I opened this tread to talk about calculations of Bass-Reflex enclosures.
Is there a differencies between results we got with simulations, and the best results for some bass unit in real? If there is differencies, why is that? And how we can correct calculations to got the best in real, without much eksperiments with different enclosure volumes, and tube lenghts for the best Fb.
Is there a differencies between results we got with simulations, and the best results for some bass unit in real? If there is differencies, why is that? And how we can correct calculations to got the best in real, without much eksperiments with different enclosure volumes, and tube lenghts for the best Fb.
Example
For example. Vifa XG18WH00-08. Data sheet.
Nominal impedance [ohm] 8
Voice coil resistance [ohm] 5.7
Sensitivity [dB] 86
Free air resonance [Hz] 34
Voice coil height [mm] 14
Air gap height [mm] 4
Voice coil inductance [mH] 0.53
Eff. diaphragm Area [cm2] 129
Moving mass [g] 11.8
Force factor [Bl] 5.6
VAS [l] 45
Qms 1.77
Qes 0.46
Qts 0.36
Optimized for 8-14 ltr
reflex enclosures
With simulations we got enclosure Vb=30 L. Vifa recommands BR 8-14 L. I am sure this smaller is better.( and higher Fb then simulated) I try it in many aplications, and it is like that. Higher Qts bigger deviation from calculation. What now? What with simulations ?
For example. Vifa XG18WH00-08. Data sheet.
Nominal impedance [ohm] 8
Voice coil resistance [ohm] 5.7
Sensitivity [dB] 86
Free air resonance [Hz] 34
Voice coil height [mm] 14
Air gap height [mm] 4
Voice coil inductance [mH] 0.53
Eff. diaphragm Area [cm2] 129
Moving mass [g] 11.8
Force factor [Bl] 5.6
VAS [l] 45
Qms 1.77
Qes 0.46
Qts 0.36
Optimized for 8-14 ltr
reflex enclosures
With simulations we got enclosure Vb=30 L. Vifa recommands BR 8-14 L. I am sure this smaller is better.( and higher Fb then simulated) I try it in many aplications, and it is like that. Higher Qts bigger deviation from calculation. What now? What with simulations ?
Hi Notax,
I plugged those values into speaker workshop and tried different alignments and I see what you mean, however I would say it is more likely that they have made an error on the datasheet than that there are errors in the calculations/simulations
If the sims are out by a fatcor of 100% there would be a lot of very dissapointed DIY'ers out there
I simulated and built a tuned enclosure for a Vifa M26WR-09-08 with a 70L enclosure (recommended between 50L - 100L by Vifa) and it worked out very close to the simulated one.
sure you will get some small differences in tuning freq due to not 100% knowing the exact box volume, but these should be minor.
Tony.
I plugged those values into speaker workshop and tried different alignments and I see what you mean, however I would say it is more likely that they have made an error on the datasheet than that there are errors in the calculations/simulations
If the sims are out by a fatcor of 100% there would be a lot of very dissapointed DIY'ers out there
I simulated and built a tuned enclosure for a Vifa M26WR-09-08 with a 70L enclosure (recommended between 50L - 100L by Vifa) and it worked out very close to the simulated one.
sure you will get some small differences in tuning freq due to not 100% knowing the exact box volume, but these should be minor.
Tony.
Data sheet parameters show average parameters for drivers, not the parameters of the one driver you have.
Parameters change once any power is applied and once the cone moves from rest.
Losses in the box and port effect the tuning.
How the port is shaped and how it is mounted (what's at the ends) effects the tuning.
If you are going to be really picky about your tuning, you need to be more careful with your measurements of driver parameters and your modeling of what you are building. A simple model will probably not be exactly right because it doesn't account for everything that exists in reality.
Parameters change once any power is applied and once the cone moves from rest.
Losses in the box and port effect the tuning.
How the port is shaped and how it is mounted (what's at the ends) effects the tuning.
If you are going to be really picky about your tuning, you need to be more careful with your measurements of driver parameters and your modeling of what you are building. A simple model will probably not be exactly right because it doesn't account for everything that exists in reality.
