ATH Gen2
Variable-Throat Waveguide Kits

Axisymmetric/R-OSSE G2 Series

A400G2
⌀400 x 130 mm

A460G2
⌀460 x 160 mm

A520G2
⌀520 x 174 mm
1.4" / 36 mm

T520-36-STD-1

T520-36-EXT-1
1" / 25 mm

T520-25-STD-1

T520-25-STD-2

T520-25-EXT-1
Special

T520-ROSSO-STD
Part No. / Link Input ⌀
[mm]
Low-frequency point (1) Input angle (2) Length
[mm]
Driver mounting (3)
[mm]
T520-36-STD-1 36 580 Hz 8.8° 41 4xM6/⌀102
T520-36-EXT-1 36 360 Hz 150 4xM6/⌀102
T520-25-STD-1 25.4 670 Hz 12° 61 4xM6/⌀76 + 3xM6/⌀57
T520-25-STD-2 25.4 740 Hz 31° 46 4xM6/⌀76 + 3xM6/⌀57
T520-25-EXT-1 25.4 440 Hz 3.2° 135 4xM6/⌀76 + 3xM6/⌀57
Part No. / Link Matching
Compression Driver
Input ⌀
[mm]
Low-frequency point (1) Length
[mm]
Driver mounting
[mm]
T520-ROSSO-STD SB ROSSO-65CDN-T 26.9 645 Hz 46 4xM6/⌀102
(1) Frequency at which the throat acoustic resistance reaches 20% of its asymptotical value.
(2) Total angle value.
(3) Mounting to the waveguide is 8xM6/⌀102mm.

The Waveguides

This set of waveguide kits offers a unique solution to the DIY audio hobbyists, especially to those who like to experiment in a long run. The main, big and the most demanding "mouth" parts needs to be made only once, while the adapters for different throat sizes, extensions and shapes are easily replaceable. If, for example, a driver calls for a special throat plug, or a different exit angle, a new adapter can be easily adopted. The kits are provided as STL files for 3D printing, together with full Ath scripts allowing further custom modification and BEM simulation.

ATH A520G2 is a kind of "no-compromise", relatively large waveguide (520 mm / 20.5" outer diameter), while the A400G2 is basically a reduced-size version, still performing extremely well though. Both waveguides can be used with a wide range of compression drivers. A460G2 is the latest iteration, maintaining virtually the same performace of the A520G2 down to around 600 Hz, but in a noticeably smaller format.

Although the waveguide profiles are specified as R-OSSE [1] (which it is for the common part), the individual throat adapters are of more general shapes, each smoothly connected to the common profile. Those throat parts marked as 'STD' are the standard, non-resonant types. The parts marked as 'EXT' are extended-throat versions, improving on the low-frequency capabilities [2]. Each combination is optimized to provide an exceptionally low-diffraction, i.e. a very smooth, frequency response and a balanced, well-controlled directivity with a mildly rising directivity index through the whole operating range. Figure 1 shows a (BEM-calculated) benchmark of the performance (T520-25-STD-1).

Fig.1 - Calculated polar map (A520G2)

The whole range of the T520 adapters can be used with all the three waveguides. The corresponding ANSI/CEA-2034-A [3] reports for the waveguides alone are shown in Figure 2. The data for these reports has been normalized for 5° off axis, meaning that they represent the performace of the waveguide with the frequency response at 5° equalized to being perfectly flat.



Fig.2 - ANSI/CEA-2034-A reports

It is to be noted that the directivity (and all the curves in Fig.2) stays virtually the same for all the throat adapters, irrespective of a compression driver used or its exit size. There may be small differences above ~15 kHz but overall the performance is very similar. This is possible because the waveguides were designed mainly with 1.4" drivers in mind, with a usable crossover frequency around 500 Hz and with the directivity adjusted accordingly. Smaller drivers with 1" exit can also be used however, with the possible advantage that the top octave may be a bit cleaner. The extended 1" throat still allows even some of the 1" drivers, if not most of them, to be used down to perhaps 600 Hz.

Figure 3 shows the (normalized) throat acoustic impedance for both A520G2 and A400G2, using the standard 1.4" adapter. Maybe not anticipated by intuition, but the impedances are basically the same. In fact, the low-frequency point(1) of the smaller waveguide is actually a tiny bit lower in this case.

Fig.3 - Throat acoustic impedances


References

[1] R-OSSE Acoustic Waveguide (https://at-horns.eu/release/R-OSSE%20Waveguide%20rev7.pdf)
[2] Extended-Throat Waveguides (https://at-horns.eu/exar-story.html)
[3] ANSI/CEA-2034-A (https://webstore.ansi.org/standards/cea/cea20342015ansi)


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Marcel Batík, Czechia, May 2024