Wednesday, August 6, 2014

Capacitors: All Things are NOT Created Equal - Part 1

When buying speakers or electronics, designers always begin a project with a budget and an idea of what can be done with that budget. In the lab behind the curtains, designs take on a very different look than what finally gets out the door. Bread-boarded amplifiers, cobbled-together cabinets, and recycled power supplies are common items jockeyed about in an engineer's inventory never seeing the light of day, at least from the consumer's eyes.

Part 1 - The Budget

An evolution occurs in an audio design where once a fundamental parts count is achieved, juggling the costs of these individual parts makes a difference in how the total comes in line with the budget. For example, the budget for a small 2-way speaker could look something like this (actual values vary but this is just an example to make a point):

  • 8" Woofer: 20%
  • 1" Tweeter: 10%
  • 2,500Hz BW-2 Crossover: 15%
  • Cabinet: 20%
  • Shipping box:: 5%
  • Misc. Materials: 5%
  • Labor: 25%
  • Raw Cost: 100%
  • Selling Price: 5 X Raw Cost

A similar budget is created for any piece of electronics gear with a far greater granularity but you get the general idea. What typically happens is that one of these items runs over budget so the designer must start juggling numbers to stay within the budget OR increase the budget (something the bean counters hate). So if the design must stay within budget, something has to give if the design is to make money. What typically is compromised first is the quality of the parts used in the design (what is on the bench rarely makes it into production).

If the designer can make a crossover network for less and stay within budget, this is one option OR use cheaper drivers OR a cheaper cabinet. Sourcing alternative drivers while possible is harder to do if a certain sound is to be maintained and the price difference for a typical 8" woofer can range from $20 to $80 (4x difference). Sourcing alternative cabinets is an option, especially if cabinet resonances can be disregarded and price differences are similar (2x-4x). 

But the cost of a quality capacitor or inductor can be hundreds of times higher than the least expensive form available. For example, a 2.2uF/100V Deuland Cast PIO is $443, a 2.2uF/600V Mundorf Supreme is $22.50 (about 20 times lower in cost), and a 2.2uF/100V non-polar electrolytic is $0.50 (886 times lower in cost). So the parts count can be identical but the cost for these parts can be adjusted so that the design stays within budget.

But what do you give up for the lower cost?  A good analogy to answer this question is to look at the MSRP of new automobiles. All of them will go 70mph but why does the TATA Nano have a MSRP of $3,056 (the cheapest car built) and the LAMBORGHINI Veneno have a MSRP of $4.5 million (BTW, the last one of the nine built sold for $7.6M)? Is the Veneno worth 1,472 times more than the Nano? Well that is what the high-end is all about, right? And if you believe that the Veneo is worth it, then you are truly a hopelessly hooked audiophile like me.



The Lamborghini Veneno

So to build a Nano, one uses the lowest cost parts possible (here the non-polar electrolytic capacitors) and does not include much in the way of an audio system and for the Veneo cost is truly not an issue (here the Deuland Cast PIO capacitors). BTW, the Veneno comes with a hand-tuned custom-built Monster(TM) audio system and the Nano doesn't. So it is with audio design and the allocation of its budget that determines what is and is not possible. (Does your automobile go from 0-60 in 2.9 seconds? The Veneo does and the Nano doesn't!)


 


The Tata Nano

Matching the links in the audio chain becomes as important as the budget to which this speaker is assigned. Again as an analogy, would you suspect to find the Monster audio system of the Veneo in the Nano? No. And why not? I am sure that this question need not be answered.

The question becomes one of applying the right money to the right design to fit into the right system. One would not appreciate the $400 Deuland capacitor in a $100 boom box but even in this modest system one would appreciate a minor capacitor upgrade from a $0.50 non-polar electrolytic to something a little more esoteric at just twice the price.

So now that you understand a little about budgets and how things fit into the overall audio chain, in the next part we will look at why it costs what it does to make a good sounding capacitor and what some of the major design considerations are.

