M Series. Loudspeakers. White Paper

Transcription

1 M Series Loudspeakers White Paper

2 Introduction The M Series is a family of loudspeakers is designed to provide maximum enjoyment while listening to music, watching movies or simply enjoying any other source of audio in your home. This document touches on the specific features, advantages and benefits of the core technologies and design approaches utilized in the Boston Acoustics M Series. It is not intended to replace the product brochure and should be viewed as a supplement to the information provided in that document for individuals interested in learning in more detail about the M Series. What Makes Good Sound At Boston Acoustics the goal of all our products is to provide sound reproduction that is accurate, natural and true to the original vision of the artist. In many ways loudspeakers are the most challenging audio products to design as they have the extremely difficult task of converting electricity into the room-filling sound we actually hear. To be faithful to the source, a loudspeaker must do several things well. First, it must be able to reproduce the entire audio spectrum from the lowest bass notes to the highest treble notes and everything in-between. Second, it must be able to convey the extremely wide range of loudness from the softest notes to the loudest crescendos - and be able to so even when the listener is playing the system near or at its maximum volume level. Third, it must be able to create an accurate sonic image in the listening room by reproducing all of the sonic elements at the correct size and in the correct position in the listening room. The width, depth, and height of the soundstage depends heavily on the phase/timing accuracy of a loudspeaker and more accurate designs make the sound appear as though it is not originating from them. Finally, the loudspeaker must be able to do all of these things without adding or removing anything to the sound it is reproducing. In short, it must allow the sonic performance to come through in a compelling way that allows the listeners to forget that they are listening to a loudspeaker system and to simply experience the emotion of the sound they are hearing. Much easier said than done! What is a Loudspeaker? A passive loudspeaker consists of 3 things; the transducers (aka drivers), the crossover network and the enclosure. The transducers (as the name implies) are the parts that actually convert electricity into sound. A transducer functions by vibrating in direct response to the electrical signal being fed to it. The crossover network (usually located inside the enclosure) connects all of the transducers together and provides a connection (the input terminal) to the power amplifier. The crossover filters the sound for the transducers so they reproduce the sound that 2

3 they are best able to reproduce. Sound waves vary in length from ~.5 to more than 50 feet and reproducing them usually requires multiple transducers that are best able to reproduce specific ranges of sound. Tweeters are relatively small (typically ) and can vibrate very quickly to reproduce high frequency sounds. Woofers are relatively large (typically 4+ ) to move more air, and are best able to reproduce low frequency sounds. And midrange drivers which are typically 3-6 are best at reproducing the middle frequency sounds. Note: the above is in reference to a typical dynamic passive loudspeaker system using dome and/or cone shaped drivers. There are several other types of speaker and drivers that are not discussed here. The Technology Behind M Series The M Series utilizes several advanced parts and design techniques to achieve its high level of sonic performance. In the transducer category, the M350, M340, M250 and M25 loudspeakers utilize the Extended Wide Bandwidth (EWB) Tweeter. The flagship M350 and M340 also feature the Mass-Loaded Midrange. The MCenter and MSurround employ an innovative Balanced Mode Radiator (BMR). In the enclosure category, all of the M Series products (except for the MSurround and MSubwoofer) utilize Lo-Q cabinet construction. The M Series loudspeakers also feature several design techniques including and the quad woofer array and narrow baffle design to achieve their acoustic goals. Boston Lo-Q Cabinet Construction The ideal speaker enclosure is one that holds the drivers and network crossover in their desired positions without adding anything to the sound of the loudspeaker. In reality, all speaker enclosures vibrate in measurable ways in response to the vibrations of the drivers. You can experience this by simply placing a hand on any loudspeaker enclosure that is playing at a reasonable level and feel how it is vibrating along with the sound. This is cabinet distortion and it can be measured with astonishing accuracy by utilizing a laser vibrometer. Damping adhesive layer Cabinet distortion varies from surface-to-surface and from product-to-product. If cabinet vibrations are high in overall level or the response curve of the cabinet vibrations is ragged looking when measured using a laser vibrometer, the cabinet itself can detract from an otherwise high-performing loudspeaker. A loudspeaker that has a lot of vibration can be described as having a high Q. The traditional loudspeaker design method to minimize cabinet vibration is to use thicker material. For example; using 1.5 thick material instead of.75 thick material. While this method is effective at reducing cabinet vibration, there are a few drawbacks with this technique; the speaker gets larger (and heavier) which may be a cosmetic concern, the interior volume is reduced which can negatively impact the low-frequency capability of the loudspeaker. And finally, increasing the thickness of the material can move cabinet vibrations into the critical midrange frequencies where they will be the most audible. 3

