Abstract : Based on a survey and testing of the noise status of hundreds of punch presses in China, and referring to a large amount of domestic and international data, this paper comprehensively analyzes the causes, spectral characteristics, and propagation paths of punch press noise. The resulting patterns can be used as a reference for formulating punch press noise reduction schemes. Keywords : punch press; noise; spectrum I. Noise Source Analysis of Punch Presses Sound waves are generated by the vibration of objects in a medium. Sound waves are mechanical waves, which are the result of the mechanical vibration of objects propagating to distant places through an elastic medium. The vibrating system that generates sound waves is called the sound source. As can be seen from the mechanism of sound wave generation, sound waves are emitted wherever there is vibration or air disturbance. For punch presses, the sound pressure level of their punching noise is generally between 90 and 110 dB. Punch press noise can be divided into operating noise and working noise. Operating noise is the noise of the punch press running under no-load conditions, which includes the noise generated by the motor, belt, gears, crank connecting rod, slide block and bearing clearances, clutch, etc. Among them, the main noises are clutch and gear noise. Working noise is the noise generated during the stamping of a punch press. The noise generated by processing the same material using different stamping processes (such as blanking, deep drawing, bending) on ​​the same punch press is inconsistent. [align=center] Figure 1 Working noise history and sound energy history [1] (a) Working noise history of punch press (b) Sound energy history [/align] Figure 1a is the sound pressure duration curve of a working cycle of a punch press measured [1]. The figure clearly shows the clutch noise, unloading noise, collision sound of the punch contacting the workpiece, and the noise emitted when the material breaks. It can be seen that the noise emitted when the material breaks and the noise generated when the clutch is working are the most intense (large amplitude and large attenuation). Figure 1b is the corresponding sound energy duration curve [2]. Its sound energy expression is: E＝∫p2dt (1) Where E—total sound energy, N2m-4s p—instantaneous sound pressure Figure 2 shows the sound energy distribution of each frequency band during a working cycle of a punch press. The polyhedron in the figure represents the equivalent sound energy of the sound source. Since the peak sound pressure level is related to its frequency, some essential characteristics of the sound source can be identified. The figure also shows that the punching noise and clutch noise have the largest polyhedral volume and the highest peak value. Therefore, punching noise and clutch noise are the two main noise sources of the punch press. Figure 3 shows the measured noise results when punching 2mm×φ30mm A3 on a 100kN punch press using the separate operation method. [align=center] Figure 2 Noise energy distribution diagram during punch press operation Figure 3 Measured values ​​of punching noise from a 100kN punch press[/align] 1. Noise of clutch engagement and disengagement Commonly used clutches in punch presses include jaw clutches, friction plate clutches, and keyed clutches. Among them, the keyed clutch is the most widely used in small and medium tonnage punch presses, and its noise is relatively high. The essence of the engagement of the keyed clutch in a punch press is that the key engages with one of the keys of the middle sleeve with three (or four) key positions. Clutch engagement noise is caused by a series of impacts. During engagement, there are three main impact phenomena [3]: (1) impact between the key and the intermediate sleeve, that is, the collision between the steady key and the rotating intermediate sleeve during engagement; (2) impact between the key and the crankshaft, that is, while one side of the key impacts the intermediate sleeve, the other side impacts the crankshaft under the reaction force of the former; (3) impact between the crankshaft and the sliding bearing it supports. Figure 4 shows the location of the first two impact phenomena. The impact between these punch press components generates forces acting on the large gear (large flywheel) and the bed. These forces are the root cause of the clutch engagement noise. [align=center] Figure 4 The two impact zones 1. Impact zone between the key and the crankshaft 2. Impact zone between the key and the intermediate sleeve[/align] The formation of clutch engagement noise can be illustrated by Figure 5. Under the action of excitation force F(ω), the punch press generates an excitation velocity V1(ω). The vibration energy is transmitted inside the machine bed. When it is transmitted to the sound-generating surface, it generates a vibration velocity V2(ω), which disturbs the air medium in contact with the surface and generates a pressure change P(ω), thereby radiating sound waves. [align=center] Figure 5 Block diagram of punch press clutch noise generation[/align] In Figure 5, B(ω), T(ω), and σ(ω) are three important transfer functions that illustrate the excitation force-vibration-sound conversion characteristics of the system. They are all complex functions of frequency. B(ω) is the mechanical admittance of the interaction