Sound-absorbing cotton problem in the subwoofer cabinet

Generally, the volume of a 12-inch speaker is 30 to 40 liters, but it is mainly determined by looking at the QTS (Q value) of the speaker. The formula for calculating the volume of the speaker is based on the condition that there is no filler in the box, or that the glass wool of no more than 1 inch thick is placed on each side of the speaker as the damping effect of the standing wave. In theory, the correct speaker design can achieve its design goals without additional processing. However, in the real world, the technique of inserting sound-absorbing cotton into a speaker is very useful, and it can achieve a specific speaker size and Q value, thereby changing the response of the speaker, and there is no other way. In addition to greatly suppressing the problem of sound and light caused by reflections in the speaker (refer to the fourth section of this chapter), adding sound-absorbing cotton to the speaker will also affect the following parameters:

Damping loss

If the sound-absorbing cotton is filled too densely and close to the frame of the unit, it is easy to cause friction loss. According to the tests performed on the Loudspeaker Design Cookbook, the frequency response of the different sound-absorbing cotton fillings will change. For example, 50% of the filling amount is not obvious for the standing wave inside the speaker. In order to enhance the sound absorbing effect, a high-density sound absorbing cotton should generally be adopted. The 100% filling amount will obviously improve the standing wave of the speaker, and the combination of the sound-absorbing cotton mixing ratio of 50/50 for two different materials will also have this phenomenon. In summary, from the characteristics of car audio, in order to use a smaller box and reduce the standing wave in the box to the lowest value, 100% sound-absorbing cotton filling is a good choice.

2. Increased flexibility

The use of low-density, specific heat-resistant materials (glass fiber, Dakron synthetic fiber and long fiber, wool, cotton) can improve the acoustic compliance of the speaker, theoretically equivalent to increase the volume of the speaker up to 40%, practically An equivalent volume increase of about 15%-25% can be achieved.

3. Increased efficiency

Proper selection of fill volume, material form, and placement in the speaker can increase efficiency by as much as 15%.

4. Mass Change Filling the sound absorbing cotton changes the total mass that can be moved within the system. This phenomenon is generally thought to be related to the air flow on the back of the unit. Increasing effective mass results in reduced efficiency, but not as much as efficiency gains due to changes in compliance. Because we don't want to reduce efficiency, there are two ways to limit this effect. The first was used by Advent, with a support frame at the back of the woofer to separate the sound-absorbing cotton. The second is to use a material that is not compressed and has a lower density than the sound-absorbing cotton, and is directly attached to the basin frame of the unit as a buffer between the sound-absorbing cotton and the monomer with higher density.