Passive Crossover Networks

 

LA IMPEDANCIA DEL ALTAVOZ

En este momento se intentará destruir un mito, por el cual, la impedancia de un altavoz corresponde con lo indicado por el fabricante; creer en esto es como creer en la cigüeña.

Realmente, la impedancia nominal que se especifica en los catálogos, tiene el único propósito de dividir los altavoces en familias, depending on the type of use which they are intended for. So we'll have 4Ω speakers for car-audio, 8Ω speakers for home hi-fi and 16Ω or more for professional applications.

El dato nominal would approximate el mas cercano de los valores de impedancia verdadera, measured on the moving coil at ciertas frecuencias, típicamente 100Hz, 400Hz, 1KHz, but not only, according to the builder and the type of speaker. Being the nominal datum referred to an only frequency, what's more in approximate way, and being instead the impedance variable with the frequency in real life, it stands to reason that you can't refer to this value for the calculation of the filter. If we then add it's not rare to come up against builders that pass off for 4Ω components speakers of even double impedance, the picture is complete.

Se puede describir la impedancia assumed by the moving coil at different frequencies in graphical form on a cartesian plan, with the scale of frequencies in abscissa (Hz) and the impedance values in ordinate (Ω). The drawing that results is called curva de impedancias y representa visualmente la modificación del valor óhmico asumido por impedancia en diversas frecuencias. Vea por ejemplo la gráfica de un altavoz de alcance medio de cono, clasificó para 6Ω nominal:

Impedance Curve

Un detalle que se notará inmediatamente es un pico de impedancia with an ascending front and a descending front specular between them (well, this specularity is verifiable only if the frequency scale in abscissa is logarithmic). To it corresponds la frecuencia de resonancia del componente. Todos los transductores presentan un abrupto incremento de impedancia en la frecuencia de resonancia. Moving toward the right, that's going up with the frequency, we meet a saddle where it should be possible to read a value near to that declared as nominal — see what has already been said about some builders — after which the effect of the moving coil inductance begins to make itself felt, in the form of a gradual increase of measurable impedance.

A través del próximo gráfico podemos encontrar la diferencia de respuesta entre un filtro pasa-banda de segundo orden calculated with the provided formulas, cargado en la salida con una resistencia (verde) y un altavoz de alcance medio real (amarillo):

Resistor vs/ Speaker

A la frecuencia de resonancia del altavoz de alcance medio y a frecuencias mas altas the filter sees a greater load impedance and filters less. Result: Las curvas resultan iguales solo en la octava 1KHz–2KHz, mientras que las frecuencias que el filtro debió rechazar are instead in overbearing evidence. Really awful!

To make the response of the set filter/speaker coincide to the model theorized by formulas debemos modificar el altoparlante de forma tal que su impedancia resulte lo mas resistiva posible, that's to say constante con la frecuencia. Este efecto se consigue con las redes de ecualización