Q. What do time response and phase response mean and how
do they affect speaker performance? Why do THIEL speakers have
A. THIEL uses the trademark "Coherent Source"
to describe the unique technical performance of our products.
This phrase is descriptive of the time and phase coherence which
gives THIEL products the unusual ability to accurately reproduce
musical waveforms for extremely realistic, three-dimensional
reproduction. The two factors of time and phase response are
interrelated since phase shifts in the crossover network usually
cause energy to be smeared in time by more than the time errors
caused by the non-aligned drivers. However, it is still helpful
to think about time and phase independently, especially since
there are speakers that incorporate driver alignment but which
are not phase coherent. While these factors alone do not guarantee
a good sounding loudspeaker, if all other parameters are successfully
addressed, time and phase coherence add a dimension of clarity
and spatial realism that cannot be achieved by other means.
Each musical sound is comprised of several different tones,
or harmonics, each having its own amplitude, time and phase relationships
with the others. To completely preserve the unique character
of each sound, it is necessary to preserve all of this information.
In other words, the loudspeaker's amplitude, time and phase response
must all be accurate. Conventional speakers can potentially do
a good job of preserving the amplitude relationships of music,
but they fail at preserving time and phase relationships.
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 sound, their outputs must be heard in unison
to preserve the structure of the sound.
Timing errors also cause the loss of much directional or imaging
information. With most speakers, the only dependable clues you
are given about the location of the sound are contained in the
loudness of each speaker. If the left speaker is playing a given
instrument louder than the right, then the sound of that instrument
seems to be located closer to the left speaker. This is why the
"sound stage" that most speakers produce exists only
between the speakers. In contrast to this loudness type of imaging
information, your ear/brain interprets real life sounds by using
timing information to locate the position of a sound. In real
life, your ear perceives a sound as coming from the left because
your left ear hears it first. That it may also sound louder to
your left ear is secondary. Your ear is set up for, and is much
better at, determining location from time information rather
than loudness information.
THIEL incorporates two methods to achieve time coherence:
positioning the speakers along a sloped baffle; and mounting
Positioning Drivers Along A Sloped Baffle
In most speakers, the tweeter is closer to the listener's
ear so the sound's upper harmonics are heard before the lower
harmonics, significantly reducing realism
To eliminate this problem, the drivers in THIEL speakers are
mounted on a sloped baffle to position them so the sound from
each reaches the listener at the same time. This positioning
accurately preserves the time information, giving the sound focus
and allowing spatial detail to come through.
The second method THIEL uses to achieve time coherence is
through the use of coaxially mounted drivers
The CS6's coaxially mounted tweeter
and midrange drivers
The THIEL models CS7.2, CS6, MCS1 and SCS3 use coaxially mounted
drivers to achieve time coherence. This type of driver is perfectly
time aligned since the sound sources of both drivers are in the
same location, and therefore the outputs of both reach the listener
at the same time.
The CS6's coaxial mounting of its tweeter and midrange drivers,
shown on the left, is unique in that the midrange uses a specially
shaped, three-layer diaphragm. This shallow design eliminates
the frequency response errors of conventional designs, which
are created by the horn loading effect of the midrange's diaphragm
on the tweeter.
To preserve the synchrony of the sound's harmonics, all drivers
must move in and out in step with each other and with the speaker's
input signal. THIEL speakers accomplish this by using very wide
bandwidth drivers in conjunction with special crossover systems
designed to provide phase coherent transitions between drivers.
Since it is necessary to optimize separate drivers for different
frequency ranges, a crossover network is needed to direct the
incoming frequencies to the appropriate driver. A problem arises
because any network which discriminates between frequencies will
also cause the phase of the signal to be shifted, which causes
the drivers to move out of step with each other.
For example, a second order network which reduces the high
frequencies to one-fourth for each doubling of frequency (12
dB/octave) will cause the phase of the higher frequencies to
be shifted almost 180°. A fourth order network will cause
the higher frequencies to lag almost 360°, or one complete
cycle. Since one cycle represents a different amount of time
for different frequencies, the network smears the frequencies
Other than time smear, phase shift also causes the individual
harmonic components of each sound, reproduced simultaneously
by different drivers, to lose their synchronous structure. This
loss is caused by the negative and positive motions of the drivers
being out of step with the input signal. This changes the waveform
and results in the loss of spatial and transient information.
There is a type of crossover system that does not introduce
any phase shift or time smear, although it is expensive and difficult
to execute. Well known as a technically perfect solution, it
was often considered impractical. This is the first order (6
dB/octave) system that THIEL has perfected for practical use
and employed in THIEL's Coherent Source systems since 1977.
A first order system achieves its perfect results by keeping
the phase shift of each filter to less than 90° so that it
can be canceled with a filter that has an identical phase shift
of the opposite direction. The phase shift is kept low by using
very gradual (6 dB/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.
For any other type of crossover system it is not possible to
completely eliminate time smear and phase shift.
To properly execute this system in practice requires very
high quality, wide bandwidth drivers and that the impedance and
response variations of the drivers and the cabinet be compensated
for across a wide range of frequencies. This is a complicated
task since what is important is that the acoustic output of the
drivers roll off at 6 dB/octave and not simply for the networks
themselves to have 6 dB/octave roll-offs. For example, if a typical
tweeter with a lower roll-off of 12 dB/octave is combined with
a 6 dB/octave network, the result is an acoustical output which
rolls off at 18 dB/octave. To achieve a first order system in
practice, the tweeter must have a very low and very well damped
resonance with high output capability and the network must in
fact have a complex response. Both of these requirements are
expensive to implement.
What is phase shift?
Imagine holding a spring with a weight attached to its end.
If you move your hand up and down very slowly, the weight will
move up and down in perfect step, or synchrony. If you increase
the speed (or frequency) of the up and down motion, the motion
of the weight lags behind your hand's motionas your
hand begins its movement down, the weight is still moving up.
This lack of synchrony is called phase shift.
|However difficult and expensive
phase and time coherence are to achieve, THIEL believes that
the sonic improvements are well worth the cost. By accurately
preserving all of the information contained in a musical recording,
THIEL speakers are able to accurately reproduce the space of
the original recording environment. Instruments can easily be
placed outside the speakers and there is no sense of the sound
stage being limited or compressed by the speakers. Also, transient
details are more focused and clearly delineated. Time and phase
coherence provide a more natural and complete musical experience.