I think Andy is essentially saying the port with a narrower center cross-section will have higher loss / more damped. A port with an unusually narrow center will also have unusually high damping, thus SPL will be lower at tuning. This might be a reason to avoid such a design.
In a 2-way, it may not be possible to shorten the port enough to push pipe resonance out of band. You'd need a large cabinet.
Actually, I think this mostly a problem in just 2-way systems.
A 3-way system is often already bigger, plus you also cross lower.
Typically around 300-800Hz.
I think we were to a fair extent at cross purposes since I was referring to what happens when similar ports differ in size/narrowness. Stv I think was referring to ports that have significantly different shapes/discharge coefficients as well as one being smaller/narrower than the other.
My point was mainly that the oscillating (resonant) flow, not forced by pressure, behaves very different from a pressurized flow through a pipe.
in essence I try to prove that a high velocity flow is not lossy because of "friction" at the tube walls but because of turbulent flow at the terminations.
Thus the problem could be solved by reducing the velocity at the terminations (even if the narrow middle port section keeps the high velocity).
I will not (yet) state that a narrow middle port section is lossless.
But it surely has several other benefits that would justify some losses:
But as I said already, there are only very special cases where such a very narrow middle section port is feasible (very small low Vas woofers tuned low).
I'll see if I can find such an application.
in essence I try to prove that a high velocity flow is not lossy because of "friction" at the tube walls but because of turbulent flow at the terminations.
Thus the problem could be solved by reducing the velocity at the terminations (even if the narrow middle port section keeps the high velocity).
I will not (yet) state that a narrow middle port section is lossless.
But it surely has several other benefits that would justify some losses:
- lower enclosure resonance transmission
- reduced port length
- probably increased flow resistance for very low frequencies
But as I said already, there are only very special cases where such a very narrow middle section port is feasible (very small low Vas woofers tuned low).
I'll see if I can find such an application.
Talking around examples rather than in general may be wise. I had another go trying to clarify losses, turbulence,... in a general way but deleted it thinking it likely better to wait for concrete examples before trying to improve a specific explanation of what is going on.
Thank you @andy19191
I think I owe you an apology.
I'll not post any more measurement results for now, but in fact there seems to be a sensible max air velocity around 20 m/s for middle sections of flared ports.
I'll post an update soon!
I think I owe you an apology.
I'll not post any more measurement results for now, but in fact there seems to be a sensible max air velocity around 20 m/s for middle sections of flared ports.
I'll post an update soon!
During the last weeks I made several measurements of three very different enclosures with drivers using different ports leading to very different tunings - including very low level inputs, to get a useful collection of data to evaluate.
The missing low level measurement previously led me to an overly optimistic evaluation of very small ports.
FIrst image:
Left a very big 50 liter enclosure with a used, old SONY 6.5" driver. This is an "old school" big volume low port length box built from scraps, to be used as a "subwoofer" tuned to ~45 Hz.
Center: my soon 🙂rolleyes🙂 to be completed 2-way speaker with 4" MONACOR midwoofer and 9 liter enclosure tuned to 50 Hz, already used previously for testing port geometries.
Right: a miniature speaker prototype with 3" sica midwoofer/fullrange driver, to be tuned to around 65 Hz.
back sides with port openings.
I installed a bunch of different ports into the boxes that led to very varying tunings. play dough is a very useful sealing and temporary attachment tool for these experiments!
I will post my results and interpretations during the next days - for now just a few observations:
--------------
the audibility of port lenght resonance chuffing, wide range wind noise and low frequency turbulence noise is not directly correlated to compression.
Some port geometries and (small) sizes do compress already considerably, but without annoying noises or distorsion.
Other ports with sharp edges instead create very ugly noises while still having no or very low compression.
--------------
I think that the three noise characteristics as mentioned above can be related to specific properties of ports.
My preliminary noise classification would be:
a) port lenght resonance chuffing: most annoying and noticeable, starts at very low levels. created mostly by sharp port endings without roundovers or any protruding edge (inside and/or outside). straight tubes are prone to longitudinal resonance which creates the characteristic "one note whistling", modulated by the bass frequency. As far as I see this effect is one of the main reasons for the bad reputation of vented speakers.
b) wide range wind noise: not as offensive as the single resonance pitch described above. created by high air velocity inside the port or turbulent flow outside the port terminations. this will usually happen at high levels to ports with no or low wall curvature (low NFR) and with roundovers. This noise can be acceptable and may even be desirable for some people, amplifying the sensation of "strong bass".
c) low frequency turbulent noise: this effect is not very audible at low tuning frequencies and at low levels. it only starts at a specific level, but increases very rapidly. it can get quite disturbing, because of strong "clipping" effect leading to a helicopter-like sound with strong H3 distorsion. it's caused by flow separation inside the port flare, essentially raising the enclosure tuning frequency. it's not entirelly clear to me whether the clipping is related to a vent compression/detuning effect or the driver exceeding it's Xmax. it happens with high curvature wall (flared) ports. to avoid this effect it's essential to keep the strouhal number above 1,5 - 2. If this requirement is met these ports can reproduce uncompressed and very low distorsion signals.
