Richard's dipole project

[richardlevesque]

I have read a lot about dipole systems and it appears they bring some real value to an audio system.
I have studied the fancy multi amp systems and all the tech info that I could get my hands on and have come up with my own alternative.
My objectives:
a) Passive system
b) Single stereo amp
c) Single point sound approach (trying to avoid crossovers in the critcal part of the spectrum)
d) Not too big
e) Not too expensive

Here are my design principles:
1) Use of full range speaker to avoid crossovers and tweeters (didn’t quite work out)
2) Use multiple speakers to enhance bass response
3) Use of digital equalizer to enhance bass response and fix speaker quirkiness
5) Mid size open back box to avoid losing too much bass response
Here is my final product
_ Open back box 18 wide, 24 inch tall, with 10 inch sides and top.
_ 2 ten inch full range speakers
_ One of the ten inch speakers is choked with a coil to create a low pass filter at about 100hz to increase bass response but not interfere with the primary speaker that covers most of the spectrum
_I discovered that the full range speakers didn’t handle high frequency that well so I added a cone tweeter with a simple capacitor designed for a 10k crossover.
_I added a Behringer DEQ 2496 digital equalizer with RTA (real time analyzer)
RESULTS:
I am not quite finished with the setup but tests so far are very satisfying. My biggest problem is the quality of the full range speakers. They have a fair bit of peakyness and honkiness in the response but the equalizer fixes this nicely.
I found that that room placement has a significant impact on the transparency of the sound. If setup a certain way, the speakers disappear and the sounds appears to be coming from the wall.
My room is about 12 by 20 with 8 foot ceilings and if I place the speakers in the corners, about 3-4 feet away from the walls, the sounds bounces around the corner and creates a very broad sound source.
I will report back when I get more test results.
Richard

[richardlevesque]

Here I am replying to my own message

Following some critical listening tests, I have been very pleased with the responsiveness of the system and the bass response in particluar is excellent.

What I am not happy with is the difficulty of finding a sweet spot to place the speaker and the poor mono ventral image within the stage, unless the speakers are positioins just right.

Looking into this matter a bit more I suspect that the box design is leading to a cardiod dispoersion instead of a dipole dispersion thereby reducing the intended spaciousness of the sound.

I have decided to modify the box structure to free up the space above and to the sides of the full range speaker by elmininating the top plate and cutting the sides to a triangular shape instead of full 10 inch panels.

In the end I end up with a front panel with 2 10inch speakers with triangular sides and no top.
I expect that the bass response will fall off a bit but I can correct with EQ. I hope to achieve a broader sound stage and less finicky speaker positioning.

[rob]

Hi Richard,
I haven't read the Linkwitz site for years but note the speakers there don't have sides around the mid and tweet. There is a rule about baffle width etc which I don't know off hand but removing the sides will change the frequency of reinforcement and affect the bass roll off. But there could be other reasons for the upper frequency problems, notably that you are using large full range drivers which will have a narrow dispersion pattern at high frequencies. I know you have a small cone tweeter as well but it is relatively large for a tweeter and also a cone so could be suffering the same beaming problem. There is also possibly some comb filtering happening depending on XO frequency and distance between driver/tweet and this would be exacerbated by no low pass on the full range and a 1st order on the tweet. (broader XO band)
Maybe do some research on those topics before cutting the box unnecessarily and affecting the bass especially as you are satisfied with it. I assume it integrates OK with the midrange?
Others should correct me if I'm wrong.
Cheers,
Rob