Integrated testing device and testing method for evaluating sound absorption and heat insulation performance of building material

Abstract

The invention discloses an integrated testing device and testing method for evaluating sound absorption and heat insulation performance of a building material. Wherein the sound absorption performance testing part relates to the field of acoustic detection, and the testing device comprises a detection module and a sound insulation module. Wherein the detection module comprises a data collecting and processing system, a power amplifier, a signal generator and a control computer, and the sound insulation module comprises a semi-closed box body with an opening at one end, a pair of loudspeaker units facing the opening direction of the box body and positioned at the closed end of the box body, and a plurality of microphones. The detection module and the sound insulation module are connected with each other. The thermal insulation performance testing part relates to the technical field of thermal insulation material detection and comprises a metering box, a protection box and a cold box. Wherein heaters and anti-radiation plates are arranged in the metering box and the protection box, a pair of fans is arranged in the protection box, and the sound absorption performance testing device is placed in the cold box. The metering box and the protection box jointly form a hot box, and a large box formed by the hot box and the cold box is placed on the trolley capable of freely moving to form the sound absorption and heat insulation performance integrated testing device. The integrated testing device for evaluating the sound absorption and heat insulation performance of the building material can measure different physical performance parameters in the same device, and has the advantages of being simple in structure, easy to operate, high in testing efficiency, wide in environment control range and the like.

Description

Technical Field

[0001] The present invention relates to the field of acoustic detection and the technical field of thermal insulation material detection, and in particular to an integrated testing device for evaluating the sound absorption and thermal insulation performance of building materials. Background Art

[0002] Dr. Peng Changhai from Southeast University conducted a preliminary study on the methods, errors and technologies of building energy-saving tests, focusing on the error problems of the traditional hot box method in energy-saving assessment. He pointed out that the development of fast, accurate and all-weather on-site energy-saving detection technology has become an important issue that needs to be solved urgently.

[0003] Yang Wansheng and others from Sun Yat-sen University explored a detection device for testing roof insulation modules and material performance. They made some improvements to the traditional hot box method so that it can be used to test the insulation performance of vegetation insulation roofs, evaporative insulation roofs and their combined roofs, with a wide environmental control range.

[0004] Liu Dong and Lv Yadong pointed out at the National Conference on Environmental Acoustics that changes in temperature will affect the speed of sound, density and viscosity of air, which in turn will affect the key properties of sound-absorbing structures such as the porosity, pore size and thickness of the material. Taking micro-perforated plate sound absorbers as an example, they simulated and compared the sound absorption performance under different temperature conditions. This study provides an important reference for the design and use of micro-perforated plate sound absorbers under different temperature environments.

[0005] The sound absorption performance of building materials is a factor that has a great impact on the user experience. It is very important to create a comfortable, quiet, clear and suitable auditory environment inside and outside the building to improve the quality of people's life and work. However, to fully and systematically analyze the sound absorption performance of building materials, it is necessary to consider the impact of temperature on it.

[0006] As people's awareness of energy conservation and environmental protection increases, building insulation has become one of the focuses of attention. At the same time, people are also beginning to seek materials that can take into account both thermal insulation and sound absorption properties to achieve better energy conservation and environmental protection effects. New thermal insulation and sound absorption materials that meet my country's current and future needs will promote the rapid development of green buildings.

[0007] In summary, none of the above methods provide a test solution for the change of sound absorption coefficient of materials under different temperature conditions, and the determination of two physical parameters, thermal insulation coefficient and sound absorption coefficient, cannot be completed in the same device. In addition, the traditional thermal insulation and sound absorption coefficient determination methods have certain special requirements for the site environment and need to be operated by professionals. The testing process will consume a lot of manpower and material resources, and the testing efficiency is not high. Summary of the invention

[0008] In view of the defects existing in the prior art, the purpose of the present invention is to provide an integrated testing device for evaluating the sound absorption and thermal insulation properties of building materials, which can measure different physical performance parameters in the same device, has a simple structure, is easy to operate, has high testing efficiency, and has a wide environmental control range, and has strong practical value.

