# Are decibel levels cumulative?

I have a computer with a case fan running at 26dba.

If I install more fans which also run at 26dba, will there be more noise produced?

(this looks like a really stupid question, but you wouldn’t believe how I’m second-quessing myself on this one. And not knowing anything about acoutics/physics… well… you know)

As always, thank you very much for any assistance!

But, it would not add up to 52dba

it should be at most 26 +3 = 29dba.

Making something twice as loud is a 3db increase.

If you went to 4 fans it would conceivable double again to 32dba.

In practice it will probably be somewhat less than a full 3db increase. If any of the sound destructively interferes from the two fans, it will give you less.

Good answer scotth. I’ll just note that to perceive a sound as twice as loud requires a 10db increase. Under normal environmental conditions, 3db is a just noticeable increase in volume. IOW, 3db is a doubling of sound energy but 10db is a doubling of “loudness”.

Another note:

If you can get two devices that make noise synchronized out of phase, it will result in a net reduction in loudness. If it meshes perfectly, it is conceivable to reduce the loundness to zero.

They featured a device like this on Alias recently. I found the actual application difficult to swallow. But since you attest to some basis in fact, please clarify:

1. Would it be possible for a device to hear a noise, then reproduce it out of phase, rapidly enough to achieve this masking effect?

2. Since this is apparently only a reduction in loudness, not sound energy, is it fair to assume that intruder alarms which use sound would not be fooled?

Yes, I used to have one that fit in my pocket with ear plugs. You could definitely tell the difference.

It’s easy to test the effect of out of phase noise generation yourself. Simply reverse the wires (negative and positive) to one set of speakers on a stereo. It will be most noticable on the bottom end which will drop out as some bass frequencies cancel each other out. If the opposite happens and you get more lively bass then your speakers were out of phase before.

Here are a couple links to commercial sources of noise-cancelling headphones:Pricey version for pilots and a cheaper version for general use.

One of the reviews on Amazon said that they only cancel low frequencies. This makes sense. Higher freqs would need faster processing and the shorter wavelengths could be a problem also.

And here’s an Active Noise Control FAQ.

My mom lives in Pocatello, Idaho, which is a pretty small town. There’s an international airport there, meaning that the “flying bus” airplanes it can handle have just enough range to get to Canada. One time when I went to visit, I flew to Seattle on a jetliner, then took two connecting flights on the flying buses. I call them that because they are just like buses, they even have the wide bench in the back. The first plane was incredibly noisy and vibrated a lot because of the engines, but the second was outfitted with a noise cancellation system that reduced the loudness by about 75% or so.

Sound cancelletion devices can only work everywhere if both sources are very close together (much closer than the wavelength of the sound). In that case, there actually is less sound energy produced, and correspondingly less input energy consumed. If the sources are separated by more than that, then there will be cancellation in some directions, and reinforcement in others, so the energy missing from the null spots (nodes) ends up in the loud spots (antinodes).