Dba unit of measurement. Airborne and structural noise
Very often, beginners are faced with such a concept as decibel. Many of them intuitively know what it is, but most still have questions.
Relative logarithmic units of Bela (decibels) are widely used in quantitative assessments of the parameters of various audio, video, and measuring devices. The physical nature of the powers being compared can be anything - electrical, electromagnetic, acoustic, mechanical - it is only important that both quantities are expressed in the same units - watts, milliwatts, etc. Bel expresses the ratio of two values of an energy quantity by the decimal logarithm of this ratio, and Energy quantities mean: power, energy.
By the way, this unit got its name in honor of Alexander Bell (1847 - 1922) - an American scientist of Scottish origin, the founder of telephony, the founder of the world famous companies AT&T and Bell Laboratories. It is also interesting to recall that many modern mobile phones (smartphones) necessarily have a selectable ringing (alert) sound, also called “bell”. However, Bel refers to units not included in the International System of Units (SI), but in accordance with the decision of the International Committee of Weights and Measures, it is allowed to be used without restrictions in conjunction with SI units. Mainly used in telecommunications, acoustics, and radio engineering.
Formulas for calculating decibels
Bel (B) = log (P2/P1)
Where
In practice, it turned out that it is more convenient to use the Bel value reduced by 10 times, i.e. decibel, therefore:
decibel (dB) = 10 * log(P2/P1)
Strengthening or weakening power in decibels expressed by the formula:
Where
P 1 – power before amplification, W
P 2 – power after amplification or attenuation, W
Bel, decibel values can be with a “plus” sign if P2 > P1 (signal amplification) and with a “minus” sign if P2< P1 (ослабление сигнала)
In many cases, comparing signals by measuring power may be inconvenient or impossible - it is easier to measure voltage or current.
In this case, if we compare voltages or currents, the formula will take a different form:
Where
N dB – gain or loss of power in decibels
U 1 is the voltage before amplification, V
I 1 – current strength before amplification, A
I 2 – current strength after amplification, A
Here is a small plate that shows the basic voltage ratios and the corresponding number of decibels:
The fact is that the operations of multiplication and division on numbers in the usual basis are replaced by the operations of addition and subtraction in the logarithmic basis. For example, we have two cascaded amplifiers with gains K1 = 963 and K2 = 48. What is the total gain? That's right - it is equal to the product K = K1 * K2. Can you quickly calculate 963*48 in your head? Me not. I can estimate K = 1000*50 = 50 thousand, no more. And, if we know that K1 = 59 dB and K2 = 33 dB, then K = 59+33 = 92 dB – it wasn’t difficult to add up, I hope.
However, the relevance of such calculations was great in the era when the concept of Bel was introduced and when there were not only iPhones, but also electronic calculators. Now it’s enough to open the calculator on your gadgets and quickly calculate what is what. Well, in order not to worry every time when converting dB into several times, the most convenient way is to find an online calculator on the Internet. Yes, at least here.
Weber-Fechner law
Why decibels? Everything comes from the Weber-Fechner law, which tells us that the intensity of the sensation of human feelings is directly proportional to the logarithm of the intensity of any stimulus.
So a lamp with eight light bulbs seems to us as much brighter than a lamp with four light bulbs as a lamp with four light bulbs is brighter than a lamp with two light bulbs. That is, the number of light bulbs should double each time so that it seems to us that the increase in brightness is constant. That is, if we add one more light bulb to our 32 light bulbs on the graph, we won’t even notice the difference. In order for the difference to be noticeable to our eyes, we must add another 32 light bulbs to the 32 light bulbs, etc. Or in other words, in order for us to feel like our lamp is gradually gaining brightness, we need to light twice as many light bulbs each time as the previous value.
Therefore, the decibel is indeed more convenient in some cases, since it is much easier to compare two values in small numbers than in millions and billions. And since electronics is a purely physical phenomenon, decibels are not spared.
