The parameters of the loudspeaker refer to the specific performance parameter values of the loudspeaker tested by the dedicated loudspeaker test system. The commonly used parameters mainly include: Z, F0, η0, SPL, Qts, Qms, Qes, Vas, Mms, Cms, Sd, BL, Xmax, Gap gauss. The following are the physical meanings of these parameters:
1. The physical meaning of the parameters
1. Z: refers to the resistance value of the speaker, including: rated impedance and DC impedance. (unit: ohm/ohm), usually refers to the rated impedance;
The rated impedance of the speaker Z: It is the minimum impedance modulus value after the first maximum value of the impedance curve, that is, the impedance value corresponding to point B in the figure below. It is the basis for calculating the electrical power of the speaker.
DC Impedance DCR: It refers to the impedance value measured by DC signal when the voice coil coil is at rest. What we usually call 4 ohm or 8 ohm refers to the rated impedance.
2. Fo (lowest resonance frequency) refers to the frequency corresponding to the first maximum value of the speaker impedance curve. Unit: Hertz (Hz).
The impedance curve of the loudspeaker is the curve of the loudspeaker impedance modulus measured by the constant current method or the constant voltage method with frequency under the normal working conditions of the loudspeaker.
3. η0 (the efficiency of the speaker) refers to the ratio of the output sound power of the speaker to the input electric power.
4. SPL (Sound Pressure Level) refers to the sound pressure generated by the speaker at a point 1m away from the speaker on the reference axis when a voltage with a rated impedance of 1W is applied to the speaker, in decibels (dB)
5. Qts: The total quality factor value of the speaker.
6. Qms: The mechanical quality factor value of the speaker
7. Qes: The electrical quality factor value of the speaker.
8. Vas (effective volume of the horn): refers to the volume when the sound compliance of the air enclosed in a rigid container is equal to the sound compliance of the speaker unit. Unit: liter (L);
9. Mms (vibration mass): refers to the total mass of each component that participates in the vibration of the speaker during the movement, including the drum paper part, voice coil, elastic wave, and the air quality involved in the vibration. Unit: grams (gram).
10. Cms (Li Shun): refers to the flexibility of the supporting parts of the speaker vibration system. The larger the value, the softer the entire vibration system of the speaker. Unit: millimeter/newton (mm/N).
Second, the nonlinear distortion of the speaker
When the loudspeaker reproduces the sound, there will be many additional signal components, resulting in non-linear distortion. There are mainly the following.
1. Harmonic distortion
The non-uniform magnetic field in the magnetic gap, the compliance of the edge of the vibration system and the centering piece will cause harmonic distortion in the case of large amplitude. This kind of distortion always appears in the low frequency band. The lower the frequency, the larger the cone amplitude and the more obvious the harmonic distortion.
2. Modulation distortion
The speaker voice coil inputs both low frequency and high frequency signals at the same time, such as low frequency 100~200HZ, high frequency 6~7KHZ, and the paper cone vibrates at the same time, and the phenomenon of high frequency sound vibration modulating low frequency will inevitably produce modulation distortion and deteriorate the tone. Stiff.
3. Transient distortion
The voice coil of the loudspeaker is energized to drive the cone to vibrate and produce sound. At this time, a certain inertial effect will inevitably occur. It takes a certain amount of time for the inertial system to reach stability. If this set-up time is too long, various extremely fast-changing signals will follow one after another, and the vibration of the paper cone cannot keep up with the changes in the signal, resulting in sound playback distortion, especially pulse-shaped The sound is the most obvious.
4. Subharmonic distortion
Subharmonic distortion is an inherent distortion of paper cone speakers. During the positive half cycle of the input signal, the paper cone bends upwards. In the negative half cycle of the signal, the paper cone is straightened and bent slightly downwards due to the effect of inertia. It will move downwards only when the positive half cycle of the next input signal arrives. Therefore, the movement of the paper cone is only half of the signal frequency, resulting in sub-harmonic distortion. This kind of distortion is more obvious for linear paper cones, while exponential paper cones hardly produce subharmonic distortion.
