Acoustics - Tight environments

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Let's picture a room of fixed dimensions and let's have a look at its acoustic characteristics. Depending on the material the walls are made of we will have more or less absorption [Absorption ] and reflection [Reflection ] of acoustic energy. Undoubtedly a portion of the energy will be reflected and, at certain frequencies (which depend upon the room's dimensions), the sound waves shall begin to resonate. In order to better understand this phenomenon, let's imagine a bath-tub filled up with water. If you start to move the water with a constant waving of the hand, you'll notice how, when a certain oscillation-rate is reached, the hand will move in synchrony with the generated waves (which get reflected off the sides of the bath-tub). When this occurs, it means that the hand is oscillating at the bath-tub's resonance frequency.

If we were to repeat the very same experiment in a sink, we'd notice how in order to obtain the same result we would have to wave our hand at a higher frequency; this means that the sink's resonance frequency is greater. From this example it becomes clear that the smaller an environment's dimensions, the greater its resonance frequency. Naturally a room has three dimensions, and therefore three different resonance frequencies in each of the directions in space. These are called resonance modes and we will be taking a more detailed look at them shortly. The presence of these frequencies implies that the acoustic response of an environment varies as frequency changes: some frequencies will be emphasized since they are reinforced by the resonances modes. All of this is very undesirable indeed, in that it doesn't allow for faithful reproduction of sound. In the following sections we'll take a look at the possible countermeasures we can apply to prevent these unwanted resonances from spoiling the acoustic characteristics of an environment.


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