4 During the research phase of the M Series we set out to create a better solution. We began by measuring and listening to cabinets lots of cabinets. After we had a thorough understanding of the problem, we reviewed the Plot showing cabinet vibrations of regular MDF and Lo-Q. Flatter line and lower potential solutions. After many levels are better. Light blue is regular cabinet. Dark blue is Lo-Q cabinet. months of this iterative process of building and testing various construction techniques we determined that a combination of two pieces of Medium Density Fiberboard joined to each other with a thick layer of damping adhesive in the middle provided exceptional results. Not only did this new construction significantly outperform standard MDF of the same size, it provided all of the benefits of using much thicker materials without the drawbacks. We call this new material Lo-Q. It reduces the cabinet vibrations significantly and also flattens the frequency response of the cabinet vibrations so there aren t any problem areas that may ring. In short the cabinet has low a very low resonance or a Low Q. In addition to the Lo-Q cabinet material, we also used a scanning laser vibrometer to critically position the M Series cabinets internal braces where they provided the most benefit, and added heavy duty absorptive cabinet fill to eliminate internal standing waves. The results are cabinets that are extremely quiet acoustically and do not interfere with the sound of the transducers, yet allow the speakers to provide exceptional sound quality from relatively small enclosures. Separate Midrange Chamber In addition to utilizing Lo-Q construction the M350 and M340 also utilize a separate internal chamber for the Mass-Loaded midrange. This acoustically isolates the midrange from the woofers inside the cabinet so they do not interfere with each other. As the midrange and woofers move in and out of the cabinet the energy coming off the back side of driver cones affects all of the other drivers that are not acoustically isolated. Imagine two people sitting in the same room speaking on two different telephones having two different conversations. The people on the other end of the telephone lines will not only hear the person they are speaking with directly but part of the other conversation as well. By moving one conversation to a separate room, the problem is solved. The same principal applies to the drivers. The midrange is given its own separate chamber so it is not affected by the energy coming off the back of the woofers and vice versa. This is not an issue with the tweeter since back side of the tweeter is acoustically sealed from the other drivers. Another benefit of the separate midrange chamber is that it doubles as a reinforcing brace near the top of the cabinet to add rigidity and reduce cabinet vibrations even further. 4

5 EWB Tweeter The Extended Wide Bandwidth (EWB) tweeter is different than most tweeters as it has a small dimple in the middle and a large radiating surface that extends from the center through the crease out to outside perimeter of the diaphragm which doubles as the surround. The dimple is where the center of the tweeter diaphragm is attached to a tripod shaped piece behind the diaphragm. This design significantly lowers the moving mass of the tweeter diaphragm which makes it better equipped to track subtle treble details. Since tweeters have to move back and forth up to 20,000 times per second to reproduce the upper limit of human hearing, their moving mass must be kept low or they will have increasing difficulty moving fast enough to reproduce the highest frequencies. The benefit of the EWB tweeter is that its moving mass is kept extremely low and it can reproduce the highest frequencies with ease. In fact, the EWB tweeter response is rated to 30,000Hz and is only reduced by 6dB in output at 40,000Hz. Although this extended high frequency response is well above the human hearing limit of 20,000Hz, what is more important is that the EWB tweeter is an exceptional design with stable linear motion, low distortion, high power handling capability and most importantly; superb sonic clarity. Mass Loaded Midrange The midrange used in the M350 and M340 has a unique visual identifier; four small rectangular tabs on the inside edge of the surround. These tabs add mass to the midrange to achieve a smoother frequency response at the top of the drivers operating range. This technique of using mass to smooth the response of the midrange has a specific design benefit. It eliminates the need to try and achieve the same sonic result by adding components to the crossover network. By smoothing out the midrange at the start with a mechanical solution, better consistency from unit-to-unit and superb sound quality and imaging in the critical upper midrange are maintained. Making a Better Midrange, The Mass Loading Advantage 5

