Page Title
What do the driver T/S parameters mean and will they help me choose the best driver.
The drivers Thiele/Small parameters were brought about in a bid to standardize and bring meaning to the behavior of a cone loudspeaker. Most were specified in the early sixties and seventies by A.N. Thiele and R. Small, these two also published landmark papers on vented box (ported) low frequency systems in 1961 (Thiele) and later Small in 1973, both papers were published in the JAES. While we always think about driver parameters and vented cabs as being T/S we should not forget the work that went on before these two, as vented box principles were first described and patented by Thuras in 1932, with most of the mathematical models up until Thiele coming from people like Locanthi, Van Leeuwen, de Boer, Beranek, Lyon, W.M. Leach and importantly Novak.
So what does it all mean. First I’ll give a brief description of what most of the useful T/S parameters mean and later how you can use them to evaluate drivers for selection. I will not be going into great mathematical or mechanical detail here as this is aimed at the novice.
fs Driver free air resonance.
The point at which all the moving parts of the driver sympathize or resonate. Resonance is a hard thing to explain simply, but a rule of thump is that you will find it hard to produce lower frequencies than the driver’s fs. So a driver with an fs of 60 Hz will not produce 35 Hz very well. A driver with an fs of 32 Hz will produce 35 Hz, if the box is tuned low enough. These two examples relate to closed, ported and bandpass cabinets, horns are less affected by fs as they use the driver as a piston.
Qts Driver total Q.
It had to happen at some point, we’ve hit the Q word. Q is basically a describing word, it is used to describe a quality or characteristic about an electrical or mechanical part of the driver. So Qts is the overall Q of the driver, both electrical and mechanical. Qts can be thought of as how strong the motor and magnet system are. A driver with a low Qts of around 0.20 would have a large magnet and be able to move the cone with a lot of force. This makes for a tight driver. A driver with a Qts of 0.45 would have a smaller magnet and less control over its cone. So low values of Qts give a tight and punchy sound but with little weight or low bass and high Qts values give a slow and heavy sound that will give you lots of low frequency output. Watch out for drivers with really high Qts values of 0.6 or above, these would require such a big box to work correctly that in normal size boxes you don’t get much low end. They are better of being used on the rear parcel shelf of your car, where they can enjoy a massive rear chamber.
Qms Driver mechanical Q
Qms is the mechanical Q of the speaker and only takes the speaker's mechanical properties into consideration. It is a measurement of the control coming from the speaker's mechanical suspension, which is made up of the surround and spider.
The total driver Q is Qts and is derived from the electrical Q (Qes) and the mechanical Q (Qms).
Qts is defined as 1/Qts = 1/Qes + 1/Qms
Qms is calculated
Fs sqrt(Rc)
Qms = -----------
f2 - f1
Drivers with a very high mechanical Q can sound more open, cleaner and have a better dynamic range. This is because they have less loss. The surround is more flexible, the spider is better constructed, they have better air flow and usually have higher sensitivity. So a high mechanical Q is a very good indicator of energy storage behaviour.
So Qts is just a product of Qms and Qes and an understanding of what they are is important when designing a loudspeaker system. Qts, Vas and fs are all that is needed to determine the box size, but when you get to a very advanced stage of designing, its parameters like Qes and Qms which become the foundation of what you do.
BL Driver motor strength.
The higher the value the stronger the motor. Given in tesla meters. Drivers with high BL values of around 30 or more have the ability to control their cones very accurately. These drivers will almost certainly have very large magnets and will weigh a lot. Note also that drivers with high BL values will normally have a low Qts value. Drivers with a low BL value of 20 or less will be less able to control their cones. These drivers will not feel as tight as those with higher BL’s. They will also normally have higher Qts values of over 0.28 and while at home in ported or bandpass cabinets I call these drivers mud motors because of there slow and heavy sound with a less than perfect transient response.
Vas Volume of air equal to the driver compliance.
This can be thought of as how stiff the movement of the cone is. The value is given in litres or cu inches. There are a lot of variables that determine the Vas, so you can’t really say that high values of Vas mean a certain thing or are better. A single or double suspension spider will affect Vas, so does the size of the cone. The temperature of the air and also the humidity will affect Vas and so it is one of the hardest parameters to evaluate.
Mmd Mass or weight of the speaker cone assembly.
