Hi again Tyimo,
If I model a TL in MathCad, I don't really worry about every peak or null. If the swings in the SPL are on the order of +/- 3 dB then I don't get too concerned. I think that the MathCad worksheets tend to over predict the ripples due to some of the basic assumptions made when I wrote them, for example assuming the driver and the open end are coincident.
I have encluded a post I made on a different forum explaining my understanding of peaks in a driver's impedance curve. I post this every once and a while when this question comes up. This is a combination of two posts, I try and improve the explanation each time I toss it out onto the Internet.
"If we restrict the discussion to frequencies between say 1 and 200 Hz, then the impedance can be thought of as a direct reflection of the driver's velocity. The higher the impedance the faster the driver is moving. Let us say that I am trying to measure the impedance of the driver in a bass reflex enclosure, I have decided to probe the device under test with a constant current source. I am not assuming a constant voltage source like an amp. It should not matter because the impedance is a ratio of voltage over current so both test methods will yield the same result. If I start by using a simplified set of equations (ignoring phasor notation) :
f = BL x i (f is mechanical force, i is input current)
e = BL x u (e is back emf, u is velocity)
Z = Re + e / i = Re + Bl x Bl x u / f (Bl is the magnetic flux and the length if wire in the gap product, Re is the DC resistance)
When you look at the impedance plot for an unstuffed TL, there are the two resonant frequencies of the driver and enclosure combination as seen by the two peaks. If we assume that the amp is applying constant current, then f will also be constant. Therefore, e is a function of u which is determined by the mechanical portion of the driver and enclosure as the speaker is being acted on by a constant applied force f. Hopefully that is not too confusing.
If you buy into my simplification of the electrical portion of the speaker, then the solution of the mechanical part for the driver velocity is required to determine the shape of the impedance curve for a speaker. The following is a description of a driver in different enclosures and the corresponding shape of the impedance curve.
If you see a peak in th driver's impedance magnitude curve, and an accompanying rapid phase fluctuation, then this is a sure sign of a resonance of some form. The way I see it is as follows.
1. Driver in free space or in an infinite baffle - a resonance condition will occur at or very near fs of the driver. There will be a single tall impedance peak along with a phase swing that approaches 180 degrees.
fs = (1/(2 x pi)) x (k_ms/m_ms)^1/2
k_ms = driver suspension stiffness (newton/m)
k_ms = 1/c_ms
c_ms = driver suspension compliance (m/newton)
m_ms = driver mechanical moving mass (kg)
2. Driver in a closed box - by adding a closed box to the back of the driver you are adding a second spring in parallel with the driver's suspension and raising the fs to a new frequency fc. This is predictable from the equation for the natural frequency of a spring and mass
fc = (1/(2 x pi)) x ((k_ms + k_mb)/m_ms)^1/2
k_mb = stiffness of the air in the box
k_mb = 1/ c_mb
3. Driver in a resonant enclosure - by adding a resonant enclosure, either a ported box or a TL tuned to fb, new resonant frequencies are generated. For a ported box the resonant frequenct is determined by
fb = (1/(2 x pi)) x (k_mb/m_mb)^1/2
k_mb = stiffness of the air in the box (newton/m)
k_mb = 1/c_mb
c_mb = compliance of the air in the box (m/newton)
m_mb = moving mass of the air in the port (kg)
For a straight classic TL the fundamental resonance is a function of the length
fb = 1/4 c/L
c = speed of sound (m/sec)
L = length of the line (m)
with harmonics at
fb = n/4 c/L
n = 3,5,7,9, ....
The interesting phenominon occurs when you combine two resonant systems, the driver and the enclosure, having approximately equal fundamental frequencies fs ~ fb. It does not matter if it is a ported box (bass reflex) ot some form of quarter wave enclosure, the behavior of the resulting resonances is the same. When two systems, with approximately equal fundamental resonances are combined, the resulting system will have two new resonances that bracket the original resonances as shown below.
f_low < fs~fb < f_high
The new resonances at f_low and f_high are the two impedance peaks you see for a bass reflex enclosure and an unstuffed TL.
