Does anybody know the meathod to work out the porting for my bass cabenet?

Question

The cabanet is 29" x 31" x 14"

And the speaker is has an 18" circumfrance and is 8" deep

Aslo should i place insulasion in the back to cut down vibrating or is that for hi fi speakers and some loud speaekrs only?

Help from anyone :D

Answer 1

I cannot help you with the equations for porting your box (I'll leave that to one of the car audio, speaker builders out there) but I can speak to your cabinet and speaker type, If at all possible you'll want the particular specs. of your speaker (there are more than watts and ohm load) q factors etc. and the manuf. may actually have some ideas on their website. The cabinet should be dense or thick (using mdf and some of the like wood can help reduce resonance in your cabinet) and I would s***w and silicone the cabinet interior to insure no vibration. In the past I used scoop cabinets for our 18" sun woofers and was very happy with the output. There is alot more science to this than meets the eye.

Answer 2

I have come across web sites that talk a little bit .  Just search a little . The insulation I don't think would help with vibrations much but who knows. It would slow down the sound waves tricking the speaker into thinking it was in a bigger box than it is .  What i can remember is that some expert box builders actually use lamb skin with the wool all over the inside of the box. Crutchfield has some tips for box building you could translate to it. The length and diameter of the port all has to do with octaves and really confusing stuff.

Answer 3

Is your driver 18 inches in circumference or diameter?



You did not indicate whether the enclosure dimensions that you posted are internal or external, which affects Vb, nor did you post the calculated enclosure resonant frequency fb, the driver volume displacement Vd, the internal depth available for the vent length (depending on where you would like to place your port,) and the typical environmental conditions of your listening environment, i.e., temperature and relative humidity. You also did not mention whether you intend to use round vents or some other vent shape. Do you intend to use flared ends (HIGHLY recommended) at one or both ends?



c = velocity of sound (corrected for environmental effects*)

For example, the velocity of sound c is 1128ft/s or 13536in/s at 20°C and 50% RH.

Vd = volume displacement of driver cubic meters

fb = enclosure (box) resonant frequency, aka Helmholtz resonant frequency, in Hz

Vb = net internal volume of enclosure (box)

dv = minimum vent diameter

Sv = π(Rv)² = the total vent (or port) area

Rv = effective radius of the circular equivalent of the total vent area

L'' = the total vent correction factor. L'' = 1.46 for straight ends at both ends of the vent; 1.70 for flared, flange-style ends at both ends of the vent (best choice); and 1.58 for a vent with one straight end and one flared, flange-style vent end.



First things first you will need to calculate the minimum vent (or port) area in order to minimize acoustic compression, turbulence noise and distortion. In general you should try to use the largest vent cross-sectional area (i.e. diameter) that your enclosure can accommodate up to the equivalent diameter of the total diaphragm area (Sd) of all low-frequency drivers sharing the same box volume; in your specific case 18 inches.



                          √[(411.25)(Vd)]

     dv ≥ (39.37) ——–––––––––            (1)

                          √(fb)

Next, you can calculate the vent length using equation 4 below.



              c²(Sv)                    √(Sv)

     L = ——————  - (L'') ———  Ã‚ Â Â Â Â Â (2)

              (2πfb)²(Vb)             √ π



              c²[π(Rv)²]

     L = ——————  - L''(Rv)  Ã‚ Â Â Â Â Â Â Â Â Â Â Â Â (3)

              (2πfb)²(Vb)

              c²(Rv)²

     L = —————  - L''(Rv)  Ã‚ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (4)

              4πfb²(Vb)



As a rule of thumb you should always add at least an additional 10 to 15% to the calculated vent length, L. You should then have enough extra vent length that will allow you—when combined with the proper testing—to shorten the vent length in order to match the actual resonant frequency of the enclosure.



Yes you should line the enclosure walls with a moderately thick layer of polyester fiberfill or batting such as Acousta-Stuff™ fiberfill.



######## RESOURCES ########



Loudspeaker Design Cookbook, 7th edition

By Vance Dickason

http://www.audioxpress.com/bksprods/prod...



Loudspeaker Enclosure Ports

http://www.geocities.com/f4ier/vents.htm...



Vent Tuning of Ported Systems

http://www.troelsgravesen.dk/vent_tuning...



The Subwoofer DIY Page Port Calculations

http://www.diysubwoofers.org/misc/portca...



† Collo's DIY Subwoofer Enclosures

http://www.subwoofer-builder.com

Port Flares

http://www.subwoofer-builder.com/port-fl...

Evaluation of Noise in Flared Ports

http://www.subwoofer-builder.com/flare-t...

† Commercial Port Flares

http://www.subwoofer-builder.com/flares-...

Select AES Publications on Flared Loudspeaker Ports

http://www.subwoofer-builder.com/flare-a...



Vented-Box Loudspeaker Design with a Given Driver

http://users.ece.gatech.edu/~mleach/ece4...



*Environmental Effects on the Speed of Sound

http://www.rane.com/pdf/ranenotes/Enviro...



Steve Ekblad's Free Audio Software Page

and On-Line Audio Tools

http://www.audiogrid.com/audio/



mhSoft - HiFi Speaker Design (metric version)

http://www.mhsoft.nl/BassReflexLoudspeak...



DIY Audio Projects - Technical and Reference

http://diyaudioprojects.com/Technical/in...



Information on Thiele/Small

http://en.wikipedia.org/wiki/Thiele/Smal...



Acousta-Stuff

http://www.partsexpress.com/pe/showdetl....