It's perfectly possible to design a driver which is suitable for both a sealed and bass reflex design. The cut-off frequency will be much lower on the bass reflex though, and also require a bigger box, as much as twice as big. (Sensitivity will be the same)
With the right parameters the same driver can give a 0.707 Qt closed box response in a reasonable size box, and a maximally flat bass reflex response with the right tuning.
With a Linkwitz transform the sealed box can even have it's cutoff frequency extended at the expense of power handling and excursion at the low end.
Being optimized for both a bass reflex design and a true acoustic suspension though, (rather than just "closed") is not so practical, as the extremely soft suspension compliance needed for acoustic suspension will cause a rapid cone excursion increase below box tuning frequency, even if the driver will give a nice flat response with a smooth roll off. I suspect this is what you mean.
With a subsonic filter and a relatively low box tuning a driver intended for acoustic suspension can still sometimes be used successfully with a bass reflex design, although there isn't much benefit over a very similar driver with a somewhat stiffer suspension that will be less prone to damage below box tuning, and still have a similar response above box tuning.
Well, f you want no distortion, you need linear air suspension. Means Adiabatic, means the enclosure full of damping material. So, yes it needs little cones and large Xmax.
With larger cones, you can chose a closed box for it, as long as it is ok with Fs ans Qts, but, as we all see, calculating-it, the main part of the suspension force will be from the spider, not air. Unless you chose a very high resultant resonances frequency.
I don't agree bass response not as good with bass reflex. On the contrary. It is a matter of design and calculation. With motional impedance compensation, and good damping, you can have faster and better damped bass response, without this booming and often dead sound of many closed enclosures. Plus extended low frequencies and less cone excursion. And, best of all, you can adjust your response curve to adjust it at your room response.
It's easy, they went to the same place as 3-way speakers. #1 you can't get a closed box giving f3 of < 45 Hz at reasonable SPLs with small drivers. So you need at least a driver > 10" for their spiders and surrounds giving Fs ~ 20 Hz. Some of the earliest models had a 10" frame with 8" cones as a compromise. Larger woofers with wide dispersion tweeters means costlier designs ie 3 ways VS the ubiquitous 5-6" 2 ways with similar F3's.
Nice!
I have to admit that I do not like all a.s. speakers and Acoustic Research is in that group. 🙁 Most ARs sounded rather "dead" to me, and as someone who listens to a lot of rock/certain pop music/punk music, their overly "polite" bass and dull - to my ears - higher frequencies did not allow me to fully enjoy my music. In fact, back in the early 90s a reviewer for Stereophile(?) magazine said that he thought AR speakers (and I am paraphrasing here) "sucked the life out of more music than he could remember".
But other a.s. speakers, for example Boston Acoustics and Advent did make my music sound good. Compared to the ARs, the only thing they were missing were the AR's extended bass response and a bit of accuracy. BUT...the Advents and Bostons traded those two aspects for increased efficiency so I didn't need a huge power amp with enough current to weld steel plates together. 😀
BTW I also own an "orphan" Realistic Nova 8 I purchased for $5 at a thrift store about 6 years ago. Check it out here on page 20. It is a 3-way with a 12" a.s. woofer.* Great bass, deep & clean and doesn't need much power to generate that great bass. But once my Pioneer receiver's power output meter reaches past @55 watts, the woofer's cone begins to strike the bump stops. But by that time it is plenty loud enough for me!
I am not saying I dislike b.r. speakers - presently, my main speakers are Boston Acoustic CR9s, two ways with a ported 8" woofer - I just like most a.s. speakers more.
* uses a conventional paper cone (not the thick, felt-like cone many ARs used) but paired with a very soft suspension which uses a concave cloth surround impregnated with a shiny black substance. The frame is made of cast aluminum - nice. Overall this woofer almost seems built too well for a Radio Shack speaker! Veneer is real walnut. Pretty much everyone that has seen the speaker comments about how nice it looks.
Or the Larger Advent and Dahlquist DQ-10, which both featured 12" frames with 10" cones and masonite spacers.
....and they need to bring them back. 😉
But can those 5-6" 2 ways provide the same visceral impact when playing a Minor Threat or Thievery Corporation album? In other words, can I turn them up to politically-incorrect levels and not worry about damaging the woofer?*
* and sure, technically-speaking a sub+sat system can provide the output level and bass extension of a pair of large a.s. speakers like some old Boston Acoustics A150s, but there's still something about the simplicity of a pair of speakers + stereo amp + source that I like, despite the illogic of it. 🙂 About the only aspect of large speakers like those that can be viewed in a negative light is their increased amount of materials, but hey, that's what recycling centers are for.........
