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Thiel PowerPoint 1.2 Wall-Mounted Speakers

Part II

November, 2005

Ed Mullen

 

Improved Frequency Response From 450 Driver Mounting Angle

A conventional wall mount speaker has the woofer parallel to the mounting surface. This tends to create anomalies in the frequency response due to cancellations from wall reflections. In comparison, the PP1.2 utilizes a 450 driver mounting angle. This keeps the woofer much closer to the mounting surface, and ensures that all frequencies in the woofer’s pass band remain within ˝ wavelength of the wall mounting surface. This tends to minimize the effect that wall reflections will have on the frequency response.

Provided below is a diagram illustrating these differences. The top curve shows the FR anomalies typically associated with the conventional wall mount design. The bottom curve shows the reduction of these anomalies achieved with the PP1.2 450 mounting angle design.

Improved Dispersion From 45 Degree Driver Mounting Angle

Compared to a conventional 900 wall-mount design, the PP1.2 450 mounting angle greatly improves the in-room dispersion pattern, providing more coverage for more listeners in the room. Some 900 on-wall designs use a rotating tweeter to overcome this limitation, but that often results in diffraction problems, and the PP1.2 design is ultimately a much more elegant solution to achieving an optimal in-room dispersion pattern.

Provided below is an illustration showing the limited dispersion pattern of a typical 900 wall-mount design verses the improved dispersion pattern of the Thiel 450 wall-mount design.

The PP1.2 tweeter is the same unit used in the famous Thiel flagship the CS7.2. Salient features of this tweeter design include:

- An aluminum cone with a wide-roll rubber suspension.

- High output and low distortion afforded by a 5 mm magnetic gap, and a remarkable 3 mm linear peak-to-peak excursion.

- A large, rare-earth (neodymium) ring-shaped magnet used to magnetize the gap, and a second disc-shaped neodymium focusing magnet used above the gap. Total magnet weight is 51 grams, which is much higher than the typical tweeter.

- A high thermal capacity aluminum voice coil wrapped around a Kapton former.

- A copper sleeve used around the pole to improve the stability of the magnetic field and reduce voice coil inductance, thereby reducing distortion and improving high frequency response.

The PP1.2 woofer design features include:

- An aluminum cone with polystyrene reinforcement for increased rigidity, a wide-roll rubber surround, and single spider suspension.

- A cast aluminum basket for strength and minimal resonances.

- A powerful rare-earth (neodymium) magnet which provides ten times the field strength of a normal ferrite magnet, and so can be much smaller, allowing the use of a shallow enclosure design.

- A high thermal capacity voice coil wrapped around a Kapton former.

- A copper sleeve used around the pole to improve the stability of the magnetic field and reduce voice coil inductance, thereby reducing distortion and improving midrange frequency response, allowing a smooth transition to the tweeter.

The PP1.2 has rated bass extension frequency of 75 Hz (-3 dB). Since the PP1.2 is a sealed loudspeaker, it will roll-off below 75 Hz with a 2nd order slope (12 dB/octave). Conveniently, these two features will perfectly complement the digital bass management (BM) crossover found in most AVRs and pre/pros.

A typical digital BM circuit features a 2nd order high pass filter imposed on the speakers, and a 4th order low pass filter imposed on the subwoofer. If an 80 Hz crossover frequency is selected, and sealed speakers with a rated extension of 80 Hz are used, the result is a perfect 4th order Linkwitz/Riley crossover. This will provide the best possible frequency response and the least amount of phase shift over the speaker/subwoofer transition bandwidth.

Users of the Thiel PP1.2 can therefore expect a smooth and phase-correct transition to the subwoofer when using an 80 Hz crossover in the pre/pro. This represents a significant advantage for the PP1.2 when compared to a vented tower loudspeaker, which will generally exhibit FR and phase anomalies over the subwoofer crossover bandwidth when used with the typical 2nd/4th order digital BM circuit.

For more information on digital bass management and crossover slope issues, refer to the Secrets Article Bass Management Woes: Trouble on the Slopes.

Click Here to Go to Part III.

© Copyright 2005 Secrets of Home Theater & High Fidelity

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