Waveguide surrounds - update

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12mm roundovers on edges:

Front corners will be done later when the baffle is glued in place. It will need to come off to work on the waveguide.

Template for flush mounting the midwoofer:

Rave horn

Here is a quick design for a HUGE horn inspired by this thread:
http://www.avsforum.com/avs-vb/showthread.php?t=1357500

Hornresp sim for 8 drivers loaded into a horn installed into a 1m high stage that is 7.5m wide. B&C 18TBX100

Now a tapped horn with the same driver.

Note that this is a single driver with 300w. Less power is required to find the excursion limits, but the efficiency is lower. The front horn has 107 db sensitivity while this has 100 db. This is 430L while the front horn is 7000L, in other words 16 x larger! However, the FLH doesn't increase much in size when more drivers are added. The mouth stays the same but the throat gets larger. The curious thing is that to match the output of the front horn, you need 16 tapped horns. The volume in the end is the same, but with the tapped horn you have twice as many drivers and 4.8kw of power.

So if the volume limit is the same, you can use 18 drivers + 4.8 kw of power, with 16 large boxes. Or 8 drivers into a single huge horn and 16 kw of power. 

Surround test boxes in progress

This is a test box for my 8" waveguide surround speakers. 

The baffle is double thickness under the driver and has a simple brace in the middle. 

Octagonal face plates are a pest! I've marked this one out in pencil but plan to flush mount using a router template. 

Drivers arrived! 2 x B&C 8PS21 & 8 x Pyle PDMR5

The drivers arrived today for my current speaker projects. Peavey Black Widow 18" shown also to give an idea of scale.

B&C 8PS21 - an 8" pro midwoofer shown in the front, Pyle PDMR5 (8x) shown in the middle along with a pair of B&C DE250 compression drivers. 

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

Black widow prototype - more measurements

Nearfield measurements

Step response

Magenta: Rythmik sealed

Grey: Peavey driver on an open baffle

Red: BP6 rear port

Blue: BP6 front port

Group delay

Phase response

Black widow prototype - 6th order bandpass

Before building the final version of my black widow active woofers, I decided to build a test version. I didn't fully trust the WinISD simulation of band pass boxes as it seems buggy and incomplete. This box was built, tested and then enjoyed in the one day. Many thanks to Antripodean who helped with building the box.



The box is 550mm wide x 580 deep x 620 high. The top of the box behind is about 900mm or seated ear level and you can see the 6" waveguide with foam in it at the top of the shot. The test box is similar in size to a 2 draw filing cabinet.

1. Testing driver parameters with the Dayton Woofer Tester 3

I did a quick test to make sure the parameters were close enough to the model.

Peavey Black Widow 18" (specs from Peavey in brackets)

fs: 39 Hz (36)
re: 6.5 (5.8)
Qts: 0.43 (0.37)
Le: 2.94 (2.07)

2. Simulation in WinISD

As you can see, there is a substantial difference in parameters, so I checked the simulation to see if it meant a different design. The two were quite close.



You can see a big peak from the front chamber port. This means WinISD thinks the port is too efficient, ie tuned too high for it's size, or the volume too large for the tuning point, or both. Reduce the front chamber size and you get a big dip in between the two humps. My plan was to EQ out the peak carefully, then yielding a flatter response than otherwise possible. However, the test box revealed that WinISD didn't really predict things too well.

3. Building the prototype

This is a simple unbraced box with an estimated net volume of 124L (12L for the front) with a shelf port for each. You can see the front chamber which is not much bigger than the driver:



The front chamber can be removed easily.

There isn't too much you can show when building a very simple box like this. Bunnings were out of 18mm MDF so this was built with 16mm. Not too much different, but given that this is a big unbraced undamped box, it rings like a hollow el cheapo boombox. It often can be a surprise how much you can get away with ...

Surprisingly I didn't quite manage to build the whole thing with one sheet. I had to add some particleboard flooring on the front chamber. Being a timber hoarder does have advantages.

Continue to measurements and listening impressions >