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A white paper on the performance attributes of various loudspeaker designs.
Because so many misconceptions appear to exist with respect to relevant performance features of both membrane and cone/dome drivers, I would like to share some of my thoughts on the subject with those who might be interested. Lets begin by examining some of the basic physics involved in the radiation of sound waves from all types of diaphragms: cones, domes and planar types.
A driver with a circular-shaped cone, suspended at its outer edges by a compliant "surround" and driven at or near its apex by a low-mass voice coil symmetrically immersed in a strong magnetic field, is probably the most accurate type of transducer presently available. The important caveat here, though, is whether it was properly designed, e.g., with: 1. a cone made of materials possessing the right acoustical properties (rigidity, internal-damping, etc.), 2. an outer "surround" with the correct compliance, damping, etc., 3. a voice-coil and magnet structure having an optimum "BL product" (the product of the magnetic flux and length of wire), 4. a spider assembly which properly centers the voice-coil and exhibits correct compliance, damping, etc. and, 5. a frame or basket that is rigid and anti-resonant. Of course, the driver must also exhibit the desired frequency response, impulse response, efficiency, impedance (resistance and reactance Vs frequency), resonant frequency, Qt, Qe, Qm, etc.
Contrary to popular opinion, a dome shaped radiating surface does not provide as wide an operating bandwidth or as wide a beamwidth as a well-designed cone driver having the same diameter. This is partly because the dome is driven from its outer edge, thereby defining the diameter of the radiating area at all frequencies within its operating range. Also, a well-damped dome with a diameter larger than about 2-3 wavelengths at its intended high-frequency limit, e.g. a 3" dome at 10 kHz, usually radiates relatively little energy from its center region - radiation mainly being confined to an annular region surrounding and adjacent to the voice coil. Thus, a dome tends to exhibit the beamwidth properties of a annular (ring-shaped) radiating surface possessing a "constant diameter", with a center region that radiates less energy with increasing frequency. Since the half-power (-3dB) beamwidth (in degrees) of a radiating source is approximately equal to 58/D (where D is the "effective diameter" of the radiating surface, expressed in terms of a wavelength at the frequency being evaluated, e.g., approx. 13,600/freq. in Hertz), a 3 inch dome would exhibit a beamwidth of less than about 26 degrees at 10 kHz. It is interesting to note that an annular-shaped radiating area also tends to exhibit high-level "side lobes" that alternate in phase relative to the main lobe, compared to a more uniformly-illuminated radiating surface.
By contrast, a properly damped, cone-shaped diaphragm, driven by a voice coil located near its apex, experiences a more constant beamwidth over a much wider bandwidth than a dome. This is because properly engineered cone material exhibits "internal damping properties", intended to progressively absorb more energy at higher frequencies. This confines much of it to regions closer to the voice coil.
Thus, while a 3 inch diameter cone type mid-range driver may have an effective radiating diameter of about 2.5 inches at 1 kHz, this probably reduces to less than 1 inch at 10 kHz, yielding a half-power beamwidth of about 80 degrees, nearly 3 times that of a 3 inch dome.
These relative beamwidths, between a 3 inch dome and a 3 inch cone, are only approximate and do not take into account the reduced "velocity-of-propagation" which occurs along or through a "lossy medium", such as a damped diaphragm.
A well-designed cone driver, with optimum damping and flat frequency response, can exhibit an excellent impulse response, with the first "overshoot" approximately minus 15 dB (without crossover). Further "ringing" is typically down more than 20 dB and persists for no more than 100 to 200 microseconds. (A well-designed crossover network can often improve on these values of overshoot and ringing.) Although well-designed dome drivers can also achieve a good impulse-response, they usually exhibit moderately more overshoot and ringing than a well-designed cone driver of the same size and efficiency.
As can be seen, cone and dome drivers typically perform quite differently than most audiophiles believe. While a dome might appear more "hi-tech" than a cone, it has many performance limitations and, for many applications, is inferior (overall) to a cone driver of equal "design quality". As a consequence, domes tend to perform best for tweeter applications, where the diameter of 1 inch dome corresponds to about 1.5 wavelengths at 20 kHz (yielding a half-power beamwidth of about 38 degrees).
An "inverted dome" is generally inferior in performance because the inverted shape forms a "cup-shaped" cavity that can exhibit a resonant property if the depth approaches 1/4 wavelength within the operating range (about 5/32 inch at 20 kHz).
