Saturday, July 23, 2016

Face-off: Top Midbass Speakers (Part 1)

I consider myself a wealthy person. No, not wealthy in the usual way. Instead, I am lucky to have hundreds or thousands of fantastic friendships. These friendships incalculably enrich my life. One of such friendships is with Jerry Niebur.
Mid 1990's Black and White photo of the sales floor at Stereo West in Omaha, Nebraska
Sales Floor at Stereo West in Omaha, NE
I met Jerry over 22 years ago. After I was called to talk with the bright-eyed and enthusiastic guy who wanted better sound for his red Cavalier, the conversation quickly took a very technical direction. Very smart, Jerry wanted to know much more than the average Stereo West customer. So, we discussed about a system that would deliver the goods. And did his car perform. Despite being a simple installation by our very high standards-in-sophistication, the car did everything right. This is the image that Jerry and I kept. One based on solid fundamental performance of a car that barked little while devouring the competition.
As I left the country to pursue my dreams, Jerry never lost the audio bug. So, when I received his call many years later, it was easy to take off from where we left it.
But this time, the student had become a master. Jerry had real insight and a large following among the many across the nation who love sound and who document such love through the many online forums on the subject.
As Jerry's celebrity grew, so did the size of his projects. One day, he called to say that he was attempting a test that would dwarf anything ever attempted by even the wealthiest of industry publications. Jerry wanted to end, once and for all, the perpetual debate among the audio elite. He wanted to define the best mid-bass driver the industry could deliver. Since it made no sense to include any of the substandard iterations vomited daily by most Chinese speaker makers, Jerry settled for the best available. Perhaps the only noteworthy brand missing was Focal. But Jerry had to settle for speakers he could get a hold of. You see, without tens of thousands of dollars to pour into fancy loudspeakers, Jerry had to rely on the assistance of many forum participants. Anyone with a high end pair who was willing to send their beloved drivers to Jerry for the test did so.
It takes a real understanding of the complexity behind Jerry's project to begin to see why he was more than crazy. But crazy or not, he accomplished it. And, thanks to his efforts, I am now able to bring it to you.
I write this blog with the goal of creating great archival value for the many who love audio and music. As such, transcribing the results from Jerry's experiment is perhaps the best chance I have ever had at honoring my blog's goal.
As you read through the experiments and conclusions, it is probable that you will disagree with some or all of the content. But regardless of your level of endorsement or skepticism, never forget that Jerry went through great lengths to remain as objective as possible. Be thankful that he worked very hard for all of us without demanding a single cent in compensation. Enjoy Jerry's altruistic and very valuable work:

The Art of the Mid -Woofer; 
11 Top Midbass Woofers Face-off!
By Jerry Niebur

Pioneer Stage 4
Approximately 80% of all high-end aftermarket mobile audio systems consist of a two-way (mid-woofer& tweeter) front stage with some type of bass supplementation in the rear of the vehicle. Consumers wishing to attain a level of superior musical reproduction, one that would equal a decent home audio system, typically look at purchasing products with exotic materials and certain published specifications associated with “high” performance; usually aligning certain brand names with specific parameters suggesting superior engineering and build quality.
We set out to find superior performance midbass drivers capable of producing accurate midrange frequencies while still producing authoritative, yet tonally accurate, midbass. The midbass performance is critical to high quality music playback as the speaker is asked to play 5 full octaves of music and in some cases 6 octaves! Additionally, the ability of the midbass to dig deep in the lower frequencies is critical to getting a subwoofer to blend correctly in the system. Excellent midbass performance compliments proper musical balance while minimizing the localization of the subwoofer and potential smearing in these critical lower frequencies.
11 high quality midbass drivers were chosen for this test. The mix of brands was based on any of the following criteria or a combination of the following: brand reputation, performance (company published specifications), price attributes, use in the mobile audio competition scene, and implementation of the latest technological innovations. You will also notice that we chose to test 6.5” – 7” woofers, as these are the most popular sizes. Drivers of this size typically have the ability to go deeper into the bass region and present a full soundstage upfront. The list of brands chosen is quite impressive; some of these brands garner great respect, not only in mobile audio, but also in some of the highest regarded home/professional drivers available.
You might be wondering why some brands are missing from this test? At the request of two popular manufacturers, we agreed to not include their drivers in this very competitive test.
The following drivers were selected for this test (in alphabetical order):

