Prior to 1970, there were no easy or affordable methods accepted as standard in the industry for obtaining comparative data about loudspeaker performance.  Recognized laboratory tests were expensive and unrealistic for the thousands of individuals needing performance information.   Standard measurement criteria were required to enable manufacturers to publish consistent data for customers to make comparisons between various loudspeakers.

Thiele-Small Parameters

In the early seventies, several technical papers were presented to the AES (Audio Engineering Society) that resulted in the development of what we know today as 'Thiele-Small Parameters'.  These papers were authored by A.N.Thiele and Richard H. Small.  Thiele was the senior engineer of design and development for the Australian Broadcasting Commission and was responsible at the time for the Federal Engineering Laboratory, as well as for analyzing the design of equipment and systems for sound and vision broadcasting.  Small was, at the time, a Commonwealth Post-graduate Research Student in the School of Electrical Engineering at the University of Sydney.

Thiele and Small devoted considerable effort to show how the following parameters define the relationship between a speaker and a particular enclosure.  However, they can be invaluable in making choices because they tell you far more about the transducer's real performance than the basic benchmarks of size, maximum power rating or average sensitivity. 

Fs   This parameter is the free-air resonant frequency of a speaker.  Simply stated, it is the point at which the weight of the moving parts of the speaker becomes balanced with the force of the speaker suspension when in motion.  If you've ever seen a piece of string start humming uncontrollably in the wind, you have seen the effect of reaching a resonant frequency. It is important to know this information so that you can prevent your enclosure from 'ringing'.  With a loudspeaker, the mass of the moving parts, and the stiffness of the suspension (surround and spider) are the key elements that affect the resonant frequency. As a general rule of thumb, a lower Fs indicates a woofer that would be better for low-frequency reproduction than a woofer with a higher Fs.  This is not always the case though, because other parameters affect the ultimate performance as well. 

Re   This is the DC resistance of the driver measured with an ohm meter and it is often referred to as the 'DCR'.  This measurement will almost always be less than the driver's nominal impedance.  Consumers sometimes get concerned the Re is less than the published impedance and fear that amplifiers will be overloaded.  Due to the fact that the inductance of a speaker rises with a rise in frequency, it is unlikely that the amplifier will often see the DC resistance as its load.      

Le   This is the voice coil inductance measured in millihenries (mH).  The industry standard is to measure inductance at 1,000 Hz.  As frequencies get higher there will be a rise in impedance above Re.  This is because the voice coil is acting as an inductor.  Consequently, the impedance of a speaker is not a fixed resistance, but can be represented as a curve that changes as the input frequency changes.  Maximum impedance (Zmax) occurs at Fs.        

Q Parameters   Qms, Qes, and Qts are measurements related to the control of a transducer's suspension when it reaches the resonant frequency (Fs).  The suspension must prevent any lateral motion that might allow the voice coil and pole to touch (this would destroy the loudspeaker).  The suspension must also act like a shock absorber.  Qms is a measurement of the control coming from the speaker's mechanical suspension system (the surround and spider).  View these components like springs.  Qes is a measurement of the control coming from the speaker's electrical suspension system (the voice coil and magnet).  Opposing forces from the mechanical and electrical suspensions act to absorb shock.  Qts is called the 'Total Q' of the driver and is derived from an equation where Qes is multiplied by Qms and the result is divided by the sum of the same. 

As a general guideline, Qts of 0.4 or below indicates a transducer well suited to a vented enclosure.  Qts between 0.4 and 0.7 indicates suitability for a sealed enclosure.  Qts of 0.7 or above indicates suitability for free-air or infinite baffle applications.  However, there are exceptions!  The Eminence Kilomax 18 has a Qts of 0.56.  This suggests a sealed enclosure, but in reality it works extremely well in a ported enclosure.  Please consider all the parameters when selecting loudspeakers.  If you are in any doubt, contact your Eminence representative for technical assistance. 

Vas/Cms   Vas represents the volume of air that when compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker. Vas is one of the trickiest parameters to measure because air pressure changes relative to humidity and temperature — a precisely controlled lab environment is essential. Cms is measured in meters per Newton. Cms is the force exerted by the mechanical suspension of the speaker. It is simply a measurement of its stiffness. Considering stiffness (Cms), in conjunction with the Q parameters gives rise to the kind of subjective decisions made by car manufacturers when tuning cars between comfort to carry the president and precision to go racing. Think of the peaks and valleys of audio signals like a road surface then consider that the ideal speaker suspension is like car suspension that can traverse the rockiest terrain with race-car precision and sensitivity at the speed of a fighter plane. It’s quite a challenge because focusing on any one discipline tends to have a detrimental effect on the others.    

Vd   This parameter is the Peak Diaphragm Displacement Volume — in other words the volume of air the cone will move. It is calculated by multipying Xmax (Voice Coil Overhang of the driver) by Sd (Surface area of the cone). Vd is noted in cc. The highest Vd figure is desirable for a sub-bass transducer.       

BL   Expressed in Tesla meters, this is a measurement of the motor strength of a speaker. Think of this as how good a weightlifter the transducer is. A measured mass is applied to the cone forcing it back while the current required for the motor to force the mass back is measured. The formula is mass in grams divided by the current in amperes. A high BL figure indicates a very strong transducer that moves the cone with authority!             

