Manufacturer's Specifications
System Type: Three way, reflex.
Drive Units: MS DCB 300 bass, MS DCM 135 midrange of special design, and Isophon 25-mm soft-dome tweeter.
Frequency Response: 38 Hz to 20 kHz, ±2 dB.
Crossover Frequencies: 500 and 4000 Hz.
Nominal Impedance: 8 ohms.
Power Handling: 250 watts continuous, contoured noise.
Dimensions: 810 mm (31.9 in.) H x 385 mm (15.2 in.) W x 325 mm (12.8 in.) D; stands, 230 mm (9 in.) H x 385 mm (15.2 in.) W x 350 mm (13.7 in.) D.
Weight: 29 kg (63.8 lbs.).
Price: $1,740.00 per pair with stands.
The Signifer is a full-range loudspeaker system manufactured by Mordaunt-Short Ltd., UK. Standing slightly over a meter when placed on its stand, the enclosure houses a three-way reflex system. Finish is in wood veneer and accent is provided by a brown-cloth snap-on grille.
A woofer with 25-cm active piston diameter is used in a vented design enclosure to cover the frequency range from 20 to 500 Hz. A 135-mm midrange driver covers the range from 500 Hz to 4,000 Hz, and 25-mm dome tweeter provides the very highest frequencies. The midrange and tweeter are positioned slightly to one side of the vertical axis of the en closure, and the two systems come as mirror image pairs.
The cabinet is finished in wood veneer on all sides and front. The front panel is recessed one centimeter so that the snap-on grille essentially fits flat with the sides when in place. A metal strip on the lower part of the front panel holds a five-position rotary switch labeled "acoustic environment balance control," marked in unit steps from -2 to +2.
Connection is made to push-on posts mounted in a recessed cavity on the rear of the enclosure. This cavity also houses a 3.15-ampere fuse. Connection polarity is clearly marked by a combination of red cap and raised + symbol. No difficulty should be experienced in proper electrical connection to these loudspeakers, even by those with no previous experience in hookup.
The loudspeakers come with a parts kit containing a spare fuse and a length of trimmed and tinned hookup wire. There is also a reprint of a review which this system received in the UK, but absolutely no user manual or instructions of any kind. A check with the manufacturer revealed that this was not an inadvertent oversight in shipping, and that a brochure is in process of preparation. As a consumer advocate, I personally object to any situation of this kind. Firstly, the mounting stand must be assembled by the user, without instructions. Granted, it doesn't take an Einstein, but there are some who might like a sketch of what it is supposed to resemble when assembled, and what to do with it.
Second, our correspondent did point out one potentially significant user item, to quote: "Please take note of the patented midrange drive unit. It has no conventional surround and this severely limits the excursions. This physical limitation to frequency response insures a flat, smooth midrange." Finally, the speakers are supplied with tweeter and midrange as mirror-symmetric pairs, but there is no identification on the speakers as to which should be left or right in a conventional stereo listening configuration. The correspondent pointed out this design feature and stated that the tweeters should be placed towards the middle, determining left and right. In my opinion, a simple L and R on the back of the speakers would serve the purpose nicely.
With that off my chest, I must point out that the Signifers show excellent workmanship and attention to detail.
Laboratory Measurements
The magnitude of impedance which the Mordaunt-Short Signifier presents to a power amplifier is shown in Fig. 1. Two measurements are included, equalizer at nominal and equalizer at maximum clockwise position (+2). Although this is rated as an 8-ohm system, a minimum impedance of 4 ohms is reached at 1.5 kHz in the +2 equalizer position. I recommend that this speaker therefore be considered a 4-ohm sys tem from the standpoint of drive requirements and amplifier connection.
On the matter of amplifier connection, Mordaunt-Short provides a length of hookup wire with each speaker. This wire is quite small in diameter and has a net short-circuit impedance of 0.342 ohms resistance and 5.5 microHenries inductance. In view of the extreme loudspeaker impedance change, over five to one, from 500 Hz to 1.5 kHz, I recommend that this wire not be used since its line drop will change the acoustic response by nearly 1 dB in this important part of the frequency range. Use large diameter hookup wire for this speaker.
The complex impedance for the +2 equalizer position is plotted in Fig. 2. Although the lowest magnitude of impedance is reached at 1.5 kHz, the impedance around 1 kHz will present a more severe load to many power amplifiers.