force point between components, which represents the relationship between vibration response and excitation force; the transfer ratio T(ω) represents the transfer characteristics of the system; the radiation coefficient σ(ω) describes the extent to which the vibration of the sound-generating surface of the part is converted into sound energy. The relationship between excitation force and sound pressure (i.e., F(ω) and P(ω)) can be expressed by the following formula [2]: P(ω) = F(ω). B(ω). T(ω). σ(ω)(2) In summary, the noise of the punch press key clutch is caused by a series of impacts, such as the impact between the key and the middle sleeve key, and the impact between the stopper and the key tail. These impacts between the punch press components first generate noise, and at the same time, they generate forces acting on the large gear (large flywheel) and the bed. These forces are transmitted inside the bed. When they are transmitted to the sound-generating surface, they generate vibration velocity, causing the air medium in contact with the surface to be disturbed and generate pressure change P(ω), thereby radiating sound waves. Factors affecting clutch noise include: the magnitude of the impulse received when the clutch is engaged, which determines the impact velocity under the condition of a certain mass; and also the stiffness and damping characteristics of the contact material itself. 2. Punching process noise During punching, once the punch contacts the metal sheet, the punching force begins to increase. At the same time, elastic energy is accumulated due to the deformation of the machine body and other load-bearing components. When the punch enters about half the thickness of the sheet, the punching force reaches its maximum value. The sudden breakage of the sheet metal causes the punch to suddenly lose load, releasing the accumulated elastic energy of the machine body and other components in a very short time. This excites vibrations in the machine body and its components, causing impacts between them. Simultaneously, the slide block plunges downwards at a considerable speed, causing pressure disturbances in the air around it, thus radiating noise. The noise generated by the former is called vibration noise, while the noise caused by the latter is called acceleration noise. Analysis shows that vibration noise is related to the punching force-time history that causes vibrations in the machine body and other components. In addition, punching noise includes five types of noise: sheet metal breakage sound, impact sound between the punch and the sheet metal, and air extrusion sound during their contact. 3. Motor noise: As the power source of the punch press, the motor also generates noise during operation. This includes electromagnetic noise from the motor windings, aerodynamic noise, and mechanical noise. The sound pressure level of the motor noise is related to the motor's power and speed. The electromagnetic noise of the motor is mainly generated by the interaction of alternating electromagnetic fields, which excites the vibration of the rotor and stator. Electromagnetic noise is generally high-frequency noise. The aerodynamic noise of the motor is mainly cooling fan noise; for many motors, cooling fan noise is the primary noise source. Mechanical noise mainly includes noise generated by unbalanced forces of some rotating moving parts and noise generated by vibration of some parts. The sound power level of motor noise can be calculated by the following formula [4] L＝20lgW+15lgN+k3 Where W—rated power of motor, kW N—motor speed, r/min k3—signal frequency correction value 4. Impact noise generated by the gap of the working mechanism In the crank-connecting rod-slider mechanism of the punch press, which is composed of connecting rods and crankshafts, sliders and connecting rods, there are three pairs of friction pairs: crankshaft journal and crankshaft bearing; crank journal and connecting rod big end bearing; connecting rod small end (ball end) and slider ball end seat. Due to manufacturing and assembly errors and the needs of the work itself, gaps are inevitable. Although these friction pairs all bear alternating loads, they do not necessarily cause strong impact noise. Their movement to each other may be contact movement or free movement without contact. But when transitioning from free movement to contact movement, it will inevitably bring strong impact. This noise has a wide frequency band and a strong high-frequency part. Obviously, the larger the gap, the higher the noise. Furthermore, with a fixed clearance, the higher the number of slide strokes, the higher the noise ratio. Among the various noise sources of a punch press, punching noise and clutch noise are the main noise sources. 