`
`
`EXHIBIT 2011
`
`EXHIBIT 201 1
`
`

`

`FOR MUSIC RECORDING AND REPRODUCTION
`x
`
`L. U DS PEAKE RS
`
`
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`THX Ltd. Exhibit 2011 Page 1
`|PR2014-00235
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`a
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`THX Ltd. Exhibit 2011 Page 1
`IPR2014-00235
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`

`

`Loudspeakers
`For Music Recording and Reprothiidkm
`
`Philip Newell and Keith Holland
`
`a (cid:9)
`
`AMSTERDAM (cid:149) BOSTON (cid:149) HEIDELBERG (cid:149) LONDON
`NEW YORK (cid:149) OXFORD (cid:149) PARIS (cid:149) SAN DIEGO
`SAN FRANCISCO (cid:149) SINGAPORE (cid:149) SYDNEY (cid:149) TOKYO
`
`ELSEVIER (cid:9)
`
`Focal Press is an imprint of Elsevier
`
`THX Ltd. Exhibit 2011 Page 2
`IPR2014-00235
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`

`(,( ) [i t,
`
`( jipi
`
`Focal Press is an imprint of Elsevier
`Linacrc House, Jordan Hill, Oxford 0X2 8DP, UK
`30 Corporate Drive, Suite 400, Burlington, MA 01803, USA
`
`First edition 2007
`
`Copyright ' 2007, Philip Newell and Keith Holland.
`Published by Elsevier Ltd. All rights reserved.
`
`The right of Philip Newell and Keith Holland to he identified as the authors of this work
`has been asserted in accordance with the Copyright, Designs and Patents Act 1988
`
`No part of this publication may he reproduced, stored in a retrieval system
`or transmitted in any form or by any means electronic, mechanical, photocopying,
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`Obtaining permission to use Elsevier material
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`Notice
`No responsibility is assumed by the publisher for any injury and/or damage to persons
`or property as a matter of products liability, negligence or otherwise, or from any use
`or operation of any methods, products, instructions or ideas contained in the material
`herein. Because of rapid advances in the medical sciences, in particular, independent
`verification of diagnoses and drug dosages should be made
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`British Library Cataloguing in Publication Data
`A catalogue record for this book is available from the British Library
`
`Library of Congress Cataloguing in Publication Data
`A catalogue record for this book is available from the Library of Congress
`
`ISBN-13: 978-0-240-52014-8
`ISBN-JO: 0-2405-2014-9
`
`For information on all Focal Press publications
`visit our website at www.focalpress.com
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`THX Ltd. Exhibit 2011 Page 3
`IPR2014-00235
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`

