After being impressed with the
Monolith K-BAS bookshelf speaker in our review of them earlier this year, we inquired Monoprice about reviewing a pair of the Air Motion Cinema speakers that were launched at roughly the same time as the K-BAS. Monoprice enthusiastically accepted our offer and threw in an Air Motion Cinema Center Channel speaker for good measure. On the availability of these bookshelf speakers, Monoprice would seem to have made an imprudent decision to release two bookshelf speakers at the same time and roughly the same price bracket. However, that would be a facile analysis; these are two very different speakers that serve different needs. The chief attribute and selling point of the K-BAS speakers is the low and powerful bass extension for the size and price, whereas the main selling points of the Air Motion Cinema speakers is their AMT tweeters, discreet size, and nicely curved cabinet. The Air Motion speakers give up bass extension in favor of a smaller size and more pleasing appearance. With a stated frequency response window of 65 Hz to 25 kHz +/- 3.2 dB, the Air Motion speakers benefit from the addition of a subwoofer far more than the K-BAS. Indeed, the K-BAS bookshelf speakers are an excellent choice when bookshelf speakers are needed for a system where the use of a subwoofer is not possible. However, the Air Motion speaker’s lack of bass extension pretty much necessitates the use of a subwoofer in order to achieve a full-range sound system. This of course is no consequence in a home theater environment where a subwoofer is a must anyways.
>Another important difference is that the Air Motion speakers come with a center channel speaker, whereas the K-BAS does not. Obviously, that means unless the user can fit another K-BAS bookshelf speaker in their setup as a center speaker, the Air Motion speaker set should be used for systems with center channels (and no, it is not advisable to use the K-BAS on its side, due to off-axis lobing inherent in all 2-way speaker designs when placed horizontally). But now, setting apart comparisons between the K-BAS and the Air Motion speakers, let’s see what Monoprice has achieved in these modestly sized and inexpensive bookshelf speakers and center channel speaker. The Air Motion speakers arrived at my doorstep with good packing that is sure to protect the speakers from rambunctious shipping with a large foam piece in the top and bottom of a thick cardboard box. Inside the box, the speakers were wrapped in a soft cotton cover underneath a thick plastic bag, in order to protect the speakers from scuffs and moisture. These speakers should be safe from all but the most abusive shipping.
The most distinguishing visual trait of the Air Motion speakers is the curved cabinets, particularly the top. This isn’t something commonly seen at this low price point. That is a classy touch that sets these apart. The use of a real wood Black Oak veneer definitely adds to the aesthetic- a dignified look that would allow the Air Motion speakers to fit in a wide range of decors. With the grille on, they look innocuous- not offensive but nothing that would strike a tremendous amount of interest either. Without the grilles, they have more personality and look more interesting. One nice touch is the use of magnetic grilles so there are no grille guides to mar the appearance of the front baffle of the speaker when used without grilles. The golden yellow of the AMT tweeter also aids in their stately presence. Overall, I think these are some of the best looking bookshelf and center speakers available in their price class.
>The Air Motion Cinema 5 bookshelf speakers are a 2-way design using a 5.25” woofer and an AMT tweeter. It should be said here that this speaker does not use the inverse horn enclosure that the K-BAS does; it is a simple bass reflex design. The AMT tweeter is the Air Motion’s most distinguishing trait from a design perspective, and it is where the Air Motion speakers derive their name, as AMT stands for “Air Motion Transformer”. As opposed to conventional dome tweeters that pistonically oscillates a little dome, AMT tweeters contract and expand the folds of a pleated membrane to produce sound. To explain using an example, imagine the folded surface of a half-closed curtain- then line the interior folds of the curtain with conductive rods on adjacent sides of the folds, and set two powerful magnets on the sides of the curtain. Run some alternating current through the conducting rods and their newfound electromagnetic field will rapidly collapse and expand the folds of the curtain, and, in doing so, squeeze air in and out of the folds thereby creating pressure waves that we experience as sound. See the below illustrations taken from Oskar Heil’s 1976 patent of the AMT transducer, which are cross sections of the ‘folds’ of the tweeter diaphragm, first at rest and then in motion.