5. Gear Meshing Noise During operation, the two gears of different sizes on a punch press generate pitch line impact force and meshing impact force, thus arousing gear meshing noise. Pitch line impact force is generated by the change in the direction of the friction force on the tooth surface when the two gears mesh. Figure 6 shows the change in friction force during gear meshing. The contact line of the gear moves from A to B along the meshing line during meshing. B is the contact point. During the movement from A to B, the speed gradually decreases, reaching zero at point B. During the movement from B to C, the relative speed direction changes. Therefore, point B is the turning point of the speed direction. Due to relative sliding, friction force also exists. The direction of friction force changes with the relative speed. Therefore, point B is also the turning point of the friction force direction. The pitch line impact force is related to the transmitted torque, the coefficient of friction between the tooth surfaces, and the magnitude of the relative sliding speed. The greater the power transmitted by the teeth , the greater the roughness of the tooth surface, and the higher the rotational speed, the greater the impact force of the gear pitch line. [align=center] Figure 6 Involute gear meshing diagram[/align] Meshing impact force is the impact force generated by the collision between teeth during gear operation. In reality, gears deform during operation, and coupled with manufacturing and installation errors, the gears radiate noise by colliding with each other during operation. Among them, the rotational speed of the gear has the greatest impact on its noise. When the rotational speed increases, the radiated sound pressure level increases accordingly. If ω1 represents the initial speed and ω2 represents the changed speed, then the change in sound pressure level is [4]: ​​ΔL＝20lgω1/ω2 (3) II. Analysis of the propagation path of punch press noise Noise propagates in solid, liquid and gaseous media, and is respectively called solid sound, liquid sound and air sound. There are direct and indirect propagation paths when noise propagates between the sound source and the receiver. Figure 7 is a block diagram of the propagation path of punch press noise. Direct sound propagation refers to the transmission of airborne sound generated by a sound source directly to the receiver without the intervention of any other medium. In this case, the sound energy changes direction due to reflection and intensity due to absorption. Examples include the noise from a blower nozzle and the noise from air extrusion generated by a falling slide. [align=center]Figure 7: Block diagram of the propagation path of punch press noise[/align] Indirect sound propagation occurs when sound generated by a sound source is first conducted, completely or partially, as solid-state sound, liquid-state sound, or airborne sound within the punch press body or workshop walls. Then, it radiates as airborne sound from an excited component (such as the bed, roof, etc.) with suitable radiation conditions and is transmitted to the receiver. In the indirect propagation path, multiple transformations between solid-state sound, liquid-state sound, airborne sound, and individual components can occur. III. Spectral Characteristics Analysis of Punch Press Noise Noise is composed of a chaotic combination of many components with different frequencies and intensities. The sound frequency range is very wide, reaching 1000 times higher than normal, making it generally impossible and unnecessary to measure each frequency individually. For convenience and practical needs, the audio frequency range is usually divided into several smaller segments, called frequency bands. The difference between the upper and lower cutoff frequencies of a frequency band is called the bandwidth. The bandwidth is expressed as the logarithm of the ratio of the upper and lower cutoff frequencies, usually with a base of 2, in octaves, i.e.: f1/f2＝2n or n＝log2（f1/f2） where f1, f2—the upper and lower cutoff frequencies that are octaves apart; n—the multiple between the two frequencies. n can be any positive real number. The smaller n is, the finer the division, the shorter the frequency band, and the more time is required for measurement. When n＝1, that is, when the two frequencies are 1 times apart, it is called an octave. When n＝1/3, it is called a 1/3 octave, and so on. The center frequency range of octaves is shown in Table 1. [align=center]Table 1 Frequency range of octaves (Hz) [table=98%][tr][td]Center Frequency[/td][td]63[/td][td]125[/td][td]250[/td][td]500[/td][td]1k[/td][td]2k[/td][td]4k[/td][td]8k[/td][/tr][tr][td]Frequency Range[/td][td]45～90[/td] [td]90～180[/td][td]180～355[/td][td]355～710[/td][td]710～1.4k[/td][td]1.4～2.8k[/td][td]2.8～5.6k[/td][td]5.6～11.2k[/td][/tr][/table][/align]The sound wave produced by a sound source undergoing simple harmonic motion is called a simple harmonic wave. Its sound pressure level versus time is a sine curve. This type of sound with only a single frequency is called a pure tone, which has a single pitch. Except for the sounds produced by certain instruments and musical instruments, pure tones with a single frequency are rare. Generally, sound waves are composed of simple sinusoidal components of different frequencies; such sounds are called complex tones. The relationship between the sound pressure level and frequency of a complex tone is called the spectrum, which describes the variation of sound in the frequency domain. Different sounds have different spectra, usually represented graphically as octaves or 1/3 octaves. Noise is composed of many incoherent simple spectrum waves, which in the frequency domain can be represented as discrete frequency graphs with several sound pressure amplitudes that are the same or different, and its main characteristic is that the phase angle or phase difference of each frequency is different. Since the larger the amplitude in