PERKINS PR 2 PRECISION REFERENCE LOUDSPEAKER

Sec. 1 -- Introduction
   
. . . This page will be in development for some time so will be unfinished in places..

. . . A quite extensive PDF on the PR-2's development can be downloaded as PR-2 Expanded Info; noting that the document is a combination of literature produced in 1982 and later updated to include model revisions as well as numerous relevant and worthwhile references.

. . . I've provided a lot of backup information by way of links peppered throughout the text below, fact-based marketing if you like. The titles of PDF documents are italicized whereas simple links are not; all documents and links open in new tabs.

. . .
A listing of all the included documents is seen in the REFERENCES section at the bottom of the page where those same PDFs can be downloaded singly or as a lot in one large .zip file.
. . . An interesting, little known, introductory note:

. . .
"The peripheral auditory system transforms air-borne pressure waves into neural impulses that are interpreted by the brain as sound and speech.

. . . The cochlea of the inner ear is a snail-shaped electro- hydromechanical signal amplifier, frequency analyzer, and transducer with an astounding constellation of performance characteristics, including sensitivity to sub-atomic displacements with microsecond mechanical response times; wideband operation spanning three orders-of-magnitude in frequency; an input dynamic range of 120 dB, corresponding to a million-million-fold change in signal energy; useful operation even at signal powers 100 times smaller than the background noise; and an ultra-low power consumption of 15 µW.

. . . All of this is achieved not with the latest silicon technology nor by exploiting the power of quantum computers neither has yet approached the performance of the ear but by self-maintaining biological tissue, most of which is salty water.

. . . How does the ear do it?"

. . . Christopher A. Shera
. . . Professor of Otolaryngology-Head and Neck Surgery
. . . Keck School of Medicine
. . . University of Southern California
   



     
Sec. 5 -- Closeups of the Bass Unit and Tweeter

 

. . . .Fig. 5.2 The solid Koa tweeter head with the tweeter I spent 10 years developing, the magnet structure for which is seen below. Dimension drawings can be seen in PR-2 Expanded Info.

. . . .Fig. 5.1 The felt plug is these days commonly called a phase plug, whatever that might mean. In this case the felt absorbs a weak cavity resonance at the apex of the cone, and is predictably called a cavity resonance absorber.  
 

 



     
Sec. 10 -- Details of the PEARL 200mm Bass Unit

. . . .Fig. 10.1 Above is the bass unit built from a raw casting I designed and have made in a foundry just south of Red Deer, AB. Working drawings can be seen in PR-2 Expanded Info.

. . . As described below, the unit bolts into the enclosure with nine (9) fasteners, one of which goes through a hole (A) in the center pole of the magnet structure. This is done prevent deformation and 'oil can' or 'snap through' resonant modes; as amply illustrated in this deliberately exaggerated FEA animation done in COMSOL by Ulrik Skov of www.iCapture.dk; the basket is resonating at 827Hz

. . . The motor assembly is magnetized using a pulse-discharge magnetizer I scratch built in the early '80s. An intimidating project; I'd never seen one before gathering together some 32,000uF
worth of 450V caps, a 2.5" dia. hockey puck SCR, the slew of sundry bits and just building it. The first firing was a memorable moment.

. . . When back in production the magnet structure will include extensive use of flux demodulation rings that linearize the motor's action and greatly reduce the tendency of non-linearized motors to create what I call the "pushups-on-a-trampoline" effect where the voice coil/cone's operating center is axially displaced within the voice coil gap as though by an applied DC bias. Even-order harmonics are abundantly produced.

. . . A further intent is the implementation of a newly conceived, high permeability, high saturation level, electrically resistive material in the magnetic return path pieces of the magnet structure.



. . . .Fig. 10.2 Directly above is the fairly generic bass unit from an early incarnation. It used a doped Bextrene cone, a 38mm voice coil, a stamped steel basket and a ceramic magnet structure with the center pole piece coming up only flush with the top plate.

. . . A real problem we identified was that the top 'deck' of the stamped basket was much less than a rigid platform and, as indicated above, could and did get into resonant states known as "oil can" or "snap through" modes.
. . . .Fig. 10.3 Seen above is the remedy for the "oilcan" modes just described: a simple epoxy potting.

. . . This resolved several problems I didn't know existed up to that point as I hadn't yet come up with the constrained layer enclosure material seen immediately below that simply erased a multitude of previously well masked problems.

. . . See the FEA animation previously mentioned for an analog of the oilcaning situation before remedy.
     
   



     
Sec. 15 -- Details of the Bass Enclosure Construction and Internal Acoustic Damping

. . . .Fig. 15.1 The bass enclosure is a well braced construction made of an isotropic material in a seven-ply, constrained-layer laminate (D) developed in-house to exhibit very high internal damping, rigidity and mass density. Its non-resonant performance far surpasses commonly used materials such as MDF and birch plywood. It is the best sheet material yet seen for loudspeaker enclosure construction.

. . . Reinforcements in the four corners (B) behind the bass unit provide for embedded nuts that accept 8 x 1/4" x 3" long mounting bolts. Centered directly behind the driver is a solid 2" thick brace (C) within which is embedded a barrel nut that accepts a 3/8" dia.
  non-magnetic stainless bolt inserted through a hole (A) passing through the center of the bass unit's magnet structure as seen in Fig. 10.1 above.
. . .
By means of these nine fasteners the bass unit's 8lb cast basket and magnet structure are made integral with the 65lb enclosure, thereby establishing a ratio of 73lb of reactive enclosure/bass unit mass to 1oz of active cone/voice coil mass, a greater than 1000:1 or 60dB ratio.

