Acoustic Principles and Sound Technology Encyclopedia

(1) Acoustic history
In 1915, an American named E.S. Pridham put a telephone listener of the time on a horn that broadcasts a record sound, and when the sound could be heard by a group of people celebrating Christmas in San Francisco, electroacoustics was born.

When the First World War was over, at the inauguration of President Harding of the United States, the Bell Company of the United States connected the dynamic receiver of the telephone to the horn of the record player at that time, and the sound could be transmitted to the president. A large group of people at the inauguration ceremony led to many professional audio research and development of the knowledge of sound reinforcement engineering.

The famous scientist Lord Calvin of the United Kingdom often said: "When you measure what you describe and can express it in numbers, you already have some common sense about it. But if you can’t express it in numbers, then your common sense It is still crude and incomplete; for anything, this may be the source of common sense, but your ideas have not yet reached the state of science."

(2) The difference between studio sound and live sound

The requirements of live sound and studio sound are different, so many devices are also different.

For example, the mixers used in the recording room have multiple parameter equalization for each input, so that the sound engineer can make the finest fine-tuning of each input sound source as much as possible to achieve the best sound source effect. A mixer used for live sound generally has relatively simple equalization on each input. Due to many hours, the live tuner does not have much time to fine-tune the sound source of each channel carefully. In the live audio mixer, the volume control fader of each channel can not only attenuate the volume, but also It can gain 10-14 dB. If it is a mixer used in a recording studio, this fader does not need to be gain for many hours, so the English name of this fader is fader, which means attenuator. High-power amplifiers used in live audio, they will all have fans for heat dissipation. Because live audio amplifiers are often operated at maximum power output, and there are many hours of outdoor live audio, the surrounding temperature may be appropriate high. If you are in a recording room, there must be an air conditioner. Of course, the temperature will not be too high. The power amplifier in the recording room is mainly used to push the monitor speakers. Of course, it does not need to output a lot of power, so the power amplifier only needs to be ordinary The radiator can dissipate very little heat. If the power amplifier is equipped with an electric fan, the sound from the electric fan will form noise instead, so the power amplifier in the recording room basically does not need an electric fan.

The speakers used in live sound are designed to convey a large sound pressure to distant audiences, so they require high-power sound reinforcement, but the monitor speakers used in the studio are used by the sound engineer to monitor the sound source. Or the final result of recording, the sound engineer is sitting in a place very close to the monitor speaker to monitor, so the monitor speaker is a close sound field speaker, which requires high sensitivity, which is completely different from the live sound speaker.

(3) The connection between audio frequency and wavelength
Many on-site tuners ignore the connection between audio and wavelength. In fact, this is very important: audio and wavelength are directly related to the speed of sound. Under the altitude air pressure and 21 degrees Celsius, the sound speed is 344m/s, while domestic tuner, their commonly used sound speed is 34Om/s, this is the sound speed at 15 degrees Celsius, but everyone The main thing to remember is that the speed of the sound will change with the air temperature and air pressure. The lower the temperature, the higher the density of molecules in the air, so the speed of the sound will decrease. If you do live sound in a high-altitude area , As the air pressure is reduced, the molecules in the air become sparse, and the speed of sound will increase. The relationship between audio frequency and wavelength and sound is: wavelength=sound speed/frequency; λ=v/f, assuming that the speed of sound is 344 m/s, the wavelength of 100Hz audio is 3.44 m, and the wavelength of 1000hz (that is, lkHz) is It is 34.4cm, and a 20kHz audio wavelength is 1.7cm.

(4) High, medium, and low frequencies of speakers
How do we calculate the high, medium and low frequencies of this speaker?

First of all, we have to calculate the diagonal length of the speaker panel, which is the root of 2 = 1.414m. When the l/4 wavelength of any frequency exceeds 1.414m, it is a low frequency for this speaker; if a frequency is l/ When the 4 wavelength is 1.414m, the wavelength is 4×1.414m=5.656m, this frequency=344m/s÷5.656m=60.8/s=60.8Hz, so any audio frequency lower than 60.8Hz, it is convenient for this speaker Is its low frequency. When the frequency of 60.8 Hz or lower is transmitted from the speaker, their dispersion image is spherical, which is equivalent to if we hang the speaker in a room base, the volume of these frequencies is emitted from the front, back, left, and right of the speaker. The sound pressures coming out are all the same, and the sound coming out becomes non-directional. When the 1/4 wavelength of a frequency is less than the diagonal length of the speaker panel, but this wavelength is greater than the radius of the speaker, this section of frequency is the mid-frequency of the speaker. For example, we are now using an 18-inch unit. The radius of this unit is 9 inches, which is 22.86cm=0.2286m. The audio frequency is 344m/s÷0.2286m=1505Hz. The frequency from 60.8Hz-1505HZ is the middle of this speaker. frequency. The shape of the mid-frequency dispersion from this speaker is hemispherical, that is, if we release this frequency from the speaker that has just been suspended at the base of the room, the shape of the sound dispersion from the speaker panel is hemispherical. The sound of this frequency cannot be heard behind the speaker.

The frequency of 1505Hz and higher is the high frequency of this speaker. The shape of the sound dispersed from the speaker at high frequencies is cone-shaped. The higher the frequency, the narrower the cone shape. Generally, if the frequency exceeds 4 times of the initial high frequency, the shape of the sound dispersion will gradually become a straight line without dispersion. If you are not sitting in the direction of the alignment unit, you will not be able to hear these high frequencies. Therefore, if many high-frequency units are of a paper cone type, the diameter of the paper cone is very small. The lower limit of the high frequency of this speaker should be increased as much as possible, hoping to increase the width of the high frequency dispersion.