the sound wave synthesis, the greater the impact on noise, suppressing the amplitude of this frequency may achieve the result of reducing noise. Therefore, when exploring the root cause and generation mechanism of noise and determining its working scheme, noise spectrum analysis is a very important method. Noise spectrum analysis is to analyze its frequency structure by measuring the sound pressure signal in the frequency domain. It is generally a graph with frequency as the abscissa and the main acoustic parameters (such as sound pressure level, sound intensity level, and sound power level) as the ordinate. During testing, an ND2 precision sound level meter is used, which is equipped with an octave filter. Its principle is to use an analog filter composed of electronic circuits to transform the noise signal from the time domain to the frequency domain. Its theoretical basis is Fourier analysis. As is well known, a periodic function can be decomposed into a series of harmonic components, and conversely, a series of harmonic components can be superimposed to form a periodic function. Therefore, a time-varying function X(T) with period T can always be expressed as the infinite trigonometric series of the Fourier series x(t) = a0 + a1cos2πf0t + a2cos4πf0t + … + b1sin2πf0t + b2sin4πf0t + … (4) where f0 is the fundamental frequency; a0, an, bn are Fourier coefficients determined by the following equations: After simple trigonometric operations, equation (4) can be changed to the following equation: where: If we assume f = nf0, then the periodic function represented by the Fourier series can be represented in the Cn-f plane, which is the spectrum of the function x(t). Similarly, the phase spectrum (Φn-f) can also be represented in this plane. It can be seen that the time domain signal can be transformed into the frequency domain signal through Fourier analysis. When performing frequency response analysis of noise using an ND2 precision sound level meter, simply switch the "weighting network" switch to the "filter" position. Since an octave band filter is inserted between the input and output amplifiers, rotating the octave band filter selection switch allows for noise spectrum analysis. The author conducted a survey of the noise conditions of 275 punch presses in 18 factories in Beijing, Wuhan, Nanjing, and Shanghai. The punching noise and clutch noise spectra of 15 punch presses of different tonnages under different processing conditions were tested, and the results are plotted graphically, as shown in Figures 8 to 13. Analysis of the above figures yields the following patterns: [align=center] Figure 8 Frequency characteristics of punching noise from punching presses of different tonnage[/align][align=center] Figure 9 Frequency characteristics of punching noise from punching presses of different materials[/align][align=center] Figure 10 Punching noise and clutch noise from a 400kN punching press[/align][align=center] Figure 11 Punching noise and clutch noise from an 800kN punching press[/align][align=center] Figure 12 Punching noise and clutch noise from a 1000kN punching press[/align][align=center] Figure 13 Punching noise and clutch noise from a 350kN old-style punching press[/align] (1) The peak sound pressure level is distributed between 250 and 2kHz, and the peak sound pressure level of small-tonnage punching presses is distributed in the low-frequency region (250 to 1kHz). As the tonnage increases, the peak sound pressure level moves towards the high-frequency range (500 to 2kHz). This is helpful for selecting sound insulation and damping materials. Large-tonnage punch presses generally punch larger workpieces, resulting in greater punching force and greater absorption of elastic deformation energy. The vibration caused by the rapid release after unloading is also greater, thus generating stronger radiated noise, as shown in Figure 8. (2) The punching noise varies greatly when punching different materials. Generally, the noise sound pressure level is higher when processing hard and brittle materials than when processing soft materials (see Figure 9). Moreover, when punching soft materials, the peak octave band sound pressure level is distributed in the low-frequency range; while when punching hard materials, the peak sound pressure level frequency range is generally wider, appearing in the range of 125 to 8 kHz. (3) Usually, the punching noise sound pressure level is greater than the clutch noise. However, for old-style small-tonnage punch presses, the opposite is true, with the clutch noise being greater than the punching noise. The reason is that old-style punch presses generally use rigid clutches. *References for projects funded by the Environmental Protection Fund of the former China Aerospace Industry Corporation [Jianzi (1993) 028]: 1. Zhang Chongchao et al. Engineering of Noise Control for Mechanical and Electrical Equipment (First Edition). Beijing: Light Industry Press, 1989: 176-179 2. Li Pumin. Research on Noise and Control of Punch Press in Punching Process. Master's Thesis, Harbin Institute of Technology. 1991: 67-79 3. L L Koss, RJ Alfredson. Identification of Transient Sound Sources on a Punch Press. Journal of Sound and Vibration. 1985, 34(1): 11-13 4. L L Koss, RJ Alfredson. Transient Sound Radiation by Sphere Undergoing an Elastic Collision. Journal of Sound & Vibration. 1973, 27(1): 59-75