Best port location?

celef

Member
2012-11-26 7:40 pm
Is it good to locate bassreflex ports near the floor for a one meter tall two way loudspeaker or is it better to place the ports near the midwoofer? I have read that placing the ports near the floor makes the speaker position less critical, is that correct? I also want to use four small ports instead of one large, i want to arrange them so one facing forward, one facing backwards, and the other two located to either sides. I imagine that placing the ports near the end of a tall enclosure and the midwoofer at the other end makes the acoustic damping inside the enclosure tricky, hard to deal with standing waves, is that something to be concerned about?
 

Galu

Member
2018-04-17 6:50 pm
As far as acoustic damping is concerned, it should be placed on the walls so as not to impede the flow of air to the ports.

Placing a port close to the floor is likely to result in greater bass reinforcement compared to mounting it higher up on the enclosure.

A port of large diameter may be preferable to multiple ports of small diameter in order to avoid the air turbulence associated with smaller ports contributing audibly to the sound.
 

celef

Member
2012-11-26 7:40 pm
Thank you!

I am concerned about the internal top-to-bottom-resonance, around 150 hz, when having the driver and the ports on each ends. normally i use i thick piece of acoustic damping at the bottom and move the port upwards accordingly, but now there is no room for any such and that concerns me.

Yes there will be increased floor gain but that is not the reson, some loudspeaker mfr’s claims that their ports-near-the-floor makes their speakers less critical to room placement and that the low freq response gets more linear, i can not tell if this is true or just marketing. Some room acoustic experts even says that best response is normally achieved when speakers are placed well away from boundarys, i think this also means the ports?

I have heard that many small ports can handle higher velocities than a single large port of the same total area, someone made a comparison to water fountains and their nozzles for laminar flow, i do not know if that is accurate or not.
 
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Is it good to locate bassreflex ports near the floor for a one meter tall two way loudspeaker or is it better to place the ports near the midwoofer? I have read that placing the ports near the floor makes the speaker position less critical, is that correct? I also want to use four small ports instead of one large, i want to arrange them so one facing forward, one facing backwards, and the other two located to either sides. I imagine that placing the ports near the end of a tall enclosure and the midwoofer at the other end makes the acoustic damping inside the enclosure tricky, hard to deal with standing waves, is that something to be concerned about?

Near/at the floor since higher up adds an additional reflection [notch in the mid-bass].

When down firing, since the speaker/floor gap fans out/damps the vent's upper harmonics that otherwise allows us to better locate it in room.

As already noted, these add acoustic resistance that requires the vent to be longer and to keep vent mach [speed] low will require the sum total vent area to be larger than a single big one, which will further increase its length. Kind of a 'dog chasing its tail' scenario.

If the driver is at the top, then the pipe's 1/4 WL action [pipe harmonics] will be greatest, so will require significant damping for smoothest response. If the driver is offset down at the 1/3rd-2/5ths distance, the harmonics are averaged out somewhat, allowing much less damping.

In either case it's normally just enough to remove any 'hollowness'. Any more and you're just rolling off its bass response.

GM
 

celef

Member
2012-11-26 7:40 pm
Thank you!

So if i mount the midwoofer a bit down from the top, say at the 1/4 point, and the internal resonance still visible in a impedance plot, do i not need to worry about it?

Is it best to spread out the acoustic dampening evenly in the enclosure or is i better to concentrate all to one or two spots?

I think it would be handy to have a removable baffle for these experiments
 

celef

Member
2012-11-26 7:40 pm
It is surprising how much (detrimentalal) higher frequency internal box sound propagates out of the port.
I have found lining the port with pipe rubber thermal insulation quietens this unwanted Mids/HF sounds very effectively, but sealed boxes is still my preference.



Dan.

Yes, i like sealed enclosures too, but for a midsized two way the sound just gets to small for me
 

Galu

Member
2018-04-17 6:50 pm
I also want to use four small ports instead of one large, i want to arrange them so one facing forward, one facing backwards, and the other two located to either sides.
I'm interested in your thinking behind that arrangement.