[0009] The present application provides an integrated testing device for evaluating the sound absorption and thermal insulation performance of building materials, which comprises:

[0010] Sound absorption performance test part and thermal insulation performance test part. The sound absorption performance test device includes a detection module and a sound insulation module. The detection module includes a data collection and processing system, a power amplifier, a signal generator and a control computer. The sound insulation module includes a semi-enclosed box with one end open, a pair of speaker units facing the open direction of the box and located at the closed end of the box, and a number of microphones. The detection module and the sound insulation module are connected to each other. In addition, the thermal insulation performance test device includes a metering box, a protection box and a cold box. There are heaters and radiation protection plates in the metering box and the protection box. A pair of fans are provided in the protection box, and the sound absorption performance test device is placed in the cold box. The metering box and the protection box together form a hot box. The large box composed of the hot box and the cold box is placed on a freely movable trolley to form an integrated sound absorption and thermal insulation performance test device.

[0011] On the basis of the above technical solution, a pair of fans are provided on the upper and lower surfaces of the protection box, which are turned on after the protection box heater starts working.

[0012] On the basis of the above technical solution, the metering box heater and the protection box heater are provided with radiation protection plates.

[0013] On the basis of the above technical solution, there are corresponding card slots between the protection box and the metering box to ensure overall stability and space sealing.

[0014] On the basis of the above technical solution, the sound insulation module part is in the shape of a hollow hexagonal column as a whole.

[0015] Based on the above technical solution, a layer of sealing strip is provided at the open end of the sound insulation module.

[0016] On the basis of the above technical solution, an arc-shaped diverging body is provided on the inner surface of the sound insulation module, and the arc-shaped diverging body is composed of different sizes.

[0017] As a second aspect of the present invention, a method for testing the sound absorption performance of a material at different temperatures is also provided. The method uses the above-mentioned building material sound absorption and heat insulation performance integrated testing device, and the testing method comprises the steps of:

[0018] Operating a sound absorption performance testing device in the cold box, and measuring the reverberation time T1 without placing a test material by means of the detection module, the microphone and the speaker unit;

[0019] After the test material is placed in the groove of the embedding part and assembled into a large box, the reverberation time T2 when the test material is present is measured by the detection module, the microphone and the speaker unit;

[0020] According to the sound absorption coefficient calculation formula, the sound absorption coefficient when the sound absorbing material is placed is calculated. The sound absorption coefficient calculation formula is:

[0021]

[0022]

[0023] Wherein, a1 represents the sound absorption coefficient when no sound-absorbing material is placed, a2 represents the sound absorption coefficient after the sound-absorbing material is placed, V represents the volume inside the sound insulation module, S represents the cross-sectional area of ​​the sound insulation module, S2 represents the area of ​​the sound-absorbing material, T1 represents the reverberation time when no sound-absorbing material is placed, and T2 represents the reverberation time after the sound-absorbing material is placed.

[0024] On the basis of the above technical solution, before measuring the reverberation time T1 when no test material is placed, the method further includes the step of: calibrating the microphone.

[0025] On the basis of the above technical solution, the step of measuring the reverberation time T2 when the test material is placed comprises:

[0026] The cold box and the hot box are spliced ​​together to ensure that a closed space is formed inside the large box. Then, all components in the detection module and the three microphones in the sound insulation module are activated;

[0027] The metering box heater and the protection box heater start to work, and the fan in the protection box starts to work until the temperature in the hot box reaches the design temperature and remains almost constant;

[0028] Send a signal through the signal generator and start the speaker unit. When the test values ​​of the three microphones in the sound insulation module are close to the design value and reach a stable state, stop the speaker unit. Record the time required for each microphone to decay by 60 decibels from the stable state, and take the average of multiple experiments as the reverberation time T2.

[0029] On the basis of the above technical solution, the step of calculating the sound absorption coefficient a2 when the test material is placed comprises:

[0030] After the heat box reaches the designed temperature and remains almost constant, the signal generator is controlled to emit a signal of a specific frequency;

[0031] Measuring the reverberation time T2 corresponding to the specific frequency, and calculating the sound absorption coefficient a2 corresponding to the specific frequency;

[0032] The specific frequency is adjusted, the above steps are repeated, and the average value of all the sound absorption coefficients a2 is taken as the sound absorption coefficient when the sound-absorbing material is placed.

