Plugging a Port - What is Happening to Your Speaker
One of the forums I browse recently had a question come up and I didn’t realize how confusing it was to most people until then. The was regarding whether plugging a port could damage a speaker. Many manufacturers have been including plugs of some kind for their speakers over the last few years and I guess the explanation in the documentation is not thorough enough to help people understand what the purpose and effects of the port plug are.
What are the plugs used for?
There are two types of plugs that I’ve seen sent with speakers before. The first is an open cell foam plug and the second is a closed cell foam or rubber plug. Open cell foam plugs allow some air to continue flowing through the port but not at the same level as being completely unblocked. This reduces the overall port output. Closed cell foam or rubber plugs completely block airflow and make the enclosure act as a sealed design.
Manufacturers send these because they don’t know in what kind of room the speaker will eventually be placed. Ported speakers in small room can sometimes sound boomy because they excite more of the room modes or get too much low bass reinforcement from the surrounding walls and other boundaries. This is why many people think sealed boxes sound “tighter” than ported – because sealed boxes roll-off often integrates better with the room reinforcement in smaller rooms.
Many manufacturers also recommend using the plugs if the speaker will be placed close to the wall for the same reason. However, I will caution that placing a speaker right up against the wall will reinforce more than just the low bass frequencies where a port is operating. It can potentially add reinforcement all the way up to 1500 Hz and throw off the tonal balance of the speaker. This is why speaker placement can be very important.
Some subwoofer manufacturers also send plugs for their subs. In these cases, the manufacturers typically have built the enclosure with multiple ports. In this instance, when you plug one of the ports, you are changing the overall tuning of the system. I might write a future article on the principles behind this, but for now just know that when you plug one of the ports, you are lowering the tuning frequency of the subwoofer. This obviously has some benefits for HT use but it also has downsides because by decreasing the total area of port openings, you are increasing your risk of getting port chuffing.
Will this cause damage?
There are really only two things that can damage your speaker in a home environment. Either the speaker had a defect from the factory or the speaker has been over-powered. Over-powering can cause damage in two main ways. One is causing a thermal failure of components due to the heat. Typically, this causes the enamel that coats the voice coil wire to get hot enough to melt and then you get a short in your voice coil. The second is by over-excursion. Over-excursion can cause a driver to incur physical damage to the voice coil (knocking wire loose) or the suspension (spider or surround separations). In a ported system, woofers unload below port tuning. Unloading is when the box provides no restorative force on the driver and the driver shows extreme excursion. So if you have a ported pox tuned to 50 Hz and played a 30 Hz tone, you would see some extreme excursion from the driver. A sealed box does not do this. It as the woofer tries to move more, which is required to play lower tone, the air inside the box tries to resist that movement and therefore will not see the same excursion. Here are two graphs with a typical 6” woofer and at 30 watts input power to illustrate the difference between ported and sealed in the same size enclosure. The ported enclosure is tuned to maximally flat at 45 Hz and the sealed system used the same size enclosure for reference.
The point is that there is that sealing the port will not damage your speaker. In fact, you are probably reducing the chances of damage by sealing up a ported speaker. Now ideally if you were designing both enclosures from scratch, the sealed enclosure would actually end up being smaller than the ported and the excursion would be reduced even further.
What effect does this have on max SPL?
I was also asked about the maximum SPL between the two box designs. Max SPL is difficult to answer because it depends on what frequency you are looking at. In the example I showed, the input power on both the sealed and ported were the same so at frequencies below the fb of the sealed enclosure, the ported will have more max SPL output. Ports or passive radiators make a system more efficient at lower frequencies. Above the roll-off frequency of the sealed box, both systems should be identical. However, the true answer is very complex once you start looking at a musical signal with varying frequency response. The dip in excursion for the woofer in the ported graph coincides with the port resonance. At that point, the driver is almost completely stationary, and the port is providing almost all the output. If you were to only measure at this specific frequency, the ported design would have drastically more output. But once you start looking at frequencies below the port tuning, the sealed design controls the cone better and prevents the excursion from becoming as extreme. Below the port tuning, you might only be able to send 10-20 watts to a small bookshelf woofer before it starts to distort. In addition, the excursion doesn’t just cause distortion at low frequencies, but over the entire bandwidth of operation. The question most people are probably wondering now is why wouldn’t most ported speakers distort like crazy given what I just said. Well, most music content really lies at around 50 Hz or above and there is almost no bass outside of synthesized material below 30 Hz. So if a typical bookshelf is tuned to 50 Hz, you have low excursion centered at 50 Hz but some to each side because of the port output. This is right where a majority of bass content actually lies, so most music won’t cause the speaker to reach over-excursion at the 10-20 watts input power.
The overall message here is that it’s impossible to damage your speaker by plugging the port. Plugging may or may not provide tighter bass, depending on your room and speaker placement. It will roll off the bass earlier like a sealed design and it will provide some excursion protection at very low frequencies in a bookshelf. Lastly, if you don't have a plug but want to try it out to see if it helps your speakers, use a rolled up sock to stuff the port.