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28 januari, 2011

Sing along

No, I am not going to talk about the usual response I get from wife, daughter and son when I can´t help myself and sing along to a catching piece. But there is a common denominator here with what this entry is about.

Been to a hifi shop lately? See any speakers there? Ok, lots, I thought so.

A fair share of all speakers on the market employ some form of reflex function to enhance the bottom end. This function is a so called Helmholtz resonator. A cavity, open in one or both ends will resonate at a fundamental frequency plus its harmonics , related to the volume of air in this cavity.

Air has mass, approximately 1,2kg´s per cubic meter. Not insignificant. After the onset of the first tone, transient etc. the resononant cavity will at first resist to change its state, then it will gradually follow. Once the tone disappears the air in the cavity is now mass in motion and will resist stopping, gradually decaying as acoustic energy (kinetic energy in air molecules)  is converted to heat.

Back to the hifi shop, in this environment you listen to a creation of your liking, but would you believe this is the worst possible place to do it in?  Why is this so?

As the target speakers play out the remaining sets of speakers will wake up and their reflex ports and speaker drivers will come alive. What happens is that you may get either an increase or decrease in the rooms acoustics that will color the listening experience severely.  It will also impair the rooms response to impulses.

One pair plays and all the others sing along.

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