Woofer Distortion 101
Recently there was a discussion in regard to speaker distortion. It was about what happens when you have two woofers in one box, but have them crossed-over at different points.
A poster wasn't able to articulate his point, and as a result of some confusing expressions, left people thinking he was 'trolling'.
In fact, he was right on the essential point, which I'm going to illustrate with some diagrams here:
Diagram 1: Two Separate Cabinets vs. One Cabinet
What actually happens when two woofers share the same cabinet?
Well, in the above diagram, there isn't much difference between the two situations. If this isn't clear, the following diagram should help out:
Diagram 2: Front and Back Radiation
Above we assume that both speakers are wired "in phase", which means that regardless of whether they are in series or in parallel, the coils are set so that a positive voltage across the marked terminals makes both speakers move outward, while a negative voltage makes the speakers move in. You can test this with a small battery.
The sound waves are pushed simultaneously in the same direction out the front, and they make a large parallel wavefront, twice as powerful as either speaker alone. In the back, a sound wave of opposite phase is pressed against every wall of the inside of the cabinet, with only a short delay from the speaker to the cabinet-wall.
By inspection, we may note that the sound pressure is applied equally to both sides of the separating wall in the first cabinet. When the speakers are moving out, they create suction inside, and when the speakers are sucking inward, they increase pressure in the cabinet. That is, while the action inside the cabinet is the opposite of that outside, the same thing happens in both compartments at the same time, either high pressure in both, or low pressure.
This means that the pressure is equal on both sides of the separating wall at all times, and according to classical mechanics, there is then no net-force on the separating panel, and no energy will be exchanged between the two partitions. From this it should make no difference if the wall is removed, as in the speaker cabinet on the right. The forces (both pressure and vacuum) are equal and opposite, and cancel inside the box.
Distortion from Excursion
Another thing to observe, is that the loudness of a sound of a given frequency will be based on how much air is pushed, or the area of the woofer surface. It follows that doubling the number of speakers will give us one of two options.
(1) If the two speakers are in parallel, and are presented with the same voltage (speakers are voltage-operated devices), we will have twice the air and twice the power used, which means a lot more loudness.
(2) Alternately, and this is much more interesting, we can instead put half the power into each speaker, and get the same volume, with each speaker doing half the work. So what? Well, it means that each speaker only needs to physically move half as much, for the pair to move the same amount of air.
Why is (2) significant? If speakers move too much, the result is distortion. Speakers have a limited excursion, that is, they can only move so much before they put up too much resistance, and start either clipping the signal, or they might get out of control. The excursion is a measure of headroom, or freedom to respond to signals. If the speaker is already playing a loud bass-note for instance, and is moving at its maximum swing, it can't respond when another musical note is added on top. There is no more headroom, and the new signal is either attenuated, or otherwise distorted. This can be referred to as Intermodulation (or I.M.) distortion, and happens at higher volumes as the speaker puts up more resistance and changes the wave-shape, which automatically adds harmonics or sidebands, and attenuates other frequencies.
Splitting the bass-load between two speakers will then cut back a large amount of distortion, and leave headroom for more musical input, with much less I.M. distortion. Of course this is very similar to simply using a larger speaker, which can then output more bass with less motion, and less distortion.
Thus many people might presume that this can only be a good thing, and that there is no trade-off in musical quality by doubling the speakers (other than cost!). We will see if that is always the case, below.
All this is just the necessary background to the discussion at hand, which originally took place on the DIYaudio forum.
Here is the link, although some of the discussion may have been subsequently deleted.
3.5 way speaker design, should I put the woofers together?
----------------------------------------------
In the original discussion, the speaker builder wanted to have two speakers, but with only one speaker carrying some midrange also. This is done simply by having a different crossover-point for each woofer. One speaker would just carry the bass, while the other would carry both bass and lower mids.
The argument was as to whether or not there was a musical drawback to the method of doubling the woofer, operating the two woofers across different bandwidths, while having them share the same box.
The poster felt that there would be distortion because of this, and he was essentially right.
Diagram 3: Midrange Bleed-Through
Lets assume the top speaker is the one that will carry the extra midrange musical content. In the separate compartments, the out-of-phase back-wave from the speaker would mostly be absorbed by the box, and only a small amount would feedback to the originating speaker, resisting and affecting the signal radiating out the front.
