LOUDSPEAKER parameter guidelines and rules of thumb

FS – free-air resonance

The point at which moving parts are balanced. The goal is to prevent your enclosure from ‘ringing’.

As a general rule of thumb:

  • Lower Fs indicates a woofer with lower low-frequency response
  • Higher Fs indicates a woofer with a higher low-frequency response
  • Not always the case because other parameters affect the ultimate performance as well.


Qms, Qes, and Qts are measurements of the a transducer’s suspension when it reaches the resonant frequency (Fs).

  • Qms is a measurement of the the speaker’s mechanical suspension system (the surround and spider).
  • Qes is a measurement of the the speaker’s electrical suspension system (the voice coil and magnet).
  • Qts is called the ‘Total Q’ of the driver and is derived from Qes multiplied by Qms and result divided by sum.
  • 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.


Vas represents the volume of air compressed to one cubic meter exerts the same force as the compliance

  • (Cms) is the force exerted by the mechanical suspension in a particular speaker.
  • Cms is measured in meters per Newton.


This parameter is the Peak Diaphragm Displacement Volume which if the volume of air the cone will move.

  • Calculated by multipying Xmax (Voice Coil Overhang of the driver) by Sd (Surface area of the cone).
  • The highest Vd figure is desirable for a sub-bass transducer.


A measured mass applied to the cone forcing it back while the current required for the motor to force the mass back.

  • Measurement of the motor strength of a speaker in Tesla meters.
  • 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!


This parameter is the combination of the weight of the cone assembly plus the ‘driver radiation mass load’ which is the weight of the cone assembly (sum of the weight of the cone assembly components).

The driver radiation mass load is the weight of the air (the amount calculated in Vd) that the cone will have to push.


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 greater than 150 usually indicates a speaker is best suited for a horn enclosure.
  • An EBP greater than 50 but less than 100 usually indicates a speaker that is best suited for a vented enclosure.
  • An EBP less than 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.


Short for Maximum Linear Excursion.

  • Xmax: Voice coil height minus top plate thickness, divided by 2.
  • Speaker output becomes non-linear when the voice coil begins to leave the magnetic gap.
  • The point at which the number of turns in the gap begins to decrease is when distortion starts to increase. Although suspensions can create non-linearity in output,
  • The Xmax figures 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; Voice coil bottoming on back plate; Voice 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.


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


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. 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.


This specification is very important to transducer selection. 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
  • 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.
  • 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 music program is double that of our standard Watts rating.


This represents the efficiency and volume expectations from a device relative to the input power.

Typical measure standards are expressed as the average output across the usable frequency when applying 1W/1M into 2.83V/8 ohms.

By AJ Design Software

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