Thiel Coherent Source CS.5 Technical Information

Technical Information

THIEL CS.5

Coherent Source

®

Loudspeaker

This paper describes some of the technical performance aspects, design
considerations and features of the THIEL model CS.5 loudspeaker system. It is
intended to supply information for those who are interested in such matters. It is not
intended to imply that good measured technical performance is sufficient to
guarantee good sonic performance.

THIEL • 1026 Nandino Boulevard • Lexington, Kentucky 40511 • USA
Telephone: [phone] • Fax: [phone] • E-mail: [e-mail]
Web: www.thielaudio.com 5/00

THIEL DESIGN PHILOSOPHY

All THIEL speakers are intended to be precision instruments that very accurately translate electronic information into musical sound. All
our efforts have been directed toward achieving extremely faithful translation of all tonal, spatial, transient and dynamic information
supplied by the amplifier. THIEL speakers are not intended to mask or mitigate shortcomings of the recording or other components in the
music playback system. We believe this approach is the only way to provide the potential of experiencing all the subtle aspects that help
make reproduced music a most enjoyable human experience.

Performance goals

Since quality of musical performance is a very complex issue it is helpful to objectively identify the aspects involved. We believe
musical performance can be described, with not much oversimplification, as performance in four areas.

Tonal fidelity includes overall octave-to-octave balance, the fidelity of timbres, absence of vowel-like colorations, and bass extension.

Spatial fidelity includes how wide and deep the performing space seems, how convincingly instruments are placed from the center to

beyond the speakers laterally, how realistic the depth perspective is, how little the speakers’ positions seem to be the source of the sound,
and how large the listening area is.

Transient fidelity includes how clearly and cleanly musically subtle low–level information is reproduced and how convincingly
realistic is the reproduction of the initial or “attack” portions of sounds.

Dynamic fidelity includes how well the speaker maintains the contrasts between loud and soft and how unstrained and effortless is the
reproduction of loud passages.

Fundamental design considerations

In our opinion, natural spatial reproduction requires creating a realistic sound field within the listening room by mimicking the properties
of natural sound sources. These properties include wide area radiation and the absence of out-of-phase energy. To meet these requirements
all THIEL speakers employ dynamic drivers. Dynamic drivers have the advantages of providing a point source radiation pattern with good
dispersion of sound over a wide area, great dynamic capability, good bass capability and a lack of rearward out-of-phase energy. Another
advantage of dynamic drivers is that their small size allows the multiple drivers to be arranged in one vertical line. This alignment avoids the
problem of line source designs which must place their different drivers side-by-side, causing the distance from each driver to the listener to
change with different listener positions.

The major potential disadvantages of dynamic speakers are diaphragm resonances (“cone breakup”), cabinet resonances and cabinet
diffraction. Also, they share with other types of speakers the potential problems of time and phase errors introduced by multiple drivers and
their crossovers. None of these problems is a fundamental limit and all can be minimized or eliminated by thorough and innovative

engineering, allowing the possibility of a speaker system without significant fundamental limitations.

Technical requirements

The task of engineering a speaker system requires the translation of the musical performance goals into technical goals. Although there
are also many minor design considerations, the following are what we believe to be the major technical requirements that contribute to each
of the musical goals.

Tonal fidelity

• Accurate frequency response so as not to over or under emphasize any portion of the sound spectrum

• Absence of resonances in the drivers or cabinet so as not to introduce tonal colorations

Spatial fidelity

• Point-source, unipolar radiation

• Time response accuracy to preserve natural spatial cues

• Lack of cabinet diffraction

• Even dispersion of energy of all frequencies over a wide area

Transient fidelity

• Phase coherence to provide realistic reproduction of attack transients

• Very low energy storage to provide clarity of musical detail

Dynamic fidelity

• High output capability

• Low distortion

Design goals

The technical requirements result in the following major technical design goals:

