What Are The Indicators To Measure The Quality Of The Speaker?

- Mar 16, 2020-

1.Power


The sound quality of the speaker has no direct relationship with the power. Power determines the maximum sound intensity that the speaker can emit, and it feels like how powerful the sound from the speaker can be.


According to international standards, there are two ways to label power:


Rated power (RMS: sine wave root mean square) refers to the maximum electrical power of the speaker that does not cause any damage after a certain interval and repeated a certain number of times when driving an 8Ω speaker within the rated range;


After the instantaneous peak power (PMPO power) refers to the maximum power that the speaker can withstand for a short time.


The U.S. Federal Trade Commission established the power calibration standard in 1974: the effective wattage measured when two channels are used to drive an 8Ω speaker load and the harmonic distortion is less than 1% in the range of 20 to 20,000 Hz. The output power is the rated output power. In order to cater to consumer psychology, businesses usually mark the instantaneous (peak) power, which is usually about 8 times the rated power.


The power of the speaker is not as large as possible, and the application is the best. For a room of about 20 square meters for an ordinary home user, a true 60W power (referring to the effective output power of the speaker 30W × 2) is sufficient, The larger the reserve power of the power amplifier, the better, preferably at least twice the actual output power. For example, the speaker output is 30W, the power amplifier's capacity is better than 60W. For HiFi system, the power of the amplifier driving the speaker is very large.


2.Frequency range and frequency response


Frequency range: refers to the range between the lowest effective playback frequency and the highest effective playback frequency that the audio system can reproduce;


Frequency response: When a constant-voltage audio signal is connected to the system, the sound pressure generated by the speaker increases or decreases with frequency, and the phase changes with frequency. This sound pressure and The relationship between the phase and frequency (the amount of change) is called the frequency response, and the unit is decibel (dB).


The frequency characteristics of the sound system are usually described by the ordinate of the decibel scale representing the power and the frequency response curve of the frequency represented by the abscissa of the logarithmic scale. When the sound power is 3dB lower than the normal power, this power point is called the high frequency cutoff point and low frequency cutoff point of the frequency response. The frequency between the high-frequency cut-off point and the low-frequency cut-off point is the frequency response of the device; the curves of sound pressure and phase lag as a function of frequency are called "amplitude-frequency characteristics" and "phase-frequency characteristics", collectively called "frequency characteristic". This is an important indicator for examining the performance of the speaker. It has a direct relationship with the performance and price of the speaker. The smaller the decibel value, the flatter the frequency response curve of the speaker, the smaller the distortion, and the higher the performance. For example: the frequency response of the speaker is 60Hz ~ 18kHz +/- 3dB. The two concepts are sometimes not distinguished and are called frequency response.


In theory, a frequency response of 20 to 20,000 Hz is sufficient. The sound below 20Hz can be heard but the other sense organs can perceive it, that is, the so-called bass strength can be sensed. Therefore, in order to perfectly play various musical instruments and language signals, the amplifier must achieve the goal of high fidelity Each harmonic of the tone is reproduced. Therefore, the frequency band of the amplifier should be extended, the lower limit should be extended below 20Hz, and the upper limit should be increased above 20000Hz. The representation of the frequency response of the signal source (receiver, recorder, compact disc player, etc.) is different. For example, the frequency response of FM stereo broadcasting specified by the European Broadcasting Union is +/- 2dB at 40 ~ 15000Hz, and the minimum frequency response specified by the International Electrotechnical Commission for the recording stand: +/- 2.5 +/- 4.5dB (general Band), the actual achievable indicators are significantly higher than this value. The upper limit of the frequency response of the CD player is 20000Hz, and the low frequency end can be very low, only a few hertz, which is one of the reasons for the good playback quality of the CD player.


However, the harmonic components that make up the sound are very complicated, and it is not that the wider the frequency range is, the better the sound is, but this is basically correct for the low-end multimedia speakers. In labeling the frequency response, we usually see the terms “system frequency response” and “amplifier frequency response”. We must know that “system frequency response” is always smaller than the range of “amplifier frequency response”, so we only label "Amplifier frequency response" has no meaning, it is only used to fool some unsuspecting consumers. The current speaker manufacturers generally mark the system frequency response too wide, and the difference in high frequency is not much, but it is extremely unrealistic on the bass end. Foreign famous HiFi (high fidelity) speakers are only marked around 4, 50Hz. , And the domestic two to three hundred ordinary wooden speakers even dare to label this data, it really makes people laugh! So I would like to warn everyone that the low-frequency sound must be true to your ears, and do not easily believe the value on the leaflet. The music in the multimedia speakers is mainly based on MP3 or CD music, songs, game sound effects, background music, and human voices and environmental sound effects in movies. These sounds are mostly high and high. Therefore, when selecting multimedia speakers, It should be more concerned about its performance in the middle and high frequency bands, rather than the low frequency bands. If you really want the theater effect, then a powerful subwoofer can definitely meet your needs.