Yours for higher fidelity,
Philip Rastocny

Skeptics are essential to keep us sane; skeptics do little to keep us inspired. Philip Rastocny, 7-16-2014

I do not use ads in this blog to help support my efforts. If you like what you are reading, tell your friends and please remember to reciprocate by purchasing some of my highly affordable works at Amazon.com. My titles include:

Sunday, July 20, 2014

Capacitors: All Things are NOT Created Equal - Part 0

I have been struggling with how to go about this series since so much has already been written on the subject. I don't want to repeat what so many have contributed but I still find myself looking for someone to say it the way I would explain it with no results. So here it goes: a brief high-level explanation of the most commonly-used component in audio gear next to the transistor: the CAPACITOR.

Part 0 – Introduction


This series helps to demystify the claims manufacturer’s make about their extreme and justify the cost of their pricey capacitors. This series is a bit technical, focusing on more than just aesthetics, but the summary to this series will provide an insight to everyone explaining what works well and why.

It is true that capacitors sound “different” and there have been many subjective studies regarding this topic. Audiophiles and engineers argue back and forth about specifications and observed results with both sides quoting their own sets of tests confirming or disproving their particular position. (This is sort of like saying: I love standards because there are so many from which to choose.

But this series takes a slightly different approach where I believe that the assertions from subjective observations are true AND that the skepticism from engineers is also deserved.  Is this an oxymoron? Nope! Read on and see why.

When someone claims they observe something, in order for an engineer to accept that claim it must be reproducible and repeatable. The standard way in which such assertions is confirmed is through double-blind testing. But there is something that happens in the behavior of the results of a double-blind test when the results are repeated many, many times: the engineer assumes that the results will ALWAYS be the same as that of these undisputed samples. Is this true OR is the sample size just too small OR is the design of the double-blind test itself in some way flawed OR are the participants? Good questions and this reminds me of the Kodak-Nikon test.

Let me summarize this test briefly for those of you unfamiliar with it. A while back Kodak sponsored a test to see if someone could tell the difference between various 35mm cameras taking pictures using their film. The test was to see if someone could pick out the ones taken with a Nikon camera and lens. What sounded like a slam dunk readily revealing that the subjective evaluations of photographs is in itself flawed because of the biases of the observers turned up one interesting twist. While almost all of the major photo critics of the time failed the test as anticipated, one picked out the Nikon photograph 100% of the time. That's right, zero errors.

What does this mean regarding such tests? Even tests to prove people are biased can themselves be biased if the sample is too small. Just imagine what conclusions would have been reached if this one person had not succeeded? Moral: Tests must be constructed very carefully, moreso than most are.

This series takes a different slant: it is an objective study that uses these subjective preferences as a starting point, and focuses on how the design of a capacitor preferred by these subjective ratings may gain a sonic edge over another. This approach will hopefully point you to a manufacturing process (aka a manufacturer’s series or material composition or whatever) that will give you the sound you want, or at least close to it, without having to go through the evaluation process yourself. It will also demystify how a capacitor is made and the planning involved in making a really good one (as opposed to a quick-and-dirty one).

Before I begin, I wish to provide some links to the works of others in this field acknowledging their superb efforts in analyzing the sound various capacitors possesses and the consistent way in which they have subjectively described the sound between them. It is a massive undertaking to describe the sonic attributes of anything, much less take the time to understand the audible effects of swapping a single capacitor in an electronic circuit. However, some folks have done a superb job in such an effort. Unfortunately by the definition of the word “subjective” this does not mean you can correlate one person’s views or impressions with another. Although they both may arrive at the same conclusion that Brand X sounds better than Brand Y, the reasons for saying so will probably differ.

In my opinion, the best audio-grade signal-path capacitor review is by the Netherlands’ based Humble Homemade HiFi analyzing an amazing diversity of products and providing a uniform – although subjective – rating system.  The second purely subjective multi-part article was written by Jon L. of Enjoy the Music while less thorough brings up very valid points. The great Joseph Levey also ventured into the fray with his Great Capacitor Shootout recommending specific brands over others.