Decibels and amplifier frequency response
As you remember in the previous example with an op-amp, our non-inverting amplifier amplified the signal 10 times. If you look at our plate, it turns out to be 20 dB relative to the input signal. Well yes, that's how it is:
Also in dB on some frequency response graphs the slope of the frequency response characteristic is indicated. It might look something like this:
In the graph we see the frequency response of the bandpass filter. Signal change +20 dB per decade(dB/dec, dB/dec) tells us that for every increase in frequency by 10 times, the signal amplitude increases by 20 dB. The same can be said about the signal decay of -20 dB per decade. With each increase in frequency by 10 times, the amplitude of the signal will decrease by -20 dB. There is also a similar characteristic dB per octave(dB/oct, dB/oct). Here, almost everything is the same, only the signal changes with each increase in frequency by 2 times.
Let's look at an example. We have a first-order high-pass filter (HPF) assembled on an RC circuit.
Its frequency response will look like this (click for full opening)
We are now interested in the inclined straight line of the frequency response. Since its slope is approximately the same up to a cutoff frequency of -3 dB, you can find its slope, that is, find out how many times the signal increases for each increase in frequency by 10 times.
So let's take the first point at a frequency of 10 Hertz. At a frequency of 10 Hertz, the signal amplitude decreased by 44 dB, this can be seen in the lower right corner (out: -44)
We multiply the frequency by 10 (decade) and get the second point of 100 Hertz. At a frequency of 100 Hertz our signal decreased by approximately 24 dB
That is, in one decade our signal increased from -44 to -24 dB per decade. That is, the slope of the characteristic was +20 dB/decade. If +20 dB/decade is converted to dB per octave, you get 6 dB/octave.
Quite often, discrete attenuators (dividers) of the output signal on measuring instruments (especially generators) are calibrated in decibels:
0, -3, -6, -10, -20, -30, -40 dB. This allows you to quickly navigate the relative level of the output signal.
What else is measured in decibels?
Also very often expressed in dB (signal-to-noise ratio, abbreviated SNR)
Where
U c is the effective value of the signal voltage, V
U sh – effective value of noise voltage, V
The higher the signal-to-noise value, the clearer the sound provided by the audio system. For musical equipment, it is desirable that this ratio be at least 75 dB, and for Hi-Fi equipment at least 90 dB. The physical nature of the signal does not matter, it is important that the units are in the same dimensions.
As a unit of the logarithmic ratio of two physical quantities of the same name, neper (Np) - 1 Np ~ 0.8686 B is also used. It is based not on the decimal (lg), but on the natural (ln) logarithm of the ratios. Currently rarely used.
In many cases, it is convenient to compare not arbitrary values with each other, but one value relative to another, called conventionally reference (zero, base).
In electrical engineering, a power value equal to 1 mW allocated across a resistor with a resistance of 600 Ohms is chosen as such a reference or zero value.
In this case, the base values when comparing voltages or currents will be 0.775 V or 1.29 mA.
For sound power, this basic value is 20 microPascal (0 dB), and the threshold of +130 dB is considered painful for a person:
More details about this are written on Wikipedia at this link.
For cases when certain specific quantities are used as basic values, even special designations for units of measurement have been invented:
dbW (dBW)– here the countdown is relative to 1 Watt (W). For example, let the power level be +20 dBW. This means that the power has increased 100 times, that is, by 100 watts.
dBm– here we are already counting relative to 1 milliwatt (mW). For example, a power level of +30dBm will be correspondingly equal to 1 W. Don’t forget that these are energy decibels, so the formula will be valid for them
The following characteristics are already amplitude decibels. The formula will be valid for them
dBV– as you guessed, the reference voltage is 1 Volt. For example, +20dBV will give - this is 10 Volts
From dBV other types of decibels with different prefixes also follow:
dBmV– reference level 1 millivolt.
dBuV (dBμV)– reference voltage 1 microvolt.
Here I have given the most commonly used special types of decibels in electronics.
Decibels are also used in other industries, where they also show the ratio of any two measured quantities on a logarithmic scale.
There is also an interesting video on YouTube about decibels.
With input from Jeer
The physical characteristic of sound volume is the sound pressure level, in decibels (dB). “Noise” is a disorderly mixture of sounds.
