Q. What do time response and phase response mean and how do they affect speaker performance? Why do THIEL speakers have sloped baffles?

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.

TIME COHERENCE
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 drivers coaxially.

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

Coaxial Driver Mounting

 

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.

 PHASE COHERENCE
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 in time.

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.


| Home | What's New | Products | Tech Talk | Where To Buy | Contact THIEL |