Arcam Muso speaker design concept
The MUSO speaker is designed to perfectly complement SOLO Music and Movie systems. As such it is designed to have the same audiophile sound quality and unobtrusive but stylish aesthetic as the rest of the SOLO product range.
Muso is designed to fill two roles; to be a high quality home cinema speaker for 5.1 (or more) channel systems working with its dedicated subwoofer Logo and to be a full range mini monitor for smaller systems where a very high playback level is not required.
To this end, Muso is based upon a medium power mid/woofer and companion tweeter. The result is a discrete speaker that can reproduce bass information down to 70Hz and still have good dynamics and accurate midrange replay.
When used with a sub woofer in a surround sound system, such as the solo Movie, the Muso speaker is rated at 100wpc. This is because a large proportion of the audio power developed by Sub/Sat systems is delivered by the sub-woofer. By using a satellite speaker with a better bass response, it is much easier for Muso achieve a good integration between it and the Logo sub woofer. This is because Muso still enjoys a flat response in the region where the crossover operates rather than starting to roll off.
When used as a full range speaker, the MUSO is rated for working with systems of up to 30W using the IEC power rating system. Muso can cope with systems up to 50W, so long as it is not played at high volumes for extended periods of time.
Muso cabinet design
Muso was developed using many of the same anti-resonance technologies as used in Arcam's electronic equipment. For example, Muso's cabinet utilizes dissimilar materials in its construction to reduce the propagation of vibration around the cabinet. The side walls are an aluminum extrusion and the front baffle is made of machined steel. The top, bottom and binding terminal plate are die cast aluminum.
To further reduce the propagation of sound waves around the cabinet, its component parts are held together with an absorbent compliant adhesive. This means that at each transition between different panels resonances are damped by the absorption in the material holding the panels together.
To reduce any build up of panel resonance, the cabinet walls use constrained layer damping. This is based on separate layers of materials of different thicknesses bonded together with vibration absorbent adhesive. It works in the same way as the Sound Dead Steel Arcam uses in its chassis designs. As the vibration moves through the region with the different thickness layers of material, the vibration is damped down by the conflicting resonant frequencies of the two panels.
- Compliant adhesive around the top of the cabinet.
- Constrained layer damping on the cabinet wall at the bottom of the photo.
- Fixing of steel baffle to aluminum extrusion. This uses compliant adhesive and threaded fixings with nuts.
Internally all the walls of the cabinet are covered in acoustic foam. This has two beneficial effects; firstly it reduces the build up of standing waves inside the cabinet as high frequency sound is absorbed by the foam. This reduces the energy inside the cabinet so that less sound is radiated from the cabinet walls. Secondly, the foam slows the speed of sound, as sound moves much slower through the foam than it does through air. The sound waves therefore take longer to hit the cabinet side walls thus increasing the apparent volume of the cabinet and extending the low frequency response.
Photo shows use of acoustic foam.
The combination of these effects is a cabinet with very low radiation of sound and good bass response. The low radiation of sound from the cabinet significantly improves the accuracy of the sound the listener hears as it is not contaminated with sound radiated from the cabinet.
MUSO drive units
The Muso Mid / Bass drive unit?
The mid/bass drive unit in Muso uses a polymer and aluminum composite cone. This delivers a breakup characteristic much better controlled than a pure aluminum cone. Pure aluminum tends to “ring” in the breakup region thus adding harmful sonic distortion. The improved breakup characteristic simplifies the design of the crossover giving a speaker with a well controlled response and a more dynamic sound.
The large inverted dust cap on the Muso is made of doped paper as this is the lightest material available for this application. Using different materials across the driving surface of the speaker also reduces the build up of resonances in the cone.
The drive unit’s chassis is constructed from die-cast magnesium alloy. This is used over a cheaper steel chassis as it is an inherently better damped material. In addition it allows for much thinner parts to be used. Having thinner parts allows sound to pass from the back of the speaker cone to the inside of the cabinet more easily and reduces reflected sound back towards the cone which, in turn, would add distortion.
To reduce the build up of air pressure behind the dust cap the magnetic pole piece is vented so that air can pass through it into the cabinet behind. This reduces compression distortion in the drive unit particularly at higher volumes.
