Introduction to speaker earphones, talk about impedance, impedance curve and sensitivity of headphones

This series of articles will make the product parameters, test methods and test results better combined. The subjective sense of audio equipment is by no means a metaphysical. Do not think that the objective test data, product parameters and subjective sense of hearing are far apart, that is Because our test methods are not good enough, the test items are not accurate enough, and the understanding of the product parameters is not deep enough.

Today we are going to talk about the two most common data "impedance" and "sensitivity" in headset products. The explanation of these two concepts is in Baidu Encyclopedia, but the noun explanation is obviously not the purpose of this article. If you have enough Headphone experience will reveal that these two values ​​are not used every time when you choose the earphones to judge whether the headphones are good or not. So how much reference value do these two parameters have? Why do all the earphone products have to be used? List these two parameters?

First, let's briefly introduce the concept of impedance under existing headphones and industry test method specifications.

Impedance: the rated impedance. This value generally represents the lowest impedance value in the ideal state when the maximum power is reached within the approved frequency range. Because there is a margin, the value should not be higher than 20% of the lowest impedance value. Related test methods are based on moving coil headphones. The impedance curve of electrostatic headphones and moving iron headphones will change greatly with frequency, and the test and definition methods may be different. From the perspective of existing products, it may take 1 kHz or The impedance at a certain frequency is used as the rated impedance.

The above definition actually finds that there are many points to explain. First, we call it impedance, which means that no matter what kind of speaker, it is not a purely resistive circuit. In a circuit with resistance, capacitance and inductance, the circuit The current blocking action is called impedance and the unit is ohm Ω.

Where R is the resistance, ωL is the inductive reactance, and 1/(ωC) is the capacitive reactance. The inductive reactance is the blocking effect of the inductor on the current in the AC circuit, and the capacitive reactance is the blocking effect of the capacitor on the current in the AC circuit. Since it is not a physics class, the concept introduction ends here.

Although industry testing generally stipulates that the impedance should be tested at the lowest value [20% deviation] in a range of frequencies, whether it is in a speaker or a headphone speaker, it seems that the rated impedance of the headphones is now taken from the value at 1 kHz. Perhaps the impedance curves of moving coil and flat-panel headphones are relatively flat, and many of them will be rated at 1 kHz. In order not to cause too much trouble for ordinary users, it is estimated that the dynamic headphones will not be labeled. The pile impedance value is for your reference.

Impedance curve: For the earphone products, the impedance curve is relatively rarely mentioned. The impedance curve of the speaker refers to the "impedance/frequency" curve of the speaker exhibiting different impedance values ​​when inputting different frequency signals. Due to the dynamic earphone speaker The impedance curve is relatively flat at 20Hz-20kHz or higher, so there are not many indicators mentioned. In the speaker used in the speaker, the impedance curve is an important indicator. Generally, it will have a large near the low frequency. The bulge then quickly falls back [with the first trough as the rated impedance], and then the impedance may increase to varying degrees as the frequency increases. We reprint the test data at below to help you understand the difference. The difference in headphone impedance curve. We are preparing this test system.

Innerfidelity puts the relationship between frequency and impedance, and the relationship between frequency and phase in a table. Phase is the phase/frequency curve, and impedance is the impedance/frequency curve. The graph says the 600Ω output impedance test. It means that the output level of the audio source is 0dBu. [Level concept, later article].

What inspired us to play headphones?

First, the rated impedance of the earphone is an ohmic number of a point obtained by a comparison method such as comparison or replacement.

Second, the moving coil, flat panel, static electricity, moving iron earphone unit will change in the effective frequency [can take 20Hz-20kHz, but also higher, such as 50kHz] impedance, instead of being immutable.

Third, the impedance curves of moving coil headphones, flat diaphragm headphones, and dynamic earplugs are relatively flat. However, the impedance curve of moving coil type large speakers [speaker use] is far less than that on headphones.

Fourth, the impedance curve of the moving iron unit is very exaggerated. When a single moving iron unit covers the whole frequency band, the high-frequency impedance will increase sharply. The multi-unit moving iron is actually improving this problem, but it still varies greatly.

