From: DPierce@world.std.com
Subject: Vented Box myths, was Celestion SL6S port recommended
Originator: tjk@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Mon, 14 Sep 1992 12:58:57 GMT


Tom Matthews writes:

>I agree that a tuned port (also called "bass reflex") can often be 
>the best compromise for a given driver and enclosure volume.
>
>Bear in mind, however, that a ported enclosure adds a low-Q
>resonance to the system's response.  The effect of this resonance 
>is that the energy radiated by the port is approximately
>in phase with the energy radiated by the driver for
>a particular range of frequencies.  If that range is
>carefully chosen, the port can extend the low-frequency response
>of the system.  However, below a certain frequency range, the 
>energy radiated by the port is more out of phase with the energy
>radiated by the driver; at very low frequencies the port does 
>not help, it hurts.  If the port is helpful at all frequencies
>of interest (above the designed low cut-off frequency), then
>its use involves no compromise.

No, this is not correct at all. The purpose of the port in a correctly design
reflex system is (to use Richard Small's words) to "supplant the radiation
of the woofer" over a narrow range of frequencies. In such a well-designed
system (of which the ideal 4th-order Butterworth maximally-flat alignment is
the best example), as you approach the point where the prot is becoming
active (the box resonance), the excursion of the woofer AND ITS OUTPUT
approach (and may reach, in the B4 alignment) zero! In fact, in order to
provide the maximum benefit, the port/enclosure system has to be fairly
high-Q. Low-Q port/enclosure resonances have little effect. In fact, a
port/enclosure resonance of 0 is about the same as a leaky sealed box.

This has several advantages. Because it is now the port that producing the
majority of the acoustical output, we no longer have to depend upon the very
large mechanical excursions of the woofer. This reduces distortion and can
improve the mechanically-limited power handling capability of the system.
Secondly, it is the port that now handles the output below the mechanical
cutoff frequency of the woofer (which is what normally determines the low
frequency cutoff).

The notion that a "no compromise port" can have beneficial effects over a wide
range of frequencies is not correct. The best a port system can do is provide
less than an octave of improvement.

>The subsonic motion you observe is possible because 
>the stiffness of the air in the enclosure is no longer
>restricting the cone's motion, but the system is probably
>not an effective radiator at those frequencies.

Yes, by definition, that's true.

>An analogy can be made to high-pass filters, where a sealed
>enclosure might be well modeled by as first-order response
>with some -3db frequency and a ported enclosure might be 
>modeled by a second order filter with a lower -3db frequency
>but with a steeper slope.  

Wrong! Sealed box systes are veryt nicely modeled as second order high-pass
filters, ported enclosures (vented box, bass reflex, what have you) are
quite nicely modeled by fourth order systems. Your original description
(the port adds a low frequency resonance) belies yor conclusions. A resonance
is a second-order system. In a reflex enclosure, you have two resonant systems.
The first is the driver mass/box compliance parallel resonance (the same as
in the sealed box case. The second is the box compliance/port mass series
resonance. Two second order system make for a fourth order system.

>It may well be that the right port helps a particular speaker,
>but in general the choice to port or not depends on many
>of the design parameters.

Well, with just a bit more emphasis (printed nicely and used to wrap the
head of a sledgehammer!), I would agree.

|                Dick Pierce                |
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