1. Very uniform frequency response

2. Time response accuracy

3. Phase response accuracy

4. Low energy storage

5. Low distortion

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THIEL CS.5 SPECIFICATIONS

Bandwidth (-3dB)
Amplitude response
Phase response
Sensitivity
Impedance
Recommended Power
Size (W x D x H)
Weight

55Hz - 20KHz

55Hz - 20KHz ±3dB

minimum ±10°

87dB @ 2.8v-1m

4Ω, 3.2Ω minimum

30-150 watts

8 x 11 x 31 inches

35 pounds

Driver Complement:
Woofer

61/2" (5" radiating area) with treated paper cone, cast frame,
1" diameter voice coil. Underhung coil (short coil/long gap) motor
system. Linear travel 1/4" pk-pk. Two magnets with total weight of

1.4 lb. Copper pole sleeve.

Tweeter

1" aluminum dome with short coil, ferrofluid, vented pole to rear
chamber, reinforced chamber cup.

DESIGN AND ENGINEERING FEATURES

FREQUENCY RESPONSE

Since frequency response errors are a measure of tonal imbalances which alter music's tonal characteristics, we believe that accurate
frequency response is an absolute requirement for a truly good speaker. In our opinion the human ear is sensitive enough to the balance
between component harmonics of musical sounds to detect frequency balance errors of as little as 0.2dB if they are over a range of an
octave or more. Therefore, even more important than the maximum amount of response error at any frequency is the octave averaged,
octave-to-octave balance which has a very high correlation with perceived tonal balance. Our design goal for the CS.5 was to achieve
octave-averaged response within ±1dB from 100Hz up to 10KHz with even tighter tolerance within the midrange from 200Hz to 3KHz.
Therefore, any deviation more than these limits is confined to only a narrow frequency range and therefore will have less effect on the
perceived balance.

Achieving these goals requires the use of drivers with very uniform responses, drivers with high consistency (so that few units need be
rejected), reduction of usual cabinet diffraction which causes response errors, and an unusual degree of compensation of driver response
anomalies in the electrical network.

Driver response

The major cause of nonuniform driver response is diaphragm resonances. These resonances are also the major energy storage
mechanism. All THIEL tweeter diaphragms are constructed of aluminum which provides much higher stiffness and compressive strength
than conventional diaphragm materials. The primary benefit is that the lowest internal resonance is much higher than with other materials.
Below this lowest resonance there are no resonances to store energy and cause ringing. An additional benefit is that the aluminum’s much
higher compressive strength results in almost all the energy of a transient attack being transferred to sonic output rather than being
absorbed in compression of the diaphragm material. In the tweeters the lowest diaphragm resonance occurs above the range of hearing at
26KHz. Therefore, there are no resonances in the audible range to cause energy storage or response irregularities.

Diffraction

Diffraction causes frequency response and time response errors and therefore a reduction in
tonal, spatial, and transient fidelity. Diffraction occurs when some of the energy radiated by the
drivers is reradiated at a later time from cabinet edges or other sudden change of environment. For
musical signals that remain constant for a few milliseconds, diffraction causes, by constructive and
destructive interference, an excess of energy to the listener at some frequencies and a deficient
amount of energy to the listener at other frequencies. Diffraction also causes all transient signals to
be radiated to the listener a second (and possibly a third) time, smearing transient impact and
distorting spatial cues.

To reduce diffraction the CS.5 employs a grille board that fits around (rather than on) the baffle
and one that is curved at the edges so energy radiated along the baffle can continue into the room
without encountering abrupt cabinet edges.

Off-axis response

In addition to on-axis response accuracy, it is also important that the off-axis response be even,
without major dips, for two reasons. First, listeners may be located far from the optimum position
and therefore will be hearing the speaker as it performs off-axis. Secondly, off-axis response is an
indication of the uniformity of the speaker’s total energy response. Since the total energy (in all
directions) radiated from the loudspeaker determines the amount of reverberant energy in the room,

First-order system on-axisHigh-slope system on-axis

First-order system off-axisHigh-slope system off-axis

it is important that the off-axis response be uniform to
avoid changes in perceived character and spatiality at
different frequencies.

