My open baffle dipole with Beyma TPL-150

[StigErik]

Cuibono,
Thank you!
Beyma claimes that the Xmax of 18P1200nd is 19 mm peak-peak (+/- 9.5 mm), so I would think RMS displacement is to be doubled - 6.7 mm. That would add an other 6 dB. Am I right?

106 dB @ 25 Hz is not bad at all, should be sufficient for my needs.

[cuibono]

Ah, yes, you're right. Pretty good, I'd say!

[mige0]

One additional suggestion, if you still have spare time and energy for experiments:

Try a second pair of nude 21" placed behind the others - half way in between to the front wall and let them come in at roughly half your XO at 6dB/ oct

should do for a quick try though I use the split power technique (parallel shunt capacitor) - shelfing the front pair by -6dB accordingly

Works pretty well for me at the moment...

This balances excursion and also seems to have a minor but positively audible influence in exciting room modes.

According to JohnK and my own lousy simus the two dipoles are more or less acting like one.

Michael

[skorpion]

Rudolf, you may very well be right about Martin's presentation, but I seemingly only can change the grid resolution not really the absolute scale ?

I think it would be best if Martin himself could explain his use of the RMS statistic. Notation is more pointing to Absolute Value. Cuibono's reasoning I think is more apporpiate with regard to electrical calculations with sine waves.

To give an example the RMS statistic for an observation series like:
+ 5, -5, + 5.2, - 5.1 would be SQR( (5^2 + 5^2 + 5.2^2 + 5.1^2)/4) = 5.076

MJK also has a guide how to use two speakers in his models: http://www.quarter-wave.com/General/Two_Drivers.pdf . I have tested this with the OB models for one and two bass units and the two methods produce identical results for parallel units. My conclusion is that it is alright to model two identical units as a compound unit.

/Erling

[mige0]

Originally Posted by mige0

Stig Erik, you are amazing - would be great you could do a 0 15 30 45 polar measurement for each setup for us to compare.

Below my polar measurements of a 8" and 6" in a 30cm / 12" wide baffle and without any baffle taken indoor with a gating of ~4ms (so forget anything below ~300Hz).
8" is the Jantzen JA8008
6" is the B&C 6PEV13

To make even more clear what Sitg Erik and Cuibono have outlined I normalised FR to on axis response.

What we see is a virtually perfect equalised an axis response and the corespondent polars at 10, 20, 30, 40, 50, 60 deg




Above the 8" Jantzen JA8000 in a 30cm / 12" wide OB





Above the 8" Jantzen JA8000 in a 30cm / 12" without any baffle





Above the 6" B&C 6PEV13 in a 30cm / 12" wide OB





Above the 6" B&C 6PEV13 in a 30cm / 12" without any baffle



Uups – the top end polar's are way better without a baffle (as already been said)
Comparing the two different speaker size' - both without any baffle - it comes as a surprise that the 8" is *smoother* (up to ~2500kHz) than the 6" !
seem to be perfectly reasonable to still cross the 8" at ~ 1500Hz

Comparing the two speaker in the same baffle the 8" speaker clearly wins by a large margin.
To be fair, we possibly would have to measure the 6" in a proportionally smaller baffle.

Also can be seen the dipole –6dB at 60deg isn't true for *both* speakers. I think this is due to the same reason as 90deg never measures 0dB – simply the asymmetry due to the motor structure.


One conclusion seems to be a safe bet, taking a smaller speaker does not necessarily turn out in smoother top end dipole polars - its individual for any speaker and subject to measurement

What can't be seen here is that the penalty is roughly a - 6dB SPL loss for the nude speaker variant

Michael

[cuibono]

Nice work Michael.

I agree, that the basket has a big effect on off axis response, esp at larger angles. The sharp dip at 1500 for the 6" looks to me like acoustic interference. Could it be the magnet?

Did you make sure to rotate the speakers around their acoustic center? What distance did you measure at? Both of these make a difference to whether the off axis response drops as expected. The JA8008 looks to have only a 30mm (1.2 inches) wider frame than the BC - I wonder if this leads them to look more similar than one would expect.

I'm also concerned that a gate of 4ms leads gives too low resolution below 1000Hz. The graph will have a resolution of about 250Hz, so there are only a few data points below 1000Hz.