. . . Any rocking motion of the cabinet on its 20lb sand-filled stand is thereby reduced to insignificant levels.
   
Sec. 15.1 -- Sound Absorption Materials
. . . Moving on to the important matter of enclosure damping materials we have Kapok and Other Sound Absorbing Materials, a 333pg. collection containing the following papers:

. . . 01 - Factors Influencing Acoustic Performance of Sound
. . . . . . . Absorptive Materials;
. . . 02 - Investigation on Sound Absorption Properties of
. . . . . . . Kapok .Fibres;
. . .
03 - Sound Absorption Properties of Kapok Fiber
. . . . . . .. Non-woven Fabrics at Low Frequency;
. . . 04 - A Preliminary Investigation on Kapok/Polypropylene
. . . . . . . .Non-woven Composite for Sound Absorption;
. . . 05 - Analysis of the Bending Property of Kapok Fiber
. . . 06 - Kapok or Capok Fibers;
. . . 07 - Kapok Fibre: "A Perspective Fibre";
. . . 08 - Recent Advances in the Sound Insulation Properties
. . . . . .. . of Bio-based Materials;
. . . 09 - Characterization of the Thermophysical Properties of
. . . . .. . .. Kapok;
  . . . 10 - Utilizing Malaysian Natural Fibers as Sound Absorber;
. . . 11 - Absorption Characteristics of Glass Fiber Materials at
. . . . . ... .Normal and Oblique Incidence;
. . . 12 - Attenuation of Noise by Using Absorption Materials
. . . . . . . . and Barriers;
. . . 13 - Review in Sound Absorbing Materials.

wherein we can read much about the sound absorptive qualities of many types of materials with kapok, degreasing cotton (aka: upholstery felt), multi-lumen polyester and common, insulation grade fiberglass figuring prominently amoung them.

. . . Figs. 15.2 thru 15.6 below provide much visual information in respect of the physical makeup of uncommonly used acoustical damping materials and measurement data regarding their frequently misunderstood sound absorption coefficients.
. . . 

     
Sec. 15.2 -- Theory and Measurement of Sound Absorption Coefficient

. . . .Fig. 15.7 The vintage Brüel & Kjaer test equipment setup seen above consists of a Type 4002 Standing Wave Apparatus in the foreground and left to right in the background, a Type 2305 Level Recorder, a Type 1022 Beat Frequency Oscillator and a Type 2107 Constant Percentage Bandwidth Analyzer.
. . . The 1022 BFO drives a full-range loudspeaker within the mahogany enclosure to produce plane waves down the length of
  the black standing wave tube. The 2107 analyzer is configured as a frequency selective microphone amplifier tuned to the BFO's output frequency, while the 2305 recorder is set up to provide an easily read indication of the probe microphone's output level as its carriage is moved to and fro along the guide rail amoung the minima/maxima of node/anti-node sound pressure levels (SPLs).

. . . .Fig. 15.8 The Standing Wave Apparatus was the subject of Per Brüel's doctoral dissertation, the Dr techn; work on which began in 1939, continued thru WWII and was successfully defended in 1945 or '46. The result of his dissertation, "Application of the Tube-Method in Room Acoustics", became the Type 4002, a product that was sold by Brüel & Kjær for many years. For more on this fascinating story I highly recommend this six page biographical sketch Dr. Per V. Brüel - 100 Years. An obituary for Viggo Kjaer is here, noting that he lived to the age of 99.
     
. . . A schematic of the Brüel & Kjaer Type 4002 Standing Wave Apparatus is directly above. A loudspeaker produces an acoustic wave which travels down the tube and reflects from the test sample. The phase interference between the waves in the tube which are incident upon and reflected from the test sample will result in the formation of a standing wave pattern in the tube. If 100% of the incident wave is reflected, then the incident and reflected waves have the same amplitude; the nodes in the tube have zero pressure and the antinodes have double the pressure.
. . . If some of the incident sound energy is absorbed by the sample, then the incident and reflected waves have different amplitudes; the nodes in the tube no longer have zero pressure. The pressure amplitudes at nodes and antinodes are measured with a probe microphone the active element of which is contained within a car
  that can be rolled along a graduated ruler. The ratio of the pressure maximum (antinode) to the pressure minimum (node) is called the standing wave ratio (SWR). This ratio, which always has a value equal to or greater than unity, is used to determine the sample s reflection coefficient amplitude R, its absorption coefficient sigma, and its impedance Z(w).
. . . By means of the animation seen here the process described above is easily understood. Hit the "Start" button and let the default settings run. In a few seconds the incidenting wave (red) will be seen to hit the reflective surface at the RH end of the virtual space, whereupon a reflected wave (blue) is created followed by a zero-to-double amplitude standing wave (black). Unchecking the "Incident-" and "Reflected wave" boxes isolates the standing wave, making crystal the reasons for its being so named.


Fig. 15.9
Incidenting wave. See the animation app here.
 
Fig. 15.10
Incidenting, reflected and standing anti-node waves.
 
15.11 Incidenting, reflected and standing node waves
 
15.12 Standing anti-node wave.

   



 
Sec. 16 -- Loudspeaker Enclosure Rigidity, Panel Resonances and Chlandi Patterns
 
. . . When a bass driver is rigidly fixed to a wall of a wooden bass enclosure and driven from a sine wave source, measurements taken on a conventional enclosure's panels invariably reveal a multitude of strong, high Q resonances.

