Loudspeakers: Objective evaluations - Part 6: Other measurements

[Part 1 begins with a look at the two basic radiation patterns of sound sources. Part 2 answers the question "If we want to measure a loudspeaker, and from those measurements try to anticipate how it might sound in a room, what should we measure?" Part 3 reviews results of the author's elaborate subjective-objective investigation into loudspeaker evaluation. Part 4 reviews the measured performance of some real-world consumer loudspeakers - some good and some not so good. Part 5 examines the objectives and measured performance - with examples - of professional monitor loudspeakers.]

18.6 OTHER MEASUREMENTS: MEANINGFUL AND MYSTERIOUS
So far we have talked about frequency response as if it were all that matters. Actually, it is almost true, in the sense that if the set of frequency response measurements that has been displayed does not look "right," almost nothing else matters. This assumes that nonlinear distortion is not gross and that neither loudspeakers nor amplifiers are hitting their limits. These frequency response curves tell us how the products will sound, within reason, but there are other dimensions to be looked at.

18.6.1 Phase Response - Frequencies Above the Transition Zone
The combination of amplitude versus frequency (frequency response) and phase versus frequency (phase response) totally defines the linear (amplitude independent) behavior of loudspeakers. The Fourier transform allows this information to be converted into the impulse response, and, of course, the reverse can be done. So there are two equivalent representations of the linear behavior of systems: one in the frequency domain (amplitude and phase) and one in the time domain (impulse response).

To put it in slightly different terms, the accurate reproduction of waveforms is possible only if the signal is delivered to the listener's ears with perfect amplitude and phase responses. The obvious question is: do we hear waveforms? All of the evidence in this chapter indicates that listeners are attracted to linear (flat and smooth) amplitude versus frequency characteristics.

Toole (1986) shows phase responses for 23 loudspeakers arranged according to subjective preference ratings. The most obvious relationship was that those with the highest ratings had the smoothest curves, but linearity did not appear to be a factor. The agreement that smoothness is desirable argues that listeners were attracted to loudspeakers with minimal evidence of resonances because resonances show themselves as bumps in frequency response curves and rapid up-down deviations in phase response curves.

The most desirable frequency responses were also horizontal straight lines. The corresponding phase responses had no special shape other than the smoothness. This suggests that we like flat amplitude spectra and we don't like resonances, but we tolerate general phase shift, meaning that waveform fidelity is not a requirement.

Loudspeaker transducers, woofers, midranges, and tweeters behave as minimum-phase devices within their operating frequency ranges (i.e., the phase response is calculable from the amplitude response). This means that if the frequency response is smooth, so is the phase response, and as a result, the impulse response is unblemished by ringing.

When multiple transducers are combined into a system, the correspondence between amplitude and phase is modified in the crossover frequency ranges because the transducers are at different points in space. There are propagation path-length differences to different measuring/listening points. Delays are non-minimum-phase phenomena. In the crossover regions, where multiple transducers are radiating, the outputs can combine in many different ways depending on the orientation of the microphone or listener to the loudspeaker.

The result is that if one chooses to design a loudspeaker system that has linear phase, there will be only a very limited range of positions in space over which it will apply. This constraint can be accommodated for the direct sound from a loudspeaker, but even a single reflection destroys the relationship.

As has been seen throughout Part One of this book, in all circumstances, from concert halls to sound reproduction in homes, listeners at best like or at worst are not deterred by normal reflections in small rooms. Therefore, it seems that (1) because of reflections in the recording environment there is little possibility of phase integrity in the recorded signal, (2) there are challenges in designing loudspeakers that can deliver a signal with phase integrity over a large angular range, and (3) there is no hope of it reaching a listener in a normally reflective room. All is not lost, though, because two ears and a brain seem not to care.

Many investigators over many years have attempted to determine whether phase shift mattered to sound quality (e.g., Greenfield and Hawksford, 1990; Hansen and Madsen, 1974a, 1974b; Lipshitz et al., 1982; Van Keulen, 1991). In every case, it has been shown that if it is audible, it is a subtle effect, most easily heard through headphones or in an anechoic chamber, using carefully chosen or contrived signals.

There is quite general agreement that with music reproduced through loudspeakers in normally reflective rooms, phase shift is substantially or completely inaudible. When it has been audible as a difference, when it is switched in and out, it is not clear that listeners had a preference.

Others looked at the audibility of group delay (Bilsen and Kievits, 1989; Deer et al., 1985; Flanagan et al., 2005; Krauss, 1990) and found that the detection threshold is in the range 1.6 to 2 ms, and more in reflective spaces.

Lipshitz et al. (1982) conclude, "All of the effects described can reasonably be classified as subtle. We are not, in our present state of knowledge, advocating that phase linear transducers are a requirement for high-quality sound reproduction." Greenfield and Hawksford (1990) observe that phase effects in rooms are "very subtle effects indeed," and seem mostly to be spatial rather than timbral. As to whether phase corrections are needed, without a phase correct recording process, any listener opinions are of personal preference, not the recognition of "accurate" reproduction.

In the design of loudspeaker systems, knowing the phase behavior of transducers is critical to the successful merging of acoustical outputs from multiple drivers in the crossover regions. Beyond that, it appears to be unimportant.