It is an inductor in series with a resistor. Hence the name Pleiades L,R or R,L. The free ends are connected to pins 2,3. Ie right at the mic output or head amplifier input.
At some low frequency (how low depending on inductance L) the impedanc of the inductor becomes 0Ω. It is more correct to say the reactance of the inductor is 0Ω. Ie the impedance of the R,L filter is (R,0Ω). If there is no R the impedance is (0Ω,0Ω). The magnitude of this vector ie the magnitude of impedance is 0Ω. So the microphone sees a short circuit and there is a complete voltage drop at its output due to the fact that it has a finite output impedance usual 200Ω.
As frequency rises the reactance in ohms of the inductor rises so does the output of the mic. This is at 6dB per octave. This is the max slope.
If R is not 0 we never have the case of 0 impedance or short circuit. So the bass decreasing slope is less than 6dB per octave depending on the value of R and output impedance of the microphone.
(x,y) or (R,L) or R+jL or R+iL ie complex number or vector notation is used because inductors or capacitors have a steady state phase difference between voltage applied to their terminals and current flowing through them of 90 degrees. Resistor current and voltage is in phase. Hence the notation of complex numbers to denote vectors.
In mathematics or nature 2 plus 2 does not always make 4. For examole one hand pushing against the other by 2 Kgf the resultant force being 0Kgf. If they push at other angles the resultant force is a different direction vector. Vectir addition is used to find the result. Vector angle is a different name for phase.
In engineering vectors are thought to rotate. If we take the projection in a straight line we have a sinusoidal oscilation. A sine wave.
Rotating vectors are called phasors.
Any signal can be described as a sum of infinite sinewaves. Hence the use and importance of phasors in engineering. To predict the outcome.
Since in sound engineering the end product is the lister's brain perception we must always take this too into account. Hence the importance of flat frequency response from producer's brain to listener's brain. For the frequency response of our brain perception depends on acoustic intensity (Equal Loudness or Fletcher Munson curves). (Usually between production and reproduction the acoustic intensity levels are very different. For example a very soft delicate voice is reproduced at high acoustic intensity) [Lowe, Morgan].
In order to feel the difference between resistance and reactance (90 degrees out of phase) or the difference between resistors and inductors in electricity or friction and mass in mechanical systems a favourite Pleiades example-experiment is vibrating a door by hand. As we try to move the door back and forth at a higher frequency it becomes progressively difficult (load impedance changes) and we can also feel the phase difference. The mass (or inductance) of the door is doing its thing.
Impedance across any 2 points in universe is defined as the vector of voltage applied across these point devided by the current flowing through them. It is in itself a vector. Has both magnitude and direction or phase.
Bibliography:
Engineering circut analysis - Kemmerly
Introduction to system dynamics - Searer, Murphy, Richardson -
Elements of Acoustical Engineering (or dynamic analogies) - Olson
Flat frequency response from producer's brain to listener's brain, Sound Picture Recording and Reproducing Characteristics - D.P. Lowe, K. F. Morgan
Sound and Hearing - Stevens. Warshopsky - Life science library
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