. . . These are due to the bending forces exerted on the structure by the reaction of the bass unit's magnet and chassis assembly to the motion of the cone, as well as acoustic energy radiated from the rear of the driver into the enclosure cavity.

. . . Due to the low mechanical loss coefficient (tan ´) of commonly used enclosure materials, energy is readily stored and only slowly released by the enclosure structure. A substantial amount of acoustic radiation is thereby generated with investigation showing that in a two octave band centered roughly on middle C (125 to 500Hz) the acoustic power re-radiated by the enclosure is often equal to that radiated by the bass unit itself.

. . . This was an exceptionally difficult problem and one I researched and prototyped for years on end, coming finally to the constrained layer solution seen in (D) above.

. . . Constrained-layer damping (CLD) is a mechanical engineering
  technique for the suppression of vibration. Typically, a viscoelastic material is sandwiched between two sheets of material of moderate to high Young's modulus that, other desirable characteristics notwithstanding, lack sufficient tan delta.

. . . As a CLD structure undergoes vibration, the high tan delta viscoelastic layer between the two constraining materials is subjected to shear strains, with their vibrational energy being converted to heat. The great advantage of CLD treatment is the realization of exceptionally high tan delta in the composite plate or beam without significant degradation of stiffness or flexural Young's modulus or increase in mass density of the composite system.

. . . As implemented in the PR-2 enclosure, the CLD system consists of seven layers of high-density fibre board bonded together with a "secret sauce" viscoelastic adhesive.

. . . In concert with its robust internal bracing, the resulting enclosure is both surpassingly rigid and supremely non-resonant.

. . . To get an idea as to the sorts of patterns seen in all kinds of driven, conventional planes, i.e. loudspeaker enclosures, driver diaphragms, etc, click the Chladni pattern thumbnail below.
     
Sec. 16.1 -- Supplementary Info & Animations

COMSOL Chladni FEA

 

Enclosure FEA

 

Chladni Patterns

 

 

 

 

 

 

 

. . . Fig. 16a.1 A COMSOL page on the FEA generation of Chladni patterns. A COMSOL DAY keynote address with René Christensen of Acculution.   . . . Fig. 16a.2 An FEA animation of the sorts of deflections seen in enclosures less stiff than they ought to be..   . . . Fig. 16a.3 A video of a plate center driven at various frequencies, and described in Chladni Patterns in Vibrated Plates.

   



     
Sec. 20 -- Notes on the PEARL Polymer-Graphite 1" Dome Tweeter

. . . .Fig. 20.1 The tweeter was years of work that in 1986 resulted in the first hard dome that worked properly, which is to say; without an egregious, high-Q, ultrasonic peak some 20 to 35dB high at 20 to 35KHz.

. . . A 1" dome, it's all-up moving mass is 150mg, less than half that of an equal sized soft dome, its fundamental resonance is 280Hz with a very flat frequency response to well over 20KHz, gently rising beyond 30KHz to a +6dB, 1/3 octave wide peak at 39KHz; numbers almost unheard of to this day. An anechoic, swept-sine frequency response curve is seen in Fig. 25.1 below, noting that the minor horizontal divisions are 1dB.

. . . 50 microns (0.002") thick, the formed dome weighs merely 50mg and due to PEARL's unique vacuum forming technique shows a <±5% thickness constancy from periphery to apex.

. . . The very low fundamental resonance implies a suspension compliance some ten times that seen in conventional 1" devices. Low moving mass, high compliance and low mechanical resistance combine to evince truly exceptional low-level resolving power.

. . . The dome is precision, hot cavity vacuum formed from a remarkable graphite-based polymer that is 70% the mass density of aluminum, has the same flexural modulus (stiffness), 25 times the internal damping, (tan ´), and thermoforms with relative ease.

. . . Broadly speaking, this material might be seen as a precursor to modern-day graphene nano-composites, relying as it does on the extreme in-plane, inter-atomic strength of its laminarized graphite's graphene 'a-b' planes; see below.

. . . First realized by repeated Scotch Tape exfoliation (yes, really) in the 1990s, graphene is a single layer of graphite; a hexagonally bonded X-Y lattice of single carbon atoms extending without theoretical limit across two dimensions.

. . . Natural graphite then, is many layers of one atom thick graphene typically seen offset-stacked one atop the next in what is known as Bernal stacking, as illuminated in J. D. Bernal's groundbreaking 1924 paper, The Structure of Graphite, and expanded upon in, Properties and Characteristics of Graphite.

. . While graphene's hexagonal inter-atom bond structure is one of the strongest in Nature, its inter-layer strength in what is known as the 'c' or 'stacking' axis arises almost entirely from relatively weak Van der Waals forces and is only about 2% that of graphene's extraordinary in-plane strength.

. . . Then and now the weak 'c' axis bond is a major problem in the implementation of graphite's otherwise beguilingly attractive single carbon atom thick, hexagonally bonded 'a-b' layer.

. . . Rather than attempting to bond atom-thick layers of graphene that in any case wouldn't be realized until the mid '90s, Pioneer's mid '70s workaround was to polymer-bond and calendar laminarize high aspect ratio graphite micro-platelets. See Figs. 20.3 and 20.3 below.

