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?
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.
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.
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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?
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?
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.
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.
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 .
I have been considering the idea of adding a helmholtz resonator to a BLH design I've been working on lately and was wondering if the resonator would add anything to the horn's impedance outside its "working" freq range.
The reason I ask is because designing a helmholtz resonator for a certain frequency and BW is rather easy with a lot of online calculators available, but a potential pitfall would be the resonator's effect on the rest of the horn's BW.
Furthermore, would there be an ideal placement for the resonator port along the horn length (pressure maximum, or velocity maximum of the resonator's design freq)?
The reason I ask is because designing a helmholtz resonator for a certain frequency and BW is rather easy with a lot of online calculators available, but a potential pitfall would be the resonator's effect on the rest of the horn's BW.
Furthermore, would there be an ideal placement for the resonator port along the horn length (pressure maximum, or velocity maximum of the resonator's design freq)?
Since a back-horn is by definition a relatively narrow-BW gain device, I wouldn't be too concerned over the former. It's unlikely to have an negative impact. For the rest, to an extent it depends what it is you're trying to do with it, but again, it's not likely to so severely affect its operation as to negate its use, so I don't think you need to be overly concerned about it.
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Thanks for weighing in.
I'll be using the resonator to get higher low pass slope to the horn at its higher cutoff frequency, and hoping to tame any higher frequency resonances using stuffing.
The design (a very compromised horn profile) calls for a horn working between 40 and 150hz, and the first resonance happens at around 500hz (this will need to be ironed out using stuffing
Possibly not, if you get your acoustic low-pass right in the first place. Leaving the drive unit aside, as you've obviously considered this, you need a reasonable low-pass slope in place anyway at 150Hz, preferably 2nd order or higher. That should put the horn output ~20dB down at 500Hz in itself. You can then add a Helmholtz at or slightly above your chosen Fh depending on phasing etc. which should increase the slope and place the resonance you are concerned about at a sufficiently low level that you may not need any damping, beyond the usual. YMMV of course & it will depend on the low-pass slope you start out with.
FWIW, I haven't tried measuring one myself, but AFAIK the physics of the situtation are thus: The internal Helmholtz resonanater is effectively an acoustic notch-filter. If you measure the output of a horn containing one, you will likely discover that it resembles a Cauer filter, with the internal resonantor helping provide a steep initial rolloff, but above this, a secondary post-notch peak since it (the internal absorber) is a narrow band device. Therefore what you need to do is ensure that the rolloff is steep enough to shunt this peak sufficiently low in level to be unobtrusive. Not too hard in itself, but it's not a panacea, and you need to select the XO frequency carefully because you don't want this secondary peak to coincide with an existing peak in the output, since when combined, the levels may be increased sufficiently to be audible.
FWIW, I haven't tried measuring one myself, but AFAIK the physics of the situtation are thus: The internal Helmholtz resonanater is effectively an acoustic notch-filter. If you measure the output of a horn containing one, you will likely discover that it resembles a Cauer filter, with the internal resonantor helping provide a steep initial rolloff, but above this, a secondary post-notch peak since it (the internal absorber) is a narrow band device. Therefore what you need to do is ensure that the rolloff is steep enough to shunt this peak sufficiently low in level to be unobtrusive. Not too hard in itself, but it's not a panacea, and you need to select the XO frequency carefully because you don't want this secondary peak to coincide with an existing peak in the output, since when combined, the levels may be increased sufficiently to be audible.
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Good question. Correct me if I'm wrong, but I suppose the shape of the hole doesn't matter provided it tunes the chamber to the correct frequency.
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