The lower resonance, f_low, is the driver moving into the enclosure pushing air out of the open end or port and this produces the 24 dB/octave roll-off of a bass reflex or TL design. The mode shape (vibration theory term - the motion of vibrating systems can be completely described by their natural frequencies and mode shapes) has the driver mass moving into the enclosure and the open end air mass moving out of the enclosure.
The higher resonance, f_high, is the driver and the air at the enclosure opening moving out of phase combining to produce SPL. As you move up in frequency the driver's output dominates and you get the SPL curve of the driver. The mode shape has the driver mass moving out of the enclosure and the open end air mass moving out of the enclosure.
The common misconception is what happens at fs~fb which is the minimum between the two impedance peaks. This is not a resonance condition in the combined driver/enclosure system. This is the point between the two resonances where the mode shapes combine and result in the driver mass almost stopping (mode shapes cancelling the driver motion) while the motion of the open end air mass combines (mode shapes reinforcing the motion) to be a maximum. When the driver almost stops moving the only significant impedance is the resistance of the voice coil which is the minimum between the two resonant peaks.
Adding stuffing to the bass reflex or TL enclosure will tend to damp out the first resonant peak. Many people claim a TL has only one resonance peak which is incorrect. As you add more and more stuffing you tend to attenuate the lower impedance peak, at f_low, resulting in a single humped impedance curve. To determine the number of resonaces and mode shapes analyze the system without damping present, for a TL this means empty.
4. Driver in a horn - if the horn is sized correctly it acts as a pure resistance above the lower cut-off frequenct fc. So combining a horn with a driver, when fs ~ fc, you just add an acoustic resistance to the driver. The resulting impedance curve will have a peak at the driver fs but it will be lower magnitude and broader. I have included some interesting response curves for horn speaker designs in the recent additon of horn theory on my website."
I hope that helps answer your question, point out what is confusing and I will try again.
All I can say is that Jon Risch and I disagree on most aspects of TL design. I really don't want to get into specifics and can only suggest that you read both approaches and use the one that you feel makes the most sense.
I have not done any simulations for the driver you are interested in using. I get several requests a week for help designing TL's and just cannot accomidate doing the design simulations anymore. I am having trouble finding the time to answer e-mail questions, respond on forums, and continue to work on my own theory and project ideas. Couple those demands with a full time job, a wife, and three little kids and I am stretched very thin. Sorry I cannot help more with your design. Try the worksheets, it is not that difficult and there are many very knowledgable and talented people on this forum who can assist.
Hope that helps,
If I model a TL in MathCad, I don't really worry about every peak or null. If the swings in the SPL are on the order of +/- 3 dB then I don't get too concerned. I think that the MathCad worksheets tend to over predict the ripples due to some of the basic assumptions made when I wrote them, for example assuming the driver and the open end are coincident.
I have encluded a post I made on a different forum explaining my understanding of peaks in a driver's impedance curve. I post this every once and a while when this question comes up. This is a combination of two posts, I try and improve the explanation each time I toss it out onto the Internet.
"If we restrict the discussion to frequencies between say 1 and 200 Hz, then the impedance can be thought of as a direct reflection of the driver's velocity. The higher the impedance the faster the driver is moving. Let us say that I am trying to measure the impedance of the driver in a bass reflex enclosure, I have decided to probe the device under test with a constant current source. I am not assuming a constant voltage source like an amp. It should not matter because the impedance is a ratio of voltage over current so both test methods will yield the same result. If I start by using a simplified set of equations (ignoring phasor notation) :
f = BL x i (f is mechanical force, i is input current)
e = BL x u (e is back emf, u is velocity)
Z = Re + e / i = Re + Bl x Bl x u / f (Bl is the magnetic flux and the length if wire in the gap product, Re is the DC resistance)
When you look at the impedance plot for an unstuffed TL, there are the two resonant frequencies of the driver and enclosure combination as seen by the two peaks. If we assume that the amp is applying constant current, then f will also be constant. Therefore, e is a function of u which is determined by the mechanical portion of the driver and enclosure as the speaker is being acted on by a constant applied force f. Hopefully that is not too confusing.
If you buy into my simplification of the electrical portion of the speaker, then the solution of the mechanical part for the driver velocity is required to determine the shape of the impedance curve for a speaker. The following is a description of a driver in different enclosures and the corresponding shape of the impedance curve.
