Explanation of sound
We have been providing silencing calculators for various projects for 35 years. Here, I will explain the basic knowledge of the items and sounds used in the silencing calculator so that those who are not specialized can understand.
What is “sound” in the first place?
Sound is often referred to as “air vibration.” The vibration is transmitted to the eardrum and microphone, and is recognized as sound. If the cycle (frequency / Hz) of the vibration is fast, the sound will be high, if it is slow, the sound will be low. If the pressure (Decibel / dB) is high, the sound will be loud, and if it is low, the sound will be low. Depending on the combination of this frequency and the decibel, the sound will be heard as various tones.
So what is “noise”?
The general term for sounds that give unpleasant feeling to the listener is called “noise”. It is said that it is almost impossible to clearly define noise by objective physical quantities because people have subjectiveness and the listener decides whether the sound is good or bad. . People who don’t want to listen to beautiful music seem to be noisy, and even the noise of the construction site seems to be less loud to those who are used to it. Thus, noise is very complex and requires careful prediction. For us involved in building equipment, the “ noise ” is the indoor noise of the air-conditioning and ventilation system, the noise from nearby equipment such as rattles, and the noise from outdoor equipment such as outdoor units and pumps.
The frequency (Hz / hertz) that represents the pitch of the sound indicates “how many times a second the air density repeats.” “1Hz” means “once per second”, and “1K (kilo) Hz” means “1,000 times per second”. It is said that the average adult can hear the sound of “20-20,000Hz” (audible range), but the range becomes narrower with aging. Recently, there has been a topic such as “preventing youth hangout by emitting mosquito sound (sound of about 1.7 KHz)”, but this is to emit high frequency sound that can be heard by people up to about 20 years old It is one of the means to make you feel uncomfortable and not to stay longer. Unfortunately, if you are over 40, you cannot hear anything. The decibel (dB) that represents the loudness of a sound is “the pressure of the target sound is multiple times when the pressure of the smallest sound (minimum audible value) that can be heard by the human ear is set to 1. Is displayed in a logarithmically compressed form. ” Originally, it is better to express the real number of “how many times”, but it is difficult to understand because a very large number must be used, and the decibel value is adopted for easy viewing. For example, suppose you have a number [3,162,277]. This is a representation of [10 to the 6.5th power], but in order to make it easier to see, the exponent is multiplied by 10 and is expressed in decibels. In other words, the sound pressure at 3.16 million times the minimum audible pressure is called 65 dB.
Type of decibel
Sound level meters that measure the loudness of sound can measure three types of characteristics: “F scale”, “C scale”, and “A scale”. “F scale” displays the purely generated sound pressure. The “C scale” has been corrected so that the frequency range below 50Hz and above 5,000Hz is displayed lower. It is said to have the effect of suggesting the effects of noise due to wind and other noises inside the sound level meter, and is used for general indoor noise measurement. The “A scale” displays the value corrected to the decibel value (audibility correction) that can be heard by human ears for each frequency range, and is mainly used when measuring outdoor noise. What we used to call “hon” in the past means this characteristic. As mentioned earlier, the decibel characteristics must be selected when measuring. Use “F or C scale” for indoor measurement and “A scale” for outdoor measurement.
Judgment of noise
Generally, there are two types of criteria for determining the noise level: “dB” and “NC”.
Used as a standard for outdoor noise. The standards are set by each administrative district as ordinances under the Noise Control Law. It is necessary to check each time what kind of area is applicable. Here, the Tokyo standard is shown for reference. To do. Measured values are rounded off and judged with integer values.
|Low-rise residential area
|Area for exclusive use of middle and high rise housing
|Commercial and semi-industrial areas
Used as a standard for indoor noise. It is determined at the time of design according to the purpose of the room, but if not specified, please refer to the table below. Note that any frequency of 63 “‘8KHz must be below the NC value.
