FAQs & Advice

No good guide to building a subwoofer box would be complete without a list of commonly asked questions that can help you out. That is why this section is designed to help you in dealing with frequently asked questions and odd little bits of advice.

1. Tuning How do I check the resonance frequency?

At the resonance frequency of the subwoofer system, the impedance will either peak at its highest level (sealed enclosures), or it is going to dip to its lowest level (ported and band pass enclosures). 6th order band pass enclosures will have two resonance frequencies, one for each volume.

If you want to accurately determine the resonance frequency of the system, set the multimeter to current measurement, and then hook it up in series with the subwoofer to your amplifier, then you can use the frequency generator to drive the subwoofer for you.

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, you can easily 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.

2. What happens if the box is too small?

If the box that you make is too small it will typically result in a boomy system that appears to have strong midbass and less low bass. Usually power-handling is improved though, so unless the box is REALLY too small, its fine. 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.

3. What happens if the box is too large?

Usually the low frequency that comes with the system improves a little, but it will happen at the expense of power handling. The bass will also sound tighter and drier than it should, which might not be what you wanted. In the case of vented systems, a box that is too-large may introduce a response peak around the resonance frequency of the system.

4. Can I use two short ports to replace one long port of the same diameter?

No you cant! The resonance frequency is completely 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 be increased also, because otherwise Fb will go up!

5. What if I'm using more than one driver?

This is much easier because all you have to do is multiply the driver's Vas by the number of drivers you intend to use, and 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 you will have to use 3x2=6 cu.ft. in your calculations. The only exception to this rule is if you're going to use an isobaric arrangement.

6. Where should I place the port?

If your system requires you to use ports, these can be placed on any side of the enclosure, once the intended location meets these requirements:

The exit of the port should be at least one port diameter away from any external surface and the entrance of the port should be at least one port diameter away from any internal surface, including the driver.

7. Is the shape of the box important?

The shapes of the boxes you build are not important, once the system is only going to be used to produce low bass frequencies. However you have to make sure that you leave enough space between the driver's magnet and the rear panel to allow unrestricted operation of the pole piece's vent which is usually about 3.

8. Which is better, driver X or driver Y?

This is a very common question, and it is also difficult to answer correctly without some real-world experience with the drivers in question. T/S parameter analysis can give a good idea of each driver's capability, so it may be best to use a software design tool to determine how each driver performs in a suitably designed enclosure, and then choose the one that more closely meets your requirements.

9. Which is louder, driver X or driver Y?

This is a lot like the answer to the above question. 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.


10. Can I use a car audio driver in a home made subwoofer?

Maybe. Typically, car audio drivers will not do well as drivers for a home subwoofer system, because they are usually designed to produce a good response when a car's cabin gain is taken into consideration. However, I can say that there are a few for which the T/S parameters suggest that they can produce good results in the home.