`

`oneepts so tally 111 Me 000K anu Lucre are oLlier, less C0111111011 ones)
`establish the point that at the very heart of all loudspeaker systems
`nperfect components, and, as mentioned earlier, that the art of the
`ce of the designs is to find the best compromise for any individual
`rement.
`
`rences
`
`loms, M., ’High Performance Loudspeakers’, 5 Edition, John Wiley & Sons,
`chester, UK (1997)
`gs. G., ’Loudspeakers, The Why and How of Good Reproduction’, Fourth
`tion, Wharfedale Wireless Works Ltd, Bradford, UK (1955)
`wick, J., ’Loudspeaker and Headphone Handbook’ Second Edition,
`Lpter 2 (By Stanley Kelly), Focal Press, Oxford, UK (1994)
`
`ography
`
`pter 3 of Reference 3, above, contains what is perhaps the definitive work
`clectrostatic loudspeakers, written by the late Peter Baxandall. In the Third
`tion of the book, published in 2001, Peter Walker somewhat modified the
`Either edition, in its entirety, is recommended reading for anybody wishing
`lelve deeper into the world of loudspeakers and headphones.
`books mentioned in References 1 and 3 above
`ale, J., ’Loudspeaker Handbook’, Chapman and Hall, New York, USA and
`idon, UK (1997)
`wick, J., ’Loudspeaker and Headphone Handbook’, Third Edition, Focal
`ss, Oxford, UK (2001)
`
`Loudspeaker cabinets
`
`3.1 The concept of the infinite baffle
`
`When the diaphragm of an open-framed driver moves forwards, the com-
`pression of the air at the face of the diaphragm is accompanied by a
`rarefaction at the other side of the diaphragm, and the natural tendency is
`for the pressure difference to equalise itself by a movement of air around
`the sides of the driver. At frequencies whose wavelengths are large com-
`pared to the circumference of the diaphragm, the equalisation is almost
`perfectly accomplished, and so almost no sound is radiated. It is therefore
`necessary to discourage this pressure equalisation if low frequencies are
`to be radiated. The simplest means of accomplishing this is to mount the
`loudspeaker in a large, rigid board, or baffle, as shown in Figure 3.1. If
`the board were to extend in all directions to infinity, it would be a true
`infinite baffle. It would cause no change in the air loading on each side of
`the diaphragm, it would exhibit no resonances, it could cause no diffrac-
`tion, and, with a good quality driver (or drivers) would sound excellent.
`Unfortunately, its great drawback is that it is a rather impractical concept.
`The two practical realisations of this idea are the finite baffle, where a
`baffle of perhaps a metre square is employed, or the so-called infinite baffle,
`which is, in fact, a sealed box. The radiation pattern of the finite baffle is
`shown in Figure 3.2(a). The cancellation around the sides of the extended
`plane of the driver cause response nulls to the sides, in the direction of the
`plane of the baffle, resulting in a three-dimensional figure-of-eight pattern
`in free space. The low frequency cut-off is determined by the size of the
`baffle. The final rate of low frequency roll-off is 18 dB per octave, but
`some measures can affect the nature of the entry to the roll-off. Varying
`the Q of the driver resonance, by mechanical and/or magnetic changes, can
`yield response shapes such as those shown in Figure 3.2(b). By placing the
`driver off-centre, the cut-off can be made more gradual due to the distance
`from the driver to each edge of the baffle being different. Open baffles are
`rarely used in recording studio control rooms because of the problems of
`where to site them and how to control the rear radiation, but they find use
`in listening rooms and domestic high-fidelity systems. In these instances the
`baffles can be sited somewhat more flexibly than in an equipment-loaded
`control room, and the loudspeaker and listener positions can usually be
`found which give good results. Subjectively, open baffles tend to sound
`very clean and, not surprisingly, open. They are largely free of resonances,
`so their time-domain responses are limited only by the drivers and the
`
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`66 Loudspeakers
`
`’-r
`
`Figure 3.1 An open baffle of Wharfedale design from the 1950s. The front panel was a sand-
`filled plywood sandwich, to damp resonances. The upward-pointing tweeter was to generate
`a more diffuse high-frequency response
`
`a) (cid:9)
`
`b)
`
`0
`
`D\2/
`
`10 (cid:9)
`
`100 (cid:9)
`Frequency (Hz)
`
`1000
`
`Figure 3.2 Directivity and roll-off of open baffles, a) Radiation pattern polar plot of an open,
`finite baffle. b) Low frequency response roll-off of an open baffle, showing the effect of the
