Speaker principle impedance curve

Impedance curve is the curve of the impedance of a loudspeaker with the change of frequency. The impedance curve of the speaker is shown in Figure 3-1, which increases sharply near the lowest resonance frequency, and increases with the increase of the inductance in the high frequency part.

In Figure 3-1, the vertical axis represents the impedance（Ω），The horizontal axis represents the frequency（Hz），Log scale.The curve peak is caused by a cone, a voice coil, the damper vibration resonance of the system. In the middle of the curve, the minimum value is equivalent to the rated impedance of the loudspeaker, which is usually 10% ~ 30% of the DC impedance. According to the speaker DC impedance estimation speaker impedance, the speaker impedance actually consists of three parts, as shown in Figure 3-2 a line DC impedance loudspeaker coil, do not change with the frequency (speak temperature strictly); B line inductance part, according to the characteristics of the inductive inductance with frequency increased. And, the voice coil winding, turns on; C line back EMF said, when the voice coil vibration will generate a counter electromotive force, current and input current direction of anti electric heat produced the opposite, in fact equivalent to reduce the input current, in other words, increased resistance. At the resonance frequency, the vibration is the maximum, and the value of the resistance increases. Impedance curve is a window to understand the performance of a speaker.

1．Resonance frequency

The impedance curve of 3-1 shows that the maximum value in the low frequency of a certain frequency impedance of this frequency is called the resonance frequency of loudspeaker, denoted as FO, namely the speaker impedance curve on the impedance values in the corresponding first peak with frequency rising frequency. The speaker is a vibration system, and the quality of the resonance frequency of loudspeaker, the quality of vibration system is bigger, cone, folding ring along the heart piece more soft, more and more low frequency resonance, and the resonance frequency is higher. In the formula of PI was fO=1/2 SQRT (1/m*c) quality type in M0 - vibration system; C0 vibration system compliance.

We often want to reduce the resonance frequency of the speakers, but there is a certain limit. The increase of the vibration system can reduce the resonance frequency, but the increase of the mass will decrease the sound pressure of the loudspeaker, and increase the frequency of the vibration system, but the increase of the frequency will increase the vibration amplitude and the vibration system. In general, the larger the size of the speakers, the lower the resonance frequency. The resonance frequency is the starting point of the speaker's reproduction and the lower limit of the low frequency reproduction. At the resonance frequency, the sound pressure of the loudspeaker decreases with the square of the frequency.

The resonance frequency of the speaker changes with the temperature and humidity, which is obviously in the whole paper basin. As the air humidity is too large, the vibration absorbing moisture makes the quality increase, and the bending ring is soft, the resonance frequency decreases and the amplitude is close to 10%. Temperature rise also has a trend to reduce the resonance frequency. Some people think that listening to music in the hazy rain do not have a taste, in addition to the mood, environmental factors, subtle changes in the resonant frequency of the loudspeaker is also a kind of opportunity.

In recent years, the improvement of membrane material and process, such as polypropylene, carbon fiber, metal and so on, all kinds of composite bending ring, the improvement of the vibrating film moisture proof agent and wet strength agent, all of which make the resonance frequency of the loudspeaker is stable. The resonance frequency of the speaker changes with the size of the input power and the length of working hours. According to our experiments, the resonance frequency will decrease slightly in a period of time, and then tend to be stable. Some people bought speakers love to add power to work for a period of time (called burn, we neither against nor advocate), its role is to enable the speaker resonance frequency stability.

2．Power

The speaker's power is one of the important indicators to select and use the speaker. V (A), W (V) to indicate that the speaker is used in power, it is more appropriate to use the A. Original power has a precise definition, the international and domestic have the basis of the standard. Due to the interests of some domestic and foreign manufacturers, power labeling is quite confusing. Both deceived customers, but also to their own signs spilled water. Here we are based on the authority of the IEC268-5 (1989), GB/T9396-1996, the definition of each power to be described.

（1）Rated noise power (power). In the rated frequency range fed to the speaker to provide a simulation program signal, and not produce heat and mechanical damage to the corresponding electric power. U/R is defined as U, R is the rated noise voltage, the is the rated noise resistance.

