Like the record I think it MAY in the main stream but not with die hard audio fans IMHO. I will always like the option using different amps, ect...
55 responses Add your response
As Chad mentions it is cheaper - it is easier to build a higher performance amp with limited bandwidth powering one driver than the same quality performance with very large bandwidth driving several drivers as well as a lossy passive crossover. Usually when something is both cheaper and better in performance it tends to win out in the market place. I suspect active speakers will win out eventually if not for performance but more likely because of lower cost for similar performance.
What goes on in the pro marketplace does not necessarily translate to the consumer marketplace. I think that powered speakers have a chance in the consumer marketplace for the reasons Shadorne mentions and also for the fact that they are more neat and tidy for a home decor--no big boxes cluttering up the living room. B&O has been quite successful with this approach, for example, in the less-than-high end, and Meridian and ADC have had some excellent designs using this approach that I think most of us would consider high end. I doubt, though, that those of us in the lunatic fringe (and I include myself in that group) of perfectionist audio will completely embrace the technology, as it limits our choices (no tubes, as Rockadanny gently points out) and ability to "upgrade", and the perception (right or wrong, and Meridian's and ADC's powered speakers make a strong case that perhaps the perception is either wrong or overrated) that constraining an amplifier to such a small, vibration-prone space will be deleterious to the ultimate sound from the speaker.
Rcprince brings up the typical objections to powered speakers. However, when carefully examined, the advantages of active speakers with active crossovers will far out weight the drawbacks. And, what is too frequently overlooked is that not all active speakers are powered speakers with built-in amps. The active speaker with outboard amps provides both the superior performance and the choice of amps, cables, and the option of experimenting all you want.
vibration-prone space will be deleterious to the ultimate sound from the speaker.
It certainly would if you used tubes. In that case you can still go active but they would need to be 'outboard' amps dedicated to each driver (this would still make the design active and is actually how active speakers started)
Active loudspeakers are IMO long overdue for penetration into the consumer marketplace - there's virtually no aspect of amplifier, loudspeaker, or crossover performance that isn't improved by using a separate amp for each frequency range with the crossover being performed before amplification.
The main reason that it isn't more common is both cultural and economic. For starters . . . loudspeaker companies and electronics companies have different resources and abilities - speaker companies are usually mainly woodshops, and electronics companies stuff circuit boards into sheetmetal enclosures. Most of the companies that build active speakers rely on an outsourced "plate-amp" module (either off-the-shelf or custom) that's easily incorporated into their conventional design/manufacturing methods, and most electronics companies that have loudspeaker lines outsource the cabinetry from a woodshop.
In an economic sense, if you're going to integrate two things together, the receiver/passive-speaker has some clear advantages over the tuner-preamp/active-speaker combination -- putting like things together affords considerable savings. (Look at a cheap mini-system with biamplified speakers - the amps are always in the main unit, with two sets of speaker wires.) For custom home installation, it's also much cheaper to run low-voltage speaker wire everywhere, than to use a high-quality balanced line-level distribution system, plus AC power at every speaker location.
And there's also the cultural difference in the distribution side - it takes much more thought and effort for a salesperson to convince somebody to replace their amplifier(s) (that they may be attached to) when they're looking at making a speaker purchase. And in the high end, there are many symbiotic relationships between amp and speaker companies, that share resources at shows, and serve the same dealer network -- an active speaker product can upset these relationships.
But although I feel that the active speaker approach is in general a better way of doing things, I still think that there are a LOT of really poor products in the "professional" ranks, even some very high-priced ones . . . i.e. I feel passive ATC SCM20 or SCM50 (or even the old JBL 4435 running passive) absolutely smoke the now-ubiquitous active Genelecs. And don't get me started about the cheap "active monitors" (really overgrown computer speakers) that mail-order music stores ship by the truckload . . .
Vibration is hardly an issue *only* with tubes. Every single type of electronic component exhibits decreased performance in a high-vibration environment.
A passive crossover is not a big deal if it's simple, as it should be for many other reasons. My experience has proven that they can be far less destructive than running the tiny, low-level signal through an active crossover of any type - a necessary evil with any type of multi-amping.
I have found that most people who are so enamored with powered speakers are those who tend to be impressed by measurements over subjective sound quality and tend to listen mostly to the processed, modern music that masks differences in electronics quality in the first place.
I'm sure there are counter-examples.
An active monitor that is worth its salt will require a balanced input, like the ATC.
OTOH, field coils are making a huge comeback right now. Active speakers are not likely to see innovations like that since they are closed systems. So you are limited by the limits of the internal amp, and the drivers themselves.
IMO they have their place, and some of them are excellent, but they are a long, long way off from describing state-of-the-art!
