8" waveguide surrounds

I'm currently working on a very dynamic active surround speaker that will feature a waveguide for controlled directivity paired with a compression driver (B&C DE250) and 8" pro midbass (B&C 8PS21). 

B&C 8PS21 midwoofer

 Goals include:

• THX reference levels of 105 dB at 3m from 80 Hz and upwards with significant headroom
• low profile for on-wall mounting
• voice matched to mains (active coaxial waveguide/horn with the same compression driver)
• neutral and natural sound with a slight bias towards a mellow sound rather than a forward, bright or detailed presentation
• sharp and focused imaging

The drivers

The compression driver is an obvious choice for a couple of reasons. I was able to get a pair second hand at a very attractive price and they are a perfect match to the mains. They easily meet all the goals and I've found this driver to be excellent value and sonically very good. 

 B&C DE250

 The woofer is the same one used by Geddes in his Harper speaker. It was also measured by John Krutke (Zaph Audio) and it performed very well in distortion tests. The response in his measurements is flatter than the datasheet indicates. Overall it appears well designed and the xmax is decent at 5mm.



The waveguide


The waveguide isn't yet decided. I'm considering either a custom made 8" oblate spheroid, or one based on the Pyle:

The Pyle is a low cost plastic waveguide and in previous tests at the waveguide events, it performed well both sonically and in measurements. The oblate waveguide requires a fair effort to build and has the axial dip caused by diffraction, probably a non-issue in this design where the off axis response is the focus. The Pyle does not have this issue, but it requires an adaptor for my waveguide and falls short of the standard of finish I'm aiming for. One option is to buy the Pyle and use it to create a mould, then casting the waveguide into the baffle resulting in a "one piece" look.

SPL estimate

WinISD estimate of output with different power levels - all at 1m

Cone excursion is kept under control by a high pass at 80 Hz. Even at high power levels, excursion is well within limits with a high pass filter in place.

At 3m, the output is reduced by ~4.5 dB, so to reach my goal of 105 dB, around 110 dB is required. Around 40W is needed to achieve this level and at the maximum simulated output, there is around 7 dB of headroom. This design will easily reach the targets.

Coming up:

1. Measurements when the drivers arrive
2. Initial prototype version
3. Experiments with placement and toe down