S9 OB - Revisiting narrow baffles

[gainphile]

This build is to revisit the previous design which sounded most natural and smooth. So much that the further effort to obtain dipole bass quantity had to be scrapped.

From measurement of different OB implementations I learnt that driver-baffle relationship is very important. If the inherent response is not smooth, no amount of EQ would result in good sound. 'Good' in here refers to the believable reproduction of performances.

These guys provided a good read on how to evaluate driver-baffle relationship. They experimented with 30 Linkwitzlab baffle variances. Smooth response relates to smooth polar measurements on and off axis. So a delta of 0 and 20 degress measurement should be flat, as well as 30 and 40 degrees.


So what makes this build special among the others I've done so far (about 15+ I think )

[Moondog55]

That second link is 404

[gainphile]

Sorry... it's fixed.

Gotta run home (and fix the flat tyre first ) ... will continue later.

[gainphile]

Following the method use by DonWM on the link above, I compiled measurement data which was taken outdoor. They are not gated so diffraction and reflection have some influence. But it is important to get the details within the frequency response.

The data were taken at 0, 20, 30, and 45 degrees. Delta of on-axis vs. off-axis then calculated in an excel file. The 30 and 45 deg. measurement was reduced by 10db to show clarity on the graph. The resulting graph is below.



The resulting graph shows that from 250Hz up to 2kHz the polar response is practically flat for the combination of P13WH woofer and thin 19cm baffle. I really did not want to measure 30 baffles so was really pleasing to have this.


As a comparation, take a look at original Phoenix measurement, showing fairly flat polar response but with 700Hz peak.



Also, DonW's modified Phoenix showing good result from 200Hz to 1.7kHz. It's curious that his best result was with much narrower baffle compared to the original Phoenix as well.




DonW's method is really helpful in designing driver-baffle combination. A usual response plot can be used too but not as visible.

[gainphile]

Next I experimented on the midrange equalisation. This is the second trickiest part of working with OB.

There are few schools of thoughts:

1. No EQ

2. Room EQ, measure from listening position

3. EQ the dipole peak (JohnK)

4. EQ the driver basket resonance (SL)


First one - No EQ.

The sound is subjectively acceptable. I actually swithed EQ and no-EQ just tu make sure. The smooth response helps but definitely the sound is coloured. If I put a mic in my listening room I can see broad peak at 600Hz. This manifests in piano recording. Check with headphones also confirms this.

[fergs1]

Greetings Gainphille, good to see you back in the cockpit. Didn't you sell your Alphas, what are you going to do about the lower end?
cheers fergs

[cuibono]

Nice work, keep it coming!

[jerome69]

Good job Gainphile

Optimize dipole is a very hard job !

I think it's important to suppress the dipole peak.

[gainphile]

Thanks. At the moment the low-end is supported by single Jaycar 10" in an H-Frame baffle. It does ok but severely limited in SPL as the equalisation is -3db at 20Hz. It struggles on organ music but with appropriate sound level it's sounds very deep. The woofer section is a later part of the development as I think it's quite easy. I am looking at Beethoven-type woofer approach.

I'll try to take some pics during the build. Been really lazy.

Anyway on to the 2nd type: Room EQ. Basically measurement taken a sitting position and equalised according to that.



Green is the equalised response. With software parametric EQ it is easy to make changes and listen to the difference. But the problem with this type of approach is high-Q peaks are almost impossible to identify. They would be diffused by the room and showing up as broad peak.

Take an example: If a driver is exhibiting high Q peak such as those metal cone resonances, they would not be picked up by the microphone at listening position as the original peak. Thus the equalisation applied will be wrong.

"Playing" with the parametric EQ also is difficult. The graphs keep moving and at best the result will only be close approximation of what should be. My ears couldn't judge what's correct and what's wrong after few minutes.

I found out this method is not good.

[cuibono]

I've been doing room EQ just this last week. I've been getting excellent results - not subtle differences. I use 110ms gating time, so basically ungated. I place the mic at the listening position, but next I'm going to try spacial averaging to see if I can spot low Q peaks/dips. I use soundeasy's mls signal, and the results are very repeatable.

My graphs look a bit rougher than yours, even though I use 1/3 octave smoothing. Are yours smoother because of gating? Just guessing off the top of my head, I'd say I EQ'ed out some peaks with a Q of 3-5. A thought I had was that dipoles take ungated EQ well because their off-axis reflections should be spectrally similar to the on-axis.

Just a thought. Let us know how it progresses.