If you see a peak in th driver's impedance magnitude curve, and an accompanying rapid phase fluctuation, then this is a sure sign of a resonance of some form. The way I see it is as follows.
1. Driver in free space or in an infinite baffle - a resonance condition will occur at or very near fs of the driver. There will be a single tall impedance peak along with a phase swing that approaches 180 degrees.
fs = (1/(2 x pi)) x (k_ms/m_ms)^1/2
k_ms = driver suspension stiffness (newton/m)
k_ms = 1/c_ms
c_ms = driver suspension compliance (m/newton)
m_ms = driver mechanical moving mass (kg)
2. Driver in a closed box - by adding a closed box to the back of the driver you are adding a second spring in parallel with the driver's suspension and raising the fs to a new frequency fc. This is predictable from the equation for the natural frequency of a spring and mass
fc = (1/(2 x pi)) x ((k_ms + k_mb)/m_ms)^1/2
k_mb = stiffness of the air in the box
k_mb = 1/ c_mb
3. Driver in a resonant enclosure - by adding a resonant enclosure, either a ported box or a TL tuned to fb, new resonant frequencies are generated. For a ported box the resonant frequenct is determined by
fb = (1/(2 x pi)) x (k_mb/m_mb)^1/2
k_mb = stiffness of the air in the box (newton/m)
k_mb = 1/c_mb
c_mb = compliance of the air in the box (m/newton)
m_mb = moving mass of the air in the port (kg)
For a straight classic TL the fundamental resonance is a function of the length
fb = 1/4 c/L
c = speed of sound (m/sec)
L = length of the line (m)
with harmonics at
fb = n/4 c/L
n = 3,5,7,9, ....
The interesting phenominon occurs when you combine two resonant systems, the driver and the enclosure, having approximately equal fundamental frequencies fs ~ fb. It does not matter if it is a ported box (bass reflex) ot some form of quarter wave enclosure, the behavior of the resulting resonances is the same. When two systems, with approximately equal fundamental resonances are combined, the resulting system will have two new resonances that bracket the original resonances as shown below.
f_low < fs~fb < f_high
The new resonances at f_low and f_high are the two impedance peaks you see for a bass reflex enclosure and an unstuffed TL.
The lower resonance, f_low, is the driver moving into the enclosure pushing air out of the open end or port and this produces the 24 dB/octave roll-off of a bass reflex or TL design. The mode shape (vibration theory term - the motion of vibrating systems can be completely described by their natural frequencies and mode shapes) has the driver mass moving into the enclosure and the open end air mass moving out of the enclosure.
The higher resonance, f_high, is the driver and the air at the enclosure opening moving out of phase combining to produce SPL. As you move up in frequency the driver's output dominates and you get the SPL curve of the driver. The mode shape has the driver mass moving out of the enclosure and the open end air mass moving out of the enclosure.
The common misconception is what happens at fs~fb which is the minimum between the two impedance peaks. This is not a resonance condition in the combined driver/enclosure system. This is the point between the two resonances where the mode shapes combine and result in the driver mass almost stopping (mode shapes cancelling the driver motion) while the motion of the open end air mass combines (mode shapes reinforcing the motion) to be a maximum. When the driver almost stops moving the only significant impedance is the resistance of the voice coil which is the minimum between the two resonant peaks.
Adding stuffing to the bass reflex or TL enclosure will tend to damp out the first resonant peak. Many people claim a TL has only one resonance peak which is incorrect. As you add more and more stuffing you tend to attenuate the lower impedance peak, at f_low, resulting in a single humped impedance curve. To determine the number of resonaces and mode shapes analyze the system without damping present, for a TL this means empty.
4. Driver in a horn - if the horn is sized correctly it acts as a pure resistance above the lower cut-off frequenct fc. So combining a horn with a driver, when fs ~ fc, you just add an acoustic resistance to the driver. The resulting impedance curve will have a peak at the driver fs but it will be lower magnitude and broader. I have included some interesting response curves for horn speaker designs in the recent additon of horn theory on my website."
I hope that helps answer your question, point out what is confusing and I will try again.
All I can say is that Jon Risch and I disagree on most aspects of TL design. I really don't want to get into specifics and can only suggest that you read both approaches and use the one that you feel makes the most sense.
