Thiele/Small parameters
Many of the parameters are strictly defined only at the resonant frequency, but the approach is generally applicable in the frequency range where the diaphragm motion is largely pistonic, i.e., when the entire cone moves in and out as a unit without cone breakup.
Thiele/Small parameters are named after A. Neville Thiele of the Australian Broadcasting Commission, and Richard H. Small of the University of Sydney, who pioneered this line of analysis for loudspeakers.
The 1925 paper[1] of Chester W. Rice and Edward W. Kellogg, fueled by advances in radio and electronics, increased interest in direct radiator loudspeakers.
1869178) his "Sound Translating Device" (essentially a vented box) which was evidence of the interest in many types of enclosure design at the time.
Progress on loudspeaker enclosure design and analysis using acoustic analogous circuits by academic acousticians like Harry F. Olson continued until 1954 when Leo L. Beranek of the Massachusetts Institute of Technology published Acoustics,[2] a book summarizing and extending the electroacoustics of the era.
J. F. Novak used novel simplifying assumptions in an analysis in a 1959 paper[3][4] which led to a practical solution for the response of a given loudspeaker in sealed and vented boxes, and also established their applicability by empirical measurement.
In 1961, leaning heavily on Novak's work, A. N. Thiele described a series of sealed and vented box "alignments" (i.e., enclosure designs based on electrical filter theory with well-characterized behavior, including frequency response, power handling, cone excursion, etc.)
[5] This paper remained relatively unknown outside Australia until it was re-published in the Journal of the Audio Engineering Society in 1971.
[6][7] It is important to note that Thiele's work neglected enclosure losses and, although the application of filter theory is still important, his alignment tables now have little real-world utility due to neglecting enclosure losses.
From 1968 to 1972, J. E. Benson published three articles[8] in an Australian journal that thoroughly analyzed sealed, vented and passive radiator designs, all using the same basic model, which included the effects of enclosure, leakage and port losses.
Beginning in June 1972, Richard H. Small published a series of very influential articles on direct radiator loudspeaker system analysis,[9] including closed-box,[10][11] vented-box,[12][13][14][15] and passive-radiator[16][17] loudspeaker systems, in the Journal of the Audio Engineering Society, restating and extending Thiele's work.
These articles were also originally published in Australia, where he had attended graduate school, and where his thesis supervisor was J. E. Benson.
Richard H. Small and Garry Margolis, the latter of JBL, published an article in the Journal of the Audio Engineering Society (June 1981),[18] which recast much of the work that had been published up till then into forms suited to the programmable calculators of the time.
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: These values can be determined by measuring the input impedance of the driver, near the resonance frequency, at small input levels for which the mechanical behavior of the driver is effectively linear (i.e., proportional to its input).
In addition, power compression, thermal, and mechanical effects due to high signal levels (e.g., high electric current and voltage, extended mechanical motion, and so on) all change driver behavior, often increasing distortion of several kinds: Some caution is required when using and interpreting T/S parameters.
Parameters values are almost never individually taken, but are at best averages across a production run, due to inevitable manufacturing variations.
From this example, it is seen that the measurements to be preferred while designing an enclosure or system are those likely to represent typical operating conditions.
Level-dependent nonlinearities typically cause lower than predicted output, or small variations in frequency response.
Level shifts caused by resistive heating of the voice coil are termed power compression.
Paper, a popular material in cone fabrication, absorbs moisture easily and unless treated may lose some structural rigidity over time.
The suspension experiences fatigue, and also undergoes changes from chemical and environmental effects associated with aging such as exposure to ultraviolet light, and oxidation which affect foam and natural rubber components badly, though butyl, nitrile, SBR rubber, and rubber-plastic alloys (such as Santoprene) are more stable.
The changes in behavior from aging may often be positive, though since the environment that they are used in is a major factor the effects are not easily predicted.
The proprietor of the firm GR Research has publicly reported several such investigations of several manufacturers' drivers.
This variability is largely related to the particular characteristics of specific materials, and reputable manufacturers attempt to take them into account.
) and result in minimal net changes (small fractions of a dB) in frequency response.
If the performance of speaker system is critical, as with high order (complex) or heavily equalized systems, it is sensible to measure T/S parameters after a period of run-in (some hours, typically, using program material), and to model the effects of normal parameter changes on driver performance.
The impedance may be measured in free air (with the driver unhoused and either clamped to a fixture or hanging from a wire, or sometimes resting on the magnet on a surface) and/or in test baffles, sealed or vented boxes or with varying amounts of mass added to the diaphragm.
Although tedious, and not often used in manual measurements, simple calculations exist which allow the true impedance magnitude and phase to be determined.
When this method is used manually, the result of taking the three measurements is that their ratios are more important than their actual value, removing the effect of poor meter frequency response.
