# Relationship between and damping factor microphone

Calculation of the damping bridging circuit devices impedance matching - no matching in on the difference between Z1 (Zin) and Z2 (Zout) with the bridging factor: In recording studio technique the microphone Zsource (Z2) is less than Besides the capsule itself, the design and construction of the mic body and as a result of the pressure difference (pressure gradient) between the two sides. there's the spacing between the diaphragm and the back plate, the damping. If you want to get the best out of your microphones, it is a good idea to learned about some of the most important factors in room acoustics that This is why it is recommended to apply mechanical damping of microphones. to be very efficient, there should be no fixed connection between the two rooms.

Kind of supports the idea of using self-powered subwoofers, or at least putting the subwoofer amps as close as possible to the subs. What if we choose an amplifier with a higher damping factor spec. Assuming this amplifier can drive a minimum 2 ohm load, we find the output impedance would be 0.

Plugging the numbers into our single loudspeaker with 12 ga. Hmm, not such a big deal. That higher amplifier damping factor only improved our system damping factor by 0. What if we use the amplifier with the 3, DF spec in our self-powered sub with 2 feet of 14 ga. Remember our calculation using the DF amplifier was Essentially, in sound reinforcement systems where we have some significant length of wire between the amplifier and the loudspeaker, the amplifier DF spec has little affect on the performance of the system.

So what have we learned? In live sound reinforcement systems, damping factor is really driven by the length and size of our wire and the impedance of the loudspeakers we connect at the other end.

For Rochelle salt, d14 is about 2. For a strain ofan electric field of about The exact value of d depends on the temperature and the circumstances of the crystal, but this gives an idea of its magnitude. The formula at the right in the diagram for the converse effect serves to find the change in length for any applied field.

## 10 IMPORTANT FACTS ABOUT ACOUSTICS FOR MICROPHONE USERS

Of course, the strain and stress are related by the elastic constants of Rochelle salt, but we will not go into that here. The Curie temperature of ADP is Ceramics like barium titanate, BaTiO3, which are ferrielectrics two lattices oppositely polarized spontaneously; the observed polarization is the differencealso are strongly piezoelectric.

If a microphone is described as "crystal," it usually contains ADP; if it is called "ceramic," barium titanate is the active element. Any piezoelectric device is reversible; if a voltage is applied to a piezoelectric microphone, it will emit sound. It is also strictly a passive electroacoustic transducer, and the output power cannot exceed the acoustic input power.

Rochelle salt first became more than a curiosity aroundwhen it was applied by Langevin to ultrasonic acoustic transducers, or hydrophones, for the detection of submarines.

Not only could strong signals be created in water by the converse piezoelectric effect, but the same crystals could be used to detect the reflected waves. This was, in fact, the origin of the important field of ultrasonics, which used acoustic waves of greater frequency than 20 kHz, which were inaudible but very useful. The impedance mismatch between air and a diaphragm is much greater than the mismatch between water and a crystal hydrophone, so microphones are much more difficult to devise.

A much more sensitive arrangement was a "bimorph" of two cemented X-cut plates with one thin electrode between them.

They could be arranged to bend or twist, and could be operated from a diaphragm through mechanical leverage. The capacitance measures pF, which gives a capacitive reactance of Piezoelectric microphones give low output at a moderate internal impedance, and must always be used with amplification.

Piezoelectric transducers were used as analog phonograph pickups, giving a much higher output than dynamic pickups. As driven elements, piezoelectric devices are used as telephone receivers, acoustic transducers and record cutters. They are used for small loudspeakers, although dynamic loudspeakers give much better results. Pierce invented the acoustic interferometer, which uses an X-cut plate and a parallel reflector to measure the speed of sound with great accuracy.

He devised an oscillator that was very sensitive to the reaction of the air on the crystal. Cady developed the quartz crystal resonator at about the same time, which has had widespread application as a frequency-control device. The Condenser Microphone The condenser, or capacitor, microphone was perfected by C. Wente in as a much-needed low-noise substitute for the carbon microphone, and as a standard microphone for acoustical measurements.

It was then used in broadcasting for a few years, until replaced by the dynamic microphone, which is much easier to use. The capacitor microphone is very simple in principle, and is still used for acoustical measuring instruments. The recent development of the electret capacitor microphone ECM has overcome all the inconveniences of the traditional capacitor microphone, and it is now used almost universally in general applications, as in telephones and consumer electronics.

An excellent, easy to use microphone can be purchased for a dollar or two, and operated on 5 V at 0. The ECM depends on two recent developments, the polymer electret film, and the field-effect transistor. We shall discuss it after looking at the traditional capacitor microphone. A capacitor microphone consists of a metallized diaphragm that forms one plate of a capacitor, a backing plate forming the other.

The diaphragm is tightly stretched to have a high resonant frequency, and is placed very close to the backing plate. Grooves are cut in the backing plate to control the mechanical impedance of the diaphragm. The microphone can be represented as a Norton equivalent circuit with a current generator i in parallel with a capacitance C. The sensitivity is proportional to the bias voltage V and the diaphragm area A, and inversely proportional to the separation h and the stiffness s.

