MERIDIAN AUDIO L2 Speakers Overview data

L2 Series Moving-coil Loudspeakers –
Technical Overview

1. Introduction

The Quad L2 series is the latest incarnation of the now
legendary range of Quad moving-coil loudspeakers. The
range has been improved in almost every area to fulfil a
number of key design criteria.

a) To improve on all previous performance

criteria with wider bandwidth, lower
distortion, better mid-range integration
through the use of the latest advances in
manufacturing and acoustic understanding

b) To retain and enhance the traditional aesthetic,

whilst manufacturing a cabinet that has less
colouration and a more rigid construction

c) To extend the range with a series of new

models designed for a range of new
applications.

2. History of the Range

The Quad L2 series loudspeaker range is actually the third generation of moving­coil loudspeaker launched by Quad. The first of these, the 10L was launched as
part of the 77 Series range of electronics and consisted of a single 2-way model.

In 2002, Quad launched the next generation the L-series, which consisted of a
range of models starting with the 11L and moving up to the flagship 2.5way floor­standing 22L loudspeaker. The range was supplemented with a dedicated centre
channel loudspeaker and 300W RMS powered subwoofer. The introduction of the
L-series sparked a revolution in the hi-fi industry. Never before had such quality
been available at such a competitive price. The technology introduced by the L­series remains at the very leading edge of acoustic science. A unique weave of
high-grade Kevlar, the introduction of the most rigid bass driver chassis in its
class, a superb new high-resolution tweeter, and the lustrous high-gloss piano
lacquered finish all set them apart from their competitors.

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Design Challenges - The L2 series

3. Stereo imaging

Design Challenge

For some years now, it has been widely accepted by acousticians that the ideal
loudspeaker should be a pulsating sphere, creating compressions and rarefactions
within the air with the sound radiating from a point source. When using two such
point sources, the coincident waves arrive at our ears and are interpreted as stereo
sounds by our brains. The quality of the stereo ‘image’ or soundstage is defined
by how closely the loudspeaker in question conforms to this ideal point-source
theory.

Solution

In addition to the widely adopted method of
using a dome tweeter to create a hemispherical
radiating pattern, the high-frequency driver and
mid/bass drivers are very closely integrated
within the design. Complimentary alloy
components are used, allowing the main bass
driver chassis and tweeter dispersion plate to
lock with each other for the closest possible
coupling of drive units. This also aids dispersion
(see below)

4. Dispersion Characteristics

Design Challenge

Whilst bass frequencies are essentially ‘directionless’ and fill a room very easily,
a good dispersion characteristic is very difficult to achieve at higher frequencies,
yet for a consistent in-room response it is essential. Problems associated with
beaming high-frequencies include a very small ‘sweet-spot’ or listening position,
complex room reflections leading to an inconsistent room response, and frequency
response that becomes very susceptible to room furnishings and layouts.

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Solution

Our new tweeter, designed in synergy with the whole of the L2 series has quite
remarkable dispersion properties. The attached graph (of the bookshelf 12L2
model demonstrates this, showing the forward dispersion characteristic being
exceptionally uniform, even at high frequencies and up to 45degrees off-axis. (See

Appendix 1 –Polar response graphs)

5. Frequency response

Design Challenge

An important, but by no means the overriding, aspect of loudspeaker design is the
frequency bandwidth it is capable of achieving. To achieve the same perceived
amplitude of bass frequency as treble, a logarhythmically larger volume of air has
to be moved. This requires either the use of a larger cone to move the air, or for
the smaller cone to travel a greater distance. In the case of the latter, much more
powerful magnet systems are required. As well as the force required to move the
air, it also has to be borne in mind that the cone itself has a degree of inertia due to
its inherent mass. The cone has to be effectively damped against over-movement.
Other factors such as the port tube tuning arrangement and internal cabinet
volume all dictate how easy or difficult it is for the bass cone to respond
effectively to the frequency it is presented with.

At the other frequency extreme the problems are also those of mass, inertia and
magnet strength. To be able to reach the very highest frequencies above 20kHz,
the diaphragm need to be able to accelerate and decelerate 40,000 times per
second or more.

