even better at reducing lobing at higher frequencies!
Let’s take a look at one such large-format, trapezoid point-source loudspeaker design. Having been a student of loudspeaker design and been mentored by several of the industry’s best loudspeaker designers during the past four decades, I have developed a keen eye for superior loudspeaker designs.
After discovering (a decade ago) that Florence, Italy-based B&C Speakers (bcspeakers.com) has been a leading OEM manufacturer of premium loudspeaker drivers for the worldwide live sound market, I specified them in custom loudspeakers I had manufactured for my projects. I later got onboard as one of their OEM dealers, to use their drivers in my own custom loudspeaker projects. So, I’ve been following the development and modeling of subs with their new drivers for many years now. I was particularly interested when B&C offered its new state-of-the-art coaxial mid-hi frequency driver, and encouraged the company to develop a companion large-format horn and crossover.
Recently, I was doing preliminary work on designing another premium point-source P.A. speaker system using the new large-format coaxial horn and drivers from B&C Speakers. I was particularly interested in how similar the design of the B&C 215-DCX developed, as compared to the new loudspeaker design I was considering for a house of worship client.
Fig. 3: 3-D Drawing of the B&C 215-DCX project
DIY Speakers Over the Years
B&C speakers is not the first company to offer DIY loudspeaker system plans to incorporate drivers they manufacture. In fact, DIY loudspeaker projects were even popular with hi-fi hobbyists half a century ago, yet I’ve never before seen DIY plans published for such a high-performance and large-format point-source P.A. loudspeaker system. This B&C 215-DCX project (shown in Fig. 3, page 32) allows live-sound pros to build their own premium point-source P.A. loudspeaker systems, without having to do the design work. The B&C 215-DCX may be an ideal choice for many modest-sized venues, immersive systems and as a replacement for a little/short line array (that doesn’t really perform like a line array except at high-frequencies) and is possibly the widest frequency range, high-output trapezoidal box design available.
Fig. 4: Horizontal polar response of the B&C 215-DCX
If you are the DIY type, B&C Speakers provides some design suggestions that use their drivers. Here’s a link to B&C Speakers’ suggested designs page: bcspeakers.com/en/resources/suggested-designs. Just from looking at the CAD 3-D drawing of the 215-DCX, it may not be obvious just how large the components in the system are. It uses the ME464 large-format horn, rated down to 300 Hz; the DCX464 coaxial mid-high driver; and two 15SW100 15” woofers. This combination of components yields both a very full-range response and high SPL output. The ME464 horn dispersion is rated at 80° x 60°. Fig. 4 (above) and Fig. 5 (below) show how the polar plots are controlled over a wide-range of 500 Hz to 16k Hz (but it seems to be 90° at more frequencies than 80°).
Fig. 5: Vertical polar response of the B&C 215-DCX
This is by no means a generic P.A. loudspeaker design. The state-of-the-art 1,500W 15-inch woofers are angled forward to limit the box to just over 48 in. high and the depth is 32.6 inches. Mounted at the heart (center) of the loudspeaker, between the two woofers, is the coaxial MH (mid-high) frequency driver and MH horn — supporting a crossover to the woofers at 300 Hz and a crossover between the coaxial mid and high frequency diaphragms at 4 kHz (with the optional FB4648 passive crossover) — in a point-source (or MTM) loudspeaker configuration.
B&C designed the DCX464 coaxial ring radiator (MH driver) from scratch to advance the state of the art. The DCX464’s midrange diaphragm covers 300 Hz to 5.5k Hz with 111.1 dB (1 watt/1 meter) sensitivity, and its 100mm voice coil handles 220 watts. The 64mm coil high frequency diaphragm covers 3k Hz to 18kHz with 111.4 dB (1w/m) sensitivity and handles another 160 watts. A patent-pending midrange integrator allows both diaphragms to be time-coherent over a very-wide bandwidth, for greater combined output and crossover flexibility. We dig a deeper into this unique DIY loudspeaker design offered by B&C with the following Q&A with Bennett Prescott.
