About dB

There are too many uncertainties in sound. Despite this, engineers still try to set some rules. You should know that these "rules" are not very old and may not be as old as they used to be. Otherwise, the formulation of these rules might have been based on your opinions.

dB is called "decibel" in Chinese. It has no special meaning. It is just a unit like "cm" and "kg". Weight is expressed in "kg" and length is expressed in "cm". Sound also needs a unit, so let's use "decibel"! This decision scared away a group of people. Those who stayed continued to "regulate" and "define" without knowing the world. It's too late for us to change it. Now people talk about "decibel" and "dB". We netizens should not be old-fashioned. "Beethoven" and "decibel" are not related.

How long is 4 cm? Think about it, if you don't have the concept of how long 1 cm is, you definitely don't know how long 4 cm is. How loud is a sound? Do we also have some preconceived concept? Let's first define how loud 1 watt of power is, and then see how loud 4 watts is. That's what the engineers thought back then. 4/1 = the size we want (size is determined by comparison!) It was originally a very simple concept, but Mr. Bel insisted on using a log (educated - no, at least you have to graduate from high school to understand what "log" is). "Log" is called "logarithmic function" in Chinese.

log(4/1) = 0.6021,

He also said that it is inconvenient to have decimals, so we should multiply by 10! (So far, it has been Bel who has been talking, and I really don’t know why people in later generations should listen to him.) So the standard of sound volume was established, decibel (dB) = 10 times log (power 1/power 0), power 1 = 4 watts, power 0 = 1 watt. Now let’s describe how loud 4 watts is:

10xlog(4/1) = 6dB

The sound produced by 4 watts of power is 6dB louder than that produced by 1 watt of power.

According to the above calculation, our experience is that for every doubling of power, the sound pressure increases by 3dB.

Csp58: 1 watt is 102.5dB, 2 watts = 102.5+3; 4 watts = 102.5+3+3; 8 watts = 102.5+3+3+3; 16 watts = 102.5+3+3+3+3. (We had a lot of 2/2/2/..., 3+3+3+... before, so it turns out to be like this!)

Standard calculation formula for power and sound pressure level:

decibel dB = 1 watt sound pressure level + 10xlog power

The sound pressure level of the Csp58 speaker at 1 watt = 102.5dB. The sound pressure level calculation at full power of 300 watts is:

300 watts sound pressure level dB = 102.5+10xlog300 = 102.5+10x2.4772 = 127.5d

The sound pressure level of Csp58 speaker 1 watt = 102.5dB, the sound pressure level calculation of maximum power 600 watts:

600 watts sound pressure level dB = 102.5+10xlog600 = 102.5+10x2.7781 = 130.3dB

Or if the power is doubled, the sensitivity will increase by 3dB. The sound pressure level at 600 watts = 127.5 + 3 = 130.5dB

(Some manufacturers also follow others to mark "maximum output sound pressure level", and I don't know where they copied it from---we don't mark this parameter---copying data is nothing, they also steal the trademark. I heard that they are engineers, and the mistake is too outrageous. I hope someone will inform them and correct it quickly! We don't want too many laymen. It seems reasonable that Pulse sells hundreds of thousands of speakers, at least they understand dB.---This seems to be too far off the topic, don't mind 2001.12.3.)

It’s not over yet. Sound propagation is related to distance. I didn’t mention the distance before because I didn’t want to scare you away. Now that you have learned this far, let’s learn more (which is more difficult than before).

Sound propagates in the air, with a point as the center, and spreads outward in a spherical shape (this has nothing to do with whether the type of sound propagation is a longitudinal wave). Assuming the radius of the ball is 1 meter, the surface area of ​​the ball = 4x3.14x12 = 12.56M2. If the radius doubles to 2 meters, the surface area of ​​the ball = 4x3.14x22 = 50.24M2; 50.24/12.56 = 4, which means that the surface area increases four times when the distance (radius) doubles. If the power remains unchanged and the area increases four times, then the power per unit area is only 1/4 of the original (the original power is 1 watt, and this 1 watt of power is distributed over an area of ​​1M2; now the power is still 1 watt, but the area has increased to 4M2, so the power per 1M2 of this 4M2 = 1/4. The most important thing here is how we understand that the power drops to 1/4 due to the four-fold increase in area. Use your imagination, and it won't be difficult after you figure it out).

Every time the power doubles, the sound pressure level increases by 3dB; conversely, every time the power decreases by 1, the sound pressure level decreases by 3db, 1 doubled = 1/2, 1/2 doubled = 1/4, 3dB+3dB = 6dB, because it is a decrease, add a "minus" sign in front. Use the previous formula to calculate:

The distance doubles the sound pressure level = 10log(1/4) = -10x0.6021 = -6dB

Our experience is that the sound pressure level decreases by 6dB for every doubling of the distance.

Standard distance and sound pressure level calculation formula:

decibel dB = 1 meter sound pressure level - 20xlog distance

(The negative sign indicates a decrease, 20xlog distance = 2x10xlog distance ≥ 10xlog power 2)

Derivation of the sound pressure level in dB at 1 meter and 1 watt from the sound pressure level at 2 meters and 1 watt (from multiplication by 10 to multiplication by 20)

1W∞1W

10log4пr2∞10log4п(2r)2

10log4пr2∞10log(4пr)2

10log4пr2∞2x10log4пr

10log4пr2∞20log4пr

For example, Csp58, the sound pressure level at 1 meter = 102.5dB, the sound pressure level at 40 meters:

= 102.5-20xlog40 = 102.5-20x1.6021 = 70.5dB

Calculation of sound pressure level at 40 meters with full power of Csp58 at 300 watts:

First calculate the full power sound pressure level at 1 meter (127.5dB), then apply the "Distance and Sound Pressure Level" formula = 127.5-20xlog40 = 95.5dB

●Summary:

Double the power and increase the sensitivity by 3dB

Double the distance, reduce sensitivity by 6dB

Now, you will thank Mr. Bel because the originally complex sound has now become simple.

