Audio Physic Step loudspeaker Measurements

Sidebar 2: Measurements

The Step was moderately sensitive for a tiny speaker, at an estimated 84dB/W/m (B-weighted). Its impedance characteristic, however, will make demands on the partnering amplifier, as shown by the plot of impedance magnitude and phase against frequency (fig.1). The magnitude drops below 4 ohms throughout the lower midrange, reaching a minimum value of 3.4 ohms at 200Hz. In the treble, however, the impedance stays above 10 ohms. The shape of the plot suggests that tube amplifiers with high output impedances will make the Step sound up-tilted. There are a few wrinkles in the traces, presumably due to resonances of various kinds.

Fig.1 Audio Physic Step, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The tuning of the Step's rear-facing port is revealed by the valley between the twin bass peaks to lie at a highish 60Hz. This is confirmed by the responses of the woofer and port, measured in the nearfield (ie, with the microphone almost touching the radiating surface). These are shown to the left of fig.2: the port response is the bandpass peaking between 60 and 70Hz—this commendably free of midrange pipe resonances—while the woofer is the trace with a null in this region. The third low-frequency trace in fig.2 is the complex sum (magnitude and phase) of the woofer and port outputs. Slightly underdamped, this rises to a slight peak between 70 and 100Hz compared to the lower-midrange level, with then a steep rollout. The –6dB point (ref. the 200Hz level) lies at 53Hz, which is actually quite low for such a small speaker.

Fig.2 Audio Physic Step, anechoic response on tweeter axis at 45" averaged across 30° horizontal window and corrected for microphone response, with nearfield woofer and port responses below 300Hz and 1kHz, respectively, and their complex sum.

To the right of fig.2 is shown the Step's quasi-anechoic response on the tweeter axis, averaged across a 30° horizontal window. The upper midrange appears to be somewhat boosted in level, which normally correlates with a subjective impression of "forwardness." As JE found the balance to sound distant, however, I suspect he was responding to the relative lack of energy in the lower mids—the saddle in fig.2 between the upper-bass and upper-midrange prominences—but perhaps more so to the mid-treble suckout.

As JE noted, this is an on-axis phenomenon, the cursor position in fig.3 showing that the suckout fills-in to the speaker's sides. (Only the differences between the on- and off-axis responses are shown in this graph, which is why the reference response in the center appears to be a straight line.) Other than that, the Step's horizontal dispersion appears to be typical of a minimonitor: an even rolloff of the highs as the listener moves to the speaker's sides. (The apparent ultrasonic peak in fig.3 is again due to an on-axis notch filling to the speaker's sides.)

Fig.3 Audio Physic Step, horizontal response family at 45", normalized to response on tweeter axis, from back to front: differences in response 90°–5° off-axis; reference response; differences in response 5°–90° off-axis.

Vertically (fig.4), the Step's tonal balance does change quite a lot with listener height. Listen too high and a monstrous suckout occurs, centered on 2.5kHz (the crossover frequency, I assume). Move down just below the tweeter, as would be the case with the speaker on its back-tilted stands, and the mid-treble suckout fills in, giving the flattest treble response. This can be seen in fig.5, which shows the overall response on an axis 10° below the tweeter, which is that provided by the dedicated stand's tiltback. While the upper midrange is still a little forward, the treble is much smoother, overall. Sit much below the woofer axis, however, and the crossover notch reappears.

Fig.4 Audio Physic Step, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 45°–5° off-axis above; reference response; differences in response 5°–45° off-axis below.

Fig.5 Audio Physic Step, anechoic response on axis 10° below tweeter at 45", corrected for microphone response, with complex sum of nearfield woofer and port responses below 300Hz.

In the time domain, the Step's step response on the tweeter axis (fig.6) suggests that the drive-units are connected in opposite acoustic polarity—as would be required by a second-order crossover. The tweeter's output is the negative-going spike; the woofer's is the slower, broader, positive-going pulse. Moving the microphone down to the optimum axis gave a better step response (fig.7), due to the approximated drive-unit time alignment this gives. The speaker will still not be 100% phase-coherent on this axis, due to the conflicting drive-unit/crossover polarities, but I measured no more than –70° of excess phase in the mid-treble, which is good. The speaker's cumulative spectral-decay, or waterfall, plot (fig.8) reveals a degree of treble and upper-midrange hash, but the first 12dB or so of decay is clean.