"Jazzy-looking" cone and dome materials (often made from yellow-colored, "bullet-proof" materials, such as Kevlar) usually exhibit poor internal damping properties, resulting in an impulse response characterized by considerable overshoot and subsequent ringing. Kevlar, and similar materials, while providing rigidity, lack proper internal-damping properties required for good impulse response and truly flat frequency/phase response. As a consequence, Kevlar cones (and domes) are generally confined to applications in loudspeaker designs using higher-order crossover responses, where impulse, step and phase responses are not considered important by the designer. Cones and domes made of metal (such as titanium) or ceramic are even worse because these materials provide virtually no internal damping properties, resulting in poor impulse response, poor step response, etc.
While the cones with the best measurable and audible performance may appear to be made of a dark-gray colored "paper" material, they are most likely a complex formulation of felt, cellulose/carbon fibers, a binder, and a coating that provides optimum "damping", minimum formation of undesirable modes at higher frequencies, excellent impulse/step response and very flat frequency/phase response, etc.
Most audiophile-quality cone and dome drivers exhibit a Sound Pressure Level (SPL) of about 88-90 dB, at an on-axis distance of 1 meter, for an input of 1 watt (2.83 volts RMS across 8 ohms). This is many times the efficiency of typical planar diaphragm radiators, having the same radiating area. However, cone and dome drivers with SPL's higher than about 91 dB (re: 1 watt) usually possess a Qts (total Q factor) that yields poor damping and less than good impulse and step responses.
Lets now switch to loudspeakers with "planar diaphragms" (flat plastic membranes) and examine how they work and perform, compared to drivers with cones or domes. To begin, there are typically two classes of loudspeakers or loudspeaker drivers based upon planar diaphragms or membranes: 1) electrostatic types, and 2) magnetic types.
An electrostatic type usually employs a thin, light weight, plastic film diaphragm (possessing low-loss dielectric properties), stretched mid-way between two parallel "wire grids or metallic mesh". A D.C. polarizing voltage, typically several thousand volts, is connected between the diaphragm and both grids. The audio input signal is fed to the two grids, out-of-phase, typically through a step-up transformer.
The magnetic type of planar loudspeaker usually makes use of a thin plastic diaphragm with an electrically-conductive coating of parallel wires (other configurations are possible and may be used). This diaphragm is tightly stretched between an array of small magnets that are configured adjacent to and on either side of the parallel wires. When a signal current passes through the wire grid, it is attracted to one array of magnets and repelled by the magnets on the opposite side, creating sound. A signal current flowing in the opposite direction causes the diaphragm to move in the other direction. (Early models made by one manufacturer used magnets on one side only, resulting in high levels of even-order harmonic distortion.)
Electrostatic and magnetic types of planar loudspeakers have been designed with several different configurations. Some use a single planar diaphragm with a large area (several square feet) that radiates all frequencies. This, of course, results in undesirably narrow beamwidths, at high frequencies, in one or more planes. An improved variation of the single diaphragm uses a narrow width with a large height, which increases the horizontal beamwidth at the expense of decreasing the vertical beamwidth (which might reduce some undesirable multipath effects attributable to floor and ceiling reflections. A second version uses a wide, tall panel to radiate bass and lower mid frequencies, while a tall but very slender strip is used to radiate higher frequencies. Other variations, such as arrays of small, individual panels, have been designed to satisfy certain performance criteria deemed important by the designer.
So! What about the popularly held belief that most membrane type loudspeakers exhibit properties that are measurably and audibly superior to loudspeakers using cone and dome type drivers? The most universally held convictions are 1) that a super-light radiating surface or membrane can accelerate much faster than a cone or dome, thereby providing superior impulse response and more accurate reproduction of complex musical sounds, 2) that the much larger surface area of a planar membrane radiates a much broader beam of acoustical energy, 3) planar membrane type loudspeakers exhibit a very flat frequency response, 4) the bi-directional radiation pattern of planar loudspeakers provides a more natural and realistic sound in most listening rooms.
Lets examine each of these assumptions to determine if any are valid.