The maximum permissible sound levels (LAmax, dBA) are 15 decibels more than “normal”. For example, for living rooms of apartments, the permissible constant sound level during the daytime is 40 decibels, and the temporary maximum is 55.
Inaudible noise - sounds with frequencies less than 16-20 Hz (infrasound) and more than 20 KHz (ultrasound). Low-frequency vibrations of 5-10 hertz can cause resonance, vibration of internal organs and affect brain function. Low-frequency acoustic vibrations increase aching pain in bones and joints in sick people. Infrasound sources: cars, carriages, thunder from lightning, etc. High-frequency vibrations cause tissue heating. The effect depends on the strength of the sound, the location and properties of its sources.
Powerful fans in bakeries, mills and other enterprises where exhaust hoods are used can make a lot of noise, and the wind blows from their side - wave-like increases the propagation range. A possible reason for their noise is improper installation and the design itself, broken bearings, misalignment, or simple wear and tear of the equipment. You may be fined for this.
High-frequency sound and ultrasound with a frequency of 20-50 kilohertz, with modulation of several hertz - are used to scare away birds from airfields, animals (dogs, for example) and insects (mosquitoes, midges).
In workplaces, the maximum permissible equivalent sound levels for intermittent noise are: the maximum sound level should not exceed 110 dBAI, and for impulsive noise - 125 dBAI. It is prohibited to stay even briefly in areas with sound pressure levels above 135 dB in any octave band.
The noise emitted by a computer, printer and fax in a room without sound-absorbing materials can exceed a level of 70 db. Therefore, it is not recommended to place a lot of office equipment in one room. Equipment that is too noisy should be moved outside the premises where the workplaces are located.
You can reduce the noise level if you use noise-absorbing materials as room decoration and curtains made of thick fabric. Anti-noise earplugs will also help.
When constructing buildings and structures in accordance with modern, more stringent sound insulation requirements, technologies and materials must be used that can provide reliable protection from noise.
For fire alarms: the sound pressure level of the useful audio signal provided by the siren must be at least 75 dBA at a distance of 3 m from the siren and no more than 120 dBA at any point in the protected premises (clause 3.14 NPB 104-03).
A high-power siren and a ship's howler - the pressure is more than 120-130 decibels.
Special signals (sirens and “quacks” - Air Horn) installed on service vehicles are regulated by GOST R 50574 - 2002. Sound pressure level of a signaling device when a special sound is emitted. signal, at a distance of 2 meters along the horn axis, must be no lower than:
116 dB(A) - when installing a sound emitter on the roof of a vehicle;
122 dBA - when installing the radiator in the engine compartment of a vehicle.
Fundamental frequency changes should be from 150 to 2000 Hz. Cycle duration is from 0.5 to 6.0 s.
The horn of a civilian vehicle, according to GOST R 41.28-99 and UNECE Rules No. 28, must produce a continuous and monotonous sound with an acoustic pressure level of no more than 118 decibels. This is the maximum permissible value for car alarms.
If a city dweller, accustomed to constant noise, finds himself in complete silence for some time (in a dry cave, for example, where the noise level is less than 20 db), then he may well experience depression instead of rest.
A sound meter device for measuring sound and noise levels.
To measure the noise level, a sound level meter is used, which is produced in different modifications: household (approximate price - 3-4 tr, measurement ranges: 30-130 dB, 31.5 Hz - 8 kHz, filters A and C), industrial ( integrating, etc.) The most common models: SL, octave, svan. Wide-range noise meters are used to measure infrasonic and ultrasonic noise.
Low and high frequency sounds seem quieter than mid frequency sounds of the same intensity. Taking this into account, the uneven sensitivity of the human ear to sounds of different frequencies is modulated using a special electronic frequency filter, obtaining, as a result of normalization of measurements, the so-called equivalent (energy-weighted) sound level with the dimension dBA (dB(A), then yes - with filter "A").
A person can hear sounds with a volume of 10-15 dB and higher. The maximum frequency range for the human ear is from 20 to 20,000 Hz. Sound with a frequency of 2-3 KHz is better heard (common in telephones and on radios in the MW and LW bands). With age, the auditory range of sounds narrows, especially for high-frequency sounds, decreasing to 18 kilohertz or less.