The stiffness is raised if h is reduced, so they cannot be varied independently. This happens to be a relatively typical value for a capacitor microphone. The capacitance of the microphone will be about The microphone cannot be located far from the amplifier. Furthermore, the bias supply must be extremely well regulated and ripple-free, since every variation will be combined with the acoustic signal.

An electret is a body with a permanent polarization, analogous to a magnet which has a permanent magnetization. Polarization P is dipole moment per unit volume, and has the dimensions of surface charge density. An electric field exerts a torque on a dipole tending to align the dipole with the field, just as a magnetic field acts on a magnetic dipole. The bar magnet and bar electret shown establish fields in space, and these fields have energy.

AMPLIFIER से SPEAKER CABLE Length कितना होना चाहिए ?DAMPING FACTOR क्या हे ?

Some of this field the "demagnetizing field" acts in the reverse direction to the polarization or magnetization, tending to reduce it. A soft iron bar may be placed over the poles of a magnet as a "keeper" through which most of the field will pass.

Magnetic pole strength can be considered to be induced at the ends of the keeper, which will neutralize some of the pole strength of the magnet, reducing the demagnetizing field. Exactly the same thing occurs with an electret when it is placed between the plates of a shorted capacitor.

The surface charge due to the polarization is neutralized by the surface charge of opposite sign induced on the capacitor plates.

This happens naturally when an electret is exposed to the air, as it collects charged particles floating as ions and dust. A magnet is not neutralized in the same way, because there are no free magnetic charges.

### 10 important facts about acoustics for microphone users

This capacitor is carefully sealed away from floating charges so that the electret does not become neutralized. When the plates are shorted to each other a "short" in this case can be a resistance of many megohoms the voltage difference between them becomes zero. An opposite charge will appear on the top of the electret. If the upper plate is the diaphragm of a microphone, the electric field E plays the same role as the field produced by the bias voltage in an ordinary capacitor microphone.

Elimination of the bias voltage and all its inconveniences makes the electret capacitor microphone a very desirable device. Since the capacitance is small, even with the dielectric properties of the electret, the ECM still has a very high internal impedance that is almost entirely capacitive. This drawback is eliminated by putting an FET right at the diaphragm. The output voltage also increases proportionately to the drain resistance, of course. This microphone is remarkably small, only 9 mm in diameter and 6 mm tall.

Diffraction effects will be negligible, so this omnidirectional microphone will probe an acoustic field without perturbing it. Its bandwidth is 20 Hz to 12 kHz, and its sensitivity is advertised at dB without specifying the load resistor, which is unfortunateor 0.

The power supply range is 2 to 10 V, and the current drawn is less than a milliampere. The ECM is a worthy successor to the carbon microphone for general uses. The Dynamic Microphone The principle of the dynamic microphone was known in when Bell developed the telephone, but it was impossible to use because of the lack of electronic amplification.

In all dynamic transducers, a coil of fine wire is free to move in a strong annular magnetic field. If the coil is moved by a diaphragm, a voltage is induced in the coil. If a current flows through the coil, forces are exerted that cause the coil to move. Fv is the rate of doing mechanical work, and ei the rate of doing electrical work. The signs are such that when mechanical work is done, an equal electrical work appears, and vice versa.

This shows very clearly that we are working with a reversible effect and that the conservation of energy is observed. The word "dynamic" simply refers to the role of motion in the device; it is not a very well-chosen term, but always refers to a moving-coil device. It is clear from this that if the diaphragm is a simple oscillator, the output cannot be independent of frequency, since the magnitude of Z' is least at resonance, and increases rapidly for higher and lower frequencies.

Dynamic microphones became possible when it was realized that by making the diaphragm oscillator include air volumes within the microphone, the response could be flattened very nicely, at the expense of some sensitivity. The structure of a typical dynamic microphone is shown in the figure. The domed diaphragm acts like a rigid piston, and carries the coil of wire, which moves in the annular gap of high magnetic field.

The pole pieces of the microphone are soft iron, with permanent magnets providing the field. The mounting rim of the diaphragm contributes stiffness and a little resistance s0, r0while the diaphragm itself contributes mass m0. It is acted upon by the acoustic overpressure, so that the microphone is a pressure microphone, with omnidirectional characteristics.

The air in the small volume beneath the diaphragm acts as another stiffness element s1while the kinetic energy of the air moving in and out from under the diaphragm through the silk cloth contributes mass m1as well as a larger resistance r1.

The result is two coupled oscillators, whose parameters can be varied to get the best results. The electrical analogy was a help in designing dynamic microphones, since the results of different arrangements could be studied easily without building actual microphones.

The port V is to help low-frequency response. On the other hand, the sensitivity is quite low, no more than dB or dB, so amplification is essential. At least 40 dB can be gained with transformers, bringing the output up to dB when applied to the amplifier, which is not too bad.

Dynamic microphones are low-noise, require no bias voltages or other nuisances, and are relatively rugged. They replaced capacitor microphones almost completely in broadcasting and recording.