Another work of important note is that of Martin Collums who way back in 1985 did what it is that I am attempting to do now. He risked his reputation back then from the professional audio community (few people desire being labeled a heretic by their peers) daring to claim that capacitors sounded different. History tells us now that Martin was indeed not a heretic but a man of science way ahead of his time attempting to correlate subjective impressions with objective measurements. He believed similarly to my own philosophy: what someone hears, we should be able to measure. The trick is to find what or how to consistently measure the observation, something that can prove to be far more complicated, elusive, and uncommon.

In his 1985 conclusions, Collums identified the following measurements and values associated with a quality capacitor, the last two of which are unfortunately non-standard specifications:
  • Dielectric Absorption of less than 0.1% (the property of retaining an electrical charge)
  •  Higher voltage than the designed circuit minimum (e.g., use 200V rating or higher in a 50V circuit)
  •  Low Equivalent Series Resistance (the losses incurred by adding the capacitor to the signal path)
  •  Low inductance (the electrical property that restricts transient response and high-frequency extension)
  •  Low internal leakage (self-discharge rate NOT the same as the ESR)
  •  Low piezo (mechanical-resonance and microphonic) effects
  •  Low delay error (ability to instantaneously track an abrupt change or impulse)

As you can see, there is a lot that must be examined to grasp the characteristic behavior of the lowly capacitor. For you to understand what I will explain, you must learn a few fundamental things about electronics (don’t worry, I’ll keep this to a minimum and make it as painless as possible). For those of you allergic to math, no worries here either. And for all of you, if you read this faithfully, your knowledge of how capacitors work will be greatly enhanced far beyond Wikipedia or other similar online helpful aids. 

So in Part 1, I will go over the generic fundamentals of a capacitor – any capacitor. In other parts of this series I will describe how engineers have tweaked these basic concepts to give the high-end community a superior product. So hang on…you are in for the ride of your life. Above all, remember to listen with your ears, not your eyes.

Yours for higher fidelity,
Philip Rastocny

Skeptics are essential to keep us sane; skeptics do little to keep us inspired. Philip Rastocny, 7-16-2014

I do not use ads in this blog to help support my efforts. If you like what you are reading, tell your friends and please remember to reciprocate by purchasing some of my highly affordable works at Amazon.com. My titles include:

Wednesday, July 16, 2014

Frugal and Effective Sound Damping in Cars

I have a new ride, a 2014 Sportwagon. I love the car but absolutely hate the road noise.

I measured my old Dodge Journey interior noise at 78dB at 70mph, a reasonable level. I measured the SW at 89dB on this same road (yikes!). So I decided to start adding sound proofing/damping to my ride and wanted to share the results.

First, from behind the front seats to the tailgate is basically not damped so start by taking out the rear seat and seat belts (not hard, just requires a triple-square bit tool I purchased at Advance Auto Parts for about $13US). Next, yank out the carpet and side panels (save the clips) to expose the rear wheel wells.

I ordered two 4'x8' sheets of 1/4" high-quality closed-cell neoprene foam, similar to the stuff VW uses in the engine compartment and front fender wells. I covered as much of the interior as possible including the spare tire well by taping it in place with aluminum foil HVAC tape. This wasn't as hard as it sounds since the foam cuts easily with a standard carpet knife. I did not glue this down (contact adhesive is usually employed for this application) but the results were excellent with just the foil tape.

I made certain that the fender wells were covered as close to 100% as possible and the rest about 90-95%. The spare tire area was easy to cover 100% and the 1/4" foam added no issues to recovering the carpet or interior panels in exactly the same location.

Road noise now measures the same 78dB and I am a happy camper. The remaining noise appears to be coming from the door panels and the hood and once I upgrade the speakers I will address these issues.

I chose this particular foam since it is used to make wet suits for scuba divers and was very water repellant. I anticipated that one day water would find its way to the interior (windows left rolled down, etc.) and wanted to use something reasonably safe but yet highly effective. This material seemed to do both while still being a frugal choice.

I can now easily converse with the passenger at highway speeds without having to raise my voice to reach above the annoying road noise. It makes for a far more pleasant driving experience and the audio system will sound better since you can hear more dynamics in the music.

I am looking at a universal hood pad too but have yet to take the plunge.

