Thank you Haron,
Much appreciated, and yes, it will take a while to adjust.
Does a sealed enclosure present a higher impedance drive for an amp, compared to a vented type or any other for that matter?
In a vented box, the resonance of the cone and diaphragm produces a very sharp impedance rise. This is normally flattened with a Zobel, a C and R in series across the voice coil, which neutralises the reactive peak of the speaker voice coil and mechanical assembly acting together.
In a sealed box, this peak is much reduced because there is no resonant conjunction of the cone and the vented aperture. Damping is far tighter because the mass of air within the box is locked in and acts as a spring, preventing resonant overshoot. This reduces the impedance peak, as the excursion of the voice coil is far less and thus back EMF is less. Thus the Zobel can be much milder.
Andy describes it well. In subjective terms, the vented enclosure has a flatter response, but the resonant peak makes the bass somewhat muddy, less defined, more 'one note'. The sealed box, however, needs more and more power with reducing frequency, and the slope, which is around 12dB/octave, starts around 2 x fs. But it's more musical - swings and roundabouts.
Some here know more about the speaker mechanisms, and might like to correct/add to my comments.
As an aside, we might well ask what the problem is with changes in speaker impedance. Seldom do I see explanations of why this important.......
Rises and falls of impedance on a driver reflect the inductance of the voice coils, the phase changes introduced by the passive crossover, and the back emf induced in the voice coils by reflected mechanical energy finding its way back into the system after the initial event. This is highly complex, in a real and an imaginary sense, if you get my drift (picture the 'imaginary' resistance of an Argand diagram used to depict reactive voltage and current flow). However, for an amp using global negative feedback, changes in impedance are very serious.
Impedance is AC resistance, and it is a vector, described by both voltage and current, which act at an angle to each other, called phase. Picture a resistor. As voltage across it varies, so too does current, and they are in synchrony. A cap is different, however. When we apply voltage to a discharged cap, current is immediately very high as it charges, but voltage across it builds slowly as it charges. Thus we say that current leads voltage; current is maximal when voltage is minimal, and vice versa, since when it's fully charged, current through it is zero. An inductor has precisely the opposite effect; a large current change through it causes a large voltage to appear across it; this is the principle of spark ignition systems in automobiles; the current through the coil is interrupted, a large voltage appears by Lenz' law across the primary, and the transformer action of the secondary is then used to apply a very large voltage across the sparking plug.
When the phase angle of a feedback amplifier is changed, as it is at the speaker terminals by the reactive nature of the transducer, the feedback mechanism is subtly modified. Feedback amps, of which both the AKSA and the Lifeforce are examples, require overall negative feedback to operate correctly, and this means that the feedback must be exactly 180 degrees out of phase with the input. If this requirement is altered by a reactive speaker, then what was negative feedback can become positive, and this causes massive, irreversible and destructive positive feedback, which must be avoided at almost any cost.
Thus while some phase shift is acceptable, large changes in speaker impedance is asking for trouble, and an attempt to flatten the impedance curve will reduce this phase shift, making the speaker generally easier to drive. In subjective terms, a well designed speaker will return much better spatial characteristics, that is, the imaging will be much superior.
There are some speakers in the market which show alarming impedance changes across the audio range. One notable speaker I know of ranges from 4 ohms to 42 ohms, an absurd range, and yet costs thousands of dollars. This sort of poor design is quite common.
I hope this goes part way to explaining the question, Haron, but one could fill a book on this stuff....
Cheers,
Hugh