Most high-quality microphones are still dynamic microphones, and are relatively expensive. A dynamic microphone in reverse becomes a loudspeaker. Of course, the designs are quite different, because they must be optimized for different things. A small loudspeaker makes a very passable microphone, and can be used for this purpose, as in an intercom system. Such small loudspeakers radiate poorly at low frequencies, so this is compensated by making them resonate at a few hundred Hz.

This is easily recognized in the oscilloscope traces when the output of a small loudspeaker is used as a microphone, since it tends to ring at this frequency. A transformer of turns ratio 1: The same amplifier that drives the speaker from a line of this impedance can be turned around to drive the line in turn when a button is pressed.

The Ribbon Microphone All the microphones so far described are operated by the acoustic overpressure acting on one side of a diaphragm, and so may be called pressure microphones. Because pressure is a scalar quantity, these microphones are omnidirectional, except for diffraction effects that depend on frequency. To realize a directional microphone, it is necessary to operate the microphone by some vector quantity. One vector quantity in an acoustic wave is the pressure gradient, which is parallel to the direction of propagation and in phase with the displacement.

A microphone operated by the pressure gradient is called a pressure-gradient microphone. Since velocity is also a vector quantity, such microphones are also called velocity microphones, but they are not directly operated by particle velocity.

This is the desired angle-dependent force. This means that v is proportional to p independently of frequency. This falloff is compensated by an increase in pressure difference due to diffraction. I have spoken of a "surface" to avoid using the word "diaphragm," which gives the wrong idea when used of a pressure-gradient microphone. The most important pressure-gradient microphone is the ribbon microphone. The surface in this case is a corrugated aluminium ribbon supported in a strong magnetic field.

The emf generated when the ribbon moves is proportional to v, and so to the overpressure p. That this ribbon acts like a thick surface is harder to realize.

The ribbon is exposed to pressure equally on front and back, and the distance L is determined by the size of the baffle in which the ribbon is suspended. L turns out to be roughly equal to the radius of a circular baffle.

The ribbon acts like a coil of only one turn, so the generated emf is very small. On the other hand, not only is the internal impedance very small less than an ohmbut the velocity can be made higher by reducing the mass m to a minimum value. The sensitivity of a ribbon microphone may be dB or dB, but more than 40 dB can be gained at once with a transformer, so its output of dB is comparable to that of a dynamic microphone. There is usually a transformer at the microphone to match it to a transmission line, and another transformer at the amplifier input.

This pattern is not dependent on frequency, as are diffraction effects. The ribbon microphone discriminates by a factor of 3 against isotropic noise. It can also be turned so that noise sources can be put in a zone of low sensitivity. These features made the ribbon microphone the standard for broadcasting, and the lozenge-shaped shiny microphone a familiar sight. This curve is a cardioid. A cardioid microphone at the front of a stage will pick up sounds originating onstage, and reject those coming from the direction of the audience.

Polar plots of the amplitude sensitivity of the ribbon and cardioid microphones are shown at the left. The amplitude response of a pressure gradient microphone changes sign for a reversal of the direction of the wave, while that of a pressure microphone does not.

This has a strange effect in a standing wave, where waves are moving in opposite directions. At a point where the pressure microphone gives a maximum signal, the pressure-gradient microphone gives a minimum signal, and vice-versa.

The Hot-Wire Microphone The hot-wire microphone is not like the other microphones we have studied. It does not reproduce sound pressure variations electrically, but is more of a detector of sound and an indicator of its energy. Since the name does crop up from time to time, we'll describe it here for completeness.

It is specifically used for low frequencies and for infrasonic signals. It was developed during the war as a sound ranging device, for acoustic location of artillery to aid counterbattery fire. After the war, Tucker and Paris perfected the hot-wire microphone for infrasonic detection, publishing their results in An example of a hot-wire microphone is shown at the right. It consists of a very fine platinum wire placed over the neck of a Helmholtz resonator and heated by a current passed through it.

How do I calculate the damping factor DF for example, at 1 kHz, if neither the impedance of the source Z2 nor the impedance of the load Z1 is known? Allow the source to send out of a 1 kHz sine tone and measure the resulting voltage V0 at the output without any load.

Then measure at this point the voltage VL, when the load is applied. The damping factor is: Power matching or impedance matching Power matching is a connection of the electronics design practice for telephone lines and radio frequencies of setting the input impedance ZL of an electrical load equal to the fixed output impedance ZS of the signal source to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load.

This includes all digital device connections interface. The other configuration, especially for audio and sound recording is an impedance bridging, voltage bridging, or simply bridging is a connection which maximizes the transfer of a voltage signal to the load. Quite often this matching is erroneously demanded connecting the power amplifier to the loudspeaker.

Where does this wrong "knowledge" come from? There are no 4 ohm amplifiers. There are really no 8 ohm amplifiers for the 4 ohm or 8 ohm loudspeakers.

We got speaker impedance bridging. There is no speaker impedance matching.