Alta Audio Adam loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system, a calibrated DPA 4006 microphone to measure the Alta Audio Adam's frequency response in the farfield, and an Earthworks QTC-40 mike for the nearfield response of the woofer, both microphones used with an Earthworks preamplifier. For logistical reasons, I measured a different sample from those auditioned by Rogier van Bakel; its serial number was ADM2015.

No preferred listening axis is mentioned in the Alyssa's manual, but the center of the Adam's tweeter is 37" from the floor, which is close to the typical seated listener's ear height: 36", according to a survey performed for Stereophile by Thomas J. Norton in the 1990s. I therefore performed a complete set of measurements with the microphone placed on that axis. Except when noted otherwise, all the measurements were taken without the grille.

Alta Audio specifies the Adam's free-field sensitivity as a high 91.5dB/2.83V/m. My B-weighted estimate was significantly lower, at 84dB(B)/2.83V/m, which is 3dB lower than the average sensitivity of the speakers I have measured. I repeated the estimate using a different technique; this confirmed the low sensitivity.

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

The Alta speaker's impedance is specified as 4 ohms. My impedance measurement, taken with Dayton Audio's DATS V2 system, indicated that the impedance magnitude (fig.1, solid trace) remained above 4 ohms throughout the audioband, dropping below 6 ohms only at low frequencies. The minimum value was 4.26 ohms at 76Hz. The effective resistance, or EPDR (footnote 1), lies below 3 ohms in the bass, with minimum values of 2.46 ohms at 30Hz and 2.6 ohms at 64Hz. The EPDR lies above 4 ohms above 200Hz, however, meaning that the Adam is a relatively easy load for the partnering amplifier.

Fig.2 Alta Audio Adam, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of rear panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

There is a wrinkle just below 200Hz in the fig.1 traces. When I investigated the cabinet's vibrational behavior with a plastic-tape accelerometer, I found a resonant mode with a high Q (Quality Factor) at 195Hz on the rear panel (fig.2). The sidewalls were extremely inert, however.

Fig.3 Alta Audio Adam, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the midrange unit (black), woofer (red), top port (green), and bottom port (blue), respectively plotted below 500Hz, 400Hz, 800Hz, and 525Hz.

There is a saddle centered on 25Hz in the impedance magnitude trace, which in a reflex design would indicate that this is the tuning frequency of the port. However, as with Alta's two-way Alyssa model, which Herb Reichert reviewed in November 2020, the Adam uses what Alta describes as Extended Transmission Line, or XTL. The speaker's low-frequency drivers are loaded with a short transmission line that terminates in a port tube. As Alta's Mike Levy explained to HR, "Basically, the port tunes the speaker from resonance on up as it would in a properly tuned ported speaker, and the transmission line tunes the speaker from system resonance down. This extends the response significantly."

The problem I found with the Alyssa was that pipe resonances in the transmission line result in significant peaks and dips in the measured output of the woofer and the port (footnote 2). I found the same thing with the Adam. The complicated-looking traces below 500Hz in fig.3 show the outputs of the midrange unit (black trace), the woofer (red trace), the port at the top of the rear panel, which loads the midrange unit (green trace), and the port at the base of the rear panel, which loads the woofer (blue trace), all measured in the nearfield. In the Manufacturer's Comment in which Mike Levy responded to the Alyssa review, he wrote that the close-miking used for nearfield response measurements is "like placing the microphone close to a hollow space. Resonances are amplified." This hasn't been my experience, though the nearfield technique does boost low frequencies, due to the assumption that the drive units are mounted in a baffle that extends to infinity in both planes.

The woofer's output has the expected notch at its port tuning frequency of 25Hz, and the output of its associated port peaks just below that frequency, which implies extended low frequencies. However, both the woofer and its port have peaks and dips at higher frequencies. Looking closely at fig.3, it appears that the peaks in the port's output don't boost or cut the woofer's output at their frequencies; instead they coincide with the highest rate of change (with frequency) in the woofer's output. The same is true for the peaks in the response of the port that loads the midrange unit. Predicting the result of this low-frequency behavior at the listening distance is therefore difficult; I will be interested in reading RvB's auditioning comments.