Research now indicates that loudspeakers which have a response way beyond
these traditionally understood to be the limits of human hearing (around 18kHz)
have a subjective performance advantage. This is a result of their improved ability
to follow complex waveforms at even high-frequencies.

Solution

All of the L2 series cabinets are ported, with the smaller bookshelf models having
a unique twin port arrangement. This allows the bass driver greater freedom of
movement due to decreased internal pressure resistance, and is tuned to create a
remarkably low frequency response in respect to the dimensions of the cabinet.

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The long-throw Kevlar driver is capable of terrific dynamic attack, mounted on a
strong spider suspension and sealed with a twin butyl-rubber roll-surround. The
cone itself is manufactured in-house from a bi-direction Kevlar weave – extremely
rigid and linear in motion even with large-scale dynamic pieces.

A brand new tweeter has been developed for the L2 range, featuring a new
Aluminium voice coil. This is capable of higher power handling and has better
thermal properties. The Neodymium ‘Rare-earth’ magnets employed are almost
twice as strong as those
used in the previous L­series range. The stronger
field the voice coil sits in
means it responds much
faster to the music signal
and explains the
exceptional detail and
high-frequency response
the new L2 series are
capable of – well beyond
the traditionally accepted
range of human
audibility.

6. Dynamic Range

Design Challenge

As well as frequency range, each loudspeaker will have a maximum possible
dynamic range – the difference between the smallest and largest wave amplitude
possible. This is a direct function of how much air it is possible to move and how
strong the magnet system is to push this air.

Whilst many loudspeakers are capable of impressive macro-dynamics – when the
amplitude is push high and very dynamic pieces of music are played, it is a
question of control and refinement as to how the loudspeaker performs in terms of
‘micro-dynamics’ – the subtle nuances and finer detail that can be lost in larger
loudspeaker systems.

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Solution

The powerful magnet systems in the Quad L2 series pack a terrific punch ­enough, so that even at bass frequencies where large volumes of air have to be
moved, they retain their composure – especially the large 21L2 and 22L2 models.

The real improvement in the L2 series though is in terms of the micro-dynamics.
The new tweeter unit is capable of superlative detail with extended HF capability.
This means it can track complex waveforms such as those produced from piano
and violin, maintaining their tonal integrity for a much more believable, natural
performance.

7. Distortion

Design Challenge

There are many possible sources of distortion with a moving-coil loudspeaker
system. All are notoriously hard to control and require careful engineering
solutions and a thorough understanding of materials science to resolve. Typically
one of the hardest sources of distortion to control is the cone diaphragm itself. The
voice coil typically drives a small area in the middle of the cone, then relies on the
stiffness of the cone to react to this impetus. This causes problems, because cone
flex causes distortion; if the cone is too stiff, then transverse waves radiate
through the cone and reflect back, causing distortion. The cone needs to be
damped to prevent it ‘ringing’, but if over-damped, the cone suffers from inertia
problems due to it being too heavy.

Solution

Our unique bi-directional Kevlar
weave is a superb solution to the
problems of cone distortion. Kevlar, of
course, has incredible tensile strength,
but very little structural rigidity, and
needs to be woven in bundles of fibres
to form a structure. This structure then
needs to be resin impregnated to
produce a cone of the appropriate
stiffness. The resulting cone is very
light and strong. Moreover, the nature

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of our weave damps the transverse
waves radiating from the voice coil
when the cone moves, leading to a
cleaner sound with much lower
distortion. The graph to the right
shows the distortion measurements
for the centre channel through the
most sensitive vocal range.

8. Driver integration /

Crossover design

Design Challenge

Due to the problems of cone inertia mentioned above, each drive unit typically
only has a limited frequency response. Therefore, to achieve the desired
bandwidth, multiple drivers are required. Whenever multiple drivers are used
(usually a tweeter for high frequencies and a woofer for low frequencies), there is
a frequency range in between them when both drive units are contributing to that
frequency. This area is also typically in the region where the human ear is at its
most sensitive. Constructive and destructive interference between the coincident
waves from each of the drivers can cause phase problems. These problems are
exacerbated because most manufacturers (even the larger ones) use bought-in
drivers, rather than manufacturing themselves (a huge amount of investment is
required to be able to produce your own drive units). In order to compensate for
the inevitable miss-match between bought-in drivers, other loudspeaker
manufacturers tend to try and correct these problems through the crossover,
maybe using high-order crossovers (creating a faster crossover slope). That can
compound these phase problems and create further frequency response problems
leading to poor mid-range integration.