Has anyone heard of "closed resistance" box? Enclosure where is two compartments and "leaky wall" (holes+damping) between them? AFAIK the idea is that enclosure "seems" smaller as frequency rises. That could make air spring more linear?
Still SB Acoustic 29NRX75-6 has more gradual roll off and accepts smaller enclosure. It allows 40 liter unstuffed for same kind on response than TVM12" in 68 liter unstuffed. Although 12" is 2,5 dB more efficient so may be it is still better choice...
One smaller driver you can imagine A.S. 2-way speaker that goes low is Scan Speak 18W8535 (26 Hz / Qts 0,38) . It gives F10 --> 29 Hz in 30 liter unstuffed box so maybe 26 liter stuffed could be nice. Still I don't believe it can give such an impact that 12" gives and 3 dB more efficient can give. At least it has to be put in wider box (than 190mm) for more directivity in low mids.
They aren't. There are sealed boxes such as infinite baffle enclosures that are nothing like AS speakers. Large Bozak speakers like Concert Grand are an example.
Here's how Edgar Villchur's invention from 1954 works and why it works (he got the right answer but didn't fully understand why it was right...but it was still the right answer.)
One problem for producing bass tones is that when the sound from the rear of the speaker meets the sound from the front of the speaker it cancels it because it is out of phase. Seen from the back when the speaker is pushing air to the front it's pulling it from the back and visa versa. The cancellation effect increases as frequency goes down. that's why speakers without enclosures have little bass. One method of preventing this is to prevent the rear wave from ever getting to the front. It's as though the baffle board stretched out to infininty in all directions. In practice, a very large enclosure can do much the same. In the early days of Hi-Fi, people would cut a round hole in a closet door and mount the speaker on the door. That worked well but not practical for stereo. Another is to put a speaker in a wall between rooms leaving the rear of the speaker open to the other room. That also works well. Using an attic or basement as the other room works well too. Dayton claims to be the only current American manufacturer of drivers optimized for infinite baffles.
The bass reflex design, grandaddy of ported designs relies on the fact that when a sound wave hits a wall, the reflected wave is 180 degrees out of phase with the incident wave. Therefore in a box speaker the sound coming through a front port will be in phase with sound from the front of the driver reinforcing it rather than cancelling it. But there are problems besides the chuffing that can occur from the air being squeezed through the opening. The port is "tuned" which means that at some frequencies it's easy for air to move through it easily, at others it's difficult. This is because the port creates a resonant air column like the pipe in a pipe organ. At the port's resonant frequency output is high, and at multiples of this frequency too. Midway between these resonant octaves output is low. So the response is irregular. This is why ported designs of the 1950s acquired the derogatory term "Johnny One Note." This may be great for rock music with its droning thump thump thump bass. It's not good at all for classical music. Another problem is that ported designs fall off steeply below resonance at 24 db per ocatve so the frequency of system resonance is the low cutoff frequncy. To produce deep bass, this type of speaker usually had to be very large, not practical in most homes when two are requried for stereo. Another problem was that the mechanical suspension of the cone was stiff and as frequency got lower, its restrictive force on cone movement increased. It was also not a constant with frequency.
Enter Edgar Villchur with an idea major manufacturers of the day rejected and said wouldn't work. Villchur's highly compliant long throw woofer is installed in a small sealed box with a lot of fiberglass stuffing. The combination forms a tuned system whose behavior can be explained according to Newton's Second Law of Motion applied to forced oscillation. This can be found in any good first year college physics textbook along with a detailed explanation and how system resonance and damping are controlled. The equation deals with three variables, mass, displacement, and velocity related damping (all speaker systems can be analyzed using this equation including ported systems, it is universally applicable. TS parameters are merely cookbook shortcuts for Newton's second law used for woofer/enclosure design.) One advantage is that the restoring force on the speaker is the compression and rarifaction of air in a sealed box which is not a function of frequency as ported systems are. Instead it's and independent of frequency and compltely linear according to the ideal gas laws (Boyle's and Charles' Laws) unlike the back pressure from ports and mechanical suspensions. The fiberglass stuffing forces the driver to push and pull air between the fibers creating a frictional aerodynamic drag. This loss controls the damping. The amount of material and nature of the fibers and how densely it's packed etc controls the effect. Therefore by adjusting these three factors alone, any arbitrary response can be obtained from any size driver. The system resonance relative peak Q as well as the resonance point F3 can be tuned. Optimally Q=.707. This gives the lowest F3 (3 db down point) without a bass FR hump. Some speakers were deliberatly designed with this hump having an intended Q of 1 because it made them sound punchier to those who used them for rock. AR1 and its descendants had drivers with a free air resonance of 16 to 20 hz and a system F3 of 42 hz. Q=.707. AS systems fall off linearly at 12 db per octave and can be equalized for an extra octave or more to extend their bass response. This is how Bose 901 series 1 and 2 achieve their low bass response.