OTOH, field coils are making a huge comeback right now. Active speakers are not likely to see innovations like that since they are closed systems.Historically, the vast majority of field coil speakers were indeed used in "closed" systems . . . such as radios, instrument amplifiers, organs and Leslie cabinets, etc. etc. This had the added advantage of the amplifier being able to use the field coil as a power-supply choke - effectively working to reduce the cost of two parts (choke and speaker magnet).
The main push for the adoption of permanent-magnet speakers came after World War II, with the demand for larger separate speakers for movie theaters and music production - many the very earliest examples of these at first had separate field-coil power supplies. The introduction of Alnico V as a magnetic material (itself developed during WWII) was the main reason that field-coil speakers were abandoned.
But as always . . . audiophiles have very fickle preferances, and while some may find it interesting and comforting to experiment continually with the amp/loudspeaker relationship, it seems to me that a significant number of posts here on the Audiogon forums is by people who are asking advice on trying to get this right -- maybe some of them would enjoy a product where this was already done for them as part of the product engineering.
Kirkus, can you elaborate on your opinion of the current Genelec's?
I happen to use the small 8020's (with a sub) in my video editing system, but often listen to music through them in the nearfield.
While nothing like the performance I get with my tube based audio system
I found them to be better than my previous very high quality passive mini monitors, I felt due to the Genelec's sculpted cabinet and waveguide tweeter.
I don't think that Genelec makes a bad speaker . . . it's just that as far as I can tell, the way they (and their knock-offs) implement the waveguide around a direct-radiating driver does not make really make it a constant-directivity system. Rather, it seems to simply to reduce the effects of the cabinet edge diffraction on the directivity. These are my rather informal observations based on hearing them in a decent handfull of studio control rooms, and measuring their response in two.
And while their idiosyncracies aren't really all that different from most direct-radiating studio monitors, Genelec specifically touts these features as making their monitors less sensitive to control-room acoustics and speaker placement, which I don't think holds up in practice. They also freely recommend most of their two-way nearfields and three-way mid-fields for horizontal configurations, which severely compromises the performance of virtually all speakers of this type. The result is that it's quite common to see Genelecs in a studio that give a very poor rendition of what the mix sounds like anywhere else.
I will concede that I do have some fairly strong opinions about both the environment and methodology of studio recording, and I feel that a true constant-directivity monitor (like the old JBL 4435) soffit-mounted in a competently-designed control room is the most neutral, consistent representation of what's actually on the master tape. Having a pair of good nearfields (NOT NS-10s) is a nice second perspective.
For a small home studio, relying solely on nearfields is frequently the only option, and the Genelecs aren't a bad choice . . . though I would personally prefer a pair of Meyers or ATCs. The main advice I would offer is to orient the monitors vertically, and keep your monitoring SPL as low as you're comfortable with. Also, pay close attention to your impressions when you take your mix to other systems, and adjust the monitor placement to get consistency between what you observe both inside and outside your studio.
Kirkus, you and I have similar ideas about monitors in studios. I use the High Emotion Audio S7- clearly the most revealing and neutral monitor to come along in a long time.
Regarding field coils, while the prior art was indeed integrated into cabinetry to reduce the power supply costs, modern FC systems usually have a regulated supply of their own. There are high excursion units now, beryllium-dome compression drivers and the like that simply did not exist 60 years ago. My understanding was the industry abandoned the art for the less expensive (and lessor performing) permanent magnet systems.
The reason I brought this up is that field coils are a rising star in high end audio right now and are an example of how having a closed system of amp and speaker will limit the ability of the end user to upgrade the system. If you recall the old powered Acoustats, the issue is similar- if you want a speaker that can play louder, or one that is **actually** full range (plays bass), or so on and so on, you have to change both the amp and speaker at the same time to get there.
Integrated systems have their place, especially when space is limited, but by definition they will never be state-of-the-art.
Atmasphere, I asked this question over at AA and I'd be interested to hear your opinion:
I'm curious who believes that a FC motor is inherently superior to even the best AlNiCo magnets, and who, conversely, believes that there is no such inherent superiority, and the only advantage of the field coil is the ability to tune parameters with the coil voltage/current.
Hi Paulfolbrecht, I for one believe the FC technology to be generally superior, but like all other things in this sport, a lot relies on execution. So its dangerous to make generalizations on that account.
FWIW though, you can't really 'tune' the parameters once the motor is designed- if you have that ability, it means that the gap isn't saturated, and if the gap isn't saturated, the motor will not be performing very well, field coil or no.
I wasn't alive back then, but I think that field-coil speakers in the first part of the 20th century were actually significantly cheaper than permanent-magnet types of similar performance . . . they were NOT a "high end" design. This was an era when the labor and expertise for winding coils was cheap and plentiful -- even budget radios were chock full of inductors, chokes, and RF transformers that were typically wound in-house. On the other hand, high-permeability magnetic materials were VERY expensive or even unavailable - the infrastructure for securing the raw materials (especially cobalt) and making high-quality alnico alloys . . . this is all very high-capital-investment stuff.
