A loudspeaker in cross section as seen from the side. The voice coil is a moving part attached to the apex of the cone below the domed dust cap; it does not touch the magnet structure in which it is very precisely seated. In the most common form of blown speaker, the voice coil is damaged such that it does rub or scrape against the magnet structure.
In the home and automotive audio world, “blown speakers” are fairly common. Many use the term in a sort of generically non-technical way to describe speakers that aren’t working right or aren’t working at all. An unfortunate inconvenience, mostly.
For musicians, though—especially working musicians—blown speakers in a guitar amp (or bass amp or PA system) are far more than an inconvenience; they can be a major problem that threatens a gig or a session. It happens in music equipment far less often than it does in home and automotive audio, fortunately, but it does happen and it raises several questions: What does the term “blown speaker” actually mean? What does a blown speaker sound like and how do you know if you have one? What blows a speaker? And what should you do if you have a blown speaker?
Let’s take these one at a time.
What does the term “blown speaker” actually mean?
A “blown speaker” is one that doesn’t work right or doesn’t work at all. It’s an umbrella phrase that encompasses several problems that could cause a speaker to sound unpleasant or go silent, but suffice it to say that a blown speaker makes either bad sound or no sound.
What does a blown speaker sound like, and how do I know if I have one?
Oh, you’ll know. The most common aural indication of a blown speaker is an unpleasant buzzing or scratching sound, by itself or roughly at the pitch of the note the speaker is attempting to reproduce. Or there could be no sound at all.
What has gone wrong in a blown speaker?
Could be several things. Usually, the speaker’s voice coil has been damaged such that it rubs against the walls of the magnet structure in which it is seated—that’s what causes the buzzing or scratching sound.
Let us explain some basic speaker anatomy here. The voice coil consists of wire coiled around a cylinder (called a former); this assembly is attached to the apex of the loudspeaker cone (see photo) and is a delicate moving part that is neatly seated in a narrow circular gap in the speaker’s magnet structure without actually touching it (see diagram). Speaker anatomy is a very exact science—the voice coil fits in that narrow gap with only 0.010” to 0.020” clearance on either side, moving in and out as the signal dictates and thus providing the electromagnetic force that moves the speaker cone in and out, thus reproducing the desired sound. Other speaker parts such as the cone, frame (or basket), surround and spider (see diagram) work together to hold the voice coil in the correct orientation while allowing it the required freedom of movement.
An upended loudspeaker assembly on which the voice coil (red), former (green), spider (yellow), cone (dark gray) and surround (light gray) are clearly visible.
Voice coil damage can be thermal or mechanical. In thermal damage, so much electrical power is dissipated in the voice coil in the form of heat that, if the temperature gets too high, one of two bad things happens. One, the adhesive that binds the wire coils to the former fails and the wire literally comes off of it. Two, the wire insulation fails, short-circuiting the coils and interrupting the circuit so that no signal reaches the cone (resulting in silence). In mechanical damage, the speaker cone itself or its support structures (such as the spider, the surround or the basket) have been bent, broken, torn or otherwise damaged such that the voice coil rubs against the inner or outer magnet structure walls.
What blows a speaker?
Blasting it with too much power for too long.
It’s worth noting here that in properly matched combinations, speakers are designed to handle whatever their amps can dish out at extremely high levels and for far longer periods than would ever likely be encountered in everyday use. Amp manufacturers use intensely rigorous testing procedures to ensure this level of quality and compatibility, making blown instrument amp speakers a highly improbable occurrence. Nonetheless, it is in the nature of any technology to experience occasional problems, and speakers do blow once in a great while despite the best efforts of the amplification industry to ensure otherwise. It’s improbable but not impossible.
What if it’s just a tweeter that’s blown—how would I know and what should I do?
Setting aside the interesting question of why most electric guitar amps don’t have tweeters, it’s easy to tell if your bass amp or PA rig has a blown tweeter because they either work or they don’t (there’s no buzzing or scratching as with coned speakers), which should be obvious on close listening. Fortunately, tweeters are comparatively simple devices that are repaired or replaced relatively inexpensively.
What are my options if I have a blown speaker?
Repair or, more likely, replacement. More expensive component loudspeakers that are sold individually, such as those by JBL and Electro-Voice, can often be repaired and re-coned (re-coning a speaker means not just replacing the speaker cone, but replacing all the moving parts that constitute the speaker cone assembly; this includes the voice coil).