`Q (degree of sharpness of resonance) of the driver. The final roll-off tends towards 18 dB per
`octave below the driver resonance
`
`rooms in which they are placed. The ways in which they couple to the
`rooms will be discussed in Chapter 7.
`When mounted on the floor, the solid surface below the open baffle
`acts like an acoustic mirror, so a baffle of one square metre placed on the
`floor behaves like a baffle of two square metres in free space. This enables
`baffles of practical size to be useful down to frequencies of 40 Hz or below,
`but the lack of anything other than atmospheric loading on the rear of the
`diaphragms and poor efficiency of radiation may lead to over-excursion
`problems with high sound pressure levels at low frequencies. The resonance
`
`Loudspeaker cabinets 67
`
`frequency of the driver on an open baffle will be that of its free-air res-
`onance. Because the open baffle mounting does not push up the free air
`resonance of the driver, and the back-pressures are not augmented by any
`constraint of the air behind the diaphragm, lighter moving assemblies may
`be used. Driver cooling is also something that poses no problem with open
`baffles, so power compression problems are rarely encountered. The open
`baffle, in the hi-fi world, still enjoys a devoted following of aficionados.
`
`3.2 The sealed box
`
`The practical realisation of an infinite baffle (the sealed box) is rarely
`large enough to avoid significantly loading the rear of the loudspeaker,
`so is best called what it really is, a sealed box. Just as open baffles tend
`to sound ’open’, sealed boxes often tend to sound ’boxy’. However, this
`need not be the case if the box and driver are of adequate size, well-
`matched, and if sufficient attention is paid to the suppression of resonances
`within the box. The constraint of the air within a sealed box causes it to
`act like a spring, which reacts against the movement of the diaphragm in
`either direction. This effectively stiffens the suspension of the drive unit,
`and raises its resonant frequency. The smaller the box, the stiffer will be
`the spring, so the higher will be the resonant frequency of the driver/box
`system. As the system resonance defines the frequency at-which the low
`frequency roll-off will begin, then for any given driver the low frequency
`response will become progressively more curtailed as the box size reduces.
`The only way to counteract this tendency is to use drivers of lower free-air
`resonance frequency, which means using a driver with a heavier moving
`assembly or, to a lesser extent, a more compliant suspension, but both of
`these characteristics have their drawbacks.
`A heavier cone takes more energy to move it, so it will need more
`amplifier power to drive it to produce the same SPL as a lighter cone.
`A more compliant suspension will be much less rugged than a stiffer
`suspension, and will tend to be much more easily damaged in the event
`of an overload. What is more, a very loose, flexible suspension may not
`be able to adequately resist the pressure changes inside the box at high
`SPLs, and may physically deform, giving rise to non-linearities in its travel
`and non-linear distortions in its radiation. This will all be discussed in
`much more detail in Chapter 11, but suffice it to say here that a small
`sealed box must suffer from either poor system sensitivity (due to its poor,
`overall electro- acoustic conversion efficiency) or a low frequency roll-off
`that begins well into the musical spectrum. The roll-off exhibits a rate
`of 12dB per octave below its frequency of resonance, but considerable
`roll-off may begin well above this frequency, depending on the system
`Q. Some typical roll-off curves are shown in Figure 3.3. Nevertheless, the
`time responses (transient responses) of well-designed sealed boxes with
`correctly matched drivers and adequate damping can be very accurate.
`Largely for this reason, sealed boxes have a strong following, and large
`sealed boxes can be the bases of excellent loudspeaker systems.
`A sealed box system is said to be critically damped when its size and
`the driver resonant frequency are matched such that the overall response
`
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`68 Loudspeakers
`
`dB120 ............................................
`........
`(cid:9) ...
`118 ......................................................................................................
`116 ............ .....................................H....................................... ............
`
`114 ... ...............
`
`112 .(cid:149)............
`110 (cid:9)
`-. (cid:9)
`.
`
