NSM Model 25 Bookshelf speakers
NSM Loudspeakers manufactures its speakers in matched pairs for more accurate stereo imaging in the home listening room. Since this requires preserving and recreating the symmetry of the recorded program, NSM has gone to considerable lengths to control production tolerances in its drivers and crossover-network components.
NSM says it carefully matches every resistor, capacitor, inductor, and driver in each loudspeaker system to the corresponding part in the other speaker of the pair. High-quality components and drivers are used throughout, including metallized polypropylene and Mylar capacitors and air-core inductors. The speakers have 14-gauge internal wiring, and the drivers are mounted flush with the panel to minimize diffraction.
Like other NSM speakers, the Model 25 has a narrow (9-inch-wide) cabinet for optimum dispersion and imaging as well as high rigidity. It is constructed of 1-inch and 3/4-inch-thick fiberboard, tuned with open-cell foam and Dacron fiber, and finished in wood veneer. The cloth grille, retained by plastic snaps, is easily removable.
Recessed into the rear of the cabinet are two pairs of gold-plated five-way binding posts on standard 3/4-inch centers. These are normally strapped together; removing the jumpers enables the system to be driven in either a biwired or biamplified mode.
The Model 25 is a two-way acoustic-suspension system using a 6.5-inch plastic-cone woofer constructed on a die-cast magnesium basket and a 1-inch soft-dome tweeter with ferrofluid cooling. Relatively insensitive, with a rated sound-pressure level (SPL) of 82 dB at 1 meter with an input of 2.83 volts, it is recommended for use with amplifiers rated between 50 and 150 watts. Nominal impedance is 8 ohms, and frequency response is given as 35 to 20,000 Hz +/-3 dB at an 80-dB SPL.
The manufacturer suggests placing the speakers on 24-inch stands 18 inches from the wall behind them and 8 feet apart, but the user is encouraged to experiment with placement for best results (good advice with any speaker). Each NSM Model 25 speaker weighs 27.5 pounds. Pairs are shipped in a single 60-pound carton.
We placed the NSM Model 25 speakers on 26-inch stands and positioned them as recommended by the manufacturer. Their room response was exceptionally flat and smooth, varying only +/-4 dB from 65 to 20,000 Hz. A close-miked measurement of the woofer response showed a maximum output between 70 and 170 Hz, falling off at 12 dB per octave at lower frequencies and more gradually at higher frequencies.
Our composite response curve, formed from these two sets of data, was flat within +/-3 dB from 50 to 20,000 Hz. For most of that range the response was extremely flat, although there were some minor irregularities between 150 and 1,000 Hz.
Response measurements with a stepping one-third-octave band of pink noise were quite similar, within +/-3 dB from 58 to 20,000 Hz. The tweeter's horizontal directivity was excellent; the on-axis and 45-degree off-axis responses diverged only 2 dB at 7,000 Hz, 3 dB at 10,000 Hz, and 10 dB at 20,000 Hz.
We also attempted frequency-response measurements using the quasi-anechoic MLS program of our Audio Precision System One test instrument. These confirmed the essential features of the midrange and high-frequency response we had measured previously (+/-2 dB from 1,000 to 20,000 Hz), but they also showed a reduced and irregular output from 500 to 2,000 Hz that was completely inconsistent with what we found in the room-response measurement (or in listening tests), although a somewhat similar effect could be seen in the close-miked woofer measurement. We have no good explanation for this apparent discrepancy, but we are inclined to accept the verdict of our ears and the room measurements, which were consistent with each other.
The speaker's impedance characteristics were quite unusual. For one thing, the minimum impedance, at 138 Hz, was about 9.6 ohms. At other frequencies the impedance varied between 10 and 38 ohms. The woofer's resonance frequency was at 60 Hz. Overall, the phase angle of the impedance varied smoothly between +30 and -45 degrees, with no evidence of the sharp jogs, indicating resonances in drivers or the enclosure, that we've seen from many speakers in this measurement. The acoustic phase linearity was also good, with a group-delay variation of +/- 250 microseconds from just over 1,000 Hz up to 20,000 Hz.
The NSM Model 25's sensitivity was (as rated) very low, with an output SPL of 81 dB at 1 meter from a 2.83-volt input of pink noise. Since our woofer-distortion measurements are based on a nominal 90-dB SPL, we measured distortion at 8 volts input. It was very low, rising from 0.3 to 0.5 percent between 120 and 2,000 Hz to 1 percent between 70 and 100 Hz and 4 percent at 50 Hz. At the system's rated lower limit of 35 Hz, the output was down 10 dB from the upper-midrange average (instead of 3 dB), and the distortion was 20 percent.
The Model 25 was able to handle large input levels in our single-cycle tone-burst tests. At 1,000 and 10,000 Hz, the amplifier clipped at 220 to 240 watts into its 30-ohm impedance. At 100 Hz, the woofer began to sound hard well before its cone reached the suspension limits, and to avoid possible damage to the speaker, we did not attempt to go higher than about 265 watts into its 11-ohm impedance.
The NSM Model 25 is not a product that follows all the usual patterns of loudspeaker design. It is one of the few speakers we have tested that gave seemingly irreconcilable results with some of the very different measurement techniques we use (which one would expect to yield fairly similar data). It is also an exception in its very low sensitivity, which makes the 50- to 150-watt amplifier power recommendation one to be taken seriously. The Model 25 requires four to eight times as much power as most other speakers to achieve the same output level.
The Model 25 sounded very easy, clean, and smooth. There was no apparent excess or deficiency in any part of the audio spectrum, and its sound-stage was superb. You could not ask for a more listenable speaker, although it is not one that can be used to generate life-like sound levels.
It also did not achieve any miracles in the bass range. The 35-Hz claimed lower limit is unrealistic; a 50-Hz limit would be more accurate and more consistent with the size of the woofer and enclosure. Indeed, NSM makes a subwoofer for those who want or need more volume or deeper bass.
But within the limits of what it can do, the NSM Model 25 is a first-rate speaker whose sound justifies its price. To our ears it sounded as flat and uncolored as our measured response curve implied, which places it in the ranks of some far more expensive, very highly regarded speakers.
