When measuring the phase shift caused by the 24 dB/octave (or higher) low-pass filter, most likely set in the 15 kHz to 18 kHz range, you might well be concerned that the phase shift introduced by the filter could cause less than optimal results.
There are two things to consider here:
1) The phase shift that would be introduced by the breakup mode of the driver will almost always be far worse.
2) The effect of the phase shift can be decreased, possibly to a very small level, by simply employing the adjustable phase filters and/or all-pass filters on the DSP.
Is the result of using all-pass and phase filters audible? Yes, in quiet, controlled conditions while listening to acoustic instruments, albeit subtlety.
In large-scale sound reinforcement conditions with very loud rock or electronic music, improvements will most likely be inaudible by engineers and patrons alike.
Phasing Out Anomalies
Let’s take a closer look at how equalizing a frequency response anomaly can actually improve the phase response, in addition to fixing the amplitude response.
In Figure 4 we see how the loudspeaker’s amplitude-versus-frequency response (upper trace) is largely flat from 43 Hz to 400 Hz. However, the loudspeaker exhibits one prominent dip centered at 175 Hz, followed by a reciprocal peak that starts at 210 Hz and doesn’t settle out until about 290 Hz. This is very typical of 12-inch cone drivers in moderate-sized enclosures.
Figure 4: The top trace depicts the acoustic response of the cone driver showing a -4 dB dip at 175 Hz followed by a broader +4 dB boost centered at about 210 Hz. The lower trace shows the corresponding phase response. The horizontal scale ranges from 0 Hz to 400 Hz, while the vertical is scaled at 6 dB per major division. (click to enlarge)
It’s also an important part of the frequency spectrum that covers baritone vocals and the fundamentals of many instruments. Notice how the disruption in the phase versus frequency response (lower trace) perfectly mirrors the anomalies in the amplitude versus frequency response (upper trace).
By the way, note that the numerous phase “wraps” that occur at about 60 Hz in the LF section and dissolve into something akin to “phase noise floor” as the frequency decreases are the result of the phase-inversion that occurs in any ported enclosure. This is not to be feared! Ported enclosures can sound very, very good when ported properly and “steered” with wise choices on the front-end signal processing. It’s hard to imagine a non-ported pro audio high-power loudspeaker design, though some have been tried in the past, such the Meyer-influenced “System 80” used by McCune and FM Productions in the late 1970s.
In Figure 5, we’ve fixed the amplitude response anomaly with a simple pair of parametric EQ filters (one cut and one boost). Lo and behold, the phase response has been fixed as well! When one listens to compare the sonic output, with and without these two filters, the difference is profoundly slanted towards the much better sounding “fixed” version.
Figure 5: The top trace depicts the acoustic response after the application of the two parametric EQ filters mentioned in the text, while the bottom trace reflects the improved phase response. The H and V scaling is the same as in Figure 4. (click to enlarge)
Although the phase response could still benefit from additional filters to flatten it throughout its operating range (Figure 6), it now follows a gradual trajectory instead of exhibiting a distinct ripple, as it did prior to the introduction of the parametric EQ filters.
Figure 6: The top trace depicts the acoustic response after the application of the two PEQ filters as in Figure 5, while the bottom trace shows the electrical response of the two PEQ filters, right out of the DP548. The overall system response can be further flattened by adding additional filters throughout the loudspeaker’s operating range. (click to enlarge)
Though a great many factors determine how a given system will sound, the informed usage of DSP to optimize loudspeakers should be high on the list of tools that can improve your craft.
Ken DeLoria is senior technical editor for Live Sound International and has had a diverse career in pro audio over more than 30 years, including being the founder and owner of Apogee Sound.