To reduce any build up of pressure behind the spider (the part of the driver which holds the voice coil in place), the magnesium chassis has holes to allow air to pass out the sides of the chassis. This reduces also distortion and improves sound quality as the spider can move freely.
The MUSO Mid / Bass drive unit is shielded so that the speaker can be placed close to a TV or monitor screen.
Photo of MUSO Mid / Bass drive unit showing vented pole piece (the hole in the center of the large magnet system).
The MUSO tweeter
The MUSO tweeter is a pure aluminum dome tweeter. As a tweeter dome is much smaller than a bass/mid cone the breakup region of the tweeter cone is outside the region of human hearing. This means that the tweeter behaves as a piston throughout the audible range giving the most accurate presentation of the sound. Even outside the human hearing range, the ringing caused by breakup can still cause some audible problems, so the MUSO crossover is designed to electronically reduce the ringing that would otherwise be present.
The Muso tweeter is acoustically damped on the rear surface of the magnet using specialist dampening rubber. The high frequency sound waves from the tweeter pass in both directions out of the front of the speaker and to a certain extent backwards through the tweeter itself. When the sound traveling backwards through the tweeter reaches the back surface of the tweeter some of it is radiated into the cabinet and some of it bounces off the back surface and starts traveling forward again. This delayed sound causes confusion in the sounds presentation. The damping on the back of the tweeter significantly reduces this distortion improving the clarity of the sound, particularly fine detail.
The tweeter uses a neodymium magnet that is magnetically shielded so it can be placed close to a TV or monitor screen. The voice coil of the tweeter is Ferro fluid-cooled to reduce any problems with a build up of heat. This heat would otherwise cause the resistance of the voice coil to vary. The Ferro fluid is held in place by the magnetic field of the magnet. It is designed to have good thermal conductivity so that heat is carried away from the voice coil.
Photo of the back of the MUSO tweeter showing the damping material on the rear of the tweeter.
MUSO bass alignment
The MUSO speaker uses a front facing port so that it can be used close to walls without the wall interfering with the port response. The port itself extends the bass response of the speaker. The port frequency is set to 63Hz. To achieve this without causing “chuffing” a long port of a large cross sectional area is needed. As the Muso box is not deep enough to accommodate a long port, the port is bent inside the cabinet to achieve the required length. The port has a small flare at each end to reduce any noise caused by air entering and leaving the port. The port uses extra thick side walls so it can support itself with minimal vibration even when the speaker is playing loud. The port combined with the acoustic foam enables the MUSO to have a bass response much deeper than would be expected for such a small unobtrusive speaker.
Photo shows MUSO bent port tube with extra thick side walls, also note die cast terminal connector with blind screw holes to prevent air leakage (Foam removed for clarity).
The crossover in MUSO was designed using a combination of computer simulation and many hours of listening tests. This enabled Arcam to develop a crossover that not only achieves a superb frequency response but also has a balanced sound, integrating the drive units together perfectly. The crossover uses eight parts rather than the usual five to provide additional control over the tweeter in the supersonic range where it can start to ring, and also to control the Mid / Bass drive unit in the breakup region.
The crossover is built on a fiberglass PCB to provide a good firm base for the parts. Each part within the crossover is carefully matched to a very specific requirement. The tweeter uses air cored inductors for their improved linearity and polypropylene capacitors for their stable high frequency response and sound quality. The parts within the crossover are damped, where required, to reduce any micro-phonic interaction between the sound field inside the cabinet and the parts of the crossover.
Photo of MUSO crossover showing:
- Air cored tweeter inductor
- Polypropylene capacitor
- Damping material used on sensitive components
MUSO acoustically damped mat
Muso is supplied with an acoustically damped mat to stand the speaker on. If Muso is being used bookshelf or other flat surface this affords the surface protection from scratching. In addition, the mat decouples the speaker from the surface with a “lossy” material, so any vibration in the speaker is attenuated and does not cause vibration in the supporting surface. This reduces confusion in the sound as any sound radiated from the supporting surface is significantly decreased.