One last suggestion is a spray-on/paint-on/roll-on product called Noxudol. You must be really careful with this one since any overspray could cause serious damage to your shiny exterior paint. Here is an impressive demo of its sound-damping effectiveness.


Anyway, I thought that this would help those of you who prefer a quieter ride without having to take out a second mortgage on your home. Enjoy!

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please tell your friends and remember to reciprocate by purchasing some of my very affordable works at Amazon.com. My titles include:

Sunday, July 6, 2014

The Lost Era

I have been around the audiophile block a few times following its meager beginnings in the era of the monophony to its stellar accomplishments in stereophony. During this time (roughly from 1955 to 1985) I have observed many changes - creative comings and fads fading - and even proven technologies fade like dying echoes in a concert hall. Many of these proven technologies die because of the NIH ego of electrical engineers (Not Invented Here). It is sad since much about the current high end is a rediscovery of old ideas.

For example, who of you can recall the home 1/2" video tape recorder wars of the early 1980s? For those of you who cannot, it was a time when video recording standards had two camps: one with superior video quality (Sony's BETA) and one with longer recording time (JVC's VHS). Guess which won? The one with longer recording time. Sales of the lower-quality 2/4-hour tapes somewhere were on the order of 20:1 and BETA lost the battle; consumers decided who would win with their dollars.

A huge lesson was learned in the industry that people wanted convenience and not quality so - badda boom, badda bing - along came the digital compact disk. Insisting on a size (a diameter of 120mm) the CD physical size was born (whatever CDs would be, they must also be playable in a single-DIN slot in an automobile). And assuring at least an hour of playing time, the math was pretty simple to determine sampling rate and word size given these requirements. Leveraging off of this VHS-BETA lesson seemed to be an assured success - give the public what it wants and compromise higher audio quality - and so it was.

There was no reason to use a 16-bit word or a 44.1kHz sampling rate other than that was the result of how much digital data space was available (640Mb). (See http://makbit.com/articles/cd-overview.pdf to understand the complete specification). A larger word size and a higher sampling rate could have been used, but then the CDs playing time would be shorter AND with the lesson of the video tape wars was still bouncing around in the heads at Sony, no one wanted to make this mistake again.

So the digital age settled on this 44.1kHz/16-bit standard and in 1982 the Red Book era was born. Mastering studios all around the world eagerly jumped on this new standard and soon analog productions fell by the wayside. Skyrocketing sales despite high prices (albums were about $5 and CDs about $15) resulted and the recording industry underwent a transition that locked us into the Red Book format for decades to come.

But an interesting thing happened along the way: people started listening to the music and hearing the faults in the playback approach to the Red Book standard. A few artists insisted on simultaneously recording analog and digital versions to assure that - in the future when a new standard arose - their efforts would not be "lost." At the time, no one could foretell if any improvement could be made to playback this format simply because it was brand new. But I digress since playback improvement is not the point of this entry.

Those who did not do this were literally locked into a format that had zero hopes of ever  appreciating the full sonic benefits and nuances of a performance. Those who did this were shunned and considered radicals being put down by the vast majority of the engineers and artists as silly or weirdos. These radicals were indeed the saviors of the high-end.

But those who did not indeed were proved to be wrong. Today, there is a resurgence in analog recording and playback and many artists demand much higher recording rates than the Red Book standard. Someone once asked me a related question: What is an infinite sampling rate and word size called? ANALOG!

On the surface, this question is very telling of the issue since any digital sample is by definition an approximation limited by its word size and sampling frequency. While this approximation is closer to the original with higher-resolution recordings and better Analog-to-Digital Converters (ADCs), they are still approximations and still prone to errors. Even digital clock jitter and similar artifacts are reportedly measurable and audible in both the recording and playback equipment.

As the last of the analog recordings were abandoned, a day came that no one was recording with analog gear. Digital mixing consoles, digital signal processors, digital recorders, and the like took over the industry and before long, the only way you could get an analog recording is to do one yourself. I know that people will argue  measurable errors do not translate into audible errors and from the objective position they are correct. But those who listen subjectively disagree even though these same people cannot consistently tell which is which in a double-blind test. While it is easy to dismiss such subjective claims as unfounded, the numbers of those forming these subjective complaints are growing.