Dynaudio Esotar
Audio Technology C-Quenze 18H
Dynaudio Esotar2 e650
Eighteen Sound 6ND430
Exodus Ex-Anarchy
Hybrid Audio Technologies L6SE
JBL 660GTi
PHASS MD0790
Pioneer TS-C172PRS
Scan-Speak 18WU/4741T-00
Seas Excel W18NX-001
Vifa NE180W-04

WHAT IS A MIDWOOFER?
A mid-woofer (also known as a midbass) is any speaker that is capable of playing both midrange and midbass frequencies at the same time. The mid-woofer is usually used in a component speaker system consisting of the woofer, tweeter, and passive crossover. In a 2-way component system, a typical mid-woofer is usually responsible at least six octaves, ranging from 80Hz to 2.5kHz and sometimes higher. In a 3-way component system, a mid-woofer usually is used as a dedicated mid-bass and responsible for a general range between 80Hz and 250Hz, while some still go as high as 1000Hz (usually when a smaller midrange is used). The mid-woofer can range in size from 5.25” to 8” in diameter, with some exceptions.
The typical car audio mid-woofer that is the most popular is 6.5” to 7” in diameter and used in the lower front door of most vehicles.

TEST METHODOLOGY
As with our first monumental test – The Ultimate Midrange Shootout, strict but statistically accurate guidelines were used in our testing methodology. Various methods (as outlined in detail in this section) were employed to prevent biased data reporting, errors in reporting and influence from external variables, which could skew actual performance of each midbass driver.

Focus Mastering’s state of the art studio
PHYSICAL ENVIRONMENT
Careful consideration was placed in the physical testing environment, the associated equipment, and with the actual testers. To further the accuracy of the test Mr. Doug Van Sloun, owner and engineer of Focus Mastering, was recruited to assist in this test.
Doug donated his time, expertise and his sophisticated, state of the art recording studio.
Focus Mastering utilizes velocity-type bass trapping as well as the new RPG pressure zone bass trapping for smooth, extended low frequency response. The reflection free zone extends all the way to the client couch and provides a wide, accurate, and enveloping sound field. The quality of the room allowed the test to be conducted on the mid-woofers themselves, without the room adding any coloration, making for a near perfect physical test environment.

One of the test enclosures pictured
TEST EQUIPMENT
As you will see from the impressive equipment list below, considerable attention was paid to the associated test equipment, further fostering the best performance playback for each midbass in the test.
One of the most difficult choices was finding a suitable reference level tweeter. After much research the Dynaudio’s Esotar T330D was chosen. This tweeter is known as one of the best tweeters ever made and has been used in the highest-end offerings from Dynaudio as well as other loudspeaker manufacturers worldwide. The Esotar Tweeter exhibits an extremely low resonant frequency (fs) of 750 Hz, which allows it to mate very well with all the mid-woofers tested.
Focus Mastering also boasts an impressive equipment list (please visit www.focusmastering.com and select the “equipment” tab for a complete list) including a Krell KSA-250 Class “A” amplifier, which was used to power the test drivers. Class “A” has been long regarded as the best amplifier topology for high-end sound reproduction. This model from Krell has 250 watts per channel available for the test drivers.
Midwest Sound & Lighting, in Omaha, NE donated two DBX 240 DriveRack complete equalization and loudspeaker management processors for the test. These units were used to perform all crossover and individual speaker level duties.
Balanced XLR cables were used throughout the entire signal chain ensuring a strong, clean signal, and all the speaker wire used was high-end oxygen free copper (OFC) wire.

Exodus Ex-Anarchy & ScanSpeak
SPEAKER ENVIRONMENT
It was decided to test the mid-woofers in sealed enclosures, which were made to mimic the approximate size and shape of a typical vehicle door where most midwoofers are mounted. It is ideal to test all drivers in an ideal enclosure size, giving the drivers a QTC of 0.707, which is considered to provide optimum transient response. Since it is impractical to build a specific test enclosure for all mid-woofers tested, the enclosure was constructed large enough to have little overall effect on the “Q” of the speaker and designed more to emulate a typical automotive sealed door environment. The enclosures were constructed from !” medium-density fiberboard (MDF), used for its strength and very dense properties, and heavy internal bracing was used to minimize cabinet resonance. The enclosures were made with 3 cubic feet internal airspace and measured 6” deep, emulating the depth of most vehicle doors. To prevent speaker cone break-up due to standing waves and enclosure resonance, Cascade Audio Engineering Deflex PowerPads were installed behind the mid-woofers (in an up-coming article, we will discuss the science behind these pads). All of the midwoofers were mounted to 1” MDF trim rings that were sealed air tight to the enclosure using gasketing foam.