Now look at the diagram on the right:
Without the barrier, the back-wave from the top speaker will also reach the bottom speaker, and radiate out through the cone. From the point of view of the cone, it matters not whether the signal is imposed upon it from an electrical signal via the coil, or mechanically from the back via the air. The cone will move (vibrate), and the sound will radiate out. The cone is a relatively thin and pervious veil over the lower hole, and permits sound to escape the box.
The Flange-Effect
However, this new copy of the midrange signal is not identical to the original. It has now been time-delayed, much as the sound from a port is. Here however, the midrange sounds are very short wavelengths, and will alternately cancel and reinforce the original signal, depending upon wavelength. This acts as a fixed-frequency comb-filter, as well as a phase-shifting filter.
Compounding this comb-filter/delay flanging effect will be the secondary I.M. distortion added by the moving lower speaker, pumping bass. Almost the same amount of I.M. distortion being added to the midrange already via the top speaker will also be added to the inverted signal radiating out the bottom.
This does not negate the gains achieved by doubling the bass-woofer. But now the midrange being reproduced by the system is accumulating distortion of an entirely different kind than would have originally been the case for a single driver with no radiator or port. And it should be remembered that the same gains in lowering I.M. distortion could have been achieved with a single larger woofer.
Thus two questions need to be asked:
(1) Why would you choose to have two smaller woofers rather than one larger one? The answer ought to be, that some other benefit is achieved, namely a different frequency response, or cabinet resonance, or speaker specs, which would move the builder to take on the extra work of designing for two speakers.
(2) Secondly, can anything be done to minimize the drawbacks introduced by two speakers in one cabinet? Well, the answer is a definite yes! The simple act of dividing the cabinet into two compartments, and stuffing the top with absorbent materials, should eliminate the undesirable 'flanging effect' we can expect from single compartment.
Thus, that poster was right when he insisted that the design could be significantly improved by partitioning the cabinet.
Recently there was a discussion in regard to speaker distortion. It was about what happens when you have two woofers in one box, but have them crossed-over at different points.
A poster wasn't able to articulate his point, and as a result of some confusing expressions, left people thinking he was 'trolling'.
In fact, he was right on the essential point, which I'm going to illustrate with some diagrams here:
Diagram 1: Two Separate Cabinets vs. One Cabinet
What actually happens when two woofers share the same cabinet?
Well, in the above diagram, there isn't much difference between the two situations. If this isn't clear, the following diagram should help out:
Diagram 2: Front and Back Radiation
Above we assume that both speakers are wired "in phase", which means that regardless of whether they are in series or in parallel, the coils are set so that a positive voltage across the marked terminals makes both speakers move outward, while a negative voltage makes the speakers move in. You can test this with a small battery.
The sound waves are pushed simultaneously in the same direction out the front, and they make a large parallel wavefront, twice as powerful as either speaker alone. In the back, a sound wave of opposite phase is pressed against every wall of the inside of the cabinet, with only a short delay from the speaker to the cabinet-wall.
By inspection, we may note that the sound pressure is applied equally to both sides of the separating wall in the first cabinet. When the speakers are moving out, they create suction inside, and when the speakers are sucking inward, they increase pressure in the cabinet. That is, while the action inside the cabinet is the opposite of that outside, the same thing happens in both compartments at the same time, either high pressure in both, or low pressure.
This means that the pressure is equal on both sides of the separating wall at all times, and according to classical mechanics, there is then no net-force on the separating panel, and no energy will be exchanged between the two partitions. From this it should make no difference if the wall is removed, as in the speaker cabinet on the right. The forces (both pressure and vacuum) are equal and opposite, and cancel inside the box.
Distortion from Excursion
Another thing to observe, is that the loudness of a sound of a given frequency will be based on how much air is pushed, or the area of the woofer surface. It follows that doubling the number of speakers will give us one of two options.
(1) If the two speakers are in parallel, and are presented with the same voltage (speakers are voltage-operated devices), we will have twice the air and twice the power used, which means a lot more loudness.
(2) Alternately, and this is much more interesting, we can instead put half the power into each speaker, and get the same volume, with each speaker doing half the work. So what? Well, it means that each speaker only needs to physically move half as much, for the pair to move the same amount of air.