Most speakers with high-slope crossover systems cannot maintain uniform off-axis
response because the dispersion of a driver narrows as frequency increases toward the
crossover frequency. Above the crossover frequency the radiation of the next driver is again
wide since it is operating at the low end of its range. First-order crossover systems have an
advantage in this regard. Since a significant part of the total energy below the crossover point
is radiated by the upper driver, the narrowing of the dispersion of the lower driver has much
less effect on the total output. Speakers with first-order crossover systems therefore can
achieve a more uniform off-axis response.

Cabinet-edge diffraction

tweeter

2

Results

The end result of reducing diffraction, reducing diaphragm resonances and correcting

response anomalies in the network is a speaker with very accurate tonal characteristics.
Figure 1 shows the on-axis frequency response of the CS.5. It is uniform within ±2dB
from 23Hz to 17KHz. Subjectively, even more important is the octave-averaged
frequency response. Figure 2 shows this response to be within ±1dB from 100Hz to
10KHz and within ±0.5dB from 200Hz to 3KHz. These measurements indicate very
accurate overall tonal balance. Furthermore, as a result of gradual crossover slopes, the
off-axis frequency response of the speaker system is also smooth and well balanced. This
unusual performance is important for producing a uniform amount of ambient energy at
all frequencies, necessary for natural spatial reproduction. Figure 3 shows this octave­averaged, 30° off-axis response to be within ±1.5dB from 70Hz to 10KHz, showing very
uniform dispersion of energy at all frequencies.

TIME RESPONSE

In most loudspeakers the sound from each driver reaches the listener at different

times causing the loss of much spatial information. One problem caused by different

Time correction

arrival times from each driver is that the
only remaining dependable locational clue
is the relative loudness of each speaker.
Relying only on loudness information
causes the sound stage to exist only between
the speakers. In contrast to this loudness
type of imaging information, the ear–brain
interprets real life sounds by using timing information to locate the position of a sound. The
ear perceives a natural sound as coming from the left mainly because the left ear hears it first.
That it may also sound louder to the left ear is of secondary importance.

Another problem is that for realistic reproduction, it is important that the attack, or start, of
every sound be clearly focused in time. Because more than one driver is involved in the
reproduction of the several harmonics of any single sound, the drivers must be heard in unison
to preserve the structure of the sound. Since, in most speakers, the tweeter is closer to the
listener’s ear, the initial attack of the upper harmonics arrives a substantial part of a
millisecond before the body of the sound. This delay results in a noticeable reduction in the
realism of the reproduced sound.

To eliminate both these problems the CS.5’s drivers are mounted on a sloped baffle to
position them so the sound from each reaches the listener at the same time. The sloping baffle
arrangement can work perfectly for only one listening position. However, because the drivers
are positioned in a vertical line the error introduced by a listener to the side of the speaker is
very small. Also, the error introduced by changes in listener height are small within the range
of normal seated listening heights provided the listener is 8 feet or more from the speakers.

Figure 1 On-axis frequency response

25
20
15
10

5
0

Amplitude — dB

-5

-10

Figure 2 On-axis octave-averaged frequency response

25
20
15
10

5
0

Amplitude — dB

-5

-10

Figure 3 30° off axis octave-averaged frequency response

25
20
15
10

5
0

Amplitude — dB

-5

-10

10020

10020

10020

Frequency

Frequency

Frequency

1K

1K

1K

10K

10K

10K

20K

20K

20K

PHASE RESPONSE

We use the trade mark Coherent Source to describe the unusual technical performance of time and phase coherence which gives

THIEL products the unusual ability to accurately reproduce musical waveforms.

Usually, phase shifts are introduced by the crossover slopes, which change the musical waveform and result in the loss of spatial and
transient information. The fourth-order Linkwitz-Riley crossover is commonly used in high performance speakers and is sometimes
promoted as being phase coherent. What is actually meant is that the two drivers are in phase with each other through the crossover region.
However, in the crossover region neither driver is in phase with the input signal nor with the drivers’ output at other frequencies; there is a
complete 360° phase rotation at each crossover point.