. . . An ideal material in this and many other high-performance audio applications, polymer-graphite is the result of genius-class materials science work by Pioneer thru the mid '70s. Lamentably, other than Pioneer themselves and their well respected pro audio division, Technical Audio Devices, TAD, no one used it. PEARL, INC. was Pioneer/Mogami's worldwide distributor Shima Trading's only polymer-graphite customer. Pioneer discontinued production in the early '80s and scrapped the costly, high precision machinery necessary for its production. A great loss of a material I have some 25 years study into putting back into production, this time using PEEK as the bonding or intercalating polymer.

. . . Interestingly, PEEK (polyetheretherketone) was invented in Nov. 1978 just before Pioneer began to publish on their PVC-based polymer-graphite. Amoung graphene-loaded PEEK's many virtues is the fact that it will run at >90% of full modulus at temperatures above 150°C.

. . . Please see rebuilt and expanded copy of Pioneer Electronics, Engineering Research Laboratory's original 1979 AES Convention presentation. A succinct and informative read where a scan through the cited references, some dating back to the 1920s, illuminates the very basic nature of Pioneer's research.

. . . Here is copy of Pioneer's subsequent 1980 AES Journal paper and numerous patents on polymer-graphite and related materials from the late '70s through the early '90s, another very informative read.

. . . .Fig. 20.2 A somewhat schematized but nonetheless accurate illustration of the structure of graphene is shown above.   . . . .Fig. 20.3 Shown is an edge-on photomicrograph of polymer graphite. The 100 micron scale equals 0.004" or the nominal diameter of a human hair.
   



     
Sec. 25 -- Measurements of the 1" Polymer-Graphite Dome Tweeter

. . . .Fig. 25.1 Shown above is the anechoic, near-field, swept sine, response of the PEARL 1" polymer graphite dome tweeter mounted in its essentially diffraction-free solid hardwood, early-80s vibration-isolated tweeter head.

. . . The minor divisions are separated by 1dB, clearly showing the first major axial breakup at +6dB x 1/3 octave wide. Note that while other less severe resonances begin to occur at approximately octave-lower frequencies, as they do in any hard
  dome driver, they're not much apparent in the frequency response due to polymer graphite's very high internal loss factor. They do however make themselves somewhat evident in the CSD, acoustic phase and group delay plots seen below in Figs. 25.4 and 25.5.

. . . Generated in PEARL's lab anechoic chamber seen in Sec. 60 below, the frequency response curve above was taken using "old school" swept sine methodology with no smoothing, averaging or other data massages.

.

The data below was taken in 1993 with a MLSSA system and on account of the measuring system is somewhat dated.

. . . .Fig. 25.2 Shown above is the unit's response to a very short interval pulse, which data is compressed in both the X and Y directions. The X axis should display perhaps 4mSec while the Y axis should be in ±dB with a range of about ±40dB. In all then, a not very useful presentation. Nice eye candy though . . .   . . . .Fig. 25.3 This display shows excellent performance out to the MLSSA system's upper limit of 30KHz.
. . . Seen from 0.34 to 1.2mSec at some -30dB is the dome getting into some ringing from 15KHz to 30KHz, a minor matter I'd like to deal with nonetheless. Some artifacts are seen from 200Hz to 1.2KHz where a possible reflection crept into the analysis. I didn't do the measurements seen in Figs. 26 thru 31 so don't know the cause.

. . . .Fig. 25.4 In 1993 acoustic phase was a difficult measurement which to this day is often incorrectly done.

. . . Here we used Heyser's suggestion to arrange compensation for the air path delay such that the phase angle read +90° at the drive unit's fundamental resonance. We were a little off from the actual 280Hz resonant point but at 300Hz we were in the ballpark given what we knew at the time.

. . . The perturbations starting about 13KHz, indicate the onset of  breakup modes almost two octaves below the first, obvious peak in response seen at 39KHz seen in Fig. 25.1 above.
  . . . .Fig. 25.5 Group delay, acoustic or otherwise is hardly a topic for a paragraph or two in a figure caption.

. . . For the moment then, here is, Delay Alignment of Top- and Sub-loudspeakers Systems A Survey of Old and New Methods by Bävholm and Grenander of WaveCapture fame who provide very clear illumination of the much bandied about and misunderstood concepts of time alignment, group delay and minimum phase, along several excellent pages of cited references, some dating to the mid-19th century.
. . . Sadly, it appears WaveCapture ceased operations at the close of 2020.

. . . .Fig. 25.6 The tweeter's impedance magnitude, Z, where the tweeter's 280Hz fundamental resonance, is clearly seen. Notice as well the only slight, 20% rise in Z at 20KHz due to the solid copper electroplating of the whole of both the top and back poles of the motor assembly. To prevent oxidation in humid environments, which was discovered to congeal the FerroFluid placed in the magnet's gap, these are subsequently nickel flashed.   . . . .Fig. 25.7 The electrical input phase magnitude appears above, showing by its maximum ±18° variation a very tractable load for the crossover designer. Note as well the curve's intersection with 0° at 280Hz, another indication of its uniquely very low fundamental resonance.
   



     
Sec. 30 -- PEARL Designed and Built Hot Cavity Vacuum Former

. . . .Fig. 30.1 Shown with its top and front covers removed, our forming machine (A) was built to precisely vacuum form polymer graphite diaphragms. A closeup of the control panel is seen below.

. . . Typically, vacuum forming involves heating a sheet material to its plastic temperature then sucking it into or over a cold die, where it immediately freezes. This has the effect of concentrating material thinning in the areas last pulled onto the die, with the result that the formed part's thickness is inconsistent. PEARL's automated hot cavity process involves first heating the die, then forming, then cooling/annealing, then part removal and a final punching step to finished shape. Although time consuming, the process result in parts of very consistent cross-section thickness, an important specification.