[0033] Compared with the prior art, the advantages of the present invention are:

[0034] (1) An integrated testing device and testing method for evaluating the sound absorption and thermal insulation performance of building materials. The sound absorption performance test part involves the field of acoustic detection, and the testing device includes a detection module and a sound insulation module. The detection module includes a data collection and processing system, a power amplifier, a signal generator and a control computer, and the sound insulation module includes a semi-enclosed box with one end open, a pair of speaker units facing the open direction of the box and located at the closed end of the box, and a plurality of microphones. The detection module and the sound insulation module are connected to each other. In addition, the thermal insulation performance test part involves the field of thermal insulation material detection technology, including a metering box, a protective box and a cold box. The metering box and the protective box are both equipped with a heater and a radiation-proof plate, and a pair of fans are arranged in the protective box, and the sound absorption performance testing device is placed in the cold box. The metering box and the protective box together form a hot box, and the hot box and the cold box together form a large box placed on a freely movable trolley to form an integrated sound absorption and thermal insulation performance testing device. The invention provides an integrated testing device for evaluating the sound absorption and heat insulation performance of building materials, which has the advantages of measuring different physical performance parameters in the same device, simple structure, easy operation, high testing efficiency, wide environmental control range, etc.

[0035] (2) The integrated testing device for evaluating the sound absorption and thermal insulation performance of building materials of the present invention has no special requirements for the testing site and operators. The testing environment only needs to be relatively quiet. In addition, exploring the sound absorption performance of materials under different temperature conditions is significantly helpful for a more comprehensive understanding of the energy-saving level of the testing materials, and has high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a cross-sectional front view of the entire device in an embodiment of the present invention;

[0037] Figure 2 This is an enlarged view of the details of the sound insulation module in an embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of the structure of the sound insulation module in an embodiment of the present invention;

[0039] Figure 4 It is a three-dimensional cross-sectional schematic diagram of the entire device in an embodiment of the present invention.

[0040] In the figure: 1-protection box; 2-metering box; 3-cold box; 4-detection module; 5-bracket; 6-test material; 7-metering box heater; 8-sound insulation module; 9-protection box heater; 10-fan; 11-arc-shaped diffuser; 12-sealing strip; 13-glass fiber cotton layer; 14-microphone; 15-speaker unit. DETAILED DESCRIPTION

[0041] See also Figure 1 As shown, an embodiment of the present invention provides a material sound absorption performance testing device, which includes: a sound insulation module 8 and a detection module 4,

[0042] An integrated testing device for evaluating the sound absorption and heat insulation performance of building materials, which includes a sound absorption performance testing part and a heat insulation performance testing part. The sound absorption performance testing device includes a detection module 4 and a sound insulation module 8. The detection module 4 includes a data collection and processing system, a power amplifier, a signal generator and a control computer. The sound insulation module 8 includes a semi-enclosed box with one end open, a pair of speaker units 15 facing the open direction of the box and located at the closed end of the box, and a plurality of microphones 14. The detection module 4 and the sound insulation module 8 are connected to each other. In addition, the heat insulation performance testing device includes a metering box 2, a protection box 1 and a cold box 3. The metering box 2 and the protection box 1 are both equipped with a heater and a radiation-proof plate, and the protection box 1 is provided with a pair of fans 10, and the sound absorption performance testing device is placed in the cold box 3. The metering box 2 and the protection box 1 together form a hot box, and the hot box and the cold box 3 together form a large box placed on a freely movable trolley to form an integrated sound absorption and heat insulation performance testing device.

[0043] The working principle of the material thermal insulation performance testing device provided in the embodiment of the present invention is:

[0044] The total heat input of the resistive heater and fan 10 equipment in the metering box 2 is recorded as Q1. By controlling the ambient temperature in the protective box 1, the unbalanced heat flow Q2 in the test material and the heat flow Q3 through the wall of the metering box 2 can be minimized. Assuming that the test material is homogeneous, when the temperature inside and outside the metering box 2 is the same, and the temperature outside the test material and its surface heat transfer coefficient are uniform, the air inside and outside the metering box 2 will reach a state of equilibrium, at which time Q2 and Q3 are both zero. This means that the total heat flow Q through the test piece is P Will be equal to the total heat Q1 input into metering box 2.

