Fostex FE208-SOL box?

[GM]

To GM:

Well, front firing won't load as smoothly as one designed to be fired in the corner till the WLs get large enough not to notice the side gaps, but our hearing acuity is so poor down low that you notice anymore than you [don't] notice how badly most rooms modify the bass line.

The horn is designed under the assumption that it will be driven by a high output impedance such as you propose, but if you don't there will be a wide band 'sag' in the driver's critical mids BW like when driven with a SS or high DF PP tube amp and tone controls won't fix it unless it has a separate mids tone control or at least a 1/3 octave graphic EQ.

GM

 

[GM]

The depicted horns are quite long but have small mouth area so the mouth area is the limiting factor for bass.

FWIW I prefer to view them as ducted port [mass loaded] TQWT alignments [ML-TQWT]. Would have to sim them, but maybe better bass 'horns' overall. It's what I use, though with a much shorter taper to suit my app, hence not as much wide band gain.

The Fostex won't have such limitations, so will need all the wide band horn gain it can get.

GM

 

[vcelkamaja]

Most basshorns both work as pipe resonators and horns. To work as a horn it has to both be long enough and have bieg enough mouth area. The depicted horns are quite long but have small mouth area so the mouth area is the limiting factor for bass. To complicate things further: a horn placed close to a corner works as horn with twise the mouth area placed at a wall. So in this case placing the horn close to a corner will make the horn go a bit deeper in the bass.

Thanks a lot for explanation - the mouth area for the bigger one is 910 cm2 (original Fostex design 1027 cm2). What happens if I just make it bigger while keeping other dimensions as they are (Bigger opening in the front panel - rectangular hole)

 

[vcelkamaja]

Any feedback please? What happens if I make the mouth area bigger while keeping all other dimensions unchanged?

 

[GM]

Do mean making the small cab's vent duct bigger? If so, it will be tuned higher, maybe enough to make it sound 'boomy'/'flabby' [under damped response].

GM

 

[DrBoar]

To be a ML-TQWT they need a severe constriction at the open end, usually a pipe/vent that gives the mass loading.
A straight pipe will have a resonance close to a frequency corresponding to a wavelength 4 times the lenght of the pipe (hence quarter wave resonator). Tapering the a pipe will lower the Q of the resonance that is widen the frequency range of the resonance. Negative taper lower the resonance a positive taper (bigger at the open end) increase the frequency. This increase can be counteracted by constricting the open end down to perhaps 50% if the surface area of the cone of the driver.

So a Pipe with a positive flare will act as a horn where the diameter of the horn mouth is a sizable fraction of the wavelength. How big depends on how the mouth is placed relative surfaces (hanging in space, on the floor freestanding, on the floor against a wall or in a corner). Below this frequency the flared pipe can still act as quarter wave resonator if it is long enough. So any bass horn of a practical size work as a hybrid with both horn features and pipe features.

Bigger mouth = more horn features in the bass. Something like the "Schmacks horn" will work like a horn a bit deeper than the Fostex designs, but then the volume is roughly twise as much.

 

[vcelkamaja]

Hi again,

coming back to this design Google Translate

there is one question coming to my mind - while the mouth area is mentioned to be 910 cm2 there is a duct in front of it - front panel make the opening area about half only ~ 450 cm2. What will be the impact on sound in this case? Is there any good reason to use this duct at the end of the 211 cm horn?

 

[Scottmoose]

It's mass-loaded (very mildly), i.e. the terminus is constricted to provide a degree of low pass filter effect, making it a variation on the ML-horn (ML-TQWT in MJK parlance) -there is no stipulation as to the degree required in order to make it fit that moniker. I haven't checked, but it probably isn't sufficient to significantly impact on Fp.

 

[vcelkamaja]

Thank you Scott. So if I understand it right the duct creates a low pass filter in front of the horn, right?

What will be the impact on sound on this BLH with and without this duct? Any audible difference? Impact on bass?

I’m wondering why one should use the duct and what is benefit of it?

 

[Scottmoose]

I can't really offer more than my above. Note that the design you are referring to is not 'with or without': it was designed to have a ducted terminus, as you can see from the internal panel that helps form the duct itself. This isn't something that has been tacked on as an afterthought or as a 'modification'. Assuming you could somehow remove it, you'd probably have more midband output (as one might expect by removing a low-pass filter ), probably a slightly raised tuning frequency & higher acoustic efficiency, although that may not be usable efficiency.

 

[vcelkamaja]

I can't really offer more than my above. Note that the design you are referring to is not 'with or without': it was designed to have a ducted terminus, as you can see from the internal panel that helps form the duct itself. This isn't something that has been tacked on as an afterthought or as a 'modification'. Assuming you could somehow remove it, you'd probably have more midband output (as one might expect by removing a low-pass filter ), probably a slightly raised tuning frequency & higher acoustic efficiency, although that may not be usable efficiency.

Thanks a lot. I have found one article describing the effect of duct damp added to the back loaded horn here:

Google Translate

 

[Scottmoose]

No mysteries here.