I have not done any simulations for the driver you are interested in using. I get several requests a week for help designing TL's and just cannot accomidate doing the design simulations anymore. I am having trouble finding the time to answer e-mail questions, respond on forums, and continue to work on my own theory and project ideas. Couple those demands with a full time job, a wife, and three little kids and I am stretched very thin. Sorry I cannot help more with your design. Try the worksheets, it is not that difficult and there are many very knowledgable and talented people on this forum who can assist.
Hope that helps,
Not really, you just have to use a driver that either has the right specs or can be tweaked to yield the desired ~IB response.MJK said:
For LF, either mass loading a driver or using a low Fs one with significant series R is usually what's required, then using multiple drivers to get the desired output. For direct or output transformer coupled amps this is a desirable design approach, but for an SS amp's vanishingly low output impedance it's a waste of $$/effort IMO beyond using enough damping to get a decent impulse response, i.e. 0.5-0.7 Qp depending on room loading.
GM
Hi Gm,
You are correct of course. I guess I was looking for the TL to add bass to compensate for the drivers rolling off low end response. If the infinite baffle response of the driver is the goal, then adding more stuffing will accomplish this while also flattening the impedance curve. If you are looking for the TL output to provide most of the bass then I think my summary is correct.
You are correct of course. I guess I was looking for the TL to add bass to compensate for the drivers rolling off low end response. If the infinite baffle response of the driver is the goal, then adding more stuffing will accomplish this while also flattening the impedance curve. If you are looking for the TL output to provide most of the bass then I think my summary is correct.
Hi Martin!
Thank you very much indeed! Since a long time I was looking for these informations what you gave me so quick! I think it could help many people to understand the TL behaviour!
To suggesting something to my driver I have meant only that what kind of design would be good for my driver Qts: Tapered Classic, TQWT, or straight etc...
One more question: I saw a lot of TQWT desing what is build with some kind of port on the bottom. So, in this way are they MLTQWTs and just the designers doesn't know it, and call them TQWTs?
Thanks!
Tyimo
Thank you very much indeed! Since a long time I was looking for these informations what you gave me so quick! I think it could help many people to understand the TL behaviour!
To suggesting something to my driver I have meant only that what kind of design would be good for my driver Qts: Tapered Classic, TQWT, or straight etc...
One more question: I saw a lot of TQWT desing what is build with some kind of port on the bottom. So, in this way are they MLTQWTs and just the designers doesn't know it, and call them TQWTs?
Thanks!
Tyimo
Hi GM!
Thanks for your kind help! I would like to use your help, but I wouldn't like to start a new thread because there are so many threads what are just keeping the free place in the forum and after a short time get abandoned.
So my driver data:
Sd=132cm2
Fs=59Hz
Qts=0.36
Vas=20.63L
Rc=6.2Ohm
BL=6.83 Vs/m
Mme=8.61gr
Qe=0.43
Qm=2.51
92dB
I would like to reach ca. 40 Hz on F3 without room gain, if it is possible....
But if you realy mean that it would be good for a new thread I will do!
Thanks
Tyimo
Thanks for your kind help! I would like to use your help, but I wouldn't like to start a new thread because there are so many threads what are just keeping the free place in the forum and after a short time get abandoned.
So my driver data:
Sd=132cm2
Fs=59Hz
Qts=0.36
Vas=20.63L
Rc=6.2Ohm
BL=6.83 Vs/m
Mme=8.61gr
Qe=0.43
Qm=2.51
92dB
I would like to reach ca. 40 Hz on F3 without room gain, if it is possible....
But if you realy mean that it would be good for a new thread I will do!
Thanks
Tyimo
Hi Tyimo,
Looking quickly at your driver's T/S parameters, I think it would work well in any one of the types of TL's that you list. My current favorite is the ML TL style of enclosure. Easy to design and build, very flexible to tweak, and seems to produce deep and tight bass. The easy to design and build are my two favorite characteristics for an enclosure.
The label ML TQWT is something I made up to describe my Fostex FE-164 design. It was my attempt to describe the physics behind how the enclosure worked. Adding a constriction to the end of a TL or TQWT does the same thing, it mass loads the enclosure. When does a TQWT become a ML TQWT is not so important, understanding the physics and using them to your advantage is the important part. So those other designs, with a restricted opening, are behaving in the same way as my ML TQWT design.
