When planning this feature article in the early stage, it seemed too professional and complicated, because it involved the design of the loudspeakers, the production and manufacture of the components of the driver unit. For our audience, the vast majority of them are salespeople, or AV custom installation technicians, and some AV enthusiasts, who are not professionals engaged in speaker development, production, and manufacturing, so they often exceed the scope to a certain extent in terms of speaker distortion content.
As an audio-visual custom installation technician, you often configure, install and debug audio-visual equipment for customers, and it is inevitable that you will come into contact with audio-visual equipment and audio test equipment. During debugging, you need to test the audio equipment, on the one hand, to ensure that the product in the user's hand is in a normal state, and on the other hand, to help you set the parameters of the system, so as to better optimize the performance of the audio system. During maintenance, it is also necessary to troubleshoot the system through testing. I think understanding speaker distortion can help you solve some of the problems in your work. As a salesperson, you also often come into contact with customers, and you need to sell your products to customers, and knowing more can increase your own conversation. As a lover of audio and video, you may wish to learn more, from which you can understand your own speakers, and even in the future when you go deep with the sound system, this special content can also help to a certain extent, maybe when you read the content of this issue, it will change some of your past perception of speakers.
The loudspeaker is the final playback component of the sound system and is responsible for the electro-acoustic conversion function. From the outside, it is a box with a drive unit, and if you open it, you can see the crossover. There are quite a few audio companies in the world that design and produce loudspeakers, and a large number of people in the audiophile circle are DIY. In fact, anyone who has ever done a loudspeaker knows that loudspeakers are really very difficult to do well, so speakers seem simple, but they are actually easy to make and difficult to refine. The distortion of a speaker is one of the indicators of a loudspeaker's performance, and it is also a very important one. Regardless of whether it is a manufacturer or an enthusiast DIY, various efforts are made to control this indicator within the allowable range.
The main contents of this topic include:
First, the source of the distortion of the speaker - the magnetic circuit part
Second, the source of distortion of the speaker — the vibration part
Third, the source of distortion of the loudspeaker — the crossover, the cabinet section and the inverter
Fourth, the distortion of the speaker — total harmonic distortion
Fifth, loudspeaker distortion — impulse response distortion
Sixth, the distortion of the speaker — phase distortion
Seventh, the distortion of the speaker — intermodulation distortion
Eighth, the distortion of the speaker - the distortion of the sub-harmonic
Ninth, the distortion of the speaker - high-power harmonic distortion
Where does the distortion of the speakers come from?
The primary source of distortion in a loudspeaker must be the driver unit in the first place, because it is the part of the speaker that really implements the electroacoustic conversion, which affects the sound quality of the entire speaker. At present, the dynamic speaker is one of the most widely used types of driver units, and in the past topic, it has been said that the dynamic speaker must undergo two conversions from electricity to sound, first converting the audio electrical signal into mechanical motion, and the mechanical movement drives the diaphragm to move and then drive the air, and finally transmits it to our ears, so all energy conversion will produce distortion.
Above: The length of the voice coil is equivalent to the length of the magnetic gap; Middle: short voice coil (voice coil length is less than magnetic gap length); Bottom: Long voice coil (voice coil length is greater than magnetic gap length), and three different designs of voice coil displacement in the magnetic field
Distortion caused by the magnetic circuit system
Turn a drive unit behind and you can see its magnet (also known as "magnet"), the material of the magnet is usually ferrite magnet, neodymium magnet, a few cobalt magnets, and even excitation (electromagnets). In addition, you can see parts such as voice coils, centering pieces (also known as "bullet waves"), T-irons, and magnetic plates.
If you look closely at the voice coil and gently press the diaphragm with your hand, you will see that the voice coil moves in a gap formed by T iron and a magnetic plate, called an "air gap". Under the action of magnets, the air gap will form a magnetic field, so when the audio electrical signal passes through the voice coil, it will be displaced in the magnetic field under the action of electromagnetic induction.
From a perfect theoretical point of view, the magnetic flux distribution must be uniform so that the voice coil can be evenly forced, theoretically without distortion. However, in fact, the magnetic flux distribution is not uniform, located in the middle of the air gap, and the two ends are uneven. In other words, the middle part of the air gap is a linear region, and the ends are nonlinear regions. As a result, the distortion is minimal when the voice coil is displaced in the linear region, and increases when the displacement range increases beyond the linear region. When designing the drivers, engineers wanted to have as much displacement range as possible in the linear zone of the voice coil. A typical approach is the "short voice coil, long magnetic gap", called "underhung coil", which is to shorten the length of the voice coil so that it is smaller than the length of the magnetic gap.
The force factor Bl(x), which indicates that the voice coil length is 10 mm (red line), and the Bl(x) curve in the magnetic gap of 10 mm (blue line), 5 mm (green line), 15 mm (pink line) and 20 mm (black line) lengths, respectively
To represent the characteristics of the voice coil in the magnetic gap, it can be represented by the force factor Bl(x). It can be seen that in short voice coils, "short voice coils, long gaps" (10mm voice coils/15mm gaps, or 10mm voice coils/20mm gaps), you can see that the Bl(x) curves have a long steady state, but once they leave the linear displacement range, the Bl(x) curves will decrease sharply, showing a hard nonlinear state. In contrast, the stationary state of the long voice coil is relatively short, and the Bl(x) curve decreases relatively slowly when the voice coil leaves the displacement range, showing a soft nonlinear state.
"Magnetic field modulation" due to voice coil
When an audio electrical signal passes through the voice coil, it also generates a magnetic field that spreads in the magnets, magnets, and air. The magnetic flux of the voice coil is related to the position of the voice coil and the current intensity, and the magnetic field of the voice coil will affect the magnetic circuit, which can increase the total magnetic field strength to saturation or decrease, and the magnetic flux density will change, which is called "magnetic field modulation", and will also produce distortion. Therefore, it is necessary to reduce the inductance of the voice coil and control the total magnetic field strength. Many drivers incorporate short-circuit rings close to the voice coil, which are usually made of copper or aluminum rings (or caps). When a short-circuit ring is added, the alternating magnetic field of the voice coil generates a current in the short-circuit ring and forms a reverse magnetic field, which reduces the inductance of the voice coil and reduces distortion.
The energized voice coil itself also generates a magnetic field, which modulates with the magnetic circuit of the driver unit, resulting in magnetic circuit distortion
Miller&Kreisel MX300系列里面使用的中低音驱动器,Aluminium Shorting Ring标识的就是它的短路环
Deformation of vibrating parts
The vibrating parts of the drive unit include the diaphragm, voice coil skeleton, dust resistance, suspension system (centering support and folding ring), etc., which will be deformed due to force when the drive unit is running, and in the case of high stress, these vibrations will be nonlinear, resulting in distortion. For example, at high frequencies, components such as diaphragms, dust caps, and voice coil frames will vibrate separately, and at low frequencies, the suspension system will be non-linear. In order to reduce the distortion caused by vibrating parts, it is necessary to work hard in terms of materials and processing technology, and pursue parts that are both lightweight and hard enough, so diaphragms and other parts made of beryllium, diamond, boron carbide, titanium, etc., will appear to reduce deformation and distortion.
Vibrating components, including diaphragms, centering tabs, and folding rings, are all deformed when the driver emits sound, as is the case with split vibrations
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To be continued