INSPECTING THE MAGNETIC SYSTEMS OF ELECTRODYNAMIC LOUDSPEAKERS
BY GENERAL PURPOSE GRAPHIC MAGNETOMETERS

 

Electrodynamic loudspeaker or simply loudspeaker – is the device transforming electrical signals to sound waves through oscillations of voice coil, driven to motion when electrical signals are passed through it with the presence of constant external magnetic field (fig.1).

Fig.1. Background of electrodynamic loudspeaker

 

Although the efficiency of this transformation of energy is only 2-3%, the electrodynamic loudspeakers at present are most popular devices to play sound owing to optimal balance between cost of production and quality of sound.

In order to obtain the best sound features of loudspeakers the weight of voice coil should be extremely small and the induction of magnetic field into gap of magnetic system should be as high as possible, so developing high-quality acoustic systems both of these parameters should be measured.

So the two main technical problems are raised up: a smallness of gap the magnetic field transducer injected into (about 0.7 – 0.9 mm), and high strength of magnetic field to be inspected (induction ranges about 1 – 2 Tesla), which are solved easily by high strength field imaging magnetometer (fig.2).

Fig.2. Magnetic system of electrodynamic loudspeaker inspection
by high strength field imaging magnetometer

 

Dynamic mode of measures built into the magnetometer and shown on the fig.2 simplifies significantly inspection of magnetic fields topography and speed up the search of local extreme points.

On the photos following the results of magnetic field induction maximum values measures into air gaps of most popular among radio amateurs magnetic systems produced in Russia and USSR:

25GDN-5 – 0.804 Tesla                                    10GDSH-1 – 0.891 Tesla

4A32 (UUU) – 0.906 Tesla                                    10GD30B – 0.982 Tesla

CODA+++ – 0.993 Tesla                                    GD3030 – 1.052 Tesla

25GD-18-22 – 1.147 Tesla                                    4A32 (LOMO) – 1.301 Tesla

1A20 – 1.760 Tesla

It should be noted that magnetic systems presented are built on the basis of identical typical permanent magnets from «Alnico» alloy, conventionally used at «KinAp» factories, however differs significantly from each other by dimensions of magnetic concentrators:

            1А20 system (1.760 Tesla) has following features:
            - height of magnetic concentrator flange 3 mm;
            - width of magnetic gap 0.8 mm.

            4A32 system (1.301 Tesla) has following features:
            - height of magnetic concentrator flange 8 mm;
            - width of magnetic gap 1.0 mm.

            25GDN-5 system (0.804 Tesla) has following features:
            - height of magnetic concentrator flange 10 mm;
            - width of magnetic gap 1.5 mm.

Results presented could be one more experimental confirmation of known theoretical conclusion that induction of magnetic field into the gap is inversely proportional to width of magnetic gap and height of magnetic concentrator flange.