Application of Thiele/ Small Parameters

Hybrid Audio Technologies driver specifications are available by following this link.

Acoustics pioneers Neville Thiele and Richard Small developed a way to predict speaker performance and frequency response. These parameters are known collectively as “Thiele/ Small Parameters,” and are divided into physical characteristics and response parameters:

The Physical Characteristics of a speaker are:

Re: The D.C. resistance of the voice coil measured in Ohms.
Sd: The surface area of the speaker’s cone.
BL: The magnetic strength of the motor structure.
Mms: The total moving mass of the speaker including the small amount of air in front of and behind the cone.
Cms: The stiffness of the driver’s suspension
Rms: The losses due to the suspension.

The Thiele/Small Response Paramters are:

Re: The D.C. resistance of the voice coil measured in Ohms.
Sd: The surface area of the speaker.
Fs: The resonant frequency of the speaker.
Qes: The electrical “Q” of the speaker.
Qms: The mechanical “Q” of the speaker.
Qts: The total “Q” of the speaker.
Vas: The volume of air having the same acoustic compliance as the speaker’s suspension.

Enclosure Recommendations

The mechanical and electrical parameters of the majority of midrange and midbass drivers by Hybrid Audio Technologies are amenable to a variety of different installations and speaker locations. In a typical vehicular installation, Hybrid Audio Technologies recommends that these drivers be placed in an “infinitely large” enclosure, which is more notably known as “infinite baffle.”

Small sealed enclosures are not needed to be constructed, nor are typically recommended in most circumstances for any of Hybrid Audio’s midrange and midbass range of products (there are some minor exceptions, based upon application, intended use, power handling, and etc., please contact us for details). Hybrid Audio has also had great success incorporating its midrange and midbass drivers in dipole configuration (detailed below), transmission lines, and in larger vented enclosures.

In a typical installation, Hybrid Audio midrange and midbass products should be mounted with unrestricted access to airspace to ensure the speaker’s ability to effectively reproduce its wide frequency bandwidth. The reason why the speaker was designed in this way is highly empirical. When a speaker is mounted in a small closed box, it radiates as much energy forward of the cone as it does rearward of the cone. All speaker cones and dust caps (diaphragms) are a weak sound barrier at best, and the result of the high amount of energy being “pushed” into a small enclosure is the energy transmitting through to the outside of the cone (an additive phenomenon to the incidental wave). It is conjectured that this effect is most notable in the low hundreds of Hz region, where acoustical stuffing materials are ineffective and the internal dimensions are not small enough for the internal air volume to act as pure compliance. Consequently, Hybrid Audio has designed this speaker to work well without an enclosure, and as such, should not be significantly prone to enclosure back-pressure and sound coloration when placed infinitely baffled. The “infinitely large” enclosure, per se, improves spectral response and power response variation between high and low frequencies. And in the case where an infinite baffle operation is difficult or impossible to achieve in your car’s environment, we highly suggest the use of acoustic resistors (aperiodic membranes or trade name Variovents®) in sealed enclosures to help dissipate the back wave energy. If you absolutely must use a sealed enclosure, we recommend that you contact us for details and assistance in targeting a sealed enclosure volume applicable for your intended purpose. In all cases, the use of loosely-packed fibrous damping materials, such as fiberglass, Dacron, or long-fiber wool will also significantly improve the final installation, no matter what type of baffle and enclosure configuration is chosen.

Dipole Applications

While infinite baffle or resistive sealed enclosures are highly recommended for this driver, you might find that in the rare instance that the Legatia SE drivers can be placed in a completely open baffle, i.e. dipole. A word of caution: Legatia SE drivers mounted in an open baffle have to move more air than a similar Legatia SE driver mounted in a resistive sealed enclosure, or infinitely baffled, just simply for the fact that there’s a progressive acoustic short circuit between the front and back waves below the speaker’s resonance frequency (Fs) in the open baffle configuration. Great care must be taken in this instance because there is no acoustic compliance afforded in a dipole configuration, and the speaker will reach its mechanical limits much quicker (and will net increased distortion). However, in certain instances, a dipole-style midrange may work well in a vehicular installation, in that radiation from the rear of the baffle, having undergone enough phase shift as it comes around to the front, adds to the total sound at off-axis angles. Of great importance in the dipole midrange configuration is the setting up of crossovers and weighing the negative effects of distortion of elevated amplitude levels.

Continue to the next section.


Hybrid Audio Technologies Specifications and Parameters Spreadsheet (Google Sheets)Advanced System InstallationLesson One: Off-Axis ResponseLesson Two: Equalization of Pathlength DifferencesLesson Three: The Effect of HRTF, ITD, and IIDLesson Four: Point-SourcingLesson Five: ReferenceMounting Baffle ConsiderationsCrossoversTime CorrectionAcoustic TreatmentAcoustic TreatmentConclusion