Another advantage of AS designs is that the restoring force is applied uniformly over the entire area of the cone. This is because it is due to air pressure, not a mechanical system that may not be uniform around the circumference or from the near to center spider to circumference. Imbalances in mechanical suspensions work to twist the cone causing it to flex and break up into Bessel function modes like a drumhead. A purely AS design therefore has no force gradient from any part of the cone to any other part. Typical for AR3 was 5% THD at 30 hz, much lower than any other speaker in its day.
A few years after the AR1W was introduced, the NY Audio League, predecessor to AES took 4 of them and some Western Electric 150 watt amplifiers to Riverside Church in Manhattan. There they played an LvR demo against an Aolean Skinner pipe organ. The results were reportedly startling.
Villchur and his colleagues struggled to create a full range speaker using this woofer. They tried an 8" Western Electric tweeter, a similar Altec tweeter, some tried Jantzen electrostatic tweeters. Around 1960 Villchur introduced AR3 which incorporated another of his many innovations, the dome driver. He used a dome tweeter and dome midrange. Although the tweeter had a known high end rolloff and the woofer response was irregular near its 1Khz crossover frequency, dispersion was outstanding and a slight boost of the treble control flattened response. The speaker was relatively inexpensive, about $450 a pair, was small at about 2 cubic feet, produced the best bass competing with and equaling if not beating the largest and most expensive giants, and was manufactured to the highest standards of quality and uniformity. As a result it became the reference standard for many and is exhibited in the Smithsonian Institution as an example of excellence in American innovation, design, and manufacturing. For many professional musicians and serious listeners of both classical and jazz it was the speaker to own. About 7 years later Villchur introduced an improved midrange and tweeter with the woofer crossed over at 575 hz and this became AR3a. Villchur is credited with many other innovations including ferrofluid cooling of tweeters.
Many of today's best and most expensive speakers quietly use the AS design without advertising it.
It seems it is over simplified. At the resonance frequency, the Bass reflex gives a charge to the loudspeaker. So the move of the driver is greatly reduced, and the impedance peak too, the damping maximised. Most of the acoustic power comes, at this frequency, from the vent. On harmonics, it is not true at all, just a matter of design, diameter of the vents, damping material and you can see a lot of bass reflex enclosures with very very flat response curves. For sure, this harmonics problem occurs with quarter wave lines designs.
Some can argue than, with closed box, you get a lot of dragging and a pic of impedance/ X replacement, at the resonance frequency.
As far i'm concerned, i do not like to lose power at low frequencies in a closed enclosure, and i greatly appreciate the extra low frequencies i can save and the better efficiency that gives a higher (faster) resonant loudspeaker. Together with the near open charge at upper frequencies.
I do not understand this rock/classical distinguish. We all try to get as deep, strong, solid linear and fast basses as we can. Percussions are the same in both musical worlds.
One thing is amusing, the mood, nowadays, is back to the beginning of the hifi story: Bass reflex and (good) horns, hight efficiency loudspeakers and enclosures ;-)
On my side, i will never return back for basses to closed enclosures, while i designed a lot of those at the beginning of my career both for my personal use and for the research and development office i was working in. With an exception for my computers little loudspeakers .
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Is this what you are referring to?
Dynaco A-50 and A-40XL
Btw I always thought of the A-25's design with its stuffed port, another type of aperiodic enclosure, as an acoustic-suspension system using a very small enclosure but specially constructed to make the woofer "believe" it was operating in a larger enclosure.
Edit: the A-25's crossover was simplicity itself, one capacitor for the tweeter. That's it (there is also a level control for the tweeter consisting of several resistors + five position switch). Woofer was allowed to operate "full range". Though if I remember correctly, SEAS designed the output of the woofer and the tweeter so they worked well together and a more complicated xover simply was not needed, important for this speaker which was intended for the audiophile-on-a-budget.
Check out this forum for detailed discussions about the Dynaco.