ELECTRONIC ENVIRONMENT
With one goal of this test being to minimize possible variances that could skew the results, a passband that favorable to all drivers was selected. The passband is the frequency range over which the mid-woofers would be played. Before selecting the passband, manufacturers data was review to ensure each driver was operating in a passband well within the limits of the driver.
Typically, using an 80 Hz crossover point will allow the woofer to mesh well with the subwoofer, while going low enough to facilitate the illusion of upfront bass. 80 Hz allows the mid-woofer to play authoritative midbass, while still protecting the speaker from over excursion, which can introduce large amounts of distortion and/or damage the speaker. A sealed enclosure is known to roll-off (meaning volume attenuates) at 12dB/octave, so applying an electronic crossover slope of 12dB/octave yields an acoustically in phase 4th-order Linkwitz-Riley impulse response.
The low-pass crossover was selected based on the physics of dynamic drivers. It is known that all drivers begin to narrow their dispersion of sound, or "beam", at higher frequencies. The frequencies at which they begin to beam is related to their cone diameter. This phenomenon is an inescapable fact in physics, and cannot be engineered out of drivers. For 6.5” to 7” drivers, beaming becomes significant at around 2,678 Hz, based on the actual diameter of the cone. Since we are mating the mid-woofer with a tweeter that has an extremely low resonant frequency of 750 Hz, it made sense to set the low-pass at 2,500 Hz. Is it generally accepted in most audio circles to set a 12dB/octave slope between the mid-woofer and the tweeter for a more natural blending between the two drivers.
The drivers were tested in the passband of 80 Hz and 2,500 Hz with 12dB/octave electronic crossover slopes, and all drivers were properly broken in using pink noise and random music offerings for the manufacturers recommended time frame and volume.

Picture of all the speakers in this test comparison while waiting to be review. Audio Technology C-Quenze 18H; Dynaudio Esotar2 e650; Eighteen Sound 6ND430; Exodus Ex-Anarchy; Hybrid Audio Technologies L6SE; JBL 660GTi; PHASS MD0790; Pioneer TS-C172PRS; Scan-Speak 18WU/4741T-00; Seas Excel W18NX-001; Vifa NE180W-04
An Audiophile’s Dream
TEST PROCEDURES
Test procedures guarantee the actual test was performed accurately to ensure all the results are reliable and all the mid-woofers were tested using the same methods.

LEVEL MATCHING
Industry experts generally agree that components under evaluation should be level matched to within 0.2 decibels (dB). To ensure that all drivers were precisely level matched, a real time analyzer (RTA) was used to verify the volume of the drivers tested. The RTA's microphone was positioned 10 feet centered between both drivers to measure the total sound pressure level (SPL) while pink noise (20 to 20k Hz, Alpine Speed of Sound CD, Pink Noise –Ref -0dB) was played. Each mid-woofer was level matched with the tweeter (due to the variance of the sensitivity of the mid-woofer) before the overall volume was precisely adjusted so its total un-weighted SPL was 82.0dB ± 0.2dB. This volume was derived from considerable experimentation and evaluation before the test began.
It’s also important to note that no equalization was used and no other drivers, other than the tweeters already mentioned, were used in conjunction with the midwoofers drivers tested.

Picture of The beautiful Pioneer TS-C172PRS & JBL 660 GTi
Pioneer & JBL
LISTENING TEST
The listening test consisted of a single blind listening session and technical measurements of all the drivers. The drivers were covered with acoustically transparent grill cloth so that none of the blind listeners could see the drivers and the testers were required to leave the room for each speaker change; even the Hybrid Audio’s beautiful copper colored phase plug was not visible, essentially guaranteeing there was no bias. The drivers were assigned a random number and a control driver was chosen at random and re-played during the testing session to validate the tester’s ability to accurately rate each set of drivers (the control driver scored identically all times it was tested thereby validating the listeners’ accuracy and repeatability). The testers were given score sheets to judge the drivers based on various aspects with a section to write down thoughts and opinions.