6 BMR Driver The Balanced Mode Radiator (BMR) is utilized for high frequency sound reproduction in the MCenter and MSurround. The BMR is very similar to a traditional midrange driver in construction except that it utilizes a flat panel diaphragm that operates in two modes simultaneously; it moves in and out like a piston similar to a traditional driver to reproduce the lower frequencies and to reproduce the higher frequencies, the diaphragm actually bends. This significantly extends the operating range of the driver allowing for a much lower crossover point and also provides unusually wide high-frequency sound dispersion characteristics. Typical high frequency drivers tend to have significantly reduced output level at the top of their range as your ears move off axis from the driver. In other words, if the center of the driver is pointed directly at your ear, it will sound correct but as you move to the side so that your ear is at an increasing angle to the driver, the high frequencies will diminish more quickly compared to the lower frequencies. The wide highfrequency dispersion characteristic of the BMR provides a more consistent surround sound experience for all of the listeners in the room. The other advantages of the BMR in the MCenter and MSurround are smaller overall cabinet sizes and lower crossover points. Instead of having a stacked tweeter and midrange combination, the BMR allows the cabinets to be shorter, allowing them to be placed in the listening room more easily an important consideration for center channel and surround loudspeakers that often need to fit on a shelf under a display or hang on a wall to the side or behind the listening area. The lower crossover point provides remarkably clear sound reproduction in the critical midrange frequencies where the human ear is most sensitive. This is especially noticeable in the MCenter where dialog is reproduced with remarkable clarity even when sitting off center. What is a flat panel driver? Directivity of 65mm-fullrange driver measured from -90 to +90 (range 50dB, colour scale 1dB/div) Directivity of BMR driver measured from -90 to +90 (range 50dB, colour scale 1dB/div) 6

7 Quad woofer array The M350 and M340 both employ quad woofer arrays for low frequency reproduction. There are a few advantages to using multiple low frequency drivers; they allow for a narrower cabinet front (baffle) and smoother in-room response. The four 51/4 woofers in the M350 have approximately the same surface area as a single 10 woofer. The four 41/2 woofers in the M340 have approximately the same surface area as a single 8 woofer. By using multiple smaller drivers, the width of the baffle can be kept very narrow which also improves the radiation pattern of the entire loudspeaker and enhances the visual appeal of the loudspeaker making it easier to integrate into more listening rooms. Another advantage is that the first floor reflection (aka floor bounce) of each woofer in each speaker is slightly different which helps to smooth the in-room response of low frequencies in comparison to using a single larger driver. In addition all of the woofers in the M350 and M340 utilize very large magnets and voice coils with powerful motor characteristics to provide a combination of sonic accuracy and very high output capabilities. Crossover If the drivers are the heart, and the cabinet the body, the crossover is the soul of the speaker. The crossover is an electronic circuit that directs the appropriate sound range to each transducer and maintains the phase/timing accuracy between all of the transducers. If the crossover is designed poorly, the loudspeaker will not sound good even with high-quality transducers and cabinets. With a well-designed crossover, the speaker becomes greater than the sum of its parts. The key is to divide the acoustic range so the right amount of acoustic energy comes from each driver, evenly across the frequency spectrum and with an even coverage around the listening room. All the parts in the crossover resistors, capacitors, inductors have an effect on the sound, and choosing the right parts will deliver the sound that the system is truly capable of. Special elements such as low loss film capacitors and low distortion laminated silicon steel core inductors have been used at all critical areas. On some models, only the combination of two different capacitors in parallel gave the sound character matching our goal. We even went so far as to optimize the location of the components relative to each other, to minimize electromagnetic interaction that could degrade the sound. On the M350 and M340 the entire woofer crossover network is located on a separate board to eliminate the potential of stray fields from the large inductors on the woofer network from cross-talking with components in mid/high frequency crossover network. All of the M Series loudspeakers also utilize high quality 5-way binding posts to allow for a wide range of connection types and wires. 7

