The sound of an audio system is most heavily influenced by the devices that generate the sound - the speakers.
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In Jan 2023, Joe Begin updated his 2019 article on The Keys of Loudspeaker Electroacoustic Measurements. This gives examples of the problems incurred when trying to get accurate measurements of loudspeaker performance. (Part 2) (Part 3)
When it is deemed necessary to use a large number of speakers, a Constant Voltage System is usually needed. Commonly called a 70-volt system, these systems are also used with 25 and 100 volts or even higher as well. Actually, these systems are more properly called Constant or High Impedance Systems, or High-voltage Audio Distribution Systems. but the older term is more widely used.
These systems use transformers to control the power to each speaker. The diagram to the left shows how this is done. While this shows a transformer at the amp, amplifiers can be designed to operat6e a constant voltage system directly. In most cases, the transformer at each speakers has taps for different power levels. As the diagram shows, each speaker can be set for a different power draw, but the total power needed to drive the speakers must not exceed the power capability of the amp driving the system.
A Bose radio utilizing its waveguide design for improved bass response.
^ A 3-way Passive speaker Crossover Network ^
where ρ is the density of air (1.184 kg/m3 at 25 °C), and c is the speed of sound (346.1 m/s at 25 °C). Using SI units, the result will be in cubic meters. To get Vas in liters, multiply by 1000. Cms is the Compliance of the driver's suspension, in meters per newton (the reciprocal of its 'stiffness'). This illustrates how the enclosure must be matched to the characteristics of the driver. Consult Wikipedia for more details on TS parameters. An online calculator for TS parameters is here. More that 130 calculators for various design purposes can be found at HiFi Audio Design.
One thing Thiele found when developing his equations is that there is an inherent trade-off between box size, low-frequency response, and efficiency. You can design for any two but not all three at the same time. At the time, efficiency was very important due to amplifier power limitations, Now, with things like class D amps, efficiency is no longer as critical.
Alpha Sound produced a video in July 2022 that shows some interesting experiments with bass speakers. He demonstrates physically the effect of the bass reflex port. He also demonstrates the use of multiple speakers and digital delays to give a bass speaker some directionality and how two speakers can be set up to provide a cardioid directional pattern. An example is the QSC cardioid sub shown below.