midrange sound

Topic: Big woofer available to R.A. Johnson HS for Career Day, to demonstrate engineering in operation.

Subtopic: The "midrange sound" source, above the frequency range of the subwoofer.

The midrange sound is to come from two Omega BU-69.3 speakers from Anything Audio in Augusta. I would have purchased an 8" midrange speaker but that wasn't available at low cost. The BU-69.3 are 3.55 ohms DC 20 W "system" power, which might mean 3 W RMS (see comments below). 3 oz. ceramic magnet, which is lightweight. Voice coil diam looks to be .65". You can find on-line the better BU-69.10 which has 10 oz. magnet, at $14.

20W peak may mean peak power of an isolated spike of voltage, where average power may be quite low. The 20W could be a rating for not rupturing the cone or tearing the voice coil off the cone, but 20W is likely to be for a stereo pair of speakers, so rupture threshold may be 10W.

20W "system power," as rated on these speakers, may refer to the power fed to the amplifier (this idea comes from Wikipedia, Audio Power), which could mean 5W to the speaker, or even 2.5W per speaker in stereo system. If 20W refers to peak power of a sine wave, the peak voltage for 3.55 ohms is 8.4V. RMS voltage is 5.9V. Power RMS would be V²/R = 10W. Ten watts in these little speakers would certainly bake the voice coil.

I purchased these speakers to act as the midrange speakers, complementing the JL subwoofer. Should these speakers go series or parallel into a sealed enclosure? For 20W peak power in a stereo setup, that would be 10W peak per speaker. Using P = V² / R, V peak = 5.9V. This is a very low peak voltage if supplies are +-24V, so series would work. But if the midrange power amp is the 2012 DIY stereo amp with +-13V supplies & +-7V peak swing, parallel is better, though 3.55 ohms/2 = 1.78 ohms is rather low for that amp. Sounds like use of a new amp for the midrange might be best, but output transistors will be air-cooled, not oil immersed.

The new amplifier will be Class AB (look this up on Wikipedia, under Amplifier). It will be a custom design, to use the existing +-24V commercial supplies. The volume control will act independently from the subwoofer volume control. The amplifier will incorporate an electronic, high-pass filter to work as crossover (look up http://electronics.howstuffworks.com/speaker8.htm) with the low-pass filter ahead of the subwoofer amplifier. This high-pass filter for midrange must have several, fine-tuned cutoff frequencies, selectable with jumpers, to get the best fit to the subwoofer filter. (And in particular to work with the two frequencies I want to switch between for the subwoofer's crossover, like 80Hz and 140Hz).

The sealed enclosure for the midrange sound might be cheap but effective, such as I did for my "speaker chair." (See https://sites.google.com/site/solderandcircuits/example-of-homemade-sound-system). Start with a corrugated cardboard box about 18" on each side. Glue on extra layers of corrugated to increase stiffness. Cut the oval holes for the speakers, with care because the speakers mount from the inside of the box. The exposed corrugated cardboard can be trimmed up simply with some sort of glued-on paper trim. Make a hole in the back for access to wiring & the wires that come from the amplifier. Loosely fill the inside with rags to avoid resonances. (Glue some rags onto the underside of the top of the box). Provide a red LED & resistor to see the signal strength; do two LEDs parallel-reversed to handle the AC.

There is a speed-of-sound-in-the-air effect that is important to crossing over speakers. This can also be termed the phasing of the speaker cones as frequency increases through the crossover. Let's say that the woofer to midrange crossover is at 80Hz. The wavelength of 80Hz, using 1130 ft per sec as speed of sound, is wavelength = 1130 ft/sec / freq cycles per sec = 1130/80 = 14.1 feet per cycle. If you were to place two midrange speakers 14.1 feet apart & feed the same sine wave to each, then walk in a circle around them, out about 40 feet, you would hear different reinforcement or cancellation of the sine wave. This can be shown better with a diagram, as seen at http://www.physicsclassroom.com/Class/light/u12l1b.cfm . This can really only be heard with the ears in an outdoors space that has no sound reflection from walls & ceiling. Now consider that one speaker is the 15" woofer and the other is the midrange enclosure. The same reinforcement & cancellation will be heard, and audiophiles consider that this is a sound-system defect! Audiophiles would prefer that what you hear is independent of where you sit in the room, especially since the lines of interference in red below vary with frequency, to the extent that at 1500Hz you can move your head just 9" and hear significant change in the sound. In a concert hall with normal music or voice, this is heard to be more a property of the room than a defect of the sound system.

Let's go on to say that the woofer is mounted in the door of a school classroom, as is the intention for the 15" speaker demonstration. The midrange enclosure, a box of 3 cu. ft., has to sit some feet out from the door. If it is 14 feet out, there will be major destructive interference at 80Hz, which is less than two octaves below middle C. If the midrange box is just 3 feet out, there will still be a significant interference pattern at 80Hz, since 3 feet is 21% of the wavelength. If one is trying to adjust the highpass crossover freq of the midrange, the 3 feet of spacing is going to have a significant effect on the adjustment, & the adjustment will be invalidated if you move the midrange enclosure more than two feet. Speaker phasing amounts to a significant technical challenge. A related effect can be heard in theater speakers. If the tweeter location is moved, for example, 15 feet toward the audience compared to where the woofer cones are, an impulse sound (a click) will be heard 13 milliseconds different between the woofer and tweeter. This is heard by some in the audience as a double click, and some people are bothered by that. So theater speaker arrangement is not a simple thing. The sound competitions conducted by IASCA, International Auto Sound Challenge Association, are sometimes won or lost depending on speaker phasing, and the first tracks of the official sound reference CD are devoted to stereo speaker phasing. The lady explains that, if the left and right are not correct, the rest of the CD doesn't matter, you really can't proceed with the evaluation if left and right are reversed.

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