Regardless of who is right, one fact remains: ever since the last analog recordings were abandoned, all of the performances recorded in the Red Book format can never sound better than they currently do. While playback improvements can make the Red Book recordings sound better, they cannot by definition be less error prone than a digital recording of higher resolution. One day in the near future, audio will undergo the same transition that HD video is now experiencing with UHD (1k to 4k). This means that an entire era of recordings will always be locked (lost) in this limited format and one day the audio industry will look back at this era as one of tedious but necessary growing pains. Are those who pooh-pooh subjective listeners wrong? Yes, because one day in the future the standard will change.

So what can be done? If the current digital era will one day be viewed as archaic and lost, what can be done to plan for this inevitable obsolescence? Simple: record in as high of a resolution possible knowing that even it will one day be viewed as inadequate.

The high-end is constantly evolving, undergoing improvement, and experiencing change. But that does not mean we should rest on our laurels in any given standard or achievement. We should insist on Constant and Never-ending Improvement to push the state-of-the-art forward, to make it better, to make it more realistic. After all, isn't that what the high-end is all about? I would rather have someone look back and say "They did their absolute best" rather than have them say "They did what made sense at the time."

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please tell your friends and remember to reciprocate by purchasing some of my very affordable works at Amazon.com. My titles include:


Thursday, July 3, 2014

Crossover Networks, Zobels, and Life Lessons

With my newest adventure with the ongoing Bozak restoration project, I had a terrible time taming the interaction between drivers and I questioned the wisdom behind the manufacturer's choice of a first-order (-6dB/octave) crossover network. It is true that the technology of the time (1958) is not state-of-the-art but there are a lot of design considerations then that still hold true today.

For example, Bozak's use of a light-weight cones provides better transient response than one of greater mass. Newton's second law of physics reads "Force is equal to Mass times Acceleration (F=MA)" meaning that if you increase the mass, it requires more force to get the same amount of acceleration. For loudspeakers this is still true today and it is why light-weight ribbon technologies are still superior sounding (cleaner or crisper) when compared to heavier cone or dome technologies.


One of My Hero's: Sir Isaac Newton

But how did I decide this? Did I just want to "give it a go" or was there more? Sir Isaac himself was quoted as saying (paraphrased here) "There is a huge difference between believing (or suspecting) something and knowing something to be true" and his efforts helped mankind better grasp its understanding of our complex world. Reading about my situation and what I did may help you know whether or not to make changes in your speakers. If nothing else, it will help you better understand why any manufacturer chooses the crossover network design that they do. So let's see what my issues were, what I did, and the reasoning behind my choices.

Remember, when changing something, you can always put it back to the way it was so don't be too worried about making a mistake. Just take notes, take a lot of before-after pictures, and remember what you did. Make one change at a time and who knows? Maybe you can make a silk purse out of a sow's ear! So with that, let's begin. This is a very simple thing you can do to improve the sound of your own speakers.

The RTA measurement below is of my Bozak B-209A midrange driver in its enclosure with no crossover network attached. This tell you a lot about this particular driver, especially how aggressive it is above the manufacturer's chosen crossover frequency of 2.4KHz. Remember to disregard the data below 300Hz since my cell phone (an AT&T Samsung Galaxy S-III) incorporates a built-in non-defeatable bandwidth limiter to make voice recognition easier. It is reasonably accurate above 300Hz and you can observe the relative changes rather than make absolute conclusions.


Bozak B-209A with No Network

This is a 6.5" conventional dynamic driver with an aluminum cone and an aluminum dome functioning as the central dust cap. This means that there is a certain amount of acoustic energy contributed by both the cone and the dust cap each acting independently of the other. This is also why the driver's sound pressure extends uniformly well above the manufacturer's 2.4KHz crossover point (most of the higher-frequency energy is emitted by the metallic dome). However, it is wise not to push any driver to operate at its limits (operating at a range well inside its capability). To do this, an engineer typically chooses a crossover point generally an octave below that upper limit and an octave above that lower limit.