8 MSubwoofer The function of a subwoofer is to effortlessly reproduce the lowest octaves of sound while seamlessly integrating with the other loudspeakers in the system. Its goal, as with the other loudspeakers, is to disappear in the room so it performs its function without revealing itself as a source of sound. For a subwoofer to perform its function well it must be capable of reproducing the lowest audio frequencies and be able to do so at relatively high volumes levels. If you have ever watched the transducer on a subwoofer, even when playing at low volumes, you may have noticed that is was probably in a constant state of motion. Subwoofers are usually working hard all of the time as they have to move a lot of air to reproduce low frequency sounds with accuracy and authority. This dictates the use of large transducers and very powerful amplifiers. The MSubwoofer is relatively small in size yet performs like a much larger product. The internal power amplifier is rated at 500 watts and provides 1000 watts of peak output when needed. The amplifier is mated to a specially designed 10 woofer capable of handling this much power. The woofer features high excursion capabilities, a heavy duty surround and a large voice coil. There are also two 8 passive radiators one on each side of the cabinet that allow the subwoofer to play the lowest notes and to reproduce the loudest peaks with very low distortion in a relatively small enclosure. This configuration puts the passive radiators mechanically out of phase, to cancel out vibrations that otherwise could make the cabinet walk across the floor. The internal power supply features a large toroidal transformer to keep plenty of power on reserve so the subwoofer can handle even the most demanding music or film soundtrack with ease. In order to blend seamlessly with the other speakers we have included a variety of connection and adjustment options on the MSubwoofer. There are two types of input connections; The LFE ( Low Frequency Effects ) input is designed for use with electronics such as Preamp/Processors and Audio/Video Receivers with their own built in crossover controls. The LFE input bypasses the subwoofer s own crossover control and is the best choice for home theater setups. The Left and Right inputs are a good choice for stereo systems where the listener may optimize the crossover (dividing frequency) between subwoofer and main speaker by adjusting the crossover knob on the Msubwoofer. The MSubwoofer incorporates a 4th-order (24 db/octave) adjustable low pass filter, allowing a sharp roll-off the subwoofer s response as a sound s frequency increases out of the subwoofer range. The benefit of such a sharp filter is that you may tune the sub s output to match the bass extension of the main speakers for instance if the main speakers themselves have excellent bass extension like the M350, M340 floor standing speakers then the subwoofer 8

9 may be set to produce only the lowest tones. If you are using the M250 floor standing speaker or the M25 bookshelf speaker then the subwoofer may be set for a higher roll-off providing the best balance for these smaller loudspeakers. Thank you for taking the time to learn more about the Boston M Series loudspeakers. They were designed by a dedicated team of people from a brand with more than 30 years of experience of designing and building high-performance audio products. We sincerely hope that they bring you many years of listening enjoyment! 7 Constitution Way, Woburn, MA bostonacoustics.com 2012 All rights reserved. Boston, Boston Acoustics, the B/A ellipse symbol, BassTrac, and Deep Channel are registered trademarks of Boston Acoustics, Inc.