I try not to place too much importance on labels, different people have different naming conventions and arguing over them is a waste of time. Who cares? I sure don't. But I know that there are people that will argue endlessly over some very rigid personal definition of a particular style of enclosure. Not me. One good example of this is the difference between a BR and a ML TL, the exact point at which a BR (constant pressure in the box) becomes a ML TL (standing quarter wave pressure in the box) is not clear. Every once and a while somebody really takes issue with my calling the design a ML TL versus calling it a BR enclosure. The line between the two styles of enclosure can become very hazy. Bottom line, the speaker does not know it just makes music.
Looking quickly at your driver's T/S parameters, I think it would work well in any one of the types of TL's that you list. My current favorite is the ML TL style of enclosure. Easy to design and build, very flexible to tweak, and seems to produce deep and tight bass. The easy to design and build are my two favorite characteristics for an enclosure.
The label ML TQWT is something I made up to describe my Fostex FE-164 design. It was my attempt to describe the physics behind how the enclosure worked. Adding a constriction to the end of a TL or TQWT does the same thing, it mass loads the enclosure. When does a TQWT become a ML TQWT is not so important, understanding the physics and using them to your advantage is the important part. So those other designs, with a restricted opening, are behaving in the same way as my ML TQWT design.
I try not to place too much importance on labels, different people have different naming conventions and arguing over them is a waste of time. Who cares? I sure don't. But I know that there are people that will argue endlessly over some very rigid personal definition of a particular style of enclosure. Not me. One good example of this is the difference between a BR and a ML TL, the exact point at which a BR (constant pressure in the box) becomes a ML TL (standing quarter wave pressure in the box) is not clear. Every once and a while somebody really takes issue with my calling the design a ML TL versus calling it a BR enclosure. The line between the two styles of enclosure can become very hazy. Bottom line, the speaker does not know it just makes music.
Greets!Tyimo said:
I would not have asked you to if I did not mean it. One, you are hijacking another's thread, but more importantly, if you put the make/model of your driver in the subject of your own thread maybe there are others who will help or share their designs if they have used these drivers. Second, I currently have 680 sims so I am not doing anything until I know what your drivers are since I may already have done the work.
GM
Hi Martin!
Thanks!!
YesterdayI did some simulations with the Offset Driver worksheets and the reasult is much better than with the Open End version!!!
Could it be realy so much differences between the 2 worksheets?? Anyway, it is looks so, that I will try the tappered version (Classic TML) with ca. 4Sd to 1 Sd and around 150-170 cm line lenght.
How could I calculate a design with 2 paraleldriver? ( like the Thor with MTM arrangement)
Hi GM!
Thanks too!
O.K. I will make a new thread, but before, I will make same more simulations.
Tyimo
Thanks!!
YesterdayI did some simulations with the Offset Driver worksheets and the reasult is much better than with the Open End version!!!
Could it be realy so much differences between the 2 worksheets?? Anyway, it is looks so, that I will try the tappered version (Classic TML) with ca. 4Sd to 1 Sd and around 150-170 cm line lenght.
How could I calculate a design with 2 paraleldriver? ( like the Thor with MTM arrangement)
Hi GM!
Thanks too!
O.K. I will make a new thread, but before, I will make same more simulations.
Tyimo
Hi Tyimo,
Yes, the difference can be very big. Offsetting a driver in a TL is an important variable that can really improve the overall response. It is most effective at reducing the ripples. If you look at the attachments in my alignment table article there are several case studies that attempt to give the reader a feel for what can be adjusted and what effect the adjustment has on the SPL response.
Under my General Speaker Related Articles link is a short write-up showing how this is done with the MathCad models. There is a sample problem that shows the inputs and the results to expect.
Hope that helps,
Yes, the difference can be very big. Offsetting a driver in a TL is an important variable that can really improve the overall response. It is most effective at reducing the ripples. If you look at the attachments in my alignment table article there are several case studies that attempt to give the reader a feel for what can be adjusted and what effect the adjustment has on the SPL response.
Under my General Speaker Related Articles link is a short write-up showing how this is done with the MathCad models. There is a sample problem that shows the inputs and the results to expect.
