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Frequently Asked Questions

For a better understanding of what I had in mind when I designed the PHOENIX loudspeaker system, you should read the <Concepts ...> pages, which give you general observations that influenced this unconventional speaker project. The reasons behind specific design solutions are addressed on the | Design Models | page and in Ref.2. Measured frequency response curves for the PHOENIX are shown on the | System Test | page. 
The project documents the evolution from a previous active speaker system in which I used two small satellites and a closed box woofer with a novel equalizer to extend its low frequency response
. You will find helpful information about many issues in speaker design in the description of that earlier project, Ref.12. Take a look at page 1 or the full article.

In general, I tried to give specific meaning to every sentence on this web site. If you do not find your answer or are confused after reading what I said, then do not hesitate to send me your question. And, for my benefit, let me know what you did with my response. I always appreciate your recommendation for a well recorded piece of music to listen to. 

Should you have built the PHOENIX or parts thereof and made interesting observations or found ways to improve the design, then send me a note. I am always interested in new insight on how to move sound reproduction closer to the original. But please, don't ask me what changes are necessary to accommodate your favorite driver. I think I have outlined in detail how you can determine that for yourself. If not, then stick exactly to the PHOENIX project.


Q1 - Why do you use 8" drivers for the midrange when 6.5" or 5" drivers would give better off-axis response?

Q2 - My room size is less than 250 ft2 (22m2) and I need a smaller speaker. How can I modify the PHOENIX design?

Q3 - How do I upgrade the system for rooms larger than 600 ft2 (55m2), or because I like to play it larger than live?

Q4 - I want to use woofer XYZ which has larger Xmax than the 1252DVC. Which other parameters are important?

Q5 - How much sound output can you get from the open baffle woofer?

Q6 - How much power will it take to drive the woofers?

Q7 - The price of the Scan-Speak drivers is a problem. Is there any valid substitute without changing the design?

Q8 - Diffraction. Would a narrower (<12.5") main panel give sharper stereo imaging, due to reduced delay of edge diffraction?

Q9 - Could the length of the main panel be extended so that separate stands are not required?

Q10 - Could the woofer be adapted to work with existing panel speakers - e.g. Quad ESL63?

Q11 - What is the total cost of all the hardware for building a pair of PHOENIX loudspeakers?

Q12 - What happens to the parameters Vas, Qms, Qes and Qts when you connect the two voice coils of the 1252DVC in series?

Q13 - Is a printed circuit board for the crossover/eq available?

Q14 - What is the low frequency sound pressure level from an open baffle speaker with a given effective piston area and excursion?

Q15 - How would you modify the crossover/eq circuit to work with a "regular" small monitor, instead of the main panel, and use the dipole woofer?

Q16 - Could you drive the main panel with a tube amplifier?

Q17 - Why do you not show waterfall response plots for the PHOENIX when your test equipment is capable of producing them?

Q18 - Which types of distortion do you measure?

Q19 - Why do you use a 12 dB/oct crossover between woofer and midrange when the crossover to the tweeter is at 24 dB/oct ?

Q20 - What is the sound pressure level at 1 m for 1 W of power under anechoic conditions?

Q21 - Why does SPL increase 6 dB for two drivers in parallel when the electrical power consumed only increases by 3 dB?

Q22 - How would you describe the sound of the PHOENIX?

Q23 - Can you point me towards people who have built the PHOENIX?

Q24 - What is your process for designing an open baffle speaker?

Q25 - Why do you not use a rear firing tweeter or a dipole ribbon tweeter?

Q26 - Why use active crossovers when passive crossovers require fewer components and amplifiers?

Q27 - How do I test the PHOENIX crossover/equalizer circuit board?

Q28 - How high in frequency can you push the dipole woofer?

Q29 - How do you measure cone excursion of a driver?

Q30 - Can a dipole woofer be placed in a room corner?

Q31 - Is there an optimum room placement for a dipole?

Q32 - How would you increase the output capability of the Phoenix?

Q33 - Is a pre-assembled crossover/equalizer available?