This is how heavy the cone, coil and other moving parts are. An 18” driver with a Mmd of around 100 grams will have a light cone and will usually be more efficient than a driver with a heavy cone. A light cone can also move quicker. Light cones are usually found in higher Qts value drivers, but not always. This would appear to give them the advantage of having a quicker transient response as the cone is light, but the weak motors found in higher Qts drivers offsets any advantages of having a lighter cone. Drivers with Mmd’s of over 200 grams will have heavy stiff cones. They will usually be less efficient, have double spiders and have lower Qts values. Drivers with heavy cones should have a slower sound, but not if they also have a low Qts and high BL. The strength of the motor system is able to counteract the high cone weight and still give a fast transient response. Do not confuse Mmd with Mms. Mms is the total cone assembly mass including radiation mass. Some loudspeaker design programs will want you to enter the Mmd and will calculate the Mms for you, while others will want the Mms and will calculate the Mmd for you.
Sd Effective driver radiating area.
Given in sq cm or sq inches. Basically means how much area the cone has to move air with. Larger cones will have bigger Sd’s and smaller cones will have smaller Sd’s. An average Sd for an 18” cone would be 1150 sq cm and a 15” driver would have an average Sd of around 890 sq cm. But the depth of the cone also has to be taken into account. A deeper cone will give you a higher Sd for the same diameter. So that’s why you see different Sd’s for same size drivers. The ones with the higher Sd’s have deeper cones or have less surround material or both.
xmax The amount of voice coil overhang.
Boring description for the parameter some of us love the most. Usually given in mm it represents the distance over which the coil can travel in one direction and maintain a constant number of turns in the gap. So a driver with an xmax of 10 mm can move is cone twice as far as one with an xmax of only 5 mm. Do not confuse xmax with maximum excursion. Maximum excursion is how far the cone will travel before 1. the coil hits the back plate or 2. the cone moves so far that it is limited by its suspension. xmax is how far the cone can travel with the coil still in the magnetic gap. There’s no point in driving the coil outside of the gap as it will no longer be under control form the motor system. More xmax means the cone can move in and out further whilst still being under control. Maximum excursion is meaningless as no sane person drives their cones to full extension all the time, although I have seen it done with a recone kit needed soon after. Note that the xmax figure is for one direction only, so an xmax figure of 5 mm means the cone can travel 5 mm outwards and 5 mm inwards past it’s resting place whilst still being under the control of the motor system. If the xmax figure is not given but the voice coil length and gap height are given you can work out the xmax from these. Take the voice coil length and subtract the gap height then dived by 2. This is how most manufacturers determine the xmax, some will also add 15% to the figure to allow for 3% third-harmonic distortion. So if you do the calculation and the manufacturers xmax figure is higher than predicted they have added 15%.
What do the driver T/S parameters mean and will they help me choose the best driver.
The drivers Thiele/Small parameters were brought about in a bid to standardize and bring meaning to the behavior of a cone loudspeaker. Most were specified in the early sixties and seventies by A.N. Thiele and R. Small, these two also published landmark papers on vented box (ported) low frequency systems in 1961 (Thiele) and later Small in 1973, both papers were published in the JAES. While we always think about driver parameters and vented cabs as being T/S we should not forget the work that went on before these two, as vented box principles were first described and patented by Thuras in 1932, with most of the mathematical models up until Thiele coming from people like Locanthi, Van Leeuwen, de Boer, Beranek, Lyon, W.M. Leach and importantly Novak.
So what does it all mean. First I’ll give a brief description of what most of the useful T/S parameters mean and later how you can use them to evaluate drivers for selection. I will not be going into great mathematical or mechanical detail here as this is aimed at the novice.
fs Driver free air resonance.
The point at which all the moving parts of the driver sympathize or resonate. Resonance is a hard thing to explain simply, but a rule of thump is that you will find it hard to produce lower frequencies than the driver’s fs. So a driver with an fs of 60 Hz will not produce 35 Hz very well. A driver with an fs of 32 Hz will produce 35 Hz, if the box is tuned low enough. These two examples relate to closed, ported and bandpass cabinets, horns are less affected by fs as they use the driver as a piston.
Qts Driver total Q.
It had to happen at some point, we’ve hit the Q word. Q is basically a describing word, it is used to describe a quality or characteristic about an electrical or mechanical part of the driver. So Qts is the overall Q of the driver, both electrical and mechanical. Qts can be thought of as how strong the motor and magnet system are. A driver with a low Qts of around 0.20 would have a large magnet and be able to move the cone with a lot of force. This makes for a tight driver. A driver with a Qts of 0.45 would have a smaller magnet and less control over its cone. So low values of Qts give a tight and punchy sound but with little weight or low bass and high Qts values give a slow and heavy sound that will give you lots of low frequency output. Watch out for drivers with really high Qts values of 0.6 or above, these would require such a big box to work correctly that in normal size boxes you don’t get much low end. They are better of being used on the rear parcel shelf of your car, where they can enjoy a massive rear chamber.