|Use of living room (we recommend)
|1/1 octave band center frequency (Hz)
|Recording studio, Anabooth, Sound laboratory
|Music Hall (Audience Seat) / Photo Studio
|Music hall (stage), multipurpose hall, boardroom
|Large conference rooms, boardrooms, hotel rooms, sick rooms, cinemas, prayer rooms
|Meeting rooms, classrooms, wedding halls, banquet halls, lobbies, libraries, museums
|Office / Corridor / Other general living room
|Office, waiting room, laboratory
|Office in the factory • Other backyard
|Factory / computer room
※Effect of background noise
Sound other than the target sound is called background noise. When determining the operating noise of the equipment, the influence of background noise must be corrected. Basically, the noise generated from the equipment is constant, but the background noise fluctuates. For example, suppose that when measuring the noise during equipment operation, the background noise was 53 dB and the background noise was 49 dB. Generally speaking, “if the background noise is lower than 10 dB, it can be ignored”, but in this case, the difference is 4 dB, so correction is required. This correction is not a normal four arithmetic operation but a “subtraction of decibel value”, so a function calculator is required. Recently, I think that I often calculate with spreadsheet software such as Excel, so I will write an equation.= log10 (1 QA (53 [dB value of measurement result] / 10) -1QA (49 [dB value of background noise] / 10)) * 10
As mentioned in the unit section, the dB value is displayed by multiplying the exponent by 10, so divide the dB value of the measurement result by 10. The real number obtained by exponentiating the exponent is also log-compressed by subtracting the real number raised to the power of the background noise, and the exponent is multiplied by 10 to display a decibel. The procedure is as follows. This can be used for “decibel sum (synthesis)” by turning subtraction into addition, so please refer to it.
About silence calculation
This section describes the typical items used in the silencing calculation sheet that we have created. The formula in this book is quoted from “Air-conditioning silence design (Rigaku Kogyo)”.
※Basics of silencing calculation
For all silencing calculations,Sound pressure level (SPL) = generated noise power level (PWL) + distance attenuation (or radiation coefficient)One of the major factors that makes the silencing calculation book look complicated is that the numerical value in the book is PWL or SPL, but in any case, the PWL of the noise source was first calculated. The SPL must be obtained by adding the final distance attenuation or radiation coefficient (equivalent to the distance attenuation considering the reflected sound in the indoor environment), taking into account the transmission path above. The distributed silencing calculator is a mixture of cobblestones, and the one that does not adhere to this basic rule is logically incorrect, so it is an important point that customers who are readers should be aware of . What I want to say here is that the silencing calculation is a fictional sequence of numbers. Starting with the PWL provided by the manufacturer, the damping factor in the road and indoor environment is only an estimate. All we know correctly is the damping performance of the silencer and the distribution coefficient. The noise maker’s mission is to show how these estimates can be made closer to the expected noise that is closer to reality. If the result derived from the silencing calculation is the one and only correct result, the silencing maker’s own calculation method is meaningless and an imaginary calculation method should be adopted. However, in reality, the results of the silencing calculation submitted by each manufacturer are various, and even if the same system is calculated in the same way, the results will differ. This simply means that “silencing calculation is a calculation of expected noise that is based on the experience of each company and which can be held on its own responsibility.” Behind the calculation, there is a history of matching a huge amount of calculation results with the measurement results after it was installed. We have been continuing this work since our founding, and we have been working to ensure that the expected noise level and the actual noise level match.
Generated noise power level (PWL)
The sound output (W: watts) generated from the sound source is displayed in decibels. It does not indicate the “sound pressure level (SPL)” felt by the ear or sound level meter, but rather the sound energy generated by the sound source. The PWL of the blower we use for silencing calculation at the time of design calculates the total PWL from the air volume and the total static pressure, and performs calculations taking into account the frequency characteristics depending on the type of blower. In recent years, the use of expected PWLs provided by blower manufacturers has increased, but their sizes vary depending on the manufacturer, so verification is required. Another common mistake is to treat the device manufacturer’s SPL value as PWL. Note that SPL is a value that takes into account the distance attenuation at the time of measurement in PWL, so it is necessary to add the value of the distance attenuation and return it to PWL when calculating silencing.
※Sound pressure level (SPL)
Sound pressure level (SPL) = generated noise power level (PWL) + distance attenuation (or radiation coefficient).This SPL reaches ears and sound level meters. Means “sound”.