`108...................... .......................................................
`106 .......................................................................
`104 ............... .. i... .,,. .........................................................
`..................
`. (cid:9) .....
`102 ..............,
`. .....
`...
`100 (cid:9)
`98 ..., ................................................................
`.. .......................................................
`
`96 ...
`
`.............................................................................
`
`.
`
`.
`
`A
`
`90
`
`92.
`F (cid:9)
`3D: C .3 (cid:9)
`20 (cid:9)
`
`
`200 (cid:9)
`
`300 (cid:9)
`
`
`
`10 (cid:9)
`
`30 (cid:9)
`
`50 (cid:9)
`
`70 100Hz (cid:9)
`
`500 700 (cid:9)
`1k
`(after Eargle 1 )
`Figure 3.3 Typical low frequency roll-off behaviour of a sealed box loudspeaker, showing
`the responses of the same driver in cabinets of six different volumes. All measurements in
`free-field conditions
`A. 7L. B, 14L. C, 28L. D, 56L. E. 112L. F, 224L
`Loudspeaker free-air resonance 20 Hz
`
`is already 6 dB down at the resonant frequency. With this alignment, the
`transient response can be exemplary, with no perceptible ringing. The total
`System 0TC (Or quality factor of resonance) is 0.5. The Butterworth ’B2’
`(maximally flat) alignment is very popular, with a 0Tc of 0.7. This exhibits
`a system response which is 3 dB down at the resonant frequency, and
`still has a transient response which is extremely well controlled. The low
`frequency responses can be extended downwards with alignments where
`the QTC is set at 1, or even up to 2, but as the Q increases, so does the
`tendency for the transient response to become extended, and for audible
`ringing or ’boominess’ to become obtrusive. The outcome of these rela-
`tionships is that if the low frequency (cid:151)3 dB point is to be dropped to 30
`Hz, and a fast, well-damped transient response is required at the same
`time, then the box must be big. If high SPLs are required, then the only
`solution to the compromise of a low resonance driver with an adequately
`robust construction and a good sensitivity is that the driver must also
`be big.
`A 15 inch (380 mm) driver, of high quality, with a 20 Hz free air
`resonance in a 500 litre enclosure can yield some very impressive bass.
`However, ’impressive’ in this context means full, flat, fast and low distor-
`tion - in other words, ’accurate’. Unfortunately, many sealed boxes get
`
`Loudspeaker cabinets 69
`
`themselves a bad name by trying to use ’boomy’ alignments in forlorn
`efforts to keep the size down whilst seeking to extend the low frequency
`response to frequencies that the box size cannot really support. The penalty
`paid is in terms of low sensitivity and poor transient response. It must be
`thoroughly understood that there is no clever computer program which
`can solve this problem. The restrictions that we must accept are deeply
`entrenched in the physical laws of the universe in which we live. They are
`that fundamental(
`Some manufacturers have tried to sacrifice system sensitivity by low-
`ering the magnet flux in order to lower the system Q. There is a strong
`’amplifier power in cheap’ lobby, who believe that lower efficiency systems
`can exhibit higher Os, and hence can be extended in their low frequency
`range. What they often seem to fail to realise is that a heavier current in the
`voice coil and a lower power magnet will drastically alter the ratio of the
`fixed magnetic field to the variable magnet field. The much higher variable
`field due to the voice coil current can severely distort the position of the
`flux lines of the weak, permanent magnet, and give rise to loss of low level
`detail in the sound and increased levels of intermodulation distortion. This
`highlights perhaps one of the worse aspects of the use of programmable
`calculators or computers in the wrong hands they can lead to good results
`on paper, but they can give rise to unpleasant side-effects in practice.
`Figure 3.4 gives a graphic illustration of the connection between system 0
`(QTc) and the transient response. The QTC is derived from the electrical,
`magnetic, mechanical and acoustical properties of the tot al.,system - electro
`
`Q=0. 50
`
`10
`
`CL
`
`b6
`
`:
`
`5=20
`
`: (cid:9)
`
`01; (cid:9)
`tI2itT (cid:9)
`time (cid:9)
`
`tI27tT
`time
`
`(after small 2)
`
`Figure 3.4 Transient response of a sealed box enclosure as a function of the total system
`Q (Qrc)(cid:149) As the (cid:176)TC increases, the transient decay time also increases
`
`THX Ltd. Exhibit 2011 Page 6
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`(cid:9)
`(cid:9)
`

