Tapping the woofer with a finger is just the reverse of it reproducing a kick drum; both are impulses that stimulate the woofer and cabinet resonance. Accuracy of reproduction is not always what sounds best to the ear; an under-damped loudspeaker will be the dreaded sonic descriptor, “warmer.” The best way to tame the speaker’s self-expression is by minimizing the cabling between it and the power amp, hence the concept of self-powered monitors (or the use of “monstrously” thick cable).
A car outfitted with a spring-only suspension system would bounce all over the road, a spring being a high-impedance device compared to a shock absorber. The amplifier’s extremely low-source impedance appears as a “short circuit” to the woofer’s natural mechanical resonance. You could describe both the shock and the amp as “low impedance devices that provide damping and stabilization to what would otherwise be a bouncy ride.”
Note: The need for damping is the reason a 600-ohm terminating resistor should be connected to the output of transformer-based gear, such as the venerable UREI LA-3 limiter, when interfaced with modern gear. A transformer consists of two coils of wire, the electronic equivalent of excitable springs.
| Load Resistor | Frequency @ -3 dB |
| 100 ohm(see note) | 800 Hz |
| 1.1 kilohm | 160 Hz |
| 2.1 kilohm | 80.5 Hz |
| 3.1 kilohm | 52.5 Hz |
| 4.1 kilohm | 40.0 Hz |
| 5.1 kilohm | 31.5 Hz |
| 6.1 kilohm | 26.5 Hz |
| 7.1 kilohm | 22.5 Hz |
| 8.1 kilohm | 19.5 Hz |
| 9.1 kilohm | 17.5 Hz |
| 10.1 kilohm | 15.5 Hz |
|
Table 1: Various load resistors interact with the series output capacitor to affect the low-frequency roll-off as shown in Figure 3. At -3 dB, the signal level is reduced to half of the power before the roll-off began. |
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NON-PLUSSED
That impedance varies with frequency should be more tangible now, what about phase? The magenta square in Fig. 1 shows how a crossover network—consisting of inductors, capacitors and resistors—affects both impedance and phase response, but to make it more tangible…
Connect a battery to a woofer and watch how it moves in or out depending on polarity, staying there until the power is removed. Oversimplified, phase is the minute delay of the cone as it attempts to travel to its destination. Once there, the speaker has a strong desire to return from such an exaggerated excursion, acting as a generator when it does. This example should also help to visualize what simple expressions such as “E-L-I the I-C-E Man” did for engineering students. Don’t laugh! Type “ELI the ICE Man” into a search engine, and you’ll be surprised as I was. The best link, http:// ewhdbks.mugu.navy.mil/elecform.htm, yielded a fabulous collection of electronic formulae, rules of thumb and mnemonics.
E-L-I reminds us that Voltage-Leads-Current by 90° (the Phase angle) in an inductor, a coil of wire designated as “L,” “E” stands for voltage and “I” for current. I-C-E reminds us that the reverse is true for capacitors, where Current-Leads-Voltage by 90°, where “C” stands for capacitor. E-L-I the I-C-E is deep, man, but memorable for the purpose of “concept retention.”
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Reprinted with permission from Eddie Ciletti, Tangible Technology, June, 2001
© 2001, All Rights Reserved
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