Bass reflex port question

[Javachip]

Since the back wave of a driver is anti-phase to the front wave, are the sound waves coming out of a port likewise anti-phase to the waves coming off the front of the driver? If so, does bass cancellation occur with a front-ported cabinet design? I assume that sound waves produced by a passive radiator are also anti-phase to the front wave from the active driver -- does that mean that a front-mounted PR would also cause bass cancellation? Thanks in advance.

[AndrewT]

Hi,
You are right.
But the cancellation effect comes in at low frequency.

I think the reason a ported cabinet rolls off so steeply is due to this cancellation effect. I also think the rapid changes in reactance at the double hump resonance is also linked to this. And why the phase angle of a plain vented driver is so hard on amplifiers.

But just prior to this the bass is enhanced compared to a sealed cabinet.

[Javachip]

Well then, how does one decide whether to place a port (or passive radiator) in the back of the cabinet or, as I have noticed recently in some models, in front? Do ports and PR's behave similarly in terms of bass cancellation?

[Ron E]

It doesn't matter where you put the port or PR in relation to the driver, the response is the same whether it is on the back or the front or side or top or bottom of the enclosure. There are practical reasons to put it in some of these locations - front back and bottom are the most likely.... but the decision is mainly an aesthetic one.

Ports and PR's act in much the same way, but PR's have a more complicated response because they have a springiness that ports do not have.

[Javachip]

Thank you for the info. I am still confused, though, because I would think that a front-mounted port or PR would cause more bass cancellation than a rear-mounted one. The whole purpose of the baffle/enclosure is to prevent the back wave of the driver from cancelling the front wave, right?

[xstephanx]

the idea is that with such long wavelengths (say 40hz) even a difference in positioning of 3 feet can have pratically NO impact on phase cancellations of the two transducers, remember if you want the ouput of the two transducers to cancel, they would need to be a 1/2 wavelength away from each other, and a 100hz wavelength is about 3.3m long, just to give you and idea of what kinda wavelenths were talkin here.

think of your hand as a driver, the air inside the enclosure as an elastic band, and the air in the port as a weight on the end of the elasticband. if you move your hand REALLY fast and slowly decelerate until you reach 0 hz you will notice a patter like this. at first the movement of your hand will make NO difference in the place of the weight, BUT eventually it will start to move a little...and as you get closer and closer to the resonant frequency it will move more and more untill it starts moving less, and then in complete unison with your hand. This is to say that above a ports tuning the box acts like a sealed box, when you get close to and at teh port tuning, the air inside the port mvoes in and out withteh driver offering some free displacement, but below it jstu acts like a leaky sealed box, actually detracting from output by letting the reversed displacement of the air in teh port to detract from the displacement from the front of the driver.

thats not a very good explanation and the spelling is iffy but i hope it lets you understand a bit of whats going on, remember air is ELASTIC.

[clem_o]

The port is an air-mass that interacts with the box so that the resulting sound output of the port at its resonance is IN-PHASE with the driver.

In this case, it's easy to see why placing the port at the front or back of the enclosure shouldn't make any difference. If its difficult visualize, simply imagine the front of the port as a cone, which travels in-phase with the driver. As the driver moves outward from the box, so does the "cone" of the port. This in turn cause the pressure in the room to INCREASE (and the box innard becomes a vaccuum).

Now imagine the situation with the port at the back of the enclosure. As the driver moves OUTWARD from the box, so does the imaginary cone of the port - the net result - the pressure in the room INCREASED by the same amount as having the port at the front.

Ok, so much for that - now "in practice":

Having the port at the back or the front DOES in fact make some slight differences. Some authors favor having the port to the back as this reduces the so-called port noise (as air rushes out - made worse when the port is too small). OTOH having the port at the back also does cause slightly less direct-radiated bass, causing a slight drop in perceived bass augmentation.

Cheers!

Clem

[Javachip]

Clem, my passive radiator behaves in a manner opposite to your example. When I touch a 9 volt battery to the binding posts of my subwoofer, the active driver cone pops outward, and the PR simultaneously pops inward. So they appear to be anti-phase, not in-phase. So the air volume and pressure in the room and within the cabinet remain constant, except for the small lag time as the PR catches up to the active driver. This is the case with a fixed voltage direct current input; I do not know if this always remains true with an oscillating musical signal of varying voltage and frequency.

[AndrewT]

Hi Javachip,
what you found is exactly as described by Stephan.
At low frequency the output from the port is out of phase with the driver.
You tested at DC and you found it out of phase, perfect.

[clem_o]

Hi Javachip ,

Your findings are right.

The In-phase response only happens close to resonance of the port, below that (and essentially down to DC, where your battery test happens), the output of the port is phase-inverted (and thus indeed cancels the loudspeaker output).

Unfortunately it's difficult to see 40 or 50Hz cone motion, but if you could, you'd see that the PR is in-phase with the woofer...


Cheers!

Clem