At this time the rated frequency range is defined by the manufacturer of the frequency range of the speaker; rated noise voltage refers to the rated frequency range fed to the speaker to provide a simulation program signal, and not produce heat and mechanical damage signal voltage". The analog signal is a signal that is more close to the actual use of the situation, which is obtained by the pink noise signal through a special filter. In the design of the speaker and the design of the design and production of stereotypes, the speaker at the rated input of the noise power 100h, which is a strict, responsible for the requirements. Through the 100h test, the speaker can ensure the safety of the speaker in normal condition. In a large number of production available 1.2 times the noise power test 48h.

（2）Long term maximum power. The electric power corresponding to the long-term maximum voltage, which is defined as the U2/R, the U is the maximum input voltage, the R is the rated impedance. The long-term maximum voltage here refers to the speaker can withstand the duration of the 1min, the interval is 2min, repeat 10 times the analog signal, and does not produce permanent damage to the maximum signal voltage. The maximum power of the long term is the upper limit of the speaker's long time.

（3）Short term maximum power. Power corresponding to short term maximum input voltage. U is defined as U2/R, is the largest input voltage, the R is the rated impedance. Short term maximum input voltage refers to the speaker's ability to withstand the duration of the 1s, the interval is 60s, repeat 60 times the analog signal without permanent damage to the maximum signal voltage. It means that the speaker can withstand the upper limit of power in short term.

In some of the information, "instantaneous power", "music power", "peak power" meaning for short-term maximum power, these power value is greater than the rated noise power. The problem of speaker power is important, which is related to its life and reliability. The reason why the speaker rated noise power is limited, the increase of the input power will cause the increase of the temperature and the distortion of the voice coil.

The efficiency of an electric loudspeaker is very low, usually only a few of the energy, and most of the energy is converted into heat energy. A part of the heat dissipation to the space, a part of the voice coil temperature rise. The temperature rise of the voice coil can cause the softening of the adhesive and the expansion of the sound. Under normal circumstances the input power increases, the temperature rise is higher; the input power increases, the temperature rise faster. The greater the size of the speaker, the corresponding input power is also large.

In addition, the speaker has a maximum linear range in vibration, and the distortion will be increased over this range. There is a mechanical vibration loudspeaker allowed beyond this speaker will produce mechanical damage and even irreparable damage. From this point of view, the importance of the rated power index is the first of the.

3．Characteristic sensitivity

The speaker as the electroacoustic transducer we are naturally concerned about the efficiency of it, for it by speaker sound pressure level and the sensitivity to represent more convenient, several concepts are introduced below.

（1）Characteristic sensitivity within a specified frequency band. In the specified frequency band under the free field condition, the input to the speaker power is equivalent to the pink noise signal of the 1W, and the sound pressure of the reference point 1m is on the reference axis. The so-called free field refers to the space without sound reflection. In the space, the relationship between P and R, which is radiated by a point sound source, should be satisfied with the P and R.

（2）Characteristic sensitivity level within a specified frequency band. The characteristic sensitivity is expressed in logarithmic. The characteristic sensitivity and the ratio of the reference sound pressure are multiplied by 20, using dB. The reference sound pressure is 2 * 10-5Pa.

（3）The pink noise mentioned here is a kind of noise signal, which is used to measure the frequency of the frequency band width, and the spectrum is continuous and uniform. In other words, it is in the broad band and the same proportion of bandwidth and energy of the same noise.

Refer to the reference axis, which is parallel to the speaker's definition of a reference plane, through the loudspeaker axis and the reference plane perpendicular to the reference axis, the reference axis and the reference plane of the reference point.

（4）Often encountered is that when the input power of loudspeaker is not 1W, but other power value, then the output sound pressure level can be calculated as

SPLmax=Lp+10lgW0

Type Lp——Characteristic sensitivity level；W0——Input power.

In addition to the output sound different input power level differences, different distance to listen to the sound pressure level is also different. The square is proportional to output sound pressure level decreases with the distance characteristic sensitivity level of loudspeaker is too small, when in use will consume more power. But it is not the higher the better high sensitivity, easy distortion increase. The characteristics described here are based on the IEC268-5 and GB/T9396-1996 standards, which are different from that of the Japanese standard.

According to the Japanese JISC553 standard, said the sensitive index called the output sound pressure level, refers to the provisions of the frequency or frequency, power supply speaker 1W, the average distance reference point 1m SPL values in the reference axis, is usually an average of 4 point frequency values, i.e. 200Hz, 250Hz, 300Hz, 400Hz. The input sinusoidal signal.