3. The maximum linear displacement volume of the loudspeaker
The maximum linear displacement Xmax refers to the unidirectional displacement limit of the cone when the speaker unit is working, and the unit is MM. When the one-way amplitude of the speaker cone exceeds this limit, the number of turns of the voice coil that cuts the magnetic field decreases, resulting in a decrease in the driving force of the voice coil, and the output sound pressure of the speaker enters a non-linear state, and the distortion increases significantly.
The result of the maximum linear displacement Xmax of the loudspeaker unit and the effective vibration area Sd of the loudspeaker unit is what we usually call the maximum linear displacement volume Vd, which determines the maximum volume of air that the loudspeaker can push when radiating sound.
The maximum linear displacement volume of the speaker largely determines the maximum output sound pressure level of the speaker unit in the morning at the lowest frequency end. The larger the maximum linear displacement volume value of the speaker, the sound pressure that the speaker unit can radiate at lower frequencies Higher. Loudspeakers with larger diameters not only have a larger effective vibration area, but also have a larger maximum linear displacement. Therefore, this kind of speaker has the advantage of higher low-frequency output sound pressure level.
The equivalent volume of the speaker unit means that after a certain speaker unit is placed in a cabinet with a certain internal volume, if the sound compliance of the air in the cabinet is exactly equal to the sound compliance of the speaker unit used, then the internal volume of the cabinet is The equivalent volume of the speaker unit, the equivalent volume of the speaker is referred to as Veq.
The equivalent volume Veq of the loudspeaker unit and the quality factor Q of the loudspeaker together with the resonance frequency F determine the low-frequency characteristics of the loudspeaker. Therefore, it is also one of the important parameters for the design of speakers.
Four, speaker power characteristics
The power parameter value marked on the speaker is one of the important indicators of the product. Due to the inconsistency in the definition of power quality indicators for speakers at home and abroad, it has caused confusion in the specifications of the same product. In order to eliminate this confusion, my country promulgated a new national standard in 1985, passed the new national standard GB9396-9400 in 1988, and revised it to the new national standard GB9396-9397-1996 in 1996.
In the new ministry standard and the new national standard, combined with the "Electroacoustic Devices-Loudspeaker" and "Minimum Requirements for Loudspeakers in High Fidelity Equipment" issued by the International Electrotechnical Commission, the speaker quality index-power is defined as characteristic power and maximum Noise power (rated noise power), maximum sinusoidal power, long-term power, (rated long-term power) and short-term power, etc.
1. Nominal power
The nominal power is measured by continuous, sine wave effective value power to determine its distortion value, and it is determined by the speaker distortion index. For example, if a speaker`s mark distortion is less than 3%, the distortion is 3% when it is set at 5W, then the speaker`s distortion value is 3%. The nominal power is 5W.
2. Characteristic power
Characteristic power refers to that in the frequency range of 100~8000HZ, the measuring instrument inputs the pink noise signal to the speaker system, and a characteristic sound pressure level of 94dB is generated at a distance of 1M from the sound source. Its value depends on the sensitivity of the speaker.
3. Maximum noise power (rated noise power)
The speaker system is within a certain rated frequency range, and it is specified that a special test noise signal is added to the speaker 100H for testing (the spectrum distribution of the noise signal is closer to the actual program signal), the result is no overheating and mechanical damage, and long-term safe operation is achieved , The power obtained by this test is called the rated noise power. This power has nothing to do with distortion, so it is often 2 to 4 times larger than the nominal power. Foreign speakers generally identify this power, and domestic speakers are gradually using the fixed value of this power.
4. Maximum sine power
The loudspeaker system is fed with continuous sinusoidal power test within a certain frequency range. As a result, the loudspeaker voice coil vibration should not produce bottoming sound, and there is no overheating or mechanical damage. Since the power is not limited by the distortion value, the power is higher than the nominal power.