ELECTRICAL TESTING
All drivers were individually tested in free air and each driver was measured in the listening standardized test enclosure using the Dayton Audio Test System (DATS). During the free air test the drivers were suspended to ensure a proper reading from those drivers with a vented pole piece. The 1,000 " reference resistor, provided with the tester, was used to calibrate the leads of the DATS system, as instructed by the manual. The DATS tests the drivers for the following Thiele/Small parameters:

Vifa’s Pentacone cone technology midbass
Re – Measured in Ohms ("), this is the DC resistance of the voice coil.
Fs  The frequency at which the combination of the moving mass and
suspension compliance maximally reinforces cone motion, called the
resonance frequency, measured in hertz (Hz)
Qts – Total Q factor. A unitless measurement characterizing the combined
electric and mechanical damping of the driver.
Qes – Electrical Q factor. A unitless measurement describing the electrical
damping of the loudspeaker.
Qms – Mechanical Q factor. A unitless measurement characterizing the
mechanical damping of the driver, that is, the losses in the suspension
(surround and spider.)
Le – Measured in millihenries (mH), this is the inductance of the voice coil.

WHAT IS BEAMING & HOW DO YOU CALCULATE IT?
The upper crossover limit for a woofer is governed principally by its polar response. At frequencies corresponding to wavelengths that are large relative to the woofer’s diameter, the sonic radiation pattern is spherical. As the frequencies rise, and their corresponding wavelengths become smaller relative to the woofer’s diameter, the radiation pattern begins to narrow with increasing frequencies. This phenomenon is commonly called beaming, and is known to occur when the wavelength of sound is approximately equal to the circumference of the driver. This narrowing dispersion can adversely affect how the woofer integrates with the dispersion pattern of its companion tweeter, given it is generally accepted that the dispersion patterns of both drivers should be as identical as possible. Since the tweeter’s dispersion at typical crossover frequencies is essentially spherical, it is desirable that the woofer’s dispersion also be spherical.
One useful criterion for determining the extent of beaming defines the amount of allowable attenuation at 45 degrees off axis. It is generally accepted that attenuation ranging from -3 to -6 dB at 45 degrees off axis is an acceptable amount of attenuation to limit the audible effects of beaming. (6) Vance Dickason presented both polar plots of directivity and tabulated data defining reasonable upper limits for low-pass crossover frequencies6. Vance’s tabulated data was fitted with a smooth function and transformed into graphical form as shown in the figure below:
Driver Directivity by Vance Dickason

The chart above shows the independent variable, driver diameter in inches, on the abscissa, and the dependent variable, frequency in Hertz, on the ordinate. The solid red line represents the upper limit for the low-pass crossover frequency based on the less stringent criterion of -6 dB, while the dotted green curve represents the more stringent criterion of -3 dB. The graph clearly illustrates that a driver approximately 6 inches in diameter should be crossed-over to the tweeter no higher than about 3 kHz, and preferably at about 2 kHz, if possible. This phenomenon highlights the dilemma loudspeaker designers face: finding a tweeter with sufficiently robust performance parameters capable of withstanding the preferred crossover frequency, and thus why some manufacturers use well designed 3-way systems with cone or dome midrange speakers.
Some of the loudspeakers in this test utilize “phase plugs”, devices believed to improve the high-frequency dispersion of the drivers. Research has shown that the effects of such devices are rather minimal, or even insignificant, for this purpose, thus rendering the term “phase plug” somewhat of a misnomer. Phase plugs are, however, effective at transferring heat away from the voice coil, and in some cases, are also used to mitigate the unwanted effects of a dust cap. The debate between the merits of dust caps versus phase plugs is a complex issue with proponents for each implementation. (7) In the end, either approach can be implemented effectively with appropriate engineering considerations.
To make sure you are playing your drivers below the point of beaming, you can calculate that point. According to NASA, the speed of sound at sea level is 1,116 ft/s (feet per second). Next take 1,116 x 12”, since speakers are sold in inches. This equals 13,392 inches per second. Now all you have to do is divide 13,392 by the diameter of the “cone”. A 6.5” diameter driver with a cone diameter of 5” would start to beam at 2,678Hz (13,392/5).

