All about T/S parameters, AKA Thiele Small Speaker Measurements - Cutting Through the Confusion!So you are curious about building speakers. But now you have run head on into Thiele Small speaker measurements (parameters) and don't know what they are, or why you should care?Can you break the Thiele Small speaker measurements or rules? Sure it’s done all the time. Breaking rules on purpose is one thing. Breaking them because you didn’t do your homework is just silly. Take an hour or so and get to know these terms. Understand that it takes some time to understand what you are doing with speaker building. For example: in some cases a high Q system is wanted or even necessary to limit driver excursion by using high pressures within the cabinet. The Mini Cube Speaker was designed this way. They have less driver low bass output down low, but that is not where these speakers were designed to play. Some often missed measurements you need to build a speaker They were designed to be used with a subwoofer. So a high Q system works fine, and since we were trying to use a really small cabinet a high Q was just an added bonus. Sealed cabinets are not nearly as fussy when it comes to the amount of air volume behind the driver. That’s not to say that you can ignore it. But can design the box to work best for your specific purposes. T/S or Thiele/Small, what does It mean?"Thiele/Small" commonly refers to a set of electromechanical parameters that define how a loudspeaker driver performs. They are useful when designing speakers because they are more easily determined experimentally than more fundamental mechanical parameters. They are named after A. N. Thiele of the Australian Broadcasting Commission, and Richard H. Small of the University of Sydney, who pioneered this line of analysis for loudspeakers. Fundamental small signal mechanical parameters...These are the physical parameters of a loudspeaker driver, as measured at small signal levels, used in the equivalent electrical circuit models. Some of these values are neither easy nor convenient to measure in a finished loudspeaker driver, so when designing speakers using existing drive units (which is almost always the case), the more easily measured parameters listed under Small Signal Parameters are more practical. Sd - Projected area of the driver diaphragm, in square metres. Mms - Mass of the diaphragm, including acoustic load, in kilograms. Cms - Compliance of the driver's suspension, in metres per newton (the reciprocal of its 'stiffness'). Rms - The mechanical resistance of a driver's suspension (ie, 'lossiness') in N•s/m Le - Voice coil inductance measured in millihenries (mH). Re - DC resistance of the voice coil, measured in ohms. Bl - The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field, in T•m (tesla•metres).
Large signal parameters...These parameters are useful for predicting the approximate output of a driver at high input levels. Xmax - Maximum linear peak (or sometimes peak-to-peak) excursion (in mm) of the cone. Note that, because of mechanical issues, the motion of a driver cone becomes non-linear with large enough inputs, ie those in excess of this parameter. Xmech - Maximum physical excursion of the driver before physical damage. With a sufficiently large input, the voice coil and cone will cause voice coil damage or to some mechanical part of the driver. Pe - Thermal power handling capacity of the driver, in watts. This value is difficult to characterize and is often overestimated, by manufacturers and others. Vd - Peak displacement volume, calculated by Vd = Sd•Xmax Descriptions...FsAlso called F0, measured in hertz (Hz). The frequency at which the combination of the moving mass and suspension compliance maximally reinforces cone motion. A more compliant suspension or a larger moving mass will cause a lower resonance frequency, and vice versa. Usually it is less efficient to produce output at frequencies below Fs, though motion below Fs can cause uncontrolled motion, mechanically endangering the driver.Woofers typically have an Fs in the range of 13–60 Hz. Midranges usually have an Fs in the range of 60–500 Hz and tweeters between 500 Hz and 4 kHz. QtsA unitless measurement, characterizing the combined electric and mechanical damping of the driver. In electronics, Q is the inverse of the damping ratio. The value of Qts is proportional to the energy stored, divided by the energy dissipated, and is defined at resonance (Fs). Most drivers have Qts values between 0.2 and 0.8. QmsA unitless measurement, characterizing the mechanical damping of the driver, that is, the losses in the suspension (surround and spider.) A typical value is around 3. High Qms indicates lower damping losses, and low Qms indicates higher. The main effect of Qms is on the impedance of the driver, with high Qms drivers displaying a higher impedance peak. One predictor for low Qms is a metallic voice coil former of a particular configuration. These act as eddy-current brakes and increase damping, reducing Qms. The same former, with an electrical break in the cylinder (so no conducting loop) avoids these losses. QesA unitless measurement, describing the electrical damping of the loudspeaker. As the coil of wire moves through the magnetic field, it generates a current which opposes the motion of the coil. This so-called "Back-EMF" decreases the total current through the coil near the resonance frequency, reducing cone movement and increasing impedance. In