Since 1978 THIEL has employed first-order (6dB/octave) crossover systems in all our Coherent Source speaker systems. A first-order
system is the only type that can achieve perfect phase coherence, no time smear, uniform frequency response, and uniform power response.

A first-order system achieves its perfect (in principle) results by keeping the phase shift of each roll-off less than 90° so that it can be
canceled by the roll-off of the other driver that has an identical phase shift in the opposite direction. (Phase shifts greater than 90° cannot be
canceled.) The phase shift is kept low by using very gradual (6dB/octave) roll-off slopes which produce a phase lag of 45° for the low
frequency driver and a phase lead of 45° for the high frequency driver at the crossover point. Because the phase shift of each driver is much
less than 90° and is equal and opposite, their outputs combine to produce a system output with no phase shift and perfect transient response.

Figure 4 graphically demonstrates how the outputs of each driver in a two-way speaker system combine to produce the system’s output
to a step input. The first graph shows the ideal output. The second shows the operation of a time-corrected, fourth-order crossover system.
The two drivers produce their output in the same polarity and both drivers start responding at the same time. However, since the high-slope
network produces a large amount of phase shift, the tweeter’s output falls too quickly and the woofer’s output increases too gradually.
Therefore, the two outputs do not combine to produce the input step signal well but instead greatly alter the waveform. The third graph
shows how, in a first-order crossover system, the outputs of the two drivers combine to reproduce the input waveform without alteration.

3

In practice, the proper execution of a first-order system requires very high quality, wide
bandwidth drivers and that the impedance and response variations of the drivers and the
cabinet be compensated across a wide range of frequencies. This task is complex since what is

Figure 4

+

necessary is that the acoustic driver outputs roll off at 6dB/octave and not simply for the
networks themselves to roll off at 6dB/octave. For example, if a typical tweeter with a low
frequency roll-off of 12dB/octave is combined with a 6dB/octave network, the resulting
acoustical output will roll off at 18dB/octave. Therefore, in practice, the required network
circuits are much more complex than might be thought.

The result of phase coherence (in conjunction with time coherence) is that all waveforms

Output

-

Time

Ideal step response

will be reproduced without major alterations. The speaker’s reproduction of a step waveform
best demonstrates this fact since, like musical waveforms, a step is made up of many
frequencies which have precise amplitude and phase relationships. For a step signal to be
accurately reproduced, phase, time and amplitude response must all be accurate. Because this

waveform is so valuable, it is commonly used

-

tweeter output
woofer output
combined output

Figure 5 CS.5 step response

to evaluate the performance of electronic
components. It is not typically used for speaker
evaluation because most speakers are not able

Time corrected fourth order crossover system

to reproduce it recognizably. That THIEL

Output

speakers reproduce the step so recognizably is
the result of accurate phase, time and

0.5 1.0

1.5 2.0 2.5

Time – msec

amplitude response. Figure 5 shows the step
response of the CS.5.

ENERGY STORAGE

Any part of the speaker that absorbs energy will reradiate it later in time in a highly
distorted manner. Although not loud enough to be consciously heard, stored energy causes
significant detrimental effects by obscuring the music’s subtle detail, causing both a reduction in clarity and loss of spatiality. The main
storage mechanisms are the driver diaphragms and cabinet walls, especially the baffle.

Figure 5 CS.5 time response

5
0

-5

-10

-15

-20

Output — dB

-25

-30

-35

0.5

1.0

1.5 2.0 2.5

Time – msec

To reduce cabinet wall vibration the CS.5 speaker utilizes cabinet walls constructed
with very thick, 1" MDF. Also, to increase the mechanical rigidity and therefore reduce
unwanted vibration, all THIEL drivers incorporate chassis of cast magnesium or aluminum
rather than stamped steel or plastic. The results are shown in Figure 6 where it can be seen
that the output of the speaker falls to -40dB in 1 millisecond and to -20dB in only 400
microseconds. This performance provides very clean reproduction of music’s subtle
information.

