The Unlikely Dipole (a garage experiment)

[y8s]

Not much to post other than a few pictures of something I whipped up with spare sparts.

The small driver is an Aura NSW2
The large driver is a Stryke (AE) AV15

Power is from a 41Hz Amp10Basic (2x100W max at 8ohm)
filtering is a behringer dcx2496
source was just some internet radio

The arrangement is basically a flat dipole up top and a U dipole on the bottom (12" deep, 16" diameter).

The sound was pretty good for not doing any concrete measurements or really setting the EQ properly. One of the biggest issues I think would arise from this kind of layout is the compromise of crossover frequency. The little Aura can play low but probably wouldn't sound good below 3-400 Hz. The woofer-in-a-tube gets a little ugly above 120 Hz.

It also plays surprisingly loud--even down below 40Hz. Consider that neither the aura, nor a dipole woofer are tremendously efficient or going to win any SPL contests, it's pretty darn good for what it is.

OH, I also stuffed some random eggcrate foam into the back of the U-woofer and it reduced the honky sound somewhat above 120Hz but I'm sure it's less than ideal. Garage experiment!

Here are the photos:





I also stuffed some random eggcrate foam into the back of the U-woofer and it reduced the honky sound somewhat above 120Hz but I'm sure it's less than ideal. Garage experiment!



Note the Aura is an open-back driver--just a screen to keep... fingers out I guess.

Matt

[lolojr1]

the open back speaker looks interesting, never thought it would be like that.

[LineArray]

Hi Matt,

one of the most beautiful Garage Experiments i have
seen in the last time ... Congratulations !


Some Ideas to fill your gap between say 100 and 300 Hz:

The freq. of the quarterwave resonance of the tube could be
doubled by modifying the U-Frame into a H-Frame.

With same length of the tube resonant frequency should
double. But unlike the U-Frame which exhibits a cardioid
charcteristic you will get a dipole bass characteristic then.
I don't know if this is intended by yours.

Maybe you could lower the Q of the remaining quarterwave
resonance by making the H-frame asymetric
(front tube length != rear tube length).

Bass performance in a living room will improve if you rotate the
axis of your woofer 90 degrees to get it parallel to the
rear wall (when choosing dipole radiation). Otherwise
the distance to the rear wall has to be fairly large for
low cutoff frequency ...

Its difficult to get down below 100 Hz with a narrow baffle
like your midrange baffle.

Maybe this is interesting for you:

dipol-audio.de

You could add some AURA drivers to compensate baffle
rolloff, if you don't want to make a large midrange baffle.

[bonjonno]

I dig it!

[CLS]

Originally posted by LineArray
....

Bass performance in a living room will improve if you rotate the
axis of your woofer 90 degrees to get it parallel to the
rear wall (when choosing dipole radiation). Otherwise
the distance to the rear wall has to be fairly large for
low cutoff frequency ...

But wouldn't this make the listener face the dipole null?

[LineArray]

Originally posted by CLS



But wouldn't this make the listener face the dipole null?

Yes. But in the bass range up to say 200Hz you won't
realize this, because bass response is due to excitation
of room modes anyway and you face the "null" only
when sitting e x a c t l y in front of the speaker when the
speaker is radiating into free space.

Try to find the "null" below 200 Hz in a living room,
you will not be able to determine the "figure 8" shape
of radiation in a living room.

You can rotate the radiation axis of a dipole sub to control
the extent in that certain room modes are excited.

This is a great advantage over a monopole sub IMO.
You can influence room excitation by rotating it, not only by
moving it around.

In fact this is IMO a great disatvantage of dipole woofers
mounted fixed to the same frame and direction axis with the
midrange dipoles.
You can find this configuration e.g. in the Apogee Fullrange
ribbons, most ESL and many dynamic OBs. When
typically placed around 1m in front of a wall, the result in
the midrange might be OK, but bass performance will be
suboptimal. Best is to have the subwoofer flexible to
point into different direction as the midrange dipole.

This technique is only usefull with a subwoofer. If a dipole
subwoofer is placed near a wall, the radiation axis should
be parallel to that wall. This way radiation of low frequencies
is more efficient.

[peufeu]

Cute !!!!!!!

But you'll always get honky sound from a dipole woofer in a pipe like you did, there will always be a very strong resonance... the pipe helps for low bass but annihilates the midbass... so since you have a digital EQ, I suggest making it a 3 way, use another driver for the midrange (on flat baffle) and crossover at 100 Hz and then 800 Hz for the tweeter or something. Digital EQ is great for messing with crossovers.

[CLS]

I found the bass is very weak when I am in the dipole null.

Now my main system comes with OB bass. It faces the couch in the living room. Meantime, on the side of the speaker, by the dinning table, I face the the null. At this position, the bass level is much less than the main listening position.

The very directional bass is a strong point of dipole bass. I can get a very satisfied SPL at the listening position without bothering others off axis. But if now I myself face the null, then others would be in the main "beam". I doubt that would be a good idea.

[LineArray]

@CLS:

I think, what you feel as sound pressure are overtones.
There is directivity in upper bass and midrange.

In the modal frequency range of the living room you cannot
localize the dipole null. At low frequencies the SPL depends
on excitation of room modes. It is only important, that room
modes are excited in a balanced manner.

How large is Your room ?

[y8s]

I've got to disagree with the listening axis being "in the null" as well. A major point of dipoles (at least per Linkwitz and others) is that the reduced omnidirectional radiation also reduces room mode excitation.

And I've also experienced the loss of response in the nulls with both this woofer arrangement and a Linkwitz-style W-frame using the same 15 (x2).

...

I think as a next garage step, I will take some measurements and see if I can't damp the tube resonance with some stuffing (per John K I believe?) or perhaps examine the distortion present in the aura driver at lower crossover frequencies.

Another option (since I have 10 of the auras) is to make them as .5-way mids. I have a similar panel with four mounted in it already. reducing excursion is an easy way to reduce distortion. I think having them all be "tweeters" though will ruin the vertical polar response.

---

The original idea with these drivers was not to use the aura in the first place. It was to put two of the 15s together with either of the following:

Hiquphon OWII and Peerless HDS 6.5 (old model with phase plug)
or
Peerless/Vifa XT25 ring radiator and HiVi M8a 8" midwoofer.

I think the former would be the "sweeter" arrangement but it remains to be seen.

The real question at that point would be: WTMW or TMWW.