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Loudspeaker efficiency: power, sound pressure (dB) and impedance.

Teufel Effekt
This article will update you about the basic concept related with Loudspeaker efficiency such as power, sound pressure (dB) and impedance.

The efficiency shows the ratio with which a loudspeaker converts electrical into acoustic power. The higher the loudspeaker efficiency, the more efficient a sound transducer works.

Loudspeaker efficiency

A measurement of the sound pressure at a distance of m from the loudspeaker with a simultaneous electrical power supply of 1 watt has established itself . According to the definition of Ohm’s law and electrical power, this results in a required input voltage of 2.83 volts for 8 ohm loudspeakers and 2 volts for 4 ohm loudspeakers 

A 90 dB converter is perceived twice as loud as an 80 dB box with the same power input. Another interesting connection: To make an 80 dB loudspeaker sound just as loud as one with 90 dB efficiency, you need a 10 times higher power supply.

What is impedance ?

It is Reactance, capacitive resistance, inductive resistance, phase shift, reactive power. 

What makes impedance so special and what does it have to do with hi-fi?

In contrast to the pure ohmic resistance, the impedance is dependent on the frequency of the applied voltage. The magnitude of the resistance or the impedance opposing the current is therefore not constant in relation to different frequency values, even if these are present at the same time. It is physically responsible for the fact that the impedance is made up of a reactance in addition to the pure ohmic resistance (the effective resistance).

With regard to the latter, a distinction is made between inductive and capacitive resistances. High inductive values ​​are typical for electronic components such as coils or chokes, high capacitive values ​​for capacitors . These elements are now among the basic building blocks of hi-fi equipment. However, even the simplest components such as cables not only have a pure ohmic resistance, but also capacitance and inductance .

Inductance or the related reactance is caused by the build-up of the magnetic field which surrounds every conductor through which current flows. The above-mentioned coil / choke , which consists of a correspondingly long, twisted conductor, reinforces this effect (long conductor in a short space). Inductance behaves proportionally to the applied frequency, since every change in polarity “tears” the magnetic field that has just been created again and then allows it to build up again.

phase shift between current and voltage (voltage leads) caused by inductance is also typical .

Capacitive reactances are exactly inversely proportional to the applied frequency. Why? The cause here is the build-up of an electric field. Explained a little more clearly: The capacity of a body or component reflects its ability to absorb electrical charge. The more frequently a current changes direction or polarity, the less charge it “floats” onto this body per unit of time. In other words: A frequencyless DC voltage would be a pure capacitive resistance up to the capacitance Let the stop “run full”, whereby this would then have “no more space” for subsequent charging or subsequent electricity. This means that the purely capacitive resistor would block completely, ie it would have an infinitely high resistance. A phase shift between current and voltage caused by capacitance is also typical (current leads).

The whole thing is made even more complex by the fact that such systems send reactive power back into the network to the actual source due to the current-voltage phase shift ( capacitor : discharges; coil : reduction of the magnetic field) This can lead to disruptive resonances.

Back to practice:

After what has just been described, it becomes clear that coils or capacitors are ideally suited to block certain signals depending on their frequency or to pass them “preferentially”. These modes of action are now also being used in hi-fi technology, the most popular is probably the use in loudspeaker crossovers . Often, for example, tweeters are only preceded by a single capacitor that blocks lower frequencies (high pass). Basically, however, crossovers are connected in a much more complex way.

Regardless of the complexity of their crossovers, loudspeakers often have different impedance values ​​in their product descriptions: 4 ohms or 8 ohms values ​​are common. What is significant in this context is that, as described in the basic electrical terms, resistance or impedance has a significant influence on the electrical power consumed. If the impedance of a system changes depending on the frequency (in our example a loudspeaker), the power consumption and thus the current flow and voltage curve over the frequency range are anything but constant.

That this poses a special challenge for the supply source (here: the amplifier) ​​becomes more apparent the greater these impedance fluctuations (caused by multi-frequency music signals). Actually, 4 ohms or 8 ohm boxes should be within a 20% tolerance limit, i.e. not fall below 3.2 ohms or 6.4 ohms over the entire frequency curve. Nevertheless, there are a large number of loudspeakers on the market that penetrate into the 2 ohm range (and not only the legendary “Infiniti Kappa” is to be thought of, even a modern Thiel 2.4. Presents itself in this way). This is where the power supply capability of an amplifier comes inspecial importance to. Simply relying on a powerful amplifier is not necessarily sufficient; load stability is important if you want to avoid tonal discolouration, etc.

But it also becomes clear what influence cables can have. Because, as mentioned above, these also have capacitances , inductances and ohmic resistance values . From the perspective of the supply source, there is no distinction between cable and box or preamplifier in this regard . From the amplifier’s point of view, both elements merge into one unit. This makes it clear which importance is attached to appropriately selected hi-fi connectors and that there cannot be a blanket “bad cable” or “good cable”. Of course, it should also be mentioned that the supply source also has an impedance that plays a role in the system.

In all these contexts, the much-cited damping factor gains importance , which, among other things, is based on how well reactive power thrown back into the network by capacitances / inductances can be “digested” by a supply source.

In general, what has been said does not only apply to the loudspeaker-amplifier relationship. Even if this is a particularly clear example, the relationships apply to every device connection (CD player preamplifier, preamplifier output stage, etc.) and the corresponding cable connections.

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