Ported vs Sealed speakers bass response more than meets the ear? (1 Viewer)

[Roger Kint]

Do the ports in ported speakers produce bass that is 180 degrees out of phase with sealed speakers or ?? That is the impression that I get.

Yes, that makes sense to me. Wouldn't there be some cancellation between the port and the speaker though?

 

[Chris PC]

If the above is true, then there would be a cancelation between ported and sealed speakers I think. This means, to me, that there is some dicontinuity between ported speakers and sealed subwoofers or visa versa. But if you compare the port and driver of the same ported speaker, its different. The ported enclosure is tuned such that the driver moves very little when reproducing frequencies that are in the ports range. The wave exiting the port is tuned so that it does not "cancel out" any of the drivers waves. Its a matter of tuning. Like I said, though, since the driver hardly moves when the port is working, then the forward wave wouldn't be large enough to cancel the port wave even if they were out of phase. It would affect it, but not cancel it and as it happens, they are tuned to work a certain way.

Where can I go to learn the precise difference between ported speakers and sealed speakers in terms of bass and time frequency domains?

 

[Chris PC]

Is there anywhere on the net a comparison of ported and sealed speakers in terms of frequencies and time. Like a spectrum analysis over time or something like that? A waterfall of frequencies is good but not enough. I'm curious about the actual waveforms generated over time. It would be one comparing each speaker type

 

[Michael R Price]

OK I'll take a shot. But the article on HT-Hifi explains it very very well.

Sealed speaker: Driver reacts to signal, moves forward and back. This compresses the air in the box which tends to push the driver back to a neutral state, just like a spring. The driver on a spring is being forced to move outward or inward by the electrical signal and the spring (air pressure) is pushing the driver back.

Ported speaker: Driver reacts to signal, moves forward and back to follow the electrical signal.

Above resonance, port doesn't do much so it acts similar to a sealed box.

At resonance the port sees the air in the box vibrating at its frequency and starts resonating... pretty much by itself. Then this vibration in turn creates more pressure in the enclosure, strengthening the spring holding the driver back. The driver and port are in phase.

Below resonance the port can't help but oscillate like crazy. It's also out of phase with the driver. Since the output is out of phase, the back wave from the port into the box is in phase with the driver motion. The air in the box has become a very strange spring. It pushes the driver back and forth as it moves, causing high excursion and little output since the port is out of phase.

As for how long it takes for the port to 'kick in,' I don't know. I don't think it's enough time to matter. And there is little cancellation between the port and driver until you get below resonance.

 

[kevin_tomb]

The air in the port doesnt go in or out of the enclosure as much as it just sorta moves back and forth while staying in the port. For every puff of air felt moving out of the port an equal amount is pulled back in on the opposite part of the sound wave. And yes polarity matters....some speakers move backwards on the first part of a sound wave....its called absolute polarity. You are only assuming the first part of the wave is positive....it could be negative also. The speaker cone moving either backwards or forwards causes the movement of air and resulting sound waves generated.

The reason for the port is is slows the air movement back and forth and causes it to be in phase with the drivers bass output over a certain range. Yes a port cancels some of the bass over other ranges, but to a much smaller degree....mostly below its tuning range, thats why its responcse drops off at a faster rate among other things...

 

[Warren_Sc]

Try the University of Toronto. I know they have a HUGE anechoic chamber and their aerospace program has course-work in acoustics. (My Aunt worked there.)

 

[Chris PC]

Geez, thats a good idea. I was thinking of checking into that. I didn't realize they had an anechoic chamber I went there but our campus didn't have anything like that for physics! The NRC has a really awesome one of course, but its in Ottawa I believe

 

[Michael R Price]

Toronto also prohibits subwoofers in dorms. :frowning:

 

[Chris PC]

Hmm, I sense bitterness. Did this cause you to choose a different University?

 

[Ron Shaw]

The direction of cone travel depends on input signal. For a positive voltage swing out of the amplifier, the cone should move forward. For a negative swing, it should move back. If your speakers are hooked up 'in phase', that is what you get. It is totally dependent on input signal. For a system hooked up in phase, a kick drum, for instance, should result in a cone moving forward (just like the kick drum diaphragm), when standing in front of the drum. This is, of course, if the microphone and everything else in the recording chain was in phase. In a ported system, the output of the port is in phase at Fb, and out of phase below that (which is why ported systems roll off faster below Fb than a sealed box). The proper name for a ported enclosure is a 'phase inverter', more commonly called 'bass reflex'. The name says it all. You get port activity for both direction of cone travel, not just negative (backward) motion. The output of the port is delayed 180 degrees, so there is a slight delay (1/2 cycle) in port output. Above Fb, the port becomes invisible, and the box looks sealed.

