You are hero of the internet today for actually correcting your post and admitting you were wrong. 🥇🏆
Accurate points. However I suggest there is less need for boost higher in the frequency range above the tuning since excursion will naturally be lower (and port Q is typically more damped and broad IRL than sims) and what goes on below depends a lot on tuning, source material and system design / dsp etc..And now show the same measurement with a 40Hz signal.
Of course does the port help arpund the resonance frequency - but ONLY around the resonance frequency! Doesn't do anything above and it's horrible below without protection filter.
Turning to the original topic I certianly see why ports are more popular than sealed in the most ubiquitous 2-way stand-mount or small floorstanding speakers. In that format, even with DSP and class-D amps there is great benefit to the efficiency boost of a port.
I don't trust indoor measurements of the direct radiator and port stitched together....they never seem to match up very well with what I measure outdoors.
But here' one, (my latest sub that I recently posted about over in subs), that I'm getting very clean measurements indoors with mic 4" from sub up off the floor, that match outdoors at 4m with mic on ground.
Green trace is mic centered directly between drivers and port (as shown in pict), and is the one that matches outdoors at 4m.
Blue is mic raised to level w drivers' centers.
And red is mic lowered to be centered on port.
1/24th smooth, no gate,
(hard to read mag scale on left ....sorry still learning new software)
I'm guessing this sub has good indoor vs outdoor match, is probably due to the big port so closeto the somewhat recessed drivers.
(Dunno really, but I do know I'm loving the sound.)
Anyway, I've played with sealed vs ported a lot, using these same drivers and all kinds of filter strategies to try to make apples to apples comparisons.
Personally, I think if ported is well done, sealed has no sonic advantage. And i find sealed is about twice as expensive to implement to get it to match ported's low corner and SPL.
Would also like to say, I've come to think of ported boxes as sort of 'two-ways' all on their own.
The direct radiator, and then the port.
Helps me think about where i want the port(s), what size/length, and how I expect it to work given the acoustic path length differences.
So no one in the history of audio, has built or is actually capable of building a sealed enclosure with a woofer that does not leak is your position? If so, I call bollocks to that.
If you have a paper cone, doped fabric surround and porus dustcap sure, I can say that enclosure would be leaky. However, with a rubber surround and a metal or carbon fiber cone, it is entirely possible to build an enclosure that is air tight.
A phase shift is a delay, 1/4 cycle (90°) at 40Hz is 6.25ms. It might not be constant with frequency and it cannot be referred to as "group delay" but as another way of expressing phase shift, delay is a valid term.
I'm also in the camp that thinks equating phase rotations with delay is a mistake. ...where 'delay' is a fixed measure of time.
I think it is a mistake that I accepted as true for a long time and was a source of immense confusion, for me, when trying to sort out time and phase.
I understand that delay is the change in phase with respect to frequency (dphi/domega) but practically speaking in the port/cone discussion that I snipped; it seems to me that if there is a phase shift that can be related to a time then it can be quantified as a delay.
If nothing else we proved Cunningham's Law
If nothing else we proved Cunningham's Law
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I can give David McBean's explanation from Hornresp help:
"Group Delay:
Negative derivative in milliseconds of the loudspeaker system phase response, versus frequency
in Hertz.
Group delay is a measure of the rate of change of phase with respect to frequency, and is positive when the phase slope is negative."
The word "resonance" is from the Latin resonantia "echo", or "resound".
The box/port air spring Helmholtz resonance delays the output from the back wave of the speaker by 180 degrees, bringing it in phase with the front output. Voilà, ~+6dB combined output.
The combined output of the direct radiator and port can be viewed as one group delay of the two different arrival times.
A sealed box can be viewed (roughly) as a second order (two pole) IIR (infinite impulse response) high pass filter, the combined response of a direct radiator and a port as a fourth order (four pole) IIR high pass filter.
Given enough processing time (longer for lower frequencies) FIR (finite impulse response) filters can correct the phase issues inherent in IIR filters and direct radiator speaker output, but they can't (as far as I know) make the port output appear at the same time as the direct output.
Art
Air tight for a subwoofer and a corner frequency of the pressure chamber of maybe 0,1Hz. Maybe even 0,01Hz.