`

`70 Loudspeakers
`
`magnetic damping, mechanical stiffness and air loading. Note that as the
`QTC increases, the transient response becomes longer. This is perfectly
`logical because the transient response becomes more resonant as the sys-
`tem QTC becomes more resonant. The amplitude response is boosted and
`extended downwards by keeping the energy responding for a longer time,
`and not by instantaneously boosting the level. As stated before, in order
`to boost the level, and nothing else, a bigger box and driver are needed.
`Small sealed boxes with relatively high rates of roll-off can be mounted
`near to room boundaries, where the constraint of their radiation angle
`can boost their low frequency output, acoustically, without suffering time
`penalties (boundary effects are discussed in Chapter 7), but on pedestals in
`the centre of a room, the low frequencies from small sealed boxes will be
`found to be either weak, resonant or both. On the metre bridge of a solidly-
`built mixing console they can also receive some low frequency support, but
`colouration problems due to the reflective surface being between the small
`loudspeaker and the listening position can be a problem. This ’acoustic
`mirror’ concept was discussed in the previous section, but when applied
`to floor standing open baffles, the mid and high frequency drive units
`are usually mounted well clear of the floor. When a small sealed box is
`placed on top of a mixing console, the sources of mid-range and high
`frequencies are inevitably close to the reflecting surface, so comb-filtering
`of the response is the likely result.
`To put things into proportion with respect to size, a cabinet which is
`3 dB down with a given low frequency driver at 80 Hz would need to be
`4 times larger if it were to be 3 dB down at 40 Hz and 16 times larger to be
`3 dB down at 20 Hz, so sealed box sizes do tend to get larger very quickly
`if lower roll-of frequencies are required.
`One advantage of sealed box systems is that they are relatively self-
`protecting in terms of excessive cone excursions. Compared to the open
`baffle, which offers almost no protection, an input signal below the resonant
`frequency of a sealed box system will tend to drive the cone at a constant
`excursion for any given input level, independent of frequency. (See Note 1
`at end of chapter.) The thermal overload of the coil is therefore the biggest
`risk factor in terms of driver integrity at input level extremes.
`The lining materials in the boxes also have an effect on the low fre-
`quency response. Although they are primarily intended to prevent cabinet
`resonances at mid frequencies, which may colour the sound by passing to
`the outside via a relatively acoustically transparent cone, the lining mate-
`rials can also affect the low frequency damping and total system Q. They
`should not be too tightly packed, or effectively they will be more or less
`solid and will reduce the enclosure volume. Neither should they be able
`to move en masse, or they can introduce non-linear distortion due to their
`somewhat erratic movement. Given just the right quantity, however, they
`can not only reduce box resonances but can also make the boxes appear to
`be up to around 20% acoustically bigger due to their ability to act as heat
`sinks and slow down the speed of sound by absorbing heat on compression
`half cycles and releasing it on rarefaction half cycles. The tortuosity of
`the path through the pores or fibres also gives rise to sound absorption.
`The density and quantity of the absorbent material inside a sealed box
`
`are therefore chosen for the parts that they play in the air loading and
`damping calculations for the whole system.
`
`Loudspeaker cabinets 71
`
`3.2.1 Acoustic suspensions
`Developed in the 1950s by Edgar Villchur, in the USA. The principal is
`to use a very low resonance loudspeaker in a small sealed box. The air in
`the box may push up the resonance by an octave, or more, and is the pre-
`dominant restoring force for centralising the diaphragm, because the low
`resonance suspension is very weak. Generally, although an acoustic suspen-
`sion system is a sealed box, the specialised term is normally only used when
`the ratio of the cabinet (air) compliance to the driver’s compliance exceeds
`a factor of about 4 to 1 1 . In the late ’SOs and early ’60s, the company Acous-
`tic Research enjoyed very great success with these designs by making huge
`improvements in the low frequency fidelity of small loudspeaker systems.
`
`3.3 Reflex enclosures
`
`Also known as ported enclosures, vented boxes or phase inverters, reflex
`enclosures use openings, or ports, to tune the cabinet resonance to a desired
`frequency. Effectively, the air in the port, which may be a simple hole
`or a tube, acts as a mass which resonates with the spring created by the
`air inside the cabinet. In Figure 3.5, a mass is shown suspended below a
`spring. Almost everybody will intuitively know what would happen if they
`were to pull down on the weight and then let go - the weight would spring
`back and the system would go into oscillation until the energy was finally
`
`I
`
`Figure 3.5 A mass/spring system. A mass suspended beneath a spring. It is easy to imagine
`how pulling down on the weight and releasing it would set up an oscillation due to the
`mass-spring interaction
`
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`

