برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

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