5. Long-term power (rated long-term maximum power)
The loudspeaker system feeds a special noise signal power test within a certain frequency range. The loudspeaker will not produce permanent mechanical damage within 1 minute of this power. The test is performed once every 2 minutes and repeated 10 times. This power is much larger than the rated noise power.
6. Periodic power
In a certain frequency range, feed the noise signal power specified by the loudspeaker system for testing. If the loudspeaker bears this power within 1S, it will not cause permanent mechanical damage, then this power is short-term power. It has the largest standard value among all named powers, which can be 8 to 10 times larger than the nominal power.
7. Music power
The power mainly depends on the speaker's ability to withstand short-term sinusoidal signal frequencies below 250HZ. The loudspeaker bears this power, and through actual tests, there is neither obvious distortion, nor overheating and mechanical damage. The standard value of music power is derived from the German DIN45500 standard, which is the actual standard value of integrated power.
At present, most of the noise signal generators are used to test the power of speakers. Noise is an irregular and intermittent signal. Generally, there are two types of noise signals: white noise and pink noise. The words "white" and "pink" are determined by the frequency spectrum of these two kinds of noise.
White noise is a kind of random noise, this kind of noise signal contains various frequencies from 20HZ~20KHZ, and the energy of these signals is uniformly distributed; pink noise is also a kind of random noise, but its energy distribution is inversely proportional to frequency, so There are many low-frequency components in pink noise. A certain signal power is fed to the speaker under test by the power amplifier during the test. The ratio of the square of the effective value of the voltage input at both ends of the speaker under test to the impedance of the speaker under test is the speaker's withstand power .
Five, speaker quality factor
The quality factor of the speaker unit is one of the important parameters that must be understood before designing and making speakers. On the impedance characteristic curve of the speaker unit, it indicates the sharpness of the impedance peak of the impedance curve at the resonance frequency. It reflects the damping state of the loudspeaker vibration system to a certain extent, referred to as the Q value.
The quality factor of the speaker unit reflects the speed at which the speaker cone returns to a static state after the electrical signal input to the speaker unit disappears, or the damping effect of the electromagnetic system of the speaker unit on the vibration system, which reflects the transient response of the speaker unit Good or bad. The higher the quality factor of the speaker unit, the slower the energy consumption of the speaker vibration system, and the less easy to control the resonance.
The low-frequency characteristics of the loudspeaker are usually determined by the quality factor Q of the loudspeaker unit and the resonance frequency F to determine the influence of the Q value on the output sound pressure at F:
When the Q value is too low, the output sound pressure of the speaker drops rapidly before it reaches F, and the speaker is in an over-damped state, resulting in excessive low-frequency attenuation.
When the Q value is too high, the output sound pressure of the loudspeaker will have a peak at F, the loudspeaker is under damped, and the low frequency is excessively strengthened. The larger the Q value, the steeper the peak.
Therefore, the quality factor Q of the speaker cannot be too high or too low. We generally take its critical damping value, that is, Q equal to 0.2~0.6 as the best value range.
Speaker impedance characteristics
The impedance marked on the speaker, in Ω. The impedance of a loudspeaker unit is usually expressed by its impedance characteristics and rated impedance, but the two are completely different concepts.
1. Impedance characteristics
The speaker voice coil is a coil made of enameled wire on a cylindrical skeleton. In addition to a certain DC resistance, it also has a certain inductance. When the audio signal is input, the speaker voice coil vibrates up and down in the magnetic air gap. Due to the inductance of the voice coil, an induced voltage opposite to the audio signal will be induced in the voice coil at this time. This induced voltage opposite to the audio input signal will weaken the current in the voice coil, thereby increasing the impedance of the voice coil. Increase.
As the frequency of the audio signal increases, this effect will become larger and larger, which makes the impedance of the speaker voice coil increase with the increase of the audio signal frequency. The law that the impedance of the loudspeaker unit changes with the frequency of the signal is called the impedance characteristic of the loudspeaker unit.
