Basso Profundo
By Martin Colloms   •   December, 1991 Bass constitutes one of the least understood aspects of sound reproduction. Opinions vary greatly on matters of bass quality, quantity, and perceived frequency range or response. Moreover, the bass region is subject to the most unwanted variation in practical situations due to the great influence listening-room acoustics have on loudspeaker performance. Every room has its different bass characteristic, and changes in the position of speakers or listener also constitute major variables at low frequencies.

What constitutes good bass?
Limits: For this discussion, the bass range has been taken as 20Hz to 160Hz---no less than three octaves. (Hz = Hertz, or cycles per second.) We know that bass sounds can occur low enough in frequency---in the 20-30Hz region---to be felt as well as heard. The sensation of pitch is lost below 20Hz, and we hear instead the individual bumps of the acoustic pressure waves. The vast proportion of recorded music has little content below 40Hz, but when music does venture down into the lowest octave---and I repeat the term "octave"---big audio systems can reproduce subjectively exciting things. The program acquires scale and weight when this range is properly reproduced. A large orchestral bass drum can sound strongly in the 35Hz area, and while the bottom E on a conventional double bass or electric bass guitar is customarily 42Hz, synthesizers and a large pipe organ can play much lower, to 20Hz at least. Fig.1 shows the frequency ranges covered by common instrumental sounds.

Fig.1 Frequency range required for realistic reproduction of various sounds (after The Audio Encyclopedia)

Slam: Several subjective characterizations are relevant in the bass. "Slam" is an interesting one, for me conveying an awareness of a massively realistic bass percussive sound---one which is deep, fast, and yet has a close association with the upper harmonic information that conveys the instant of impact. "Loose" bass indicates a softer, slower bass separated from the impact. In comparison with real life, can the audio system effectively reproduce the heavy slam of a big door? At least one system can: the WATT/Puppy, in which bass speed was a crucial design factor. Accurate presentation of time and rhythm information is this system's forte.

Timing: "Timing" is a term gaining increasing importance. "Good timing" generally indicates that temporal aspects of the music---tempo and rhythm---sound realistic.

Rhythm: Fundamental to rhythm is the perception that the rhythmic aspects of the music, particularly in rock and jazz, are presented in unison, coherently locked together over the entire frequency range. For example, a good pattern on a snare drum must be allied to pedal bass drum on the beat; likewise, the complementing sounds from the cymbals must tie in with the other two. When timing is right, the whole sound seems to slide into temporal focus. One is aware of the rhythmic flow, the ebb and surge of the syncopation, the expressive playing of great performers. For many listeners, this is just as important as the optimum reproduction of the classic aspects of stereo staging, transparency, neutrality, and detail.

When all is right, the reproduction is imbued with an appropriately realistic sense of pace, the music driving forward with life and energy, confidently drawing in the increasingly involved listener.

Slow, soft, loose, overhung bass is a killer in this respect. Designers who specialize in getting good rhythm have learned that if a given system cannot reproduce bass well, it's better to leave well enough alone and not attempt the impossible. Stiffening a small speaker's suspension prevents the driver cone from moving too much. In direct contradiction of the classical theory, controlled short throw (and, by implication, a restricted bass response to, say, above 55Hz) has the following benefits: The upper-range bass can be designed for better damping since the whole system can be adjusted for a higher conversion efficiency.

Reduced cone excursion, especially for drivers working over the bass and midrange, has the benefit of reducing a host of modulation effects both dynamic and magnetic. Higher efficiency promotes lower voice-coil temperature and improves subjective dynamic accuracy. Distortion is also improved since it is strongly proportional to input power, this much reduced with higher system efficiency. Abandoning the lower bass register allows the smaller system to use a lighter cone with a superior transient response in the midrange, again contributing to system "speed."

Striking the balance between classical virtues and timing
How do you determine the optimum balance between life, pace, and speed, and bass extension, low coloration, and transparency? The last three are classical virtues of a high-quality audio system, yet the first three are also vital if those intellectual qualities are to remain satisfactory. A good novel is not just fine prose---it must also have a gripping story if it is to entertain. Indeed, a great story can still be involving even if the writing is not so good.

Why that stress on the word "octave" in the introduction?
Audio engineers concentrate much of their attention on the midrange, officially centered on 1kHz but in practice encompassing the 200Hz-2kHz decade, about four octaves in all. A great deal certainly happens in this range in musical terms; the majority of sounds, voices, and musical instruments play their tunes here. However, the bass is often neglected by comparison, possibly because the number of Hertz is less. It's easy to imagine a full musical scale over the 400Hz-800Hz range with 400Hz to play with. The 30-60Hz bass range has just as many tones and semitones, etc., but makes do with only a 30Hz interval. We need to view low-frequency responses with greater precision, and try to discount the idea that only higher numbers of Hz matter.