The early Lansing alnico designs specified a flux in the magnetic gap of something like 13,000 gauss . . . I'm skeptical that any field-coil design of that era could even produce half of that. And as I understand it the move to ferrites in the 1970s was a reaction to geo-political events - the main deposits of cobalt in the world fell under the control of regimes sympathetic to the Soviets, and the price of alnico alloys shot through the roof in a very short time.
I am also quite skeptical of the claims for the superiority of ferrites vs. neo vs. alnico vs. field-coil arrangements . . . but all of these methods of making the magnetic flux allow very different approaches to the design of the motor structure itself, and this does have a huge, fundamental impact on the driver characteristics. The first generation of JBL professional drivers that used ferrites were very carefully designed to have the same magnetic characteristics as their alnico predecessors, and I've mixed and matched them in sound-reinforcement systems and couldn't tell a bit of difference (except for the odd alnico driver that's lost some of its flux).
But I will agree that the Alnico magnetic structures are so much more elegant in an engineering sense . . . with no stray field, and much easier on one's back when moving them around. And I can see some similar appeal to a modern field-coil speaker.
It is always nice to see a good discussion on pros and cons. There is definitely
more than one way to skin a cat. I tend to agree with Kirkus about the older
generation Genelecs but I happen to like their active newer 8050 and 8020 small
monitors. In fact, I'd be happy with a great many many speaker designs both
active and passive, horn or panel etc. - so it is not like it is ever as clear as
"night and day". In many cases, it is all a matter of combining as
many small incremental improvements as possible - soffit mounting is one,
room acoustic treatments is another, and going active speakers is just one more
increment (similar to tri-amping) etc. etc.
For example, if you prefer Tube power amplifiers then the incremental benefit of
active speakers may be outweighed by the loss of that tube sound...
Most of the active systems I have heard/used have been for the pro market. So the likely hood of using a Tube amp in them is zero. Amp technology has come along way since the bad old days so I think the idea that because it is active means it has less than great amps is probably old now.
All the characteristics can be worked around to make the design work in most situations. You just have know what you are chasing.
The Genelec monitors I have used in studios over the years have always been "nice" sounding, but I have always found them hopeless to mix on. They sound great and musical with that flabby mid bass which gets in the way when you really need to know what is happening in the mix. I feel the same way with the Mackie monitors too (last generation Mackies). They sound great and musical, but take the mix somewhere else and it sounds flat. I do like the ATCs as they seem to have less of the color of the Genelec but still musical and detailed.
I have yet to find a great powered monitor for the studio although I do like the Adam active monitors. Being an Apogee and planar fan, I guess the Adam ribbon tweeter had the detail I crave to an extent.
I have never heard a modern Field Coil speaker, but I see Focal are making one which I am told is good although expensive. I believe the idea is to bring the design to the lower range soon.
I recently bought Avantgarde Trios for my Hifi. A 110db/watt hornloaded speaker with active sub bass after years of various planar speakers. Many of the traits I thought were the domain of the Planar/electrostatic were easily achieved by the Trio, but with the dynamic swing no planar I know is capable of. I am sure if I used Avantgardes own amp design for the passive part of the trio, essentially making it all "active" you would never be able to say active speakers are not or cannot be SOTA.
I kindly disagree with some of the comments concerning Genelec waveguides. In non-anechoic conditions we always listen both direct and reverberant field, most often the reverberant field is dominating. The reverberant field balance depends on speaker system's power response, i.e. total radiated power in all directions, not only on axis. Systems without any directivity control exhibit uneven power response having dips around crossover frequency(-ies) due to the simple fact, that lower frequency driver is more directive at its high end cutoff, than next smaller driver at its low end. The waveguide limits the directivity of the higher frequency driver (MF or HF) to the same as that of the lower frequency driver, and makes the total directivity uniform. This happens with the Genelec waveguides and for example with the JBL Bi-radial horn used in 4430/35. The frequency range depends on the size of the waveguide. The other important consequence is reduction of diffraction from the cabinet edges, and this is obvious in stereo imaging. A trivial mono signal reveals the truth easily.
If the power response is not uniform, the perceived balance depends heavily on the room and listening distance, and these simple facts are source of endless discussions.
As for field coil drivers, they still suffer from the very basic source of nonlinearity, iron. Regardless of that, field coil drivers would be easiest to use with active speakers, as power supply is inherently available. The drawback is naturally heat dissipation.
IMO active speakers can definitely be better performers than passive - there are many inherent technical reasons for that - but in addition to skills in electroacoustic design it requires a second skill set for proper amplifier design.
K_ilpo_p, I once heard a two way-speaker that used a waveguide on the tweeter the same size as the woofer, to even the power response as you say (and get the other benefits of horn-loading such as the increased dynamics and higher sensitivity). It was one of Duke's (Audiokinesis) creations and it was one of the best speakers I've ever heard in virtually every way. I probably would have bought a pair then & there (RMAF) had I not already had something like 5 pairs at home.