But that's FAR from "atmospheric pressure changes" air tight!
I had to build a small pressure chamber for getting pressure DC values for at least a few 100 seconds ... that was way harder as I thought! A 50L wooden box ...
Test it - put a pressure sensor inside the cabinet, seal it properly and give a DC jump on the speaker. Try to keep that pressure for a minute ... report back what it needed to do so!
Here you are, woofer and port at tuning frequency of 70 Hz recorded with two mics simultaneously and normalised to the same steady-state level. I've also attached the files so you can look for yourself. Let me know what you see here!
My impression:
There is approx. 1 ms between the woofer and port rise off the line. The woofer and port exit are physically separated by about 20 cm. This is clearly not a delay equal to 180 degrees at 70 Hz, which would be closer to 14 ms. The propagation of sound over 20 cm however would only be about 0.6 ms, so there is a small delay I can not account for there.
It is true the port takes a few cycles to build up. This is clearly seen in the woofer amplitude being higher on the first cycle and then reducing as as the port builds up.
I think, we can say the combined response is minimum-phase and the system phase is simply linked to the frequency response. This does not change the fact the woofer will have to work harder on the first few cycles of any waveform until the port resonance is established. Of course, it would need to work harder all the time, without a port. I don't really see any 'time smearing' that would not also be caused by EQ'ing a sealed system.
What do you all think?
My impression:
There is approx. 1 ms between the woofer and port rise off the line. The woofer and port exit are physically separated by about 20 cm. This is clearly not a delay equal to 180 degrees at 70 Hz, which would be closer to 14 ms. The propagation of sound over 20 cm however would only be about 0.6 ms, so there is a small delay I can not account for there.
It is true the port takes a few cycles to build up. This is clearly seen in the woofer amplitude being higher on the first cycle and then reducing as as the port builds up.
I think, we can say the combined response is minimum-phase and the system phase is simply linked to the frequency response. This does not change the fact the woofer will have to work harder on the first few cycles of any waveform until the port resonance is established. Of course, it would need to work harder all the time, without a port. I don't really see any 'time smearing' that would not also be caused by EQ'ing a sealed system.
What do you all think?
With FIR the arrival time could be aligned a bit earlier at the critical frequencies giving a better average result?
Because subwoofers?
I was writing up a long explanation about how the reliance on subwoofers has changed speaker design, because subwoofers force us to use a ~100hz crossover. And then we try to push smaller drivers down to 100hz by porting them, because it doesn't seem to make sense to make a 3 way design where the 3rd driver "only" extends down to 100hz. ETc, etc.
But yeah, I blame subwoofers. That's easier to say and less controvertial!
(I'm not sure that sealed = punchy, or that ported = un-controlled. I think the "punch" comes from having a larger driver who's sole purpose in life is to play the low frequencies we perceive as punchy base, say from 50-250hz? And this type of speaker really is out of fashioned...)
well ... I was just doing something very similar!
here are my graphs, recorded with my current investigation loudspeaker, as shown here.
I used two sine burst signals at 50 Hz (bass reflex tuning frequency of my speaker), one with short fade-in and fade out cycles and one with hard start and stop (of course this includes high frequency contents, but the midwoofer has quite some frequency extension).
bass reflex measurement with faded-in and out 50 Hz sine burst:
bass reflex with hard cut sine burst. I put the original source signal as overlay at the top graph. the slight differences come from AC couling by input caps, I presume.
for comparison, the same speaker, but sealed (fb is around 100 Hz, so this is one octave below fb), again faded and hard-cut versions:
and finally, sealed speaker roughly at tuning frequency (100 Hz):
as can also be seen in @Tenson's graph, during the first cycle of bass reflex speakers the driver has an increased excursion, because the resonator does not yet provide the counteracting air pressure.
I also observed something similar with my test bandpass enclosure, see here.
apparently the well known excursion limiting effect of resonator enclosures cannot protect a driver from transient high excursions.
this, in combination with air turbulence noise might be one of the reasons many people dislike bass reflex systems, even if the increased group delay cannot be heard.
please note that the driver and port of the recorded speaker are quite near eachother. so the signal separation port/driver is not at all perfect.
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