. . . This page will be in development for some time so will be unfinished
. . . 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
. . . "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
. . . Christopher A. Shera
. . . Professor of Otolaryngology-Head and Neck
. . . 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
. . . .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
. . . 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
. . . 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"
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.
. . . 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
. . . . . . . .Non-woven Composite for Sound
. . . 05 - Analysis of the Bending
Property of Kapok Fiber
. . . 06 - Kapok or Capok Fibers;
. . . 07 - Kapok Fibre: "A Perspective
. . . 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
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
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
. . . 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. 15.10 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
. . . 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. 20 -- Notes on the PEARL Polymer-Graphite 1" Dome
. . . .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.
. . 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. 25.2Shown 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. 25.3This 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.
. . . .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
. . . .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.
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:
. . . .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.
. . . .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."
. . . .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
. . . 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
. . . 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.'
Sec. 75 -- Brüel Acoustics Cremer 'Acoustic Jungle' Anechoic Rooms
. . . .Fig. 75.1Brüel 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
. . . 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.
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.