. . . Seen at (B) is the family of forming die shapes we investigated. Formed by the simple expedient of changing the former seen fitted in the center of the heated/cooled forming head (C), we were able to cost effectively work though many shapes to find an optimum. Once the forming procedure was working well, a task in itself, the process took about a month; while concurrently running the company.

. . The range of torispherical shapes investigated and the algebraic method of their generation can be seen in Dome Former Profiles while The Free Vibrations of Cylindrical Shells with Various End Closures provides mathematical analyses of several shells of rotation atop parallel-walled cylinders. The paper describes the
  performance of a wide range of shapes, torispheres amoung them. The authors Galletly and Mistry went on work with Dr. Don Barlow on, "The Resonances of Loudspeaker Diaphragms" which is included with several other cited papers in the Bank and Hathaway compendium, below.

. . . Noteworthy are that facts that in the early '80s scanning laser Doppler vibrometry, SLDV, and computationally intensive finite element analysis, FEA, tools were unavailable to any but academic and cutting edge, industry researchers.

. . . Piggybacking the earlier work of Bank and Hathaway, then at Rola Celestion in the UK, I developed a range of what Bank called blended radius, or torispherical shapes and by an expedient, empirical methodology worked my way through a range of possible shapes to find an optimum.

. . . In 1981 Bank and Hathaway produced an AES publication, Three Dimensional Inteferometric Vibrational Mode Display that was one of the seminal works of the day. This is a 70MB download because it contains an extensive addition of references cited in the paper itself and their sub-references.

Martin Colloms was privileged to be able see the B/H system in action at Celestion and told me on the phone one day that soft dome tweeters, " . . . vibrate like bowl of jelly, they are no part of pistonic radiators".



     
Sec. 35 -- Bare Tweeter Dome being Fitted with Voice Coil
. . . .Fig. 35.1 Dome ready for voice coil.   . . . .Fig. 35.2 Dome and voice coil glued and ready for surround.


. . . The dome-to-voice-coil glue joint is done by hand with a 3cc syringe and a #28 hypo' needle as the mandrel slowly rotates in a little Unimat jewelers' lathe rebuilt to run at very low speeds. All the jigging parts are Teflon. The Ferrofluid-resistant glue used is a well-known Loctite product used throughout PEARL's the driver assembly processes.
     



   
Sec. 40 -- Tweeter Dome Fitted with Ultra-high Compliance, Acoustically Transparent Surround

. . . .Fig. 40.1 Dome in surround fitment jig, ready for glue.   . . . .Fig. 40.2 Dome with its ultra-high compliance, acoustically transparent surround fitted, ready for clamping while the glue cures.

. . . .Fig. 40.3 Weighted Teflon clamp on the glue joint   . . . .Fig. 40.4 The dome/surround assembly affixed to its mounting card, which is then glued onto the magnet assembly top plate and hand aligned.
     



   
Sec. 45 -- Classic Holbrook C10 - 12" x 20" High Precision Toolroom Lathe

. . . .Fig. 45.1 All the metal parts shown above were made with the toolroom lathe seen above. I bought it good condition then took it completely apart to thoroughly clean it and replace every ball bearing in it and to regrind and refit various surfaces.The machine is presently part way through another teardown and rebuild that this time involves a strip down to bare cast iron and a repaint and refit to make it look like the other rebuilt pieces here.

. . . Time permitting I might well have the bed, cross slide and compound ways reground back to new specification and fitted
  with Turcite B, a, " . . . high performance thermoplastic material for use in linear bearing applications such as the guideways of machine tools." Basically the idea is that the material machined away from worn, mating surfaces is replaced with a material purpose-designed for better performance in the guideway application than the original base materials, usually cast iron.

. . . An example of the installation of Turcite B is seen below, note that by fitting this material the reworked parts can be brought back to their original working tool heights and centerlines, a crucially important requirement. A Turcite B brochure is here.

. . . Throughout its decades-long history Holbrook consistently produced machines of the highest caliber, albeit at great purchase cost. When new in the mid-60s the machine above likely cost some $US60 to $100K in today's dollars.   . . . For those with an interest, several Holbrook brochures from the 1950s & '60s are available below:
               
Holbrook Minor - A pristine, extant example.
Holbrook Minor - Operator's & Service Manual.
Holbrook C10-13-16 - Sales Brochure & Operator's Man.
  Holbrook Major - Sales Brochure.
Holbrook D13-15-18 - Sales Brochure & Operator's Man.
Holbrook H15-17-20 - Sales Brochure.
               



             
Sec. 50 -- PEARL's Acoustics and Electronics Lab

. . . .Fig. 50.1 Text to follow  
 

. . . .Fig. 50.2 Text to follow  



   
Sec. 55 -- 25 foot (7 Meter) Free-field Outdoor Measuring Lift

 

 

 

   
. . . .Fig. 55.1 The piece to the right allows one to lift a 500lb payload some 25 feet off the ground in order to make accurate low-frequency measurements in what is called free space. It was equipped with upper and lower limit switches that were part of the hoist control electrics. As well, provision was made for a control cable one could run back into the lab so the lift could be remotely controlled.