If there are no sound-absorbing materials (carpets, special coverings) on the walls of the premises, the sound will be louder due to multiple reflections (reverberation, that is, echoes from the walls, ceiling and furniture), which will increase the noise level by several decibels.
Noise scale (sound levels, decibels), in the table
| Decibel, | Characteristic | Sound sources |
| Can not hear anything | ||
| Almost inaudible | ||
| Almost inaudible | quiet rustle of leaves |
|
| Barely audible | rustle of leaves |
|
| Barely audible | human whisper (at a distance of 1 meter). |
|
| human whisper (1m) |
||
| whispering, ticking of the wall clock. |
||
| Quite audible | muffled conversation |
|
| Quite audible | ordinary speech. |
|
| Quite audible | normal conversation |
|
| Clearly audible | conversation, typewriter |
|
| Clearly audible | Upper standard for class A office premises (according to European standards) |
|
| Norm for offices |
||
| loud conversation (1m) |
||
| loud conversations (1m) |
||
| scream, laugh (1m) |
||
| Very noisy | scream, motorcycle with muffler. |
|
| Very noisy | loud scream, motorcycle with muffler |
|
| Very noisy | loud screams, freight railway car (seven meters away) |
|
| Very noisy | subway car (7 meters outside or inside the car) |
|
| Extremely noisy | orchestra, subway car (intermittently), thunder Maximum permissible sound pressure for the player's headphones (according to European standards) |
|
| Extremely noisy | on an airplane (until the 80s of the twentieth century) |
|
| Extremely noisy | helicopter |
|
| Extremely noisy | sandblasting machine (1m) |
|
| Almost unbearable | jackhammer (1m) |
|
| Almost unbearable | ||
| Pain threshold | plane at the start |
|
| Contusion | ||
| Contusion | sound of a jet plane taking off |
|
| Contusion | rocket launch |
|
| Concussion, injuries | ||
| Concussion, injuries | ||
| Shock, injuries | shock wave from a supersonic aircraft |
|
| At sound levels above 160 decibels, rupture of eardrums and lungs is possible, |
||
Sound volume - noise level.
Before moving on to the results of measuring the noise characteristics of Titan coolers, let's take a closer look at the tasks and methodology of these studies.
Relevance
As the performance of computer processors increases, including due to an increase in the number of active elements in the chip and an increase in operating frequency, the amount of heat generated by the processor also increases. This, in turn, leads to the need to intensify cooling, which until recently, in relation to household personal computers, was achieved by increasing the effective area of radiators and increasing the speed of the fan blowing the radiator. The latter leads to a significant increase in emitted noise. And now, in many offices with a large concentration of computers, the noise in the room is determined not by the remnants of noise penetrating from the street through sealed plastic windows, but by the computers themselves. But noise is one of the important factors determining human performance! There is a subconscious desire to move the system unit far away.
Wanting to change the situation and being in conditions of fierce competition, manufacturers of cooling systems began to introduce technologies into household personal computers that have proven themselves in professional electronic equipment for various applications. Cooling systems based on heat pipe technology and water cooling systems have appeared on the market. A comparative analysis of three systems produced by Titan Computer GmbH in terms of heat removal efficiency is given in the article “Review of Titan coolers”. The following were tested: Siberia - a representative of the traditional cooling system, Vanessa S and L-type cooling system based on heat sink pipes and the TWC-A04 water system. Issues of measuring the noise characteristics of the above systems will be discussed in the article “Measuring the noise characteristics of Titan cooling systems.”
Noise characteristics. Physical and psychological perception of noise by humans.
The passport data of cooling systems or fans most often provides an integral assessment of the noise level, measured in dBA, less often in dB (read, decibel). This is a logarithmic value that determines the noise level relative to the threshold of human audibility. The difference between dB and dBA is that in the latter case, a uniform frequency sensitivity characteristic (for example, like an ideal microphone) is adjusted to suit the human auditory perception. At noise levels emitted by computers, auditory perception has an increased sensitivity threshold at low and high frequencies with a maximum ranging from 400 Hz to 4 kHz.
