You can get some good sound for not much money with a bass reflex box using Radio Shack's 40-1909 5-inch fullrange. Price $9.99 according to the 1997 Radio Shack catalog.
This thing is likely obsolete now.
This speaker can only handle continuous power to about 2 watts, and peaks to about 10 watts RMS. It may suffer distortion and/or thermal power compression with wattages much over about half these.
Note that these designs have some significant resonant character centered at a frequency very slightly below the port frequency. Depending on your tastes, this is not too likely to be objectionable to those who like bass reflex speakers. This resonance is of slightly higher Q than average for bass reflex (ported) speakers.
Thiel-Small Parameters for the 40-1909 (updated slightly 6/19/2011):
Vas=.31 cubic foot
Reference Sensitivity = 90.5 dB at 1 watt, 1 meter
(Due to above-average ratio of RE/Znom, its "reference sensitivity" is less than that predicted by my "SPKR" software.)
Nominal Sensitivity = 91 dB 1W, 1m
This speaker gives fairly full bass with fairly good efficiency. The midrange response is fairly good, especially if the speaker is not equally distant from the four sides of the box. The treble response is good enough to hardly need a tweeter.
For best results, a small amount of damping material in the box is recommended. About half an inch of fiberglass or about an inch of "Polyfill" on the bottom, back, and one side of the box should be sufficient. One should use a minimum of damping material, just enough to make standing waves not have a noticeable effect on the sound of the speaker when the speaker is operated in the box. The box itself, with the speaker removed, should have a fairly bass-resonant sound with a slightly noticeable but rather damped standing wave character. Excessive damping material will reduce the bass response.
Use a slightly larger box volume of .68 cubic foot and a lower port tuning of 52 Hz. Boost the 63 Hz control of the octave equalizer by 3 dB. This extends the bass -3dB point to (updated slightly 3/24/2015) to 45 Hz.
Internal dimensions that make .7 cubic foot: 15.5"H * 10.875"W * 7.0625"D
External dimensions using 3/4 inch wood: 17"H * 12.375"W * 8.5625"D
Port dimensions for 52 Hz with .68 cubic foot: 2" inside dia. * 2.75" long.
With a .065 cubic foot box, bass response has a hump peaking at nearly +3 dB at about 230 Hz, and the bass -3 dB point is at 145 Hz - when the box is against or recessed into a wall.
When such a box not coupled to any surfaces, the bass hump is nearly nonexistent and the -3 dB point is about 180-190 Hz. Without coupling to any surfaces, an enclosure as small as .055 cubic feet has only a very minimal bass (lower midrange?) hump peaking at about 270 Hz and -3 dB at about 190-195 Hz. This frequency response is surprisingly good for male voices, even music and musical instruments such as guitars and bass guitars. It is recommended in this case to filter out frequency content below 150-160 Hz so that amplifier power is not wasted on heating voice coils at frequencies where loudspeaker efficiency is low.
As for sealed enclosures for the 40-1909 operating against or recessed into a wall: .23 cubic foot is sufficient to have the bass hump nearly nonexistent and the bass -3 dB point at about 108 Hz. Filtering out frequencies lower than 60-70 Hz or so is recommended to conserve power of low power amplifiers and/or to minimize stress to the loudspeaker. Bass response will remain impressive for a loudspeaker enclosure of such size with nominal sensitivity being a true 90.5-91 dB @ 1W, 1m.
As for larger unported enclosures? Going larger than .23-.25 cubic foot does not gain a lot. .45 cubic foot is enough to get the bass -3 dB point within 1 Hz of the lowest possible for an unported enclosure with this loudspeaker in a usual environment as well as getting the bass resonance very-well-damped. The bass -3 dB point is about 101-103 Hz with .45 cubic foot, 100-102 Hz with .6 cubic foot, and increase of unported box volume from .6 to .8 cubic foot will actually very slightly decrease bass efficiency in the 100-103 Hz range as well as in the 100-200 Hz octave. For that matter, efficiency in the 100-200 Hz octave is maximized with unported box volume not far from .23 cubic foot.
As for what unported enclosure volumes optimize the -6 dB and -10 and -12 dB frequencies? 1st, I find it does not pay to enlarge an unported loudspeaker enclosure from 85% or even lower percentage of what it takes to minimize the bass -3dB point or to damp down the "bass hump" above that frequency to nonexistent. Especially not for the 40-1909 due to its higher Qts greater than .4 and its low ability to pump out much bass below ~60 Hz in unported enclosures.
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Written by Don Klipstein.
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