[0045] When the heat box reaches a stable heat transfer state, its energy balance relationship can be expressed by the following formula:

[0046] Q1=Q P +Q2+Q3 (1)

[0047] Where Q1 represents the heat input of the heater per unit time (W); Q P represents the total heat flux passing through the specimen (W); Q2 represents the unbalanced heat flux passing through the specimen per unit time (W); Q3 represents the heat dissipated through the side wall of the metering box 2 per unit time (W).

[0048] When the heat transfer of the metering box 2 and the test material is in a stable state, the total heat flow through the test material can be calculated by measuring the heat input by the heater and the heat dissipated by the side wall of the metering box 2. Furthermore, by measuring the temperature inside the metering box 2 and the comprehensive temperature of the atmospheric environment in contact with the test material, the comprehensive heat transfer coefficient of the test material can be calculated using formula (2).

[0049]

[0050] The temperature control in the hot box is controlled by a precision temperature controller, and the power supply of the heat-dissipating fan is provided by a 12V DC power supply. The control temperature range in the box is ambient temperature ~ 70℃: the fluctuation range of the air temperature in the box is less than ±0.5℃, and the heat transfer coefficient of the box is less than 5%; the total heating power of the hot box is controlled to be 500W.

[0051] One end of the data collection and processing system is connected to the microphone 14, and the other end is connected to the control computer; one end of the power amplifier is connected to the speaker unit 15, and the other end is connected to the signal generator; in addition, the signal generator is also connected to the control computer.

[0052] In the embodiment of the present invention, a pair of speaker units 15 and three microphones 14 are provided, wherein the pair of speaker units 15 are symmetrically mounted on the upper and lower sides of the closed end of the sound insulation module 8 through the bracket 5, and the three microphones 14 are respectively mounted in the middle of the sound insulation unit through the bracket 5. The inner wall of the channel of the sound insulation module 8 is provided with a wiring hole, and the speaker unit 15 and the microphone 14 are connected to the detection module 4 through the wiring hole, and are sealed with rubber at the wiring hole and the connecting wire.

[0053] As a preferred implementation, the control computer in the embodiment of the present invention is a tablet computer, which is easy to operate and has intuitive data.

[0054] In the embodiment of the present invention, three microphones 14 are installed in the middle of the sound insulation module 8, and the two speakers are both facing the middle of the channel of the sound insulation module 8 so that the microphones 14 can receive sound waves.

[0055] In the embodiment of the present invention, the channel wall of the sound insulation module 8 is divided into three layers, the outer surface is the first layer, the glass fiber wool layer 13 is the second layer, and the third layer is attached to the speaker unit 15. The glass fiber wool layer 13 has a good sound insulation performance due to its barrier effect on sound waves.

[0056] Furthermore, in an embodiment of the present invention, there is an arc-shaped diverging body 11 on the first layer, and the arc-shaped diverging body 11 is composed of several different sizes.

[0057] In the embodiment of the present invention, the arc-shaped diverging body 11 diffuses the sound waves in all directions, so that an effective diffuse sound field can be formed in the sound insulation module 8.

[0058] In an embodiment of the present invention, the step of measuring the reverberation time T1 when no test material is placed comprises:

[0059] Before measuring the reverberation time T1 without placing the test material, the microphone 14 needs to be calibrated. This step should be completed independently in the cold box 3, and all components in the detection module 4, including the data collection and processing system, power amplifier, signal generator, control computer and all microphones 14 in the sound insulation module 8, are started.

[0060] First, the signal generator is controlled to send a signal and the speaker unit 15 is operated. When the test values ​​of the three microphones 14 in the sound insulation module 8 are close to the design values ​​and reach a stable state, the speaker unit 15 is stopped. The time required for each microphone 14 to decay by 60 decibels from the stable state is recorded, and the average value is taken from multiple experiments as the reverberation time T1.

[0061] Further, in an embodiment of the present invention, the step of measuring the reverberation time T2 when the test material is placed comprises:

[0062] The cold box 3 and the hot box are spliced ​​together to ensure that a closed space is formed inside the large box. Subsequently, all components in the detection module 4 and the three microphones 14 in the sound insulation module 8 are activated;

[0063] The metering box heater 7 and the protection box heater 9 start working, and the fan 10 in the protection box 1 starts working until the temperature in the hot box reaches the design temperature and remains almost constant;

[0064] The signal generator sends a signal and starts the speaker unit 15. When the test values ​​of the three microphones 14 in the sound insulation module 8 are close to the design values ​​and reach a stable state, the speaker unit 15 is stopped. The time required for each microphone 14 to decay by 60 decibels from the stable state is recorded, and the average value is taken from multiple experiments as the reverberation time T2.

