How do I Calculate the Size of a Ported Box Speaker?

Written by dan ravens
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The size of a ported box speaker is calculated in two ways: mathematically using established equations, or via measurements provided by established Thiele-Small principles for specific loudspeaker sizes. Thiele-Small parameters, (T/S parameters) were developed to standardise and ease speaker building procedures for do-it-yourselfers who are not experts in speaker design and engineering.

Building a speaker is challenging, but the cost savings are dramatic--you can save as much as 90 per cent over an equivalent manufactured speaker.

Skill level:
Moderate

Things you need

• Loudspeaker/driver
• Thieles/Small parameters
• 3/4-inch birch plywood or MDF
• PVC pipe
• Hacksaw

Instructions

1. 1

Buy the component loudspeaker(s). The loudspeaker is the main factor in calculating the size of a ported box speaker.

2. 2

Employ the Thiele/Small parameters. T/S parameters are provided by speaker manufacturers and measure the performance characteristics of the component loudspeaker that dictate the box size.

3. 3

Determine the port shape. Speaker ports are generally round, but they can also be quadrangles (called vents rather than ports). Easy formulas help determine port dimensions.

4. 4

Use PVC pipe for round ports. PVC pipe is dense and economical. Calculate the port diameter and length with this formula:

LV= [(1.463 x 10^7 x R^2) / (Fb^2 x Vb)] - 1.463xR

Fb = the desired tuning frequency of the enclosure in Hertz.

LV = the port length in inches.

R = the port's inside radius.

Vb = the internal volume of the box in cubic inches. To convert cubic feet to cubic inches, multiply by 1.728.

Note: Height x Width x Depth = Cubic Volume. But remember, speaker manufacturers usually recommend box volume dimensions for the component loudspeaker in the T/S parameters.

5. 5

Use the same wood as used for the speaker cabinet to construct squared ports. Square ports are an aesthetic choice over round ports, provide more port area where cabinet size is limited, but are more prone to air turbulence (noise) than round ports as air moves through the port.

Calculate the port internal volume and length with this addition to the formula in Step 4:

This provides the value of R:.

R = square root of (A/Pi)

A = the area of the square vent (height x width)

Pi = 3.141592.

6. 6

Calculate the size of a ported box speaker with all the above numbers. Cut the box material to size and build the speaker box.

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