2. Rated impedance
The rated impedance of the loudspeaker unit is the resistance value of a pure resistance, which is the first minimum impedance of the measured loudspeaker unit after the resonance frequency. At this time, the back-EMF generated by the self-inductance of the voice coil and the back-EMF generated by the voice coil vibration are equal in magnitude and opposite to each other, so that the impedance of the speaker is most approximately equal to the DC resistance of the voice coil.
The national standard GB9399-88 "Main Technical Parameters of Loudspeakers" stipulates that the preferred series of rated impedance values of loudspeaker units are: 2, 4, 8, 16 and 32Ω. At present, the rated impedance of most speakers at home and abroad is 4Ω or 8Ω.
The rated impedance of the speaker is given by the speaker manufacturer and marked on the product trademark or the lower magnetic plate of the speaker. The rated impedance of the speaker can also be estimated based on the DC resistance of the speaker voice coil. Multiplying the DC resistance of the speaker voice coil measured by the multimeter by 1.1 to 1.3 times is the speaker frequency characteristic of the rated impedance of the speaker.
The frequency characteristic of the loudspeaker refers to the law that the output sound pressure of the loudspeaker on the reference axis changes with the frequency of the input signal when the signal voltage of the input loudspeaker is constant. The frequency characteristic is also an important indicator to measure its quality. The requirement for the speaker frequency means whether the frequency range of the reproduced audio signal can meet the requirements, so the actual frequency characteristic curve must be measured.
1. Frequency response curve
The test must be carried out in an anechoic room. At present, the frequency response curve of most speakers is measured under the condition of 1M*1W, that is, the signal power input to the tested speaker unit is 1W; the measurement microphone is 1M away from the emitting surface of the speaker along the reference axis of the tested speaker unit; signal generator The output signal is amplified by the power amplifier and then fed to the speaker under test. The acoustic signal radiated by the speaker under test is received by the measuring microphone and converted into an electrical signal, which is processed by the measuring amplifier and sent to the level recorder. When the frequency of the output signal of the signal generator changes, the corresponding changes in the output sound pressure of the loudspeaker are simultaneously recorded by the level recorder. This is the frequency response curve of the measured loudspeaker unit.
2. Effective frequency range
Choose an octave wide band in the area with the highest sound pressure level on the frequency response curve of the tested loudspeaker unit, (octave refers to the frequency interval of two frequency ratios of 2, for example, 500~1000HZ, 3000~6000HZ are all one Octave) calculate the average sound pressure level of the bandwidth, and then use this average sound pressure level as a reference, and draw a straight line parallel to the X axis at 10dB below the average sound pressure level. The frequency range between the two points corresponding to the intersection of the horizontal straight line and the frequency response curve of the loudspeaker is what we often call the effective frequency range of the loudspeaker.
When the frequency of the signal input to the dynamic speaker is lower than its resonant frequency, the output sound pressure of the speaker drops at a rate of 12dB per frequency range. Therefore, the International Electrotechnical Commission IEC stipulates that the resonant frequency of the speaker unit is the lower frequency limit of the speaker. , And the intersection of the high-frequency end of the loudspeaker unit's frequency response curve is taken as the loudspeaker's high-frequency upper limit frequency. The frequency range between them is called the effective frequency range of the speaker.
Sometimes the speaker manufacturer also provides an index called the rated frequency range of the speaker. The effective frequency range of the loudspeaker and the rated frequency range are two different concepts. The former means that the loudspeaker unit can effectively reproduce the signal frequency range that meets the requirements of a certain sound pressure level. The latter means that the loudspeaker manufacturer is based on the product standards and specifications of the loudspeaker unit. The best operating frequency range specified by the application.
3. Unevenness
Ideally, the flatter the speaker frequency characteristic curve, the better, but the usually measured speaker frequency response curve is an irregular continuous curve with many peaks and valleys. Within the effective frequency range of the measured speaker, the difference between the maximum sound pressure level and the minimum sound pressure level on the frequency response curve is called the unevenness of the speaker unit. The peaks and valleys narrower than 1/9 octave can be ignored Excluding.