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Newton's second law of motion is not an oversimplification. Over hundreds of years of experimentation it has proven to correlate excellently with real world experience. The cost of adequate amplifier power for operating an AR3 in 1961 was a real problem for many. Fifty years later it isn't but a 100 wpc HT receiver won't work, they don't have the power bandwidth and they aren't stable with very low impedance loads. Horn speakers are much more efficient over their operating range than AS speakers but to build one that performs as well at very low frequencies requires it to be far larger and more expensive.
The current market trend seems to be towards small slender 2 way or 2 1/2 way 6" or 8" ported stand mount or tower designs that are far overpriced, often shrill sounding with their attention getting focused tweeters, and have little deep bass. If you ask the manufacturer why, he'll tell you to buy a subwoofer which he'll gladly sell you. Those are often AS designs with their own dedicated high power amplifiers.
Not the law, but the real world's acoustic phenomenas in YOUR specific enclosure;-)
I you put a measurement microphone at the output of my vents, you will see mostly the resonant bump, and, if any harmonics, they are hidden in the acoustic leakage of the driver above it. If you dump the vents, you will not see any noticeable change on the response curve above 100hz neither.
One of the major bad sounding causes with bass reflex is the way they are damped.
The good way is to put the damping curtains where air speed is maximum, in the middle of each volume's dimension (making crosses in the middle), and not on the walls of the enclosure: you will immediately notice a 2/3 db decrease of frequencies levels in the 100-300hz range. And no more box's sound. It acts too as a closed resistance and random harmonic resonances of the vent.
This method helps a lot with closed boxes as well, dumping the Q of the main resonance, reducing the apparent volume for upper frequencies. It need less dumping material too, so you don't loose volume. If your enclosure is transparent, you will notice how the vertical dumping panel moves at the low frequencies and absorb energy here. Again, not usable for air suspension where you need light damping material in all the volume.
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It's also very difficult to find a non-reflex design in the world of mixing audio , near field monitors ; Surprisingly , half of the models in this arena ( where it's almost a certainty that the box is going to be against a wall in a home studio ) have the port in the back . The more expensive models may include some shelving to adjust for the half space application , But it would seem that the consumer's appetite for accurate monitoring is bridled by the constraints of size .. the port in back makes for a smaller box .
Many lower market priced near field monitors do have bad bass transient smearing ; so much so in fact that Martin Walker ( long time contributor at the U.K's Sound on Sound ) has stated in his new book Mixing Secrets for the small studio that one shouldn't even take the risk of buying any N.F. monitor that is a bass reflex design unless they cost at least $1500 us a piece !!!
I hate to admit it , but my old Alesis monitor ones where within a few inches of the wall for quite some time , and I spent allot of wasted time putting in notches in my bass eq settings to try and kill the one note bass !!!!
Many lower market priced near field monitors do have bad bass transient smearing ; so much so in fact that Martin Walker ( long time contributor at the U.K's Sound on Sound ) has stated in his new book Mixing Secrets for the small studio that one shouldn't even take the risk of buying any N.F. monitor that is a bass reflex design unless they cost at least $1500 us a piece !!!
I hate to admit it , but my old Alesis monitor ones where within a few inches of the wall for quite some time , and I spent allot of wasted time putting in notches in my bass eq settings to try and kill the one note bass !!!!
The rear ported design relies on the fact that the sound emerging from the port will undergo a phase reversal the same way it does in a bass reflex enclosure except that one is not within the designer's control.
There is no getting around the fact that the resistance to air flow through any opening in a ported design relies on a resonant air column whose very nature means that the resistance to air flow will vary with frequency. It also relies on a mechanical suspension whose spring constant can also vary with frequency. This is where the AS design beats the ported design hands down.
It should be pointed out that no design is purely AS there is invariably some mechanical restoring force. If a design were purely AS and absolutely air tight, it would be subject to a mannometer effect where the neutral point of the voice coil would change as external air pressure changed. A minute leak and some mechanical restoring force is all it takes to be certain the woofer will return to the desired neutral point when the applied voltage is zero.
Transient response and FR are two ways of looking at the same thing. The best transient response comes with flat FR and no bump. The AS concept deals with the mechanical resonances of the moving system. The electrical resonances are the easy part, they are corrected with passive or active filtering (also known as equaliziation or crossover design.)
There's no such thing as a "fast" or "slow" speaker driver. Two speakers driven to the same linear displacement amplitude at the same frequency will have exactly the same velocity at every point in their travel. What is different is the propagation delay, the time between application of voltage and corresponding mechanical response. The goal of so called time corrected or phase coherent designs is to match the propagation delay of the woofer with the tweeter at the crossover frequency. This "group delay," that is the difference between them should be as small as possible, their absolute value being irrelevant to the concept. BTW, the concept is fatally flawed anyway. Not only is there no evidence that any attributes of so called phase coherent designs are not caused by other factors but unless both drivers have the same point of origin, that is on the same axis (coaxial) and the tweeter's signal is delayed in time since its propagation delay is invariably shorter and it will be in front there will be interference patterns in the resulting field around the crossover frequency. MTM designs don't eliminate them either, they just make them symetrical with respect to the plane through the perpindicular bisector through the center of the tweeter.
