Electrodynamic loudspeaker

A dynamic speaker works on the electro-dynamic principle and is the most widely used speaker type. This transducer is also called electro-dynamic speaker, which is thus alluded to the physical drive principle of electrodynamics and not the design is meant. All dynamic speakers are driven electro- dynamic and can be subdivided on the basis of their design on.

Classification dynamic speaker

• Moving coil speakers, including: Cone loudspeaker
• Dome speakers
• Horn speaker
• Flat-panel loudspeaker

Types of cone loudspeakers

Cone loudspeaker comes in two types, both work according to the electrodynamic principle.

Electrodynamic cone loudspeaker

This design had a separately excited field coil, which was located in an iron core and the required constant magnetic field generated electrically. These speakers were used, among other things in old tube radios.

Permanent Dynamic cone speaker

In this design, no magnetic field coil is required, the magnetic coil has been replaced by a strong permanent magnet. Since there from about 1950 was technically possible inexpensive to produce strong permanent magnets as standard, which required constant magnetic field was provided by the permanent magnets available. The first cone speaker of this type were introduced in 1938.

The principle of electrodynamics

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

Function

Since electrodynamic working speaker to use as a power source, the Lorentz force being required a constant as possible stator field ( strong magnetic field ), which is usually formed by a permanent magnet.

Construction of a moving coil loudspeaker

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

As a magnetic material ferrite, aluminum -nickel-cobalt (Alnico ) or neodymium -iron-boron (NdFeB) can be used. " Neodymium " magnets is characterized by an extremely high field strength in a small package from, but the Curie temperature is only 200 ° C. At this temperature, the magnet is demagnetized and the speaker unusable. Already 100 ° C reduce the magnetic field of Neodymium permanent. Therefore, Neodymium is only limited and can only be used with special cooling for high-quality speakers.

The voice coil is located on a support which in turn is attached to the diaphragm (English cone ). The membrane consists of an outer and inner regions, the inner region is often referred to as cap and dust cap ( engl. dust cap) refers to a spider (English spider ) and the bead ( engl. surround) are for the return of the membrane into the rest position and responsible for centering the voice coil. The bead also prevents a direct exchange of air between the front and back of the membrane. Beads were sometimes made ​​of non -aging plastic and can break down after a few years. In early speakers a flat, stamped cardboard leaflet was glued into the cone center, which was centrally screwed in front of the magnet and which was called centering spider because of the wound arms.

Electrical ratings

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

An improvement of the cooling by increasing the air gap reduces the efficiency because the magnetic field is weaker and requires more power, which causes even more temperature rise - it goes round in circles. One way is to mount the voice coil on an aluminum sheath; it is used inter alia in broadband speakers. The resulting reduction in coil inductance linearized frequency response, however, the moving mass is increased, which reduces efficiency. Another very efficient way to dissipate the heat loss is filling the air gap with a ferromagnetic liquid ( ferrofluid ) - so are three effects to be achieved:

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

This is only possible when the deflection is far below one millimeter, are used only for the tweeter into consideration.

In dynamic speakers, temperatures of about 200 ° C may occur during prolonged operation. Overloading leads in extreme cases to a " burn-through " of the voice coil, which usually only burn up the insulation, leading to a short circuit and / or the voice coil wire melts. However, it is generally first to a softening of the adhesive and thereby loosening of the coil wire on the coil support, so that the driver will be unusable.

An often-overlooked role of the resistance increase of the wire by the elevated temperature. Because of the temperature dependence of the electrical resistance of the resistance of a voice coil, for example, 8 Ω at 20 ° C grows at 1.7 times at 200 ° C and then was 13.6 Ω. With an unchanged output voltage of the amplifier the power consumption drops to 59%, the speaker is quieter. To compensate for the amplifier is set louder, and the temperature rises to even further.

The specification of a sine wave output (power at a fixed frequency), as it is eg in amplifiers usual is not appropriate for the determination of thermal load in speakers, since under certain circumstances, even at low temperature due to excessive deflections using the mechanical destruction. In addition, conventional music signals in the time average is more like a 3 dB / octave sloping mixture of frequencies are similar; see 1/f-noise ( pink noise). Here one must note: the permissible thermal power with a pink noise, limited to the specified frequency range, measured and expressed as mean PRMS. This means that a tweeter for the frequency range 8 kHz to 16 kHz gets from the maximum noise power by filtering only one hundredth off.