The impedance of around 4 ohms with a capacitive reactance phase shift of nearly 45 degrees may cause difficulty for some power amplifiers, particularly those amplifiers with internal current limiting protection circuits. High-quality power amplifiers, capable of driving low impedances, should be used with this loudspeaker.
The one-meter on-axis frequency response is shown in Figs. 3 and 4. Figure 3 is the sound pressure amplitude for a constant-voltage drive corresponding to one average watt into an 8-ohm resistance. The equalizer is set to its nominal position for these measurements. Low frequency response is uniform down to a cutoff of around 40 Hz. With the exception of a broad dip at 5 kHz, the anechoic response is quite smooth out to 15 kHz.
Fig. 1--Impedance measured at the terminals for two equalizer settings.
Fig. 2-Complex impedance at the terminals for the +2 equalizer setting.
Fig. 3--Anechoic frequency response at one meter on-axis for a constant-voltage
drive level corresponding to one average watt into 8 ohms.
Fig. 4--Phase response measured in the same position as Fig. 3, corrected
for (A) acoustic time delay of woofer, 3.9170 mS; (B) acoustic time delay
of midrange, 3.3482 mS, and (C) acoustic time delay of tweeter, 3.0965 mS.
Fig. 5--Three-meter room test.
Fig. 6--Horizontal polar energy response.
Fig. 7--Vertical polar energy response.
The one-meter anechoic phase response is shown in Fig. 4.
Since acoustic positions differ for woofer, midrange, and tweeter, three phase measurements are shown, and each phase measurement is corrected for the appropriate air path and time delay. On this axial location, the sound from the tweeter arrives before the sound from the midrange and woofer drivers. The midrange sound is delayed 0.2517 millisecond relative to the tweeter, and the woofer is delayed 0.8205 millisecond. The phase measurement shows that acoustic crossover frequencies are 1 kHz and 7 kHz. The polarity of drivers is such that a positive-going voltage on the correspondingly marked loudspeaker terminals produces an in-phase positive-going sound pressure at the listening location for sound from the woofer and tweeter, and negative-going for the midrange. The sound from each driver (woofer, midrange, and tweeter) is minimum phase, but the combination of polarity reversals and differential time delay cause the net response to be of non-minimum phase type.
The three-meter room response, Fig. 5, is the frequency spectrum of the first 13 milliseconds of sound which arrives at an average listening position three meters from the loudspeaker and one meter above a carpeted floor. The system was placed 25 centimeters in front of a rear wall for this test. The upper curve is the response directly in front of the loudspeaker. The lower curve, displaced 10 dB for clarity of presentation, is the response 30-degrees off-axis with the speaker in a left-channel stereo listening position. This room response is smoother in upper register than the anechoic response, which is quite unusual. The difference is the listening position relative to the front of the enclosure. Direct on-axis room listening provides better high-frequency performance than off-axis listening, as was confirmed by the earlier listening test. The speakers should be rotated toward the listening location for best high frequency performance.
Horizontal and vertical polar energy responses are shown in Figs. 6 and 7 respectively. These measurements verify that significant reduction of direct sound energy occurs for horizontal listening positions more than 20 degrees off the geo metric axis. The Signifers should be rotated toward the preferred listening position for best stereo balance as well as frequency response. Since there is a substantial amount of sound energy launched upward, as well as forward, these speakers should not be placed directly under acoustically reflecting objects, such as overhanging shelves.
Harmonic distortion for the musical tones of E1, A2, and A4 is shown in Fig. 8. Distortion components remain moderately low up to an average power of 60 watts. Above this level, the tones of E1 and A2 show some acoustic distress.
Intermodulation on A4 (440 Hz) by low E1 (41.2 Hz), when both are mixed at equal drive level, is shown in Fig. 9. Up to 10 average watts, intermodulation is principally phase modulation of the higher tone by the lower tone. Above this drive level, a small amount of amplitude modulation begins to creep in to add to the phase modulation. The phase modulation does not significantly rise for levels above 20 watts. A burst of 100 average watts produces 3 degrees peak-to-peak phase modulation and 10 percent amplitude modulation with a net 5 percent reduction and 1.5 degree phase advance of the tone of A4.
We measure musical dynamic capability of a speaker by the extent to which a low-level inner musical voice has its level changed by the presence of broad-band high-level sounds not otherwise related to that tone. It is a measure of dynamic compression which we call a crescendo test. The Signifer passed this test exceedingly well. A tone of middle C (262 Hz) had its level changed less than 0.1 dB by the addition of white noise with an rms level 20 dB above that of the tone. This property was maintained up to peak levels of over 250 watts. This means that solo instruments will not jump kangaroo fashion on the stereo stage with loud orchestral outbursts.