Distance attenuation and radiation coefficient
When a sound wave is generated, it has the property of expanding into a sphere. Based on this property, distance attenuation is calculated by calculating the area of the sphere whose radius is the distance from the sound source to the receiving point. Please understand that the sound wave density becomes thinner as it spreads.1Olog [Q / 4π r2] Q; directional coefficient, r; distance (m2)
[4π r2] is the formula for calculating the area of a sphere. By using this as a denominator, by assigning the directivity coefficient to the numerator, “what percentage of the spherical surface the sound wave spreads out” is calculated. In an environment without reflected sound, such as an anechoic room, it spreads out into one complete sphere, so Q = 1. If the sound source is on a semi-anechoic room with a concrete floor, or on the floor or ceiling, it expands to a half sphere, so Q = 2. Q = 4 because it will be expanded to the shape cut into / 4. The fact that sound waves that should originally spread on a 1/1 spherical surface expands only on a 1/2 or 1/4 spherical surface means that the density of the generated sound waves will be doubled or quadrupled, so the nature of decibels The difference comes out by the upper 3dB. For example, when the distance r = 1m and the directivity coefficient Q = 2, the distance attenuation is △ 8.0dB, but when Q = 4, it becomes △ 5.0dB.In this way, the shape of the diffusion changes depending on the position of the sound source, so it is important how to handle the directivity coefficient (Q). When predicting indoor noise, it is necessary to consider “reflected sound in the room” in addition to the aforementioned distance attenuation. Sound waves travel at a speed of about 340 m / s at 15 ° C • 1 atm (1013 hPa). Will be heard. The radiation coefficient is calculated by calculating the attenuation due to sound absorption, taking into account the degree of sound absorption depending on the dimensions and finishing of the target living room, and the direct distance attenuation.1O lag [Q / 4π r2 + 4 / R)] R (room constant) = aΣS / (1-a) a; sound absorption coefficient, ΣS; total surface area of the chamber (m2)
※Distribution ratio coefficient
Since the silencing calculation of the duct system calculates the route to the specific ventilation opening of the target room and finally combines the number of units, the distribution ratio coefficient is indispensable. When sound waves diverge in the traveling direction, sound energy is divided in proportion to the cross-sectional area of the duct, so it can be calculated by the following formula.1O log [S / ΣS] S; duct area to be calculated after branching (m2), ΣS: total duct area to branch (m2)
Since the duct area of the air-conditioning ventilation system is usually proportional to the amount of air passing through it, the airflow ratio and the number ratio are often used when calculating the distribution ratio coefficient, but there is no problem in using it as an approximate value. .
※Open end reflection
When sound waves propagating in the duct are radiated from the end opening, they are canceled out by reflection from outside air. The smaller the aperture size, the more attenuation there is in the low frequency range, and the larger the aperture, the less attenuation. In our silencing calculations, we may use corrected values according to the shape of the opening and the surrounding environment.
※Natural noise reduction in ducts
When sound waves propagate through the duct, they are converted to plate vibration energy of the duct or transmitted through the duct, reducing the relative sound energy. Generally, this is often ignored on the safety side, but at our company, if the duct path is long, in order to avoid excessive installation of silencers, に て 0.3-0.SdB per 1m of straight pipe duct length May be calculated. Also, in the curved pipe section, the attenuation in the high frequency range tends to be large, but we may use the value adjusted after considering the entire path.
※Various regenerated noise
For more than 30 years, we have been calculating the path from the PWL of air conditioners and blowers to the air intake at the end, but in the past, we have subtracted the attenuation factor in the road from the PWL and made it the calculation result, and it was calculated as the allowable noise The mainstream was to calculate the required attenuation with the difference. The disadvantage of this calculation method was that it did not take into account the regenerated noise of roadside dampers and equipment. In the living room where the NC-20 is targeted, this re-generated noise often causes problems. From a comprehensive point of view, we also raise issues regarding duct dimensions, damper positions, and equipment shapes and dimensions.
・Regenerated noise of damperPWL=L0 + 1 O logA+SSlogV
PWL: Airflow noise overall PWL
V: Average wind speed in the duct (m / s), A: Duct cross-sectional area (m2)
L6: Constant (dB) determined by damper blade angle (0)
・Instrument re-generation noisePWL=101ogA+a • logV+b
PWL: Airflow noise overall PWL
V: Ventilation wind speed or neck wind speed (mis)
A: Cross-section area or neck area (m)
a, b; constants (dB) experimentally determined for the type of air vent
It is calculated by taking into account the relative value of frequency from these overall PWLs. We perform calculations in line with the realities while reviewing constants in view of past examples.
※Sound transmitted from duct
About 20 years ago, when the method of calculating and combining the noise from each ventilation opening and calculating the expected noise in the target living room was the mainstream, we began to propose that “ from ducts Transmitted sound. ” Low-frequency noise is often a problem in rooms below the duct path, even without ventilation. We quickly pointed out that this was a problem with transmitted sound and established a calculation method for it. We do not offer any dramatic countermeasures because there are various solutions. While asking the customer’s wishes, we will formulate the best method for each case. In addition, even now, we continue to review the calculation method from the measurement results and pursue more accurate calculation of expected noise.
※Sound transmitted from the machine room
Although the bulkhead of the machine room has been dry for a long time, the problem of sound transmitted through the walls has become apparent. As the day progresses, the machine room becomes smaller and closer to the living room. Also, when the machine room is large or small, and when the air conditioner is near or far from the wall, there is a difference in the way the sound spreads. We have devised various calculation methods not only for duct noise, but also for transmitted sound from the machine room wall. The status of each room in each project varies widely. We are committed to addressing issues from both the architectural and the equipment side, and we are committed to helping the entire project resolve issues in a consistent manner.