There is no getting around the fact that the resistance to air flow through any opening in a ported design relies on a resonant air column whose very nature means that the resistance to air flow will vary with frequency. It also relies on a mechanical suspension whose spring constant can also vary with frequency. This is where the AS design beats the ported design hands down.
It should be pointed out that no design is purely AS there is invariably some mechanical restoring force. If a design were purely AS and absolutely air tight, it would be subject to a mannometer effect where the neutral point of the voice coil would change as external air pressure changed. A minute leak and some mechanical restoring force is all it takes to be certain the woofer will return to the desired neutral point when the applied voltage is zero.
Transient response and FR are two ways of looking at the same thing. The best transient response comes with flat FR and no bump. The AS concept deals with the mechanical resonances of the moving system. The electrical resonances are the easy part, they are corrected with passive or active filtering (also known as equaliziation or crossover design.)
There's no such thing as a "fast" or "slow" speaker driver. Two speakers driven to the same linear displacement amplitude at the same frequency will have exactly the same velocity at every point in their travel. What is different is the propagation delay, the time between application of voltage and corresponding mechanical response. The goal of so called time corrected or phase coherent designs is to match the propagation delay of the woofer with the tweeter at the crossover frequency. This "group delay," that is the difference between them should be as small as possible, their absolute value being irrelevant to the concept. BTW, the concept is fatally flawed anyway. Not only is there no evidence that any attributes of so called phase coherent designs are not caused by other factors but unless both drivers have the same point of origin, that is on the same axis (coaxial) and the tweeter's signal is delayed in time since its propagation delay is invariably shorter and it will be in front there will be interference patterns in the resulting field around the crossover frequency. MTM designs don't eliminate them either, they just make them symetrical with respect to the plane through the perpindicular bisector through the center of the tweeter.
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Good grief.
A big long authoritative sounding post on bass reflex and closed boxes that is absolutely riddled with factual errors and misunderstandings. Where to begin in debunking this one ? 🙄
It also makes no difference to the phase of the bass emanating from the port (or reaching the listener) whether it is situated on the front or the back of the cabinet relative to the driver. (Which by implication phase reversals at reflection boundaries would cause) The box is too small in wavelengths to have any effect like this, unless its made long enough to be a transmission line. (Rather long)
The rear radiation of the cone as you pointed out in the previous section is naturally 180 degrees out of phase with the front - leading to cancellation if there is an easy path for the rear radiation to travel around to the front - such as an open baffle.
The additional phase reversal required to bring the cones frontal radiation and the port radiation into phase above the box tuned frequency is provided by the Helmholtz resonance itself.
The image I've attached clearly shows the rapid phase transition that occurs around the box tuned frequency. In this example there is a 45 litre box tuned to 43 Hz, with red showing the phase of the ports output measured near-field, and yellow a near-field measurement of the drivers cone.
The two are exactly 180 degrees out of phase below about 38Hz, and exactly 360 degrees (in phase) above 46Hz with a relatively narrow transition region.
Tuning the box's resonant frequency will shift this inflexion point, changing one of the dimensions of the cabinet such as depth (while keeping the same box volume) will not.
By the way, I don't understand why you're trying to draw an artificial distinction between "bass reflex" and "ported" ? They're the same thing, unless you're also lumping transmission line designs into "ported", however most people would use the word ported to describe a bass reflex design, not a transmission line.
The resonance that occurs in the bass region (say 40Hz) as part of the bass reflex operation is not a pipe resonance, but rather a mass spring resonance where the moving mass is the mass of air contained within the port tube (and extending a bit beyond it each way) and the spring is the volume of compressible air within the cabinet. (excluding the air within the port)
The two work together to set the resonant frequency - varying the length or diameter of the port, for the same box volume or varying the box volume whilst keeping the port the same will both change the box tuning frequency.
Technically its wrong to say that the "port" is tuned to a certain frequency, as it's the port and box volume together that resonate, and if you have multiple ports of different lengths (not a good design idea) there is still only one resonance at one frequency not multiple resonances. It's normally referred to as the box resonance for that reason.