Q34 - What is the optimum Qts for the drivers of a dipole woofer?

Q35 - How does the new ORION compare to the PHOENIX?

Q36 - Are there better drivers for the PHOENIX?

Q37 - What cables and interconnects do you recommend?

Q38 - How do diffraction effects show up?

Q39 - How much power does it take to drive a dipole woofer?

Q40 - Can the PHOENIX be build with passive crossovers?




Q1 - Why do you use 8" drivers for the midrange when 6.5" or 5" drivers would give better off-axis response?

A1 - I am using 8" drivers because the crossover to the woofer will be around 100 Hz and because an open baffle speaker has to move more air than a box speaker due to the progressive acoustic short circuit between front and back. The open baffle gives significantly wider dispersion at high frequencies than a box speaker with the same 8" driver on the same baffle, see Ref. 2, Design Models F. This is caused by the radiation from the rear of the baffle, having undergone enough phase shift as it comes around to the front, so that it adds to the total sound at off-axis angles. See Dipole Models
    For almost all drivers, as you go down in driver size, the excursion
capability drops. Thus, the linear volume displacement - which makes SPL - goes down even faster than diameter squared. What is gained in wider dispersion, which is inversely proportional to diameter, is more than lost in increased distortion. Low distortion is higher on my priority list than dispersion. Since dispersion is widened with a properly sized open baffle, there is no good reason for using a smaller diameter driver other than if the 8" driver had poor behavior in its cone breakup frequency region. 
    A 8" driver will typically have twice the useable volume displacement of a 6.5" driver, but the effective diameter increase is only 30%. Thus, when used in an infinite baffle or in a box, the 8" driver will have the same dispersion at 1.5 kHz as the 6.5" driver has at 1.95 kHz. The smaller drivers are so popular in commercial products because narrow loudspeaker cabinets are much more acceptable in the home for visual reasons, a trend that is conveniently supported by theories about the detrimental effects of edge diffraction. Note also how many of these speakers use two drivers in MTM or MMT layouts to obtain adequate output. 
    Check out my distortion test results for midrange drivers.  Top


Q2 - My room size is less than 250 ft2 (22m2) and I need a smaller speaker. How can I modify the PHOENIX design?

A2 - You could eliminate one of the 8" drivers and make the main panel only 16" tall. This will sacrifice 6 dB in acoustic output capability for the same non-linear distortion level. The gain of the midrange channel in the crossover/eq needs to be increased 6 dB. I would not recommend to also go to a single woofer, i.e. 6 dB less bass output, because subjectively this is a much more significant reduction in output capability (observe the close spacing of the equal loudness contours at low frequencies, Fletcher-Munson). The reduced height main panel would also make for a high performance center or surround speaker with very uniform and smooth off-axis response, but for distortion should not be pushed below 100 Hz. 
    A second alternative is to eliminate the woofers all together and to extend the main panel response from the present 100 Hz down to 50 Hz. To maintain the same SPL down to 50 Hz, the 8" drivers must move with 4x the excursion of 100 Hz. Thus, for the same distortion, output is reduced by 12 dB, but probably more like 20 dB, because distortion increases much more rapidly with increasing excursion. For the crossover/eq circuit you would need to change the midrange high-pass filter 90HP to 50 Hz  and the 90-500LP to a 50 Hz to 500 Hz low-pass of appropriate gain.   Top


Q3 - How do I upgrade the system for rooms larger than 600 ft2 (55m2), or because I like to play it larger than live?