It seems to me that the best-sounding speakers are the ones that pay give proper attention to power response, which is indeed rare. The AN/E is another example.
Duke's speakers are indeed special. I own the Jazz Modules and in addition to the waveguide design and power response (summed omnidirectional response), another factor that contributes to the sound of these speakers is the fact they have negligible power compression. Duke uses pro sound drivers with the woofer being a TAD model with a alnico magnet and the tweeter using a neodymium magnet. The result according to Duke is that sound pressure levels up to 112 dB are reproduced with less than 1 dB of power compression.
When it came down to me choosing new speakers to replace my Spendor 1/2e, both Duke's and the Audio Note AN/E were on my shortlist.
It seems to me that the best-sounding speakers are the ones that pay give proper attention to power response, which is indeed rare.
Dr. Floyd Toole devotes quite a lot of time to this important aspect in his new book: Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms. ATC have been preaching about even power response for about 30 years. Careful attention to driver size and use of waveguides can create a smooth continuous off axis response which means the speaker will sound consistent in almost any room and from any position in the room. Ignoring the off axis response or power response can create an exciting and impressive speaker due to the nasty discontinuities like this huge hole or "scoop" in the mid range - this can make the speakers seem artificially "revealing" or amazing but will make it very difficult to setup as the user will really struggle to find a good placement which creates a balanced sound from primary and reflected energy as well as a good stereo image.
The result according to Duke is that sound pressure levels up to 112 dB are reproduced with less than 1 dB of power compression.
Although this thread seem to be digressing there have been many excellent comments.
Duke's claims are extrodinarily good. Most people would be surprised how effortless undistorted and uncompressed sound can be - even at what would seem to be very high SPL's (provided a good recording of course).
Sadly most speakers do not come with anything like high quality pro type drivers but at least they usually come with nice cabinets and great veneers...
I haven't heard the Audiokinesis loudspeakers, but I have spent a little time with Dr. Geddes' "Summa" loudspeaker, on which (as I understand it) the Audiokinesis designs are based. The Summa's horn/waveguide is a true constant-directivity design, and this is immediately evident in their excellent imaging, and very consistent tonal balance. I actually feel that Dr. Geddes' research in this area is some of the most interesting, competent, and relevant work in loudspeaker design in recent years.
But the "waveguide" designs in the Genelec monitors bear very little resemblence to a true constant-directivity spherical or bi-radial "waveguide" horn, in both the theory and the way they behave. This is mainly because they don't use compression drivers - and the waveguides are so short that the directivity characteristics of the driver itself dominate the polar response of the loudspeaker. Genelec's "waveguides" do seem to clean up the directivity performance of their drivers at the more extreme realms of their off-axis response, but they are NOT constant-directivity.
I'm really not trying to slam Genelec in general, and when used in the vertical configuration, their directivity characteristics similar to many well-behaved direct-radiating monitors. I just feel that some of their recommended setup configurations give lackluster performance, and their marketing material seems to imply that they are truly constant-directivity (even though they don't actually make that claim).
I don't know if it's accurate to say Duke's designs are "based" on Geddes' work, but he certainly pays tribute to the man. (I mean he pays vocal respect and acknowledgment, not that he pays tribute in the Cosa Nostra sense.)
Yes, Dr. Geddes obviously understands acoustics very well and it is enlightening to read his material.
But the "waveguide" designs in the Genelec monitors bear very little resemblence to a true constant-directivity spherical or bi-radial "waveguide" horn, in both the theory and the way they behave.
Good Discussion. Yes but....horns are different in that they use a compression chamber. For horns the exact mathematics and expansion design become critical, however, for a normal non-compression driver a simple conical expansion often has teh best performance and is enough to control dispersion such that a smooth power response is achieved.
and see K&H O500C - click on the "measurements" link in thr right hand corner for directivity plot. This is an example of an impressive response that is extremely smooth and wide both on and off axis and illustrates the use of the "waveguide".
Shadorne, great links! Thanks for the interesting reading.
One source of confusion is that there's a lot of ambiguity between in the terms "horn" and "waveguide" (add "lens" and it gets worse) - I tend to use them rather imprecisely as well. But to differ with AeroNET article (if I was to attempt to be precise), I think that the difference lies not in the efficiency or the type of driver used, but rather in the theoritical basis for its shape. A "horn" is usually based (at least loosely) on Webster's horn equations, which were derived in the early 1920s mainly to calculate the load the horn presents to the driver, for the purpose of maximizing efficiency -- this is the origin of the classic exponential shapes. However, there's very little theoritical basis here for understanding the horn's directivity characteristics, which is why horns for the first half of the 20th century used other techniques (multi-cellular construction, or a slant-plate lens) to control directivity without a good understanding of how the contour itself affects this.