B&C’s Bennett Prescott
Bennett Prescott, director for B&C North America, agreed to discuss the design process and benefits of the B&C 215-DCX three-way, point-source P.A. loudspeaker. The following is a series of questions I posed to Mr. Prescott, regarding its design philosophy, design process and laboratory measurements:
FRONT of HOUSE: Should we assume that since the 215-DCX is not marketed as a complete loudspeaker system — but rather more of a DIY kit — we should not expect the complete/typical set of specifications for complete loudspeakers?
Bennett Prescott: For sure, especially because I expect that some enthusiasts will modify the plans to suit their exact needs! [More on that below] One of the selling points of putting all the effort in yourself is to get exactly the loudspeakers you want. Our suggested designs are quite complete and — in the case of the 215-DCX especially — contain quite a lot of extra effort to ensure even pattern control in both axes. They’re not a complete loudspeaker system, though. The cottage manufacturer also has to choose its own hardware, processing and amplification, which will determine the capability of the finished speaker system. If built exactly to plans, the speaker sounds great and has incredible output, no qualms there, so take it as a suggestion or take it as the whole project and run with it as you like.
The large-format ME464 horn is a key component of the 215-DCX project
FOH: I would assume that the 15-inch woofers are angled, not only reduce the cabinet height but also bring the acoustic centers closer together, to minimize off-axis comb filtering, is that right?
Prescott: The goal was to spread out the higher frequency lobe coming off the cones, to widen the frequency range where the crossover is well behaved both on- and off-axis. Reducing cabinet height was also helpful — the cabinet is about the size of an adult human now, which I always think sounds realistic for many sources. Vocals and bass instruments radiate sound in a pattern determined by their size. Small speakers are nearly omnidirectional at low and mid frequencies, which makes it hard for them to sound convincingly like the original source.
FOH: With the use of two 1,500W 15-inch woofers, it would seem the system would have incredibly low-frequency extension. How low is this loudspeaker enclosure tuned to extend?
Prescott: B&C’s Francesco Mazzini and I chose a tuning frequency in the low-40 Hz range. That’s low enough to be considered full range and render subwoofers an effect or unnecessary. There are plenty of subwoofers with similar -3 dB points, and for normal music sources, the box will sound impressive and balanced even at maximum output. Home theater or dance music users may want to still use subs, or extend the port dimensions to achieve a lower tuning. We have a new isobaric subwoofer design which could be an ideal pairing.
The B&C website has detailed construction plans for 34 speakers (including the 215-DCX, shown here)
FOH: From the drawings, it would appear that even though the woofers are angled, it’s a pretty big loudspeaker system with large-format components. How heavy is it when complete with drivers?
Prescott: When built with 18mm birch plywood, the whole system weighs around 190 pounds. There’s no way to make something this large really portable for one person, but anyone who has wrestled an arena-capable trap P.A. will be familiar with how to deploy it. My hope was that it would come in lighter than an EAW KF850z without flyware, and be similarly capable in output. I think we achieved that, plus an octave more LF extension — 35 years or so later!
FOH: Based on the polar data of the ME464 horn, the dispersion is 80° x 60°. How much lower than the 500 Hz horn data does the loudspeaker system maintain vertical directivity?
Prescott: The directivity starts to open up around 350 Hz, and smoothly increases from there — although the vertical pattern is still ±60° at 200 Hz. Even with such a low crossover point in a box of these dimensions, it’s a challenge to keep side-lobes under control, everything has to be in balance! It’s a really interesting experience, because this much control in a box you can walk around, is unusual. There’s a whole layer of room excitement and resonance smack in the middle of the vocal range, which is removed. It sounds a little strange and maybe even thin at first indoors — until you realize how much detail is normally masked by energy coming back off the room. It’s really clean and revealing, which isn’t necessarily the expectation from such a sledgehammer of a box.
Unfiltered frequency response for two 15SW100s in parallel (in red) and the DCX464 with a passive filter in blue.
FOH: From what I understand, not only is the design available in a PDF form, but soon also as an AutoCAD file; how about as a more accessible SketchUp file as well?
Prescott: The design was conceived out of 2D materials (plywood) so a comprehensive 3D model wasn’t in the plan.
FOH: Together with the drivers, horn and crossover, it would seem that not much else would need to be sourced (such as cable, connector, hardware and grilles) to create a complete loudspeaker system, is that about right?