. . . At the time I designed and built this lift it was likely the only such dedicated apparatus in the Canada. Its construction was inspired by a mid-80s trip to the Canadian National Research Council's facility in Ottawa, Canada where anechoic measurements of my speaker in their inadequate to the very low-frequency measurement task chamber plainly showed me what needed doing.

. . . I ultimately sold it to the University of Alberta's Mechanical Engineering Acoustics and Noise Unit, MEANU, some 25 years ago; an interesting facility housing two reverberation chambers, the only such rooms in the country so far as I know. Built in the mid '70s by Bolstad Engineering of Edmonton, AB, their original paper was A New Acoustic Test Facility in Western Canada.

. . . A compendium of articles on Measuring Loudspeaker Performance at Low Frequencies is both interesting and useful.

 

 

 

 

 

 

 

 

 

 

. . . .Fig. 55.2 Although loathe to do so, I'll blow my own horn and say that this particular free-field response might well be unique in all of direct radiator loudspeaker audio. The reason is the beautiful 8db/oct rolloff from 60 to 20 hz. Nothing does that, not reflexes, sealed boxes, horns, passive (re)radiators, trans lines, multi-chamber reflex contraptions . . . nothing. This is a straight ahead, swept-sine, real world measurement, no weightings, no fudge factors, just the straight up Real Deal.
. . . And there is no. mystery. here; I've been trying to give this away for 35 continuous years, almost no one listens; everyone knows more, knows better, knows it can't be done, knows it was done and didn't work or, "That's not a Thiele & Small alignment!". And no, it isn't because we are not trying to create a resonant system, we're meaning to damp one. The list of excuses and/or Reasons Why Not is endless and all that despite the fact that the first papers describing the basics were written in 1951, '53, '54/55, '55 and '56, all of which are found in "DAMPS Early Papers", where on the last page of the last article E. J. Jordan, then chief engineer at Goodmans Industries in the UK told us in 1956 that:

. . . "The performance of Axiom [ARU- or DAMPS-loaded] enclosures has been compared with that of other types. Listening tests have shown that the bass radiation is somewhat better than that from the reflex type cabinet at middle bass frequencies and considerably better at the low frequencies, thereby imparting a warm, well-balanced quality to the reproduction. Tests with an oscillator showed that a strong, pure 20c/s fundamental note could be radiated without excessive cone movement. Transient curves taken showed a very short decay time, characteristic of non-resonant conditions.

. . . This is the more interesting when one realizes that the volume of this type of enclosure is about half that of a correctly designed reflex cabinet for the same speaker."

. . . Low frequency complex impedance in particular and complex Z in general substantially impact the 'ease' with which a loudspeaker can be driven by a power amplifier, with wild swings in impedance and phase angle presenting difficult loads; a serious problem all too often seen. Following on then, here are, "Between Amplifier and Speaker", "Heavy Loads How Loudspeakers Torture Amplifiers." and, "Power Amplifiers and the Loudspeaker Load" all of which are excellent reads.

 

 
  . . . .Fig. 55.3 Below 200 Hz this impedance curve is as remarkable as the free-field response above because through the region of fundamental resonance the curve is flat, not peaked. The broad peak between 10 and 20 KHz is due to a notch filter used to suppress output from a breakup mode common to soft dome tweeters such as I was using at the time. With the implementation of the polymer-graphite tweeter I eliminated both the problem and the filter.  
 

 
 

. . . .Fig. 55.4 The input phase angle data above is as noteworthy as the data in the preceding Figs. 55.2 & 55.3. Here we see an essentially resistive load through the region where a bass unit would typically be going through its fundamental resonance and swinging wildly from inductive to capacitive and back. A resonant system will generally be resistive at its fundamental and here we can barely discern that zero degrees occurs at about 23Hz.

Sec. 55.1 -- Further information on the need for acoustic resistance venting of enclosures from Sigfried Linkwitz:

. . . Acoustic absorption and acoustic resistors

. . . When a speaker driver is mounted in a box it radiates as much energy into the space in front of the cone as it does into the much smaller space behind the cone. What happens to the air borne energy inside? At long wavelengths it is common practice to store it in resonant structures to extend the steady-state low frequency response of the speaker. In general, the energy leads to very high sound pressures inside the box. A small amount of the energy is lost as heat in the stuffing material, some in the process of flexing the cabinet walls. Much of it reappears outside the box, because the thin cone presents a weak sound barrier. Just how much is difficult to measure, but it is a contributor to the frequency response. I am of the opinion that the effect is most notable in the low hundreds of Hz region, where stuffing materials are ineffective and the internal dimensions not small enough for the internal air volume to act as a pure compliance. Consequently, enclosures should be either very small (less than 1/16th of a wavelength) or extremely large, both of which are not very practical for different reasons.

. . . To make progress with box speakers an acoustic resistor is needed that can more effectively dissipate energy in the 80 Hz to 800 Hz frequency range at high volume velocities. Such device would not only be useful for closed box speakers, but also for speakers that use the rear radiation from the driver to form a specific polar radiation pattern, such as a cardioid. A cardioid speaker can be made with two opposite polarity monopole sources separated by a distance D, and with the signal to one of the sources delayed by a time T = D/c. An implementation of this concept could be a driver in a box of depth D where the rear wall is an acoustic resistor R. At long wavelengths the box internal air volume behaves as a compliance or acoustic capacitor C. The acoustic output from the rear of the box is low-passed by the RC filter and delayed relative to the front output by T = RC.
.. . .The acoustic resistor should be purely dissipative, with vanishing reactive component, and be independent of frequency. It also should be linear over the range of volume velocities encountered for high SPL. Traditionally cloth type materials have been used for cardioid speakers. Long fiber wool, synthetic fibers or fiber glass matting have been used to attenuate sound inside enclosures. The properties of these materials are neither frequency independent nor linear.