Mms   This parameter is the combination of the weight of the cone assembly plus the ‘driver radiation mass load’. The weight of the cone assembly is easy: it’s just the sum of the weight of the cone assembly components. The driver radiation mass load is the confusing part. In simple terminology, it is the weight of the air (the amount calculated in Vd) that the cone will have to push.      

EBP   This measurement is calculated by dividing Fs by Qes. The EBP figure is used in many enclosure design formulas to determine if a speaker is more suitable for a closed or vented design. An EBP close to 100 usually indicates a speaker that is best suited for a vented enclosure. On the contrary, an EBP closer to 50 usually indicates a speaker best suited for a closed box design.  This is merely a starting point. Many well-designed systems have violated this rule of thumb! Qts should also be considered.         

Xmax/Xlim   Short for Maximum Linear Excursion. Speaker output becomes non-linear when the voice coil begins to leave the magnetic gap. Although suspensions can create non-linearity in output, the point at which the number of turns in the gap (see BL) begins to decrease is when distortion starts to increase.  Eminence has historically been very conservative with this measurement and indicated only the voice coil overhang (Xmax: Voice coil height minus top plate thickness, divided by 2).  The Xmax figures on this website are expressed as the greater of the result of the formula above or the excursion point of the woofer where THD reahes 10%. This method results in a more real world expression of the usable excursion limit for the transducer. Xlim is expressed by Eminence as the lowest of four potential failure condition measurements:  spider crashing on top plate;vVoice coil bottoming on back plate;vVoice coil coming out of gap above core; or the physical limitation of cone.  A transducer exceeding the Xlim is certain to fail from one of these conditions. High pass filters, limiters, and enclosure modeling software programs are valuable tools in protecting your woofers from mechanical failure.

Sd   This is the actual surface area of the cone, normally given in square cm.

Usable frequency range   This is the frequency range for which Eminence feels the transducer will prove useful. Manufacturers use different techniques for determining ‘Usable Frequency Range’.  Most methods are recognized as acceptable in the industry, but can arrive at different results.  Technically, many loudspeakers are used to produce frequencies in ranges where they would theoretically be of little use. As frequencies increase, the off-axis coverage of a transducer decreases relative to its diameter. At a certain point, the coverage becomes ‘beamy’ or narrow like the beam of a flashlight.  Following is a chart that demonstrates at what frequency this phenomenon occurs relative to the size of the transducer.  If you’ve ever stood in front of a guitar amplifier or speaker cabinet, then moved slightly to one side or the other and noticed a different sound, you have experienced this phenomenon and are now aware of why it occurs.  Clearly, most two-way enclosures ignore the theory and still perform quite well. The same is true for many guitar amplifiers, but it is useful to know at what point you can expect a compromise in coverage.

Power handling  This specification is very important to transducer selection. Obviously, you need to choose a loudspeaker that is capable of handling the input power you are going to provide. By the same token, you can destroy a loudspeaker by using too little power.  The ideal situation is to choose a loudspeaker that has the capability of handling more power than you can provide lending some headroom and insurance against thermal failure. To use an automobile as an analogy; you would not buy a car that could only go 55mph if that were the speed you always intended to drive.  Generally speaking, the number one contributor to a transducer’s power rating is its ability to release thermal energy. This is affected by several design choices, but most notably voice coil size, magnet size, venting, and the adhesives used in voice coil construction.  Larger coil and magnet sizes provide more area for heat to dissipate, while venting allows thermal energy to escape and cooler air to enter the motor structure.  Equally important is the ability of the voice coil to handle thermal energy. Eminence is renowned for its use of proprietary adhesives and components that maximize the voice coil’s ability to handle extreme temperatures.  Mechanical factors must also be considered when determining power handling. A transducer might be able to handle 1,000W from a thermal perspective, but would fail long before that level was reached from a mechanical issue such as the coil hitting the back plate, the coil coming out of the gap, the cone buckling from too much outward movement, or the spider bottoming on the top plate. The most common cause of such a failure would be asking the speaker to produce more low frequencies than it could mechanically produce at the rated power. Be sure to consider the suggested usable frequency range and the Xlim parameter in conjunction with the power rating to avoid such failures.  The Eminence power rating is derived using an EIA 426A noise source and test standard. All tests are conducted for eight hours in a free-air, non-temperature controlled environment. Eminence tests samples from each of three different production runs and each sample must pass a test exceeding the rated power by 50 to 100W. The Eminence music program is double that of our standard Watts rating.

Sensitivity   This data represents one of the most useful specifications published for any transducer. It is a representation of the efficiency and volume you can expect from a device relative to the input power.  Loudspeaker manufacturers follow different rules when obtaining this information — there is not an exact standard accepted by the industry. As a result, it is often the case that loudspeaker buyers are unable to compare 'apples to apples' when looking at the sensitivities of different manufacturers’ products.  Eminence sensitivities are expressed as the average output across the usable frequency when applying 1W/1M into the nominal impedance. ie: 2.83V/8 ohms, 4V/16 ohms.