Hope that helps,
Hi Martin!
Thanks!!
Is it possible to use your MLTQWT worksheet and methode for a classical TML arangement? I mean a taperd line with the driver on the closed and wide end, the port would be on the smal end ? (The worksheet is asking only for smal end and large end parameters) I ask this because I got pretty good reasult with the offset driver simulation on tapered design and I would like to build the box so that later I could put a port on it and try how sounds the "ML-TML" too!
Thanks!
Tyimo
Thanks!!
Is it possible to use your MLTQWT worksheet and methode for a classical TML arangement? I mean a taperd line with the driver on the closed and wide end, the port would be on the smal end ? (The worksheet is asking only for smal end and large end parameters) I ask this because I got pretty good reasult with the offset driver simulation on tapered design and I would like to build the box so that later I could put a port on it and try how sounds the "ML-TML" too!
Thanks!
Tyimo
Hi Tyimo,
Yes, you can change he large and small ends in the ML TQWT worksheet.
You might want to try the Ported Box worksheet as this one allows you to specify the port position. A little more accurate then the ML TQWT. If you use the Ported Box worksheet, remember to review the stuffing locations on the second page (the default is the entire enclosure) and zero out places where you don't want the stuffing.
Yes, you can change he large and small ends in the ML TQWT worksheet.
You might want to try the Ported Box worksheet as this one allows you to specify the port position. A little more accurate then the ML TQWT. If you use the Ported Box worksheet, remember to review the stuffing locations on the second page (the default is the entire enclosure) and zero out places where you don't want the stuffing.
MJK said:
whilst talking about the port position is there anything i should be looking for when placing it?
ie in terms of canceling harmonics etc
i've just been looking at bob brines site and his very informative tutorial on your sheets and TL design however he doesn't really mention the port position at all
oh, and thanks for the sheets, great work!
dave
Hi Dave (synergy),
To be honest, I have not made an in depth study of the port placement variable. I do know that in the last ML TL I designed I saved the port variable for last and was able to tweak a little bit better resposne by positioning it near the bottom of the enclosure. It seems to have an impact on the higher harmonics and can be used to tame some of the wiggles. My advice is to use the Ported Box worksheet and see how it goes, you can always push the port to the bottom and duplicate the ML TQWT worksheet.
I think Bob wrote most of his site before this worksheet was available.
Thanks for the positive feedback,
To be honest, I have not made an in depth study of the port placement variable. I do know that in the last ML TL I designed I saved the port variable for last and was able to tweak a little bit better resposne by positioning it near the bottom of the enclosure. It seems to have an impact on the higher harmonics and can be used to tame some of the wiggles. My advice is to use the Ported Box worksheet and see how it goes, you can always push the port to the bottom and duplicate the ML TQWT worksheet.
I think Bob wrote most of his site before this worksheet was available.
Thanks for the positive feedback,
TML result
Hi!
I have played a bit with the worksheats and I have now same design for starting. The qustion is : which would have better transient response? The classic tappered, the MLTQWT or the MLTL?
For Dave's port question: I read many times that the port on the bottom give very good bass and dynamic strenghtening. The Ariel builder believe this: "...noticed that an interesting boundary effect occurs close to the floor. Get the driver or vent within a critical distance of zero to three inches, and the whole room lights up, almost as though the bass waves were travelling freely along the entire room periphery ... move the driver or vent just a few inches further away from the floor, the effect disappears, and the speaker starts sounding like a minimonitor again....
I have build some basreflex box in this way too with very good result.
Greets:
Tyimo
Hi!
I have played a bit with the worksheats and I have now same design for starting. The qustion is : which would have better transient response? The classic tappered, the MLTQWT or the MLTL?
For Dave's port question: I read many times that the port on the bottom give very good bass and dynamic strenghtening. The Ariel builder believe this: "...noticed that an interesting boundary effect occurs close to the floor. Get the driver or vent within a critical distance of zero to three inches, and the whole room lights up, almost as though the bass waves were travelling freely along the entire room periphery ... move the driver or vent just a few inches further away from the floor, the effect disappears, and the speaker starts sounding like a minimonitor again....
I have build some basreflex box in this way too with very good result.
Greets:
Tyimo
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