Cogito wrote:
Tom,
I am confused because, the spread sheet you gave me hows the resonance frequency going down as the volume of the sealed box increases. I am equating the resonance frequency with bass response. Chris's modeling shows there is not particular correlation between volume an f3.
Which is more relevant for determining bass response, resonance frequency fs or f3?
BTW, I found references online that xmax 7mm in each direction or 14mm total.
Shashi:
The resonant frequency does in fact go down to a point where it intersects with Fs or the resonance in free air. Resonant frequency only is an indicator of the degree of low extension, not the quantity of bass. Resonance is only the point where the totol mass of the moving parts and the total stiffness or inverse of compliance create a natural increase of movement of the cone because of the combination of those factors storing energy on each cycle. Think of a car with no shock absorbers (dampers) bouncing down the street after hitting a bump at a constant frequency.
I disagree with Chris on his assessment of changes of resonant frequency based on Q. Resonance is only a factor of mass and compliance. The volume of air inside the box is part of that compliance. Q is the behavior of the driver at resonance and around that point. It is easy to make the assumption that Q affects resonant frequency because both the resonant frequency and Q change as the box volume changes.
I agree with Chris that the term cutoff frequency refering to F3 is misleading. When you cut off a water valve no more water comes out of the hose. The F3 point as he says is where the output drops to -3dB with additional output below that. The slope of the decrease depends on the type of enclosure with sealed boxes having a more gradual decrease in output than ported boxes below the port resonance. Also ported boxes unload the driver possibly leading to damaging excess excursion. A better term to use is Fb which is the resonance of the driver/enclosure combo. That leaves the slopes below resonance and the position of F3 up to the box type.
The term Q has it roots in the early days of radio where it was very important to have the ability to build tank circuits with very steep slopes to allow greater selectivity. If the circuit had a very high peak and narrow bandwidth at resonance it was said to be a very high quality circuit. Thus the term "Q" relating to the quailty of the circuit in it's ability to have very narrow bandwidth and high amplitude at resonance. The Q had nothing to do with the frequency at which the circuit resonated, only the behavior at that frequency. Although high Q circuits are very good in radio tuning, they are bad in speaker/enclosure systems because of the deviation from flat response at resonance. While generally negative, this has been used to advantage in some cases. When early tube amplifiers had output transformers that were lacking in low frequency response, a bump at resonance would add some bass back that was being rolled off by poor transformer response. That came at the expense of transient response. Also, Carver used very high Q woofers in his Amazing design to help compensate for the dipole cancellation of the panel configuration. Everything is a compromise.