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Accurate box simulations
Accurate box simulations
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Old 23rd December 2018, 09:49 PM   #11
kimmosto is offline kimmosto  Finland
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Quote:
Originally Posted by bolserst View Post
so likely easier to get going with
I really don't buy this, though it's not big secret that I'm the author of VituixCAD.
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Old 24th December 2018, 06:37 AM   #12
kimmosto is offline kimmosto  Finland
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Something to read why simulators might be different: About the ports and differences between various equations and design software calculations by J.Ahonen

I guess majority of simulators calculate for example Fb of vented box:
Cab=Vb/ro/c/c
Map=ro/Sp/n*(Lv+K*Dv)
Fb=1/2/pi/sqrt(Cab*Map)

User enters Vb, n, Dv, Lv and end correction K which is probably the main source for error along with different losses (Rab, Ral). Especially end correction K varies depending on shape and location of the port(s). Calculated Fb is typically higher than actual which leads to suspect that common default value K=0.732 is too small also for ports with single free end. Therefore default value in VituixCAD is 0.8. User is finally responsible to study and give parameters which are suitable for box and vent construction.

Lossy inductance has also some effect but that is quite easy to eliminate by measuring free air impedance response and extracting Leb, Le, Ke and Rss with suitable software, if not given by manufacturer.

Last edited by kimmosto; 24th December 2018 at 06:46 AM.
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Old 24th December 2018, 02:52 PM   #13
JMB is offline JMB  United States
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I agree Kimmo. Classic models do not really account for flare of the port well (with different degrees of flare impacting the tuning a little differently). The proximity effect (how closely the port is placed to a boundary or the number of boundaries close to the port) are also not well accounted for. I seem to remember Salvatti, Button, and DeVantier may have written something about it with some fine tuning formulae.
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Old 31st December 2018, 11:32 AM   #14
celef is offline celef
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I use 'basta'! and it has a 'le loss' feature which i use when simulate, i change the 'le value' and the 'le loss value' to track an actual free air impedance measurement curve of the woofer used. Then, when i simulate this woofer in a test cab (sonotube) i just can not get simulation to match actual impedance measurements for the woofer in the test cab regarding box q and box resonance.
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Old 31st December 2018, 11:44 AM   #15
kimmosto is offline kimmosto  Finland
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^Could you give internal dimensions of sonotube, tell something about filling and attach or upload impedance measurements in free air and boxed?
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Old 3rd January 2019, 11:52 PM   #16
bolserst is offline bolserst  United States
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Originally Posted by celef View Post
…just can not get simulation to match actual impedance measurements for the woofer in the test cab regarding box q and box resonance.
The magnitude of the impedance peak will not match unless you are using a very stiff enclosure and no damping material. Wood and cardboard enclosures are flexible enough to reduce the magnitude of the peak a significant amount. An added complication is that the suspension damping is assumed to be constant when in reality it trends slightly with frequency. The magnitude of the peak is a measure of the mechanical Q of the driver/enclosure system. Fortunately, when driven by a voltage source the SPL response is dictated by the total Q of the system which is generally dominated by the electrical Q. In short if it is the impedance magnitude you are having problems matching up, that is to be expected and not an operational concern.

The frequency of the impedance peak should match very well if you determined correct values for driver compliance (Vas), the enclosure volume(Vb), and aren’t using any damping material. Damping material can effectively increase the acoustic size of the enclosure. If you aren't using any damping and the frequency doesn’t match, either Vb or Vas is in error.

I agree with kimmosto, if you can provide your measured impedance data and enclosure dimensions it would greatly aid in resolving the discrepancy you are seeing. It would also be helpful to include the impedance data and details of how you determined the TSP of the woofer you are using for modeling.

BTW, if interested in seeing impedance measurements showing damping trends in an extremely stiff aluminum enclosure, see presentation here:
"Losses in Loudspeaker Enclosures," MOC Presentation 2011, Claus Futtrup
http://www.cfuttrup.com/about/15-Scan_Speak.pdf
http://www.cfuttrup.com/about/scan_speak.mp3 (audio of presentation)
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Old 4th January 2019, 12:47 AM   #17
olsond3 is offline olsond3  United States
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I also found Hornresp to be accurate. It showed the internal reflections/resonances that matched my measurements.
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