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发表于 2023-5-17 08:00
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16.2 THE DAMPING FACTOR DECEPTIONOne of the universal compliments attached to audio products, including wires, is that it results in “tighter bass.” In the case of loudspeaker wire, it seems as though there might be some truth to it because of its role in the loudspeaker/amplifier interface and damping. Damping unwanted motion of a loudspeaker diaphragm is undoubtedly a good thing. In 1975, I wrote an article for AudioScene Canadacalled “Damping, Damping Factor and Damn Nonsense.” I still like the title because is a succinct statement of reality. The point of the article is summarized in Figure 16.2c.
The internal impedance of the power amplifier is used to calculate something called the damping factor (DF) of the amplifier (DF = 8 divided by the output impedance); the number 8 was chosen because it is the nominal load (resistive) used to measure the power output capability of amplifiers.The logical inclination is to think that larger is better. Solid state amplifiers have damping factors ranging from about 200 to 800, using the impedances quoted earlier in this section. Tube amplifiers in my survey ran from 2.4 to 11.4 because of their high output impedances.
Figure 16.2c shows the complete circuit involved in the electrical damping of loudspeakers—it does not mysteriously stop at the loudspeaker terminals. Current must flow through components and devices inside the enclosure. After flowing through the wire,it typically passes through an inductor, part of the low-pass filter ahead of the woofer in a passive system. Then, inside the woofer there is the voice coil. The inductor resistance is commonly around 0.5 ohm, and the voice coil resistance can have different values but is commonly around 6 ohms. So, let us examine all of the resistances in the circuit to arrive at the following progression of damping factor changes:Amplifier internal impedance: 0.01 ohm DF = 800（=8/0.01）Add wire resistance: 10 ft of 10-gaugeBoth conductors: 0.02 ohm DF = 266（=8/0.03）Add crossover inductor resistance:0.5 ohm (typical) DF = 15（=8/0.53）Add voice-coil resistance: 6 ohms (typical) DF = 1.2(=8/6.53)
Obviously the resistances inside the loudspeaker are the dominant factors. Even eliminating the inductor and driving the woofer directly changes things only slightly. The article (Toole, 1975) shows oscilloscope(英[slskp]美[slskop]n.示波器：示波器是一种用来测量交流电或脉冲电流波的形状的仪器，由电子管放大器、扫描振荡器、阴极射线管等组成。) photographs of tone bursts of various frequencies and durations while the damping factor of the amplifier was varied from 0.5 to 200. At damping factors above about 20 (internal impedance less than 0.4 ohms), no change was visible in any of the transient signals, and changes in frequency response were very much less than 1 dB, and then only over a narrow frequency range. On music no change in sound quality could be discerned, including attentive listening for “tightness.” Because 0.4 ohms is at least a factor of 10 higher than internal impedances found in typical solid state amplifiers, it means that, from the perspective of damping the transient behavior of loudspeakers, the wire resistance can be allowed to creep up substantially. However, as shown earlier, doing so can change the frequency response of the loudspeaker and that, we know, is audible. In summary, with tube amplifiers, the internal impedance is already so high that damage is done to the frequency responses of loudspeakers having normal impedance variations. Added losses in wire simply make the situation worse. Listeners do not hear the loudspeaker that the manufacturer created. With solid-state amplifiers internal impedances are negligibly low, so wire resistance must be controlled in order to minimize corrupting the frequency response of loudspeakers. How low? It depends on the variations in the impedance of the loudspeakers being used, and how low those impedances are—wire resistance represents a higher percentage of low impedances. For example: a loudspeaker ranging from 3 ohms to 20 ohms (not unusual for consumer loudspeakers and a moderately demanding situation) would experience about 0.6 dB variations in a system with 0.2-ohm wire resistance.
Section 4.6.2 shows that this is slightly higher than the detection threshold for low-Q spectral variations in quiet anechoic listening.
Twelve-gauge wire would allow for a run of 0.2 / 0.0032 = 63 ft (19 m). Obviously this is not very restrictive. Loudspeakers having nearly constant impedance (a few exist) can tolerate large wire losses, sacrificing牺牲 only efficiency up to the resistance at which damping is affected. If compelled强迫 to do better than this suggestion, more copper, shorter runs, or higher-impedance loudspeakers are the solutions.【Reference】《Sound Reproduction》第三版 |
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