Click here to purchase the entire book in PDF format. Filter model In 1999, Carsten Fog and Torben Holm Pedersen from Delta Acoustics in Denmark published a paper describing what they call the filter model of the perception of sound quality [Fog and Pedersen, 1999]. This model is an excellent way of describing how sound is translated by the nervous system, and how that is, in turn, translated into a judgment of quality or preference by your cognition. These two translation systems are described as ``filters'' which convert one set of descriptions of the sound into another. This is shown in Figure 5.15. Figure 5.15: The ``Filter model'' showing the relationship between objective and subjective descriptions of a sound. Note that the objective contains two components, physical and perceptual descriptions [Fog and Pedersen, 1999]. In the far left side of Figure 5.15 we can see that the initial description of the sound is a physical one. This can be as simple as recording of the sound, an equation describing it (as in the case of a sinusoidal waveform, for example) or a set of measurements on the signal (i.e. frequency response, THD+N etc...). In a perfect world, this would tell us everything there is to know about how we perceive the sound to be, but in reality this is very far from the case. For example, I could tell you that my loudspeakers have a frequency response from 20 Hz to 20 kHz $\pm$0.001 dB and you'll probably think that they're good. However, if their THD is 50%, you might think that they're not so good after all. So, the sound comes out of the gear, through the ear and reaches our eardrums. That signal gets to the brain and is immediately processed into perceptual attributes. These are descriptions like ``bright'' and ``dark'' or ``narrow'' and ``wide.'' These perceptual attributes are pretty constant from listener to listener. We can both agree that this sound is wider than that one (whether we prefer one or the other comes later). (An good example of this is if we do a taste test. We can say that one cup of coffee tastes sweeter than another. We'll both agree on which one is sweeter - but that doesn't mean that we'll agree on which one is better...) So, Filter 1 in the model is our sensory perception - this translates technical descriptions of the sound hitting our eardrums into concepts that normal people can understand. Note that this filter actually contains at least two components - the first is the limitations of our hearing system itself. Maybe you can't hear a 40 kHz tone, so you never perceive it. The second component is the translation into the perceptual attributes. Then we get to the far right side of the model. This is where you make your hedonic (as in ``hedonism'' - the pursuit of pleasure of the senses) or affective judgment of the sound. You decide whether you like the sound or not - or whether you like it more than another sound. The filter that is used to translate the perceptual attributes into preference (your subjective evaluation of the stimulus). Going back to the coffee example, we'll both agree that one cup of coffee is sweeter than the other, but we might disagree on which one each of us prefers. So, from this model, we can see that we can make an objective measurement of a perceptual attribute. If you do a test where you live using your group of subjects, and I do a test where I live on my group of subjects, and we both use the same stimuli, we'll get the same results from our evaluation of the perceptual attributes. Both groups will independently agree that 3 spoonfuls of sugar in a cup of coffee makes it sweeter than having none. However, if we do a subjective test (``Which of these two cups of coffee do you prefer?'') we might get very different results - particularly if you have a group made of Greek vari glykos drinkers and Danes (my experience in Denmark is that many people don't even bother putting cream and sugar on the table when they serve coffee because almost everyone drinks it black). So, be careful when you talk about doing subjective testing - or at least make sure that you mean ``subjective testing'' when you use that phrase.