Why is (2) significant? If speakers move too much, the result is distortion. Speakers have a limited excursion, that is, they can only move so much before they put up too much resistance, and start either clipping the signal, or they might get out of control. The excursion is a measure of headroom, or freedom to respond to signals. If the speaker is already playing a loud bass-note for instance, and is moving at its maximum swing, it can't respond when another musical note is added on top. There is no more headroom, and the new signal is either attenuated, or otherwise distorted. This can be referred to as Intermodulation (or I.M.) distortion, and happens at higher volumes as the speaker puts up more resistance and changes the wave-shape, which automatically adds harmonics or sidebands, and attenuates other frequencies.
Splitting the bass-load between two speakers will then cut back a large amount of distortion, and leave headroom for more musical input, with much less I.M. distortion. Of course this is very similar to simply using a larger speaker, which can then output more bass with less motion, and less distortion.
Thus many people might presume that this can only be a good thing, and that there is no trade-off in musical quality by doubling the speakers (other than cost!). We will see if that is always the case, below.
All this is just the necessary background to the discussion at hand, which originally took place on the DIYaudio forum.
Here is the link, although some of the discussion may have been subsequently deleted.
3.5 way speaker design, should I put the woofers together?
----------------------------------------------
In the original discussion, the speaker builder wanted to have two speakers, but with only one speaker carrying some midrange also. This is done simply by having a different crossover-point for each woofer. One speaker would just carry the bass, while the other would carry both bass and lower mids.
The argument was as to whether or not there was a musical drawback to the method of doubling the woofer, operating the two woofers across different bandwidths, while having them share the same box.
The poster felt that there would be distortion because of this, and he was essentially right.
Diagram 3: Midrange Bleed-Through
Lets assume the top speaker is the one that will carry the extra midrange musical content. In the separate compartments, the out-of-phase back-wave from the speaker would mostly be absorbed by the box, and only a small amount would feedback to the originating speaker, resisting and affecting the signal radiating out the front.
Now look at the diagram on the right:
Without the barrier, the back-wave from the top speaker will also reach the bottom speaker, and radiate out through the cone. From the point of view of the cone, it matters not whether the signal is imposed upon it from an electrical signal via the coil, or mechanically from the back via the air. The cone will move (vibrate), and the sound will radiate out. The cone is a relatively thin and pervious veil over the lower hole, and permits sound to escape the box.
The Flange-Effect
However, this new copy of the midrange signal is not identical to the original. It has now been time-delayed, much as the sound from a port is. Here however, the midrange sounds are very short wavelengths, and will alternately cancel and reinforce the original signal, depending upon wavelength. This acts as a fixed-frequency comb-filter, as well as a phase-shifting filter.
Compounding this comb-filter/delay flanging effect will be the secondary I.M. distortion added by the moving lower speaker, pumping bass. Almost the same amount of I.M. distortion being added to the midrange already via the top speaker will also be added to the inverted signal radiating out the bottom.
This does not negate the gains achieved by doubling the bass-woofer. But now the midrange being reproduced by the system is accumulating distortion of an entirely different kind than would have originally been the case for a single driver with no radiator or port. And it should be remembered that the same gains in lowering I.M. distortion could have been achieved with a single larger woofer.
Thus two questions need to be asked:
(1) Why would you choose to have two smaller woofers rather than one larger one? The answer ought to be, that some other benefit is achieved, namely a different frequency response, or cabinet resonance, or speaker specs, which would move the builder to take on the extra work of designing for two speakers.
(2) Secondly, can anything be done to minimize the drawbacks introduced by two speakers in one cabinet? Well, the answer is a definite yes! The simple act of dividing the cabinet into two compartments, and stuffing the top with absorbent materials, should eliminate the undesirable 'flanging effect' we can expect from single compartment.
Thus, that poster was right when he insisted that the design could be significantly improved by partitioning the cabinet.
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re: 'significantly' - can you put numbers on this?, the theory is right but in practical terms I have my doubts (but happy to be proven wrong...), if you were really troubled by this effect you'd go for a 3 way instead of 2.5 way, or 4 way instead of a 3.5 way...???
There can be no 'numbers' with a general overview, or abstract analysis. Scientific analysis begins by simplifying and isolating effects and parameters of interest.
However, while the 'numbers' in a specific case might be 'small', the effect may still be audible, and be judged to have a negative impact on the listening experience.
In this case, many factors could come into play to coincidentally improve the situation, in a real-life example:
(1) The speakers could be low-wattage, and the sound-pressure levels might minimal enough to relieve concern.