The membrane can be mechanically damaged by excessive deflections. This occurs mainly at the lowest allowable frequencies. To this may also be relevant to a sinusoidal signal. In high and midrange can be stated to large deflections usually at the drastic increase of the harmonic distortion for bass you can measure the achievement of the maximum allowable deflection of light. Unfortunately, this data is never provided to us by the manufacturers, but they can usually be calculated from other data. Typical is the case of high and midrange through the crossovers mechanical overload associated with the thermal. An exception are horn driver. These are designed for small deflections and large acoustic loading. Operation without this, ie below the horn cutoff frequency, or even without a horn, can lead to immediate failure despite still uncritical temperature.

For effective protection of low-frequency speakers, both the thermal and the Auslenkungsgesichtspunkt observed. High level only can be useful if they involve protection device also provides the heat capacity into account. Thus it can be operated for several tens of seconds with a power such as a woofer perfectly, which is significantly above the permanent load index. The voice coil takes time to warm up. The smaller drives of tweeters have significantly lower time constants and require even more caution.

Must be warned of the mistaken belief that one could speakers by under-performing amplifier against overload: When overload ( clipping) create these distortion products, especially in the higher frequency range, which often result in multi - way speakers for the destruction of the tweeter also heavy-duty boxes. It is nevertheless useful to select the amplifier power is less than the speaker load, since then the playback quality is higher - provided that the power is below the amplifier limits.

The indication of a permissible peak power can be - with the efficiency listed in the specifications - a maximum achievable sound pressure calculated. In practice the sound pressure is, however, often limited by compression and distortion to a lower value, since the voice coil leaves the uniform magnetic field, and the cone suspension is mechanical limits. The specification of a peak power " PMPO ", as it is found in speakers in the lowest price class, does not follow a protected definition and thus is not meaningful.

Efficiency

The efficiencies, even the most efficient dynamic speakers, are very low ( 0.2-5 %, to 20 % near resonance points ); it is not customary to quote them. The speaker efficiency is indicated by the mean sound pressure.

The considered acoustic transducers are all characterized by a very low energy efficiency. Although other parameters ( frequency response, distortion ) play especially in the hi- technology a more substantial role, but the efficiency is for several reasons a role: An efficiency- low converter ( eg, a dynamic speaker with a weak magnet ) requires considerable amplifier services, which must be dissipated as heat by the power converter in order to avoid damage to the drive. Higher amplifier power is required inter alia in battery-powered applications, a disadvantage caused in turn heat or require amplifiers with high efficiency, which do not always possess good transmission characteristics.

The efficiency of a dynamic loudspeaker is increased by:

• High strength and large surface area of the magnetic field ( rare earth magnets, high magnetic fluxes to about 1.2 Tesla, large voice coil diameter )
• High copper fill factor of the air gap ( small air gap, large ratio between the wire and the base material, partly using rectangular wire, precision manufacturing, precise suspension )
• Light membrane (eg, titanium, carbon fiber composite ) and light voice coil ( contrary to the above point)
• Effective coupling of the speaker to the air (eg, bass reflex principle, large baffle, large volume in closed boxes, exponential )

The first three factors are at play in each case a positive effect on the quality of reproduction, since this also is the coupling factor and the internal damping can be improved. In contrast, the efficiency improvement can result through better air - coupling may also result in a distorted frequency response: Distinct resonances of small boxes or volumes of the bass reflex pathway leading to a selective increase in volume, but also to a deterioration of the impulse response.

Large deflections cause, inter alia, in dynamic speakers and high intermodulation distortion because the voice coil comes into areas with a weaker magnetic field and the ratio of electricity / force is no longer constant. Great efficiency and good sound reproduction is therefore achieved with large speakers (lower deflection at the same sound level ); However large designs are often not desirable, they are more expensive or have other drawbacks (eg partial oscillations of the membrane ).