A "waveguide" on the other hand is designed with mathematics that are derived from other fields, using techniques designed to accurately predict the directivity based on the waveguide's contours. The specifics of these maths are way over my head, but I think it's accurate to say that waveguide theory isn't limited to lower rates of expansion or lower acoustic gains.
The root of my skepticism with the direct-radiator/short-waveguide configuration (for which I've made the Genelec monitors the poster-boy) is that with my (admittedly VERY rudimentary and imprecise) understanding of both waveguide techniques and Webster's equations . . . all of it assumes a specific wave-front propegation for the horn/waveguide to work as intended. So for true constant-directivity performance to be possible, the wave-front propegation has to be constant with frequency . . . and the conventional direct-radiating cones and domes used in such configurations do NOT acheive this. Rather, they exhibit the classic increase in directivity with increase in frequency, just like all domes and cones.
The author of the AEROnet article does make mention of this, but then goes on to say that for wide-angle waveguides with direct-radiating drivers "that most of the mathematical detail can be side-stepped." Huh???? If you side-step the mathematical detail, then what you have is simply a random curvy recessed cabinet-front, and NOT a waveguide. He then goes on to make some measurements of some purely emperically-derived combinations . . . and while the final results look nice and smooth, this seems to be the obvious result of simply changing the way the waveguide is illuminated, thereby effectively altering its curve in a theoritical sense.
The K&H monitor does indeed have very smooth directivity plots, but the directivity still increases quite steadily with frequency . . . as one expects with direct-radiating domes. I would say that the smoothness of these plots (compared to i.e. a Genelec 3-way) is a testament to the excellent performance of the drivers themselves, and well-implemented crossover design . . . and the cabinet contours help out at the extremes. But they're also endorsing my other pet-peeve -- horizontal placement, for which they give no directivity plots. Suffice it to say it will be worse, and its horizontal polar response will then exhibit some of the inconsistencies found in the vertical directivity plots. Maybe they can clean this up a bit in the DSP, by changing some of the crossover slope characteristics . . .
I've also heard Duke's speakers at length, and they perform with some of the very best and IMO perform well outside their price points. He definitely did his homework.
FWIW in the old days, field coils and a lot of the other in-house coils were made in a very different economic environment. If we are talking about the 1920s, for example, the dollar now buys less than 1/10th of what it did back then. As the buying power of paper money declined, the industry looked for economic solutions- IOW permanent magnets are a **lot** cheaper to build.
I have an old RCA FC speaker from the 30s that is purported to have about 18,000 gauss once energized. The magnet structure on the thing is immense- so large that it is used for mounting the speaker, not the basket. IOW it puts a lot of Lowthers and the like to shame.
I have an old RCA FC speaker from the 30s that is purported to have about 18,000 gauss once energized. The magnet structure on the thing is immense- so large that it is used for mounting the speaker, not the basket.Wow, holy crap! I have never seen a field-coil magnet structure that size. If you ever have pictures . . . it's be cool to see.
Kirkus, again I have to kindly disagree on some points. Also omnidirectional source directivity is constant, and in power response terms the driver type has no inherent effect. In most practical cases the limited radiation angle is achieved in a certain bandwidth, not from 20 to 20k. Horns and waveguides can both be mathematically modeled and optimized and there are several BEM packages doing this. They usually model also the diaphragm radiating into the waveguide, as without that the picture is incomplete. The modern directivity control appeared first in sound reinforcement field, where the coverage is important. Altec made their Mantaray horns, using basically two flares and an abrupt joint between them. First expansion created the vertical pattern, opening into a narrow arc-like slit - being in itself a (horizontally) wide radiator- and the second flare controlled the horizontal pattern. The JBL Bi-radial horns use the same principle. In very simple terms a waveguide could be interpreted as the outer flare of a constant directivity horn, if you so wish. The fact that the throat area is equal to diaphragm area, i.e. there is no compression, mainly affects efficiency, which is higher with horn drivers. However, also waveguide improves efficiency in two ways: the actual radiating impedance is better matched, and the same energy is radiated into smaller solid angle than without waveguide. These factors are connected and the net result can be seen in the raw responses of the link, the sensitivity at the low end of the tweeter passband is higher. This is very good result as the improved sensitivity is easy to equalize and it improves tweeter reliability.
I do not understand what you actually mean with saying, that "for true constant-directivity performance to be possible, the wave-front propegation has to be constant with frequency." For constant directivity the radiation angle has to be independent of frequency. In practice the radiation angle vs. frequency has some ripple, as can be seen also from the O500 graph. O500 has a 300 mm woofer and its radiation angle (-3 dB) can be seen changing from 180 degrees at around 140 Hz( there seems to be measurement errors due to room limitations) to about 80 degrees at 500 Hz, then there is small ripple at upper crossover around 3 kHz,because the soft dome midrange becomes a more directive ring radiator, then it is getting narrower to about 40 degrees and again widening to 80 degrees at 15 kHz and getting narrower above that. As a whole this is can be regarded a good practical approximation of constant directivity, just like the performance of its predecessors, Genelec 1037, having also a 300 mm woofer and a bit smaller MF/HF waveguide, and 1038, having 385 mm woofer and larger waveguide. Those came to market already in early 90's (about 10 years before K+H O500) while their first predecessor, Genelec 1022A came to market in 1985.