A3 - Now you are into a 4-way system. You could add two 10" drivers (e.g. Scan-Speak 25W8565) in the space below the main panel as was done in the Audio Artistry Beethoven and double up on the woofers. If that is not enough capability, then you might add two more 10" drivers on top of the main panel and use eight 12" drivers for each woofer as in the Audio Artistry Beethoven-Grand loudspeaker system. Extreme as these designs may seem, the goal here is to keep non-linear distortion low at high volume levels or, conversely, have very low distortion at normal levels. 
    I think it is important to be able to play back at live like levels. If the system has low distortion it will not sound loud, just natural. The level needs to be realistically high, so that the ear produces the same distortion and timbre as it would at a live event. Then you can experience maximum involvement and enjoyment. 
    The crossover/eq for a 4-way system is considerably more complex than the 3-way circuit for the PHOENIX. But, if you like a serious challenge, then you might follow my  4-way  functional block diagram and use circuit topologies from the PHOENIX for the individual blocks. You can use two printed circuit boards to realize all functions. Be sure to perform free-field and ground plane acoustic measurements of the mounted drivers before, during and after designing the circuits to ensure proper polarity and addition of all driver outputs and flat frequency response. Design Models F1
    CAUTION: Such 4-way open-baffle speaker project will lead to endless questions and frustration unless you have good electrical and acoustic measurement capability and a solid understanding of what each element in the system is supposed to do. You cannot adjust relative channel levels by ear and expect to obtain a speaker that is true to the original. Unless you have these resources your chances of improving on the PHOENIX are slim and you might even end up with lower performance than if you had followed the PHOENIX design.
    I consider the 3-way PHOENIX speaker system described here as more practical and useful for a wider range of audio enthusiasts, particularly since its output capability can be increased comparatively easily.   Top


Q4 - I want to use woofer XYZ which has larger Xmax than the 1252DVC. Which other parameters are important?

A4 - I use two drivers to double volume displacement and to reduce even order harmonic distortion by running them with opposite cone motion. The requirements for an open baffle driver are quite different from those for a closed or vented box design.
-  Fs < 20 Hz 
Qts = 0.5 - 0.8   
Smaller values of Qts increase roll-off towards the low frequency end and require more equalization than the 6 dB/octave boost for dipole cancellation that I used in the 10-300LP stage of the Crossover/EQ. Most drivers have too strong a motor for dipole applications and are over-damped in an open baffle cabinet. When the driver is mounted into the woofer cabinet, air mass is added to the free-space moving mass of the driver. This lowers the mechanical resonance frequency Fs and increases Qts by approximately the same percentage. The amount of change is difficult to predict accurately and best measured using f0Q0.gif from Ref.12
I do not recommend the use of drivers with very small magnets and Qts >> 0.8 in an attempt to compensate for the 6 dB/oct open-baffle roll-off with a rising driver response. Such equalization is crude at best. I prefer active 6 dB/oct low-pass filter equalization. The filter cut-off frequency should be set in conjunction with Qts and Fs to avoid a peak in the group delay of the resulting 3rd order acoustic high-pass filter. See FAQ19 for acoustic effects.
-  Sensitivity: <10 W for Xmax at Fs
-  Power handling >50 W
-  Frequency range 15 -100 Hz
-  Xmax > 6 mm
Note that   Xmax = 0.5 [(voice coil height) - (air gap height)]   is merely proportional to the linear excursion. Different drivers with the same Xmax do not necessarily have the same linear excursion range! The design and construction of the spider, surround, and magnet gap region have strong influence on the actually useable excursion range before distortion becomes excessive. 
A driver with Xmax = 12 mm will generate quite a bit of aerodynamic noise, i.e. distortion, at such high excursions. The space behind the spider needs to be open to the outside to minimize noise. The pole piece should have a large vent, or the cone should have no dust cap, so that air trapped behind the dust cap does not contribute noise.
    For a qualitative evaluation of a new woofer perform a simple test. Put a dot of white paint on the cone or dust cap to monitor excursion. Connect the driver to a power amplifier and drive it from a signal generator set to 35 Hz. Hold the unbaffled driver in your hands. Increase the generator output and observe how quiet the driver remains and how much excursion it can handle before sounding obnoxious. Remember that an open baffle cabinet will not attenuate this noise. Lower the frequency and repeat the test. Be careful not to overheat the voice coil or to destroy the driver mechanically!! The burst capability of the Agilent 33120A Arbitrary Waveform Generator is wonderful for this work. Unfortunately, the majority of drivers that I have evaluated required no further investigation after this test. 
    The Gefco X6100 and especially an Eminence Speaker Corporation 12-6676 proto type performed well and are very applicable. The Gefco X6100 driver is available from Madisound
    The Eminence 12" proto type was built on a similar platform as the Adire Audio "Shiva" driver, but specifically optimized for open baffle application. It would require a slightly wider cabinet, but should give 3 to 5 dB more SPL for the same distortion level as the Gefco X6100.  Cost, though, is at least twice that of the X6100. If you listen to a lot of organ music and woofer cabinet size is very important to you, then instead of stacking two PHOENIX woofers with four X6100, you could have nearly the same SPL output from a pair of theses drivers in a smaller cabinet.    For an update and somewhat disappointing conclusion of this project see Woofer2 and for a real, higher output alternative see Woofer3
    There are possibly other 12" or 10" drivers on the market with equal or larger useable, linear volume displacement than the 1252DVC and X6100 . These two drivers, though, have proven themselves for open baffle application and provide a sensible tradeoff between cabinet size, distortion performance and cost. 
    I measured the following parameters for these two drivers:
1252DVC:  Re = 5.5+5.5 ohm, Fs = 17.4 Hz, Qts = 0.5, Xmax = 0.5"p-p.
X6100:  Re = 11.4 ohm, Fs = 12.7 Hz, Qts = 0.43, Xmax = 0.5"p-p, voltage sensitivity when mounted in woofer baffle is about 1 dB lower than for the 1252DVC.  Top


