Another try, this time giving quite different results. Short throat vs long throat:
I quit 🙂
I quit 🙂
At present, my attempt to understand what's going on with the 1/4WL distance is the air trapped by the horn throat is hard enough to reflect the sound pressure coming from the Midrange ports. That's why we don't need to measure all the way down to the compression driver diaphragm to determine where the 1/4WL notch will occur. In contrast, if you placed a speaker in front of a wall the sound pressure would travel back and scoot the air out of the way until it finally hit a hard surface. The air trapped in the horn throat has no where to go so it becomes the hard surface like the wall. And air compression might be involved as well because the simulations with different throat sizes changed the reflection point but I don't know if that is an accurate way to understand what's happening.
You can apply a lumped Element Script to feed those driven sources and use that to simulate the bandpass chambers. Varying those parameters would not need a complete resolve of the BEM only the LE.
Because a flat source in a wall is not quite the same as something of the same size driven by an LE script. The bandpass chamber can be modelled as an LE script instead of all the internal surfaces being meshed.
But does it make a difference for the examination of the notch frequency?
Sure, if you are not interested what effect the cut out has on the HF response. I am.
I think it is relevant because the filter created by the bandpass has an effect on the overall output. The more things are isolated and less like reality the less reliable the simulation becomes.
The comment was made in response to a situation where you were not simulating the effect 🙂
Ah yeah nice "imagination to sleep" stuff recently in this thread 😀
The throat reflection looks peculiar, great cancellation on-axis and less towards off-axis, but still at ~same frequency towards all axis. Referring to images here:
https://www.diyaudio.com/community/...-design-the-easy-way-ath4.338806/post-7269222
It means that path length difference of direct and reflected sound to any direction is roughly the same, but intensity of either the reflection or direct sound varies with direction. Notch, destructive interference, is strongest when interfering sounds are equal in amplitude. If either the direct sound or reflected sound is attenuated, the cancellation is less.
Waveguide makes less sound toward off-axis naturally, perhaps the reflection is not waveguided at all and is ~as loud to any direction? Perhaps its the other way around, reflection is waveguided but the direct sounds are not. Polar graphs show wider response above the first notch.
It would probably be better to have mid taps so close to the throat both direct and reflected sound get waveguided roughly the same to keep point source idea. Single sound should happen, not two (direct and reflected) with different DIs. Perhaps it doesn't matter if steep xo filters.
Well not sure if there is too much value in what makes it, the reflection, otch is there at some frequency anyway. We probably want the waveguide to be so that it makes good response for tweeter and position of the holes is more or less dictated by the mid drivers fit and response and it looks like everything is working together, crossover is possible.
The throat reflection looks peculiar, great cancellation on-axis and less towards off-axis, but still at ~same frequency towards all axis. Referring to images here:
https://www.diyaudio.com/community/...-design-the-easy-way-ath4.338806/post-7269222
It means that path length difference of direct and reflected sound to any direction is roughly the same, but intensity of either the reflection or direct sound varies with direction. Notch, destructive interference, is strongest when interfering sounds are equal in amplitude. If either the direct sound or reflected sound is attenuated, the cancellation is less.
Waveguide makes less sound toward off-axis naturally, perhaps the reflection is not waveguided at all and is ~as loud to any direction? Perhaps its the other way around, reflection is waveguided but the direct sounds are not. Polar graphs show wider response above the first notch.
It would probably be better to have mid taps so close to the throat both direct and reflected sound get waveguided roughly the same to keep point source idea. Single sound should happen, not two (direct and reflected) with different DIs. Perhaps it doesn't matter if steep xo filters.
Well not sure if there is too much value in what makes it, the reflection, otch is there at some frequency anyway. We probably want the waveguide to be so that it makes good response for tweeter and position of the holes is more or less dictated by the mid drivers fit and response and it looks like everything is working together, crossover is possible.
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Adding the chambers in full detail into the model really makes no difference to the notch.
This is for a full chamber (red) and a flat source in the wall (blue) (levels shifted for the comparison). In this case the first notch pretty much determinates the usability.
The acoustic low-pass itself seems to be pretty smooth - that's the difference between the two curves.
This is for a full chamber (red) and a flat source in the wall (blue) (levels shifted for the comparison). In this case the first notch pretty much determinates the usability.
The acoustic low-pass itself seems to be pretty smooth - that's the difference between the two curves.
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Yes. I used to do that test a lot, when crossing the compression driver straight to 8"-12" low-mids. (without any small mids)
It could make quite a bit of difference, as to how high in frequency they extended, where the port was in relation to the cone center.
As could the well known technique reducing volume of the pre-chamber (space between cone and outer horn wall), to raise the frequency of the acoustic low-pass.
The notch was always pretty easy to see on those larger drivers, as a separate dip in the acoustic attenuation (like mabat shows in #11,871)
Never bothered with the small mids on a piece of wood......
I guess I need to reduce volume under the mids on my syn10 that I've been hunting for the notch. I guess I need to raise the acoustic low-pass given, to be able to see the notch, given the proximity of the ports to the throat/CD.
Although, all this said....I'm not too much concerned with the notch, given I can't find it ! and like where the acoustic-low pass is !
Yes, thx fluid. All of what you say came flooding back to mind, the moment ChristianZoch posted.
And I had made a blind association with the term "phase-plug", not understanding PB's post.
Dementia here I come Lol
Derived wg profile:
The flare rate is is then calculated from the exponential horn equation and derived:
The highest flare rate / cut off frequency occurs at about 8 mm from the throat.
However, the flare rate is constantly changing (since it is not an exponential horn) and my guess is that the frequency where the notch ends up therefore depends on where the mid ports enters the wg in relation to the over-all wg profile.
The closer the mid ports are the wg apex, the less difference in flare rate and thus the better match between the lambda/4 and the actual notch frequency.
The further away from the apex the mid ports enters the wg, the more difference in flare rate and reflection therefore occurs more in front of the apex as could be seen from your simulations.
The flare rate decrease closer than 8 mm from the apex, and since the wavelength is about 30 cm of the frequencies of intrest I assume that the sound does not "see" this decrease. My believe is that the sound see the wg profile as if the blue curve would continue upwards from 8 mm to 0 mm (apex). This cause an almost complete reflection and helps explain why reclection can occur even if absorbent material is put at the apex or the driver is physically removed, leaving the apex open. It also explains the little difference between open and closed apex.
Does it make any sense or have I gotten it all wrong....?
BTW, notice that the pressure is still the highest right at the throat - I now doubt it can be otherwise.
(Only the midrange ports were driven in all the above examples.)
(Only the midrange ports were driven in all the above examples.)
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