most drivers, Qes is the dominant factor in the voice coil damping.BlMeasured in tesla-metres (T•m). Technically this is B x l (vector cross product or B * l * sin(θ)), but the standard geometry of a circular coil in an annular voice coil gap gives sin(θ)=1. Bl is also known as the 'force factor' because the force on the coil imposed by the magnet is Bl multiplied by the current through the coil. The higher the Bl value, the larger the force generated by a given current flowing through the voice coil. Bl has a very strong effect on Qes.VasMeasured in litres (L), is a measure of the free air 'stiffness' of the suspension -- the driver must be mounted in free air. It represents the volume of air that has the same stiffness as the driver's suspension when acted on by a piston of the same area (Sd) as the cone. Larger values mean lower stiffness, and generally require larger enclosures. Vas varies with the square of the diameter.MmsMeasured in grams (g), this is the mass of the cone, coil and other moving parts of a driver, including the acoustic load imposed by the air in contact with the driver cone. Mmd is the cone mass without the acoustic load, and the two should not be confused. Some simulation software calculates Mms when Mmd is entered.RmsUnits are not usually given for this parameter, but it is in mechanical 'ohms'. Rms is a measurement of the losses, or damping, in a driver's suspension and moving system. It is the main factor in determining Qms. Rms is influenced by suspension topology, materials, and by the voice coil former (bobbin) material.CmsMeasured in metres per Newton (m/N). Describes the compliance (ie, the inverse of stiffness) of the suspension. The more compliant a suspension system is, the lower its stiffness, so the higher the Vas will be.ReMeasured in ohms (Ω), this is the DC resistance of the voice coil. American EIA standard RS-299A specifies that DCR should be at least 80% of the rated driver impedance, so an 8-ohm rated driver will have a DC resistance of at least 6.4 ohms, and a 4-ohm unit should measure 3.2 ohms minimum. Advertised values are often approximate at best.LeMeasured in millihenries (mH), this is the inductance of the voice coil. The coil is an inductor in part due to losses in the pole piece, so the apparent inductance changes with frequency. Large Le values limit the high frequency output of the driver and cause response changes near cutoff. Simple modeling software often neglects the effects of Le, and so does not include its consequences. Building a copper cap into the magnet structure can reduce this effect.SdMeasured in square metres (m²). The effective area of the cone or diaphragm. It varies with the conformation of the cone, and details of the surround. Generally accepted as the cone body diameter plus half the width of the annulus (surround). Wide roll surrounds can have significantly less Sd than conventional types.XmaxSpecified in millimeters (mm). In the simplest form, subtract the height of the voice coil winding from the height of the magnetic gap, take the absolute value and divide by 2. This technique was suggested by JBL's Mark Gander in a 1981 AES paper, as an indicator of a loudspeaker motor's linear range. Although easily determined, it neglects non-linearity’s and limitations introduced by the suspension. Subsequently, a combined mechanical/acoustical measure was suggested, in which a driver is progressively driven to high levels at low frequencies, with Xmax determined at 10% THD. This method better represents actual driver performance, but is harder and more time-consuming to determine.VdSpecified in litres (L). The volume displaced by the cone, equal to the cone area (Sd) multiplied by Xmax. Any particular value may be achieved in any of several ways. For instance, by having a small cone with a large Xmax, or a large cone with a small Xmax. Comparing Vd values will give an indication of the maximum output of a driver at low frequencies. High Xmax, small cone diameter drivers are likely to be inefficient, since much of the voice coil winding will be outside the magnetic gap at any one time and will therefore contribute little or nothing to cone motion. Likewise, large cone diameter high Xmax drivers are likely to be more efficient as they will not need, and so may not have, long voice coils.η0Specified in percent (%). Comparing drivers by their reference efficiency is more useful than using 'sensitivity' since manufacturer sensitivity figures are too often overly optimistic.Source: Wikipidea When you understand why these are important you can start to use them in your designs. The more information you have when designing speakers the better chances you have for success.Still, some builders don’t have the patience to learn all about Thiele/Small parameters. That’s fine, just stick with a sealed box and use drivers that are best suited to sealed cabinets. You can do a lot worse, believe me. Many speakers on the market were designed without any knowledge of T/S parameters. But, if you are after the last degree of resolution, low distortion, most transparent bass, and a high quality music experience, you have little choice but to learn about measuring speakers. Return toChoosing a speaker crossover frequency using wavelength Return to speaker box design page from; Thiele Small speaker measurements Return to Home Page - Home Speaker from thiele small speaker measurements |
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