1. The ability to rapidly accelerate any type of diaphragm, planar, cone or dome. is dependent mainly upon two parameters: the moving mass (or weight) of the radiating element and the total "force" (electric or magnetic) acting upon the full surface of the diaphragm. The simple expression from basic physics, F=MA (or A=F/M), tells it all. From this expression, it can be seen that, although the mass M of a planar diaphragm is very small, the forces acting on the mass are even smaller - compared to an ordinary cone or dome type of driver with a large magnet and a voice coil with a large number of turns (a large "BL Product"). The larger BL Product of cone and dome drivers provides a 6-10 dB advantage and is the reason why membrane type loudspeakers require a large "radiating area" to obtain reasonable efficiency and satisfactory sound levels.
The lower "efficiency versus radiating area" of membrane loudspeakers frequently equates to a relatively poor "damping factor", due mainly to the lack of any "restoration force" needed to "restore" the membrane to its original position, after the input signal drops to near-zero at the end of a transient.
Further, all membranes must somehow be mechanically attached to a rigid frame. Thus, when the membrane is set in forward motion by a signal, it's surface becomes curved because the edges adjacent to the frame are not free to move. As a consequence, sound energy is reflected from the frame back toward the center of the membrane. This reflected energy combines with the original "incident" energy to form "standing-waves", with maximums and minimums spread across the membrane at intervals of one-half wavelength at each frequency of excitation. This is, perhaps, one of the reasons that many manufacturers have chosen to build their membrane loudspeakers as an array of separate modules of smaller size, with resonances and standing-waves that are easier to control. It is also possible that some manufacturers have discovered unique means for acoustically damping the edges of the diaphragm where it is attached to the frame so as to absorb sound energy and reduce reflections that might create standing-waves.
2. Because of the much larger radiating area required by membrane type loudspeakers to reach acceptable levels of efficiency, a design using a single, large membrane (to radiate all frequencies) exhibits a beamwidth that approximately drops in half for each octave above the frequency where the membrane measures approximately one-wavelength in the plane being evaluated. This assumes, of course, that the listener is located at a distance that is greater than about three times that of the largest dimension of the membrane (commonly referred to as the "far-field" distance). For a membrane 4 to 5 feet high, this would equate to a listening distance of about 12 to 15 feet.
The relatively narrow beamwidths exhibited by membrane loudspeakers can significantly alter the spectral balance for a listener seated off-axis by more than a few feet. Such narrow beamwidths can also alter the ratio of the on-axis response to the integrated room-response, causing the speaker to sound "heavy" in the upper-bass and lower mid-range regions. The half-power (-3dB) beamwidth (in degrees) of a uniformly-illuminated planar radiating surface is, as mentioned in par. 5 (above), approximately equal to 58 divided by the dimension of the surface (in the plane being evaluated), expressed in wavelengths.
One might argue that membrane loudspeakers using a narrow (but tall) ribbon for the tweeter range solves this problem of limited angular dispersion in the horizontal axis. And this is certainly true, but does not take into account that, while it significantly improves horizontal dispersion, radiation in the vertical plane remains very narrow - literally only a few degrees above about 7 kHz.
3. While Peter Walker's ESL-63 exhibited a relatively flat, on-axis, amplitude versus frequency curve (nearly plus/minus 2 dB), most membrane loudspeakers barely meet a plus/minus 4 dB spec, on-axis, at a distance of 10 to 12 feet. In reality, however, this is representative of most hi-end audiophile loudspeakers, for very, very few even approach their advertised specifications. (Sadly, as many wise audiophiles have concluded, "measurements don't lie, but measurers often do!")
4. Although more psycho-acoustic research is needed (and the jury is still awaiting additional evidence), there are good reasons to conclude that bi-directional (or dipole) radiation, while it may produce great sounding music and a wide sound-stage that pleases many devotees, is not truly accurate when compared to the original live performance. Nor does it provide a true "pin-point" center image (for well-recorded center vocals) in most rooms. The inability of bi-directional radiation to accurately emulate the original performance may partially be explained by the fact that many (if not most) musical instruments radiate their sound with a directivity pattern that is essentially unidirectional, at least at mid and high frequencies, where the relevant dimensions of the instrument become large with respect to a wavelength. Obvious exceptions, of course, are the organ, drums and string bass.