------------
Very low tuning (far below driver resonance frequency) leads to very low QL and subsequently to quite low port output. I tried to model that effect in the hornresp simulations that I used to estimate port velocity.
There is a very strange low level compression effect with the SONY driver and low tuning frequencies, something I have not yet encountered before. This leads to "inverse compression" with increasing signal, up to a certain limit. I have to admit, it's probably a rather cheap driver with a supposedly lossy suspension (?).
The missing low level measurement previously led me to an overly optimistic evaluation of very small ports.
FIrst image:
Left a very big 50 liter enclosure with a used, old SONY 6.5" driver. This is an "old school" big volume low port length box built from scraps, to be used as a "subwoofer" tuned to ~45 Hz.
Center: my soon 🙂rolleyes🙂 to be completed 2-way speaker with 4" MONACOR midwoofer and 9 liter enclosure tuned to 50 Hz, already used previously for testing port geometries.
Right: a miniature speaker prototype with 3" sica midwoofer/fullrange driver, to be tuned to around 65 Hz.
back sides with port openings.
I installed a bunch of different ports into the boxes that led to very varying tunings. play dough is a very useful sealing and temporary attachment tool for these experiments!
I will post my results and interpretations during the next days - for now just a few observations:
--------------
the audibility of port lenght resonance chuffing, wide range wind noise and low frequency turbulence noise is not directly correlated to compression.
Some port geometries and (small) sizes do compress already considerably, but without annoying noises or distorsion.
Other ports with sharp edges instead create very ugly noises while still having no or very low compression.
--------------
I think that the three noise characteristics as mentioned above can be related to specific properties of ports.
My preliminary noise classification would be:
a) port lenght resonance chuffing: most annoying and noticeable, starts at very low levels. created mostly by sharp port endings without roundovers or any protruding edge (inside and/or outside). straight tubes are prone to longitudinal resonance which creates the characteristic "one note whistling", modulated by the bass frequency. As far as I see this effect is one of the main reasons for the bad reputation of vented speakers.
b) wide range wind noise: not as offensive as the single resonance pitch described above. created by high air velocity inside the port or turbulent flow outside the port terminations. this will usually happen at high levels to ports with no or low wall curvature (low NFR) and with roundovers. This noise can be acceptable and may even be desirable for some people, amplifying the sensation of "strong bass".
c) low frequency turbulent noise: this effect is not very audible at low tuning frequencies and at low levels. it only starts at a specific level, but increases very rapidly. it can get quite disturbing, because of strong "clipping" effect leading to a helicopter-like sound with strong H3 distorsion. it's caused by flow separation inside the port flare, essentially raising the enclosure tuning frequency. it's not entirelly clear to me whether the clipping is related to a vent compression/detuning effect or the driver exceeding it's Xmax. it happens with high curvature wall (flared) ports. to avoid this effect it's essential to keep the strouhal number above 1,5 - 2. If this requirement is met these ports can reproduce uncompressed and very low distorsion signals.
------------
Very low tuning (far below driver resonance frequency) leads to very low QL and subsequently to quite low port output. I tried to model that effect in the hornresp simulations that I used to estimate port velocity.
There is a very strange low level compression effect with the SONY driver and low tuning frequencies, something I have not yet encountered before. This leads to "inverse compression" with increasing signal, up to a certain limit. I have to admit, it's probably a rather cheap driver with a supposedly lossy suspension (?).
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I wonder if this is similar to what I was curious about in this thread. I suspected the port at first but realised it affects closed box and also active speakers too. https://www.diyaudio.com/community/threads/ports-going-turbo.400699/
To avoid children trying it out on their own!
(And to protect poor loudspeaker drivers)
@stv thanks for verbalizing the test you did. 🙂
I am missing one additional thing, which is port asymmetry.
According Klippel this could be one of the main issues with ports as well.
Did you notice any difference with those as well?
What about a rounded edge on the outside, but a straight edge on the inside?
To summarize in general, port geometry is often just overlooked (or not even looked at and thought off at all)
I have seen systems with very expensive drivers totally failing because of this.
Bye bye beautiful low intermodulation distortion.
One of the reasons why I try to avoid it (or advice against it) for high-end/quality systems.
Especially in combination with how this thread started; the inner resonance of the port itself.
That combination makes it really hard not to bump into some issue left or right.
In all cases, the least thing we want, is rounded edges.
I am missing one additional thing, which is port asymmetry.
According Klippel this could be one of the main issues with ports as well.
