Therefore it's best to tune a filtered bass reflex system very low and equalize the falling slope. An eventual added high pass with the resulting group delay could then be below all useful music frequencies.
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And with all this filtering and equalizing it may be questionable why not go for a sealed, equalized system with just a bit more Sd ...
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The group delay thing has been chewed on over and over. Audibility however in normal listening rooms and for low frequencies is questionable as no research has proven otherwise. As opposed to level variations btw. Like the ones that exist in listening rooms with normal speaker systems and no LF EQ.
That’s true ONLY for frequencies within the passband tuning area of the port…..if the system is tuned to 35hz and your listening to stand up bass at say 75hz, its just a leaky box……..no free lunch fellas.
Remember….I’m a mix/mastering engineer by trade so I’m familiar with sloppy low end from playback systems……it makes it impossible to make to make decisions. For all of its flaws the NS10 did one thing very well…..and with good reason.
The entire system (driver, cabinet, port, box) in the bass is minimum phase, isn't it? It's easy to test audibility, if you have headphones and software (Equalizer APO, Foobar) to run an FIR filter on file playback. Simply generate the group delay linearization filter in RePhase in 'rotate' mode (inverted phase) and listen back to music.
I've always been amazed by how cavalierly loudspeaker design decisions tolerate greatly increased bass phase distortion and an arbitrarily higher frequency cutoff (i.e., -10 dB point) of ported speakers. They tolerate a greatly increased loss of deep bass response below the port frequency, as well as greatly increased potential for unloading the woofer at low frequencies--to the point of having to add electrical high-pass filters on their woofer channels.
TANSTAAFL.
Chris
TANSTAAFL.
Chris
I haven't tried, haha!
Can you explain what you mean? I'm not sure how damping is related here?
Ported speakers crush the first cycle of any plucked bass note that requires the port to produce it. On the first cycle of a bass note, there is effectively zero output from the port. So look at that frequency response graph and see where the woofer is down 6 dB without the port. That's what you get from the speaker until the port resonator gets pumped up to full output over a few cycles. All those nice frequency response graphs are the steady state infinite time response. So instead of getting that whack of the first cycle of the decaying exponential of a bass drum or bass guitar, you get a three cycles of smooth increase and then a delayed exponential decay. There are some nice LTSpice models of ported speakers where you can graph the transient response of the port and woofer to see the effect.
When looked at as an entire system, the bass reflex response may be considered minimum phase, but combined group delay alone does not characterize the separate arrival times of the driver and port output.
The compliance, or ”springiness" of the air inside the enclosure reacts with the air mass in the port in a similar manner as the air in a bottle resonates when a current of air is directed across the opening, exciting it’s Helmholtz resonance.
The Fb (frequency of box tuning) at which the box/port system resonates, is determined by the effective length and cross sectional area of the duct, the volume of the “air spring” in the enclosure, and the speed of sound in air.
Above Fb, the back wave of the bass driver’s sound emission through the port is delayed by the “air spring” and inverted in polarity.
Since the driver’s back wave is in opposite polarity with the front wave, this inversion brings the two emissions in phase, although the vent output lags by one wave period.
The “in phase” combination of the delayed port output and the driver’s front wave constructively reinforce each other. The constructive reinforcement produces higher output for a given driver excursion compared to a closed box, or a similar output with less excursion around Fb, which reduces driver distortion around Fb.
The drawback of the resonant reinforcement is time smearing, the vent output augments the driver’s output by adding a "resonant tail" to it.
At Fb, the port’s output is 90 degrees behind the driver’s front radiation, at frequencies below Fb, the vent polarity inversion reverts to 180 degrees, and the out of phase back wave cancels the front, reducing combined output at around 24dB per octave. Below Fb, the driver behaves as though in free air.
The bass reflex time smear drawback vs the requirement of double the sealed drivers and power for similar low frequency SPL is why neither sealed or ported cabinets have ever gone "out of fashion".
I use both ported and sealed low frequency enclosures, but don't cavalierly make the design decision of which to use for a given application
Art
[Siegfried Linkwitz about vented boxes] "I would say 90% of loudspeakers out there today use this concept to get a little more bass. Because if you put the resonance [of the vent] below the resonance of this driver in the box, and it seems that you get more bass. I find you now have bass ALL the time!"