Another deficit of a bi-directional radiation pattern is that low frequency sound refracts (bends) around the edges of the outer frame where it combines, out-of-phase, with the sound radiated from the opposite side of the membrane. This tends to seriously limit the low-frequency response, usually calling for use of a sub-woofer. But this, in turn, creates a time/phase alignment problem between most sub-woofers and membrane loudspeakers, resulting in sharp "drop-outs" in the frequency response within about one-octave of the crossover frequency.
I sincerely hope that what I have said above helps interested audiophiles to better understand the relevant differences between loudspeaker types. If I have erred in any of my comments, I would appreciate responses from competent readers to help set the record straight.
Best regards,
John Dunlavy
CEO
Dunlavy Audio Labs
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"Soft-Dome Midranges"
These are enlarged (2 to 3 inch) versions of soft-dome tweeters, using similar construction techniques with a half-roll surround acting as the combined surround and spider. Unfortunately, what works for a tweeter doesn’t work so well when scaled up for midrange use. In a tweeter, excursion requirements are modest (0.5 mm is plenty), but the requirements for the 3rd derivative of excursion (jerk) are severe, since the tweeter handles the very top of the spectrum, and is occasionally exposed to ultrasonic clicks from amplifier clipping, phono cartridge mistracking, or high-frequency noise and distortion from digital converters.
By contrast, the midrange (or midbass) driver experiences much greater demands for excursion and acceleration for two reasons: if you halve the frequency, you need four times as much excursion, and the musical spectrum carries most of its power in the lower midband. Both factors combine to make the midrange driver a device that must handle much more power than a tweeter. This imposes harsh demands on the rigidity of the diaphragm, and it exposes the simple suspension to rocking modes.
The reason conventional cones have a separate surround and an inner spider is to constrain the cone travel to a back-and-forth piston motion. Only very expensive mid domes intended for professional studio monitors (like the ATC) use a separate spider; as a result, most consumer-grade domes have serious problems with side-to-side rocking and other spurious motions. In addition, the doped-silk diaphragm is easily deformed by the high acceleration loads in the power band of the midrange. You don’t see bass drivers made out of doped silk, after all.
As a result of these problems, soft-dome midranges measure well, but sound a lot worse than conventional steady-state measurements would indicate. Even if you stick to measurements and discount all of the foregoing, they are limited bandwidth drivers, requiring a 12dB/octave crossover no lower than 500Hz (800Hz is better) thanks to a linear excursion of no more than 2mm. You’d expect a big tweeter to do well at high frequencies, but all of the soft-domes I’ve seen start to roll off at 4 to 5kHz, which is no better than good modern midbass drivers.
Of course, there are exceptions to what I’ve mentioned above. For example, there are cone-dome hybrids, such as the 5" Scan-Speak 13M/8636 and 13M/8640, and the 5" Dynaudio 15W-75. These new drivers are actually designed as high-quality miniature cone drivers, not as midrange domes. The only thing they have in common with the traditional soft dome is a large dustcap, which does indeed act as a dome radiator at the higher frequencies.
These new cone-domes have much more excursion, much lower distortion, and a much wider frequency response than the older soft-dome midranges. The cone-dome drivers are capable of realistic and transparent sound. They are described in more detail in the other sections, since they use Kevlar, paper, and polypropylene cone materials.
Another "special case" is the English professional-grade ATC 3" dome with an integral short horn. This driver uses a dual spider to eliminate the rocking problem that plagues most soft-domes, reducing the IM distortion very significantly. Ron Nelson (of Nelson-Reed) recommended this driver as one of the very best midranges around, and I take his recommendation seriously. This is a very expensive driver (around US$300/each). They also need to be hand-selected so the resonant frequencies of the left and right channels match.
Strengths are: None. Metal-dome midranges have some potential, but they require sharp crossovers on both ends with an additional sharp notch filter at high frequencies to remove the first (and worst) HF breakup mode. Note: This does not apply to the cone-dome hybrids or the prosound ATC driver.
Weaknesses are: High distortion, fatiguing sound, high crossover frequency, limited bandwidth, limited power-handling, and misleading frequency response measurements. It takes a detailed swept IM distortion measurement and laser holography to get the full story on these drivers. Note: As before, this does not apply to the cone-dome hybrids or the prosound ATC driver.
Best Examples are: ATC 3" professional-series - a totally different animal than the usual soft-domes. About 4 times as expensive, though (so what did you expect?). The Scan-Speak 8640 and 8636 are also excellent wideband midrange drivers.
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