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So how do the crossover filters compensate for the basic drive-unit behavior? Their electrical responses, measured at the speaker terminals, can be seen in fig.5. The woofer drive signal gently slopes down above 150Hz to compensate for the driver's rising on-axis response, with the upper midrange shelved down by 7dB compared with the level at 100Hz. The electrical crossover point to the tweeter is high, at 4.2kHz, with then a second-order, 12dB/octave rollout. The tweeter drive signal is peaked up in the top audio octave to compensate for the unit's dip in this region. Its high-pass slope is…
Carrying out a spatially averaged 1/3-octave spectrum analysis for left and right speakers individually in my room and averaging the results gives the curve shown in fig.10. The unevenness between 50Hz and 300Hz is due to room effects that have not been eliminated by the spatial averaging. Superbly even in balance with, as conjectured earlier, just a slight depression in the presence region, this curve ties in neatly with my listening impressions: the Silver Signature is fundamentally a neutral, uncolored loudspeaker, with good small-signal low-frequency extension to the 32Hz band.
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Because I review the very-low-distortion Velodyne DF-661 in this issue, the levels of the B&W Silver Signature's individual harmonics are given in Table 1. (Note that I've listed the speaker's impedance magnitude at each of the test frequencies. This is because the 2.83V and 8.95V voltage levels I used will only be equivalent to 1W and 10W power levels if the impedance is exactly 8 ohms.)
Table 1: Distortion Harmonic Levels in dB ref. Fundamental Level
Frequency (Impedance)Drive Power2nd3rd4th5th 100Hz (11.9 ohms)2.83V 0.67W-40.5-53.3 -75.7-68.6 8.95V 6.7W-31.0-40.0-57.6-…
There is one date I dread every year: my wife's birthday. After nearly 16 years of marriage, I have exhausted every last iota of my spousal resources in trying to think of a suitable present. Nothing too ordinary, nothing too out of the ordinary, nothing that will trigger those dreaded words, "You did keep the receipt, right?"
This year was different. First, computer catalogs started appearing in my listening room, each opened to a specific page. Of course, I didn't get to be a long-term husband by displaying too much initiative, so I asked.
"I want an iPod. The 30-gigabyte model…
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.
In the first part of this series of articles, I examined why I feel a review magazine should include measurements in its loudspeaker…
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…