There is also no port output at "multiples" of the box resonant frequency, again there is only a single resonance, and in fact the port acts as band pass filter at its resonant frequency. (With one exception I'm about to mention)
The pipe resonances you refer to are real enough, but they fall at much higher frequencies well up into the midrange. For example the box I referred to before has two ports each about 220mm long, and the first pipe mode resonance occurs at around 700Hz and is dictated solely by the length of the pipe, whereas the bass resonance is controlled by the length and diameter of the pipe as well as box volume - they're two different resonances with different causes.
This tube resonance can be a big problem, it's not uncommon for even a well lined box to have spurious midrange output from the port at this resonance frequency which exceeds the output from the driver.
By far the simplest and most effective way of turning this into a non-issue is to place ports on the back panel rather than the front...
If you're referring to the suspension stiffness changing with frequency, you're wrong, (it does change a bit with amplitude though) and if you're referring to the change in loading (apparent stiffness of the air load) then yes, it changes with frequency in a bass reflex design, being stiffest at box resonance, and then becoming a lot softer below resonance and that's how a bass reflex design works, and provides more output in that frequency range. So it's not a "problem".
Air in a closed box is typically much more linear than the suspension of drivers made in the 60s, so this was a net win in the AR acoustic suspension designs of the 60's.
However modern drivers are much more linear, and tend to get squeezed into ever smaller boxes and with longer excursions. A point is soon reached where air itself becomes a non-linear spring which will cause 2nd harmonic distortion even if the motor were perfectly linear - because the percentage change of air volume in the box is not symmetric on the in stroke and out stroke, therefore the resisting force is not equal in each stroke direction, result - second harmonic distortion.
This is a real problem with closed box sub-woofers that use a large and/or long throw driver in a very small box. This is one thing motional feedback was invented to solve, and in a large excursion/output small box some type of motional feedback system is the only way to eliminate this distortion which is introduced by the asymmetric air spring.
Air approximates a "linear" spring only if the volume displacement doesn't exceed a certain percentage of the box volume. I forget what the commonly accepted figure is but its something on the order of 10-20%.
There is nothing magical about using fibrous damping - it's just one source of damping within the box at bass frequencies, and not the dominant one. Because of the way fibres affect the adiabatic process of air compression and rareification, it's possible to make the box "look" slightly larger than it really is - up to about 15%, but there is no sonic benefit to this, it's just a different way of achieving the same response that requires a slightly smaller box. (Improvements in midrange standing wave absorption can be dramatic though)
If you have a driver in a very large box the force gradient on the cone at bass frequencies is largely determined by the stiffness of the surround and the mass of the cone, with very little effect from the air.
In some drivers the spider is made the dominant stiffness with a very compliant surround - this is often done in high sensitivity full range drivers with thin cones precisely for the reason that having a stiff surround will put much more bending strain on the cone, for no good reason.
By having most of the stiffness at the spider, and using the driver in a large box (as is usually required in a high efficiency driver) such a driver can produce reasonable bass despite having a rather thin and delicate cone - because there is very little mechanical stress on the cone. The voice coil is pushing and pulling against the spider and the cone and surround are essentially just "along for the ride".
Now try to put that same driver in a very small closed box and equalize the bass. In all likely-hood the cone will tear or deform near the edges due to the intense back pressure across the entire cone - load that wasn't there before.
Yes this extra load is uniform across the entire cone but its a lot of extra load all the same, and requires a much stiffer cone to avoid bending or tearing apart.
For proof of how much strain loading puts on a driver look at woofers designed to be used in bass horns - they require very stiff cones and strongly constructed surrounds, and even then they can still suffer from crumpling or tearing at the edges due to the intense air load provided by the horn - when the same driver will survive perfectly happily at full excursion in a bass reflex design, where the cone load is far less.
Look also at sub-woofer drivers - the strain on the cone of a large high excursion driver in a small closed box is exceptionally high, and far higher than if the same driver was used in a larger box.
Air loading in moderation can improve the linearity of a poor driver, but the better the linearity of the driver to begin with the less of a benefit it is, and if you go too far in shrinking the box size you just increase the strain on the driver and eventually reach a limit where distortion starts to increase again.
True acoustic suspension designs (Vas/Vb >3) made a lot more sense with poor linearity 60s drivers than it does with the drivers of today.
A big long authoritative sounding post on bass reflex and closed boxes that is absolutely riddled with factual errors and misunderstandings. Where to begin in debunking this one ? 🙄
This is so wrong, I don't know where to begin. There is no such phase reversal due to reflections within the box, and bass reflex systems do not rely on any internal reflections for phase reversals or any other aspect of their operation.