Did you notice any difference with those as well?
What about a rounded edge on the outside, but a straight edge on the inside?
To summarize in general, port geometry is often just overlooked (or not even looked at and thought off at all)
I have seen systems with very expensive drivers totally failing because of this.
Bye bye beautiful low intermodulation distortion.
One of the reasons why I try to avoid it (or advice against it) for high-end/quality systems.
Especially in combination with how this thread started; the inner resonance of the port itself.
That combination makes it really hard not to bump into some issue left or right.
In all cases, the least thing we want, is rounded edges.
I was pondering this recently - in a 3-way system we often think it's best to keep the port longitudinal resonance at a freq. above the xover point to stop it being directly stimulated by the driver. This makes sense.
However, when considering port noise like chuffing, this resonance will still be stimulated purely by the air flow. Would the noise actually be less audible / objectionable if it were say 500hz rather than 1500hz ?
Perhaps these two factors are in opposition?
We have focused on amplitude of port noise in this thread, but not looked much at subjective impression of the noise spectrum.
However, when considering port noise like chuffing, this resonance will still be stimulated purely by the air flow. Would the noise actually be less audible / objectionable if it were say 500hz rather than 1500hz ?
Perhaps these two factors are in opposition?
We have focused on amplitude of port noise in this thread, but not looked much at subjective impression of the noise spectrum.
I don't follow?
The port resonance will always be there and always be destructive?
Or do you mean; if we make the port longer the port noise spectrum could change accordingly?
I wrote about testing flange inside vs. outside in post #434 (including video)
That is probably the worst thing to do, even if it is very common. I can only explain it's widespread use with aesthetic reasons (a visible roundover obviously creates less chuffing), combined with the difficulty of making a port with two flanges adjustable in lenth.
The theoretical port is a volume of air being accelerated to an oscillating movement by interior enclosure pressure variation.
So there is (must be) a transistion from pressure variation to acceleration - velocity - displacement inside and back to pressure variation at the outside.
The outside usually has already the advantage of a flange (enclosure wall) and furthermore the pressure variation is low compared to the inside. so the air can flow in direction of the port.
The inside is usually a sharp edge and because of the very high pressure variations the air flow is not directed but will pass over the sharp edge of the tube, creating turbulent flow and (worst of all) very strong resonance chuffing noises.
I am not sure wether the (presubably?) more sudden transition from air velocity to air pressure at the inside end would justify a smaller flare or if the need for reduced air speed to avoid resonance chuffing would require an even bigger flare. It seems to be sensible to make the inside and outside flares similar.
just chose a sensible port geometry and size and there will be no chuffing at all!
Generous flaring is usually enough to avoid any chuffing noise.
As far as I follow @Tenson's thought:
we'd like to push the first longitudinal port resonance above woofer passband, let's say to 1.1 kHz for a 15 cm port instead of 550 Hz for a 30 cm port.
If this port starts to make chuffing noise there will now be a "whistling" noise peak at 1.1 kHz instead of 550 Hz, which will be more audible because of human perception.
Yes that's what I was thinking. If we can produce a port that does not chuff, then having a longitudinal resonance above xover is purely a good thing, but that will depends how much displacement the driver is capable of.
I keep putting high excursion drivers in relatively small enclosures, lol. I'll try STVs calculator soon and see what size port I end up with for this 8" driver in a 20L volume. Xover will be 600Hz so would like the 1st port resonance to be around 1KHz.
I keep putting high excursion drivers in relatively small enclosures, lol. I'll try STVs calculator soon and see what size port I end up with for this 8" driver in a 20L volume. Xover will be 600Hz so would like the 1st port resonance to be around 1KHz.
There is not just one harmonic in a pipe, but series. A 30cm pipe second harmonic is same as 15cm first harmonic, so in this sense it is always better to have shorter pipe, harmonics as far from pass band as feasible. Unless, it is always only the lowest harmonic that dominates and rest doesn't happen/matter.
Any sound of the cabinet is sound of the cabinet and not sound of the music, so could/would draw attention to itself and detract fromt he music. In this sense it is critical to make noiseless port for hifi application, where careful listening might spot it and ruin the "experience". For party speaker perhaps the extra noises are part of the charm, at least attention is not so much in audio quality but in having fun in many ways.
Any sound of the cabinet is sound of the cabinet and not sound of the music, so could/would draw attention to itself and detract fromt he music. In this sense it is critical to make noiseless port for hifi application, where careful listening might spot it and ruin the "experience". For party speaker perhaps the extra noises are part of the charm, at least attention is not so much in audio quality but in having fun in many ways.
Well, yes and no
The 2nd harmonic won't be as high as the first harmonic.
The better approach here would be that you run out of cone excursion before you run into port problems.
But I totally get the question, because "hiding" it can be sometimes also be a good compromise.