Questions about subwoofers

[Briankz]

Hi,

I'm curious to know if the location of the port on a subwoofer makes a difference in the bass. Rear, front, bottom, cornerport? Is the only thing that matters is where the sub is located within the room, or does the direction of the port make a difference as well?

Thanks!

[TheEAR]

Good question,the location of the port will affect the room loading.A front driver front port design is ideal for in cabinet mounting as all the energy radiatated is on the same side.

Rear ported subs will have a light,very slight advantage as often subs are close to walls.And a port very close to the floor and rear wall will have a slight advantage over a port located further from the rear wall. Place the sub sideways and it does not matter front or rear port.

Bottom porting,the port being very close to the ground,at least one solid plane for reinforcement. I think the gains or loss depending how you view it are in the order of 1-2dB.

Anyone with some input here?

[Jay481985]

it can matter as it might change room nodes and such. The RSW line had the passive radiator in front while the active in the back. This was to reduce some room nodes and other stuff I forgot that the engineers mentioned. Also having the port and speaker in the front is nice to get the placement a bit easier as then you can placed the subwoofer closer to the wall without the ports becoming blocked (unless there is an amp that needs to be cooled)

To be honest though most port placement is not as important as placement of the actual sub. There are lots of way you can port, svs has rear ports and top ports on their cylinder style. I remember some others have bottom ports, then some have front, heck the logitech z5500 has side ports. Also then there are slot loaded like the promedia ultra and klipsch THX ultra 2 kw120 subwoofers. I would like to say that port placement can have an affect but then again everything can have an affect.

[Briankz]

It's been a tough question that I've been unable to find a definitive answer to on the web. The reason I'm curious about it is because I'm currently looking to upgrade my sub and my current one has rear ports (with a downfiring driver). When moved closer to the wall I get more bass, but if I turn the ports another direction, and the sub remains in the exact same location I lose a bit of bass. So I became concerned this would make a difference. The two subs I'm most interested in right now are both front firing, and have the ports in front as well (RW-12d, and Velodyne DPS-12). Along with that, I have a shag rug in my HT room and with the ports pretty much shooting into the floor with these two subs, will the carpet affect performance?

Thanks for the input.

Brian

[Jay481985]

not really.

[mas]

As long as ports are located within 1/4 wavelength of the lowest frequency (tuned frequency will do fine for calculation) driver source, there is no difference. Generally speaking, in most cabinets this means that a port location is generally not critical.

Regarding placement of the speaker relative to the port, the only restriction would be that the exit area have an unimpeded exit area >= to the port area in order to avoid 'loading' the port. (Note, this is not exactly what is occurring here, so please take the 2-3 sentence description with a grain of salt.)

In this respect the port is similar to a driver, with the exception that you do not generally need to worry about 90 degree surface reflections back into the speaker (note, there can be and are exceptions to this!...but instead of worrying about this, simply take it as just one more reason that corner placement is advantageous.).

Thus a speaker with a port 5 inch in diameter has an area of (pi)(r^2) = 19.6 sq.in . Assuming placement near a flat surface with a circumferential access (air can move in all directions to the side between the surface and the port), the port's circumference is equal to (pi)(d) = 15.7 in.

Therefore to allow for a minimum of equal areas, the port would need to be a minimum of 1.25 inches from the surface, as (1.25in)(15.7in) = 19.6 sq.in.

I think that you can see that this is normally not a problem, as speaker cable connections, baseboard trim, etc. normally provide for an offset sufficient for port venting. Thus, in general, avoid flush mounting that obstructs the port as well as ports that 'empty into another tuned cavity such as any enclosed space such as in a cabinet.

[TheEAR]

Thanks MAS

Some good info you posted.

[Raider]

Mas:

I find your rationale solid and it bears out my experience as well. I know we are talking subwoffer ports here, but it brings up an interesting related issue.  When I spoke to Klipsch tech support a few months ago comparing the Reference and Synergy, one of the issues was that the Synergy ports fire to the front and the Reference fire to the rear. They recommended that the RF83 be placed 18" from the wall behind it.  

I was just curious about what might be the reason for the difference between recommendations.  Perhaps diffraction of higher frequencies, or other issues?  Your thoughts and comments, as well as any contributions by Klipsch Tech Support would be appreciated.  This may be a key issue for many trying to decide whether to buy the RF83 since the depth of the RF83 combined with the 18" recommendation would mean the speaker front would have to be be over three feet away from the wall behind it, and this might preclude their use in some smaller rooms of where this might interfere with the traffic flow in the room.

[mas]

I am going to have to admit to some ignorance here as I am not intimate with some (read: most!) of the non-Heritage products.

One thought...do these speakers use passive radiators? If so, reflections from the wall can act to load the radiator and effectively cause a change in tuning.

Looking up a description it mentions having 3 rear firing ports.

It may not be a port loading issue at all. Instead the recommendation may be based upon the room gain effects of placing a speaker adjacent to a surface and thus reducing the effective volume the acoustic energy is radiated into - and thus increasing the efficiency of the bass. This is typically referred to as 'room gain'. The net result is that the frequency response can be augmented in either a positive or a negative way, as the surface will act as second mirror source, redirecting much of the energy back into the space as if there were a second device.

This characteristic may potentially occur 'several times' in placement . First, when a speaker is moved from a suspended free air position away from ALL reflective surfaces and placed, say, on the floor, this effectively reduces the volume into which the acoustic energy is radiated by 1/2, doubling the effective power (as you have the same acoustic energy now being radiated into 1/2 the volume). And again, if the speaker is then placed on the floor against the wall, the volume is reduced by 1/2 again and the effective acoustic energy is again doubled. If the speaker is then placed in the corner, the radiated volume is again reduced by 1/2 with an additional doubling of effective acoustic energy. Thus, with each reduction in volume by 1/2, the same energy is effectively increased by a factor of 2 - as the same energy is radiated into one half of the volume.

Also, when an acoustic source is placed close to a reflective surface, this surface reflection reinforce an already strong bass response and render the tonal character of the speaker as having more bass - rather like EQing the speaker in a way that is less natural sounding. (Oh, and a primary reason the bass tends to be enhanced more than the mid and high frequencies is that the polar pattern radiation of the bass energy is of a lower Q, it is less focused than the higher Q 'focused' higher frequencies, so it is more available to be reflected. The higher frequency energy, by virtue of their dispersion being focused forward is typically are not as available to be reflected, thus it is not altered as much.

Now, depending on how the speaker was originally designed, this may change the tonal character of the speaker (for good or bad - depending upon how it was designed). This room gain may be desirable or not. If Klipsch recommends a further placement from the wall, I suspect that they feel that the additional room gain is not 'needed' or desireable. But I would be curious to hear what they say.

But I would feel free to experiment. Again, generally speaking, corner placement offers many advantages acoustically (while still allowing sufficient venting of the rear ports), so I would give this a try and judge for yourself. Besides, done properly, you may not mind the additional reinforced bass. (And bear in mind, all of what is mentioned is rather independent of subsequent room acoustical issues such as standing waves/resonant room modes, etc. One thing at a time!

Ironically this same technique is employed routinely in some of the Heritage line precisely for the increase in efficiency, sensitivity and a reinforcement of the lower frequencies.

That is my guess, but I would be interested in hearing the 'official' line. []

[Jay481985]

they can move alot of air. Also I have placed them about 6 inches off the wall.... It just makes it sound boomy. If you can move them away from the wall at the same corner load them will improve the bass and reduce boominess.