Thursday, January 31, 2013

Bozak Rebuild Project - Part 6

Old drivers like the Bozaks I use in my project have very little technical information available; the Thiele-Small driver parameters readily available today did not exist until 15 years after their manufacture.  But with the help of the newly acquired XTZ Real Time Analyzer, some of this parameter mystery is moving out of the darkness and coming into light.
All dynamic drivers have an impedance plot that resembles the one shown next.  Where the measurement peaks (the resonant frequency), how wide the peak is (the ratio of the bandwidth at -3dB to the frequency at the height of the peak is its Q), and how low it plunges (the minimum impedance) tells you a lot about the behavior of the driver.

Typical Impedance Plot of a Dynamic Driver

Even without knowing the impedance plot you can estimate the resonant frequency by what you observe the driver does from a SPL analysis.  For example, the Bozak B-209A midrange driver I am working with in this project was measured to have the following output within its enclosure and attached to its stock network (pink noise, all other driver loads removed from the network).
Near Field SPL Graph of the Bozak B-209A Driver
You would assume that the sonic energy suppressed by the network design (an 800Hz first-order Butterworth High-Pass Filter) should predictably suppress the sonic energy below 800Hz, and it does to some degree.  However, as you can see in this graph, the resonance of the driver is somewhere around 315Hz.  Making a system measurement as shown below reveals the effect of this sonic contribution as the acoustic energy begins to rise significantly at this resonant point.  The result is that the resonance of the driver is measurably coloring the sound.


RTA Measurement of the First-Order Network
This RTA measurement tells you what the driver is doing as opposed to what the mathematical model predicts it should be doing.  It is the non-linear impedance of the driver that alters how it behaves as opposed to how you predict it will behave.  In the case of first-order networks such as the one used in the stock Bozak design, the tradeoff gained by good phase relationships between the woofer and midrange drivers is lost by the naturally occurring rising resonant impedance of the midrange driver below the crossover frequency.  What happens acoustically is that both drivers produce sound in an overlapping region and creates louder sound in this region.
As you can see, the sound pressure rises at about the 315Hz resonant frequency of the midrange driver.  But notice that the sound pressure also dips at about one and two octaves above the driver resonance (630Hz and 1.26KHz).  Because of this harmonic recurrence, this indicates some sort of destructive acoustic or electrical phase interference at these points.  All of these problems in the uniformity of sound pressure can be attributed to the resonance of a driver.
So if you assume that the drivers are creating sound in their assigned bandwidths, here you would be wrong.  Simple mathematical calculations and even mathematical models do not accurately predict reality.
Changing the first-order Butterworth HPF to a second-order Bessel network on this midrange driver changed the behavior drastically as shown below.  The sonic contributions of the impedance resonance are now adequately suppressed and the resulting sound pressure is considerably more uniform.
Second-Order Bessel HPF Network
The results of this test show that the nonlinear sound pressure was indeed being contributed by this midrange resonance.  The 315Hz peak is now greatly reduced as is the 630Hz dip.  But as it is with most problems, fixing one can create another and the 1.26KHz dip has now become a 1.26KHz peak.  A resonant tank circuit may be necessary to completely control this sonic contribution of this driver’s impedance peak but only time will tell if the phase shifts of such a circuit will create more problems than it solves.
The results of changing the network design show a huge gain not only in the observed uniform sound pressure but also confirmed in subjective listening tests.  What I heard most definitely was a reduction in the lower mid-bass region particularly noticeable in the human voice.  Before there was breathiness to the vocal content that now sounds considerably more natural.  Overall, the system sounds more like the real thing where acoustic guitars now produce a fundamental string component that can be heard resonating in the guitar body.  Small inner detailing of fret fingering is more in the forefront and the hollowness of a clarinet in its lower registers is much more representative of the sonic truth.
The point is this: there is more to building a quality loudspeaker than stuffing drivers into a box and calculating a crossover network, much more.  Getting them to work in harmony and create the accurate sound you want without coloring it too much is where the real art lies in design.  Untold hours of testing, retesting, rethinking, redesigning, and reflection on results is where the sheep are separated from the goats.  Properly interpreting what your measurements tell you is where true art enters into the picture.
I hope to better eliminate the remaining small problems introduced by the midrange driver resonance after which I will consider its design finalized.  The sound this system produces today is light years ahead of the sound it created at its start just over 13 months ago.  I would encourage you to hack into your speakers after making similar measurements and see if you can squeeze more out of your system than available from its original design.  This experiment was fruitful and yours may differ, but the fun you have and the experience you gain is worth the effort alone.  If all else fails, you can go back to the original design so what do you have to lose besides a little time?
So I hope you enjoyed this dissertation and can benefit from what I have tried.  I will make the network pretty once I stop tweaking it and maybe add a final picture of it in Part 7 (although this may never happen, but then again I said that Part 6 would not happen either but here it is).  As I said, I am still writing and I have a life.  Besides audio I also love the beach, astronomy, and bicycling with my wife.  I have another novel and a self-help book in the works so time allocated to my blog is very limited.
Bear with me, keep checking back for updates, and you will see I have much more to share.  I have my eyes and thoughts on the tonearm of my turntable…

See also Part 1Part 2, Part 3, Part 4, and Part 5

Yours for higher fidelity,
Philip Rastocny

Copyright © 2015 by Philip Rastocny. All rights reserved.

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