Fifth, the impedance is a complex number, which is related to the signal frequency and phase.

Sixth, the impedance and the external resistance can not be equal, especially the earphones with huge changes in the impedance curve, which may bring a very strange sense of hearing.

Next, talk about the sensitivity of the headset.

Headphone Sensitivity: After understanding the impedance, let's look at the sensitivity of another impedance-related value earphone.

The sensitivity of the earphone refers to the sound pressure level that can be achieved when the earphone reaches 1mW power under the specified test conditions [IEC artificial ear and different coupling device analog human ear, or ear canal], using 1kHz sine wave AC signal. dB/mW. Since dB is used as the index unit, it must be followed by the test conditions. It is inconvenient for many ordinary users. We may save the mW unit when referring to the sensitivity. But many earphone products will write the dB SPL/ very complete. mW @1kHz, or similar writing. You can use the power formula P=U^{2}/Z to find the U voltage value. The P power requirement is 1mW, that is, 0.001W is known, that is, the maximum sinusoidal power. U is the maximum. Sinusoidal voltage, Z is the rated impedance of the speaker. It seems that there is no difference between the method of calculating the DC power.

For example, a headset with a rated impedance of 32 ohms requires approximately 0.179V [0.032 square root]; 150 ohms corresponds to 0.387V. So different impedance values ​​tell us roughly what a headset is reaching its nominal The sensitivity of the value of the sound pressure level, the difference in the required supply voltage.

Here is an additional topic to discuss, why more and more mobile phones can claim to be able to drive 600 Euro headphones? According to the above formula, if the headphone load is 600 ohms, then the power supply voltage is 0.775V, and Industry test standard The definition of 0dBu in the analog level digital standard in the past, that is, 0.775V of the analog voltage of 1mW under 600 ohm load is 0dBu [also 0dBm of 600 ohm].dBu, in fact, we usually evaluate sound card, mobile phone, decoder, etc. The product talks about the output level xxx dB. The next time I talk about this concept, the phone says that it can drive a 600 ohm headphone, which means it can provide 0dBu, which is 0.775V output voltage, and achieve 1mW output power.

What are the earphone sensitivity parameters for us to use and purchase headphones?

First, the sensitivity of the current headphones is generally around 100dB, and the sensitivity of the small earphones is easily over 100dB. While most people listen to music in a quiet environment, the sound pressure level required by the headphones is about 90dB-100dB. Currently less than 90dB sensitivity. There are not many headphones, below this value, and then look at its rated impedance, basically know if it is easy to drive.

Second, sensitivity and rated impedance are a set of related parameters. Although they are static indicators at a certain frequency point, they also need to be combined. For example, the same sensitivity is 100dB, and the impedance is 32 ohms and 300 ohms. Great difference. Since the sound pressure is an exponential unit, the impedance must correspond to the voltage value under the square root [in order to get the power index of 1mW], so for the human brain with a habitual linear relationship, the feeling is not so accurate.

Third, as mentioned above, the impedance of the speaker will change with the signal frequency. The sensitivity is only the sound pressure level at 1mW at 1kHz. It is only a quick and preliminary judgment indicator, so sometimes you will think that these two There are almost no reference values ​​for the parameters and the sense of hearing. There is a quantitative meaning for whether the headphones are good or not. The impedance is low, and the voltage for the sound pressure is lower. The sensitivity is high. Naturally, under the same input power, the sound pressure will be It's bigger. But this is only an indicator of the size of the sound, let alone a single frequency of 1 kHz. We have to consider its impedance curve and many factors.

Fourth, because of the content of the third article, everyone should be wary and understand why some audio sources can drive 600 Euro or even 1000 Euro impedance headphones. This is a statement that is not rigorous and irresponsible.

As an introduction, the information and knowledge points involved in this article seem to be somewhat complicated. As a so-called electroacoustic component with such a complex speaker, not all of the above parameters can fully summarize its performance. Frequency range, impedance and sensitivity, as a headset product. The most common three related acoustic parameters, its role needs to take into account its definition and test conditions. I hope this article related to several tips to help you understand these two parameters.