-

First order crossover system

4

DISTORTION

Driver motor systems

Unlike some sources of distortion, motor system distortion is very dependent on volume level, being low during quiet playback levels
but increasing rapidly as volume levels increase. At moderate to loud playback levels it is the major source of distortion. The CS.5
incorporates several unusual features in its woofer to decrease distortion and increase dynamic range.

The purpose of the driver’s motor system is to apply a force to the diaphragm that is directly proportional to the voltage supplied by the
amplifier as modified by the electrical network. In order for the force to be directly proportional to the voltage applied, as desired, the
magnetic field strength must be constant, the length of voice coil wire acted on by the magnetic field must be constant, and the current in
the voice coil must be directly proportional to the applied voltage. In practice, none of these three conditions actually exist but the CS.5
woofer incorporates refinements of design that greatly improve the accuracy of each of these factors.

The first distortion mechanism is that the strength of the magnet’s field is not actually constant in operation but is changed by the
current from the amplifier through the coil. This change occurs because the amplifier current through the coil generates the force to move
the diaphragm by creating its own magnetic field that “pushes” against the magnet’s field. The magnet is somewhat demagnetized by the
coil’s magnetic field when current flows in one direction and is remagnetized when current flows in the opposite direction. Therefore, since
the magnet’s field strength is not constant, the force generated is not in the desired direct proportion to the current in the coil. To greatly
reduce this effect the CS.5 woofer incorporates a copper sleeve around the center pole. With this sleeve any change in the magnet’s
strength induces an electrical current in the sleeve which generates a magnetic field that is opposed to and practically cancels the original
change.

The second distortion mechanism results from the fact that almost all woofers use a long coil/short gap motor system where the long
coil is acted upon not only by the field within the air gap but also by the “fringe” field in front of and behind the gap region. As the coil
moves forward or backward to produce bass energy, the magnetic field acting on the coil becomes less intense because the coil is further
from its rest position where the magnetic field is strongest. This weakening of field strength as the coil moves away from its rest position is
the primary distortion producing mechanism in woofers.

To eliminate this problem the CS.5 woofer uses a very unusual short coil/long gap system where the coil is much shorter than the
magnetic gap. Therefore, even when the coil moves a considerable distance from its rest position, it continues to be acted upon only by the
uniform magnetic field in the air gap and does not experience the changes in magnetic field strength with position as in the conventional
system. As shown below, the distortion produced by the CS.5 woofer’s short coil motor system at normal excursion levels is only one-tenth
that produced by the typical long coil system.

The third distortion mechanism is that the coil current is dependent

not only on the driving voltage and the coil resistance but also on the

coil

8%

6%

4%

distortion

2%

1234

peak excursion ±mm

8%

6%

4%

distortion

2%

1234

peak excursion ±mm

An additional problem is that the voice coil is an iron-core inductor. Iron-core inductors are not linear and therefore introduce
distortion. For this reason such inductors are avoided in high quality crossover systems. Nonetheless, one iron-core inductor remains in the
signal path—the driver’s voice coil. An additional benefit of the copper sleeve is that since it reduces the coil’s inductance, it also reduces
the associated distortion.

coil inductance. The problem is that the coil inductance varies with the
amount of iron inside the coil and therefore with conventional magnet
system geometry, inductance changes during the excursions necessary
to reproduce low frequencies. As the diaphragm and coil move back,
more of the coil is around the pole, increasing the inductance and
decreasing the mid-frequency output of the driver. As the coil moves
forward, less of the coil is around the pole, the inductance decreases,
and the mid-frequency response increases. By this mechanism the
frequency response of the speaker is modulated by driver excursion.
This problem has been virtually eliminated in the CS.5 woofer. The
short coil design results in the entire coil surrounding the pole in all
positions and therefore the coil’s inductance does not change with the
diaphragm position. In addition, the problem is further reduced by the
copper sleeve which reduces the inductance of the coil to a fraction of
its normal value by acting as a shorted turn of a transformer secondary
winding.

5