Two-way speakers can be designed to constant directivity as well, but due to smaller woofer, the bandwidth of this behavior is not as wide and starts at least one octave higher. Even then the practical improvements are clearly audible. To make the woofer more directive at lower frequencies you can naturally use cardioid designs and accept the consequences in available power.
K_ilpo_p, thanks for the excellent discussion . . . let me see if I can better refine and clarify my thoughts on the matter.
Also omnidirectional source directivity is constant, and in power response terms the driver type has no inherent effect.The parameter of total, summed power response as I see it is most useful in trying to correlate the perceived timberal balance vs. measured frequency response for NON-constant-directivity systems, and for establishing the optimum placement and room treatment for a given loudspeaker system. It's pretty much irrelevant for the issue of establishing the best directivity characteristics of the driver(s) themselves. Consider that (for a single driver) electronic equalisation is supremely effective in altering the summed power response, but completely ineffective at solving directivity issues.
Altec made their Mantaray horns, using basically two flares and an abrupt joint between them. First expansion created the vertical pattern, opening into a narrow arc-like slit - being in itself a (horizontally) wide radiator- and the second flare controlled the horizontal pattern. The JBL Bi-radial horns use the same principle. In very simple terms a waveguide could be interpreted as the outer flare of a constant directivity horn, if you so wish.While not all modern constant-directivity compression-driver waveguides use an abrupt change in expansion rate, I like your description, and find it a useful analogy. So I'll attempt to use it to illustrate my basic point in the whole matter - which is that to substitute a pistonic driver (cone or dome) for the compression driver and throat . . . brings out fundamentally different principles of operation in the waveguide as far as the directivity is concerned. Also, the difference between these two approaches is pretty much unrelated to the traditional view of the difference between compression drivers and direct-radiating drivers - which you accurately state as being efficiency, and acoustic impedance.
I do not understand what you actually mean with saying, that "for true constant-directivity performance to be possible, the wave-front propegation has to be constant with frequency."Fair enough . . . my use of the term "true" implies a value judgement which I did not intend.
Instead, I'll refer to a compression-driver constant-directivity waveguide system (like the big JBL butt-cheek we've been discussing) as being a "wideband constant-directivity" system. In addition to the traditional points stated above (acoustic impedance and efficiency), a compression driver strives to transform the pistonic movement of the diaphragm into a pressure wave - a wave that has a shape that is (ideally) frequency independent. Early-20th-century practice viewed these as plane waves, examples being devices such as slant-plate acoustic lenses, and the driver measurement apparatus, a "plane-wave tube". And although the plane-wave as a useful, precise mathematical model may be completely outdated (I'll again reference Dr. Geddes' work), it is my understanding that in a "wideband constant-directivity system" (my terminology), the ultimate goal is for the driver to illuminate the waveguide in a manner that is constant with frequency. The result is a device where the useable constant-directivity frequency range is limited solely by the practical size of the waveguide, the mechanical performance of the compression driver, and the compression driver/phase plug/throat meeting the goal of frequency-independent waveguide illumination.
This is in (at least conceptual) contrast to the practice of using a pistonic driver to illuminate a waveguide, because the driver/waveguide relationship isn't (and cannot be) frequency-independent. Rather, (please correct me if I'm wrong) the idea is that the waveguide should dominate the directivity at the bottom of the driver's passband, and as the frequency increases, the directivity is decreasingly defined by the waveguide, and increasingly defined by the driver . . . this occurs because a pistonic driver will ALWAYS have an increase in directivity with an increase in frequency. Thus, in order for the driver/waveguide system to have smooth, predictable directivity performance . . . it is obviously of paramount importance that the driver itself have smooth, predictable directivity performance - in exactly the same manner as it should in a non-waveguide direct-radiating system.
My general conclusion is that while a piston-driver/waveguide combination can maybe acheive "a good practical approximation of constant directivity" (your description), its ability to do this will ALWAYS be limited to a much narrower frequency range than is possible with a wideband, compression-driver constant-directivity waveguide. It's also only effective over a specific range of desired radiation angles, which thankfully correspond to reasonably useful ones for studio monitoring. In the end, the directivity characteristics of the driver itself is the tail that wags the dog, and ultimately determines the extent of effectiveness in the waveguide.
