MJK's Fostex FF125wk BLH project

[Godzilla]

Thanks Scott! One day I'll get around to building another back horn.

[chrisb]

Originally Posted by Scottmoose

Right. See the thread I linked to. It's an internal Helmholtz resonator / absorber. It works in the same way as the bass-traps sometimes used to kill a problematic room-mode, which are essentially a big empty box tuned to said problem frequency.

In this case, Martin has tuned it to, or just above, the intended Fh (upper corner-frequency) of the horn in order to provide a higher order acoustic low-pass slope. Since most such resonators are narrow band devices, the slope will ultimately resemble a cauer filter, so the trick is to shunt the secondary peak > 40dB down to maximise the benefits.

Are the funny little vented chambers in Lowther designs like the Fidelio, etc at all related to Hemholtz resonators, or something else altogether - and is it possible to model them with DIY accessible software?

[Scottmoose]

It's been a while since I looked at them, & I'm on my decrepit laptop at the moment, so probably best if I don't try now. From memory though, yes, they're a variation on the theme. Akabak can model them easily enough.

[Colin]

In Martin's article he talks about the line transitioning from TL to horn loading.

What is the benefit of the horn-loading part of this? Is it purely to provide more output for low xmax drivers?

[Scottmoose]

No, it's an inevitability with an expanding line. Martin defines horn loading in terms of impedance matching, i.e. the lowest frequency to which the pipe is impedance matched (determined by the terminus area & local boundary conditions), & anything below that as TL action.

Not that it matters, I just call anything that expands toward the terminus a horn, since it will have some degree of 1/2 wave resonant behaviour present. In both cases, the point is that in a compromised horn where the expansion area is reduced compared to the theoretical ideal, even when boundary loaded, Fo and Fc are not necessarily the same, and if that is the case, the region between the two is dominated by QW action. Martin's approach to the nomenclature makes this point somewhat more overtly.

[Colin]

Thanks. I must say I like the pragmatic approach Martin takes with his designs and the reverse approach of this one seems very practical.

Now, I'm not against horns but I presume that these days the need for a horn or horn/TL hybrid is purely to make best use of a particular drive unit, especially when operating with a single driver?

[Scottmoose]

No problem.

Reverse approach?

Depends on circumstance when it comes to 'best use.' Horns & their variations are generally efficient, & are broader band gain devices compared to smaller vented boxes, which can be useful for lower Q drivers. With that said, it's a myth that very low Q drivers are necessary or indeed optimal for back horns. If the driver's mass-corner is higher than it is practical to run a back load of any kind (once amplifier output impedance is factored in), then you'll need some Eq, or a short front horn to fill in the dip in the response.

Outside of that, many people like the sound. A horn is usually coupling to substantially more air than a small vent BR, sealed box, OB or whatever, so the in-room power-response is very different.

[chrisb]

Originally Posted by Colin

Now, I'm not against horns but I presume that these days the need for a horn or horn/TL hybrid is purely to make best use of a particular drive unit, especially when operating with a single driver?

While it's not always the case, this is particularly true when higher sensitivity FR drivers are selected - such as in combination with a cherished flea-power SE amp (say a DHT 45 or 2A3).

This class of driver, ranging from budget Fostex such as FE126, to the esoteric Lowthers, AER, Feastrex - name your poison - often require the LF gain that a BLH can provide if not supported by separate woofers .

[Colin]

By reverse approach I meant the selection of enclosure dimensions as a starting point. I'm more used to reading that you start with the horn and then figure out how to fold it into a box.

[Scottmoose]

Ah, I see what you mean. Yes, I agree, designing a horn & then trying to work out a way to fold it up afterward isn't ideal. Since I tend to use the folding scheme as a functional part of the low-pass filter, it's viewed as a component throughout the design stages.