This room is the dedicated home theater, I think the tactile impact of those low frequencies adds something special. And while the manufacturer is fine with using the port plugs, it looks to me like this mode was not an important design element given how early the problems shows up (at about 85 dB).
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Sealed vs Ported Sub
- Thread starter caioferrari
- Start date
ernestcarl
Major Contributor
That is odd, but we are just testing to see if the sub is bottoming out after all. What test signals are you using? You could use audacity to view the spectrogram of the track the SPL and frequencies peaking if you don't have an analyzer graphical view in the chain.
I'm afraid my equipment for analyzing this problem is almost literally the seat of my pants. As I've outlined above, the problem appears using the soundtrack to Bladerunner 2049, which features a subwoofer busting intro and frankly that's the kind of thing these subs were designed for. Just not with the ports plugged, apparently.
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Out of curiosity, I performed a frequency analysis of the entire "2049" track. Below is the result. There is a very narrow, strong peak at about 17.2 Hz, followed by an even stronger peak at 34.7 Hz, one octave higher.
Interesting. I realize I'm looking at the frequency spectrum, but the issue indicating my subs are having trouble occurs in exactly one spot. I haven't timed it, but on the blu-ray this spot occurs immediately after the appearance of the WB logo in Scene 1. I do think the problem is down to the fact I was using this ported sub with both ports plugged. It appears the sub struggles to cope when used in this mode. It has no problems coping with the ports open.
Keith_W
Master Contributor
Why does the sub bottom out when the port is sealed, but not when the port is open? That's what I don't understand.
Even without EQ, excursion will be lower for a ported configuration near the tuning frequency vs a sealed box.
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Without knowing the OP playback level it's hard to ratify the issue, best will be using REW RTA capture both sealed n port level during that scene.
Most ASR members probably not using BEQ for movie playback but the graph those guys capture n created actually is quite useful to know the level on bass frequency level inside a movie,below is the Blade Runner 2049 graph from 1 of them,the dotted line is the original,the level between 20+~30+Hz is quite high might cause issues if already got a house curve in the sub channel + playback at high SPL level. Left top side is after BEQ, right top side is original can see the first few minutes is very hot at those frequencies.
Most ASR members probably not using BEQ for movie playback but the graph those guys capture n created actually is quite useful to know the level on bass frequency level inside a movie,below is the Blade Runner 2049 graph from 1 of them,the dotted line is the original,the level between 20+~30+Hz is quite high might cause issues if already got a house curve in the sub channel + playback at high SPL level. Left top side is after BEQ, right top side is original can see the first few minutes is very hot at those frequencies.
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Because when the port is open, the subwoofer can play significantly louder around the port frequency with significantly less driver excursion. That is the whole point of the port.
ZolaIII
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Yes but plugin the port dosent make it become closed compression enclosure, just remove the port reinforcement, driver works almost the same. 18 dB low self to impedance/phase crossing or anything in between to FS and afterwards extending it with Butterwort is a school example to do it and to 18 dB order. Would reform XO's the same order and to Butterwort.
@arichman what's the room length and can you show us measurements?
It's often assumed that there's "less pressure" with the port open, but that is not the case. Near Fb, there's enough pressure to almost completely stop the motion of the driver.
How a Hole-in-the-Box Works - HomeTheaterHifi.com
Even though loudspeakers utilizing bass reflex techniques (i.e., ported or passive radiator designs) are quite common, or maybe even the norm if you...
hometheaterhifi.com
It's often assumed that there's "less pressure" with the port open, but that is not the case. Near Fb, there's enough pressure to almost completely stop the motion of the driver.
How a Hole-in-the-Box Works - HomeTheaterHifi.com
Even though loudspeakers utilizing bass reflex techniques (i.e., ported or passive radiator designs) are quite common, or maybe even the norm if you...
Yep.