Qms Driver mechanical Q
Qms is the mechanical Q of the speaker and only takes the speaker's mechanical properties into consideration. It is a measurement of the control coming from the speaker's mechanical suspension, which is made up of the surround and spider.
The total driver Q is Qts and is derived from the electrical Q (Qes) and the mechanical Q (Qms).
Qts is defined as 1/Qts = 1/Qes + 1/Qms
Qms is calculated
Fs sqrt(Rc)
Qms = -----------
f2 - f1
Drivers with a very high mechanical Q can sound more open, cleaner and have a better dynamic range. This is because they have less loss. The surround is more flexible, the spider is better constructed, they have better air flow and usually have higher sensitivity. So a high mechanical Q is a very good indicator of energy storage behaviour.
So Qts is just a product of Qms and Qes and an understanding of what they are is important when designing a loudspeaker system. Qts, Vas and fs are all that is needed to determine the box size, but when you get to a very advanced stage of designing, its parameters like Qes and Qms which become the foundation of what you do.
BL Driver motor strength.
The higher the value the stronger the motor. Given in tesla meters. Drivers with high BL values of around 30 or more have the ability to control their cones very accurately. These drivers will almost certainly have very large magnets and will weigh a lot. Note also that drivers with high BL values will normally have a low Qts value. Drivers with a low BL value of 20 or less will be less able to control their cones. These drivers will not feel as tight as those with higher BL’s. They will also normally have higher Qts values of over 0.28 and while at home in ported or bandpass cabinets I call these drivers mud motors because of there slow and heavy sound with a less than perfect transient response.
Vas Volume of air equal to the driver compliance.
This can be thought of as how stiff the movement of the cone is. The value is given in litres or cu inches. There are a lot of variables that determine the Vas, so you can’t really say that high values of Vas mean a certain thing or are better. A single or double suspension spider will affect Vas, so does the size of the cone. The temperature of the air and also the humidity will affect Vas and so it is one of the hardest parameters to evaluate.
Mmd Mass or weight of the speaker cone assembly.
This is how heavy the cone, coil and other moving parts are. An 18” driver with a Mmd of around 100 grams will have a light cone and will usually be more efficient than a driver with a heavy cone. A light cone can also move quicker. Light cones are usually found in higher Qts value drivers, but not always. This would appear to give them the advantage of having a quicker transient response as the cone is light, but the weak motors found in higher Qts drivers offsets any advantages of having a lighter cone. Drivers with Mmd’s of over 200 grams will have heavy stiff cones. They will usually be less efficient, have double spiders and have lower Qts values. Drivers with heavy cones should have a slower sound, but not if they also have a low Qts and high BL. The strength of the motor system is able to counteract the high cone weight and still give a fast transient response. Do not confuse Mmd with Mms. Mms is the total cone assembly mass including radiation mass. Some loudspeaker design programs will want you to enter the Mmd and will calculate the Mms for you, while others will want the Mms and will calculate the Mmd for you.
Sd Effective driver radiating area.
Given in sq cm or sq inches. Basically means how much area the cone has to move air with. Larger cones will have bigger Sd’s and smaller cones will have smaller Sd’s. An average Sd for an 18” cone would be 1150 sq cm and a 15” driver would have an average Sd of around 890 sq cm. But the depth of the cone also has to be taken into account. A deeper cone will give you a higher Sd for the same diameter. So that’s why you see different Sd’s for same size drivers. The ones with the higher Sd’s have deeper cones or have less surround material or both.
xmax The amount of voice coil overhang.
Boring description for the parameter some of us love the most. Usually given in mm it represents the distance over which the coil can travel in one direction and maintain a constant number of turns in the gap. So a driver with an xmax of 10 mm can move is cone twice as far as one with an xmax of only 5 mm. Do not confuse xmax with maximum excursion. Maximum excursion is how far the cone will travel before 1. the coil hits the back plate or 2. the cone moves so far that it is limited by its suspension. xmax is how far the cone can travel with the coil still in the magnetic gap. There’s no point in driving the coil outside of the gap as it will no longer be under control form the motor system. More xmax means the cone can move in and out further whilst still being under control. Maximum excursion is meaningless as no sane person drives their cones to full extension all the time, although I have seen it done with a recone kit needed soon after. Note that the xmax figure is for one direction only, so an xmax figure of 5 mm means the cone can travel 5 mm outwards and 5 mm inwards past it’s resting place whilst still being under the control of the motor system. If the xmax figure is not given but the voice coil length and gap height are given you can work out the xmax from these. Take the voice coil length and subtract the gap height then dived by 2. This is how most manufacturers determine the xmax, some will also add 15% to the figure to allow for 3% third-harmonic distortion. So if you do the calculation and the manufacturers xmax figure is higher than predicted they have added 15%.
