Dynamic speaker

A dynamic loudspeaker works on the electrodynamic principle and is the most widespread type of loudspeaker . This sound transducer is also referred to as an electrodynamic loudspeaker, which alludes to the physical drive principle of electrodynamics and not the structural shape. All dynamic loudspeakers are electrodynamically driven and can be further subdivided based on their design.

Classification of dynamic speakers

Moving coil speakers, these include:
- Cone speaker
- Dome speaker
- Hyperbaric loudspeaker
- Flat membrane loudspeaker
Magnetostatic speakers
Electrostatic speakers

Types of cone speakers

Cone loudspeakers are available in two types, both work according to the electrodynamic principle.

Electrodynamic cone loudspeaker

This design has an externally excited magnetic field coil, which is located in an iron core and electrically generates the required constant magnetic field. These loudspeakers are used, among other things, in old tube radios.

Permanent dynamic cone loudspeaker

With this design, no magnetic field coil is required, the magnetic field coil has been replaced by a strong permanent magnet. Since it was technically possible to produce inexpensive, powerful permanent magnets in series from around 1950, the required constant magnetic field was made available by the permanent magnet. The first cone loudspeakers of this type were introduced as early as 1938.

The electrodynamic principle

If a current-carrying conductor (wire) is held in a magnetic field, this leads to a mechanical movement of the conductor. The conductor through which the current flows moves depending on the direction of the current in a magnetic field and thereby exerts a force, this force is called the Lorentz force .

function

Scheme of a dynamic loudspeaker (cone design, sectional view)

In moving coil loudspeakers, the diaphragm, at the end of which is a coil that is in turn in the constant magnetic field of a permanent magnet, is caused to vibrate by the Lorentz force . The coil and membrane can move back and forth in the magnetic field, preferably in the direction perpendicular to the course of the field.

Since electrodynamically operating loudspeakers use the Lorentz force as a power source , a stator field that is as constant as possible (strong magnetic field) is required, which is usually formed by a permanent magnet .

Structure of a moving coil loudspeaker

Cutaway model of a dynamic bass loudspeaker

A loudspeaker consists of a loudspeaker frame, a membrane, a bead, a centering spider, a voice coil and a permanent magnet.

Ferrites , aluminum-nickel-cobalt ( Alnico ) or neodymium-iron-boron (NdFeB) are used as magnet material . “Neodymium” magnets are characterized by an extremely high field strength with small dimensions, but the Curie temperature is only 200 ° C. At this temperature the magnet is demagnetized and the loudspeaker becomes unusable. Even 100 ° C permanently reduce the magnetic field of neodymium. This is why neodymium is only of limited use and can only be used with special cooling for high-quality loudspeakers.

The voice coil is on a support, which in turn is attached to the membrane (English cone ). The membrane consists of an outer and an inner area, the inner area is often referred to as a cover cap or dust cap . A centering spider (English spider ) and the bead (English surround ) are responsible for returning the membrane to the rest position and for centering the voice coil. The bead also prevents a direct exchange of air between the front and back of the membrane. At times, beads were made of non-aging-resistant plastic and can fall apart after a few years. In early loudspeakers, a flat, punched cardboard sheet was glued into the center of the cone, which was screwed onto the center of the magnet and which was called the centering spider because of the coiled arms.

Electrical load capacity

Cut view of a loudspeaker

A loudspeaker can be thermally overloaded. Because of the low efficiency of only about 1%, most of the energy is converted into heat. This can thermally destroy the drive coil.

Improving the cooling by increasing the air gap reduces the efficiency because the magnetic field becomes weaker, and requires more power, which causes an even greater increase in temperature - you turn in circles. One way is to mount the voice coil on an aluminum sleeve; she will u. a. applied to broadband speakers. The resulting reduced coil inductance linearizes the frequency response , but the moving mass is increased, which reduces the efficiency. Another, very efficient way to dissipate the heat loss is to fill the air gap with a ferromagnetic liquid ( ferrofluid ) - this achieves three effects:

Heat dissipation through increased thermal conductivity
damping
The air gap can be reduced

However, this is only possible if the deflection is well below one millimeter, so it is only possible for tweeters.

With dynamic loudspeakers, temperatures of around 200 ° C can arise during prolonged use. In extreme cases, an overload leads to the voice coil “burning through”, whereby the insulation usually burns up first and leads to a short circuit and / or the voice coil wire melts. In most cases, however, the adhesive first softens and thus the coil wire loosens on the coil carrier, making the driver unusable.

The increase in resistance of the wire due to the increased temperature plays an often overlooked role. Because of the temperature dependence of the electrical resistance, the resistance of a voice coil increases from e.g. B. 8 Ω at 20 ° C to 1.7 times at 200 ° C and then has 13.6 Ω. If the output voltage of the amplifier remains unchanged, the power consumed drops to 59% and the loudspeaker becomes quieter. To compensate for this, the amplifier is turned up and the temperature continues to rise.

The specification of a sinusoidal power (power at a specified frequency) as it is e.g. B. is common with amplifiers, is not appropriate for the determination of the thermal load capacity of loudspeakers, because under certain circumstances mechanical destruction begins even at low temperatures due to excessive deflections. In addition, conventional music signals are, on average over time, more similar to a frequency mixture that drops by 3 dB / octave; see 1 / f noise (pink noise). It must be noted: the permissible thermal power is measured with a pink noise, limited to the specified frequency range, and specified as the mean value P RMS . That means: a tweeter for the frequency range 8 kHz to 16 kHz gets only a hundredth of the maximum noise power through the filtering.