Fig.6 Audio Physic Step, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

Fig.7 Audio Physic Step, step response on optimum axis at 45" (5ms time window, 30kHz bandwidth).

Fig.8 Audio Physic Step, cumulative spectral-decay plot at 45" (0.15ms risetime).

Although there were some wrinkles suggestive of resonances in the Step's impedance plot, using a simple PVDF-tape accelerometer to examine the vibrational behavior of the speaker cabinet's surfaces revealed it to be quite inert. The only significant resonant mode I could find was on the top, but that lay at 586Hz—high enough in frequency not to be excited by sustained musical notes.—John Atkinson

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COMMENTS
FSonicSmith's picture

Who is Jack English?

Separate and aside from that issue, what is it with S'Phile and minimonitors? While it is true that all of audio comprises compromises, I feel that for over twenty years S'Phile has unduly emphasized minimonitors. I blame JA prinicipally (how long were B&W SS's in his primary system?), and Tom Gillette secondarily. When, ever, have the reviewers or contributors of this fine magazine pointed out to and educated the reader that sound reproduction is not best considered in terms of frequency response, but based instead on energy production throughout the most important portions of the frequency spectrum? I was a long time owner of minimonitors and then I learned an important lesson; minimonitors excel at all the things that tend to distract away from the essence of the music. Things like pin-point imaging, soundstaging, wrap-around, halo efffect, and laser-like treble. At the end of the day, they are sound effects and not music. I would go so far (admittedly an extreme view to make a point) that for too many audiophiles, minimonitors incentivize the owner to set up their speakers incorrectly and to listen for all the wrong things. For those that have apartments or small listening rooms and for those that are on a low budget, they serve a great purpose. They do not deserve the emphasis they get in this magazine. 

volvic's picture

FSonicSmith, far be it from me to defend Atkinson and anyone else at S'Phile but most of us mortals live in compromised environments; small budgets, small rooms that necessitate the purchase of mini monitors.  I enjoy reading about mini monitors because I am sure like most people it is all we can afford and they are optimal for our listening environments.  Would love MBL's in my room and enjoy reading about them but get very excited when a mini monitor appears for review.  

FSonicSmith's picture

I conceded that many have small listening rooms and need minimonitors. Is that true of the majority of audiophiles in the US? Considering that the demographics demonstrate that the average reader of S'Phile is (educated guess) 38 or so and earns 100K or so, I don't think so. Let's put it simply; Stephen Mejias needs minimonitors, JA does not. Art Dudley, go figure, has his priorities right. It's about the essence of the music and not about particular facets of sound reproduction that give rise to the "gee-whiz, that's cool" response. The latter is very enticing. I learned.

John Atkinson's picture

FSonicSmith wrote:
Let's put it simply; Stephen Mejias needs minimonitors, JA does not.

I hate it when people put words in my mouth like this. You are erroneously assuming that your own needs and desires define everyone else's. A renowned audio engineer said it best decades ago: "Large speakers make large mistakes." Unlike you, I will sacrifice 2 octaves of bass extension and ultimate loudness capability in order to maximize midrange and treble purity and imaging accuracy and stability. That doesn't make me wrong; it just means my tastes and needs are different from yours.

John Atkinson

Editor, Stereophile

remlab's picture

..but if you don't know who Jack English is..

Anyway, that review was from 1994.

FSonicSmith's picture

The bad resolution photos should have been a tip-off. I usually catch the distinction between the posted-date and the original publishing date and didn't here. I feel like Roseann-Roseannadanna. Never mind. Well, mostly never mind.

volvic's picture

I thought your critique was based on mini monitors in Stereophile since 1994, guess the main thrust of your argument isn't lost whether it's 1994 or 2014, but I still like mini monitors.  

remlab's picture

You really should mark anything not current as historical. "Looking Back.." would be a good heading in bold face. To many people don't catch it and get confused. Personally, I love these period pieces. Keep it up..

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