Ants, excellent question. When a person designs a speaker, one of the considerations is how much the cone will move and the subsequent loudness for a given amount of power driving it.
If the designer uses a loose spider and fairly flexible surround on the cone, the voice coil will have an easier time moving the cone and the speaker will be loud with a small amount of power. The problem with a loose spider and surround is that as the power is increased, it takes the cone to its mechanical limit of movement and it gets harsh. This is an underdamped system and can also sound ringy. If you put this speaker in a sealed or closed back cabinet, the air that is trapped in the cabinet acts as a spring or big balloon that the back of the cone pushes against. This air mass helps control the movement of the cone and also affects the damping of the system, making the speaker sound more in control. A designer who uses a big magnet, tight surround and tight spider is designing for a low system Q, good damping, and good transient response. In other words, when you hit it with a sharp attack note, it responds and then stops very quickly rather than ringing on and on. This speaker would work well in an open back. However, in a sealed or closed back, the air mass spring we discussed earlier would add to the damping, potentially causing the speaker to be overdamped, compressed, and lifeless. It's a difficult design task to design a speaker that is a good compromise for all shapes, sizes and types of cabinet.
Take it easy...
Percy