As a final note . . . you make reference to the importance of matching the directivity of the bass driver(s) to the waveguide-loaded device(s) (something I very much agree with), and the effects of the crossover slope on the transition-band directivity. I'd be interested on how you view the common (recommended?) practice of turning i.e. the Genelecs sidewise and simply rotating the waveguide, which I feel makes a mess of these issues in both theory and practice.
Thanks - you make some really interesting points. I'd like to point out that a lot of the discussion depends on what "driver" you have to begin with. Let me explain.
To me the TWO main ideas of the short open waveguide are as described by K_ilpo_p....
1) it allows a driver to have greater sensitivity (better SPL and lower distortion) at the low end of its passband
2) it can narrow the low end of the passband radiation pattern - which enables one to match the higher frequency driver radiation pattern to that of the lower driver - for a smooth transition at crossover. Note that this type waveguide cannot help with the high end of the passband radiation pattern because that narrows anyway as the wavelengths become smaller than diaphragm.
Without digging into physics it seems clear that only a CONE shape will maintain uniform spherical wavefronts therefore if you have a dome driver (such as a dome tweeter or the midrange in the K&H O500) then you pretty much start with a spherical wave and therefore a cone waveguide is the simple answer to control dispersion (no bending of the wave is needed as in the case of a horn which has to be bent from a plane wave in to a spherical wave).
Here is some more interesting reading.
Shadorne, these are two great points, and I agree that they are very significant potential benefits of the "short open waveguide" approach. But they're not constant-directivity (which was my main point), and since as it does indeed very much depend on what "driver" you have to begin with . . . these behave fundamentally very much like a standard direct-radiating driver.
But as far as the cone vs. a dome to "maintain uniform spherical wavefronts" that's the whole problem, neither of them deliver any kind of wavefront that's consistent with frequency. Cones, domes, inverted domes, ring-radiators . . . they can all exhibit profound differences in their application and execution, but they are all of a similar ilk in their inability to deliver a consistent wavefront independent of frequency. The compression driver differs in the fact that it (at least aims to) acheive this goal.
I enjoyed Mr. White's article to which you kindly provided the link, but the main problem is . . .
The theory behind the waveguides to be described is that a dome driver produces what is fair approximation of a spherical wave over its piston rangeI simply can't conceive of this as being valid . . . I wish my knowledge of physics and my mathematical skill was sufficient to expound on this further, but I think it reasonable to say that it would be hard to build a consenus on this among those who do have competencies in these areas. Further, his calculations are based on the idea that the dome behaves as a point source . . . which is certainly impossible except perhaps for an extremely narrow range of frequencies.
After all, if a dome behaved as a point source, then simply screwing it into a baffle of appropriate size would produce absolutely perfect directivity characteristics, and we wouldn't need waveguides at all.
But they're not constant-directivity (which was my
I agree absolutely. "Constant-directivity" is indeed a term that
applies to compression horns rather than 'short open conical
waveguide". And the constant directivity in a speaker using short
conical waveguides is achieved primarily by limiting the drivers to covering
frequencies with wavelengths larger than the diaphragm diameter (this means
a three way in most cases rather than a more protypical two way
"CD" horn). The waveguide simply narrows the wide dome
dispersion so as to integrate the dome with the driver covering the lower
frequencies below the crossover.
After all, if a dome behaved as a point source, then
In general this is true - a dome works very well as a point source...this is why
they are so popular as the standard tweeter in the majority of speakers (used
within a limited bandwidth of course as they do start to become directive
somewhere above about 8 to 12 Khz and also suffer from breakup like any
regular cone at even higher frequencies and, of course, they rapidly drop in
SPL output as you go low in frequency and exceed excursion limits - however
there is not much "music" above 12 Khz anyway and they make
awesome cheap tweeters )
Large domes for covering lower frequencies also have a nice dispersion and
sound great but they have proved much less successful than the ubiquitous
dome tweeter - mainly because they are expensive to build properly (you
need a very large voice coil/motor and rocking can be an issue due to lack of
lateral support/alignment for the motion ( so some designs resort to having
two spiders) - all factors that make large domes extremely expensive
compared to a regular cone so few designers use them (awesome but way too
Power response predicts the perceived balance for all types of systems, also for those with constant directivity. Another factor affecting that balance is room absorption vs. frequency.
" It's pretty much irrelevant for the issue of establishing the best directivity characteristics of the driver(s) themselves. Consider that (for a single driver) electronic equalisation is supremely effective in altering the summed power response, but completely ineffective at solving directivity issues."
First: The Directivity Factor (DF) is the ratio of the intensity of a source in some specified direction (usually along the acoustic axis of the source) to the intensity, at the same point in space, due to an omnidirectional point source with the same acoustic power. Directivity Index (DI) is logarithm of that factor. The whole issue is defined as intensity ratio, and in itself it does not matter what kind of source you have. For constant directivity it is sufficient to have DI which does not, within tolerances, depend on frequency. In practice this can be achieved in many ways, horns and waveguides being good examples of them.