●Speaker impedance

Common places where impedance is mentioned in general audio equipment include speaker impedance, input impedance of pre- and post-amplifiers, output impedance of pre-amplifiers (post-amplifiers are usually not called output impedance, but output internal resistance), transmission impedance of signal lines (or characteristic impedance), etc. Since the unit of impedance is still ohm, Ohm's law also applies. Therefore, in a nutshell, under the same voltage, the higher the impedance, the less current will flow, and the lower the impedance, the more current will flow. The most common speaker impedance indication value is 8 ohms, which means that when the pair of speakers are tested in the factory, when a 1KHz sine wave signal is input, the impedance value it presents is 8 ohms; or it is an average impedance value within the working frequency response range of the speaker. It is not a fixed value, but varies with different frequencies. When the post-stage outputs a fixed voltage to the speaker, according to Ohm's law, a 4-ohm speaker will flow twice as much current as an 8-ohm speaker. In theory, an 8-ohm crystal post-stage with an output of 100 watts will automatically become 200 watts when connected to a 4-ohm speaker. When the impedance of the speaker drops all the way, the output of the post-stage is a fixed voltage, and the current flowing through it will become larger and larger, and finally it is a bit like short-circuiting the speaker line, so the impedance value will sometimes be as low as one ohm. If it exceeds this range, the machine will burn out. This is also the seemingly true reason why people often say: the power of the post-stage does not need to be large, but the output current must be large.

●Speaker box shape test

When designing a speaker, the traditional cone speaker unit is usually not in the following forms:

1. Closed type (also called air cushion type, suspended type or infinite baffle type).

When a single unit moves back and forth in the air, the diaphragm pushes forward to generate a strong energy in front, but there will be a short vacuum behind the diaphragm. The air pressure in front and the pressure in the back are added together to offset a lot, so the volume of the single unit without a speaker is very small. It is not a problem for tweeter or midrange units because they have short wavelengths and large diffusion areas. But bass is different. Humans are not sensitive to low frequencies and need a lot of sound pressure to satisfy them. A closed speaker completely isolates the outside air. When the single unit moves, the air in the box expands and compresses accordingly. The air controls the single unit tightly like a spring, and can get correct, fast and deep bass. The low-frequency extension of the closed-design speaker is absolutely related to the volume of the speaker. The larger the speaker, the deeper the low frequency. In order to avoid sound reflection everywhere, a lot of damping or sound-absorbing materials must be added to the speaker. In order to maximize the effect of the spring, the thickness of the diaphragm of the small air cushion speaker will increase, and they are not easy to push in comparison.

2. Reflective type.

The speaker is not airtight, but has several openings in the front or back. When the unit moves, the back wave is not absorbed, and the energy is fully utilized in a guided way. The advantage of the reflective type is that you can get more bass without a large speaker, or you can say that the same power can produce greater sound pressure. However, there are also things to note about this design. For example, the duct cannot be too large, otherwise there will be peaks, and the sound of air circulation may be too loud. The length of the duct will also affect the resonant frequency. Poor design may cause the bass to be too heavy or the speed to keep up. Some designers do not use a reflex tube, but install a paper cone without a voice coil at the exit, which is called a passive radiator, hoping to achieve the dual effects of increasing energy and maintaining speed.

3. Horn type.

It is divided into front-loaded type and front-loaded type (folded horn). The front-loaded type is to connect the drive unit directly to the horn, effectively transmit the vibration pressure to the air, and the horn has a supercharging effect. The front-loaded type mostly adopts a short horn design, but it is quite unfavorable for low frequencies. To regenerate extremely low frequencies, the horn is often only ten meters long, so it is sometimes used in conjunction with a reflex speaker. The advantage of the horn design is high efficiency. In order to avoid resonance with the driver, the horn is mostly cast in metal or cut from logs, but the cost is very high. The rear-loaded model horn speaker looks like it has no horn. Its monomer still emits sound forward, and the back wave is guided into a curved channel, and finally squeezed out from the horn-shaped opening, which can effectively extend the low frequency. The folded horn also has the problem of complex speakers and high costs, and is now rarely used.

4. Transmission Line, also known as maze design.

This is actually a modified reflex design developed by John Wright, the founder of TDL in the UK. John Wright believed that although the reflex speaker can enhance the bass energy, it cannot really make the low frequency dive very low, so he designed multiple parts in the speaker with baffles, on the one hand, to allow each unit to have an independent and ideal acoustic space, and on the other hand, to allow the low-frequency wavelength to really breathe. To reproduce the complete sound wave of 20Hz, a distance of about 17 meters is required, and the minimum distance is 8 meters for half-wave calculation. The listening space is very small and meets this condition, so why not let the speaker do it? In addition to the complex structure of the transmission line speaker, a large amount of sound-absorbing materials are also laid out inside, which absorbs the harmonics of the back wave of the unit and only allows the same low frequency emitted from the front of the unit to appear, but because of the carefully calculated length, the low frequency has become deep and clean. Therefore, the transmission line speaker can usually achieve the effect of a large speaker with a medium volume. It must be considered that a poorly designed transmission line speaker may also cause the counter-effect of too much and too slow low frequency.