The noise level of the cooling system depends significantly on the fan speed and the design of the radiator. Therefore, if it is equipped with a rotation speed regulator, then the specification indicates the minimum and maximum noise levels. For example, for the Siberia cooling system from Titan Computer GmbH, this level at the minimum rotation speed is less than 27 dBA, and at the maximum it can reach 45 dBA.
The noise level of a working modern computer ranges from 35 to 50 dBA. If your computer has a poorly balanced fan, it can reach 55 dBA or more, especially in the first minutes after switching on.
People, for obvious reasons, are most irritable to noise at night. From the point of view of sanitary standards for comfortable housing, the recommended level from ventilation system equipment at this time should not exceed 25-35 dBA. Thus, the noise of the Siberia cooling system at maximum performance is 10 dBA higher than the sanitary standard. And an increase in the sound level by 10 dBA is subjectively assessed by a person as an increase in volume by more than 2 times! Thus, using a regular computer at night can hardly be called comfortable.
If there are several computers in the room, then the total noise level cannot be obtained by algebraic addition from each. For example, if there are two computers in a room emitting 45 dBA each, then the noise level will be 48 dBA, four computers will provide a noise level of 51 dBA, and so on.
The integral assessment of the noise level (in dBA or dB) does not say anything about its spectral distribution. The noise spectrum is typically measured in spectral bands centered at 63 Hz; 125 Hz; 250 Hz; 500 Hz;1 kHz; 2 kHz; 4 kHz; 8 kHz. Also very useful are measurements of the current spectrum without averaging over bands, which make it possible to isolate the frequency components determined separately by the rotation of the fan and the components emitted when the air flow flows around the radiator. Analysis of the noise spectrum allows us to evaluate the factor of its psychological influence on a person. Knowing it for the cooling system, you can predict the overall noise of the computer system unit. In addition, spectrum analysis is necessary when selecting methods and materials for passive and/or active noise reduction.
Standards. Equipment.
Fans of cooling systems manufactured in China are certified according to the CNS 8T 53 standard, which is very close to the DIN 45635 standard. Certification measurements are carried out in an anechoic chamber (free field conditions). The level of intrinsic noise in the chamber and the intrinsic noise of the measuring equipment should not exceed 15 dBA.
These requirements are met by a large sound-measuring anechoic chamber of the Federal State Unitary Enterprise “Acoustic Institute named after Academician N.N. Andreeva". The sound-measuring anechoic chamber (SCA) is designed for carrying out acoustic measurements in conditions of a free sound field. The chamber building is installed on a separate “floating” foundation to reduce vibration levels and low-frequency noise interference; the chamber has double walls with an air gap between them.
The internal walls of the ZZK room are lined with an absorbent coating made of wedge-shaped slabs consisting of staple glass fiber glued with non-flammable resins with a specific gravity of 150 kg/m 3 and a wedge length of 1.5 m. The ZZK room has the shape of a parallelepiped, the dimensions of which are 11.7 x 8 .7 x 11.0(h) m. In this case, the useful volume is 1120 m 3. The working floor of the ZZK is a mesh made of steel cable, located at a height of 4 m from the sound-damping floor covering. The camera, together with a set of measuring equipment, constitutes a measuring stand and undergoes mandatory periodic certification by standardization bodies.
In particular , certification is carried out to determine the deviation of the sound pressure field of the sound-measuring anechoic chamber from the free field. It must comply with the requirements of GOST 12.1.024-81 “Noise. Determination of noise characteristics of noise sources in an anechoic chamber. The exact method." In this case, measurements of sound levels are carried out in one-third octave bands with geometric mean frequencies from 63 to 20,000 Hz. Deviations of the field from the free one do not exceed ±1.5 dB at the edges of the frequency range at distances of 4 m.
Measurement technique.
The cooling system is located on the desktop in the center of the chamber and operates in a standard position without additional obstruction to the air flow.
The sound pressure level is measured using a precision sound level meter 2203 from Brühl and Kjær, installed at a distance of 1 m from the test object. It is equipped with a 4145 one-inch condenser microphone and 1613 octave filters. Photo 1 illustrates the noise measurement of the Vanessa S-type cooling system.