Prescott: The grille, if desired, can be a real hassle to source for one-off speakers. Nothing in that was built into the design because I have no idea how to make it look cool, and the box itself looks pretty sharp in the flesh. Items such as terminals, handles, internal wiring — should all be pretty routine for someone ready to take on a project of this magnitude. My focus would be more on amplification and processing, which is suggested but not really specified. I always prefer DSP in the amplifier in order to gain access to power limiters, and to avoid patching problems.
FOH: Can you share anything about the design process of the DIY B&C 215-DCX point-source P.A. loudspeaker system — perhaps how the low-frequency ports were modeled (to avoid port noise) and does the low-frequency modeling predict not only F3, but also max SPL?
Prescott: Honestly, all the maximum output stuff is easiest to do by ear, which is how Francesco did it. With a box like this, you either have to get pretty far away, or have a sophisticated system listening from another room on headphones, with a high-SPL mic, but that requires a lot of experience, because you lose so much of the ear/brain interaction people normally rely on. Doing more in software would make sense if we were a professional cabinet producer, but then we could also make some other integrated choices that could make this cabinet too expensive or difficult to produce. You look at the beautiful tooled ports in some products today, and clearly a lot of modeling went into those to get them well behaved at very high velocities in the smallest space possible. There’s also $100K in tooling or more, so that’s not exactly DIY-friendly. With the 215-DCX, the ports are pretty well-behaved, because they’re pretty large, and the tuning frequency isn’t extremely low, so airflow is manageable. Rounding over the entrance and exit of the port is recommended.
FOH: Do you have other topics relevant to this loudspeaker system that you would like to add?
Prescott: Just remember this is supposed to be fun to listen to, and not too difficult to build. The components are proven, but in the end, you have to like the same kind of speakers I do — ones that put a smile on your face at eye-blurring SPLs. There are lots of design choices that could have been different, and specifications that could have been goals, but the real goal for me was to have something relatively unique, interesting, and immediately rewarding.
As Bennett said in the Q&A above, “some enthusiasts will modify the plans to suit their exact needs.” I fully intend to specify this B&C 215-DCX loudspeaker system (with a larger LF array or a modular variation of it) into many of my midsized venue projects. It’s awesome that B&C includes performance measurements and DSP settings with the 215-DCX enclosure design/plans. But, until B&C offers a diversity of dispersion options (with coverage modeling data) for its large format horns, some users may have to look elsewhere for the horn part of the project. I already have one project in mind for this very interesting loudspeaker system — and have performed dispersion modeling with a similar horn file — with very positive results.
David K. Kennedy operates David Kennedy Associates, providing acoustic and sound design consulting services, with an emphasis on loudspeaker arrays for educational and religious facilities. Contact [email protected] for help adapting, implementing or sourcing this high-performance loudspeaker system for your projects or visit him at www.d-k-a.com.
Altec Lansing 604 Duplex Speaker (1945)
SIDEBAR
Altec Lansing 604 Duplex Speaker (1945)
Altec Lansing’s famed 604 was not the first Duplex coaxial speaker; that honor goes to the company’s 1941 model 601, which mounted a high-frequency compression driver onto the back of a 12-inch woofer with a hole cut into the center of the magnet, forming a throat for a small multicell horn in the center of the cone. However, it was Altec Lansing’s model 604 coaxial that created a splash that continues to this day.
The 604 was based on a 15-inch woofer with a 3-inch voice coil within an Alnico V permanent magnet, combined with a full-size HF compression driver and a six-cell horn. The 604 was capable of a then-impressive 30 watts of power handling, but due to its high efficiency, wide bandwidth and point-source imaging, it was soon adopted as a standard monitor in studios.
Altec continued improving the 604 through the years, and the drivers have been used in numerous variants — both in the studio and for live sound — either in stock “utility” cabinets, custom enclosures or modified third-party designs (often with high-performance crossovers). During the years, the affinity for the 604 may have waned in the studio, but the speakers (with a different horn, cabinet and crossover) later regained popularity as the basis for UREI’s 811/813/815 studio monitors.
Contact [email protected] for help adapting, implementing or sourcing this high-performance loudspeaker system for your projects.
David K. Kennedy operates David Kennedy Associates, providing acoustic and sound design consulting services, with an emphasis on loudspeaker arrays for educational and religious facilities. Visit him at www.d-k-a.com.
