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Thoughts and Theories of speaker design
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Design Goals of the Musica Bella SP/AV

The all important midrange:

For several years it has been thought that the combination of an eight-inch mid/bass driver crossed over to a 1" dome tweeter was just plain wrong. This is actually true in most situations. In conventional speaker systems that utilize eight inch or larger drivers, the use of a midrange driver becomes virtually necessary. The problem with most of these systems is that a six or seven inch driver is often used to cover these frequencies up to 3 kHz… or even higher! These drivers, actually [most] cone drivers larger than 5 inches, are unable to properly cover the dispersion patterns of midrange frequencies above 1 kHz. Think about this in another way. Look at the description of any driver over five inches. They are usually designated as woofers, not midrange drivers.

As you look around and audition audiophile speakers, even systems running into the tens of thousands of dollars, you will find many manufactures actually using these "woofers" to cover midrange frequencies. In all actuality, many of these systems are utilizing multiple "woofers" to cover the critical midrange spectrum. Is this for better performance? Nope. The real reason for using these larger drivers is that they offer higher sensitivity than their typical smaller diaphragm counterparts do. This means higher efficiency across the entire operating range (including the midrange) of the driver. This makes it possible to match this makeshift “midrange” driver to the higher output/sensitivity of the larger bass drivers in the system. As a “side benefit,” it also provides a higher efficiency rating of the overall system.1

The end result of using these larger drivers for midrange coverage is a non-linear “in room” power response, producing a forward midrange or "beaming" effect of these frequencies on-axis. If all that mattered was the on-axis response, this might not be much of an issue. But considering the fact that as much as 80-90% of the sound reaching the listening position can be the result of reflected energy from the room’s boundaries, the issue becomes very non-trivial. It has been shown that a linear in-room power response that results from loudspeakers providing constant-directivity provides a much more natural and pleasant musical presentation. Larger sized cone midrange drivers cannot provide the necessary dispersion properties required to achieve a balanced in-room power response – period.

If all this weren’t bad enough, there is a secondary issue. In order for larger diaphragms (6-inch +) to reach frequencies high enough to crossover to a typical 1-inch tweeter (2 kHz +), they must be designed to “flex” at their centers. This is the area of the cone where the driver’s voice-coil is attached. When reproducing frequencies above around 1kHz, the center of the cone begins to operate independently from the rest of its surface. As the frequency increases above 1kHz, less and less of the cone is actually moving. At such a high frequency as just before the driver’s output begins to drop-off in volume, the only part moving is the portion of the cone directly in front of the voice-coil. The whole system behaves as though it possessed a form of “mechanical’ crossover. If done correctly, this method can provide a relatively smooth frequency response on-axis. Amazingly, it is not uncommon for an 8-inch unit to extend as far out as 5kHz. A rather ingenious little approach to be sure, but besides the drawback outlined above, it is fraught with yet another insidious side effect.

The problem is; if the cone can flex at all, it will do so when we don’t want it to. The effect is commonly referred to as “cone breakup.” This unwanted flexure results in distortion that is not a part of the original music signal. All cones will exhibit this at some frequency. In fact, rigid metal cones will “ring like a bell” at some frequency. They must be used well below that point and that limits the larger ones (6-inches +) to about 1kHz. The use of the Bella/SP waveguide would permit this due to its ability to permit very low crossover frequencies, but metal cones are heavier. As such, they do not offer high enough sensitivities to be used with lower powered amplifiers such as SETs.

To produce a speaker of sufficient sensitivity for use with lower power amplifiers, we have chosen a Nomex fiber cone woofer that provides a very reasonable sensitivity of 90dB @1W/1M. In order to achieve a respectably low frequency response such that a subwoofer is not necessary, an 8-inch unit was called for.

A woofer of that size and composition will “flex,” as outlined above though. This is not good…correct? Just imagine a snare drum being struck and the “splash” of frequencies generated after the initial strike. That’s what you get when these woofers are excited by a fast enough signal to get them started flexing on their own. You can be sure they will too if they are forced to operate above their “piston” range. And you can also be sure this is what you’ll get from designs offered by others that crossover much above 1kHz – using large drivers to cover the midrange. The very drive signals from the amplifier that we do want are the “stimulation source” of the “distortion splash” that we definitely don’t want. The Bella/SP designs avoid this entire scenario by crossing over around 1kHz. That way, frequencies high enough to excite these “cone-flexure” distortion products never get through to the woofer to begin with. No “drumstick” whacking the drum, no “splash” of distortion…very simple. This is midrange done right!