Solution

Quad is in an enviable position amongst loudspeaker manufacturers in having a
factory capable of complete vertical integration which extends through the R&D
process.

Every single chassis, cone, spider, magnet assembly, terminal, cabinet, crossover
and even the internal wiring is made within our own four walls. This affords our

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loudspeaker engineers remarkable freedom in
design. They have the ability to design drive units
which have natural roll-offs at particular frequency
– very exacting performance parameters which can
be defined at the outset – all of which contributes to
us requiring only the very simplest low-order
crossover.

That said, the crossover on the L2 series is of
audiophile quality in every respect. All components

are mounted on glass-fibre circuit boards with large,
non-interlacing tracks. Perfect layer, air-cored low-distortion inductors are wound
on clear plastic bobbins and mounted in such a way as to virtually eliminate
magnetic inter-modulation between
components. All the capacitors used are
metallised polypropylene low loss and
low induction of the finest quality. The
crossover is then wired internally to the
drive units using exceptionally pure
heavy gauge ‘oxygen-free copper’
internal wiring.

9. Timing and rhythm

Design challenge

To achieve a response that is accurate and
tracks the music signal precisely, a
loudspeaker needs be able to react as a
whole to the signal. Bass ‘overhang’ is
unacceptable, but furthermore there are
many other issues that affect the temporal
integrity of a loudspeaker. One key contributor to temporal smearing of the mid­range is reflections from the bass-driver chassis itself. Most drive units are
manufactured using either plastic or pressed steel chassis. This requires them to
have quite wide legs and relatively small apertures between them. The rearward
movement of the cone is reflected from such structures back through the cone,
causing temporal smearing of the mid-range akin to the ‘ghosting’ effect seen on
televisions when reflected signals occur.

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Solution

Tracking a music signal precisely
and accurately in the time domain
requires a very clean and fast
loudspeaker, free of complex
reflections and interactions. A
powerful tool for Quad in designing
the L2 series has been our degree of
vertical integration. Because we
have complete freedom of design for
our own drive unit chassis, and the
ability to manufacture complex cast
alloys into almost any shape we
desire, we have engineered a cone which is robust, strong, light with one of the
cleanest rear profiles of any chassis ever created. The chassis legs radiate from the
centre uniformly and in such a way that even leg reflections are quickly dissipated
within the cabinet. This has almost eliminated this common source of temporal
smearing.

The powerful magnets in
the bass and treble drive
units keep tight control
over any diaphragm inertia.
The attached waterfall plot
(below) from the L2
satellite speakers is a
perfect example of how
quickly and cleanly the
drivers follow the decay
from an impulse.

10. Cabinet design

Design Challenge

A cabinet around the drivers is a necessary evil of loudspeaker design and allows
the forward wave to be less affected by the rearward travelling wave produced by

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the cone movement. In all other respects however, the job of the acoustic engineer
is to minimise the effect of the cabinet.

The problems are again, numerous. The cabinet can itself suffer from transverse
waves, colouring the sound. The rearward waves travelling from the back of the
cone can create standing waves reflected back and forward inside the cabinet,
leading to slow and coloured bass. The internal volume needs to be maximised to
allow the greatest bass extension, yet for a modern aesthetic, the smaller the
loudspeaker is, the more acceptable it becomes for a domestic environment.

Solution

This was a particular challenge as we wanted to retain a very traditional aesthetic.
The cabinet itself is manufactured from Starwood MDF – a very high grade,
Eucalyptus based material, prized for its woodworking qualities and low internal
resonance.

This choice of material coupled with additional internal bracing, designed to
randomise the internal reflections and thus prevent standing waves from forming
significantly reduces cabinet colouration and resonance.

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Appendix 1

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