Hi,
We know that at the box tuning frequency the port produces all the SPL which is 180 degrees out of phase. What if we add another enclosure tuned at the same frequency to get 360 degree phase, i.e. in phase with the active driver. Lets ignore the extra space needed for the moment. Does the idea have any merit in terms of flattening the phase?
Warm Regards,
WonderfulAudio
We know that at the box tuning frequency the port produces all the SPL which is 180 degrees out of phase. What if we add another enclosure tuned at the same frequency to get 360 degree phase, i.e. in phase with the active driver. Lets ignore the extra space needed for the moment. Does the idea have any merit in terms of flattening the phase?
Warm Regards,
WonderfulAudio
No, at Fb (frequency of box tuning) the port output is 90 degrees out of phase with the driver's front output.
At Fb, the port output lags the driver by 270 degrees, 90 degrees short of 360 degrees.
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 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".
Art
Your arrangement reminds me of the double bass reflex (DBR) enclosure:
In comparison to a standard bass reflex enclosure, the DBR enclosure is said to achieve further extension of the bass response.
I've no idea how the DBR ports are tuned, but I read that it is difficult to get this more complex reflex design just right.
In comparison to a standard bass reflex enclosure, the DBR enclosure is said to achieve further extension of the bass response.
I've no idea how the DBR ports are tuned, but I read that it is difficult to get this more complex reflex design just right.
You dont get more bass, Reflex is a Reflex a speaker does what it does.
Resonant frequency or Fs is the end, and hence the resulting impedance curve.
You can get slightly more power handling, and the same bass with a slightly smaller woofer chamber.
The port lengths can be shorter, but who cares because you need a second chamber twice as big.
Something fun to do if you want a tall tower to get a tweeter at ear level.
Then fill all the needless space from the tower with the second chamber.
A high Qts speaker wont want a small woofer chamber so use a speaker with a real magnet for .3 to .4 Qts min
Virtuix cad has a calculator for this type enclosure. 2 volumes and 2 port lengths not much different than calculating
a normal vented cabinet. Just 2 volumes to calculate when building the real thing.
Your just matching the same old .707 filter Q for a ideal volume for the speaker suspension and magnet strength.
The cutoff will be 3rd, 4th or 6th order depending upon enclosure type but the out of phase portion of the vent
that creates that cutoff is un avoidable.
You can reinvent the wheel and get more output with numerous " bandpass" type enclosures.
The fart tubes get longer and the transients get worse. Make up a name, make up a new vent type
all been done to improve efficiency but destroy transient.
Same story as always, want " More Bass" buy a speaker with low Fs and enough magnet and cone weight/suspension
to handle it. And no it wont be " Small" because bass is big
The box is big enough for .7 to .8 Qtc the port is tuned slightly above or below Fs = the end
Resonant frequency or Fs is the end, and hence the resulting impedance curve.
You can get slightly more power handling, and the same bass with a slightly smaller woofer chamber.
The port lengths can be shorter, but who cares because you need a second chamber twice as big.
Something fun to do if you want a tall tower to get a tweeter at ear level.
Then fill all the needless space from the tower with the second chamber.
A high Qts speaker wont want a small woofer chamber so use a speaker with a real magnet for .3 to .4 Qts min
Virtuix cad has a calculator for this type enclosure. 2 volumes and 2 port lengths not much different than calculating
a normal vented cabinet. Just 2 volumes to calculate when building the real thing.
Your just matching the same old .707 filter Q for a ideal volume for the speaker suspension and magnet strength.
The cutoff will be 3rd, 4th or 6th order depending upon enclosure type but the out of phase portion of the vent
that creates that cutoff is un avoidable.
You can reinvent the wheel and get more output with numerous " bandpass" type enclosures.
The fart tubes get longer and the transients get worse. Make up a name, make up a new vent type
all been done to improve efficiency but destroy transient.
Same story as always, want " More Bass" buy a speaker with low Fs and enough magnet and cone weight/suspension
to handle it. And no it wont be " Small" because bass is big
The box is big enough for .7 to .8 Qtc the port is tuned slightly above or below Fs = the end
Last edited:
Driver = +90 degrees
Port = -180 degrees
Difference = 90 - (-180) = 270 degrees
Indeed
And with David's software be able to see a more realistic model of the ports response / resonant peaks.
Including numerous horn on line type ports for Bandpass response
And with David's software be able to see a more realistic model of the ports response / resonant peaks.
Including numerous horn on line type ports for Bandpass response
See bug fix:
Hornresp Update 5750-250512
Hi Everyone,
BUG FIX
Steps to generate the error:
1. Calculate bass reflex loudspeaker combined acoustical power response
2. Select menu Tools > Output > Direct Radiator
3. Select menu Window > Phase Response
4. Capture current results
5. Select menu Window > Acoustical Power
6. Select menu Tools > Output > Port
7. Select menu Window > Phase Response
8. Select menu Tools > Compare Captured
Form title should read Hornresp - Phase Response not Hornresp - Acoustical Power:
The bug has now been fixed:
Kind regards,
David
Hi Everyone,
BUG FIX
Steps to generate the error:
1. Calculate bass reflex loudspeaker combined acoustical power response
2. Select menu Tools > Output > Direct Radiator
3. Select menu Window > Phase Response
4. Capture current results
5. Select menu Window > Acoustical Power
6. Select menu Tools > Output > Port
7. Select menu Window > Phase Response
8. Select menu Tools > Compare Captured
Form title should read Hornresp - Phase Response not Hornresp - Acoustical Power:
The bug has now been fixed:
Kind regards,
David