Q5 - How much sound output can you get from the open baffle woofer?

A5 - I give a formula in Ref. 2 and in Design Models A1 that relates the sound pressure level from a driver on an open baffle to the SPL from the same driver in an acoustically small closed box, assuming that both have the same cone excursion.

        Fequal = 0.17 v / D

At frequency Fequal the SPL is the same for either driver arrangement. The open baffle driver output decreases at 6 dB/oct below this frequency relative to the closed box. Above this frequency it increases to an on-axis peak that is 6 dB higher than the SPL from the box. The peak occurs when the distance D between front and rear equals 1/2 wavelength, i.e. when the rear wave is phase shifted 180 degrees and in phase with the front wave.
For the PHOENIX woofer D = 19" (0.48 m). With v = 343 m/s

        Fequal = 120 Hz

Thus, for on-axis SPL at 120 Hz, one 12" driver in the open baffle cabinet is equivalent to the same 12" driver in a closed box. At 30 Hz, though, the open baffle SPL has dropped to 1/4, i.e. it would take four 12" drivers to maintain the same SPL as the single 12"  driver in a box, when all of them are driven to the same excursion. (See also FAQ14)
At 30 Hz the two woofer cabinets of the PHOENIX are equivalent to a single 12" in a closed box, assuming that the box speaker is flat to 13 Hz (-3 dB). Above this frequency, of course, the two open baffles generate considerably more SPL than the single 12" driver in a box. At 60 Hz the dipoles would be equivalent to two of those 12" box woofers and at 120 Hz equivalent to four in terms of maximum SPL. You can add 6 dB to this capability by doubling the dipole woofers and stacking them on top of each other for a height of 28", but this might negatively affect the WAF. The primary benefit of the stacked woofer arrangement would be reduced non-linear distortion when listening at the same SPL as before.
    A lot of air is shuffled back and forth between the two sides of the open cabinet. This sets up a strong sound velocity field but our ears only respond to sound pressure and we do not hear it. This is wasteful of cone excursion but the price to pay for natural bass reproduction. You can see why the open baffle woofer will never be a popular commercial item, when a majority of consumers loves excessive bass, and when a dipole woofer will just not get the room going in all its resonant modes for additional boom.   Top


Q6 - How much power will it take to drive the woofers?

A6 - Not much. At Fs it takes maybe around 5 W to drive a 12" unit to full excursion in free air. It takes just a little bit more in the open baffle cabinet because of the air mass loading. More power is needed as frequency goes up. I have found - by monitoring the p-p voltage across the woofer terminals - that each woofer channel is easily served by a 50 W amplifier.   Top


Q7 - The price of the Scan-Speak drivers is a problem. Is there any valid substitute without changing the design?