One thing to be aware of, for both drivers, is that their response is falling above 1000Hz. If they are crossed at 1500Hz, there will be a strong suckout off axis due to both the crossover and the driver's beaming.

Does ARTA allow you to normalize graphs in the program? Soundeasy kind of lacks for data manipulation. If so, I'm getting ARTA...

[mige0]

Originally Posted by mige0

Uups – the top end polar's are way better without a baffle (as already been said)

Just when you think you finally arrived – the game begins from start.


Below I measured - out of pure curiosity – what I'd like to call a "thong baffle" – meaning a quadratic OB the size of the basket:

Again we see the 8" Janzen JA8008

What we see is – again - a virtually perfect equalised an axis response and the corespondent polars at 10, 20, 30, 40, 50, 60 deg





Above the 8" Jantzen JA8008 in "thong OB"


For direct comparison what I have shown in my last posting:



Above the 8" Jantzen JA8008 in a 30cm / 12" without any baffle





Above the 8" Jantzen JA8008 in a 30cm / 12" wide OB





To sum up :
with a "thong OB" the smoothness in the upper FR is further improved – as a draw back a severe polar-peak arises at ~ 4 kHz

So the hunting after an optimal OB in terms of best polar smoothness isn't over IMO – it not even has started...

Michael

[cuibono]

Nice! I was just wondering if there was a middle ground between the two sets of measurements (baffle, no baffle).

What is a "thong baffle"? A square the width of the driver frame?

[mige0]

Originally Posted by cuibono

Nice work Michael.

I agree, that the basket has a big effect on off axis response, esp at larger angles. The sharp dip at 1500 for the 6" looks to me like acoustic interference. Could it be the magnet?

...

most likely but dunno – would need deeper investigation..

Originally Posted by cuibono

Did you make sure to rotate the speakers around their acoustic center? What distance did you measure at? Both of these make a difference to whether the off axis response drops as expected. The JA8008 looks to have only a 30mm (1.2 inches) wider frame than the BC - I wonder if this leads them to look more similar than one would expect.
.

No I didnt take care about the acoustic centre – the measurement distance was 1m (SPL of 90dB) so I'm not really concerned.

Yeah the JA8008 might be a "small" 8"

Originally Posted by cuibono

I'm also concerned that a gate of 4ms leads gives too low resolution below 1000Hz. The graph will have a resolution of about 250Hz, so there are only a few data points below 1000Hz.

One thing to be aware of, for both drivers, is that their response is falling above 1000Hz. If they are crossed at 1500Hz, there will be a strong suckout off axis due to both the crossover and the driver's beaming.

The 4ms is the most I can get indoors and I'm aware the resolution isn't stellar - but on the other hand it should be sufficient to make the point (no scientific precision intended).

Originally Posted by cuibono

One thing to be aware of, for both drivers, is that their response is falling above 1000Hz. If they are crossed at 1500Hz, there will be a strong suckout off axis due to both the crossover and the driver's beaming.

Don't think so – but maybe I'm missing something?
The polars I've shown are *exactly* what you get with proper on axis EQing

Originally Posted by cuibono

Does ARTA allow you to normalize graphs in the program? Soundeasy kind of lacks for data manipulation. If so, I'm getting ARTA...

Yes ARTA can!


1.) You go into "overlay" add the 0-deg measurement and set it as active > go back and select "subtract overlay" from the "edit" pulldown to make for the first flat on axis overlay. Add this one to overlay as well.

2.) Set the initial 0-deg measurement as the *only* active overlay – open your first off axis measurement > select "subtract overlay" from the "edit" pull down to make for the firsts off axis overlay. Add this one to overlay as well

repeat step 2.) as many times you have off axis measurements – finally make all overlays active to display the whole bunch and you are done

Originally Posted by cuibono

Nice! I was just wondering if there was a middle ground between the two sets of measurements (baffle, no baffle).

What is a "thong baffle"? A square the width of the baffle?

sorry - should have been " A square baffle the width of the basket "



Michael

[cuibono]

Michael, could you rotate your 'thong baffle' 45deg in the baffle plane and do a set of polars? So the frame looks like a diamond (corners point up/down)?