Some of the best large, multi-way speakers I have heard to date utilize a single, three or four inch midrange driver, dome or cone, designed for the frequencies that they are asked to reproduce. The reason we did not go with this type of design is that multi-way speaker systems are very difficult to execute properly. They require much larger enclosures and considerably more complex crossovers, translating into higher price tags. Even at that they can still suffer from poor dispersion characteristics.

Yes, the Bella/SP speakers are comprised of a relatively simple, eight-inch driver and dome tweeter. However, as outline above, the secret is in the crossover frequency and more importantly, the Waveguide. 

Waveguides 5 are similar to horns in that they change the air load that the driver "sees". The difference lies in the extremes. The SP Technology designed Waveguide, utilized on the Bella/SP Loudspeakers, are characterized by the fact that they utilize relatively low compression ratios. At higher frequencies, the air load is virtually the same as a standard "baffle" mounted driver. This eliminates the associated distortion that is common at high frequencies with traditional horns. The relatively shallow design does not provide the higher loading characteristics of traditional horns and hence, none of the distortion either. Acoustic loading is only increased at the lower frequency end of the drivers operating range. This provides added efficiency where it is needed most. The low frequency range of any dynamic driver is where its mechanical limits dominate and ultimately degrade its performance.

The SP Technology designed Waveguides should be considered as a hybrid union of technologies, bridging the gap between high-efficiency and high-linearity. Compression ratios are high enough to greatly extend the low frequency performance of the driver as well as ensure that its mechanical system remains within linear operational limits under all conditions and drive levels. At the same time, the decreasing compression ratio with increasing frequency characteristic guarantees avoidance of the non-linear regions of the air medium. Sort of the "best of both worlds" -- if you will.

The effect on Dynamic Linearity (transient response) is nothing short of incredible. Reproduction of percussive instruments is truly life-like -- almost unnerving. Critical midrange frequencies of the human voice are reproduced with virtually no distortion, imparting a captivating immediacy seldom experienced in recorded music. The masking effects of Time Domain errors are minimized -- thus permitting the natural localization of instruments and performers in 3-dimensional space. Upon scientific analysis, it will be found that this Waveguide technology is truly an optimized design that leaves very little room for improvement.

 

Low frequencies - aka: bass:

Here we are dealing with some different design theories. In regards to obtaining good dynamics from systems that realistically, cannot operate down to their rated F3 (frequency extension at minus 3 dB), we have a problem. To get lower bass, especially from smaller speakers, most manufactures will use a ported enclosure.2. In my eyes, this is fine if you are not trying to integrate a subwoofer into the system and you’re not expecting much in the way of dynamics. On the other hand, most sealed speakers, even smaller satellites, can easily produce frequencies down to at least 80 Hz. From there, a subwoofer can take over the duties of the lower frequencies without experiencing weird frequency anomalies or dynamic compression. 

What's wrong with vented enclosures? Well, to start with their bass performance is usually exaggerated, sometimes severely 3. This can lead to large deviations from a flat frequency response and/or poor time-domain/transient response. The Bella/SP loudspeakers are designed with a sealed enclosure. While sealed enclosures can and often do have their own drawbacks, such as often being somewhat inefficient and requiring high power amplification, the Bella/SP loudspeakers overcome this by utilizing a woofer of moderately higher sensitivity housed in a “larger than normal” sealed enclosure. 

 I am not a big fan of using subwoofers, but I do believe that a subwoofer that is properly integrated with a good, efficient sealed monitor speaker, can sound much more realistic than a speaker using a ported design to obtain a lower frequency extension that looks deceitfully better on a specification sheet.  The problem with subwoofers is that most consumers are buying a subwoofer designed for thunderous bass in a home theater application. These subwoofers usually produce exaggerated, “boom-y” bass and are not capable of producing the speed and tautness needed in a good music playback system. Buying a good "music" subwoofer is not as difficult as many would lead you to believe though. Consumers these days tend to look at how low a speaker is rated with regard to frequency extension, not realizing that this advertised specification is purely eye candy and is the cause of so much complaining about "boom-y" bass. This is great for the room treatment manufacturers, but much of this in-room bass exaggeration is to be directly blamed on the speakers rather than the room acoustics.4.

Now we look at the Bella/SP speaker systems. Our goal for these speakers was sonic accuracy and a flat response. We could easily take their low end down another ½ octave or so going with a ported design, but this would compromise our transient response and make the speakers sound boomy and unrealistic. However, if you want a small speaker with big exaggerated bass, well…I guess that is your prerogative. You won’t find that available from us though.

