For open-backed cabinets, you’ll need sufficient stuffing to cover half the internal cabinet surface. For closed-back cabinets, cover 70-80 percent of the internal surface area with stuffing material.
Materials like fiberglass batting or acoustic panels are recommended for stuffing. While polyfil also does the job, it's not as efficient as fiberglass.
Acoustic panels usually come with adhesive backing. For other stuffing materials, you can use spray adhesive or tack them down with staples or nails to the side panels of the cabinet. It’s crucial that the stuffing remains firmly attached to the panels.
Acoustic Insulation Sheets
Acoustic insulation sheets act as a barrier between the speaker and its surroundings. These sheets are designed to reflect sound waves, effectively dampening the effects of resonant frequencies.
The sheets can be placed directly in contact with the speaker or with an air gap in between. This will depend on the specific sound profile you are aiming for. Experimenting with the placement of the acoustic sheets allows you to tailor the overall sound of the cabinet.
If you place the sheets directly on the speaker, they act like a baffle. This affects the high frequencies, creating a more mid-range-focused sound. Positioning the acoustic sheets away from the speaker allows for more reflection and a more natural sound that’s faithful to the original tone.
Soundproofing Sheets
Soundproofing sheets can also help dampen the resonant frequency of a speaker cabinet, thanks to their inherent ability to absorb sound energy.
These sheets typically come in the form of high-density foam or heavy acoustic fabrics, making them effective sound absorbers.
Similar to acoustic insulation sheets, you can place these directly on the speaker for a fuller and warmer sound or with an air gap to retain a more natural sound of the speaker.
For both acoustic insulation and soundproofing sheets, you can use spray adhesive, nails, or tacks to keep them securely in place.
Mass Loading
Mass loading involves attaching a layer of dense and heavy material to the inside of the speaker cabinet. This increases its overall weight, which effectively shifts the resonant frequency to lower levels.
While this technique is generally effective, it can also reduce the overall responsiveness of the speaker, so it’s important to be mindful of the mass of the material you are using.
Common mass loading materials include sheet lead, sand-filled panels, or even soundproofing composites that often come in the form of heavy and flexible sheets.
Acoustic Foam Blocks/Wedges
Another straightforward solution for damping the resonant frequency is to use acoustic foam blocks or wedges within the speaker cabinet. These foam solutions trap sound waves, preventing unwanted resonances from occurring.
The placement of the acoustic foam blocks is relatively flexible, so you can experiment to achieve the desired sound.
They can also be easily removed or repositioned if you later decide to change your speaker’s characteristics. Be sure to ensure the foam blocks are stable and cannot vibrate freely to prevent any unwanted sounds or rattles.