`

`(cid:9) (cid:176)MS X QES (cid:9)
`
`In the case of a reflex cabinet, a bigger box provides a weaker spring,
`because the enclosed air is compressed or rarefied proportionately less
`than in a small box for any given diaphragm displacement. For any given
`diameter of hole (port), extending it with a tube will lower the resonant
`frequency because a greater mass of air will be trapped within it. For
`any given cabinet volume and mass of air in the port, changing the area
`of the port will also change the resonant frequency. Increasing the area
`will increase the resonant frequency. This is because there is more surface
`area in contact with the air-spring, so more force acts upon the air mass,
`effectively stiffening the spring. There are therefore three variables in the
`equation, the cabinet volume, the length of the port, and the area of the
`port - the latter two defining the volume of air in the port, and hence its
`mass. Air weighs about 1.2 kg per cubic metre, and thus about 1.2 grams
`per litre.
`The cabinet tuning frequency can therefore be calculated approximately
`from either of the two following equations, the first in imperial measure
`and the second in metric units.
`
`Loudspeaker cabinets 73
`
`Note: L allows for an end correction. The effective length of a port tube
`is: in reality, somewhat longer than the physical length, but for many
`calculations the actual, physical length can be used.
`The formulae are not precise : because there are always variables such as
`the quantity of air displaced by the drive units themselves, the air displaced
`by the port tubes, the air displaced by internal bracing, and the effect of
`the absorbent material inside the enclosure. Nevertheless, the formulae
`give good working approximations or starting points for calculations. Of
`course, the cabinet volume is calculated from the interior dimensions of
`the cabinet, not the exterior dimensions.
`In practice, when the frequency of resonance of the driver in the cabinet
`is just above the resonant frequency of the box, the port resonance gives
`rise to a high load on the diaphragm and greatly reduces the diaphragm
`excursion. In this way, the ported cabinet can protect the driver from exces-
`sive travel while still maintaining a flat response. Below this frequency,
`the driver output falls, but the port, itself, begins to radiate, thus extend-
`ing downwards the frequency response. At still lower frequencies the port
`and loudspeaker outputs occur in opposite polarity, so the response falls
`off rapidly at 24 dB per octave. Moreover, below the port resonance, air
`simply pumps in and out of the port under the influence of the driver. At
`these frequencies, the cabinet is just a box with a big air leak, and it can
`provide no loading on the driver diaphragm, which then behaves as if it
`were in an open baffle with no air loading protection, so over-excursions
`are easy to encounter in reflex enclosures unless the low frequency drive
`signal is filtered, or has no natural content, below the resonant frequency
`of the box.
`A comparison of the performance of two different low frequency drivers
`in an open baffle, a sealed box, and a reflex enclosure is shown in Figure 3.6.
`In practice, a driver would be specifically designed for each type of loading,
`because the different cabinets or baffles match more optimally with drivers
`of specific 0TC values. The QTS, which can be found in many formulae
`, which are the
`and reference texts, is the sum of the QM5 and the Q
`mechanical and electrical system quality factors (sharpness of resonance),
`respectively. The higher the Q, in each case, the more highly tuned is the
`resonance, as shown in Figure 3.7. For reference, the 0 terms commonly
`found in loudspeaker texts are as follows 2 :
`is the mechanical system Q. It is the ratio of the electrical equivalent
`Q
`of the frictional resistance of the moving parts of the driver to the reflected
`motional reactance at the free-air resonance frequency of the driver.
`is the electrical system Q, which is given by the ratio of the voice coil
`Q
`DC resistance to the reflected motional reactance at the free-air resonance
`frequency of the driver.
`QTS is the parallel combination of the Q55 and QE5, and the equation
`takes the same form as that for two, parallel, electrical resistors:
`
`4
`
`(cid:176)TS =
`
`(cid:9) (cid:176)ES
`
`(3.3)
`
`QTC is the total system 0 of the driver and the cabinet.
`
`72 (cid:9) Loudspeakers
`
`dissipated. Adding more weight would cause the oscillation to slow down,
`as would using a weaker spring. Therefore:
`
`more weight
`weaker spring (cid:9)
`
`less weight (cid:9)
`stronger spring (cid:9)
`
`) (cid:9)
`
`)
`
`slower oscillation (lower frequency)
`
`faster oscillation (higher frequency)
`
`2700A
`V(L +
`
`= (cid:9)
`
`where:
`
`= resonant frequency of box (Hz)
`A = area of port in square inches
`V = volume of box in cubic feet
`L = length of port in inches
`
`OR
`
`f1 (cid:9)
`
`where:
`f (cid:9) = resonant frequency of box (Hz)
`c (cid:9) =speed of sound in air - 340m/s
`A = area of port in square metres
`V = volume of box in cubic metres
`Le = effective length of port in metres
`
`(3.1)
`
`(32)
`
`THX Ltd. Exhibit 2011 Page 8
`IPR2014-00235
`
`

