Driver evaluation for OB project

[wowo101]

Hi all,

after having learned a lot by just reading the incredibly smart and well-researched stuff posted on dipoles and OB design around here, I decided to take the next step and start my first OB project.

First, I'll be evaluating if the drivers I've got already lying around here may be suited for the project, especially in terms of their capability for constant directivity. I'll post my finding as they come in and hope you can give me some feedback on how to proceed!

Today I took some rough on- and off-axis measurements of my little KEF iQ3s. Since the enclosures are already a bit battered and I'm not really happy with them anyway, I might consider to take them apart and use just the good parts - which, as it turns out, could be the coax drivers.

iQ_Driver_front.jpg
iQ3_Driver_side.jpg

They seem to exhibit some nice constant directivity (measurements taken at eyeballed 0°, 15°, 30°, 45°, 60°, 75° and 90° - I obviously didn't quite manage to keep measurement distance constant):

iQ3_Freqency_Response_off_axis.png

This is some quite nice off-axis response, don't you think?

The question now is: Would those drivers be suited for an OB speaker? Two obstacles will be the large magnetic shielding and the closed back of the tweeters... Maybe both could be removed.

I could only obtain the following parameters for them from teh Interwebs:

Power Handling: 15 - 120 Watts
Nominal Impedance: 8 Ohms
Tweeter: 0.75" Aluminum-Dome HF Driver At Acoustic Center Of 6.5" Midrange: Titanium-Coated Cone
Crossover Frequency: 2.8 kHz
Frequency Response: 45 Hz - 40 kHz
Sensitivity: 89 dB (2.83 Volts @ 1 Meter)
Maximum Output: 110 dB SPL @ 1 Meter

What do you think?

(Next in line: Aura NSW2 and SEAS L16RN-SL.)

Thanks in advance for any feedback!

Wolfgang

[wowo101]

So - that was a bit of premature posting: When measuring the other drivers, I learnt that measuring from the distance I did (within 1m) doesn't really tell you anything about off-axis behaviour...

To start again as is fit for a beginner like me: Are there any alternatives to outdoor measurement and greater measuring distances to get useful information about the polar response of those drivers? Would you recommend a (hopefully easy to use) simulation software for first checks?

Many questions - I hope someone can help out with some answers...

[twest820]

Yeah, near and far field radiation patterns aren't the same. 1m is usually far enough, though I generally prefer to use 2m as that's closer to my 2.5m listening distance but still makes for easy SPL adjustment to the 1m standard.

For indoor measurements, look into gating. Most any measurement program should give you good options at high frequencies. I know at least SoundEasy supports gated sweeps that can measure direct SPL at considerably lower frequencies than windowed MLS measurements. Or just get ahold of the a biggest space you can.

Disclaimer: every time I've tried to get SoundEasy to do a gated sweep it's failed with an uninformative error. Other users have reported similar problems.

[jerome69]

Hi Wolfgang,

Nice measurements.

Have you measured the T/S parameter, the distortion, CSD and the impedance of each driver ?

This coax driver could be a good midrange treble in a 3 ways.

But it could be the fact you don't like the sound of this driver, an other setup could not change anything.


Have fun

[wowo101]

Thanks for your feedback!

I fiddled a bit with the other two drivers to be tested and think at last I've got a quite solid measurement setup: 1m measurement distance, DIY turntable with degree marks, flexible driver stand made of IKEA leftovers (as freestanding as my flat allows), gated measurements with FuzzMeasure (highly recommendable, btw).

Concerning the KEFs: Measuring the T/S parameters is a bit beyond my abilities so far, but Jerome might be right anyway... So on to the next candidates.

I measured the SEAS and the Aura today and will post some pictures and graphs tomorrow - I've already learned a whole lot by getting into this and am looking forward to discussing my results!

And: fun I definitly have. :-)

[wowo101]

As promised, here are the measurements for the drivers left over from an abandoned Linkwitz Pluto project. Both have been measured from 1 m with 3,5 ms gating; FR is shown for 0°, 30°, 45°, 60° and 90°, 1/24 octave smoothing.

First the SEAS L16RN-SL:

SEAS_L16RN-SL_1m_off_axis.png

It shows pretty decent directivity up to 1.4 kHz; above that, things get messy. Also, distortion has a nasty peak just above that frequency. (Of course bass is lost in the graph due to the narrow gating.) Apart from that, the SEAS seems quite usable.

Next the Aura NSW2:

Aura mit 1cm Baffle, Gating 3,5 ms, 1-24 oct Smoothing.png

This was measured with the driver mounted slightly recessed on top of the SEAS and with a small rounded cardboard baffle (pics of the setup will follow). Obviously, the Aura has two serious problems when used as a dipole: The peak at 1.6 kHz which is caused by a pipe resonance due to the rear design of the Aura (showing primarily at angles above 15°), and the early dipole peak at around 4.3 kHz caused by the length of its enclosure (i.e. effective baffle). After the peak it becomes nearly unusable directivity-wise.

Correcting the first peak via EQ probably wouldn't be a good idea since that would reduce front radiation as well - am I right about this? The dipole peak can't be transferred much higher (even if I remove the small additional baffle) due to the Aura's enclosure, and stuffing it with damping material would destroy its dipole character if it's effective at all.

In addition, if I would use both drivers together, crossing at a higher frequency to avoid the first peak altogether doesn't seem to be an option, too, because of the SEAS's behaviour above 1.4 kHz.

So basically this means: Not a good combination for an open baffle speaker. Or am I missing something here?

The next questions for me now are:

• Is there another tweeter that a) has better dipole behaviour and b) can be crossed as low as 1.4 kHz? The latter criterion rules out the Neo 3, I guess...
• If low XO to the tweeter isn't an option, what would be a good replacement for the SEAS? It would have to be a midwoofer that goes roughly from 200 Hz to 2.5 kHz.

Any ideas?

[StigErik]

I've found that a 5" (15 cm) Seas has a really good directivity up to 1.7 kHz. XO at 1.7 kHz is quite possible with an almost perfect off-axis response, making it a bit easier to find a good tweeter. 5" may be a bit small for going down to 200 Hz, but you can compensate by using two (or more) drivers.

Dipole tweeter: there's not really any alternatives to a good AMT....

[wowo101]

Thanks, StigErik, for the suggestion! Which driver(s) exactly have you tried? I'm asking since the Excel series will quite probably beyond my budget...

I've read good things about the Peerless Exlusive 8380882 (7"), which are being used in the NaO Mini if I remember correctly - any opinion about that?

Speaking of using more drivers: Would it be an option to combine the L16 with another midrange driver for higher frequencies, or is it bad advice to mix drivers in the midrange?

And as for the tweeters: AMTs might also clash with my budget limits - are there any relatively modest options that wouldn't sacrifice too much quality?

[StigErik]

I'm using the Excel W15CH001. Expensive stuff indeed, but very good. Its the best driver I've ever measured or listened to for use below 2 kHz.

A budget option could be the very nice Seas CA15RLY or ER15RLY.

A 6" og 7" is a bit too big if you want well behaved directivity up to 1500 Hz.

With a 5" you might be able to use the Neo3, XO at 2 kHz or so. A possibility is also an even smaller midrange than 5" to push the XO freq up.

[keyser]

I'll probably try this one: http://bmsspeakers.com/fileadmin/bms...155_t_data.pdf

It's about 85 euro's in Germany.

This driver has high sensitivity, high power handling and sufficient excursion capabilities to be used in an unbaffled configuration down to below 300 hz.
If the rear construction of this driver doesn't mess up rear radiation too bad, it could be a perfect match with the Neo3W tweeter.