Fig. 8--Harmonic distortion for the tones of E1 or 41.2 Hz, A2 or 110 Hz,
and A4 or 440 Hz.
Fig. 9--IM distortion on A4 (440 Hz) by E1 (41.2 Hz) when mixed in one-to-one
ratio.
Fig. 10--Energy-time curve.
Another measurement related to subjective stereo dynamics is that of acoustic transfer gain. An ideal speaker is one for which each dB increase of drive voltage produces a dB in crease of sound pressure. If the sound pressure increment is less than a dB per dB, the effect is similar to that of compression. If the compression, or expansion, is frequency dependent as well as power level dependent, both timbre and dynamics will be adversely affected. The Signifer has a mild compression in transfer gain with power level but little change with frequency. Relative to 0.1 average watt, the one-watt sound of 50 Hz is down 0.2 dB, 110 Hz is down 0.2 dB, 262 Hz is down 0.1 dB, and 440 Hz is down 0.25 dB. At 10-watts drive the compression is 0.5 dB, 0.4 dB, 0.6 dB, and 0.4 dB for these same frequencies. Above 60 watts, the compression on 50 and 110 Hz is greater than one dB, while middle C and A4 hold up well. This indicates that the Signifer may tend to soften orchestral dynamics at high sound levels, but should cause very few timbral changes during such passages.
The one-meter energy-time curve is shown in Fig. 10. The first sound from the tweeter arrives at 3.03 milliseconds and accounts for the substantial peak of early energy. The mid range driver contributes most of the energy which is seen from 3.2 to 4 milliseconds. Subsequent energy peaks are due to sound reflecting off edges on the front of the enclosure.
All in all, this is a respectably good energy response and supports the smoothness of frequency response measured under anechoic conditions.
Listening Test
The listening tests were performed prior to measurement with the Signifers placed approximately 25 centimeters in front of a draped wall. Spacing between the speakers was such that they subtended a 60-degree listening angle at a distance of 3 meters.
The Signifers were quickly shown to have a moderate amount of horizontal beaming in the highest registers. It became necessary, in my opinion, to rotate each speaker toward the listening position in order to provide best spectral balance and most realistic stereo image.
The 5-position rotary switch labeled "acoustic environment balance control," on the front panel of each speaker system, is subtle in operation and affects only the highest registers. After considerable experimentation with various types of program material, I came to the conclusion that the central position provided the best compromise for the room in which the listening test was performed.
These speakers also evidenced a moderate amount of vertical beaming of high frequencies. Good stereo imaging could not be consistently maintained as I moved about the general listening area. Timbral changes and spatial image shift are problems caused by any large changes in listening position.
The bass is well balanced, although there is no super low end. Response extends smoothly down the register to some where near low E1 then gracefully exits to inaudibility.
The top end is somewhat bright, but clean and free of audible distortion at moderate sound levels, but becomes somewhat harsh at high drive. The upper midrange, in the octave above middle C, sounds harsh, to my ears, even at moderate level. In way of explanation, when I say moderate level, I mean loud; when I say high level, I mean darn loud.
Vocals, both solo and choral, are reproduced with good stereo illusion of presence. Stereo imaging, both in lateralization and depth, comes off well for this class of material.
In my opinion, there is a tendency for some instruments, notably horns, to smear in spatial position when reproduced at high level. Thus, while simple orchestral dynamics are well reproduced, certain loud passages showed distress. In view of the nature of impedance (which was measured after the listening test is performed), a second listening test was made to determine if the power amplifier was getting into trouble. Careful check showed that it was not the amplifier.
The Signifer does a good job of reproducing piano, which is a tough test. While one would never be fooled into believing we were listening to a live piano, the sonic accuracy is quite good, above average for speakers in this price range.
Percussive sounds are also handled well by this system. Cymbals, blocks, and hand claps come through a bit bright, but accurately located in the stereo stage.
On the average, I feel that this system provides excellent value for those who want accurate sound at less than lease-shattering acoustic levels.
-Richard C. Heyser
(Audio magazine, Feb. 1981)
Also see:
Monitor Audio Studio 10 Speaker (Equip. Profile, Jul. 1991)
Mission 735 Speaker (Jan. 1996)
NHT Model II Speaker (Jul. 1990)
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