The Box-Building FAQ
20 October 2018
1.0 TUNING QUESTIONS
- 1.01 - Choosing the right type of enclosure
- 1.02 - Tuning tools
- 1.03 - Checking the resonance frequency
- 1.04 - What if the box is too small?
- 1.05 - What if the box is too large?
- 1.06 - Can I use two short ports to replace one long port of the same diameter?
- 1.07 - What if I'm using more than one driver?
2.0 ENCLOSURE-BUILDING QUESTIONS
- 2.01 - Checking the volume of the enclosure
- 2.02 - Box-building material
- 2.03 - Where should I place the port(s)?
- 2.04 - Is the shape of the box important?
3.0 DRIVER QUESTIONS
- 3.01 - Which driver is better, driver X or driver Y?
- 3.02 - Which will be louder, driver X or driver Y?
- 3.03 - Can I use a car audio driver in a home subwoofer?
Make sure that the driver is suited for the enclosure that you have in mind! For example, just because software or equations predict that the driver may perform great in ported enclosure doesn't mean that it WILL perform well. Read the literature included with the driver (or talk to the manufacturer) to determine what's the best enclosure for the driver.
The list of basic tools that can be used for tuning your enclosure is as as follows:
- A digital multimeter (voltage, current, resistance, and if possible, frequency)
- An accurate basic frequency generator (a software-based frequency generator that works with your PC's sound card can be used for this purpose). Once such frequency generator is the NCH Tone Generator, available from the NCH site at www.nch.com.au .
- A high-power (>5 watts) low impedance resistor, ideally between 4 to 8 ohms.
At the resonance frequency of the subwoofer system, the impedance will either peak at its highest level (sealed enlosures), or dip to its lowest level (ported and bandpass enclosures). 6th order bandpass enclosures will have two resonance frequencies, one for each volume.
To determine the resonance frequency of the system, set the multimeter to current measurement, then hook it up in series with the subwoofer to your amplifier, then use the frequency generator to drive the subwoofer. At the resonance frequency, the meter will read the lowest result (sealed enclosure) or the highest result (ported and bandpass enclosures). In the case of the ported and bandpass enclosures, If the measured resonance frequency is not the same as the predicted resonance frequency, adjust the port lengths accordingly. In the case of the sealed enclosure, the resonance frequency can be adjusted by adding or removing stuffing material from the enclosure.
A more accurate version of the above method involves using a resistor in series with the subwoofer system, and connecting the multimeter (set to voltage mode) across the resistor. In this case, the meter will measure the lowest voltage at the resonance frequency of a sealed system, and the highest voltage at the resonance frequencies of the ported and bandpass systems.
If the box is too small will typically result in a boomy system that appears to have strong midbass and less low bass. Typically power-handling is improved though, unless the box is REALLY too small! You can compensate somewhat for a small box volume by adding stuffing to the box. The stuffing can make the box appear up to 40% larger to the driver. Note that sealed systems are generally a lot more tolerant of variation in box volume than are the other tuned-port systems such as the ported and bandpass systems.
Typically the low frequency extension of the system improves a little, at the expense of powerhandling. The bass will also sound tighter and "drier", which might not suit your listening tastes. In the case of vented systems, a too-large box may introduce a response peak around the the resonance frequency of the system.
No! The resonance frequency (Fb) is proportional to the cross-sectional area of the port, and inversely proportional to the length. If you increase the effective cross-sectional area by adding another port, the length will have to INCREASE as well, otherwise Fb will go up!
Simple - just multiply the driver's Vas by the number of drivers you intend to use, then use this new value of Vas in the box calculations. For example, if your driver's Vas is 3 cu.ft., and you're using two of them, then use 3x2=6 cu.ft. in your calculations. The exception to this rule is if you're going to use an isobaric arrangement (where one driver is firing into another) - in that case you divide the Vas by the number of drivers you intend to use (normally two).
Almost any material that has good stiffness and self-damping capability can be used for building a subwoofer box. Good quality 3/4 plywood or MDF are certainly suitable for building a sub enclosure, and both of these materials are usually easy to obtain. Large panels should be braced properly. Place braces asymmetrically across the inner surface of the panels. Don't forget to include the volume occupied by the bracing when determining the design's final volume!
For in-car use, fiberglass can be used to build a strong but light subwoofer box, but note that adequate bracing should be used and the use of fiberglass to make any large flat panels should be avoided.
Make sure that you add the volume displaced by the driver, ports and bracing to arrive at a final enclosure volume. This is especially important in the case of bandpass systems, where the volumes displaced by the ports and driver can have a significant effect on the enclosure volume.
You can measure the volume of the driver by
- wrapping it in plastic,
- dunking it in a bucket full to the brim of water until the mounting flange is level with the brim of the bucket
- removing the driver from the bucket and
- using a measuring cup to refill the bucket.
The volume displaced by the ports can be calculated from the following equation:
Vp = (Lv-Ex)*PI*(Dout/2)^2
where,
Vp = Volume displaced by port
Lv = port length
Ex = enclosure thickness
Dout = outside diameter of port
If your system calls for the use of ports, these can be placed on any side of the enclosure, once the intended location meets the following requirements:
- The exit of the port should be at least one port diameter away from any external surface.
- The entrance of the port should be at least one port diameter away from any internal surface, including the driver.
Box dimension ratios are not important, once the system is only going to be used to produce low bass frequencies. Just make sure that you leave enough space between the driver's magnet and the rear panel - usually up to 3" of clearance is requred, to allow unrestricted operation of the pole piece's vent (if the pole piece is vented, of course).
A common question, and difficult to answer correctly without some real-world experience with the drivers in question. T/S parameter analysis can give a fairly good idea of each driver's capability, so probably the best approach would be use a software design tool (like the spreadsheets available on this site) to determine how each driver performs in a suitably designed enclosure, then choose the one that more closely meets your requirements.
The answer to this question is similar to the one given above. However, you should use the software design tool to model the peak linear output capability of each driver with, and this is important, the amount of power that you intend to feed the drivers. For example, many of the high-Xmax drivers on the market these days also have a somewhat lower sensitivity when compared to "normal" drivers, the result being that you have to drive them with considerably more power to achieve the same SPL levels as the latter drivers.
The answer to this question is "it depends". Typically, car audio drivers will not do well as drivers for a home subwoofer system, as they are usually designed to produce a good response when a car's cabin gain is taken into consideration. However, there are a few for which the T/S parameters suggest that they can produce good results in the home. If you are thinking of using a car audio driver in your design, simply treat it as a home audio driver and design a system based on its T/S parameters to get the best results.