In the first part of this series of articles, I examined why I feel a review magazine should include measurements in its loudspeaker…
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This series of articles was initially written (in slightly different form), as a paper presented at the 103rd Audio Engineering Society Convention, New York, September 1997. The preprint, "Loudspeakers: What Measurements Can Tell Us—And What They Can't Tell Us!," AES Preprint 4608, is available from the AES, 60 East 42nd Street, Room 2520, New York, NY 10165-0075. The AES internet site, www.aes.org , offers a secure transaction page for credit-card orders.
An analogy can be drawn with the Heisenberg Uncertainty Principle: the more tightly you confine something in one domain, the more uncertain it becomes in an alternate domain. If you confine the rectangular pulse in the time domain to an infinitely narrow time, its spectrum in the frequency domain becomes infinite. Conversely, if you confine the frequency-domain representation of a signal, its spectrum, to a single line—ie, there is just one frequency present—the time-domain representation of the signal extends from the beginning of all time to the end of all time.
A typical loudspeaker's…
Fig.11 shows a good step response produced by a time-coherent, three-way loudspeaker, with the outputs of the three drive-units adding in-phase at the microphone position. There are not that many speakers that produce this good a step response. Of the speakers I have measured for Stereophile, only about 10—models from Quad, Thiel, Dunlavy, Spica, and Vandersteen—have step responses this good.
Fig.11 Good loudspeaker step response.
Fig.12 shows a more typical step response, again of a three-way loudspeaker. This time there are actually three step responses apparent in the…
Acoustic Phase Responses
Does a loudspeaker's time coherence matter? A "perfect" speaker, of course, would have both a perfect impulse response and a perfect frequency response (at one point in space). Another way of looking at a loudspeaker's time-domain performance is to examine its acoustic phase response, the phase angle between the pressure and velocity components of the sound plotted against frequency.
Does a loudspeaker's time coherence matter? A "perfect" speaker, of course, would have both a perfect impulse response and a perfect frequency response (at one point in space). Another way of looking at a loudspeaker's time-domain performance is to examine its acoustic phase response, the phase angle between the pressure and velocity components of the sound plotted against frequency.
Again, this is an aspect of loudspeaker behavior that has proved controversial. One school of thought holds that it is very important to perceived quality; another, which…
By contrast, fig.17 shows the excess-phase response of the speaker whose step response was shown in fig.11. This model uses a sloped front baffle and a crossover with first-order acoustic slopes to give a time-coherent performance on the listening axis. There's a little bit of positive error at high frequencies, meaning that the microphone is just a little high of the optimal axis.
Fig.17 Acoustic excess phase response of time-acoherent loudspeaker.
But, as I said above, the fact that almost no loudspeakers perform in this manner does not stop many of them from getting good…
By contrast, fig.19 shows the CSD of a loudspeaker whose on-axis frequency response is severely unflat. The graph is dominated by a severe mode at 3750Hz. In addition to the speaker's timbral problems, this resonance could be heard as adding a hard "zinginess" to the overall sound. However, loudspeakers with such audible resonant problems appear to be very rare these days.
Fig.19 Poor cumulative spectral-decay plot (0.15ms risetime).
To produce a meaningful CSD, the time data need to be free both from noise—it helps to average as many separate impulse response measurements…
It must be a good idea in general to reduce the amplitude of resonant modes, but how audible a high-Q, narrow resonance will be depends not only on its level and the area of the radiating panel, but also its frequency. If the resonance lies in the upper bass, say between 100 and 200Hz, it will likely coincide with specific musical notes as well as being excited most of the time. An octave or so higher, and a narrow-bandwidth, high-Q resonance might well "fall in the cracks" between the discrete frequencies of musical notes. (With equal-temperament Western music, the notes are farther apart…
References
[29] D.D. Rife, J. Vanderkooy, "Transfer Function Measurement Using Maximum-Length Sequences," presented at the 83rd Convention of the Audio Engineering Society (1987 October). Preprint 2502. J. Aud. Eng. Soc. Vol.37, pp.323-348 (1989 June).
[29] D.D. Rife, J. Vanderkooy, "Transfer Function Measurement Using Maximum-Length Sequences," presented at the 83rd Convention of the Audio Engineering Society (1987 October). Preprint 2502. J. Aud. Eng. Soc. Vol.37, pp.323-348 (1989 June).
[30] D.D. Rife, "Introduction, Theory, & Loudspeaker Time Coherence," DRA Labs MLSSA Reference Manual v.10.0A, pp.7-9, pp.145-150, & pp.159-163 (1996).
[31] J. Vanderkooy, "Aspects of MLS Measuring Systems," J. Aud. Eng. Soc. Vol.42, pp.219-231 (1994 April).
[32] F.E. Toole, "Loudspeaker Measurements and their…
Sidebar: The Fast Fourier Transform
As I have implied in this article, a representation of a signal in the time domain has a related representation in the frequency domain. There is a mathematical operation, the Fourier Transform, that can be used to calculate the frequency-domain representation—the signal's spectrum—from its time-domain representation—the signal's impulse response. In practice, a mathematical shortcut is used, leading to the designation of the mathematical operation as the Fast Fourier Transform or FFT (footnote 1).
As I have implied in this article, a representation of a signal in the time domain has a related representation in the frequency domain. There is a mathematical operation, the Fourier Transform, that can be used to calculate the frequency-domain representation—the signal's spectrum—from its time-domain representation—the signal's impulse response. In practice, a mathematical shortcut is used, leading to the designation of the mathematical operation as the Fast Fourier Transform or FFT (footnote 1).
The basic premise behind the application of the…
Nobody Wants a *" Drill Bit!
One of the most thought-provoking insights I've ever heard from anyone came from a marketing teacher. He claimed that, in the entire history of the human race, nearly all of the *" drill bits that have been bought were bought by people who did not want *" drill bits. What happens is, *" drill bits are bought by people who want *" holes in something. Thwack! Ah, enlightenment.
The makers of Stanley tools are more rewardingly thought of as being in the "hole-enabling" business than the "drill-bit" business. The value of this kind of head-thwack for…