The Loudspeaker Design Cookbook
THIELE/SMALL DEFINED
“T/S parameters” refer to a set of electromechanical parameters that define the low frequency performance of a loudspeaker driver in a closed box, and are named after A. Neville Thiele of the Australian Broadcasting Commission, and Richard H. Small of the University of Sydney, who developed the electrical and pneumatic models that define the parameters.
There are two main types of closed-box designs: the infinite baffle (“IB”) and the acoustic suspension (“AS”). Relative to the compliance of the driver’s suspension, the IB enclosure utilizes a larger box containing a highly compliant volume of air, while the AS enclosure uses a much smaller, less compliant volume of air. Generally, mobile audio woofers, as typically mounted in automobile doors, would be considered IB.
T/S parameters of the driver are used as input variables to design the driver/enclosure resonance magnification factor, or Qtc, which defines the composite response of the electrical, mechanical, and pneumatic behavior of the driver/enclosure system at resonance. Information regarding T/S parameters can be found elsewhere. (1)
Certain values of Qtc have mathematical significance. When the Qtc is equal to 0.5, the system is described as “critically damped”, and the frequency response of the system rises as rapidly as possible to its asymptotic output level with no overshoot and no settling time.
Mathematically speaking, a system is described as “overdamped” when Qtc is less than 0.5, and “underdamped” when Qtc is greater than 0.5.
When the Qtc is 0.707, the system is described as “optimally damped”, meaning the frequency response rises as fast as possible while achieving the least amount of overshoot and settling time. It is interesting to note that maximum power handling occurs when the Qtc is equal to 1.1. Qtc also defines many important loudspeaker performance parameters such as the volume of the box, closed-box frequency of resonance, roll-off slope, phase angle at -3 dB output, cone excursion, group delay, impedance, and cone velocity, to name a few. Wiebell and Bywater published an excellent paper discussing the optimization of sealed-box loudspeaker systems. (2) Numerous types of computer programs and software are available to aide in the optimization of these parameters. (3)
The mathematical values of Qtc also influence the subjective sound quality of the driver/enclosure system. Systems with a Qtc of 0.5 are sometimes regarded as “dry”, “excessively taut”, or “over-damped”, yet some renowned loudspeaker designers maintain that 0.5 to 0.6 is optimal.
Conversely, systems with a Qtc of about 1.0 sound “warm” and “robust”, and many consumers find this appealing. Many commercial loudspeakers utilize Qtc ranging typically from 0.5 to 0.9, while one notable exception, and commercially successful design, used 1.2.
It is important to realize that the T/S parameters are useful for small signal (low power) predictions. The driver/enclosure system can be expected to perform according to the calculations at 1 Watt (“W”) of power input.
However, as the power input increases, the voice coil temperature increases, which causes significant changes to the predictions at 1 W.
The maximum power input a driver can handle is limited by the driver’s ability to tolerate and dissipate heat. Typical materials and adhesives used in loudspeaker transducer design can tolerate temperatures up to about 250 °C. As the temperature of the voice coil increases with increasing power input, its electrical resistance increases, consequently decreasing the damping of the system. Research has shown that a driver/enclosure system modeled to obtain a Qtc of 0.707 at 1 W changes substantially with increasing power input. For instance, at 40 W, the voice coil temperature has been shown to rise to about 192 °C, and as a result, the Qtc rose to 1.2. Obviously, the significant rise in Qtc has a profound effect on the loudspeaker performance parameters. Since typical mobile audio systems provide tens, if not hundreds, of W to a driver, it is important to take this phenomenon into consideration. Consequently, some loudspeaker designers recommend using a lower Qtc to compensate for this effect. (4)

Reference
(1) V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, ©2000, pp. 23-32, 155-171.
(2) H. J. Weibell and R. Bywater, Practical Design of Optimal Sealed-Box Loudspeaker Systems, Audio Engineering Society, reprint 2105 (C-2), 1984.
(3) V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, ©2000, pp. 173-183.
(4) Ibid. p. 33.
(5) Vance Dickason, The Loudspeaker Design Cookbook, 6th ed., 2000, p. 155.
(6) V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, ©2000, p. 105.
(7) http://www.diyaudio.com/forums/multi-way/192215-phase-plug.html
*The above illustrates and supports your editors’ findings and conclusions derived from the “The Ultimate Midrange Shootout”.




No doubt that Jerry took every precaution possible to be subjective. nothing less would be expected. Think about it. Any of the speaker pairs included in the face-off could win it all. They are all that great. But the reality is that only one will. All others will be losers with no one else to deliver their anger to than Jerry. His are some big shoes to fill. 
Next, we should move on to his conclusion. Click this link to the second part on this quest for the best midbass speaker.

No comments:

Post a Comment