Sounds pretty simple, right? Well, there are other issues at play that interfere with a "perfect" world. You see, when a crossover network begins to attenuate (roll off) the signal to an attached driver, the driver may produce non-linear sound pressure levels for any number of reasons. Lower-order attenuation slopes may be inadequate to prevent any non-linearity from interfering with the operation of other drivers.

For example, the Bozak B-209A has a good characteristic frequency response to about 7KHz, a highly desirable property. However, when connecting the driver to the manufacturer's designed first-order network (a 2.4KHz crossover frequency), the driver inappropriately colors the sound of the tweeter in its 4-7KHz operating region. Below is a stand-alone measurement of this driver connected to this manufacturer's designed first-order network.


B-209A with -6dB/octave Attenuation above 2.4KHz

What the heck happened? This looks terrible and indeed it sounds awful. As you can see, there is a significant change in the uniformity of the driver's sound pressure as compared to when it is not connected to any crossover network. Most notable is the rising response of the driver (white dots) above the network's predicted attenuation level (red line). Although somewhat attenuated by the crossover network, the driver still contributes a CONSIDERABLE amount of acoustic energy in a region in which it is not desired. This undesired contribution will color the sound of the other driver (here the tweeter) causing non-uniformity in sound pressure and phase distortions all over the map. Yuck! No wonder it sounds bad.

To understand why this happens, we must look at what other electro-mechanical influences could disrupt the behavior of the crossover network itself. After all, it should have worked, right? The network should have attenuated the acoustic output of the driver above the crossover point as predicted. What changed that defeated the networks ability to control the driver?

Crossover networks are designed to operate at a certain impedance, here 8 ohms. However, any dynamic driver has a rising impedance which basically pulls the rug out from under the crossover network and eliminates its ability to attenuate sonic energy from the driver. The Bozak B-209A driver has a measured impedance of 8.3 ohms at 2.4KHz, 14 ohms at 8KHz, and 22 ohms at 16KHz, a value almost three times the value anticipated by the crossover network design. Could this rising impedance be the culprit? Yes it is!

So the first issue to be addressed is the rising impedance of this driver. A Bourcherot cell (commonly called a Zobel network) stabilizes the rising resistance of the driver. In reality, a Zobel network is another first-order network that introduces a mirrored resistance value to that of the rising impedance thereby normalizing the resistance observed by the crossover network (i.e., it stabilizes the rising impedance). This allows the crossover network to properly operate at its anticipated 8 ohm value. Below is a measurement of this same driver with a Zobel network in place.


B-209A with Zobel and -12dB/octave Attenuation above 2.4KHz

Adding the Zobel network helps to resolve this tweeter-interaction issue but the sound level just before the crossover point also suffers. While the Zobel is a very good idea, its influence can adversely impact the linearity of sound pressure desired from a driver. In other words, a Zobel is not a magic bullet but it can help the crossover network better control the driver's behavior. Changing the Zobel values can marginally correct for this non-linearity in the desired operating band at the expense of increasing the sound pressure above the crossover point. The only way to have it both ways (uniformity without coloring the tweeter's sound) is to move to a higher-order (steeper sloped) crossover network design (i.e., -12dB/octave or greater).

However, making the crossover network slopes steeper creates other issues such as increased cost for components and additional phase distortion at the crossover frequency (remember, in audio and in life you rarely if ever get anything for nothing). The only other option is to change the driver, something I did not want to do for this project. The logical choice to control better control this driver is to abandon the manufacturer's designed first-order network.

Finally we have reached the logical steps I followed in making the decision to change the crossover network design from manufacturer's choice of a first-order design. Although my gut originally told me something was wrong, like Newton I needed to prove the truth to myself rather than just follow my hunch. Although my hunch was indeed correct (a plus in the column for trusting your gut) objectively testing my intuition proved the truth to me (a plus in the column for trusting your mind). Both my intuition and my logic were necessary in discovering the truth for without either the truth would not have been discovered.