. . . It may not be widely known that filter media for the filtration of liquids and gases in the chemical and other industries can have applications in acoustics. Such filters may be thin sheets (<1 mm thick) of a non-woven, sintered, stainless steel fibre matrix for filtration levels from 5 to 50 micron. Airflow at a constant velocity v through the filter material causes a pressure drop Dp between input and output sides corresponding to a flow resistance Rf = Dp/v [Ns/m3]. It is common in this industry to specify an inverse quantity which is Permeability P [l/dm3/min] at 200 Pa pressure drop. Flow resistance and permeability are related by Rf = 1200/P in this case. Resistance values between 150 and 3500 Ns/m3, or 15 to 350 rayl in the older cgs system of units (1 rayl = 10 Ns/m3), are obtainable from a single filter sheet. For comparison the free-space acoustic field impedance p/v = rc is resistive and has a value of 414 Ns/m3 = 41.4 rayl. Materials are available with greater structural rigidity such as Feltmetal with thickness up to 6 mm and resistance between 6 and 50 rayl. The impedance is resistive and constant over the 20 Hz to 2 kHz range that I tested. Linearity should also be quite good, but I have not measured it. Feltmetal and filters should be readily usable for a cardioid speaker, but for a woofer application their linearity at high volume velocities needs investigation.

. . . The challenge remains to build an acoustic termination for the inside of a box.

 
         
  Sec. 55.2 -- PEARL Acoustic Resistance Unit (ARU)  
 

 
  . . . .Fig. 55.5 This construction is simplicity itself, comprising a single layer of 1/16" (1.5mm) thick dressmaker's felt sandwiched between two pieces of 1" (25mm) thick Tectum, spot glued as appropriate, with a wrap around of good grade duct tape to prevent acoustic short circuiting.  



       
  Sec. 60 -- PEARL's Lab Anechoic Chamber

  . . . .Fig. 60.1 This anechoic chamber is my third. Built from 1.5" laminated MDF and painted colors I had specially mixed to exactly match the well-known Brüel & Kjær two-tone color scheme, it breaks down to pass through a typical 32" doorway opening.

. . . .The widely appreciated B & K color scheme originated with the Danish Army during WWII, when Per Brüel and Viggo Kjær started what became their world leading company.
  . . . .As with almost everything else, B & K did anechoic chambers differently. Due to its lack of the large, plane surfaces seen in a typical 'wedge' room, the Cremer 'Acoustic Jungle' seen in Sec. 75 below has far better performance above about 300Hz, being much less reflective at higher frequencies due to the diffusive nature of its varied-density acoustically absorbent, fiberglass blocks. An important and greatly overlooked performance aspect. Here is an eight paper collection on Anechoic Chamber Design and Construction.  
         



       
Sec. 65 -- A Large Room at the National Metrology Institute in Japan

. . . .Fig. 65.1 An example of a very large and very expensive anechoic chamber, this one at the National Metrology Institute of Japan.

. . . Undoubtedly a many-million-dollar facility, and almost certainly a vibration isolated structure built within a very heavy exterior shell.

. . . Comparing the probable about 5ft height of the woman to the
  apparent length of the wedges, a best guess as to the lower cutoff frequency is a respectable 30 to 40Hz.
. . .
If however, one is prepared to work around northern latitudes' ever changing weather conditions, always point north to keep solar thermal gain minimized, work in the <1m near field at low frequencies and do some signal averaging to better one's S/N ratio, perfectly acceptable results are achievable another octave lower in frequency and at an all-up cost of around a thousand dollars.
     



Sec. 70 -- A Medium-sized Room at a European Loudspeaker Manufacturer

. . . .Fig. 70.1 This Danish anechoic chamber consists of a shell of concrete and LECA that rests on coil springs to dampen noise from vibration in the ground. The chamber is lined inside with meter-long sound absorbing wedges of damping material. The chamber is used for all kinds of free-field acoustic measurements. It should be mentioned that 'LECA' (Light Expanded Clay Aggregate) ' is a remarkable material having both sound and vibration damping qualities, very low thermal conductivity and great fire resistance. It can absorb some 15% H20 by weight while simultaneously resisting decomposition by wet- or dry-rot. It is also a hypo- allergenic building material. LECA consists of small, lightweight, bloated particles of burnt clay. The thousands of small, air-filled cavities give LECA its strength and thermal insulation properties.   . . . The base material is plastic clay which is extensively pre-treated and then heated and expanded in a rotary kiln. Finally, the product is burned at about 1100 °C to form the finished LECA product. LECA is an entirely natural, environment-friendly product providing the same benefits as conventional tile but in brick form.

. . . LECA branded product is produced in Italy, Denmark, Switzerland, Norway, Germany, Finland, Portugal, U.K. and Iran. Countries which produce very similar aggregates but with different brand names are: Russia, Poland, Sweden and China making 'Keramzite'; South Africa making 'Argex'; and Spain producing 'Liapour.'

The LECA website is here.
     



   
Sec. 75 -- Brüel Acoustics Cremer 'Acoustic Jungle' Anechoic Rooms
     
. . . .Fig. 75.1 Brüel Acoustics use the Cremer principle which has proved both useful and economic.

