# Can two loudspeakers one next to other, damage them self?

by scientist91
Tags: damage, loudspeakers
 P: 133 Can two loudspeakers one next to other, damage them self? I think the question is very clear, and if you know please answer. Thank you.
 PF Gold P: 8,963 The only way that I can think of damage occurring is if one very powerful speaker is face-to-face with a second one. In that case, the air pressure from it might tear the cone of the other. Even that isn't likely.
 P: 689 hmm, if the two speakers oscillate with some matching sound waves, could there be a catastrophic resonance?
P: 219

## Can two loudspeakers one next to other, damage them self?

 Quote by scientist91 Can two loudspeakers one next to other, damage them self? I think the question is very clear, and if you know please answer. Thank you.
No, they can't
P: 1,504
 Quote by antonantal No, they can't
Not even if they generate the same (high amplitude) signal but in phase opposition?
P: 219
 Quote by lightarrow Not even if they generate the same (high amplitude) signal but in phase opposition?
No. Why do you think they would damage each other in this case? The only thing that would happen in this case is that you would hear less sound from both of them then you would hear from one alone.
 Sci Advisor P: 5,095 The only way is if on happened to produce a resonance frequency that corresponds to the natural frequency of the cabinet or some other component. They would have to provide a decent amount of energy though.
 P: 949 There is nothing unique about having two speakers, other than the overall power you are dealing with is doubled (if they have the same power). This could potentially be a problem only if:The cabinet is sealed The speakers are operating near their maximum power The cones are placed towards eachother close enough to nearly form a seal to keep air out At least that's all I can think of. As for having two speakers, even if you have complete constructive interference at some point, the power at that point will only be twice the average power anyhow. Usually that won't damage a speaker.
 Engineering Sci Advisor Thanks P: 6,031 A loudspeaker in a properly designed cabinet does not have any undamped resonant frequencies. If it did, they would distort the sound it was producing. Also speakers are very inefficient at transferring energy into the air. Good quality speakers are often only 1% efficient. So the amount of energy the air could put back into the speaker is small compared with the amout of electrical energy driving the speaker, and "resonance" would not do any damage. However, if you take a speaker which is NOT mounted in a cabinet, it does have a natural resonance at a low frequency (lower than its operating frequency range). It's very easy to destroy a speaker in free air, by applying a sine wave signal at much less than its rated maximum power, at the self-resonant frequency. That has nothing to do with two speakers interacting with each other, though.
 Sci Advisor P: 5,095 I was really reaching with the resonance angle. I didn't say it was likely to happen, but the possibility is always there as with any structure.
Engineering
Thanks
P: 6,031
KingNothing sent me a PM with this question, and I'll try and answer it here since others might be interested:

 Hi. Could you please elaborate on speaker cabinets having no natural resonance, and also on how such a thing would distort the sound? I am familiar with sound and resonance, just not the design of cabinets.
To be specific, let's talk about a fairly large (say 12in diameter) moving coil speaker mounted in a cabinet which is a sealed box. (And I'll mention ported or "bass reflex" cabinets at the end)

Mechanically, the speaker is basically a mass (the centre part that is driven electrically) on a spring. The mass-and-spring system does have a resonant frequency, but there are two sources of damping which stop it resonating freely.

One damping effect is the speaker cone, which transfers energy to the air as it moves.

The other (usually larger) source of damping is electrical. You can think of a speaker as an electric motor. Like any motor, if you try to drive it in reverse, it acts as a generator. If the speaker cone tries to move (for example oscillating at its resonant frequency) a current is generated in the coil. This current flows "backwards" through the amplifier, takes energy out of the speaker, and damps the motion.

Note that if you experiment with the mechanical resonance of a speaker (e.g. get a 12in speaker, tap the center of the cone with your finger, and watch and listen to what happens) you won't get the electrical damping effect unless you short-circuit the speaker terminals together with a bit of wire.

The function of a sealed speaker cabinet is to absorb all the sound that would be radiated from the back of the speaker cone, otherwise the sounds from the front and back of the cone would interfere with each other and cancel out. The actual box is made of stiff material (e.g. thick wood or plastic) so it doesn't vibrate and transmit any sound from the inside to the outside of the box. The air inside the box can resonate, the same as in lab experiments showing resonance in an air column, but the cabinet contains absorbing material to damp out that resonance, and the electrical damping described above stops the air from moving the speaker cone easily.

So to summarize, the idea is to reduce all the resonances from the system as much as possible.

If a speaker system did have its own resonance, the sound quality would be poor. To reproduce a single pulse of sound (e.g. something vaguely like a bass drum) you want the speaker cone to move in a single pulse and then stop, like the red graph in the attachment. If the speaker had its own natural resonance, it would move more like the blue graph.

Finally, ported or reflex speakers. These are very common as sub-woofers on PC and home cinema systems. They are not sealed boxes. There is a large hole visible in the front or the side of the box. This is a clever design to increase the volume of low frequency sound from a small speaker.

The hole is at the end of a tube, inside the box. This tube acts like a pipe. As in lab experiments with resonance at the lowest resonant frequency of a pipe, the air is moving in opposite directions at each end of the tube. The clever bit is that the "tube" works as if it was U-shaped, so the air coming out of the hole at the front has the opposite motion the the air at the back of the speaker cone, i.e. the same motion as the air at the front of the speaker cone. The result is you get twice as much sound energy compared with a sealed box. The audio quality isn't very good, but that doesn't matter much for reproducing explosions in video games etc.
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