Enclosure design - NOOB

[daniul]

Few more questions.
When designing an enclosure, let's say Vb=114l and Fb=38Hz. When building the enclosure, you need to build a bigger enclosure like 140l and then substract the driver volume and air vent volume or i build straight an 114l box?
And I can t really understand how triangle ports work even after a lot o reading... I want to build 4 triangle ports, 15cm each side like this, but i cant figure out the length needed for 38Hz and an air velocity under 25m/s. I looked on other designs but they doesn't specify the air velocity so I can't really figure out why they made that length.. Next week i want to build the subs, I need it for my church
Edit: I atached a photo of what I want from the B&C 15TBX100, F3 is at 37Hz, I think it s ok this enclosure for this driver.

 

[Guerilla]

Bass guitar yes 30 to 40 Hz for open strings.
You actually hear the 2 nd harmonic. 60 Hz to 80 Hz
Which a pluck or picked string goes too slightly
after the initial attack.

15" cabs are tuned around 45 to 55 Hz
for bass cab and they do 60 Hz fine.
Which is what the player actually hears.

Doing live sound for many many years.
Once you put a bass player on a actual sub.
All they do is whine its too boomy.
Because they have no idea or enough note
control to handle a sub or what 40 Hz even
Feels like. Not sounds like, feels like.

For live sound bands, twack twack kick drum
or zoom zoom bass notes are 50 to 80 Hz.
Actual fretted notes and toms around 80 to 150 Hz
With modern boards I high pass both of them
because a 15" MIDBASS wont fill a room with
pressure. You hear the instrument.

Thanks!
-That was very educational to hear about your experience.

Could you elaborate a little as to which F3 is sufficient to get nice unboomy live-sound from a band with 4-string bass and a drumset? 45hz? 39, 50?

Would be nice to know when its unfeasable to aim for lower bass. Especially on a budget.
Cheers?

 

[daniul]

I tried to make an enclosure with rectangular vent on the bottom but when i tried to calculate the port length I got 2 different values... I tried using winisd and the port length formula from post #13... 2 different results... Can someone help me to calculate the rectangular port length for an Vb=100l and Fb=40Hz? I really can't get an answer

 

[techtool]

For your weight concerns consider drivers with neodymium magnets as they are much lighter than ferrite magnets. From your list the BC and Faital are neo but the Beyma is ferrite. Driver data sheets will specify the overall weight.
For the size you may also look at sealed boxes as they are usually about half the volume of vented designs and subsequently lighter. Simpler to design and build as well. A sealed box is not as efficient as a vented box but that's not usually a problem these days.

 

[DJ Stifff]

I'm not sure how higher Bl/√Re drivers will benefit Xmax limits, as these are mechanical limits for the driver. Wouldn't a high Bl/√Re driver simply reach its Xmax with lower power inputs? Also, aren't higher Bl/√Re drivers usually associated with lower Qts values, which tend towards having compact vented-box enclosures with relatively high –3dB cut-off frequencies? A high-efficiency driver with a low –3dB point would normally require a very large enclosure.

The driver with higher Bl/√Re will move less above tuning frequency. but it will have a lower SPL at displacement peak too (that's why an active filter can erase this problem). The driver power graph on hornresp shows the same driver with a lower Bl takes more watts at the same voltage above tuning frequency (that's probably the reason).
But at tuning frequency the higher Bl has higher SPL.

Edit: here's a good example with the B&C 18IPAL (huge Bl/√Re ratio) in a 144,533l DBR enclosure (internal volume) with a 70v input:

VS the same driver parameters except with a Bl reduced from 24,5 to 14,5:

BTW i'll see if i can make one with a less expensive driver that can go up to 120hz.

 

[witwald]

I have looked at your example, and the fact that the driver with lower Bl/√Re is moving less above the fundamental tuning frequency simply appears to have resulted from the low-frequency SPL peak in its alignment. Higher SPL output requires larger displacement from the same size driver. Wouldn't it be the case that if the lower Bl/√Re system were to be electrically equalized to flatten its SPL peak, then the displacement curve would become more like that of the system with the higher Bl/√Re driver?

 

[DJ Stifff]

You can flatten with an active filter that reduces the amount of power above tuning frequency but the output will still be below around tuning frequency (see the driver power graph, it's the same amount of power at tuning frequency while it's way below in SPL).

Higher Bl/√Re means stronger motor so also more displacement (which is more easily seen with smaller drivers on the displacement graph) around tuning frequency and then more SPL.

 

[GM]

BTW i'll see if i can make one with a less expensive driver that can go up to 120hz.

The box only loads the driver to its upper mass corner (Fhm), i.e. 40 Hz Fs, 120 Hz Fhm:

Qts': 2*Fs/Fhm = 2*40/120 = 0.67, so good for going less expensive, but if wanting a low(er) Qts', then either raise Fhm or lower Fs.

Fhm = 2*Fs/Qts'

Fs: Fhm*Qts'/2

(Qts'): (Qts) + any added series resistance (Rs): http://www.mh-audio.nl/Calculators/newqts.html