For simple cone or dome radiators, electronic equalization can be used to equalize the pressure response at a certain axis (mostly the main axis) of the driver. However, equalizing response very flat on one axis often means the response is worse on some other axis. This is a common problem with powerful DSP and diffraction ripple. Electronic eq cannot change how the driver does its radiation job, it can affect only the signal you put into it. Equalizing power response without simultaneously affecting on-axis pressure response is not possible.
As said, power response depends on the radiation characteristics of the driver, i.e. how much its radiation is attenuated off axis compared to on-axis radiation. Most important factor affecting this is the radiator effective diameter in relation to wavelength. When the diameter is much smaller than wavelength, the source is omnidirectional. Hence a 1" dome at 3 kHz is practically omnidirectional but at 20 kHz it is not. However, a 1" throat of a compression driver has same diameter and its directivity characteristics then depend on the shape of the wavefront propagating in the throat and later in the horn. Often also CD horns are pretty directive at 20k as well, but I agree with you that with a good 2" compression driver and well designed horn you can get about 4 octave bandwidth with good directivity control, acceptable frequency response and distortion. With a larger diameter driver you can add an extra octave on the low end at the cost of HF performance. This is a nice system if size and price are not too important. Like all drivers, compression drivers have their own set of compromises and it depends on the desired performance what features are regarded valuable. For example: compression drivers suffer from air nonlinearity causing distortion at high levels, but on the other hand, there is no better way of generating high SPLs. Therefore there have been long standing efforts to eliminate the distortion, one possible way is to predistort the signal in a predictable way. This is basically similar to analog tape recording. DSP can do audible improvements in this respect.
You are right in referring to the aim to match directivity of drivers. However, I see no practical problem in rotating the waveguide in 3-way Genelecs as the MF/HF section still remains vertical. Because the LF/MF crossover is somewhere around 400 Hz (i.e. pretty low) there will be no practical difference whether vertical or horizontal, but of course the room reflection pattern will change due to different height of the woofer. The woofer directivity remains as it is and so does the MF, the only changing parameter is their relative position. You see the same off-axis performance between woofer and MF driver either in vertical or horizontal off-axis measurement, i.e. when going sufficiently off axis you start seeing the crossover. You can naturally calculate the first off-axis zero from the driver distance. If you take 400 Hz as crossover, wavelength is 0.85 m and half of that is 0.43 m. The first zero is at angle where the distance difference is half of wavelength.
I think all two-ways should be used vertical, because of the same off-axis dip appearing at crossover. Naturally the same dip exists also when a two-way speaker is vertical but its audibility is small. Naturally if you have brickwall filters there will be no interference, then you might only notice the changing source position.
Thanks to all for the interesting discussion on waveguides in active monitors; it's given me much to think about and listen for.
However, I see no practical problem in rotating the waveguide in 3-way Genelecs as the MF/HF section still remains vertical. Because the LF/MF crossover is somewhere around 400 Hz (i.e. pretty low) there will be no practical difference whether vertical or horizontal, but of course the room reflection pattern will change due to different height of the woofer. The woofer directivity remains as it is and so does the MF, the only changing parameter is their relative position.I find myself taking over the producer's role on a small-label classical production, and was thinking about these comments a couple days ago during a tracking session . . . the mains in this studio are horizontal soffit-mounted Genelec 1038s. I still feel that they're problematic in this configuration, and much of what I don't like is in this lower-midrange area that's likely related to the low/mid crossover region. It's bad enough where for the mixing sessions we'll either have to come up with a near-field arrangement that I'm happy with, or move to another facility. Either way, I was relieved that I was only making performance decisions through those 1038s . . .
Vertical mounting of course doesn't eliminate the crossover-related off-axis lobing, but it places it entirely on the vertical axis . . . and the vertical-axis listening position in the control room varies far less than the horizontal. Perhaps the use of brick-wall filters would reduce this, but I have no experience with monitors that use them.
Kirkus, if you can, try out a set of High Emotion Audio S-7s for near-field. They will give you very reliable information about what is going on in the recording- and no lobing effects if you happen to be off-axis. They have the widest dispersion I've seen- there are no powered speakers that can compete.
Vertical mounting of course doesn't eliminate the crossover-related off-axis lobing, but it places it entirely on the vertical axis . . . and the vertical-axis listening position in the control room varies far less than the horizontal.
Good point. I agree tha this is an interesting discussion. Thx to you and to Kilopop and others.
However I woudl add that your point above is even more a problem for passive designs.
I would add that the advantage of Active speakers is that you can make a much sharper and phase compensated crossover filter precisely because it is active. This should reduce lobing. From what I know the 1038's allow the HF/MF combined unit to be rotated inside the cabinet for horizontal placement and this should ensure that any remaining lobing remains in the vertical axis where it is less significant.