All very true, but with that particular datasheet it says optimised for a 8-14L box which when moddeled seems to give very sub-optimal performance based on the TS params provided...... non-flat and very poor low end extention.... I think iether the params are incorrect or the 8-14L is incorrect either way I think they made a mistake or are trying to appeal to people who want a small box rather than an optimal one
Vifa actually has a reputation for having very good driver to driver consistency which match the published specs very well.... others are not so crash hot... but yeah the best way is to measure them yourself!
Tony.
edit: wooohooo my 1000th post
either way I think they made a mistake or are trying to appeal to people who want a small box rather than an optimal one Vifa actually has a reputation for having very good driver to driver consistency which match the published specs very well.... others are not so crash hot... but yeah the best way is to measure them yourself!
Tony.
edit: wooohooo my 1000th post
wintermute said:Hi Notax,
I plugged those values into speaker workshop and tried different alignments and I see what you mean, however I would say it is more likely that they have made an error on the datasheet than that there are errors in the calculations/simulations
Its not error. If you look at the data sheet of the other units, you will find at that the other units with High Qts recommandetions for BR enclosures much smaller then got with simulations.
Re: Example
I think it is a matter of design considerations. If you compare the Cone excursion and maximum power plots of the 30lt 38Hz Fb box vs 14lt 45Hz Fb, it shows that the 30lt box is likely to bottom out easily with bass notes. Sure 30lt gives flatter and extended FR, but at the expense of high distortion (going over of Xmax quite often) and risk of damage to the driver (going over limits of excursion).
14lt box has some peaking at FR, but will better handle the bass notes with less distortion and safer. Don't overlook other design considerations like power handling while going after the deepest and flattest FR curve.
Notax said:
I think it is a matter of design considerations. If you compare the Cone excursion and maximum power plots of the 30lt 38Hz Fb box vs 14lt 45Hz Fb, it shows that the 30lt box is likely to bottom out easily with bass notes. Sure 30lt gives flatter and extended FR, but at the expense of high distortion (going over of Xmax quite often) and risk of damage to the driver (going over limits of excursion).
14lt box has some peaking at FR, but will better handle the bass notes with less distortion and safer. Don't overlook other design considerations like power handling while going after the deepest and flattest FR curve.
wintermute said:All very true, but with that particular datasheet it says optimised for a 8-14L box which when moddeled seems to give very sub-optimal performance based on the TS params provided...... non-flat and very poor low end extention.... I think iether the params are incorrect or the 8-14L is incorrect
Opposite!
I think if you made like you got with simulation, it will be deep (by frequency) but tiny (by SPL).
I think Thiel and Small did not envelope all influences at the transfer funcion, as: many kind of compresions, termodinamic changes in enclosure, turbulencies in the BR tunel, relations between Mms and mass of air in the box (inertia), etc.
Re: Re: Example
absolutely all about trade offs
Tony.
fair comment I just did a quick sim in SW and didn't look at cone excursion. I guess that is a design consideration, if you don't want outright SPL then a design that will distort at high power may not be an issue, if the speaker will only ever be run at lowish levels.Feyz said:
absolutely all about trade offs
Tony.
pinkmouse said:
Don't knock T/S modelling that much
. It depends on the software but they can include mass of air inside box and a couple more details easily, check out for an example:http://users.ece.gatech.edu/~mleach/papers/spice_electro.pdf
It is apparent to me that Vifa is recommending such box sizes because they know their drivers and with a box that gives the flattest and deepest extension, it will distort and likely get destroyed. Think about this way: do you use a tweeter at 800Hz, just because it can play down to 800Hz flatly? Same thing here. T/S modelling is not that bad to give twice the box volume of what in reality would be. The small 14lt box WILL not play as low as the box 30lt, as the T/S modelling shows correctly. But it will play music better, because will not distort as much as 30lt box. IMHO this is about it. Don't blame it all on T/S modelling, they work. But T/S models are not only there to produce FR curves, they also produce Excursion, group delay, maximum SPL, maximum power curves as well. If you just look at FR curves of T/S models, and then your woofer box sounds lousy, who is to blame?
BTW, I find it very hard to make a good and accurate measurement of ported boxes with near mic techniques. It is not easy, there are lots of pitfalls, from the mic distance to cone differences to estimating how to adjust port SPL to woofer SPL etc.
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