It also makes no difference to the phase of the bass emanating from the port (or reaching the listener) whether it is situated on the front or the back of the cabinet relative to the driver. (Which by implication phase reversals at reflection boundaries would cause) The box is too small in wavelengths to have any effect like this, unless its made long enough to be a transmission line. (Rather long)
The rear radiation of the cone as you pointed out in the previous section is naturally 180 degrees out of phase with the front - leading to cancellation if there is an easy path for the rear radiation to travel around to the front - such as an open baffle.
The additional phase reversal required to bring the cones frontal radiation and the port radiation into phase above the box tuned frequency is provided by the Helmholtz resonance itself.
The image I've attached clearly shows the rapid phase transition that occurs around the box tuned frequency. In this example there is a 45 litre box tuned to 43 Hz, with red showing the phase of the ports output measured near-field, and yellow a near-field measurement of the drivers cone.
The two are exactly 180 degrees out of phase below about 38Hz, and exactly 360 degrees (in phase) above 46Hz with a relatively narrow transition region.
Tuning the box's resonant frequency will shift this inflexion point, changing one of the dimensions of the cabinet such as depth (while keeping the same box volume) will not.
By the way, I don't understand why you're trying to draw an artificial distinction between "bass reflex" and "ported" ? They're the same thing, unless you're also lumping transmission line designs into "ported", however most people would use the word ported to describe a bass reflex design, not a transmission line.
If the port cross sectional area is undersized - a common design mistake decades ago, but easy to avoid now that we know how to calculate the required port area for a given design. Basically a non-issue for anyone following good design practices.
Also wrong. There are no pipe mode resonances in ports at bass frequencies in any normal bass reflex design.
The resonance that occurs in the bass region (say 40Hz) as part of the bass reflex operation is not a pipe resonance, but rather a mass spring resonance where the moving mass is the mass of air contained within the port tube (and extending a bit beyond it each way) and the spring is the volume of compressible air within the cabinet. (excluding the air within the port)
The two work together to set the resonant frequency - varying the length or diameter of the port, for the same box volume or varying the box volume whilst keeping the port the same will both change the box tuning frequency.
Technically its wrong to say that the "port" is tuned to a certain frequency, as it's the port and box volume together that resonate, and if you have multiple ports of different lengths (not a good design idea) there is still only one resonance at one frequency not multiple resonances. It's normally referred to as the box resonance for that reason.
There is also no port output at "multiples" of the box resonant frequency, again there is only a single resonance, and in fact the port acts as band pass filter at its resonant frequency. (With one exception I'm about to mention)
The pipe resonances you refer to are real enough, but they fall at much higher frequencies well up into the midrange. For example the box I referred to before has two ports each about 220mm long, and the first pipe mode resonance occurs at around 700Hz and is dictated solely by the length of the pipe, whereas the bass resonance is controlled by the length and diameter of the pipe as well as box volume - they're two different resonances with different causes.
This tube resonance can be a big problem, it's not uncommon for even a well lined box to have spurious midrange output from the port at this resonance frequency which exceeds the output from the driver.
By far the simplest and most effective way of turning this into a non-issue is to place ports on the back panel rather than the front...
This comment makes no sense at all. Are you referring to the stiffness of the suspension changing with frequency, or the change in air loading on the cone with frequency in a bass reflex design ?
If you're referring to the suspension stiffness changing with frequency, you're wrong, (it does change a bit with amplitude though) and if you're referring to the change in loading (apparent stiffness of the air load) then yes, it changes with frequency in a bass reflex design, being stiffest at box resonance, and then becoming a lot softer below resonance and that's how a bass reflex design works, and provides more output in that frequency range. So it's not a "problem".
Wrong (or at least misleading) again. Air in a closed box is not completely linear, depending on the volume displacement of the cone relative to the box size.
Air in a closed box is typically much more linear than the suspension of drivers made in the 60s, so this was a net win in the AR acoustic suspension designs of the 60's.
However modern drivers are much more linear, and tend to get squeezed into ever smaller boxes and with longer excursions. A point is soon reached where air itself becomes a non-linear spring which will cause 2nd harmonic distortion even if the motor were perfectly linear - because the percentage change of air volume in the box is not symmetric on the in stroke and out stroke, therefore the resisting force is not equal in each stroke direction, result - second harmonic distortion.
This is a real problem with closed box sub-woofers that use a large and/or long throw driver in a very small box. This is one thing motional feedback was invented to solve, and in a large excursion/output small box some type of motional feedback system is the only way to eliminate this distortion which is introduced by the asymmetric air spring.