(2) A large amount of midrange energy could be absorbed inside the cabinet. Well-lined and/or stuffed cabinets will exhibit a much reduced effect.
(3) Rear-porting, rear-mounted passive radiators, or open-back situations might alleviate SPL levels exerted on the back of the lower speaker.
(4) Musical program content might be such that the most noticeable flanging would not appear in the most sensitive frequency ranges.
(5) The user may prefer music which is actually enhanced or made more interesting and novel by small spacial effects like flanging. For instance, thin recordings, or 'rock n roll' material might be quite tolerable or even improved, just as a mild reverb effect might please the listener's taste.
(6) It is well known that sound systems are signatured by their most gross flaws, and noticeable improvements should address the most prominent problems first, before attempting to identify more subtle ones.
Nonetheless flaws like this can and should be avoided in the design stage.
Such factors however, don't negate the basic analysis, and the effect will remain of interest to speaker enclosure designers who are striving for perfection in their builds.
A speaker manufacturer might avoid addressing the problem because of cost factors: It may be cheaper to add damping material for instance than to add a partition. But DIYers will not necessarily be so constricted in their design choices.
(The analyst with the best diagrams wins 🙂
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1 woofer playing bass 1 woofer playing mid = not fine, as the bass will move the mid cone
1 woofer playing bass 1 woofer playing bass and mid = fine to a point, but at intivals when there is no LF sent to the speakers, the backwave from the mid may vibrate the lower cone....
1 woofer playing bass 1 woofer playing bass and mid = fine to a point, but at intivals when there is no LF sent to the speakers, the backwave from the mid may vibrate the lower cone....
Here is another diagram to illustrate the additional distortion from the lower speaker which compounds the secondary midrange signal radiating from the lower speaker:
Diagram 4: Intermodulation Distortion
Diagram 4: Intermodulation Distortionre:;'the effect may still be audible' - not in any of the 2.5 ways I've built, MY hypothesis that it's because of your point 2, + I don't play at stadium levels.
re:'The analyst with the best diagrams wins' - no, in science the analyst with the best numbers wins....
(P.S., I think the IMD caused by the lower woofer on the upper will far outweigh any 'flanging' effect)
re:'The analyst with the best diagrams wins' - no, in science the analyst with the best numbers wins....
(P.S., I think the IMD caused by the lower woofer on the upper will far outweigh any 'flanging' effect)
Hi,
Analysis along the lines there is a problem and I'm going
to show it doesn't mean anything, its pure conjecture.
In reality there is no real problem, theory or measured.
rgds, sreten.
Analysis along the lines there is a problem and I'm going
to show it doesn't mean anything, its pure conjecture.
In reality there is no real problem, theory or measured.
rgds, sreten.
As far as things go I see it like this.
The box plays an important role with regards to modifying and controlling the drivers operation, but this is only relevant to the frequencies where the box + driver and its alignment are 'tuned'.
This is akin to a passive radiator acting as a port in a vented enclosure, or rather simply akin to the port which only responds to a narrow band of frequencies. Outside of that range though neither respond in any meaningful way.
Now naturally large pressure variations inside the cabinet will cause air to 'chuff' out of any tiny holes and would cause a smaller drivers membrane to be modulated by the variations, but that's not what we're talking about.
As was said above, woofer + midrange in the same cabinet is a problem because the woofer will modulate the midrange driver which is bad, but the midrange driver wont 'do' anything to the woofer.
The only thing that happens is that the sound radiated by the rear of the midrange would have an easier path through which to escape. Now you may say 'yes! that's the point', but is it? This isn't any different to a single midrange driver in a sealed cabinet all by itself. The sound radiated by the rear of the driver will bounce around inside the cabinet and then try and escape back through the cone, this happens in every box loudspeaker on the planet, some people go to great lengths to minimise the effect.
This shouldn't be any cause for concern in a 2.5 way either, but you might want to use some clever cabinet work just to make absolutely sure. Heavy stuffing + an angled internal brace should be more then enough mind you.
The box plays an important role with regards to modifying and controlling the drivers operation, but this is only relevant to the frequencies where the box + driver and its alignment are 'tuned'.
This is akin to a passive radiator acting as a port in a vented enclosure, or rather simply akin to the port which only responds to a narrow band of frequencies. Outside of that range though neither respond in any meaningful way.
