PORT size vs SPEAKER size

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Mark.Clappers

Mark.Clappers

Member
Joined 2010
There are a couple of things to consider:

1. Air speed in / trough the port
2. port resonance

You should design your port to fit your application! a smaller port will have higher air speed, which can become audible. Port resonance is a function of it's diameter / length.

Try to do a sim with winISD, it shows the air speed in the port.
 
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Tomom

Member
Joined 2010
You should design your port to fit your application! a smaller port will have higher air speed, which can become audible. Port resonance is a function of it's diameter / length.
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As I am talking about large size port, air speed is not a problem; port resonance - do you mean tuning? This is not exactly what I had in mind..

port area similar to front panel area will result in port volume >> speaker volume (Vbox)
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This may be true if you are tuning 6" speaker to unreasonably low frequency...what about 12" driver in 200L box...

Hi,

Port length will always be a serious problem with large diameter ports,
and simply your heading into the realms of TL speaker design in reality.
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Sreten, are you talking about so called "pipe mode resonance" of mid frequencies? And delays caused by the length of long port? As to small port will practically "unload" driver as it is driven harder and harder, will larger port, up to size of driver, benefit driver and it's efficiency and overall sound...
 
Mark.Clappers

Mark.Clappers

Member
Joined 2010
Tomom said:
As I am talking about large size port, air speed is not a problem; port resonance - do you mean tuning? This is not exactly what I had in mind..
Click to expand...

The port itself will resonate, the frequency and amplitude of these resonances are a function of it's length / diameter. These can be good (bass extension) or bad (mid bass coloration).

You want to tune the box/port system and at the same time avoid port resonances as much as possible in the mid bass.
 
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speakerdoctor

R.I.P.
Joined 2006
classicloudspeakerservices.com
The OP may find this link useful...

tech note 14

It seems the rule of thumb for vent velocity limit is in the 30 to 35 meters/sec range.

I have a simple XL spreadsheet for calculating a variety of vent dias. knowing the resonance desired and cabinet volume. Other calculable variables are also included. It's all based on the classic Helmholtz equation. PM me if you like the file to work with in your XL software.
 
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sreten

R.I.P.
Joined 2003
Tomom said:
As too a small port will practically "unload" driver as it is driven harder
and harder, will larger port, up to size of driver, benefit driver and
it's efficiency and overall sound...
Click to expand...

Hi,

A "too small" port can be optimised to block at high level and make
the loading more like sealed, it won't "unload" the driver and can be
arranged to compensate for the driver Q dropping at high excursion.

Two wrongs don't always make a right though. Port size is generally
not an issue in big speakers and a major issue in small speakers.

rgds, sreten.
 
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dumptruck

Member
Joined 2010
As far as normal bass reflex for a 12" driver, I don't think you're usually going to see much advantage of port cross section over 300-500 cm² or so, but I guess that's what you're talking about in reference to "up to size of driver"? The only problem there is major midrange leakage ("leakage" isn't even the right word, really). If you basically just have a big pipe, you need to stuff it and end up with something like a TL, as sreten said. If the BR alignment doesn't require too much length, you can do it Onken-style, though.
 
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Tomom

Member
Joined 2010
The only problem there is major midrange leakage ("leakage" isn't even the right word, really). If you basically just have a big pipe, you need to stuff it and end up with something like a TL, as sreten said. If the BR alignment doesn't require too much length, you can do it Onken-style, though.
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In regards to midrange leakage, if crossover is set at 300Hz, would there still be a significant problem with mid lekage...? And what is Onken-style...

A passive radiator is a useful substitute, for situations where a large area vent is required, as it does not rob cabinet volume and it is effective in reducing unwanted noises from within the enclosure escaping.
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I was thinking of passive radiator, but it's area has to be much larger then radiator itself to prevent over-excursion; that would ideally mean another 2 12" holes somewhere, or 1 15" with higher excursion...
 
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Andrew Eckhardt

Member
Joined 2005
The tricky part of the onken type design is you really need to get lucky with the vent spacer thickness for a random new driver, whereas a pvc pipe starting a little long cuts off real easy until you nail a tuning. I suppose you could "tune up" an onken enclosure in a similar way, but it would be a small circus.
 
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dumptruck

Member
Joined 2010
Well you can attempt to simulate with a big end correction (maybe 2.4 or something?? I think some people have attempted to put calculators together, I've never tried it) but yeah, you'd probably have to burn through some cheap MDF, to say the least. Although... if you just simulated as two big slot ports and put your margin of error well in the right direction, you could probably get away with just sliding in different sized spacers... that wouldn't be that much of a circus :D.
 
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