(5) Various different sound fields

When a cone speaker receives the signal from the power amplifier, the cone will swing back and forth. When the cone is pushed forward, the cone will hit the air molecules in front of it, and the air in front of the cone will be Add pressure, these molecules will continue to push forward, bumping the air molecules in front of them, forming a slender high pressure. When the paper cone retreats, the air molecules in front of the paper cone will have a slender vacuum, and then these molecules will follow the retreat of the paper cone, and the air formed here will have a slender pressure reduction.

But the air in front of the paper cone has just been swayed by the action of the paper cone and cannot reach the elastic force of the air itself. At this time, we have to look at the wavelength of this frequency. The sound will be heard until the interval of the paper cone is 2.5 times the wavelength. The air exerts its elastic force to form a sound. For example, for a frequency of 100 Hz, its wavelength is 3.44 meters, so the sound must be 2.5×3.44 meters = 8.6 meters away from the paper cone to be the real 100 Hz sound. If you use 100Hz to calculate, the distance from the paper cone has not reached 8.6 meters before the near sound field of 100Hz, and beyond 8.6 meters is the far sound field of 100Hz. When it comes to the near and far sound field of the speaker, the most important thing is to pay attention to the frequency and its wavelength, rather than simply looking at how far away from the speaker is equal to the far or near sound field. The most important thing is to remember that when we appreciate music, It is to be in the far sound field, not in the near sound field.

(6) Direct sound field, reflected sound field, and indirect sound field. When the speaker makes a sound in a room, the listener can hear the sound transmitted directly from the speaker. This is the direct sound field, but it can also be heard from the wall The sound reflected from the ceiling and floor is called the reflected sound field. The more the sound of the direct sound field is heard by the audience, the smaller the sound of the reflected sound field, and the better the sound. Since the sound of the direct sound field can be manipulated, the sound of the reflected sound field cannot be manipulated. The sound from the direct sound field will be added to the contrast to reduce the clarity of the original sound. Therefore, listeners who sit close to the speakers will feel a better sound effect, while the listeners sitting behind are likely to be The reflected sound field they hear is louder than the direct sound field, and the acoustic effect will be poorer and less clear.
(7)Interface disturbance
When we choose the position to place the speaker, the important thing is to notice that the sound emitted by the speaker will be affected by the interface next to it and cause disturbance. For example, for the main speakers placed on both sides of the platform, if their low-pitched paper cones are separated from the ground and the wall next to it at about 1 meter, a 4-meter wavelength audio will be disturbed by these two interfaces. The frequency of a 4m wavelength is 86Hz (344m/s ÷ 4m= 86Hz). When the 86HZ sound is released from the speaker, the large air pressure happens to hit the ground and wall within 1/4 week, and then l/ It was reflected back to the paper cone of the speaker in 4 weeks, but at this time it happened that the paper cone was going to retreat. The large air pressure reflected from the ground and the wall would be offset by the retreating action of the paper cone, and the main thing was lost. Whispered. If you encounter this situation, you should retreat the speaker 0.5-1 meters to the station, so that the sound from the speaker cannot directly hit the ground, and if you can move the speaker to the wall close to both ends, you can even use the wall The reflection system makes a louder volume. The frequency of 80-100Hz is very important. It is the consensus point of our lung space and the consensus frequency of low-pitched drums. If you don’t understand the interference of the interface and place the speakers in the wrong position, it is really not worth it. of.

(8) High and low sound effect
It is difficult for us to specify a high-pitched sound above a certain frequency or a low-pitched sound below a certain frequency. We often say that human hearing is from 20Hh-20KHz, but the frequency of 20kHz is rarely heard by people, generally only young people under 20 years old People can hear it when their ears are not damaged in any way. If an auditory test is done, the highest listening frequency is only 8 kHz. When the sound is transmitted, the high frequency decays much faster than the low frequency. If 1kHz is compared with 10kHz, when the sound runs 100 meters, the 10kHz frequency will attenuate 30-35dB compared to the IkHz volume. Compared to low frequencies, high-frequency sounds are more directional. After the high-frequency sound ran out of the unit, if it is obstructed by an object, the treble can no longer pass through. This is very different from the low-frequency sound. Because the high-frequency wavelength is relatively short, it will be affected by the object. After obstruction, it will not turn, but the low-frequency wavelength is relatively long, so in many hours, even if there is an object obstructing in front, the low-frequency can turn the past. For example, some professional speakers are planned to put a high-pitched horn in front of its low-pitched unit, but for the low frequency emitted by this low-pitched unit, it basically cannot see that something is blocking the sound in front of it, so Low frequencies can still pass the past.

From our hearing point of view, we need to hear high-frequency sounds to distinguish all kinds of different sounds, but if it is only human speech, we only need to hear frequencies of 4kHz and below, and we can immediately Distinguish who is talking. For example, the sound transmission of the phone, the high frequency only reaches 4kHz, so it is sometimes divided into a person who has not spoken to you for a long time, when he calls you, just say: [Hello], you can immediately distinguish him from you The sound of a brother who hasn't talked for a long time. We also have directionality when listening to high frequencies, which is the direction in which we can distinguish the source of high frequency sound. Since the high-frequency sound reaches our two ears, there is already a very slender time difference, so when they arrive at the ears, there are different phase changes. We can use this changed phase to determine the phase of the sound.
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