Now naturally large pressure variations inside the cabinet will cause air to 'chuff' out of any tiny holes and would cause a smaller drivers membrane to be modulated by the variations, but that's not what we're talking about.
As was said above, woofer + midrange in the same cabinet is a problem because the woofer will modulate the midrange driver which is bad, but the midrange driver wont 'do' anything to the woofer.
The only thing that happens is that the sound radiated by the rear of the midrange would have an easier path through which to escape. Now you may say 'yes! that's the point', but is it? This isn't any different to a single midrange driver in a sealed cabinet all by itself. The sound radiated by the rear of the driver will bounce around inside the cabinet and then try and escape back through the cone, this happens in every box loudspeaker on the planet, some people go to great lengths to minimise the effect.
This shouldn't be any cause for concern in a 2.5 way either, but you might want to use some clever cabinet work just to make absolutely sure. Heavy stuffing + an angled internal brace should be more then enough mind you.
Indeed; and when "theory" doesn't agree with numerous real-world examples, one has to suspect the theory.
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In speaker design there are always trade-offs.
Speakers are current devices, when driven by a voltage source the speaker's impedance acts as the R that does voltage to current conversion
In each case there is a fixed amount of mid-range getting into the cabinet. In the larger cabinet, it will travel further, have more time to pass thru damping material & thus be attenuated, so the problem of (total) time-smeared midrange leakage is reduced.
Further, one has to ask, with less than 1/2 the mid energy impinging on the back of the cone, is it easier to turn away that energy, ie is through the cone transmission linear.
The mid-bass cone is already being modulated by the amplifier, the 2nd driver should add no more since they are syncronised.
dave
Speakers are current devices, when driven by a voltage source the speaker's impedance acts as the R that does voltage to current conversion
In each case there is a fixed amount of mid-range getting into the cabinet. In the larger cabinet, it will travel further, have more time to pass thru damping material & thus be attenuated, so the problem of (total) time-smeared midrange leakage is reduced.
Further, one has to ask, with less than 1/2 the mid energy impinging on the back of the cone, is it easier to turn away that energy, ie is through the cone transmission linear.
The mid-bass cone is already being modulated by the amplifier, the 2nd driver should add no more since they are syncronised.
dave
Post #124. Jack does a nice job I think.
http://www.diyaudio.com/forums/mult...uld-i-put-woofers-together-4.html#post3067526
http://www.diyaudio.com/forums/mult...uld-i-put-woofers-together-4.html#post3067526
That was a good synopsis. The whole discussion seems to center on a solution in search of a problem.
Cal, so was yours: "those who say it has an effect can read into your explanation and say "see I told you there was an effect" and the other side can say but "It's not big enough to hear and that was the point"
Love it when we're all winners....
Love it when we're all winners....
I think most of your analysis (outside of the comments on IM) is on track but would make a small correction. It is true in the mirrored two woofer case that a barrier at the mid point has equal pressure impinging on both sides. In that regard it doesn't matter whether the barrier is there or not, assuming it is a hard barrier. As you have drawn the barriers, with damping, is a significantly different case. With no barrier each woofer gets a delayed rear wave from the opposite woofer. Depending on diaphragm mass a certain amount of sound will get through the opposite woofers. If you have an undamped barrier then the back radiation will go half the length of the cabinet, hit the hard surface and reflect, then finally exit to some degree through the woofer it started from. This is indistinguishable from the no barrier case.
Damping the barrier means that the delayed rear radiation is absorbed and this is a significantly different case.
Should we split the cabinet into two sealed cavities? There are some issues of DC offset that wil lead to misbehavior with some woofers that share an air space. This is a high level phenomenon where woofers, under the right conditions can jump in or jump out. If one wants to offset inwards then the other will happily offset outwards. The restoring force of the cabinet goes away under those conditions.
This doesn't mean that woofers shouldn't share a common cavity so much as they should be well designed to simply avoid this misbehavior.
David S.
Damping the barrier means that the delayed rear radiation is absorbed and this is a significantly different case.
Should we split the cabinet into two sealed cavities? There are some issues of DC offset that wil lead to misbehavior with some woofers that share an air space. This is a high level phenomenon where woofers, under the right conditions can jump in or jump out. If one wants to offset inwards then the other will happily offset outwards. The restoring force of the cabinet goes away under those conditions.
This doesn't mean that woofers shouldn't share a common cavity so much as they should be well designed to simply avoid this misbehavior.