Remember that low bass frequencies tend to be omnidirectional.

For example, a port on the front of the enclosure will radiate bass frequencies equally in all directions.

Provided a port is not hard up against a boundary such as the floor or rear wall, it will radiate bass frequencies equally in all directions.

If you are trying to ensure omnidirectional bass by employing the four ports as stated, then that is not necessary.
 
I also want to use four small ports instead of one large, i want to arrange them so one facing forward, one facing backwards, and the other two located to either sides.

At the risk of disagreeing with many others here...
If you're making a relatively small speaker, it should not matter much where you put the port.
But if you're making a narrow tower type speaker, then the modes triggered by a long rectangular type volume will in most cases interfere significantly no matter where you put them. If you have several small woofers, it is better to make a smaller volume for each, to shift internal modes to a higher frequency where they can be dealt with.
And unless you are very careful where you put the port, but only rely on crude calculations, you'd benefit from putting the port on the back or towards the floor.

I've tried enclosures with several distributed ports, better to just have them front OR back than on any sides IMO.

Lastly, front ports can be excellent (I'm addicted to properly designed front port enclosures, this is my first time talking in any SA (stereo addicts) meeting...), but they require a certain level of "attention to detail" to get right.
 
Can you please tell some how you did the enclosure inside with regards to damping and bracing/shelfs?

The pictures below show the internals and externals. The port tube comes up the centre and is surrounded by BAF wadding. This seems to work fine. There are no standing waves and the bass output is unaffected.
 

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If you place it too low the speaker might become a MLTL. Just to keep in mind.

I like mine downfiring, works best. Just in case, keep in mind the area from the port to the free space does not add to port length unless you rise the speaker very slightly. The standard spikes sold by most vendors will rise it too much so do not hope for shorter port due to floor extension.
 
Thank you!

So if i mount the midwoofer a bit down from the top, say at the 1/4 point, and the internal resonance still visible in a impedance plot, do i not need to worry about it?

Is it best to spread out the acoustic dampening evenly in the enclosure or is i better to concentrate all to one or two spots?

I think it would be handy to have a removable baffle for these experiments

You're welcome!

Well, at 1/4 it's still ~ same as top WRT damping requirements whereas further down at an odd harmonic [dip] much less is required.

BTW, forgot to mention that due to having a vent, the actual distance is shifted around a bit, so for a minor extra bit of accuracy [less damping required] L * ~0.349, 0.42, 0.7, 0.848 for odd and ~0.21 for the only even harmonic commonly used.

Depends on the material's acoustic properties, desired amount of damping.

During my active building 'career', 'rigid' acoustic fiberglass duct board and/or in wall, ceiling batting insulation was the norm [at least in the USA], so with my big and/or tall cabs I just lined some portion of the top, back and one side of any parallel walls.

For the very high aspect ratio pipes like common today due to drivers having much stiffer suspensions [lower Vas], lower Fs [lower box tuning] combined with folk's general aversion to the health risks of FB; loose stuffing is the norm, which usually requires being held in place with some form of netting such as cheesecloth, so can be concentrated around the driver where it's most effective overall.

FWIW, early on I chose to defy the then ~universal 'standard' of removable backs to removable front 'sub' driver, vent baffles simply because the cabs were heavy and often were placed such that they had to be moved out into the room and/or the drivers needed to be rotated periodically due to the cloth surround's sealant melting down, 'weeping' onto the mounting gasket, diaphragm, not to mention some of these now long time vintage drivers are very heavy, unwieldy, especially when moving them around in the vertical plane!

Easier overall IME to hang onto/prop up a square or rectangular baffle than a loose driver that's only a flange mount's width wider.

Regardless, IME best to have a solid cab construction with one or more removable 'sub' baffles, which will make for at least as stiff/massive [or even greater] baffle than a one piece panel and Velcro on a nice false fascia or covered grill to hide all the mounting hardware/whatever.

GM