Custom Audio Designs Ltd
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Bass Trap Basic Design PrinciplesBass Trap Basic Design PrinciplesBass Trap Basic Design PrinciplesBass Trap Basic Design PrinciplesBass Trap Basic Design Principles

Bass Trap Basic Design Principles

Acoustic membranes, wood or plasterboard sheet can be very effective at the absorption of low frequency sound especially in small spaces where studwalls can often be built from them. These walls will act as bass absorbers and so should be taken into account when constructing a studio of this type. Any sheet material will resonate at a particular frequency depending on the relationship between the mass of the material and the distance it is spaced from a solid wall surface. This resonant frequency can be calculated by dividing 60 by the square root of the panel mass (in Kg/m²) multiplied by the distance from the wall in metres.

Bass Trap Basic Design Principles

Simply put, the heavier the panel and/or the deeper the air space behind, the lower the resonant frequency of absorption will be. If mineral wool is included in the cavity space behind then the frequency band over which the absorber will work will be widened.

i.e. our 5kg soundproofing mat securely fixed over a frame of the following depths will absorb as follows:-

  • 120Hz @ 50mm
  •  98Hz @ 75mm
  •  84Hz @ 100mm
  •  60Hz @ 200mm

Fig1. shows how plywood sheet performs when used in this manner. If the plywood were substituted for soundproofing mat or plasterboard, which has a greater mass, the absorption frequency would be much lower.

For example 12.5mm plasterboard on 75mm studwork will resonate at around 63Hz. Two sheets of 12.5mm plasterboard will resonate at around 50Hz also over a 75mm cavity.

 Bass Trap Basic Design Principles

Obviously the problem with rooms built from studwork is that as a large amount of surface area is taken up with effectively a 'uniform depth bass absorber' which will have substantial bass absorption but only in a particular frequency range. This may or may not be convenient to you. In a masonary walled room the bass reflections will be that much stronger and you will almost certainly be in the need for a large amount of low frequncy absorption areas if you want a clean sound.

Fig2 shows a simple single depth absorber. It is essential that the mineral wool behind the panel does NOT touch the front panel itself otherwise the panels vibrations will be damped down.

 Bass Trap Basic Design Principles

Bass frequencies tend to collect in the corners of a room so in a small room where space is at a premium it's best to build or place Bass Traps in the wall/ceiling soffits. This also makes the maximum use of the extra depth available and because an angled panel, across a corner, has a varying cavity depth the frequencies that are absorbed will be even wider than those of a constant depth absorber.

Fig3 shows a simple corner mounted panel absorber. It is essential that the mineral wool behind the panel does NOT touch the front panel itself otherwise the panels vibrations will be damped down.

Most of our acoustic absorption and diffusion products will absorb substantially more low frequency sound if they are mounted away from the walls, or across the corners, of a room.

 Bass Trap Basic Design Principles

Probably the most efficient bass absorber is the quarter wavelengh trap. This is a large absorbent cavity which has a depth relative to the frequency in question. The outer face and the inner damping act togeather in a pumping type effect which makes the traps extremely efficient especially at the lowest frequencies whos wavelengths are extremely large.

Fig4 shows a simple quarter wavelength bass trap.

Most of our acoustic absorption and diffusion products will absorb substantially more low frequency sound if they are mounted away from the walls, or across the corners, of a room.

 Bass Trap Basic Design Principles

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