[0065] On the basis of the above technical solution, the step of calculating the sound absorption coefficient a2 when the test material is placed comprises:

[0066] After the heat box reaches the designed temperature and remains almost constant, the signal generator is controlled to emit a signal of a specific frequency;

[0067] Measuring the reverberation time T2 corresponding to the specific frequency, and calculating the sound absorption coefficient a2 corresponding to the specific frequency;

[0068] The specific frequency is adjusted, the above steps are repeated, and the average value of all sound absorption coefficients a2 is taken as the sound absorption coefficient when the sound-absorbing material is placed. Finally, the sound absorption performance of the test material at the set temperature is obtained, and the sound absorption performance of this material is comprehensively evaluated.

[0069] The present invention is not limited to the above-mentioned implementation modes, and is not intended to limit the scope of implementation of the present invention. Without departing from the principle of the present invention, several improvements and modifications may be made, and these improvements and modifications are also considered to be within the protection scope of the present invention.

Claims

1. An integrated testing device for evaluating the sound absorption and thermal insulation performance of building materials, characterized in that: It includes: The metering box (2) is provided with a metering box heater (7) on the opposite side of the test material, a radiation protection plate is placed on the metering box heater (7), and the test material is embedded in the card slot of the metering box (2). A protection box (1) is provided with a protection box heater (9) on the opposite side of the test material, a radiation protection plate is placed on the protection box heater (9), a pair of fans (10) are provided on the other two sides of the box body, and the metering box (2) and the protection box (1) together form a hot box. A cold box (3) comprises a sound absorption performance testing device, wherein the sound absorption performance testing device comprises a detection module (4), a sound insulation module (8) and a bracket (5), wherein the opening of the detection module (4) is connected to the test material, the detection module (4) and the sound insulation module (8) are connected to each other, and the cold box (3) and the hot box together form a large box.

2. An integrated testing device for evaluating the sound absorption and thermal insulation performance of building materials according to claim 1, characterized in that: The metering box (2) is stepped and is divided into an embedding part and a metering part. The metering part is inside the protection box (1). The embedding part has a groove so that the test material can remain in a stable state after being placed in the groove.

3. An integrated testing device for evaluating the sound absorption and heat insulation performance of building materials according to claim 1, characterized in that: The protection box (1) has a slot that matches the metering box (2), so that the protection box (1) can be kept in a sealed state after being spliced.

4. The integrated testing device for evaluating the sound absorption and heat insulation performance of building materials according to claim 1, characterized in that: The opening side of the cold box (3) is provided with a slot matching the hot box, so that the two can maintain a stable and airtight state after being spliced.

5. The integrated testing device for evaluating the sound absorption and heat insulation performance of building materials according to claim 1, characterized in that: The test apparatus is also equipped with a movable and easily dismantled trolley for carrying large boxes.

6. The integrated testing device for evaluating the sound absorption and heat insulation performance of building materials according to claim 1, characterized in that: The detection module (4) is composed of a data collection and processing system, a power amplifier, a signal generator and a control computer. Specifically, one end of the data collection and processing system is connected to a microphone (14), and the other end is connected to the control computer; one end of the power amplifier is connected to a speaker unit (15), and the other end is connected to the signal generator; in addition, the signal generator is also connected to the control computer.

7. The integrated testing device for evaluating the sound absorption and heat insulation performance of building materials according to claim 1, characterized in that: The microphone (14) in the sound insulation module (8) is connected to a data collection and processing system, and a pair of speaker units (15) is connected to a power amplifier.

8. A method for testing the sound absorption performance of a material at different temperatures, characterized in that: Before measuring the reverberation time T1 without placing the test material, the microphone (14) must be calibrated. This step should be completed independently in the cold box (3), and all components in the detection module (4) are started, including the data collection and processing system, power amplifier, signal generator, control computer and all microphones (14) in the sound insulation module (8). First, a signal is sent out by a control signal generator to make the speaker unit (15) work. When the test values ​​of the three microphones (14) in the sound insulation module (8) are close to the design values ​​and reach a stable state, the speaker unit (15) is stopped. The time required for each microphone (14) to attenuate 60 decibels from the stable state is recorded, and the average value is taken from multiple experiments to be used as the reverberation time T1.