David S.
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Well, my language was scientifically precise:
A speaker really is a voltage-operated device.
This is meant in the usual scientific way, the way we in electronics conventionally speak of various devices. For instance, we say a vacuum tube is a voltage-operated device, in distinction from a transistor, which is a current-operated device.
By this accepted terminology, we mean this: That each device is controlled either by a voltage signal or a current signal, in the process of amplifying it or converting it in terms of impedance or power.
That is, a vacuum tube is controlled by a small voltage signal, although it in turn controls both voltages at its terminals and currents through its elements.
In contrast, a transistor is controlled by a small current signal, although it in turn may control both voltages and currents at its other terminals.
The idea, is that one looks at either device as being controlled by an external signal in the form of a voltage or a current, but not both. The reason for this description is to distinguish the original signal from other artifacts.
In the case of the voltage-controlled tube, the simultaneous current at the same place in the controlling terminal may vary widely in form and size from the original signal, due to grid-current effects, miller capacitance, and Time-domain distortions. The original voltage remains the original signal.
In the case of the transistor, the simultaneous voltage at the input or controlling terminal may not be a true representation of the source signal, which ought to be a current-signal. There may be variations in the voltage component of the signal at the terminal which are not part of the original signal, but are artifacts as a result of current-controlled amplification.
My point here is that your reference to a speaker being a 'current device' is too vague and ambiguous to be meaningful in the context of current conventions in electronic circuit theory.
It is true that a speaker is a 'current device' in that it conducts current, and currents flow in it, and that power applied in the form of voltage and current together is converted by the device into sound-waves and acoustical power.
It is also 'true', or rather a truism, that because a speaker is a complex device involving both electric power and magnetic flux, that one can talk of a speaker as controlling currents, both in the electric wires, and in the magnetic components (i.e., flux variations), and even air currents.
Nonetheless, according to 'current' wisdom in the field of electronics, the conventional 'woofer' is classed as a voltage-operated device, not a current-operated device. The reason for this classification is as simple as it is perhaps elusive to some:
Although the signal is transferred to the speaker in a form involving both voltage and electric current, this controlling signal is primarily a voltage signal, in the sense we have been using all along here.
Namely, that portion of the signal applied to and controlling the speaker is found in its purest and primal form in the voltage component of the input signal, and not in the current component. Why? Simply because the current component is not a reliable reflection of the original input signal, since the impedance varies unequally in regard to the original signal, and so does the resultant current. For instance, the response and impedance of the speaker as a unit may vary by frequency, independently of the type of signal applied: as a result the current is attenuated, or the speaker motion is not proportional to the input signal.
In these cases, we would not describe the speaker as a 'motion device', because while it certainly moves, motion is not the key concept in explaining its ability to convert a signal from electrical energy to acoustic vibration.
Similarly, we do not normally describe a speaker as 'a current device', because this does not make plain its expected and intended means of operation, even though it certainly conducts currents (electric, magnetic, air). We also avoid such a term because it is too similar to established conventions of expression in electronics, as described above.
If Planet10 merely wanted to remind us that speakers carry current also, as part of the power-transfer process, then his informal expression would do this.
But if Planet10 wants to suggest that a conventional woofer is actually 'current-operated', then this idea is rather inappropriate for the reasons discussed above. As well as failing to give the essential means of operation of the device, and instead giving a misleading impression of its operation, this also would break convention as electronic engineers currently prefer to refer to speakers as 'voltage-controlled', not 'current-controlled' devices, in a very sensible attempt to give a sense of how the device operates by an appropriate choice of classification.
At the moment we are seeing little justification for referring to a speaker as a 'current device'.
I think we conceded from the start that 'results may vary', based on lining and stuffing methods of cabinets.
However, the idea that cabinet damping would automatically reduce the effects discussed to 'insignificant' is unrealistic.
Suppose for instance that the cabinet was actually heavily lined with an inch of wool-based felt.