WARNING: Stepping on my soap box now. This is a micro life lesson. And so it is in other walks of life where opinions, ideas, and convictions can cloud your understanding of the truth. These are sometimes called biases and have absolutely no place in science - or in life if what you desire is your highest good. Finding value and a balance in both is where solutions lie. Stepping off my soap box again.

In my case, moving to a second-order (-12dB/octave) network significantly improved the operation of the driver but more than doubled the cost of the components in the crossover network. Even though the steeper slope helped in this design, a Zobel network was still required to control the driver's rising resistance.

BTW, with a second-order network, the phase of the sound pressure at the crossover frequency also changes by 180 degrees. This means that the piston movement of the B-209A would appear to be moving backwards as the tweeter movement was forwards. To resolve this is a simple matter of swapping the polarity of the driver (put the plus wire on the minus terminal). Below is the final measurement of this driver with a well designed second-order Bessel network including a properly engineered Zobel.


RTA Measurement of the Final Design

Whoa! Where did that nasty 1KHz peak come from? As I mentioned earlier, you never get anything for nothing and the peak is one of those unexpected "gifts" (and probably why Bozak never used a second-order network). Mathematically predicting behavior and measuring reality is often two very different things as demonstrated here. There is an interaction between the higher-order network and a natural resonance and so yet another level of tuning is needed. Called a parallel notch filter, a few components in series with the driver can help tame such annoyances - at the expense of additional phase distortion. However, when such a peak persists, there is little choice to the matter.

The good news is that with the introduction of a properly designed notch filter (one involving more iteration), a driver's irregular behavior can be tempered. Below is the result of all modifications to the Bozak B-209A driver as observed from the measurement of the entire system.


System RTA Measurement 7-3-14

Although not ruler flat, the performance is vastly improved despite the remaining anomalies. Instruments have that natural timbre reminding you that you are now one step closer to headphones at the mixing console.

Summary

What I wanted to demonstrate to you was how the behavior of drivers changes when connecting them to passive crossover networks. Mathematically calculating a Zobel's value and then adjusting that value to make it work properly is an iterative operation, one requiring patience on your part to get it right. Remember, after making changes and reaffirming the chosen values, ALWAYS confirm measurements with prolonged listening tests. I cannot tell you how many times I could make the RTA measurement look ruler flat and appear visually fantastic but have the system sound absolutely terrible. Needless to say I backed out of these modification as fast as dropping a hot potato (yikes!) while learning yet another valuable life lesson.

The toughest challenge you will face in designing or redesigning your speakers is to be brutally honest with yourself. Ask yourself, "Is what you just did make your system sound better or worse?" Then, answer this question both objectively (listening) and subjectively (measuring). Just because something is predicted to work will it do so. Engage both sides of your brain and listen to your music with your ears, and not just your eyes. Get your ego out of the way and listen with your heart.

There are two quotes I find appropriate for this life lesson:

Not everything that counts can be counted, and not everything that can be counted counts.
Sign in the office of Albert Einstein

Not all that matters can be measured; not all that can be measured matters.
Elliot Eisner, Artist, Author

From these two quotes, it appears that the left and right brains can come to an agreement. I challenge you to find yours.

Related ArticlesSee all entries about speaker enclosures in Part 1Part 2Part 3, and Part 4. Also, a related article on the effects of crossover network components on driver performance.

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please tell your friends and remember to reciprocate by purchasing some of my very affordable works at Amazon.com. My titles include:

Saturday, June 7, 2014

Speaker Enclosures - Part 4

As all things must come to an end, this is the last of a 4-part series discussing issues with speaker cabinet designs. By now you should understand a little bit about how cabinets influence the sound of a speaker by what is called internal resonances. These resonances are challenging to control but easy to predict in solid rectangular cabinets. Changing the shape of a speaker enclosure from a solid rectangle to one with fewer parallel sides eliminates some of the internal resonance issues and eliminating all of the parallel surfaces eliminates one of the modes that an enclosure resonates (axial mode).

Truncated pyramids with their totally non-parallel wall structure are good choices for such designs as are variations on the basic wedge shape. But again there is more about cabinets that can be visually discerned. Scrutinize the way the baffle board is designed (the board on which the drivers are mounted) in the two cabinet designs below and note the differences. Take your time and see how many you can find just by looking at them.