A7 - The Vifa D27TG-45-06 (also the reliable D26TG-35-06) fabric dome or D25AG-35-06 aluminum dome tweeter and P21WO-20-08 woofer could be alternate drivers for the main panel. The mounting hole dimensions need to be changed and some of the component values in the crossover/eq. While this will give you an excellent, natural sounding speaker, it has not quite the same clarity and dynamics as the PHOENIX.  See FAQ22
    Many other drivers could be used with the same crossover/xo circuit topology. If you leave the crossover frequencies and the main panel dimensions unchanged, then only the woofer and tweeter gain levels, the 1440PH phase shifting networks, and the 400NF notch filter might have to be readjusted. If you change the main panel dimensions, then you are in your own ball game, but the circuit topology should still give you all the necessary functionality.   Top


Q8 - Diffraction. Would a narrower (<12.5") main panel give sharper stereo imaging, due to reduced delay of edge diffraction?

A8 - Diffraction is a difficult subject. I will try to give a simplified explanation of how diffraction works for an open baffle speaker after looking  first at diffraction in general, and then for a closed box speaker. See FAQ 38 for actual measurements of diffraction.

    Fundamentally, diffraction is about the transition between an acoustic wave propagating from one space first into another space of different volume. The effect, at an observation point at large distance from the source of the wave, is a change in sound pressure and, correspondingly, a change in the frequency response. The effect will be different whether the point is on-axis or off-axis. Take, for example, a small source on a flat baffle. At low frequencies, where the wavelength is much larger than the distance from the source to the baffle edge, the source radiates essentially unimpeded into full space (4pi). At higher frequencies and shorter wavelengths, the baffle begins to look more like a large plane because it takes increasing numbers of wavelengths to reach the edge of the baffle. The source now radiates essentially into half-space (2pi), but with delayed reflections from the baffle edge that cause ripples in the frequency response, and which become weaker as the edge recedes in terms of wavelengths. On average, the pressure at a distant on-axis point in space will be 6 dB higher at high frequencies, than at low frequencies. 
    If the baffle is not flat, but tapered 30 degrees such that the source is located at the tip of a 120 degree circular cone of finite extension, then at short wavelengths radiation will be into 3pi-space. At long wavelengths radiation is again into 4pi-space and on-axis pressure increases only 20log(4/3) = 2.5 dB on average and with ripples in the frequency response from the edge of the cone. 