Air approximates a "linear" spring only if the volume displacement doesn't exceed a certain percentage of the box volume. I forget what the commonly accepted figure is but its something on the order of 10-20%.
Yet again rather misleading. "any arbitrary response" ? Really ? So you could get a box Qts lower than the drivers free air Qts ? No, I didn't think so.
There is nothing magical about using fibrous damping - it's just one source of damping within the box at bass frequencies, and not the dominant one. Because of the way fibres affect the adiabatic process of air compression and rareification, it's possible to make the box "look" slightly larger than it really is - up to about 15%, but there is no sonic benefit to this, it's just a different way of achieving the same response that requires a slightly smaller box. (Improvements in midrange standing wave absorption can be dramatic though)
A bit misleading, I think. Yes if the suspension system is rather poor (as it may have been in 60's drivers) the air load will tend to keep the cone movement better centred and axial, but to say there is no force gradient on the cone is simply not true.
If you have a driver in a very large box the force gradient on the cone at bass frequencies is largely determined by the stiffness of the surround and the mass of the cone, with very little effect from the air.
In some drivers the spider is made the dominant stiffness with a very compliant surround - this is often done in high sensitivity full range drivers with thin cones precisely for the reason that having a stiff surround will put much more bending strain on the cone, for no good reason.
By having most of the stiffness at the spider, and using the driver in a large box (as is usually required in a high efficiency driver) such a driver can produce reasonable bass despite having a rather thin and delicate cone - because there is very little mechanical stress on the cone. The voice coil is pushing and pulling against the spider and the cone and surround are essentially just "along for the ride".
Now try to put that same driver in a very small closed box and equalize the bass. In all likely-hood the cone will tear or deform near the edges due to the intense back pressure across the entire cone - load that wasn't there before.
Yes this extra load is uniform across the entire cone but its a lot of extra load all the same, and requires a much stiffer cone to avoid bending or tearing apart.
For proof of how much strain loading puts on a driver look at woofers designed to be used in bass horns - they require very stiff cones and strongly constructed surrounds, and even then they can still suffer from crumpling or tearing at the edges due to the intense air load provided by the horn - when the same driver will survive perfectly happily at full excursion in a bass reflex design, where the cone load is far less.
Look also at sub-woofer drivers - the strain on the cone of a large high excursion driver in a small closed box is exceptionally high, and far higher than if the same driver was used in a larger box.
Air loading in moderation can improve the linearity of a poor driver, but the better the linearity of the driver to begin with the less of a benefit it is, and if you go too far in shrinking the box size you just increase the strain on the driver and eventually reach a limit where distortion starts to increase again.
True acoustic suspension designs (Vas/Vb >3) made a lot more sense with poor linearity 60s drivers than it does with the drivers of today.
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You are right on a sinus wave, but music is not of that kind. Did-you had measured transient on several Loudspeakers ? Or waterfall ? I doubt you had written there is not fast or slow loudspeakers. Fast have a steeper slope and a faster damping. And, as you said, less phase delay witch is very important.
I apologize, English is not my language, it is hard for me to explain what i want to say with elegance. "Fast" is may-be a bad translation of a French expression that everybody understand. Means a loudspeaker with light cone, strong suspension and big magnet. Hight efficiency, hight motional level when it works as a microphone, by the way. So, hight acceleration and hight damping factor.
I agree fast is not in the meaning of max speed for a car, but fast acceleration.
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I'm not surprised many monitoring speakers have the port located on the back - for reasons of midrange quality, nothing to do with bass performance. Any port will have a tube resonance at a high frequency somewhere in the midrange dependent on its length - if that port is on the front, you'll hear this resonance.
Put it on the back and it's almost completely eliminated by the baffle step effect of the cabinet.
Putting a speaker hard up against a wall is going to affect the response below the baffle step frequency regardless of whether the port is on the front or the back. Any speaker equalized to sound right in free space will sound wrong up against a wall.
Are you sure it wasn't room modes ? Even a speaker with perfectly flat anechoic bass response is going to have one or more large peaks in the bass response when shoved up against a wall.I hate to admit it , but my old Alesis monitor ones where within a few inches of the wall for quite some time , and I spent allot of wasted time putting in notches in my bass eq settings to try and kill the one note bass !!!!
Trying to get a smooth bass response with a couple of small speakers up against a wall sitting on a desk with the listener sitting at the desk is extremely difficult due to unfavourable standing wave patterns and room modes, as well as the close proximity of the wall and desk. I'm not surprised you had problems with one note bass.
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