Fun graph of excursion of a ported speaker. This is with 105dB at the port frequency (28hz)
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It might be that in sealed mode there is a lot of EQ being applied to make up for the shortfall in low-frequency output below about 50 Hz that occurs when the ports are sealed. In the simulations shown below, the solid black lines correspond to an 80-litre 12-inch sealed subwoofer. The dash-dot lines correspond to the same driver in an 80-litre vented enclosure tuned to 22 Hz with an F3 = 23 Hz. Note that the input power level had to be increased +2 dB to get the peak output of the sealed subwoofer close to that of the vented system. There is still a big shortfall in SPL relative to the vented subwoofer between 16 Hz and 40 Hz. At 30 Hz, note how much greater is the sealed subwoofer's excursion than that of the vented subwoofer, even though the acoustic output is about 2 dB less. Keep in mind that the subwoofer may also be being called upon to simultaneously reproduce signals at 17 Hz as well as 34 Hz, which would cause the excursion requirements to become significantly greater.
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Some relevant information of which I was previously unaware regarding this subwoofer. Not really sure how seriously to take these different “tunings”, but I was using the THX EQ level setting with the ports sealed. Something I don’t understand is that this setting supposedly greatly diminishes low level output under 30 hz, but in the room I was seeing undiminished output down to 10 hz, which was the HPF setting. How is this happening if the built in EQ "tuning" is rolling off at 29 hz? But assuming the setting does what it says, then is it safe to say the sub in this mode was thoroughly unprepared to reproduce a 17 hz tone? Does this setting help point out the cause of over-excursion, ie is it the amplifier coming up short or the physical properties of the subwoofer design modeled here?
Alternative level settings for Monolith M12 V2:
- Extended EQ (Both ports open): Drops the \(-6\text{ dB}\) point down to 18 Hz, allowing you to feel ultra-low frequency rumbles in movies.
- THX EQ (Both ports open): Targets a flatter studio response, reaching down to 20 Hz.
- THX EQ (One port plugged): Delivers a deeper low-end rolloff, achieving 23 Hz at \(-6\text{ dB}\).
- THX EQ (Sealed - both ports plugged): Tunes the subwoofer for the tightest, punchiest sealed response, rolling off at 29 Hz at \(-6 dB)
melomane13
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The above measurements seem to indicate that there is a 4th-order (24 dB/octave) high-pass filter present on this subwoofer. The filter would be there to reduce the cone excursion below 20 Hz, which is especially needed for vented subwoofer systems that could be expected to get a significant amount of low-frequency energy in the 10–20 Hz frequency range.
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It's certainly a little surprising that the vented setting of the M-12 V2 subwoofer system, with its steep high-pass filtering in place, is getting acoustic output down to 10 Hz. For the sealed THX subwoofer mode, the response seems to be rolling off at approximately a 2nd-order rate, so room gain could potentially be boosting the acoustic output at the lower frequencies in the roll-off region.
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I had read that SVS specifically designed their sealed subs to take advantage of room gain to achieve a very flat frequency response down to single digits (https://www.svsound.com/blogs/subwoofer-setup-and-tuning/what-is-subwoofer-room-gain) and was frankly skeptical. But see post #72 above where I found a very similar response with the Monolith M12 v2. I preferred the sealed performance, setting aside the fact that the sub in this mode cannot cope with very low frequency material without risk of over-excursion at even moderate listening level.
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melomane13
Active Member
So, to get the best of both worlds, you just need to EQ. Take a look at the first simulation, which corresponds to the ported box with ext EQ, and then the second one, after applying a 2nd-order low-shelf filter at 36Hz, Q 0.6, -10dB.
"It's very close to the sealed response, but with the ported alignment benefits—meaning less cone excursion and lower distortion."
"It's very close to the sealed response, but with the ported alignment benefits—meaning less cone excursion and lower distortion."
I use BEQ, though I'm still not entirely sure where I stand on it. I've seen arguments ranging from actual bass restoration to additional processing that may deviate from mixer intent.
What I have learned is that BEQ makes the ported vs sealed discussion more relevant. Once you start adding significant infrasonic content, excursion and protection become important considerations.
In my system I use a permanent 16 Hz high pass filter, automated BEQ loading, about 2 dB of automatic gain reduction for titles with heavy infrasonic BEQ, and a miniDSP compressor as a safety net. That has made me appreciate the headroom and excursion advantages of ported subs for demanding movie content.
My dual HSU VTF-15Hs are both run in a one-port-open configuration. Regardless of where someone stands on BEQ philosophically, it has definitely made me think more about headroom, excursion limits, and system protection than I otherwise would have.
