Philosophy

Up to this point, we in audio have been dealing largely with the soundstage (or soundspace) as a three dimensional construct. It is a construct that has proved illuminating in helping designers find their way from a two-dimensional world into one that more nearly approximates reality, as we perceive it.

But it has become increasingly clear that the best audio designers have measured the fundamentals of a soundstage, along with the goals of the last generation (flat frequency response and quite low distortion figures), and yet we still have not developed any components or systems that bear a reasonable resemblance to the feel of the real thing. Even the least sophisticated observer can hear the difference between, say, a live marching band outside the house and one being reproduced on a stereo system, of however many speakers or channels. To get closer to our goal of replicating a lifelike resemblance to the absolute sound, we have to move beyond the soundstage. The fundamental facts below is based on the hearing mechanism and not ad hoc ideas from the past.

1. The Ear's Ability to Differentiate Sounds: A principal function of the ear is to identify voices. For this use it has developed an extraordinary ability to differentiate sounds. We can separate a single sound source such as a distant voice from other sounds by concentrating our hearing apparatus on the voice and ignoring noise or other voices that we don't want to hear.

2. Volume (Intensity) Variations: The ear has little sensitivity to sound level "jumps," or to the relative loudness of different sounds that are audible at the same time. For a loudspeaker, sound output levels (amplitude) over a range of frequencies are valid criteria, but they are of lesser importance for our ears. The physical construction of the ear makes it relatively insensitive to amplitude changes. The difference in amplitude between a whisper and normal speech is not just 1:2 or 1:4, but can be as much as 1:100,000. The relative loudness of different sounds, within certain limits, is therefore not too important to us, since the ear has the ability to adjust to different levels. This explains why street noises do not necessarily disturb conversation. It also explains why we can hear an opera singer even though the sound level of the orchestra is many times that of the voice.

3. Frequency Variations: In contrast to its relative insensitivity to amplitude variations, the ear is extremely sensitive to minute fluctuations in the frequency of sounds, especially in the mid-frequency range. A half tone in the musical scale represents a frequency change of 6 percent, while the frequency shift in the vibrato of a violin is approximately 0.5 percent. In the critical midrange of 250Hz to 6,000Hz, we can differentiate between two tones even when the frequency difference is as little as 0.06 percent.

This sensitivity to frequency variations enables us to identify different voices. When we speak, we do not produce constant tones, but constantly varying tones. We can usually recognize a familiar voice immediately, even over the telephone, and we can often judge the mood of the other party by differences in speech pattern produced by the changing tension of the loudspeaker's vocal cords.

4. Frequency Variations vs. Amplitude Variations: It is commonly accepted that the smallest change in amplitude that the ear can detect is 1dB, which represents a power difference of 26 percent. Compare that to the ear's sensitivity to frequency variations of 0.06 percent. Contrasting this relative insensitivity to amplitude changes with the ear's extreme sensitivity to frequency variations, it is difficult to understand the loudspeaker industry's obsession with minor loudness variations of 1 or 2 dB in the frequency response of a loudspeaker, while ignoring the audible shifting or fluttering or high frequencies which can result from changes in materials stiffness as a sound wave spreads transversely across a transducer's diaphragm.

5. Phase Differences — The Ability to Localize Sounds: A listener's ability to localize sounds is made possible by phase differences (time delays) resulting from the differences in path lengths from a sound source to each ear. This ability is frequency-dependent and is more pronounced in the critical range of 500Hz to 3000Hz than at lower and higher frequencies. This is why the speed of response of a loudspeaker diaphragm is extremely important to the faithful and realistic reproduction of music. If the loudspeaker's diaphragm cannot respond fast enough to reproduce these transients, or if it distorts them, the listener's ability to recognize and localize the sound source is greatly diminished, and the realism of music reproduction and pleasure of listening are seriously reduced.

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