The Bella/SP speakers are housed in a finely tuned and tweaked, sealed enclosure. They may not rattle the foundation in your home, but they do have the dynamics to easily impress most critical listeners - even without a subwoofer. And you won’t have to worry about the woofer cone “jumping out of the box’ when driven by a sub-40Hz signal either. The small, ported designs offered by others are typically “tuned’ around 40Hz. If you drive them with even a moderately large signal that’s only a little lower than their tuning frequency, you’ll see that cone look like it’s going to launch right out of the box! Sealed designs are inherently protected from doing that by the “air spring” trapped inside the box.

If you want deeper bass but do not want to compromise sonic integrity, we now get into the Bella/SP Subwoofer system. The Bella/SP Subwoofer system is designed to integrate properly with the Bella/SP monitors to sound more like a properly built 3-way system, rather than a “monitor with subwoofer” design. Please DO NOT mistake this as a "3-way" speaker system though, as it is still to be considered a monitor-based system. It just doesn’t sound like one. Used alone, a Bella/SP monitor will provide many users with both natural and more than sufficient bass response. For others, the Bella/SP Subwoofer is the ultimate solution to powerful, realistic bass.

Dispersion-Angle:
Footnote 5

The SP Technology Spherical Waveguide used in the Bella/SP monitors offers many performance advantages. Sound is radiated from the Spherical Waveguide into the environment with a constant 90° angle of dispersion from 1 kHz to beyond 10 kHz - both vertically and horizontally. This means that as the frequency varies, the area that is "illuminated" by the driver remains the same over a 90° arch symmetrically and directly in front of the Waveguide. Without some form of wave "directing" device, this simply is not possible. Drivers mounted on a flat baffle produce a coverage pattern that often varies considerably. Many times they will be found to be very broad at the lower end of their operating range then begin to narrow as the frequency is increased, only to widen and then narrow again at the highest frequencies. This unpredictable behavior makes placement difficult and the optimal listening area quite narrow. The problem is highly multiplied when multi-way (3+) driver arrays are employed.

The advantage that controlled and constant dispersion offers is quickly realized when one is faced with "not so perfect" room placement options. In more ideal settings, the reduction of reflected sound from nearby walls permits the natural ambiance in a given recording to be heard clearly with less of the masking effect caused by early room reflections. A 90° dispersion angle is ideal for achieving this while still providing a very wide listening area.

Professional recording engineers will most certainly appreciate such constant dispersion characteristics. The ability to monitor recording sessions with near-field accuracy and midfield freedom of movement is a combination seldom experienced and highly sought after.

In addition, with the recent advent of home theater, designers have sought to find ways to control the vertical dispersion of their designs in order to comply with the THX™ specification. A common technique is to employ two or more drivers operating over the same frequency range that are spaced some fixed distance apart. This method causes destructive interference at angles off-axis vertically from a pre-determined listening position. While this does effectively limit vertical dispersion to some degree, it most frequently does so in a rather erratic manner. Upon close analysis it can be found to produce a vertical pattern that varies with frequency. A stable, constant pattern at all frequencies is much more preferred and is a natural by product of waveguide technology. Hence, you will find that the Bella/SP Loudspeaker products are inherently more "home theater ready" than many products advertised as such and without their inherent musical limitations. 

Expectations:

Are the Bella/SP loudspeakers the best there is? There is no accurate answer to this as there is no such thing as the "best" when it comes to audio components, especially loudspeakers. The Bella/SP loudspeakers were designed to allow the listener to be overwhelmed with the music rather than spending excessive amounts of time analyzing the equipment in order to “get it right.”  Simply put, the Bella/SP loudspeakers were design to bring the listener closer to the music. Accurate, enjoyable music…without a lot of fuss!

You can now return to the Bella/SP Home Page to choose which model would be best suited for your personal desires.

 

 

Footnotes:

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2) There are many fine examples of ported speaker designs. Before considering this technology, first take into account what your future system requirements may be. If you are using these speakers in a home theater system, consider a sealed enclosure to be better able to incorporate a subwoofer and maintain a linear presentation.

3) These designs should only be considered for those who are not requiring an accurate music presentation. 

4) Before the emails start coming in, I would like to clear this one up. Room acoustic can and do play a roll in "dips & peeks" in response where room treatments should be considered. My recommendation is to first learn how much of these irregularities are due to speaker design before "blaming" the room.

5) To learn more about the SP Technology Waveguide and see products that are the absolute best examples of this technology, be sure to check out the loudspeakers by SP Technology.