AMT tweeters were initially used to great acclaim by the speaker company ESS back in the early 70’s. AMT tweeters have had a comeback in popularity in recent years after having spent decades dormant since ESS’ demise. A major advantage in this design is that since it is the folds of the diaphragm are much deeper than they are wide, air is ejected out much faster speed than the vibration of the diaphragm itself- as much as five times faster. Another advantage is the very light mass of the diaphragm that makes it very easy to move and to change direction since it does not have the momentum of the weight of typical dome tweeters. These elements give AMT tweeters an extended response well into ultrasonic frequency ranges. Also, since AMT tweeters can have a relatively large surface coupled with the air, they can have a very wide dynamic range.
Removing the tweeter, we can see a felt pad on the back that serves the purpose of minimizing backwave pressure from woofers that share the >same enclosure. Happily for the Air Motion speakers, that is not needed, since the tweeter is in its own sealed compartment within the cabinet. We are told by Monoprice’s Senior Product Manager that this tweeter can be found on much more expensive speakers.
The 5.25” woofer uses a cone made from New Zealand pulp paper that has longer fibers than ordinary wood pulp. This makes for a lighter and more rigid cone that Monoprice claims will produce clearer mids and tighter bass. Removing the bass driver and examining it closely, we see a cast frame that Monoprice says is made from super low carbon steel. We also see a healthy-sized motor with a vented pole piece and a large spider for a driver of its diameter. We are also told that the motor uses a shorting ring to control inductance, which reduces even-order THD. Altogether, this woofer looks up to the task. This is certainly more than a cheap ‘barely adequate’ woofer, and it shows that Monoprice is serious about making a good speaker.
The Air Motion Cinema 5 speakers are crossed over at 2.8 kHz using a first-order crossover for the tweeter and a third-order crossover for the woofer. Monoprice must have a lot of confidence in their tweeter to use a first order filter on it. First-order filters allow the tweeter to playback quite a bit of content under its crossover point, since high-pass slope is a gradual 6 dB per octave.
The board itself looks reasonably well built, with a handful of inductors, a few decently large capacitors and a slew of resistors. Curiously, the Air Motion Center Channel speaker looks to have a simpler crossover pictured on its product page, even though it is a more complex 3-way design using a second-order Linkwitz-Riley crossover for the tweeter, a second-order bandpass crossover for the mid-range, and a first-order butterworth crossover for the woofers.>
Cabinet construction on the Air Motion speakers looks quite good. The Air Motion speakers are hefty little bookshelf and center speakers, weighing 14.3 and 26.5 lbs respectively. The knock test produces a dulled sound that bodes well for quiet paneling during the speaker’s operation. The cabinet panels are made from high-density fiberboard of a 21 mm thickness (or just over ⅘ inches), however, MDF has greater self-damping and is said to be a superior acoustic material. There is a window brace midway in the speaker between the woofer and tweeter that is also 21 mm HDF. As mentioned before, the tweeter has its own sealed compartment to completely block off backwave pressure from the woofer from interfering with the tweeter’s lateral travel during playback. Furthermore, the curved cabinet of the Air Motion speakers isn’t just for looks; by eschewing flat sidewalls, resonances caused by standing waves inside the cabinets can be reduced, thereby reducing coloration of the sound by lively enclosure panels. On the rear panel of the speaker, we see a 1.5” port and a couple of heavy-duty binding posts attached to a metal plate. With the drivers removed, we can see the cabinet is generously stuffed with Dacron. This abundance of filling is good for reducing mid and high frequency reflections from the back of the box, and it also helps to make the cabinet behave as though it is larger than it really is; this is called isothermal conversion. Isothermal conversion is beneficial to speakers, because a larger cabinet is usually more efficient.
Source : http://www.audioholics.com/bookshelf-speaker-reviews/monoprice-monolith
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