 

[Chris PC]

That says it all. A sealed speaker starts a 30 hz bass sound with a compression ala forward driver movement, and a ported speaker starts the bass sound with a rarefaction with forward driver movement that excites the port but in the opposite direction, hence the rarefaction. There is more to it though, as many have eluded that ported speakers delay the onset of bass often more than one cycle.

 

[Ron Shaw]

Both sealed and ported start the same. For a positive signal swing from the amp, both cones give a forward motion, no output from port yet. As the signal starts back to zero, and then negative, both cones are still in sync, and now port activity begins. The initial positive motion of the cone, now delayed 180 degrees, is a rarefraction, which now matches the negative moving cone, so the cone and port output are in phase to each other at Fb.

 

[Chris PC]

Thats true about port and driver at Fb, and that is how the speaker enclosure, driver and port are tuned.

Both sealed and ported start the same.

I take this to mean that both speaker drivers in a ported and a sealed speaker move in the same direction over time when fed the same signal. This makes sense. It is the bass they produce which is not the same analysed over time.

So with that in mind, my point was that each speaker starts bass notes (ones in the range of the ports response) differently with the sealed sending a compression to your ears first, and a ported speaker sending a rarefaction to your ears first. Not only does your ear receive a different intitial wavefront, but the different waves must be out of synch when you analyze the frequencies which blend between the port and driver. Others here and elsewhere have said its not as simple as that though, but I think as an absolute minimum, that difference in the shape of the first wavefront is the way it should work logically. It only makes sense. Further though, some say the port wave actually takes even longer to begin propogating because it takes a few cycles before the enclosure and port begin resonating enough to produce sound at the port exit. Wierd.

 

[Patrick Sun]

Ever consider the reason why sealed and ported sound different is due to the increased bass output on the low frequencies in the 20Hz range? In a sealed sub, you simply don't get that final low end octave like you do with a ported sub unless you have a prime spot that acts to boost the frequencies in the low 20Hz range for a sealed sub. The low end simply flavors the bass you here out of the ported sub over a ported one that is not doing as good a job reproducing the complete waveform if low frequency material is present in the signal.

It's not about one sub having the waves go in or out in opposite of the other type of sub, that's simply phase which is easily remedied.

 

[Ron Shaw]

I'm not sure that I understand what you are saying, Chris. Both types send compression/rarefraction wavefronts at the same time. Both drivers first move forward, sending a compression front, then they both start to move back, sending a rarefraction front. Its just the port output that isnt there on the first half cycle in a ported box, but phase is the same. Once the driver begins the second half of the waveform, the port begins to add to the total output. I have never seen any data that shows that it takes several cycles to start output. That isnt true. I think that what people hear when they come to that conclusion is just a poorly designed ported box, which can indeed sound slow and sloppy. A bass reflex enclosure (phase inverter) follows the laws of physics, and is quite predictable.

 

[Chris PC]

, sending a compression front, then they both start to move back, sending a rarefraction front. Its just the port output that isnt there on the first half cycle in a ported box, but phase is the same.

I am saying that the intitial forward motion of the ported speakers driver, in fact, most of the forward motion of the driver does not produce bass radiating out from the driver in a significant sense, at least not over the response range which the port operates.
The drivers response is not comparable to the port for the frequencies we are discussing. The port is dominant. Watch a ported speaker moving while playing a 30 hz tone, you won't see much movement. It is the port that is producing the bass and the port only produces compressions when the driver moves backwards, therefore, the first significantly audible compression produced by a ported speaker for 30 hz is not until 180 degrees after the forward motion.
Now watch a sealed speaker move. Measure the sound and match the levels. The sealed speaker will be moving more than the ported speaker. How can the forward motion of the ported speaker be responsible for the compressions you hear when it is moving much less than the sealed speaker which you have playing the same volume?
Examine the response curves of a ported speaker and you'll see that some, like the ones I am talking about which have smaller drivers, have a port which is the dominant producer of sound for the bass.
If you look at the response curve for a ported speaker, you'll see the drivers response droping from -5dB to -20dB when compared to the port. Is is for this reason that I believe you DO NOT hear the drivers forward motion as a compression. Not in the case of a speaker with a small driver which relies on a large enclosure for bass, such as the speakers I have. A subwoofer with a large 12" to 18" driver may be designed and sound different.
Does anyone else recognize this?