`

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`Figure 3.7 a) Transient response of a reflex enclosure as a function of the total system 0 (Qr).
`Compare with Figure 3.4. b) Pressure amplitude response of the C4 and B4 alignments. Note
`how the response extension of the C4 alignment corresponds with an increase in the decay
`time, as shown in a). Transient response is traded for response extension
`
`The free-air resonant frequency of the drivers for reflex enclosures may
`also need to be different to the optimum resonant frequencies for sealed
`enclosures of similar size or covering a similar frequency range. The mod-
`ern tendency is to tailor complete driver designs to given box sizes and
`system concepts, and the programs now available for computer analysis are
`very powerful and very accurate. However, careful listening tests are still
`a sine qua non because concentration on the optimisation of one aspect of
`driver design may unexpectedly change for the worse some other aspect of
`performance that was not under such close scrutiny. Unfortunately, listen-
`ing rooms are expensive to build and listening panels can be an expensive
`luxury. Computer time on the other hand is cheap, and quick, and there
`has developed a strong tendency to design systems ever more by computer
`and ever less by careful listening.
`
`THX Ltd. Exhibit 2011 Page 9
`IPR2014-00235
`
`(cid:9)
`(cid:9)
`

`

`76 Loudspeakers
`
`3.4 Acoustic labyrinths
`
`These are sometimes referred to as transmission lines, but at low frequen-
`cies they are usually not transmission lines. A true transmission line needs
`a rear cavity, straight or folded, at least a quarter of a wavelength long.
`At 30 Hz, with a wavelength of about 11 metres, the line would need to
`be around 3 metres long, and lines of this length are rare indeed. A true
`transmission line works by presenting the correct acoustic impedance at
`the rear of a driver so that all the backwards radiation propagates away
`from the driver, never to return. This can be achieved by loading the rear
`of the driver with an infinitely long pipe (which is rather impractical) or by
`some other system that absorbs all the sound energy. A finite length pipe
`can therefore be made to operate as a transmission line if it contains sound
`absorbing material strategically placed to give the correct, purely resistive
`acoustic impedance, such as an anechoic wedge. However, in order to work
`at low frequencies this pipe still needs to be very long, so some form of
`low frequency tuning is often employed.
`If, instead of an infinite pipe we think of an organ pipe, it would exhibit a
`series of resonant frequencies determined by its length. If we attach it to the
`rear of the driver and fold it round to the front, there will be interference
`between the sound from the front of the driver and that from the open end
`of the pipe. When the pipe is one quarter wavelength long, there will be a
`high acoustic pressure at the rear of the driver and a high acoustic velocity
`at the open end, which combines with a phase difference of 90 degrees with
`the acoustic velocity from the front of the driver, and this provides a useful
`boost in output. As the frequency is lowered, the output from the pipe
`increases in phase difference with respect to the direct output from the
`driver, and so tends towards cancelling the combined output. This yields a
`24 dB per octave roll-off below the tuning frequency, which leads to tran-
`sient problems not unlike those of a conventional reflex cabinet. A finite
`length open pipe, which is what the vast majority of so-called transmis-
`sion lines certainly are, is clearly not a transmission line, as it works on a
`completely different principle and yields a different acoustic performance.
`A typical cross-section of such a design is shown in Figure 3.8(a).
`In order to tame the strong resonant behaviour exhibited by the open
`pipe, absorbent material is introduced into the pipe to add damping. As
`the amount of absorbent material is increased, the acoustic performance
`lends towards that of a transmission line except at very low frequencies,
`where there is insufficient absorption. A carefully lined, or filled, open-
`ended pipe may thus exhibit some of the properties of a transmission line,
`but may also rely on the quarter wavelength resonance to supplement the
`total low frequency sound output. Most commercial ’transmission lines’
`are therefore really something between the two extremes of a transmission
`line and an open pipe, depending on the amount of absorption present at
`any given frequency.
`Some versions of ’transmission lines