Vented box impedance trace understanding

[bibio]

Hi all,

I know vented box impedance traces have been covered plenty of times, but I’m still struggling to fully square a few things, so I’m hoping someone can help clarify.
With a typical vented enclosure impedance plot you get the familiar two peaks with a dip between them. I’d always understood that the left-hand peak was associated with the port and the right-hand peak with the driver resonance. However, in an AES YouTube video it’s stated that you can’t really say which peak “belongs” to the port and which to the driver.

What’s confusing me is that when you change the port size or block the port entirely, it always seems to be the left-hand peak that shifts, while the right-hand peak stays more or less where it is. That feels like it does tell you something about which peak is which, so I’m clearly missing a nuance here.

Using the usual terminology:

• FL – left-hand impedance peak
• FH – right-hand impedance peak
• FB – box tuning frequency (the minimum between the peaks)

I’m trying to understand what you can actually take away from comparing impedance traces in a meaningful way:

• If FL, FB, or FH move left or right in frequency, what does that generally indicate?
• If the height of FL or FH changes, what does that usually point to (damping, losses, Q, etc.)?
• Does the difference in height between the two peaks tell you anything useful? For example, one peak being much higher than the other, or the two becoming more or less symmetrical.
• Finally, with speaker ageing (changes in compliance, losses, suspension softening, etc.), what would you typically expect to see happen to:
  
  FL, FB, and FH

Overall, I’m just trying to build a more accurate mental model of what changes in frequency, amplitude, and peak shape are actually telling me when looking at vented box impedance traces over time or after physical changes to the system.


Thanks in advance.

 

[Arindal]

I’d always understood that the left-hand peak was associated with the port and the right-hand peak with the driver resonance.

Theory would suggest that the port resonance does cause an impedance minimum, as the Helmholtz resonator´s combination of mass and spring reaches full efficiency at this frequency, so the port draws maximum of energy from the contracting and decontracting air inside the enclosure. So Fb is the port resonance if one follows a standard way of tuning the system.

If FL, FB, or FH move left or right in frequency, what does that generally indicate?

If the minimum at Fb moves left, the tuning frequency gets lower. Right vice versa. The peaks usually follow but that depends on the whole system.

Does the difference in height between the two peaks tell you anything useful? For example, one peak being much higher than the other, or the two becoming more or less symmetrical.

If port tuning was done according to a standard model of keeping both sources at equal SPL (like Bullock's famous charts), height in peaks should be more or less equal. If Fh peak is higher, this indicates a tuning frequency lower than the standard calculation, which in most of cases leads to a less efficient port hence reduced port output SPL. FL impedance higher Than Fh is unusual, as this means that the port resonance frequency is closer to the frequency band in which the active drivers front diaphragm is still very efficient, and the output is pretty much depending on the relative phase of the two sources.

with speaker ageing (changes in compliance, losses, suspension softening, etc.), what would you typically expect to see happen to:

No prediction, as usually several parameters (particularly Fs and Vas) are changing simultaneously, which is affecting the overall tuning of the system.

I’m just trying to build a more accurate mental model of what changes in frequency, amplitude, and peak shape are actually telling me when looking at vented box impedance traces over time or after physical changes to the system.

Impedance graph is useful to understand how and at which frequency the system is tuned, but difficult to draw any practical conclusions from if you change the tuning. Found it more useful to look at the frequency response, THD and alike to get an understanding of how much the diaphragms or the air are moving at which frequency.

 

[MJKing]

When you combine a driver with a resonant frequency fs with an enclosure like a BR or TL with a tuning frequency fb (close to fs) you get two new resonances at f_low and f_high bracketing fs and fb, the double humped electrical impedance curve. There is no longer a resonance at fs or fb for the speaker system (driver + enclosure). The lower resonance is the mass of air in the enclosure exerting pressure on the back of the cone's mass dropping the resulting frequency below fs and fb (moving mass has increased). The higher resonance is the air in the enclosure pulling against the driver cone's motion which raises the resonant frequency above fs and fb (stiffness has increased). I summarized this way of looking at the mechanical vibration behaviors of drivers in enclosures in the following presentation (in paticular slide 16).

http://www.quarter-wave.com/Project13/Part_1_The_Basics.pdf

Hope that helps,

Martin