9. A material sound absorption performance testing method according to claim 8, characterized in that: The steps to measure the reverberation time T2 when sound-absorbing materials are placed are as follows: The cold box (3) and the hot box are spliced ​​together to ensure that a closed space is formed inside the large box. Then, all components in the detection module (4) and the three microphones (14) in the sound insulation module (8) are activated; The metering box heater (7) and the protection box heater (9) start to work, and the fan (10) in the protection box (1) starts to work until the temperature in the hot box reaches the design temperature and remains almost constant; A signal is sent out through a signal generator to start the speaker unit (15) to work. When the test values ​​of the three microphones (14) in the sound insulation module (8) are close to the design values ​​and reach a stable state, the speaker unit (15) is stopped. The time required for each microphone (14) to attenuate 60 decibels from the stable state is recorded, and the average value is taken from multiple experiments to be used as the reverberation time T2.

10. The method for testing the sound absorption performance of a material at different temperatures according to claim 8, characterized in that , the step of calculating the sound absorption coefficient a2 when the sound absorbing material is placed comprises: After the heat box reaches the design temperature and remains almost constant, the signal generator is controlled to emit a signal of a specific frequency; the reverberation time T2 corresponding to the specific frequency is measured, and the sound absorption coefficient a2 corresponding to the specific frequency is calculated; the specific frequency is adjusted, and the above steps are repeated, and the average value of all sound absorption coefficients a2 is taken as the sound absorption coefficient a2 when the sound-absorbing material is placed. According to the sound absorption coefficient calculation formula, the sound absorption coefficient calculation formula is calculated as: Wherein, a1 represents the sound absorption coefficient when no sound absorbing material is placed, a2 represents the sound absorption coefficient after the sound absorbing material is placed, V represents the volume inside the sound insulation module (8), S represents the cross-sectional area of ​​the sound insulation module (8), S2 represents the area of ​​the sound absorbing material, T1 represents the reverberation time when no sound absorbing material is placed, and T2 represents the reverberation time after the sound absorbing material is placed. The integrated testing device for evaluating the sound absorption and thermal insulation performance of building materials according to claim 1 is characterized in that: The principle of thermal insulation performance testing is: In the metering box (2), the total heat input of the resistive heater and the fan (10) equipment is recorded as Q1. By controlling the ambient temperature in the protective box (1), the unbalanced heat flux Q2 in the test material and the heat flux Q3 passing through the wall of the metering box (2) can be minimized. Assuming that the test material is homogeneous, when the temperature inside and outside the metering box (2) is the same, and the temperature outside the test material and its surface heat transfer coefficient are uniform, the air inside and outside the metering box (2) will reach a state of equilibrium, at which time Q2 and Q3 are both zero. This means that the total heat flux Q passing through the specimen is P Will be equal to the total heat Q1 input into the metering box (2). When the heat box reaches a stable heat transfer state, its energy balance relationship can be expressed by the following formula: <h2 style=";text-align:left;direction:ltr">Q1=Q<h2 style=";text-align:left;direction:ltr"> P <h2 style=";text-align:left;direction:ltr"> +Q2+Q3 (1) Where Q1 represents the heat input of the heater per unit time (W); Q P represents the total heat flux (W) passing through the test piece; Q2 represents the unbalanced heat flux (W) passing through the test piece per unit time; and Q3 represents the heat lost through the side wall of the metering box (2) per unit time (W). When the heat transfer of the metering box (2) and the test material is in a stable state, the total heat flow through the test material can be calculated by measuring the heat input by the heater and the heat dissipated by the side wall of the metering box (2). Furthermore, by measuring the temperature inside the metering box (2) and the comprehensive temperature of the atmospheric environment in contact with the test material, the comprehensive heat transfer coefficient of the test material can be calculated using formula (2). The temperature control in the hot box is controlled by a precision temperature controller, and the power supply of the heat-averaging fan is provided by a 12V DC power supply. The temperature control range in the box is from ambient temperature to 70℃, the fluctuation range of the air temperature in the box is less than 0.5℃, and the total heating power of the hot box is controlled to be 500W.

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