Sidebar 4: Measurements
I performed a complete set of measurements on a Dynaudio Confidence 20 Active Space standmount loudspeaker, serial number 10177473, with the analog output of my DRA Labs' MLSSA system connected to the XLR jack on the back of the integral stand. (The speaker was set to Left/ Primary.) I then repeated some of the measurements with the MLSSA output digitized with my Ayre QA-9 A/D converter and fed to the loudspeaker's AES3 digital input. This input locked to data with a sample rate up to 192kHz. I used a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the Dynaudio Confidence 20A's farfield behavior and dispersion and an Earthworks QTC-40 mike for the nearfield responses. I disabled upsampling for the measurements.
Dynaudio doesn't specify the Confidence 20A's sensitivity. Sending the speaker a pseudorandom noise signal with a 20kHz bandwidth and an amplitude of 125mV peak–peak gave an SPL of 89.8dB(B) at 50". The Analog sensitivity was set to "Med" for this measurement. Setting it to "High" increased the SPL by 4dB; setting it to "Low" decreased the SPL by 14dB.
The nearfield response of the woofer (fig.2, blue trace) has its minimum motion notch, which is when the cone is held stationary by the back pressure from the port resonance, at a low 35Hz. The nearfield response of the flared, downward-firing port on the Confidence 20A's base (red trace) peaks between 25Hz and 55Hz with a clean low-pass rolloff, apart from a strong resonant mode at 700Hz.
Footnote 1: This means that the loudspeaker is firing into hemispherical space rather than a full sphere. A speaker that has a truly flat response in the usual "4pi" space will therefore appear to have a boosted upper-bass output with a nearfield measurement, the center frequency of that boost depending on the physical dimensions of the speaker and the woofer alignment. See this explanation. The nearfield measurement and a truly anechoic measurement are what is called "limiting cases." The speaker's in-room low-frequency behavior will be somewhere between these extremes, depending on the size of the room.
Fig.1 Dynaudio Confidence 20 Active, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall. (Measurement bandwidth, 2kHz).
I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer with the speaker's SPL at 96dB(C), slow ballistics, measured at 1m. I found a high-Q (Quality Factor) resonant mode at 137Hz on the loudspeaker's top and side panels with some lower-Q modes at lower levels (fig.1). All the modes were low in level, so it is unlikely they will have audible consequences.
Fig.2 Dynaudio Confidence 20 Active, anechoic response on the tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response with the analog input (black trace) and with the digital input at 192kHz (green), with the nearfield responses of the woofer (blue) and port (red) and the complex sum of the nearfield responses respectively plotted below 300Hz, 900Hz, and 300Hz.
The black trace below 300Hz in fig.2 shows the summed nearfield response of the woofer and port. The 3dB rise in the mid- and upper-bass regions will be due to the nearfield measurement technique, which assumes that the drive units are mounted in a true infinite baffle (footnote 1). The Dynaudio's low frequencies are impressively extended for a relatively small loudspeaker; the output is down by 6dB at 27Hz, though the high-pass rolloff has a fast, sixth-order slope.
The black trace above 300Hz in fig.2 shows the Confidence 20A's quasi-anechoic farfield response at a 50" microphone distance, averaged across a 30° horizontal window centered on the tweeter axis, taken without the grille, with the Sound Balance set to Neutral (see later), with the analog input. The response is generally even, with some small peaks and dips in the upper midrange and treble and a slight lack of energy in the presence region. The response rolls off extremely rapidly above 22kHz, reaching full stop-band attenuation at 24kHz, which indicates that the analog input is digitized with a 48kHz sample rate. Repeating this measurement with digital data sampled at 192kHz (green trace) gave an identical response except that the Esotar3 tweeter's output now extended to the 30kHz upper limit of the MLSSA measurement bandwidth.
Fig.3 Dynaudio Confidence 20 Active, anechoic response on the tweeter axis at 50" with Sound Balance set to Dark (red), Neutral (green), and Bright (blue) (2dB/vertical div.)
Fig.4 Dynaudio Confidence 20 Active, nearfield woofer response with Position set to Free (green), Wall (blue), and Corner (red) (5dB/vertical div.)
A three-way switch on the integral stand's rear panel allows the Dynaudio Confidence 20A's high-frequency balance to be adjusted. The green trace in fig.3 shows the anechoic response on the tweeter axis with the Sound Balance set to Neutral. Setting it to Bright (blue trace) increases the level in the top octaves by 1.5dB, while setting it to Dark (red) reduces the level by the same 1.5dB. Another switch adjusts the low-frequency behavior to compensate for room placement issues; fig.4 shows the woofer's nearfield response with the Position switch set to Free Space (green trace), Wall (blue), and Corner (red).
Fig.5 Dynaudio Confidence 20 Active, 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 Dynaudio Confidence 20 Active, 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 Confidence 20A's horizontal dispersion, normalized to the Neutral response on the tweeter axis, which thus appears as a straight line. The slight lack of presence-region energy fills in to the speaker's sides, which implies that the most neutral treble balance will be achieved if the speakers are not toed all the way in to the listening position. The radiation pattern is otherwise even and well controlled, a likely indicator of accurate and stable stereo imaging. The dispersion narrows above 10kHz. The Dynaudio speaker's radiation pattern in the vertical plane, again normalized to the response on the tweeter axis, which is 40" from the floor, is shown in fig.6. A suckout at 2.66kHz starts to develop 15° above the tweeter axis, which is close to the specified crossover frequency between the woofer and the tweeter of 2.5kHz.
Fig.7 Dynaudio Confidence 20 Active, analog input, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
In the time domain, the Confidence 20A's step response on the tweeter axis, taken with the analog input, is shown in fig.7. Although the speaker's crossover is implemented with DSP, this hasn't been used to adjust the arrival times of the drive units so that they arrive simultaneously at the microphone. The step response is therefore time coherent rather than time coincident; it indicates that both drive units are connected in positive acoustic polarity, with the tweeter's output leading that of the woofer. The decay of the tweeter's step smoothly blends with the start of the woofer's step, which implies an optimal crossover topology. Note that the tweeter's output doesn't arrive at the microphone until 5.4ms rather than the usual 3.7ms at a 50" microphone distance. The 1.7ms latency will be due both to the DSP-implemented crossover and to the A/D conversion. Repeating the step response measurement with digital data sampled at 192kHz gave a latency of just 1.1ms.
Fig.8 Dynaudio Confidence 20 Active, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The Confidence 20A's cumulative spectral-decay, or waterfall, plot (fig.8) is very clean overall, though some low-level delayed energy is present at the top of the midrange unit's passband. Ignore the small ridge of delayed energy close to 16kHz, which is due to interference from the MLSSA host PC's video circuitry, but note that there is a ridge of aliased energy at 24kHz.
Overall, the Dynaudio Confidence 20 Active Speaker offers excellent measured performance. The Sound Balance and Position controls will provide extra flexibility in room placement.—John Atkinson
Footnote 1: This means that the loudspeaker is firing into hemispherical space rather than a full sphere. A speaker that has a truly flat response in the usual "4pi" space will therefore appear to have a boosted upper-bass output with a nearfield measurement, the center frequency of that boost depending on the physical dimensions of the speaker and the woofer alignment. See this explanation. The nearfield measurement and a truly anechoic measurement are what is called "limiting cases." The speaker's in-room low-frequency behavior will be somewhere between these extremes, depending on the size of the room.
