Sensitivity
This parameter determines the volume of the sound of a particular speaker when a certain standard signal is fed to it. Typically, this is considered a 1 W signal with a frequency of 1000 Hz. The higher the sensitivity, the less the speaker is demanding on the power of the amplifier. So, it is believed that for acoustics connected directly to the car radio, the sensitivity should be 80 dB or more; for less sensitive systems it is recommended to use a separate amplifier.
External crossover
Crossover is part of acoustic system that divides the overall signal into separate frequency bands fed to the corresponding speakers (for more details, see "Bands"). All multi-band speakers (component and coaxial, see "Type") have their own crossover; it is usually located directly in the speaker body. However, in some models, usually the most advanced ones, the crossover can be made as a separate device, in its own body.
External crossovers are considered to provide better sound quality and introduce less distortion into it; in addition, they often provide the ability to fine-tune the sound.
Tweeter size
Size of the high-frequency speaker(s) included in the speaker design.
Size affects the amount of space required to install the speaker. However, high frequencies do not require large diffusers, so this dimensions is usually small and does not significantly affect the overall dimensions of the speaker system. The only exception is individual high-frequency tweeters (see "Type"), where the overall dimensions of the housing are directly related to the dimensions of the diffuser.
Mounting depth
The mounting depth of car audio is, in fact, the size of the speaker in depth. Note that for component sets (see "Type") this parameter is given for the largest speaker, and for case models it is generally irrelevant.
The mounting depth, along with the diameter of the speaker (see above), determines the amount of space needed to mount the speaker — in this case, how deep a niche would be required for normal mounting. At the same time, the larger the speaker, the larger, usually, the installation depth. And with similar diameters of equal size, a more powerful model is likely to be more “deep”.
The smallest amount of space —
up to 15 mm deep — is required by some tweeters. A depth
of 16 – 30mm is normal for tweeters and very shallow for general range speakers, most of these models have a depth
of 30mm or more. At the same time, full-range speakers are usually "recessed" by
less than 60 mm, component systems — by
less than 90 mm, and
more depth may be required only for some coaxial and mid-range models and, oddly enough, tweeters.
