Bass horn sub: FLH vs BLH

[paulspencer]

It seems like a simple decision - BLH for HE drivers where you want to horn load to get the bass up to match the midrange or FLH for bass horns like the lab sub.

But this is a large 20 Hz bass horn with drivers I'd like to put in a BLH for 2 reasons:

1. I want to see the driver - it looks too good to hide

2. My intuition tells me drivers can sound better when they aren't damped by a sealed box

The latter appears to be the reasoning behind Rhino Acoustic's bassmaxx subwoofers, where they claim this is better.

My own experience with open baffle dipole woofers suggests this is so. They seem to have more punch and kick without the damping effect of a sealed box.

When I model in hornresp, the difference between the two, in group delay, excursion, phase, response only appears significant below 20 Hz. I can use a rumble filter to protect the driver.

I've heard comments by Tom Danley on the benefits of the sealed chamber:

1. The air spring is more linear than the driver suspension, resulting in lower distortion
2. Power handling is increased by protecting the driver

However, in the design I have in mind, it will be loaded down to 20 Hz, with a rumble filter at that point. Simulations show that above 20 Hz there is no significant effect of the sealed chamber. Not as far as I can tell. Do his comments apply to such a design then?

While this sub will be huge, I do intend to make a detachable manifold with the drivers connected, which has a removeable panel so I can switch from BLH to FLH by removing the sealed chamber.

But still, I'd like to hear some views and advice on this one.

[Rademakers]

When I model in hornresp, the difference between the two, in group delay, excursion, phase, response only appears significant below 20 Hz.

The downside of the BLH is the dip in the frequency response. Hornresp shows the response without the cancellation, only with tools/ backhorn you can see the difference. One would expect the 3 - 6 dB dip to occur between 40 - 80 Hz, and so possible work against the kick like you know from dipole. Out of the subrange the changes are more severe.

Inside the house BLH can sound better than folded horn due to the direct radiaton for nicer bass up front.

Wkr Johan

[paulspencer]

Johan, isn't that dip normally associated with high efficiency drivers? The driver I'm using has low efficiency - a typical subwoofer driver. Hence I wonder if this dip would actually present in this case.

Also there would be eq to get it flat.

[paulspencer]

I didn't realise there was that tool. I had a look and the response gets a lot more ragged above 100 Hz. There is a slight dip, but overall much less flat. How audible it would be after eq, I'm not sure.

[RobWells]

Paul,

The back loaded horn is basically a scoop - they seem to be frowned upon due to a phase cancellation ocurring between the direct radiated bass and the horns contribution.

Theres some thoughts here


Rob.

btw, isn't there any way to use the 'void' space in your latest horn plan to improve it? Seems a shame to waste the space.

[paulspencer]

I've discovered the back horn tool in hornesp and now it doesn't look quite so good. I've read that link previously but I wonder if those comments apply as much if at all to a 20 Hz horn where eq is used. A conventional BLH has a driver with say 100 db sensitivity but no bass and the horn is used to get the bass to match in efficiency. In this case, the drivers are more like 85 db, so that dip is very small, and possibly insignificant in comparison to other abberations in both FLH and BLH designs.

I think this is one I will have to try - I'd be happy to judge it subjectively. In this case, if I'm happy with the sound of BLH then I'll go with it. This driver is just too good looking to hide! The teenager inside who wants to see the driver moving just won't let go!

Rob, I have thought about the void issue. I may be able to do it better. The problem with a project this big, is it doesn't lend itself to prototypes! I hope to be happy with the first version I build. The manifold holding the drivers is something I can do a few versions of. The horn itself I hope to get right before building. I'd like to make it all curved for stiffness and looks - probably 9mm sheets bent and glued together. Bolted together in sections.

I'll have to read and re-read everything I can find on the net before I build this.

I modelled last night with a mouth 3x as big and to my surprise, the low end didn't change. What it did is decrease the midbass efficiency and flatten the response. This was effectively simulating the effect of the walls expanding the mouth, assuming that this effect goes further than I had allowed. Eric Cowan suggested that it could in fact extend to the diagonal of the room so that I am effectively sitting in the horn mouth. I doubt it would go that far, but if it did, the output would in fact work out as less, with higher excursion, lower tuning and better behaved SPL, GD charts.

[Rademakers]

Johan, isn't that dip normally associated with high efficiency drivers? The driver I'm using has low efficiency - a typical subwoofer driver. Hence I wonder if this dip would actually present in this case.

No, the dip occurs because the front and rear of the driver are out of phase. This effect is most intens at the frequency which wavelength equals 2 x times the pathlength difference (between front and rear).
It's one of the physical property of a BLH and will happen at any level disregarding sensitivity.

Knowing this you can aim the cancelation out of the operating range or equalise it out at the cost of excursion/headroom.

Wkr Johan