Directivity Index of a Driver by it's Size

[BradH]

Do all drivers of a given size have the same directivity index? Why or why not?

I realize baffle plays a part, could we say the comparison is for all drivers on an infinite baffle?

 

[krivium]

Hi,
Not directivity index but you could guess it:
https://www.diyaudio.com/forums/multi-way/351883-3-active-horn-speaker-monitor-4.html#post6142193

If you need smaller ( or larger size) just ask.

 

[BradH]

Thanks for the link. I may have been using the term directivity index improperly. Maybe it's just directivity?

 

[krivium]

The polar maps linked are indeed the directivity behavior you can expect from 6,5" to 15".
These are approximation ( they are simulation based upon SD ( membrane surface) ).

Directivity index is used to determine a horn usually, that said you could derive it from the polar map of direct radiating driver if you want. Take a look at the thread linked, there is a link to a Jbl 2380a horn. In the datasheet you'll see directivity index. But it'll not be the usual way to plot direct radiating drivers for what i've seen.

 

[Robh3606]

Here read this also look at the overall index lots of really good info about directivity in some of the other Altec papers.

https://greatplainsaudio.com/wp-content/uploads/2017/07/TL_237.pdf

Rob

 

[AllenB]

Thanks for the link. I may have been using the term directivity index improperly. Maybe it's just directivity?

I think DI is more appropriate when you're talking about a specific directivity, and just directivity when speaking casually. And a plot of DI vs say, the DI or a figure of DI whether talking about a single frequency or a range.

 

[454Casull]

Generally, yes, as long as you're looking at the drivers when they're operating in without breakup, and the diaphragms are shallow to moderate in depth.

Things start to diverge when you enter breakup or have weird diaphragm shapes.

 

[krivium]

Here read this also look at the overall index lots of really good info about directivity in some of the other Altec papers.

https://greatplainsaudio.com/wp-content/uploads/2017/07/TL_237.pdf

Rob

It was an Altec paper! Thank you Rob, i've spent an hour looking for it believing it was from Jbl.

 

[Juhazi]

Some info here
https://www.klippel.de/fileadmin/_migrated/content_uploads/KLIPPEL_Sound_Radiation_Poster_01.pdf

We have wonderful freeware simulation software to study also off-axis radiation pattern of a driver and a driver in a baffle

Tolvan Data
Software

The problem with simulations is that they use flat piston model for the radiator. But that effect is smaller. Nowdays we have also freeware measurement software and cheap calibrated microphones...

 

[Juhazi]

Edge overlays. Spl drop is exaggerated because with mic off-axis also distance gets longer. Mic offset 0, 250, 500 and 1000mm
Baffle diffraction changes too off-axis!