Why Some Subwoofers Rattle and What to Do About It

Written by: Zach Wright

Subwoofers often rattle when there are loose components, but it they can also rattle if they are underpowered or overpowered. It’s important that you do an inspection on your subwoofers before each and every performance.

Subwoofers are crucial devices for your home theatre system. Whether you use them to play back your music or just watch movies, they can help by enriching the sounds so you can enjoy the full audio experience.

Being delicate electrical instruments, they aren’t immune to damage. Many individuals find that their woofers often start rattling. They note that these subwoofers, instead of elevating the experience, actually end up deteriorating it.

How Does Your Subwoofer Work?

a subwoofer on the ground of a street

A basic understanding of your subwoofer is important before you try to diagnose the problem. The subwoofer includes the following basic components:

  • Voice coil and former
  • Cone
  • Voice Gap
  • Surround
  • Spider
  • Backplate

For those who are unaware of the science behind acoustics, an amplifier turns sound waves into sine waves of AC current. AC current works by changing the voltage between positive and negative values. The amplifier might provide the sub with a positive voltage. In this case, the voice coil and cone move forward accordingly.

When the amp provides the sub with a negative voltage, the coil and cone move backward by that amount. The distance they move depends on the value of the voltage.

The forward and backward movement allows the cone to push the air and produce sound.

Why Your Subwoofer Might be Rattling

There may be various reasons why your subwoofer is rattling. Some of these involve issues with the components while others may be your fault. All in all, these reasons may be:

Loose Components

One of the most obvious reasons for rattling is loose components. There are various key components inside the subwoofer such as the voice coil, the voice gap, and the outer cabinet.

What to Do About It

We recommend extensively inspecting your subwoofers to find if any components are loose. First, check the feet, if they have any. If the screws on the feet are loose, tighten them with a screwdriver. Even if you find them to be intact, the loud vibrations from the subwoofers may be causing them to move too much.

The effect is amplified if the feet are unbalanced. Use a level on top of the subwoofer to see if the surface is straight. Next, unscrew the cabinet and inspect inside for any broken components.

If everything is secure in its place, tighten the cabinet and secure it. Also, check the grill and screw it down. You can even replace it if you think it doesn’t fit right. If the rattling continues, then perhaps you need to change the voice coil.

Overpowered Subwoofer

Each subwoofer comes with a certain rating. This defines how much power it can handle. If you turn the volume up too much, it might start to distort the sound. The result may be a rattling sound inside the sub. Not only does this damage the sub, but it sounds horrendous as well.

Increasing the volume of the signal results in an increase in the signal voltage. If you increase the volume more than its limits, the cone might move forward too much. Since these components were not designed to move beyond these limits, it ends up tearing your cone, surround and other components.

Similarly, the negative voltage signal will tell the coil to move backward too far. The voice coil might collide with the backplate of the subwoofer. This can severely damage the former and the coil. In fact, the coil might even end up jammed inside the voice coil gap.

What to Do About It

Check for the safety and wattage rating of your subwoofers. A good idea is to check out the RMS ratings as they are safer to use. The peak or maximum ratings may still allow you to overpower your speakers. RMS ratings are the average or the safest values to follow.

Underpowered Subwoofer or Faulty Amplifier

It is also possible to underpower your subwoofer. Playing lower than RMS ratings is not the main culprit. The real trouble is when your amplifier is sending out clipped signals causing the sub to damage itself. It can heavily affect the quality of the sound, causing rattling, sizzling or popping.

If the signal from the sound source is too high and your amplifier isn’t designed to handle such high voltage signals, it can clip the signals. It simply distorts the signal and reduces the quality of the sound.

The resulting waveform is square and not sinusoidal. When this waveform passes through the subwoofer, it will try to move the cone backward and then forward very quickly. The gradual sinusoidal slope prevented this, but the vertical sides of the clipped signal do not.

This constant quick movement easily tears these components apart or wobbles the cone too much, causing rattle sounds.

Also, note that the clipped waveform is at a constant position at its maximum and minimum values. This causes the cone to hold still for a long time. It can require an intense amount of energy, causing the coil to heat up.

The coil may end up burning all the way through, causing it to unwind or simply deform itself. Another point to note is that the square waveform has twice the RMS rating than its sinusoidal counterpart.

So, you end up with a coil that is moving from one position to the next too fast while holding itself still for a long time -- at almost twice the recommended capacity. Thus, a faulty amplifier spells a recipe for disaster.

What to Do About It

Make sure that your amplifier is capable of handling high volume signals. If it cannot, then either change the amplifier or simply reduce the volume.

Final Thoughts

All in all, subwoofers are an essential item on any audiophile’s list. On the other hand, faulty subwoofers can damage the acoustic experience rather than helping it. We recommend you religiously follow the RMS ratings on both your subs and your amplifier.  

Written By:
Zach has a vast experience in digital audio and sound design. Being a studio owner for 13 years, he actively helps musicians and producers with technical issues around musical instruments and studio and audio equipment.

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