Basso Profundo Page 3

As a result, a more distant, quiet source sounds "thinner" and "lighter" in tonal quality due to these clearly audible changes in subjective frequency response. For most of us, it comes so naturally that we are not really aware of it.

Thus one classic interpretation of the action of a volume control is the control of distance. Reducing the reproducing level alters the loudness and the simultaneously perceived tonal balance, giving the imprecision that the musicians are farther away. While this view is valid for classical program played at a natural level, the argument is weakened when rock music is involved. Very few, if any, of the sounds in modern mixes have a truly natural basis, hence the foundation for the aural recognition of tonal-balance accuracy is lost. With rock the case would seem to be the louder the better, provided that you and the audio system can cope. Nevertheless, this question of natural loudness is highly relevant to music played on acoustic instruments, including the human voice.

The timbre of an instrument is a highly variable quality, and such variation is skillfully exploited by composers and musicians for expressive purposes. Setting aside matters of musical technique---the position of a bow along the length of a violin string, or the particular selection of a reed for woodwind---there are factors which will affect sound quality and timbre, according to the loudness at which they are played. Timbre will change according to the physical energy put into a performance. This is a natural part of the dynamics of an instrumental performance and must not be masked or exaggerated by a hi-fi system. If this happens, the natural musical dynamics will be falsified. Several ailments result, including the sin of "compression." In the real world there is a general tendency for audio equipment to impair natural musical dynamics. It is quite natural for the timbre or tonal quality of an instrument to become harder, fiercer, brighter, and sharper when it is played more strongly, with greater energy, and at a greater volume.

Knowing that the perceived frequency response for the ear varies with loudness, it can be seen that our convenient logarithmic dB scale for loudness is a serious oversimplification. Looking at the curves, one can pick one convenient frequency, 1kHz, and see that the dB steps in phons or audible loudness pretty well match the log scale for sound pressure in this frequency region. However, they don't fit so well at 500Hz. For example, the aural response shape at 120dB is not repeated accurately down the dynamic range. A similar fault is present at 4kHz, where the response dip is deeper at high than at low loudness levels.

Bass at last
At last we get to the bass region, and here the characteristic is anything but logarithmic. Both spacing and the shape of the curves vary continuously with loudness. Moreover, we must remember the high audibility threshold, below which bass cannot be heard at all. Put another way, we all suffer from an increasing rate of bass rolloff with decreasing loudness. This fact has practical consequences. There's little point in producing a miniature loudspeaker of modest loudness capability which boasts an extended bass performance if the little darling can't play loudly enough for its low notes to be audible. Instead the designer should exploit the relationship between the bass extension and sensitivity, curtailing the former until the available loudness rises to a point where that bass which can be reproduced is usefully audible.

Simplifying the matter and assuming that typical room gain of 6dB by 40Hz, at the listening position a speaker will need to be capable of delivering 100dB if full-weight, 40Hz bass is to be audible with normal orchestration. At 40Hz the sensitivity threshold is 54dB, the sensitivity loss at 100dB being around 6dB. This leaves a dynamic range of 46dB, or the bass extreme compared with the 100dB available (in theory) in the midrange. At sound pressures above 90dB (1kHz, 0dB reference) the bass response becomes more logarithmic. Below 70dB, the bass lines in the characteristic responses begin to bunch up. This bunching reflects the fact that we hear changes in bass level more acutely than in the midrange.

This increased sensitivity for change could be interpreted as a compensation of the reduced dynamic range, but it is also the basis for that earlier comment that "a little bass goes a long way." Once we get into the working bass area, the ear is more sensitive to variations in uniformity and absolute balance. This is why there are more complaints about bass---whether too much or too little---than in other parts of the frequency range.

In a large hall, bass may propagate freely down to the lowest practicable frequencies; there is an open, easy, flowing quality to concert-hall bass. By contrast, listening-room bass is often lumpy, boomy, and oppressive when in excess, while in unfortunate locations it can seem infuriatingly absent. The dimensions of normal rooms are comparable with the actual propagated wavelength of the bass sounds we are attempting to reproduce. Those infamous standing-wave modes are the problem, where the distribution of bass energy varies strongly with speaker and listener position as well as with frequency. Major variables include room size and shape, and the stiffness of its boundaries. Bass is subjectively much more powerful in a brick-and-concrete building than in a frame house with stud-and-plaster walls and suspended timber floors. In many traditional American frame houses the bass energy simply leaks away through the acoustically soft structure.

Optimized speaker location
While some speakers are designed specifically to be mounted close to the wall, the majority are expected to be used in free space. What exactly does this mean? As regards acoustic theory, free space means just that---the great blue yonder---with the speakers located well off the ground, say 100' for starters. In the context of a speaker in a room, however, free space simply means moving the speaker clear of the back and side walls and mounting it on an open-frame stand appropriate for the type and at a worthwhile height. With a listening position somewhere in the last third of the room, even up to the back wall, the speakers on the other hand would be placed in the region of 1/5 to 1/3 of the way out from their nearest boundaries.

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