Typically Vb = Vas/1.44 tuned to 1.56x Fs to ensure good power handling in the 'heat of the moment', i.e. at high power thermal compression due to VC heat rise.
Power handling can require > the driver's rating once above its upper mass corner (Fhm = 2*Fs/Qts') where T/S theory peters out, so generally speaking it's good to have up to +6 dB dynamic headroom (4x low end rated power), ergo in a prosound app its Xmax is just a point to start finding what the driver can handle at continuous rated power at 'x' BW, so unless the manufacturer has provided this info it's up to you to figure it out in testing ideally to destruction if wanting the absolute max rating for a given app.
(Qts'): (Qts) + any added series resistance (Rs): http://www.mh-audio.nl/Calculators/newqts.html
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Often with advanced speaker managment there are 3 different parameters.
RMS Limit - For short transients. Often these are just stopping the ampliier from clipping.
Thermal Limit - For sustained levels where the voice coil gets heated. Has an attack & release time.
X max Limit - For limiting excursion with bass frequencies. Voltage & frequency dependent.
Of these the thermal limit would usually be the lowest voltage rating.
RMS Limit - For short transients. Often these are just stopping the ampliier from clipping.
Thermal Limit - For sustained levels where the voice coil gets heated. Has an attack & release time.
X max Limit - For limiting excursion with bass frequencies. Voltage & frequency dependent.
Of these the thermal limit would usually be the lowest voltage rating.
Im building a box for an Eminence 15 inch. So in a similar situation. My X-Max is 3mm
What I learned so far.
1. Look at the Cab recommendation files. These are box designs by Eminence for Eminence drivers.
They will typically give you 3 designs Small / Med / Large.
They share the output charts for each type. The larger the box the lower the bass. and the less power you can give it.
The smaller the box the more power you can give it. But you cant go down low. Each box comes with numbers. i.e. max power you can supply. And cut off freq filter you need to use.
The largest box I chose was safe to 100 watts with a 45 Hz high pass.
2. The max I can push my driver to without a high pass filter is about 30 watts.
3. Install Winisd and enter the TS parameters and then chk your cone excursion numbers for different watts.
Ps: I am attaching the cab file for my Gama 2s. Find one for your driver. If you send me the file I can make the project for you in Winisd or Vituixcad. Both are free s/w. To design your high pass filter use one of the free calcualtors and verify in Xsim. Or Vituix cad. I am working on a s/w where you just give it the volume and it designs a box for you. Using Golden ratio or Acoustic Ratio.
What I learned so far.
1. Look at the Cab recommendation files. These are box designs by Eminence for Eminence drivers.
They will typically give you 3 designs Small / Med / Large.
They share the output charts for each type. The larger the box the lower the bass. and the less power you can give it.
The smaller the box the more power you can give it. But you cant go down low. Each box comes with numbers. i.e. max power you can supply. And cut off freq filter you need to use.
The largest box I chose was safe to 100 watts with a 45 Hz high pass.
2. The max I can push my driver to without a high pass filter is about 30 watts.
3. Install Winisd and enter the TS parameters and then chk your cone excursion numbers for different watts.
Ps: I am attaching the cab file for my Gama 2s. Find one for your driver. If you send me the file I can make the project for you in Winisd or Vituixcad. Both are free s/w. To design your high pass filter use one of the free calcualtors and verify in Xsim. Or Vituix cad. I am working on a s/w where you just give it the volume and it designs a box for you. Using Golden ratio or Acoustic Ratio.