. . . The absorbing walls are built up with cubes which towards the center of the room are small and made of special glass fibers with a very low density. Moving outwards towards the room's boundary walls the cubes increase in both size and density. In this way one obtains an extremely good impedance matching with the heavy absorbing material in the internal part of the room. This principle is analogous to an exponential horn in front of a loudspeaker driver unit.

. . . Wedges are normally built of the same material from the base to the tip. In the loudspeaker analogy this corresponds to a linear cone in front of the drive unit so consequently the Cremer room is much better at high frequencies than a wedge room of the same size.
At mid-frequencies the two treatment types are equal, and at low frequencies the wedge type is normally slightly better.

. . . From an economic point of view it can be said that while a wedge room contains more absorbing material than a Cremer room, a Cremer room requires more sophisticated construction work which means higher labour costs.


. . . "At B & K we used only Cremer rooms because for us it is very important that the room be a good performer at higher frequencies up to 15 kHz."

. . .
Dr. Per V. Brüel
 
. . . With frequencies over 3 kHz a wedge room will always give some uncontrollable phase shifts because the wedges have large plane surfaces whereas Cremer rooms are in effect an acoustical jungle. The lower frequencies below 300 Hz are not so important as the wavelengths are long, allowing the implementation of other methods. A technical review of Brüel Acoustics Cremer rooms is found in Bruel Acoustics Anechoics.
. . . Brüel Acoustics was Dr. Per Vilhelm Brüel's retirement hobby horse, the company's website is still alive, as might be the company itself. He passed at age 100 in 2015, an obituary is seen here.
. . . Dr. Viggo Kjaer was equally long lived, passing in 2013 at age 99, his obituary is available here.
     



   
Sec. 80 - REFERENCES - Back to the top
 

 

         

00

 

Download the entire collection of references.    

 

 

05

Original Perkins PR-2 Literature - Expanded to include much additional information - 139 pgs.

10

Kapok and Other Sound Absorbing Materials - Including extensive measurement information - 333pgs.

15

 

Brüel and Kjaer Standing Wave Apparatus - Technical Review from Jan. 1955 and subsequent Instructions and Applications - 96 pgs.

20

 

Brüel and Kjaer 2305 Level Recorder - Instructions and Applications - 130 pgs.

25

 

Brüel and Kjaer 1022 Beat Frequency Oscillator - Instruction and Applications - 52 pgs.

30

  Brüel and Kjaer 2107 Frequency Analyzer - Instructions and Applications - 50 pgs.

35

  Dr. Per Vilhelm Brüel - 100 Years - A heartwarming biographical sketch - 6 pgs.

39

  The Mechanical Loss Coefficient (tan delta) - Drilling down into the origins of damping - 2 pgs.

40

  Chiladni Patterns - Studies of vibrated plates, methodologies and mathematics - 27 pgs.

45

  The Structure of Graphite - A groundbreaking work by J. D. Bernal, published by The Royal Society, UK, 1924 - 25 pgs.

50

  Properties and Characteristics of Graphite - For the semiconductor industry, May 2013 - 39 pgs.

55

  Polymer Graphite Loudspeaker Diaphragm - Electronic Engineering Laboratory, Pioneer Electronic Corp, Tokyo, Japan. AES 64th Convention, Nov. 1979 - 80 pgs.

60

  Polymer Graphite Loudspeaker Diaphragm - includes all AES & JASA papers, all available references and all known patents - 168 pgs.

65

  Delay Alignment of Top- and Sub Loudspeaker Systems - Group delay and wavelets analysis are used to evaluate the measurements - 24 pgs.

70

  Torispherical Tweeter Dome Profiles - The cohort of shapes investigated by build-and-measure, empirical means to arrive at an optimum shape for the PEARL 25mm dome tweeter - 12 pgs.

75

  The Free Vibrations of Cylindrical Shells with Various End Closures - Provides mathematical analyses of several shapes of shells of rotation atop parallel-walled cylinders - 18 pgs.

80

  Three-dimensional Interferometric Vibrational Mode Display - Possibly the first implementation of Scanning Laser Doppler Vibrometry (SLDV) in audio, including several large references - 266 pgs.

85

  Measuring Loudspeaker Performance at Low Frequencies - Four papers discussing this difficult realm of accurate loudspeaker measurement - 40 pgs.

90

  Distributed Acoustic Impedances (DAMPS) - Early Papers - The foundations of acoustic resistance (ARU) loading of bass drivers in enclosures, published in 1951, '53, '54/55, '55 and '56 - 53 pgs.

95

  Between Amplifier and Speaker - Discussion of loudspeakers' almost invariably highly reactive complex input impedances and the problems caused amplifiers driving such loads - 4 pgs.

100

  Heavy Load - How Loudspeakers Torture Amplifiers - An in-depth look at the problems discussed immediately above with a view toward solid state output stage load lines and output device safe operating areas - 10 pgs.
101   Power Amplifiers and the Loudspeaker Load - Another discussion of loudspeakers' almost invariably highly reactive complex input impedances and the problems caused amplifiers driving such loads with some emphasis on the corner cutting seen in the face of, "the [very great] temptation to design the amplifier for very high power into a resistive load at the expense of adequate and costly "elbow room" for operation into reactive loads." - 6 pgs.

105

  Anechoic Chamber Design and Construction - The title tells you what you need to know - 208 pgs.

110

  Brüel Acoustics Cremer Room "Acoustic Jungle" Anechoic Chambers - The superior high frequency performance of the Cremer room is discussed - 3pgs.