June 22, 2010

Bass horns

Do you want to experience the ultimate in tight accurate dynamic bass? You can't beat bass horns. Well, actually you can, but it takes a very extreme setup costing a great deal more. Out of curiosity I did a simulated comparison between the current LFE champion, the Exodus Audio Maelstrom 21", and a 15" Rythmik. The larger driver has nearly twice the excursion and more than twice the cone area and the coil can take much more power. But due to the wonders of bass horns, the smaller driver wins the output challenge horn loaded if we compare to a sealed or vented design. In fact, the 21" was limited to 114 db @ 20 Hz with a very high powered amplifier, while the Rythmik could hit 125 db at that point with only 370w. What's the catch? The tapped horn is huge. Both are excellent options, but the point is that horn loading can extract very surprising performance out of a driver, effectively turning a modest or small driver into a much bigger super driver.

3 types of horns

There are 3 basic types of bass horns:

1. Front loaded horn (FLH)
2. Back loaded horn (BLH)
3. Tapped horn (TH)

A conventional horn is a FLH. The driver has a small sealed chamber on one side, and a horn on the other side, starting with a throat typically half the area of the cone expanding out to the mouth which is many times the area. A back loaded horn has no sealed chamber. This means that the excursion has less control below tuning, and that output from both sides of the cone are used. This is useful for wider bandwidth use, but generally detrimental for bass horns with a limited bandwidth.

The third type of horn is slightly controversial. It is often argued that it is a transmission line or a bandpass. While there are similarities, tapped horns (THs) have their own traits and deserve their own category. They require different drivers and a different design approach and different tools to simulate. They give you a different response and I see no reason to paint them with the same brush.

Tapped horns vs front loaded horns

In general I would not use them for the same job. For the ultimate in efficiency and performance where size is no object, you can't go past a conventional FLH. Trouble is, the size is so extreme that most can't live with them. A 40 Hz horn is about the limit of what all but a few nutters could live with. It might be 0.4 x 1.2 x 0.9m. A tapped horn can cover the same range in a much smaller package. The trade offs are efficiency and bandwidth. It will more easily go lower, but it won't extend as high. A typical design will see 1.5 octaves such as 20 - 60 Hz. Beyond this range you will typically see a very ragged response that isn't useful.

This example compares the efficiency of a TH and FLH with two of the Peerless XLS 10" driver. The TH has impressive efficiency down to 20 Hz - the kind you would get from a pro 12" down to 50 Hz. It would take a massive driver to match this in a vented down down to 20 Hz. You can see that it needs to cross around 60 Hz and some EQ is needed.

The FLH has much greater efficiency - 100 db @ 40 Hz and it could be used up to 200 Hz.

So you can see that the two designs are very different. For LFE use, the TH wins. Main speakers with strong output and efficiency to 60 Hz are required. For a very dynamic system, the FLH wins. It's not a matter of one being superior. The ultimate would in fact be a TH for LFE use combined with stereo FLH.

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