OB rear wave cancellation thread.

[chris661]

A lot of people have been asking similar questions about these speakers, so here's a general reference thread - add your own tips if you wish. Questions are also welcome.

Disclaimer - these notes are from my own personal experience, using *small* speakers. Scaled up versions should work to the same rules, but I do not know this for a fact.

General notes
- these speakers aim to achieve the following:
- the open-ness of the midrange and treble generally associated with OB speakers
- enough bass to remove the need for eq.
- the two drivers should follow similar excursion curves, for both input power and frequency, which is why, in my opinion, sealed boxes should be used as the cancelling speaker.
- Using a port with the cancelling driver would change the excursion curve dramatically, leading to a lack of cancellation around port tuning, which would be compensated for by the port, leading to very little gain.
- the two drivers should not be too far apart, as a large distance would mean that, further up the cancelling range, the driver would be out of phase with the OB driver, leading to sound re-enforcement, which would mean the front wave of the OB driver would be cancelled down even more.
- The cancelling driver should be low-passed, I did around 150Hz, but youshould be able to get away with up to 400Hz (dependant on distance between drivers etc)
- The width of the baffle matters little, you will just have to raise the XO for the cancelling driver, thus meaning you'll need the to be closer together. However, you will need a decent box volume for the cancelling driver, otherwise the excursion will be a lot less than the OB driver, and you'll need to bi-amp to sort this.
- Other options for this speaker arrangement is using 2 drivers (across), and 2 cancelling drivers, which would, theoretically, increase overall volume levels and bass depth.

Just thought it would be handy to have a reference to these speakers.

Chris

PS - I am doing this to spread the word of these great speakers, and to help more people build them.

[chris661]

Listening Impressions

My speakers, shown in the pic above, are, simply put, annoying. They defy all the audio laws I know, such as big box + big driver = more bass. The speakers shown above are 12" high, 7" across and 6" deep, and go to 60Hz, using a 3" OB driver and the same one in a box.

The midrange, as is common with OB projects, is open, with very wide range stereo imaging which goes past the speakers (suspected reflections). The bass is more punchy than deep, so, while it's not all there, you get a good impression of what's going on. The treble is nice, as metal things sound metally, and drums sound like drums (you get the idea).

If you want to make some of these, the pieces of wood you'll need are:

12"x7" (1 of) - front
5.5"x6" (2 of) - sides
5"x6" (2 of) - top and bottom
6"x6" (1 of) - back
2x4.7mH inductors (series with each other and the cancelling driver) and an 80uF cap (parallel with the cancelling driver).
And 2x Visaton FRS8 drivers. This list is per speaker.

Chris