Baffle Boards 1 and 2

The first thing you should notice is that the RH baffle board flush mounts the drivers and the LH does not. Why do you suppose this is? Could there be a reason? You bet! It's called the First Reflection. As the sound moves away from the driver, it strikes something - anything - from the edges of nearby drivers to screws on the baffle board to - well you get the idea. Anything that sound can be reflected from will bounce back to the driver and disrupt the sound coming from it. The simplest thing to do is to make the front of the baffle board appear as flat and smooth as possible thereby eliminating as much of the driver-induced first reflections as possible. You can do this by recessing the drivers into the baffle board (countersink them) and to use flat-head screws instead of round-head, etc.

Good job! Now, look at how the drivers themselves. Why did the RH designer use three drivers and the LH designer two? Good question. Air is moved by the driver thereby producing sound but just like bicycle racers ride lightweight bikes to go fast, two smaller drivers are typically lighter than one bigger driver and they will move faster (have better transient response) than using a single driver. However, you never get anything for nothing. You double the cost by adding a second driver and the free-air resonance of the cone will be higher (less deep bass, all things being equal).

Good. So far you have noticed how drivers are attached, recessed, and arranged on the baffle board. BTW, why did the RH designer put the tweeter in between the two woofers? Doing so causes the relative sound wave created by the woofers to appear to emanate from the same point as that of the tweeter (called a point-source). Unfortunately, there is a compromise with such an arrangement that disperses the sound by the two drivers well horizontally but not so much vertically. So to get good listening from a wider range of listening positions, you should orient the speakers vertically as shown rather than laying them horizontally on their side.

OK, let's look at some more examples and see what differences can be observed. Again take your time and see what you notice just from the appearance of the drivers on the baffle board.


Baffle Boards 3 and 4

The most obvious difference is the way the woofers are mounted. These are both KEF speakers and it shows how the evolution of design takes place. The model on the left shows the typical convention for mounting a driver: put a screw in every hole in the driver basket. The model on the right shows a marked departure from this philosophy as a result of some serious research: 3 screws only in a 120-degree pattern. KEF found back in the early 1980s when trying to minimize basket resonances that by using three screws to mount a driver created the minimum amount of mechanical resonance. This is something to note that seems to elude speaker manufacturers today. Most still plug all of the holes with screws.

Here is a free hint: if your speakers have 8 screws in them, back off the 5 screws that are not in a triangular pattern and see if you hear a difference in the way they sound. If so, remove the driver, plug the holes with RTV, and replace the driver using only 3 screws (one on top and two at the bottom).

One last thing that you need to understand about the baffle board and that is shown in the next picture.


Baffle Boards 5 and 6

These are two versions of the B&W model 801 as they have evolved over time. The LH older model shows an early attempt to round the edges of the enclosure at the baffle board. The RH newer version shows the implementation of a no parallel sides enclosure with much more radically rounded midrange and tweeter surfaces. What B&W and many others are doing here is to remove the effects of what is called edge diffraction where the mere presence of a square edge influences how the sound radiates into free space.

Rounding the corners of the baffle board is pretty common practice today and something you can easily see when making your next loudspeaker purchase. Those with well designed boxes using flush-mounted drivers, non-parallel surfaces, and rounded baffle boards at least give the loudspeaker a chance at sounding better by minimizing the physical interactions of first reflections and edge diffractions. It does amaze me that manufacturers still insist on using more than three screws to install drivers but hey, some folks never do learn from history.

Anyway, I hope you have enjoyed this brief explanation of speaker enclosures. At least now some of the mystery behind the curtain has been exposed and you can better understand why designers make the choices they do. Most is cost driven but some are aesthetic compromises. Regardless, all are just that: compromises. And BTW, compromise is not a bad word as some may lead you to believe.

Related Articles
See all entries about speaker enclosures in Part 1, Part 2, Part 3, and Part 4. Also, a related article on the effects of crossover network components on driver performance.

Yours for higher fidelity,
Philip Rastocny

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