Higher in frequency in fig.3, the crossover between the woofer and midrange unit appears to occur between 200Hz and 400Hz, though the exact frequency is obscured by the peaks and dips in the nearfield outputs. The woofer's upper-frequency rolloff is relatively slow, however, its farfield output at 1kHz being just 10dB lower than that of the midrange unit.

Fig.4 Alta Audio Adam, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responsea plotted below 400Hz.

The black trace above 500Hz in fig.3 shows the quasi-anechoic farfield response of the midrange unit and tweeter. A major suckout is apparent just above 1kHz and another between 2kHz and 4kHz, this perhaps associated with the crossover between the two drive units. In the top audio octave, there's a 3–4dB peak. This behavior can also be seen in fig.4, which shows the Adam's farfield response, averaged across a 30° horizontal window centered on the tweeter axis. Fig.4 was taken without the grille. The grille increased the depth of the suckout between 2kHz and 4kHz by 1.7dB, but the response was otherwise identical.

Fig.5 Alta Audio Adam, lateral response family at 50", 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.

Fig.6 Alta Audio Adam, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–10° below axis.

Fig.5 shows the Adam's horizontal dispersion normalized to the response on the tweeter axis, which thus appears as a straight line. The Adam's radiation pattern at higher frequencies is relatively wide and even, narrowing only slightly in the top audio octave, though the suckout just above 1kHz in the tweeter-axis response deepens to the speaker's sides. The dispersion in the vertical plane, again normalized to the tweeter-axis response, is shown in fig.6. A suckout centered at 2131Hz appears 10° above the tweeter axis, which I assume must be the crossover frequency between the midrange unit and tweeter. The tweeter's high-frequency output starts to drop 5° below the measurement axis or rise 5° above that axis, though the Adam's lower-frequency output doesn't change significantly over that vertical window.

Fig.7 Alta Audio Adam, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

In the time domain, the Adam's step response on the tweeter axis (fig.7) indicates that the tweeter and woofer are both connected in positive acoustic polarity, the midrange unit in inverted polarity. The positive-going decay of the midrange unit's output smoothly blends with the start of the woofer's step, which suggests optimal crossover implementation. However, the tweeter's output arrives a little too soon at the microphone, which means the decay of its step doesn't quite coincide with the start of the midrange unit's step. This implies that the optimal blend of the two driver outputs, at least in the time domain, will occur below the tweeter axis, though this will be an unrealistically low listening axis. The Alta speaker's cumulative spectral-decay plot (fig.8) is relatively clean in the treble, though a strong ridge of decayed energy is present at the frequency of the on-axis suckout at 1kHz.

Fig.8 Alta Audio Adam, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

The Alta Audio Adam's measured performance is a mixed bag. The enclosure is well-damped and the speaker is relatively easy to drive, though its low voltage sensitivity will be of concern to those who use low-powered tube amplifiers. The resonances in the transmission lines that load the midrange unit and woofer may look worse than they sound—Jim Austin noted in a follow-up review of the Alyssa that the airspace resonances I measured weren't audible with music, though he could hear them as modest volume changes with a slow frequency sweep—but I was bothered by their presence, nonetheless.—John Atkinson

Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and

Footnote 2: See fig.3 here.

Alta Audio LLC
139 Southdown Road
NY 11743
(631) 424-5958

leonya's picture

these medium-sized towers sound, in a word, right

No way, not with those measurements.

Poor Audiophile's picture

So they don't/can't sound "right" because of the measurements? Oh here we go again.

georgehifi's picture

That's being diplomatic.

Cheers George

ChrisS's picture

...jacket material to cover your speaker cable, or adding an Ikea cutting board under each of your components makes a difference to the sound of your system, then building and "tuning" your speaker like a musical instrument makes sense, too.

steve59's picture

I find Alta audio speakers to sound very good at shows also. Maybe it's better not to know how something measures if you like the sound?

Bacheaudio's picture

Listen at Presentation in VPI house, Huge soundstage Very controlled bass
Excellent speakers

trynberg's picture

This level of piss-poor engineering wouldn't be acceptable on a $300 pair of speakers. What a joke. I suggest the reviewer listen to music on actual high fidelity speakers to calibrate what good sound actually is. Good grief.