The Question of Bass / Bass Instruments & Frequencies
A few issues back, in Vol.9 No.3, I used "As We See It" to clarify what Stereophile writers have in mind when they use the term "transparency" in equipment reports. This time, I'll do the same thing for the performance parameters of bass reproduction.
Perhaps more than any other aspect of system performance, LF behavior is influenced by every single component in the system, as well as by their mutual interactions. Exaggerated bass can just as readily result from acoustic feedback, or a shortage of woofer damping due to an undersized magnet, as it can from an actual rise in low-frequency response. Inadequate damping will, in fact, cause a measurable LF rise, but acoustic feedback usually won't. For this reason, and to allow anyone equipped with an octave equalizer to illustrate for himself some of the phenomena we will be discussing, I will define most of these LF parameters in terms of frequency response aberrations which can cause or replicate them, even though a frequency-domain problem may not be the underlying cause.
For example, lack of amplifier power can frequently exacerbate LF acoustic feedback, leading to a heavy, poorly defined bass. If the tendency toward feedback is there, a low-powered (under 30 watts) amplifier can be almost guaranteed to make it much worse, as it loses control at low frequencies when overdriven. In fact, bass detail and apparent impact are directly related to the amount of power the amplifier is capable of delivering at low frequencies. Current capability---the ability to deliver rated power into very low load impedances---also determines to a great extent the quality of LF performance from a speaker whose impedance falls to a very low value at very low frequencies.
The arm/cartridge resonance establishes the lowest frequency that a phono system can attain, and the Q, or sharpness, of that resonance greatly influences the smoothness and detail of the bass range. Traditionally, this resonance is tuned to between 10 and 15Hz, far enough away from the warp information peaking at 6Hz that it won't continually be excited, but still comfortably below music information. In extreme cases, it is better in terms of overall LF performance that a system response take a nose dive below 40Hz with a poorly-matched arm/cartridge pair with a large resonant peak between 10 and 20Hz, than have a well-extended response to 25 or 30Hz.
Aside from the system itself, the most important determinant of bass performance is the size and shape of the listening room. All rooms exhibit standing-wave resonances whose frequencies and distribution depend on the three dimensions of the room, and the extent to which these will impair LF performance depends on the placement of the loudspeakers in a given room. Generally, corner placement excites standing waves to their maximum, producing the greatest amount of bass but the most irregular LF response. The smoothest bass is obtained from speakers which have enough output at low frequencies to yield proper bass/treble balance without the need for corner or wall augmentation, so that they may be located well away from wall surfaces. (ASC Tube Traps, reviewed in Vol.9 No.3, are the simplest remedy for standing wave problems in small- to moderate-sized rooms.)
The LF Ranges
In high-end audio, the bass or LF range is generally considered to be that range of frequencies between about 150Hz and the bottom-most frequency of our hearing or of the reproducing system, whichever comes first. The bass range is further divided, rather arbitrarily, into three secondary ranges: the upper bass, from 150Hz down to 70Hz; the mid bass, from 70Hz to 45Hz; and the deep or low bass, from 45Hz to around 25Hz.
The term "infrasonic" is often used to describe the range below 25Hz, without regard to how much of that range may actually be audible to humans. This usage is actually correct, as infrasonic means, strictly, "below the range of audibility," but the fact that we can feel sounds with our bodies which are too deep to be heard, means we cannot rule out the significance of the infrasonic range in sound reproduction.