In Volume 31’s “Sound-O-Light Speakers,” the vented build option is not a true bass reflex design (see reader Louis Lung’s letter, below). Also, author Bill Gurstelle’s Night Lighter 36 spud gun project is from MAKE Volume 03, not 04.
In Volume 31’s “Sound-O-Light Speakers” project (http://makezine.com/projects/make-31/sound-o-light-speakers/), two diagrams on page 82 describe an acoustic suspension (sealed) alignment and a bass reflex (vented) alignment. Unfortunately, the description of the bass reflex design is based on a popular but incorrect theory — that the back wave of the driver is made available via a port or opening to augment the sound from the front of the driver.
In reality, the port is a Helmholtz resonator whose resonance is defined by its length and cross-sectional area. The port is typically tuned to a frequency that’s lower than that of the driver — it’s not a manifestation of the back wave of the driver.
The opening at the bottom of the Sound-O-Light speaker is not a port since the entire chamber would be the resonating column. It more closely resembles a transmission line speaker.
Vented designs are by their nature less forgiving and more difficult to work with. For a hobby-type speaker, a sealed design is far easier to build and get right.
I plugged the HiVi B3N driver’s parameters into a Thiele-Small calculator (you can find these online). In a sealed alignment, for a speaker box volume of 0.1ft3, the length of the 3″ pipe is a manageable 24.49″ and the max bump in low-end response is 0.98dB. This will add a little boominess to the sound and some false impact to the bass, and should be OK.
A vented alignment is not so easy. The optimum calculated volume is 0.59ft3, for a max low-end bump of 1.63dB. The vent (port) will be 1″×1.5″ (or 1.5″×4.85″ but that’s hard to fit in this design), tuned at 36.9Hz. Unfortunately, our 3″ pipe would have to be over 12′ long! If we reduce the volume to 0.1ft3, the low-end bump rises unacceptably high, and the port tuning has to be moved up, so we lose the extended low end, and the port’s output can’t match the bump; the result is a really bad response curve.
In summary, a small speaker made from this driver works best in a sealed box. If we allow for significantly more volume (a bigger box or a 3″ pipe that’s 12′ long), the vented alignment can be made to work and yield a much better low end than the sealed box. It’s a tradeoff.
Or, one could ignore the details and just build the thing and enjoy it. :)
Louis Lung, lungster.com,
Projects Editor Keith Hammond replies:
Louis, thanks for your excellent analysis. We thought we were clever to offer a vented option but it’s clearly trickier than we knew. Consider yourself an honorary member of the MAKE Technical Advisory Board, Loudspeaker Division.