3. Distortion

There are harmonic distortion, intermodulation distortion and transient distortion. Harmonic distortion refers to the distortion caused by the addition of higher harmonic components that the original signal does not have in the sound playback; intermodulation distortion mainly affects the tone aspect of the sound; transient distortion is due to the presence of a certain inertial mass of the speaker, The vibration of the basin can't keep up with the difference between the original signal and the playback sound caused by the instantaneous change of the electrical signal. It is more important in speakers and speaker systems, which directly affects the degree of reproduction of sound quality, so this indicator is closely related to the quality of the speakers. This term is usually expressed as a percentage, the smaller the value, the smaller the distortion. The distortion of ordinary multimedia speakers is preferably less than 0.5%, and the distortion of subwoofers is generally large, and less than 5% is acceptable.




4.Sensitivity of the speaker (in dB)


For every 3dB difference in the sensitivity of the speakers, the output sound pressure is doubled. Generally, 87dB is the middle sensitivity, 84dB is the low sensitivity and 90dB is the high sensitivity. The increase in sensitivity comes at the cost of increased distortion, so as a high-fidelity speaker, to ensure the degree of sound reproduction and reproducibility, you must reduce some of the sensitivity requirements. However, it cannot be said in reverse that a speaker with high sensitivity must have poor sound quality and a speaker with low sensitivity must be good. Low-sensitivity speaker amplifiers are difficult to push (requires large reserve power of amplifiers). Although sensitivity is an indicator of the speaker, it has nothing to do with the sound quality of the speaker.


5.Impedance


It refers to the ratio of the voltage to the current of the speaker input signal. The input impedance of a speaker is generally divided into two types: high impedance and low impedance. Higher impedance is higher than 16Ω, lower impedance is lower.


The impedance of headphones is generally high impedance-32Ω is very common. The impedance of a power amplifier can generally be labeled as the equivalent impedance. For example, the output of 130W at 4Ω is equivalent to the equivalent of 80W output.


There is an easily confused term called "damping coefficient", which refers to the speaker impedance divided by the internal resistance of the amplifier source, which ranges from about 25 to 1000. The speaker cone needs to oscillate several times after the electrical signal has disappeared to completely stop swinging, and the voltage and current generated by the coil can prevent this parasitic movement, which is damping. The amplitude of the current, that is, the effect of damping, depends on the internal resistance of the current flowing through the amplifier output stage. This resistance is much lower than the rated impedance of the speaker. The typical value is 0.1Ω. With the series resistance of the network, it is difficult to achieve a damping coefficient of 50.




6.Signal to noise ratio


It refers to the ratio of the normal sound signal played back by the speaker to the noise signal (power) when there is no signal. Also expressed in dB. For example, the signal-to-noise ratio of a tape recorder is 50dB, that is, the output signal power is 50dB greater than the noise power. The higher the SNR value, the smaller the noise.


The minimum requirements of the International Electrotechnical Commission for the signal-to-noise ratio are 63 dB or more for the preamplifier, 86 dB or more for the post amplifier, and 63 dB or more for the combined amplifier. The optimal value of the signal-to-noise ratio of the combined amplifier should be greater than 90dB; the radio head: 50dB of FM stereo, in fact, it is better to reach more than 70dB; 56dB (common band) of the tape recorder, but the signal noise after Dolby noise reduction The ratio is greatly improved. For example, the signal-to-noise ratio after noise reduction by Dolby B can reach 65dB, and the signal-to-noise ratio after noise reduction by Dolby C can reach 72dB (the above refers to ordinary bands); the signal-to-noise ratio of CD player can reach more than 90dB. More than 110dB. When the signal-to-noise ratio is low, the noise is serious when the small signal is input, and the sound in the entire sound range obviously feels cloudy, so speakers with a signal-to-noise ratio below 80dB are not recommended to buy! Subwoofers below 70 dB are not recommended for the same reason.