Large sound-measuring anechoic chamber of the Federal State Unitary Enterprise “Acoustic Institute named after Academician N.N. Andreeva". Vanessa S-type noise measurement.
Noise measurements are made in octave bands with center frequencies from 63 Hz to 8000 Hz and in dBA.
If the fan is equipped with a rotation speed regulator, then measurements are carried out for three rotation speed modes: High, Middle, Low.
As an example, we present the results of measuring the noise characteristics of the IH-3200C cooler manufactured by ICEHUMMER Corp. (). Its productivity reaches 90 m 3 /hour at a fan speed of 3000 rpm. The results of thermal measurements can be found in the article Coolers ICE HAMMER IH-3400WFCA and IH-3200C.
Unfortunately, the cooler design does not provide a fan speed controller. . Therefore, we used a speed controller from Vanessa S-type. The distribution of sound pressure level in octave bands depending on the position of the rotation speed controller is presented in Fig. 1.
Fig.1. Distribution of the sound pressure level of the IH-3200C cooling system in octave frequency bands.
The maximum of the fan noise spectrum is concentrated in the frequency band from 500 Hz to 4000 Hz. This is not very good from the point of view of human noise perception, since the maximum in the spectrum falls in the region of greatest hearing sensitivity 1000-2500 Hz. If we compare the IH-3200C and the Titan Computer GmbH Vanessa S-type cooling system, which has greater performance, then the noise from the Titan product will be perceived by a person as less annoying, due to the fact that its maximum spectrum is shifted to lower frequencies. You will soon be able to learn more about the noise characteristics of Titan cooling systems in the article “Measuring the noise characteristics of Titan cooling systems.”
The table shows the results of measurements of the noise level of the IH-3200C in dBA, at three positions of the speed controller.
Table. Relative noise level L emitted by IH-3200С .
The measurement results showed that the noise level of the measured image coincides with the value declared by the manufacturer.
Noise is defined as a disordered combination of different sounds having tones of varying strength and frequency. Noise levels must be measured in quantities capable of expressing the degree of sound pressure produced. Such units of noise level measurement are associated with the names of two physicists - Alexander Bell and Heinrich Hertz.
Bels, or more often decibels, express the relative loudness of a sound. At its core, a decibel is ten times the logarithm of the ratio of the intensity of existing sound energy to its value. It is not directly a unit of measurement, but rather an expression of a relationship.
The measurable characteristic of sound is the amount of energy contained in it. That is, its intensity as a flow of this energy. That is why the quantitative characteristic is, for example, expressed in watts per square meter (W/m2). However, the obtained values relative to the reference level of 10−12 W/m2 are so small and incomprehensible to most ordinary people that 1 bel was “adopted” to express the resulting ratios. For example, the noise level of a jet aircraft is about 13 bels, or in smaller units 130 decibels (dB). For the human ear, the normal range of noise is defined by the boundaries from 20 to 120 decibels. At sounds above this level, a person can suffer serious injuries to the eardrum and contusion. And 160 dB can be lethal.
All people experience noise in their homes. They consist of those directly arising in the room and penetrating from the outside. In order to protect the health and normal condition of citizens, standards for permissible penetrating noise have been adopted. This is 40 dB during the day and 30 at night. Average indicators of noise level units prove that in approximately 80% of cases, even with normal operation of the radio and TV, conversations, noise penetrating from neighboring apartments remains at a level of 40-45 dB, and sounds from the entrance (elevator movement, door slams) reach 60 dB.
In addition to sound intensity, the human ear is sensitive to noise vibrations. Hertz is a unit of frequency, equal to the frequency of the ongoing periodic process, in which one cycle of such a periodic process occurs in 1 second (that is, 1 oscillation). Therefore, for an objective characterization it is necessary to use both of these units of noise level measurement. The human hearing system is more sensitive to vibrations created by high frequencies than by low frequencies. But in industrial and living conditions, everyone is under the influence of the entire spectrum. In this regard, when comparing the sound volume level, it is necessary, in addition to the characteristics of the strength and intensity of the sound in decibels, to also indicate the frequency of vibrations per second.