Mechanical resilience

The membrane can be mechanically damaged by excessive deflections. This occurs especially at the lowest permissible frequencies. A sinusoidal signal can also be relevant for this. In the case of tweeters and mid-range speakers, excessive deflections can usually be determined by the drastic increase in distortion; for woofers, the maximum permissible deflection can easily be measured. Unfortunately, these data are never given by the manufacturers, but they can usually be calculated from other data. With tweeters and mid-range speakers, mechanical overloading is associated with thermal overload due to the crossovers. Horn drivers are an exception. These are designed for small deflections and large acoustic loads. Operation without this, i.e. below the horn limit frequency or even without a horn, can lead to immediate failure despite the temperature being still uncritical.

For an effective protection of bass loudspeakers, both the thermal and the deflection aspects must be observed. High levels can only be represented meaningfully if the protective device also takes the heat capacity into account. So z. B. a woofer can be operated for a few tens of seconds with a power consumption that is well above the continuous load specification. The voice coil takes time to warm up. The smaller drives of tweeters have considerably lower time constants and require even more caution.

A warning must be given against the misconception that loudspeakers can be protected from overload by low-power amplifiers: If overdriven , these distortion products generate distortion, especially in the higher frequency range, which in multi-way loudspeakers often leads to the destruction of the tweeter, even with highly resilient boxes. Nevertheless, it makes sense to choose the amplifier output lower than the loudspeaker capacity, as the playback quality is then higher - provided that the output is below the amplifier limit values.

A maximum attainable sound pressure can be calculated from the specification of a permissible peak power - with the efficiency listed in the technical data . In practice, however, the sound pressure is often limited to a lower value by compression and distortion, as the voice coil leaves the area of the homogeneous magnetic field and the membrane restraint sets mechanical limits. The specification of a peak performance " PMPO ", as it can be found in loudspeakers in the lowest price class, does not follow a protected definition and is therefore not meaningful.

Efficiency

The efficiencies, even of the particularly efficient dynamic loudspeakers, are very low (0.2–5%, up to 20% close to resonance points); it is not customary to show them off. The loudspeaker efficiency is given with the characteristic sound pressure .

example: An average dynamic loudspeaker with e.g. B. 87 dB / W / m requires an electrical power of about 80 W for a level of 100 dB at a distance of four meters, whereas a loudspeaker with a high level of efficiency with 101 dB / W / m gets by with 3.2 W.

The considered sound transducers are all characterized by a very low energy efficiency. Although other parameters (frequency behavior, distortion) play a more important role, especially in hi-fi technology, efficiency is important for several reasons: A low-efficiency converter (e.g. a dynamic loudspeaker with a weak magnet) requires considerable amplifier power, which must be dissipated as heat output from the converter in order to avoid damage to the drive. Required higher amplifier power may be. a. disadvantageous in battery-operated applications, in turn causes heat or requires amplifiers with high efficiency, which do not always have good transmission properties.

The efficiency of a dynamic loudspeaker is increased by:

high strength and large area of the magnetic field (rare earth magnets, high magnetic fluxes up to 1.2 Tesla, large voice coil diameters)
high copper fill factor of the air gap (small air gap, large ratio between wire and carrier material, sometimes use of rectangular wire, precise manufacturing, exact suspension)
lightweight membrane (e.g. titanium, carbon fiber reinforced plastic ) and lightweight voice coil (contradiction to the aforementioned point)
effective coupling of the loudspeaker to the air (e.g. bass reflex principle, large baffle, large volume with closed boxes, exponential horn)

The first three influencing factors increase the reproduction quality, as this also improves the coupling factor and the internal attenuation. On the other hand, the improvement in efficiency through better air coupling can also lead to a distorted frequency response: Pronounced natural resonances of small speaker volumes or the bass reflex path lead to a selective increase in volume, but also to a deterioration in impulse fidelity.

Large deflections cause u. a. Dynamic loudspeakers also have high intermodulation distortions because the voice coil comes into areas with a weaker magnetic field and the current / force ratio is no longer constant. Great efficiency and good sound reproduction are therefore achieved with large loudspeakers (less deflection at the same sound level); However, large designs are often not desired, they are more expensive or have other disadvantages (e.g. partial vibrations of the membrane).

literature

Heinz Sahm: HIFI loudspeakers. 2nd edition, Franzis Verlag GmbH, Munich, 1982, ISBN 3-7723-6522-1 .
Wolfgang-Josef Tenbusch: Basics of the loudspeakers. 1st edition, Michael E. Brieden Verlag, Oberhausen, 1989, ISBN 3-9801851-0-9 .
Helmut Röder, Heinz Ruckriegel, Heinz Häberle: Electronics 3rd part, communications electronics. 5th edition, Verlag Europa-Lehrmittel, Wuppertal, 1980, ISBN 3-8085-3225-4 .
Beckmann: Handbook of PA technology, basic component practice 4th edition, Elektor Verlag, Aachen 1989.
Eberhard Zwicker, Manfred Zollner: Elektroakustik , 4. Edition, Springer-Verlag 1998
Dieter Franz: Handbook of electroacoustics. Basics of sound processing presented in a practical way , Franzis Verlag 1995