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Sep 29, 2004

The journal Nature will be publishing an article on the source of a natural “hum” (NPR, New Scientist, Google News) present in the ground. The NPR article mentions that the earth vibrates at a rate of 1 cycle over 300 seconds which corresponds to a frequency of 3 milliHertz - not something you’re likely to measure with an accelerometer.

The source of the hum is apparently wave activity caused by seasonal storms in the north and south Pacific during the winter seasons in the respective hemispheres.


I got a response from a reader regarding his experience with Bose’s Personal Amplification System:

I only just now read your 3 APR 04 and 20 OCT 03 blogs on the Bose L1 line system PA. I’ve been fascinated with this system since I first saw it at a Guitar Center store. I’m a jazz vocalist/guitarist and not terribly concerned with frequencies below 80Hz ( the lowest E on the guitar ). The Bose sounds excellent with acoustic guitar/ voice without the bass modules connected.

We did some A-B testing (with/without a bass module plugged in) and discovered this: If you have program material (guitar, in our case) going through the system, and unplug the bass module, there is about a one second period during which the sound from the column is very tinny (lacking bass). Once the system detects the absence of the bass module, the EQ is adjusted to compensate, and it sounds good again. There is _extremely_ heavy EQ going on here.

He went on to note (as I did) that the PAS look like Bose’s earlier PA column speakers. Kinda emphasizes the point that the speaker itself is nothing new, but the application is novel.

I’m not surprised about the heavy EQ, Bose’s self-powered speakers use a lot of equalization to compensate for deficiencies in the speaker drivers. Whether this is a good or bad think I’ll leave for you to decide.


Peter Hotton of the Boston Globe (via since the original article is only available for a fee) addresses a reader’s concern that excessive ‘stomping’ can cause damage to a home. Short answer: it’s nothing to worry about.

It brings up a point that we consultants have to make time and again when it comes to vibration analysis. It can be understandably scary when some source (be it train, plane, industrial or construction) causes perceptible vibration in your home, but that doesn’t automatically mean that the vibration will cause damage.

Really, it’s true.

With the exception of extreme cases (nearby blasting, vibratory rollers at your porch, impact pile driving right outside your window), you don’t have any more reason to worry about most vibration sources causing damage to your home than, say, an angry teenager slamming his bedroom door.

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