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| By: cplkittle on Sat, 4 Oct 02:20:29 -0500
| | There are many variations on the bandpass design, and the actual design possibilities are virtually endless. I will focus on the two more popular designs without going too deep into the isobaric setups.
The 4th order bandpass is basically a driver placed in a sealed box with an identically tuned port on the other side of the sealed section. The resulting system usually provides a lower cutoff frequency, the tradeoff being a larger enclosure. The enclosure can be reduced in size by using two drivers in an isobaric configuration.
4th order bandpass systems usually demonstrate better power handling characteristics than the other main systems considered here. Its transient response is second only to the sealed enclosure systems, making it a good choice for subwoofer applications.
As all of the output of the 4th order bandpass system is via the port, the largest port diameter possible for the enclosure should be used in order to minimize port noises. The ports should be flared whenever possible, for the same reasons.
The 4th order bandpass system rarely exhibits a perfect bandpass response - there is usually some out-of-band noise present in its output. A simple notch filter can be used to reduce this noise if it is audible. Alternatively, a low-pass filter can be used in series with the driver, but the in-band response of the system may also be affected if this approach is taken.
The 6th order bandpass system is similar to the 4th order bandpass system , except in this case both the front and the rear volumes are tuned via ports. The power handling of the 6th order bandpass system ranges from excellent within its passband to poor for frequencies lower than its passband.
The transient performance of 6th order bandpass systems is usually worse than the sealed, ported and 4th order bandpass systems, making it more suitable for sound reinforcement, multimedia and other less critical applications, rather than high-end audio. Like ported systems, the driver becomes unloaded at frequencies lower than the passband.
As with the 4th order system, the 6th order bandpass system rarely exhibits a perfect bandpass response - there is usually some out-of-band noise present in its output. As mentioned before, a simple notch filter can be used to reduce this noise if it is audible. Alternatively, a low-pass filter may be used, but the in-band performance may be affected.
There is no set measurement or equation to calculate these boxes to perfection. A lot of the box design software available online will have bandpass options, but from the research I have done on this, I have found that it is still in the area of being experimental.
For the 6th order bandpass configuration, Bose claims to have a working set equation for configuring the two different ported areas, they even have patents on these. I believe this, personally, after hearing a 3” speaker pump out extremely clean bass frequencies from a Bose system. However I seriously doubt this will be released to the public anytime in the near future.
I would highly recommend using software designed to calculate volume and port size if you are attempting to build your own bandpass box. I have added 2 links below for easy reference. I have built 2 myself using the highest recommended airspace for the driver on the sealed side, and tuning the ported side by ear (trial and error). There is only one set balance for designing a 4th order bandpass box: The larger the sealed area, the lower the bandpass frequency that can be obtained.
Tips for building bandpass boxes:
-Flare all ports to reduce port noise.
-Use largest applicable port size for the same reason.
-Allow ALL adhesives to dry completely and air out before mounting speakers or plexiglass (fumes may damage or cloud some materials).
-Use 3/8” or thicker plexiglass or Lexan on windows to prevent flexing. ¼” doesn’t work, I tried.
-Always test your box and fine tune it before covering or upholstering.
Here are some good links that I came across while researching this topic.
http://www.danmarx.org/audioinnovation/theories.html
http://www.carstereo.com/help/Articles.cfm?id=27 (free calculation for 4th order BP)
http://www.carstereo.com/help/Articles.cfm?id=32 (free calculation for 6th order BP)
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