    Let's now look in more detail at diffraction at the front panel edges of a closed box speaker with rectangular baffle. Assume the driver cone moves abruptly outward and causes a  local air pressure increase. The pressure increase propagates at the speed of sound (343 m/s) away from the cone into an environment that is bounded on one side by the front panel. Until the pressure wave front reaches the edge of the panel, it looks as if the driver was radiating into half-space. When the wave encounters the edge it suddenly sees an expanded space and the pressure drops. This pressure drop occurs all around the front panel edge, though at slightly different time, depending upon the distance from a particular point on the edge to the cone. All together, the pressure is reduced to 1/2, i.e. it drops 6 dB, because the volume of space encountered by the wave has doubled. We can think of this phenomenon as if a delayed wave of half the strength of the original wave and with opposite polarity was propagating out from the circumference of the front panel.
    When we monitor this behavior from a point in front of the box (e.g. 1 m), then we observe first the arrival of the original pressure increase and a little time later a pressure decrease when the wave from the cabinet edge arrives. Note, that our conceptual model assumed that the pressure increase occurred in such a short time interval, that we can resolve the ensuing pressure decrease, which is no longer abrupt, but smeared because of the unequal distance from the panel edge points to the cone. If the panel had an effective width of 8" (0.2 m), and the cone was centered on the baffle, then the pressure decrease would occur 0.1m / 343m/s = 292 microseconds after the increase. To resolve the two events as separate from each other, the pressure must reach its final value in an interval shorter than 292 us, which means the driver must have a bandwidth greater than 1 / 292us = 3430 Hz. A slower pressure increase as produced, for example, by a 300 Hz tone would almost immediately be decreased by edge diffraction to 1/2 its starting value. 
    Thus, if we look in the frequency domain, the pressure response from an idealized point source on a finite size baffle will start at low frequencies with a value that is 6 dB lower than the value around which the response oscillates at high frequencies. The oscillation occurs due to the phasing between initial sound and edge diffraction, adding and subtracting from each other. By making the baffle narrower the transition to the oscillatory region moves higher in frequency, e.g. to above 2000 Hz in the above example. As the driver itself, due to its piston diameter, becomes more directional with increasing frequency, it also illuminates the panel edge with less strength, which then reduces response irregularities caused by diffraction. This comes into play when the effective piston diameter becomes larger than 1/3 of a wavelength. A smaller diameter driver, 6" versus 8", illuminates the the panel edge stronger than the larger driver for a given frequency and panel width, because it is less directional of itself. This effect is offset to some extend by making the cabinet proportionally narrower, which moves the response irregularities up in frequency. I think the popularity of 6" drivers is mainly driven by greater consumer market acceptance of visually less obtrusive narrower cabinets. 
    In general, the effect of cabinet diffraction can be observed and measured in the frequency domain as on-axis and off-axis frequency response irregularity. It shows especially strong in the on-axis response of symmetrical driver layouts. Diffraction effects can be seen most easily in the time and amplitude response to shaped tone bursts.
    References: H. F. Olson, Acoustical Engineering, 1957, pages 21-23. Anyone attempting to come up with a simple calculation of diffraction should first read: G. G. Muller, R. Black, T. E. Davis (all from Bell Labs), The Diffraction Produced by Cylindrical and Cubical Obstacles and by Circular and Square Plates, JASA, Vol.10, 1938.

    If I have not lost you by now, then let's talk about open baffle diffraction effects with the above in mind. Assume a small unenclosed piston source in the center of a circular, flat baffle. The source radiates towards the front and with opposite polarity towards the back. A pressure increase in front is associated with a pressure decrease in back of the baffle. Observing again the response to a stepwise increase in pressure at some on-axis distance in front of the baffle, we see the initial pressure increase, which drops to 1/2 when the edge diffracted front side wave arrives. At the same time we also receive from the backside of the baffle that 1/2 portion of the wave, which was diffracted around the baffle edge into the frontal hemisphere.  The 1/2 of the backside wave is of opposite polarity to the front wave and together with the front edge diffraction cancels the front wave completely(!). If the baffle had a 12" (0.3 m) diameter, then we would observe a  0.15m / 343m/s = 437 us  duration pulse in response to the pressure step from the point source.
    You might say this is a much stronger diffraction effect than for the closed box - and you are correct - but it is also the characteristic response of an open baffle, dipole source - after equalization. Modeling the response with a stepwise pressure increase and decrease implies an integration of the systems impulse response. Integration in the time domain is equivalent to boosting the low frequencies at 6 dB/oct in the frequency domain. The 437 us duration pulse is described by a sin(f)/f  frequency response function, which exhibits a flat response up to the frequency where the diffracted wave is delayed by 1/2 wavelength, i.e. where it adds in phase to the initial wave. For the 12" baffle this frequency is at 343m/s / 0.3m = 1143 Hz. At higher frequencies the response rolls off at 6 dB/oct with periodic dips and peaks unless the equalizing 6 dB/oct boost is leveled out. See Design Models A
    In practice you would not use a circular baffle to avoid the strong, comb filter like oscillations in response at high frequencies. A rectangular baffle smoothes out diffraction irregularities. In addition, as the driver itself becomes more directional with increasing frequency, the shape of the baffle becomes less critical because its edges are less illuminated. 
    Ultimately, it is the on-axis and off-axis frequency response of the equalized speaker that tells how well diffraction effects have been integrated into the design, regardless of the speaker type, cabinet shape, baffle width or drivers used. Narrower cabinets are just easier to work with for box speakers. A real feature of the PHOENIX is the smooth off-axis response that mimics the on-axis response, but with decreased amplitude as the angle is increased. You can see various frequency response data on the | System Test | page and in