The ability to choose the most appropriate loudspeaker is directly related to the performance data that is provided by the manufacturers. with their products. After the 1970, there were define methods that were accepted as standard in the industry for obtaining of this data. The recognized methods were expensive and often unrealistic for the thousands of individuals needing loudspeaker performance information.

These methods were presented and accepted by the AES (Audio Engineering Society) that were later know as and refered to as the ‘Thiele-Small Parameters’. Thiele and Small devoted efforts to show how these parameters define the relationship between a speaker and a particular enclosure.

Thiele-Small Parameters

FS – This is the free-air resonant frequency of a speaker. 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.  This important is helpful to prevent enclosure from ‘ringing’.

As a rule of thumb, a lower Fs indicates a better for low-frequency response. Because other parameters affect the performance, this is not always the case.

RE – 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.

Concerns are raised when Re is less than the impedance in fear overloading the amplifier. The fact that the inductance of a speaker will rise in frequency making this a non issue.

LE – The voice coil inductance measured in millihenries (mH). Industry standard measures inductance at 1,000 Hz. As frequencies increases, there will be a rise in impedance above Re because the voice coil acts as an inductor. The impedance is not a fixed resistance, but represented  in a curve that changes as frequency changes.
Note: Maximum impedance (Zmax) occurs at Fs.

Qms – A measurement of the control from the speaker’s mechanical suspension system (the surround and spider). Viewed like a springs.

Qes – A measurement of the control 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 – Called the ‘Total Q’ of the driver and is derived from an equation.

(Qes * Qms) / (Qes + Qms)

  • Rule of Thumb
    1. Qts of 0.4 or below indicates a transducer well suited to a vented enclosure.
    2. Qts between 0.4 and 0.7 indicates suitability for a sealed enclosure.
    3. Qts of 0.7 or above indicates suitability for free-air or infinite baffle applications.

Vas – represents the volume of air that is compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker.

Note: Vas is one of the trickiest parameters to measure because
air pressure changes relative to humidity and temperature.

Cms – measured in meters per Newton and is the force exerted by the mechanical suspension of the speaker. This is the measure of stiffness.

VD – Is the Peak Diaphragm Displacement Volume or the volume of air the cone will move. Calculated by Xmax * Sd and is noted in cc.
Note: The highest Vd figure is desirable for a sub-bass transducer.

BL – Expressed in Tesla meters, is a measurement of the motor strength of a speaker. Calculated by mass in grams divided by the current in amperes. A high BL figure indicates a very strong transducer.

MMS – Is the combination of the cone assembly weight plus the radiated mass load of the driver’. This is the weight of the air (the amount calculated in Vd) that the cone will have to push.

EPB – This is calculated by Fs / Qes and is used in many enclosure design formulas to determine if a speaker is more suitable for the enclosure.

  • An EBP close to 100 usually indicates a speaker that is best suited for a vented enclosure.
  • An EBP closer to 50 usually indicates a speaker best suited for a closed box design.

The Qts should also be considered as well. This is only a starting point. Many well-designed systems have violated this guideline.

XMAX – Maximum Linear Excursion. This a another guideline because the speaker output becomes non-linear when the voice coil starts to leave the magnetic gap and this is when distortion starts to increase.

Xmax = Voice coil height – top plate thickness / 2).

The Xmax is expressed as the excursion point where THD reahes 10%.

XLIM – This expression is the usable excursion limit for the transducer. Xlim expressed for the lowest of four potential failure condition and exceeding this Xlim is certain to fail from one of these conditions.

  • Spider crashing on top plate
  • Voice coil bottoming on backplate
  • Voice coil coming out of gap above core
  • The physical limitation of cone

Protection can be provided for these types of failures by using:

  • Enclosure modeling to design for protection of mechanical failure.
  • Use of High pass filters or Limiters

SD – Is the actual surface area of the cone, normally given in square cm.

FREQUENCY RANGE – This is the proven useful range of the device. Please note that manufacturers use different techniques in determining this Usable Range. As frequencies increase, the off-axis coverage of a transducer decreases relative to its diameter. Higher frequencies coverage becomes ‘beamy’ or narrow. Most two-way enclosures ignore the theory and still perform quite well but it is useful to know at what point you can expect a compromise in coverage.

POWER HANDLING – is very important to transducer selection. You will need to choose a loudspeaker that is capable of handling the input power provided. The number one contributor to a transducer’s power rating is its ability to release thermal energy. 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. 

  • This rating is expressed as continuous power not RMS
    • Most manufacturers and vendors use RMS.
    • This test is with pink noise ans is vary abusive
  • Program and Music power is defined as 2 times continuous power
    • Most vendors will relate to this as peak power.
    • This is random frequencies and is not as abusive as pink noise
    • This is used as the thermal limit for selection
  • Peak power is defined as 2 times program or music power
    • This rating is used as the mechanical limitations of the speaker

Subwoofer Applications

  • Define the required for the venue that will be provided for.
  • Use of factor of 1.5 times required power for the speaker selection
    • Low frequencies are very abusive and requires compensation
  • Use 2 times or less of the AES rating for the amplifier selection
  • Using less then 2 times AES rating covers mechanical concerns
  • Select an amplifier with DSP function for speaker protection
    • With this feature, allows for selection of amplifier that is larger then the 2 times AES rating for additional headroom

FOH Applications

  • Define the required for the venue that will be provided for.
  • Use 2 times or less of the AES rating for the amplifier selection
  • Using less then 2 times AES rating covers mechanical concerns
  • Select an amplifier with DSP function for speaker protection
    • With this feature, allows for selection of amplifier that is larger then the 2 times AES rating for additional headroom

Caution: Even though the manufacture uses EIA 426A or other testing standards, the typical test period is only two hours. If you are providing a service for greater than two hour, compensation make be considered as in the subwoofer application to provide a safety factor.

The other choice would be to select a manufacturer that conducts eight hour test on their product.

SENSITIVITY – Represents one of the most useful specifications for any transducer. It represents the efficiency and volume you can expect from a device relative to the input power.

This should be expressed as the average output across the usable frequency when applying 1W/1M into the nominal impedance.

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