What is an Induction Coil?

Induction loop systems (IL) are also called closed-circuit electromagnetic induction collective hearing aid systems. They are the earliest collective hearing aid technology. This hearing aid system consists of a main console (including amplification and FM components), a ring-shaped induction coil that is pre-positioned in indoor places such as classrooms, homes, and individual hearing aids (with a T file). It can transmit speech signals from external wired microphones or FM wireless microphones, as well as audio signals from recorders, keyboards and televisions. ^[1]

Induction coil system

Today's induction coil systems are not only used for hearing aid systems. The more important industrial application is the use of industrial heating equipment. It is an important part of industrial power supplies and industrial induction heating power supplies. The substantial progress of domestic induction heating technology is Starting from 2003, for different industrial heating workpieces, the induction coil is an important part. Generally, the induction coil will run a large current when it is working, and it needs to generate a sufficiently large electromagnetic field to heat the workpiece, so it itself will also Heat generation, cooling water needs to be used in the studio to cool down. The most typical applications are: short circuit ring brazing of industrial motors, evaporated aluminum coating, copper brazing, pre-heating of pipes, etc. Some technologies are constantly being developed!

From the principle of electromagnetics, we know that when a long straight wire has a current passing through it, there will be magnetic lines of force around it. According to the direction of the right-hand rule magnetic field lines, the shape is shown in the figure:

(A) the uniformity and direction of the magnetic field

The magnetic field of a linear current is extended outward from the current that generates the magnetic field. The direction of the magnetic field (that is, the direction of the magnetic field lines) is a closed curve that surrounds the current. The magnetic field lines and the resulting magnetic flux (can be regarded as the flow of magnetic force). , Are concentric circles lying on a plane perpendicular to the current, and flow in a ring around the current that generates them. The magnetic field is stronger near the current, and the farther away from the current, the weaker the magnetic force and current. The relationship between the direction of the magnetic field lines and the direction of the current can be determined using the right-hand spiral rule. Hold the wire with your right hand and straighten your thumb. If the thumb represents the direction of the current, the bent fingers represent the direction of the magnetic field.

When the coil is installed on the floor and the hearing aid wearer is sitting or standing, the magnetic field lines at the height of the head are mainly horizontal in the circuit. Thus, at the height of the head, a nearly continuous amount of vertical portion of the magnetic field covers almost the entire room. The exception is the one just entering the circuit, where the entire magnetic field is strong except for the weak vertical part. The above characteristics are important because the receiving coil in the hearing aid is installed vertically, and it can only pick up the vertical part of the magnetic field.

The current has been discussed in one direction along the loop, however, the sound is an audio signal, and the direction will be reversed many times per second corresponding to the positive and negative pressures in the original sound wave. Therefore, the circulating magnetic field is inverted many times per second. In fact, according to the electromagnetic field theory, it is the constantly changing magnetic current that makes the pickup coil sense, and produces an audio current (the earth's magnetic field does not affect the coil, precisely because the earth's magnetic field has continuous power and direction).

(Two) the strength of the magnetic field

The strength of the magnetic field near the center of the room is directly proportional to the magnitude of the current in the loop and the number of loops, and inversely proportional to the diameter of the loop. The international standard (IEC60118-4, BS7594) states that the long-term average output power value of a magnetic field should be 100mA / m (referring to milliamperes per meter). Must not be lower than 70mA / m or higher than 140mA / m. This value is the intensity of the vertical plane of the magnetic field measured in the loop at 1.2 meters from the floor. Peaks of intensity up to 400 mA / m are allowed in speech, and the frequency range should cover 100 Hz to 5 kHz.

At the center of the loop, the diameter is a meter, and the magnetic field strength of the loop with n cycles can be calculated by the following formula:

H is the strength of the magnetic field, expressed in milliamperes per meter, I is the root mean square of the current value, expressed in amperes, and for a square loop, the size is expressed in a meter. Its magnetic field strength is 10% less than the calculated value. If the long-term average output power intensity of the magnetic field is to reach 100 mA / m, the value of the loop output must be at least 400 mA / m (preferably 560 mA / m), so as to avoid excessive peak clipping in the speech sound with greater intensity. .

According to the electromagnetic principle, we can see that the induction loop coil is not the only wire that generates a magnetic field in a building. All wires in a building generate a magnetic field. Therefore, hearing aids can not only receive voice signals, but also other magnetic field signals. , Such as 50Hz power supply voltage signal. The problem of interference sources must be fully considered when wiring. If the audio magnetic field is too weak, the signal-to-noise ratio is not large enough. Increasing the signal transmission power can suppress interference. In some smaller hearing aids (and their coils are also small), the sensitivity of the inductive coil is obtained by using a separate preamplifier coil. Of course, for weak magnetic fields, users can also make up by increasing the volume. But this is not convenient, especially when you need to switch between the microphone and the inductor. In addition, this requires that the hearing aid has sufficient volume retention while not causing howling when sufficient gain is obtained. At the inductor position, if the gain is too large, it will also cause howling. Just as sound waves leaking back from the microphone to the microphone will cause feedback, so does the howling caused by the magnetic field leaking from the microphone to the inductor.

(Three) the frequency response of the induction coil circuit

There is a problem of matching between the frequency response received by the hearing aid through the microphone and the frequency response obtained through the induction coil. Hearing aid loudness is usually carefully adjusted to suit the wearer. The output power of the hearing aid is the same when the sound input is 70dBSPL and the magnetic field strength is 100mA / m. The hearing aid wearer can easily switch from the microphone stop. To the inductor without changing the volume. However, the frequency response of induction coil circuits and hearing aid inductance systems is still sometimes unsatisfactory. However, the sound generated when the loop response and the hearing aid inductance response are combined cannot be too different from the original sound response. There is only one exception, that is, the attenuation of sound at frequencies below 500 Hz, which may be beneficial to some people in some cases, because this frequency range is the most prone to magnetic interference. But this is also an important frequency range for people with severe hearing loss. Fortunately, multi-memory hearing aids can separately adjust the response of the microphone and the inductor, so that the best inductive response can be selected for the user. Some hearing aids even allow users to choose low-frequency attenuation when needed, such as in a room with a lot of magnetic interference (such as fluorescent lights, lights with dimmers running).

There are two reasons why users feel that there are different responses in the inductor block and the microphone block:

(1) The magnetic signal generated by the circuit for high-frequency sound is weaker than that for low-frequency sound.

(2) The hearing aid itself. Inductive coils in hearing aids have inherent voltages that increase with frequency. Hearing aid designers can partially or fully compensate for this by connecting the inductor to the hearing aid amplifier. Specific conditions such as the inductive response and the response to the microphone can be seen in the instruction manual of the hearing aid or measured in a test box. ^[2]