[FONT=Times New Roman, serif]Or rather, just make a 1' x 1' x 3' square tube made of MDF, with open ends.[/FONT]
[FONT=Times New Roman, serif]Then line this tube on the inside with an inch of wool-based felt. [/FONT]
[FONT=Times New Roman, serif]Now hold it up to someone 3 feet away and shout at them. [/FONT]
[FONT=Times New Roman, serif]Of course they can hear you, quite clearly, and your voice will easily be transported 3 feet by the tube to the ear of the listener. [/FONT]
[FONT=Times New Roman, serif]Yes, the felt takes a lot of edge off of your yelling, but it still remains loud and clear as day (perhaps clearer!).[/FONT]
[FONT=Times New Roman, serif]This shows that the back-signal from the midrange will easily traverse the short distance it needs to, even in a heavily lined cabinet, to get to the other speaker and vibrate the cone from the back.[/FONT]
Of course it will be attenuated, and less than half the front radiation.
More than that however, no one can say without giving specific examples of designs and builds.
I don't doubt that a cabinet could be lined and stuffed to a point where the effect is indeed insignificant, but hardly by accident, and this would be an unlikely build for a typical manufacturer.
The real question is however,
why wouldn't a DIYer use the simplest solution, namely partition the cabinet?
Then one has no worries, only the cost of an extra panel and some lining.
Well, again the issue of how much sound is absorbed and re-radiated out the front as opposed to reflected back into the compartment is worthy of consideration, but one cannot help but suspect that the back of any cone will have its own frequency-sensitive reflective properties, and so will inevitably again distort the original signal in a kind of 'comb-filter' way, i.e., unevenly.
While the various distortions being introduced may be minimized, its hard to be very impressed with the multiplicity of new modes and channels by which the midrange will be distorted.
As with all designing, simpler is almost always better, and simply eliminating additional modes of distortion will usually be the best choice, along with other sensible and practical techniques.
This again I think betrays a lack of understanding of how the signals will be uncontrollably distorted.
While in pure Fourier theory one would expect phase for signals of all frequencies to stay aligned, this is never the case in real-life variable impedance devices.
One will rather expect that since one woofer will have only the 'bass'-range, while the other will have a discreet midrange superimposed, the reality will be far different from oversimplified 'F-theory'.
[FONT=Times New Roman, serif]We will find that tracking the actual 'wave-shapes' of the signals, [/FONT]
[FONT=Times New Roman, serif]the two speakers will be wildly different in a very random way, [/FONT]
[FONT=Times New Roman, serif]in terms of the instantaneous voltage in the coils and also the instantaneous sound-wave in the air. [/FONT]
[FONT=Times New Roman, serif]To be blunt, the two speakers will be doing wildly different dances, and theoretical cancellations of supposedly 'in phase' bass signals will be a practical absurdity. The one thing adding higher harmonic (or inharmonic) content does to any waveform is make its instantaneous value completely random. Both the electrical signals and the acoustic waves coming from each speaker will be so different and idiosyncratic that 'cancellation' will be at best very imprecise and at worst non-existant. [/FONT]
Remember also that one speaker will be being asked to move in a ridiculously complex fashion in comparison to the other, and every change of direction, with its accompanying inertial losses and resonances will put the speakers in such a random instantaneous relation that preservation of phase over the whole bass range must be wholly dismissed as a pipe-dream.
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That was one h*** of a long post , half of which was wasted on talking about current vs voltage and the rest was just more conjecture. Show us some measurements proving your point? I don't see anyone online showing what you propose to be true.
There is no voltage without current and no current without voltage so you might argue both sides of the fence. Of course the impedance curve translates one to the other via Ohms law.
Still, a loudspeaker is primarily a current device for simple reasons:
F = ma and F = BLi.
(B is magnetic gap flux, L is coil conductor length, and i is voice coil current)
Through out the midrange all loudspeakers are mass controlled so constant force gives constant acceleration which equates to flat radiated power (not axial pressure). Force is only constant for constant current.
So constant current gives constant force, for constant acceleration, for constant radiated power. This is true from above resonance up to frequencies where ka is about 1.
By convention we drive loudspeakers with low impedance sources and tend to design for flat axial response from a constant voltage, but a loudspeaker is a current drive device.
A long answer isn't always a clear answer.
David
Indeed. I'd go further. If you can't explain something simply, then you don't understand it.
For all that, I am in the separate enclosure camp with nazaroo! 😀
Visaton - Lautsprecher und Zubehör, Loudspeakers and Accessories
Troels isn't. Or at least he doesn't care. His Ellam 2.5 design uses one box.
ELLAM-25
Troels Gravesen said:
Now there is some REAL illumination why 2.5 has something going for it. It allows for simpler filters while doing the hideous and unnecessary (IMO) bafflestep correction. 😎
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