I'm not so sure it is. You have a motor driven mass with spring and damper coupled by a spring (air in box) to a second mass (air in port tube). I suspect the system can't be inverted due to that second mass and all that phase shift/delay. It's been too long since I darkened the door way of a control system class for me to be sure.
Drive it hard, it will sound like a whistle. Of course, it may not be audible at your listening position if in the rear of the enclosure. I've heard leaks around screws with large displacement drivers.
The offset can occur from the temperature and pressure changing. You could literally have a coil in offset one day, and coil out the next. If we are already handicapped with a short stroke driver in a sealed box, why would you want to use up part of the travel and reduce linearity?
You can check yourself some relationships:
https://www.omnicalculator.com/physics/gay-lussacs-law
Maybe a 10-32 screw into a tapped hole on the terminal panel. Use an under head O-ring and flat washer and occasionally equalize the pressure.
The Etons have +/-5mm lin and +/-14mm excursion so they are suited. I should go calculate what 10degrees does (say18 to 28 C). I don’t anticipate any more that that range in my house.
The Etons have +/-5mm lin and +/-14mm excursion so they are suited. I should go calculate what 10degrees does (say18 to 28 C). I don’t anticipate any more that that range in my house.
You wouldn't, which is why in the classic reference text book "Acoustics", Leo Beranek states that for a closed-box "A slow air leak must be provided in the box so that changes in atmospheric pressure do not displace the neutral position of the diaphragm."
Generally "sealed" speakers are leaky enough without an additional air leak to compensate for barometric pressure, but pressure changes quickly generated from voice coil heat may require a small hole to equalize the pressure to avoid offset.
If the hole "whistles", stuffing it with dense foam can eliminate that problem.
https://www.diyaudio.com/community/...ric-pressure-in-sealed-cabinets.364107/page-6
I like tuning vented boxes lower than most, not for maximum flatness but more for closeness to the response of a closed box with equivalent volume. For example, an RSS460 in 4 ft^3 (110 litres) will be tuned to Fb = 19 Hz with an f6 of about 24 Hz or so, not far from what a closed box gets. However, the same output requires about 3 dB less drive in the bottom octave (the midband is unaffected, of course), and the cone excursion is reduced a useful amount for the octave above and the half octave below Fb. Since Fb is ridiculously low, even the pedal tone in Also Sprach Zarathustra doesn't bother the woofer much.
Doesn't deliberately tuning low and thus incurring early rolloff go against the maximum flatness principle of vented boxes? In isolation, sure. However, reasonably sized real rooms tend to have increasing gain below 70-100 Hz, to the point where most, perhaps all, of the rolloff is compensated by room gain, leading to effectively flat response at the listening chair.
Group delay for the vented box is greater than for the closed box, but only for the octave above Fb. Since the audibilty of group delay below 40 Hz is not proven, I'll take the hit in exchange for all the benefits.
Doesn't deliberately tuning low and thus incurring early rolloff go against the maximum flatness principle of vented boxes? In isolation, sure. However, reasonably sized real rooms tend to have increasing gain below 70-100 Hz, to the point where most, perhaps all, of the rolloff is compensated by room gain, leading to effectively flat response at the listening chair.
Group delay for the vented box is greater than for the closed box, but only for the octave above Fb. Since the audibilty of group delay below 40 Hz is not proven, I'll take the hit in exchange for all the benefits.
Most of commercial BR speakers are tuned around 40Hz, even towers, and have too small vent diameter to not puff with high spl.
BR ystem has hig GD down low and they do not shound "fast" in bass, eg. bass drum kicks.
Worst scenario is a small desktop/bookshelf speaker with BR, installed on a table or near wall - and this seems to be the most common installation! Another funny modern installation is t o have a mid size 2-way on a post far away from walls, to look "hifi"
Well done low tuned ported passive towers are ok however, unless used for HT with lots of very low rumble tones that will kill woofers
BR ystem has hig GD down low and they do not shound "fast" in bass, eg. bass drum kicks.
Worst scenario is a small desktop/bookshelf speaker with BR, installed on a table or near wall - and this seems to be the most common installation! Another funny modern installation is t o have a mid size 2-way on a post far away from walls, to look "hifi"
Well done low tuned ported passive towers are ok however, unless used for HT with lots of very low rumble tones that will kill woofers