Bass directivity and room modes

hvbias

Well-Known Member
Jun 22, 2012
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New England area
Posted by Lynn Olson on another forum

One of the real differences between direct-radiator bass and bass horns is the much higher directivity index (DI) for bass horns; by comparison, direct radiators in the 100 Hz ~ 500 Hz region are nearly omnidirectional.

Since room response is dominated by very severe modes and standing waves in the 100 Hz ~ 500 Hz region, there's a big difference between a near-omnidirectional radiator and one that is pretty directional. The modes will be at their worst with the near-omni radiator, since all the wall reflections are driven. With a higher DI, much less energy goes into the rear, side, floor, and ceiling reflections, and modes are not as strong.

In practical terms, when you allow for the contribution of room modes, the higher-DI horn system may deliver flatter response, even if the horn itself has rougher response compared to a simple direct-radiator.

Discussion open to all, I'm particularly interesting in hearing from our acousticians or anyone with experience.
 

jdza

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May 3, 2010
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On re reading this thread my post can be ignored. I would not consider the 100 - 500Hz range to be bass ,so there is clearly a difference in terminology here. Standing waves at 500 Hz is really not something I have ever worried about.Kindly ignore what follows then.

This gets back to what is a bass horn? Very few so called bass horns are true horns. I used to run Klipschorns with and without sub woofers. I never had a problem with any standing waves. Then in a divine moment of madness ( And seeing Marcel Rogerro's system) I spent an enormous amount of money and turned a big part of my house into 2 bass horns. On the very first firing up the 2 dominant room modes at 32 and 71Hz became so agitated that it was almost unlistenable.

My experience is in no way unique. In the East many ( well geez maybe one or two of the grand total of maybe 10) fellow travelers abandoned the " horn houses" for that very reason while Goto recommend the use of multiple partially filled urns and vases( Helmholz resonators) as an integral part of such an installation.

I postulate that the reason most so called bass horns are better at room modes is simply because their fundamental output in that range is poor (you can not get away from Physics) and that they actually need those modes to augment that weakened output.

When all the wild horses are finally tamed in a true horn there is nothing but absolutely nothing that even gets close to hundreds of tons of concrete horn and 10 or 12 very fast bass drivers delivering bass, but to get there there is a huge mountain of room modes to cross
 

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hvbias

Well-Known Member
Jun 22, 2012
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New England area
On re reading this thread my post can be ignored. I would not consider the 100 - 500Hz range to be bass ,so there is clearly a difference in terminology here. Standing waves at 500 Hz is really not something I have ever worried about.Kindly ignore what follows then.

This gets back to what is a bass horn? Very few so called bass horns are true horns. I used to run Klipschorns with and without sub woofers. I never had a problem with any standing waves. Then in a divine moment of madness ( And seeing Marcel Rogerro's system) I spent an enormous amount of money and turned a big part of my house into 2 bass horns. On the very first firing up the 2 dominant room modes at 32 and 71Hz became so agitated that it was almost unlistenable.

The thread I took the quote from was discussing folded midbass horns.
 

jdza

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May 3, 2010
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The thread I took the quote from was discussing folded midbass horns.

Thank you,I misunderstood. The single biggest challenge in the creation of a full range multihorn system remains how one is going to deal with the 100-500Hz range and how you are going to blend that solution with the rest of the speaker system.
 

JackD201

WBF Founding Member
Apr 20, 2010
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Waaaay back when I set up various kinds of bass horns for SR use. The ones I used most often were 18" CV Jr Earthquakes. Big and bad ass as these looked the range they covered was from the 200s and only reaching down into the 40s. One thing about these designs is that they were designed that way more for throw than actual extension. In a ball room for instance you could be very near to one of these and not feel much but it might hit you hard 20 ft or more away. I guess this ties in with jdza's postulation. At what we would consider normal listening distances in the home (6 to 12ft) I do believe that output IS inefficient. With the exception of say a single driver using a folded horn box, IMO dealing with midbass with a traditional/popular folded horn in a smallish space is a case of the wrong horse for that course unless the mouth is very wide and the horn itself is actually quite long.

I have no idea at what pint jdza crosses over to his house horns but even from the picks one can see how much farther back his array of drivers are.

Just my unsure 2 cents.
 

hvbias

Well-Known Member
Jun 22, 2012
578
38
940
New England area
Thank you,I misunderstood. The single biggest challenge in the creation of a full range multihorn system remains how one is going to deal with the 100-500Hz range and how you are going to blend that solution with the rest of the speaker system.

This particular setup was using an Azura 340 for 500 hz up.
 

Jose Almagro

Well-Known Member
Dec 10, 2010
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Madrid (Spain)
www.mundohifi.com
Posted by Lynn Olson on another forum
Discussion open to all, I'm particularly interesting in hearing from our acousticians or anyone with experience.
That's right but you can look at the other side of the coin and you'll see that when you only excite the modes of one dimension some problems can occur at higher frequencies so it's not simply better. It's just a different approach.

I use bass dipoles (Apogee Stages) and I simulate them using the REW Room Simulator. It's intended for monopoles but easy to create dipoles, cardioids, etc. here you have an explanation (Spanish)
http://acondicionamientoacustico.info/el-simulador-de-salas-de-rew-para-ajustar-dipolos/
 

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