Question about using 2 ports of unequal length. Why?

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I rescued a pair of Pioneer SCS13 2-ways from a footpath (/sidewalk /nature strip).
8" W + 2" T.
You can see in the attached photo that they have two ports of equal diameter, but unequal length.
Intriguing. Why did they do this?

Any ideas?

cheers

Doug
 

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It doesn't work full stop. The effect of two vents is to just combine the two vent masses into one single mass, and give one tuning frequency.
However if you block off the shorter vent you will change the overall vent tuning frequency. Whether you prefer the result is up to you, but you will have made a change to the sound.

Andrew
 
AndrewJ said:


It doesn't work full stop.
The effect of two vents is to just combine the two vent masses into one single mass, and give one tuning frequency.

However if you block off the shorter vent you will change the overall vent tuning frequency.

Whether you prefer the result is up to you, but you will have made a change to the sound.

Andrew

Hi,

a) No. it does not work like that. The short one has more effect.

b) True. The asssumption being the lowered frequency still works.
Blocking off the longer port will not make much difference.

c) True. but b) should go deeper with less boom.

:)/sreten.
 
I wonder what the effect would be on the BR impedance peaks with two ports of different lengths. Would the traditional two peaks broaden or even separate into more peaks?

Also, what if the 'shorter' port (tuned higher) also was of a slightly different diameter? Could this be used to counteract the tendency to favor that port as far as tuning goes?
 
thoriated said:
I wonder what the effect would be on the BR impedance peaks with two ports of different lengths. Would the traditional two peaks broaden or even separate into more peaks?

Also, what if the 'shorter' port (tuned higher) also was of a slightly different diameter? Could this be used to counteract the tendency to favor that port as far as tuning goes?

Visually they appear to be identical diameter, but I will measure them and report back.

Doug
 
A lot of misinformation:

To the box tuning two ports act like one, the masses just add (but in parrallel). The two ports do have different masses however. The shorter one will have less acoustic mass than the longer one. But why block either of them? Just leave them as they were intended to work.

With long ports one can have a substantial port internal resonances - those are not very long so different lengths doesn't make a lot of sense (except that if you used two ports of equal length, but then decided that the box should be tuned differently, just go ahead and change one port, no need to cut both.) Multiple ports all with "slightly" different lengths will have the acoustic mass of the average length, but each one will have a different internal resonance. This will tend to create a "port resonance" that is broader but lower in level. It doesn't work well, but it does work.

But blocking off the long port will substantially change the tuning. Blocking off either port will change it enough that I wouldn't do it.
 
I am not an expert, in fact I am still learning, but as I understand vents, the effect is that of the mass of air in the tube.

The mass of air is a function of diameter and tube length. It doesn't matter (really) if the vent is circular, square or oval, and in the same sense it shouldn't matter if there are two vents provided they are close to each other.

Now, and that is more of a question, if they were on opposite sides of the box?

I am thinking of an analogy, however.

If we dampen a moving mass by means of a spring, then the dampening effect is a function of the spring's K factor.

If we use two springs of different K, then we have a compound spring with a K calculated as an average (?) of the two.

However, could it be that the wrong combination of L factors could generate standing waves between the two springs?

Is the analogy between the vents and the springs valid?
 
Hi,

i agree to AndrewJ and gedlee, there will be no difference
in behaviour compared to either one single vent or two vents
of same length (which both would have to be inbetween both
present vents in length to get te same tuning frequency).

In fact the construction shown is counterproductive,
since the vents should be small against wavelength in
the whole working range of the box, which does not apply to
most BR Boxes in the midrange anyway.

With the given design one of the vents gets unnecessarily long,
which should be avoided.

If the box sounds OK, i would change nothing.

If the Box sounds boomy, there is the additional option
of gradually stuffing the shorter vent:

While closing the shorter vent will change the tuning radically
towards a lower frequency, gradually (slightly) stuffing or cloth
covering the shorter vent will

- lower the tuning frequency too.
- add a resistive leakage component lowering the boxes Q
- gradually supresses the upper impedance peak of the box

If the box is a boombox with a weak motor driver, there is
a slight chance to improve performance significantly towards
a more controlled impulse response, without loosing too much
bass extension.

Just a simple experiment without analysing the TSP of
the design.

Cheers
 
sreten said:


Hi,

Really ? I'd suggest you are adding more.

It does not stand up to the analysis of a very short and a very long port.
(of equal area)

:)/sreten.

Hi sreten,

i am sorry, but to me ALL of Mr. Geddes statements concerning
the dual port issue are simply correct.

A broadening of the resulting port internal
lamda/2 (and higher order) resonance(s) may be claimed
as an advantage of using two ports of different length.

And i may supplement my statement from above:
The design is advantageous, if you cannot avoid using the
box in the internal resonance range of the ports.

I had good experience with "length smeared" ports as well
in the past, which is basically the same as using "multiple"
ports of differing length.

But do not expect miracles occuring when using that trick.
Even in that point i have to agree to Mr. Geddes from experience.

We should not confuse the helmholtz resonance at the tuning
frequency fb with the internal pipe resonance of the port, which
occurs usually very much higher in the midrange.

If you'd have only the shorter port, the mass reactance
DECREASES by adding the longer port even though the
longer port has a higher mass reactance compared to the
shorter one. This is what Mr. Geddes meant by paralleled mass:
In the equivalent circuit, the mass reactances of both
vents are in parallel causing the resulting mass to decrease and
the tuning frequency to be lifted up.

There is no myth in these issues and no reason for confusion
or misunderstanding at all.

Kind regards
 
LineArray said:


Hi sreten,

i am sorry, but to me ALL of Mr. Geddes statements concerning the dual port issue are simply correct.


Kind regards

Hi,

They may be "correct" but they are also misleading ......

To the box tuning two ports act like one, the masses just add (but in parrallel).

What does this actually mean ?

It means the short one has most effect, the moreso the bigger the difference.
Widely different you can simply ignore the longer one.

Blocking the short port has the most effect.
Blocking the longer one has less effect.
Both drop the port frequency, which is usually not a bad thing ....

:)/sreten.
 
Hi sreten,

you were faster than my last edits.

What you say is not wrong, but lowering the tuning frequency
in a radical manner is counterproductive if the alignment
was OK before ... what we DO NOT KNOW given the example.

So your suggestion is a bit like drastic treatment if you do not
know how well the alignment of the given box works.

My "slight stuffing" approach aims into the same direction and
includes your approach: Heavily stuffing is closing - isnt it ?

Kind regards
 
Hi,

There is no harm in trying stuffing the shorter port.
Stuffing the longer one instead IMO will not change things much.

The short port looks too short to me, handwaving though .......

There is no myth in these issues and no reason
for confusion or misunderstanding at all.

Yes there is.
Unequal but near is a bit pointless, it is a lot better
to deal with a problem at source than ameliorate it,
such as the port self resonance frequency.
Widely spaced unequal ports are a complete waste of time,
and the longer useless port takes up a lot more box volume.

:)/sreten.
 
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