 

[Richard Greene]

I've never seen a discussion of exactly how a port works
but I'd like to add a few comments.
The primary audible differences between ported and sealed subwoofers, assuming a ported subwoofer with a 20Hz. port tuning frequency:
(1) The ported subwoofer will have significantly more output at 20Hz. (+6 to +8dB)
Output will be noticeably higher for about a full octave above the port tuning frequency (up to 40Hz.)
(2) A low Qtc sealed subwoofer will have more output below roughly 15Hz. but that will rarely be audible because CDs and DVDs don't have much output at 15Hz.
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Chris wrote:
"It is the port that is producing the bass and the port only produces compressions when the driver moves backwards, therefore, the first significantly audible compression produced by a ported speaker for 30 hz is not until 180 degrees after the forward motion."
******RG comments:******
I would describe this differently:
The driver vibrates the air inside its enclosure.
The air inside the enclosure vibrates the air inside the port after a slight delay.
The air inside the port vibrates the air inside the listening room.
The air inside the listening room vibrates your eardrums.
"Vibrates" means compressions and rarefactions, not just compressions.
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Chris wrote:
"Now watch a sealed speaker move. Measure the sound and match the levels. The sealed speaker will be moving more than the ported speaker. How can the forward motion of the ported speaker be responsible for the compressions you hear when it is moving much less than the sealed speaker which you have playing the same volume?"
****RG comments:****
At two octaves above the port tuning frequency both ported and sealed bass drivers will stroke about the same distance.
At one octave above the port tuning frequency the sealed enclosure driver will be stroking somewhat more than the ported enclosure driver.
At the port tuning frequency the sealed enclosure driver will be stroking a lot more than the ported enclosure driver
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Here is a URL for a website where you can see a simulation comparing maximum output of a high quality 18" subwoofer driver in a sealed enclosure versus a ported enclosure
(14Hz. box tuning frequency) in the same size enclosure:
Link Removed

 

[Ron Shaw]

Yes, a driver in a sealed box will have greater excursion than a ported box. This is due to the loading on the back side of the diaphragm in the ported box. This is why a ported box has much lower distortion numbers at Fb. Also, at Fb, you seem to think that you get more output from the port than from the speaker itself. No true. The port activity is directly related to the driver, and can never be greater. The driver is always contributing more to the output than the port is. If you are getting more from the port, where is this gain coming from? You are breaking the laws of physics. You dont get something for nothing. Because of various losses, you will actually never have equal output from the port. There are always losses, such as friction in the enclosure, port, and air particles themselves, and this is converted into heat (adiabatic energy to isothermal). Inside of a sealed box, all backwave radiation is converted from adiabatic to isothermal, except the energy that transmitts through the box walls. You can certainly have greater velocity from the port (due to smaller cross sectional area), but never more total particle movement.

 

[Chris PC]

I tried to post a picture of the response curve of a ported speaker but I have not website right now.

If you look at the graph for a ported speaker, you will see the port output is higher than the driver output for a good octave or so. Thats that. It is this part of the response that differs significantly from the response of a sealed speaker in my view. Remember, I know the speaker driver is vibrating the air in the enclosure that vibrates the air in the port which ultimately vibrates the air in and outside the port into the room. What I mean to say, is that given the same signal input, for instance, a bass sound starting with a compression, the driver in the sealed speaker moves forward and creates a compression. The ported speakers first movement is forward also, but this DOES NOT CREATE A COMPRESSION EQUAL TO THAT OF THE SEALED SPEAKER. It is the reverse motion of the ported speaker driver which moves the air in the enclosure and ultimately creates the FIRST compression which is of significant volume and that exits the port. If that isn't true, then how can the port response be greater than the driver? Certainly that is what it means. And I don't mean that the port is creating sound all by itself, I mean the sound that is exiting the port, that which is created by the motion of the driver against the air in the enclosure, is louder than the direct wave that radiates from the outside surface of the ported speaker driver, AT LEAST FOR THAT FIRST COMPRESSION. After that, the remaining forward motion of the outside driver surface is mostly in phase with the port wave.

This isn't always the case, but it is the case for a large part of the frequency response of the ported speaker, especially that which is close to Fb.

 

[Ron Shaw]

It is the reverse motion of the ported speaker driver which moves the air in the enclosure and ultimately creates the FIRST compression which is of significant volume and that exits the port. If that isn't true, then how can the port response be greater than the driver?

Well, thats because it isnt true. The port response isnt greater than the driver, as its output is directly proportional to the driver. Its physics again. You cant create enery from thin air. The driver supplies the energy, the port cannot add 'gain'.

Also, both directions of the diaphragm cause port activity, not just the reverse motion.