Vibration Sensor

Vibration Sensor Market Outlook

The global vibration sensor market was valued at approximately USD 3.33 billion in 2023 and is projected to reach around USD 5.01 billion by 2035, growing at a compound annual growth rate (CAGR) of 6.8% during the forecast period from 2025 to 2035. The increasing demand for vibration sensors in various industries, such as automotive, aerospace, and manufacturing, is propelling market growth. Additionally, the rising awareness of predictive maintenance and industrial automation is driving the adoption of advanced vibration monitoring solutions. Moreover, the increasing focus on safety standards and regulations across industries is further contributing to the demand for these sensors. The technological advancements in sensor technology, such as the development of wireless and intelligent vibration sensors, are also expected to enhance market growth significantly.

Growth Factor of the Market

One of the primary growth factors for the vibration sensor market is the escalating need for predictive maintenance across various sectors. Industries are increasingly leveraging vibration sensors to monitor equipment health and performance, enabling them to predict potential failures before they occur. This proactive approach reduces downtime and maintenance costs while optimizing operational efficiency. Additionally, the integration of IoT and smart technologies in industrial applications has led to a greater demand for sophisticated vibration monitoring systems that provide real-time data and insights. Furthermore, the growing automotive sector, particularly in the development of electric vehicles (EVs), is fueling the demand for vibration sensors for performance and safety testing. The aerospace and defense sectors are also witnessing significant investments in advanced vibration sensors for structural health monitoring, driving market growth. Overall, the increasing reliance on automation and smart manufacturing practices is expected to create substantial opportunities for the vibration sensor market.

Key Highlights of the Market

• The global vibration sensor market is projected to grow at a CAGR of 6.8% from 2025 to 2035.
• Predictive maintenance is a key driver for the increased adoption of vibration sensors.
• Technological advancements, including IoT integration, are transforming the market landscape.
• The automotive and aerospace sectors are leading the demand for vibration sensors.
• North America holds the largest market share, driven by advanced industrial sectors and technology adoption.

By Product Type

Accelerometers:

Accelerometers are a prominent type of vibration sensor that detects changes in motion or acceleration. They are extensively used in various applications, including automotive, aerospace, and industrial sectors. The ability of accelerometers to measure dynamic and static acceleration makes them crucial for monitoring equipment performance and structural integrity. In the automotive industry, they are employed to enhance vehicle safety and performance by providing data on impact and vibration levels. Furthermore, advancements in MEMS (Micro-Electro-Mechanical Systems) technology have led to the development of miniaturized accelerometers that offer high precision and reliability, thereby fueling their adoption across different sectors.

Velocity Sensors:

Velocity sensors are designed to measure the velocity of vibrating objects, providing crucial information for assessing equipment health. These sensors are widely used in industrial applications, particularly in rotating machinery, where they help monitor the performance of motors, pumps, and generators. By measuring the speed of vibrations, velocity sensors can provide insights into the operational state of machinery, enabling predictive maintenance practices. The increasing need for real-time monitoring and timely maintenance in industrial setups is driving the demand for velocity sensors, making them an essential component of vibration monitoring systems.

Displacement Sensors:

Displacement sensors are utilized to measure the physical displacement of an object, typically in relation to its baseline position. These sensors are vital in applications where precise measurements of vibration levels are necessary, such as in civil engineering and structural monitoring. Displacement sensors can detect minute vibrations, allowing for the assessment of the health of structures like bridges and buildings. The growing emphasis on infrastructure safety and maintenance is propelling the demand for displacement sensors, particularly in the construction and civil engineering sectors, where monitoring structural integrity is paramount.

Proximity Probes:

Proximity probes are specialized vibration sensors designed to monitor the position of rotating machinery components. They are particularly useful in applications where precise measurements of vibration levels and shaft movements are required. Proximity probes are commonly employed in industrial settings to monitor turbines, generators, and compressors, where they help prevent catastrophic failures by providing early warnings of misalignment or excessive vibration. The increasing adoption of condition monitoring systems in industrial applications is driving the demand for proximity probes, as they play a critical role in ensuring operational reliability and safety.

Others:

This category encompasses various other types of vibration sensors, including fiber optic sensors and piezoelectric sensors. Fiber optic sensors are gaining traction in applications where electromagnetic interference is a concern, as they offer immunity to such disturbances. Piezoelectric sensors, on the other hand, are widely used for their high sensitivity and ability to detect low-frequency vibrations. The diversification of vibration sensor technology is catering to a broader range of applications, thereby expanding the overall market for vibration sensors.

By Application

Automotive:

The automotive sector is one of the key applications for vibration sensors, where they are utilized for monitoring vehicle performance, safety, and comfort. Vibration sensors play a crucial role in detecting vibrations caused by road conditions, engine performance, and structural integrity. With the rising demand for electric vehicles and advancements in vehicle technology, the need for sophisticated vibration monitoring systems is increasing. Sensors are employed in various components of vehicles, including engines, suspensions, and safety systems, contributing to overall vehicle safety and performance. The growing trend towards automation and intelligent driving systems is further driving the adoption of vibration sensors in the automotive sector.

Aerospace & Defense:

In the aerospace and defense industries, vibration sensors are essential for monitoring the health and performance of aircraft and defense equipment. These sensors are used for structural health monitoring, ensuring the integrity and safety of critical components. Vibration sensors play a vital role in detecting anomalies or potential failures in aircraft systems, thereby enhancing safety and reliability. With increasing investments in aerospace technology and defense systems, the demand for reliable vibration monitoring solutions is on the rise. The push for smart and autonomous aircraft systems is further driving innovation in vibration sensor technology, making it a crucial component for the future of aerospace applications.

Industrial:

The industrial sector represents a significant application area for vibration sensors, as they are integral to predictive maintenance and operational efficiency. Industries rely on vibration sensors to monitor equipment conditions, detect anomalies, and minimize downtime through timely interventions. The adoption of Industry 4.0 concepts and smart manufacturing practices has led to an increased focus on condition-based maintenance, thereby driving the demand for vibration monitoring systems. From manufacturing plants to power generation facilities, vibration sensors are employed to ensure optimal performance and reliability of machinery, contributing to significant cost savings and improved productivity in industrial operations.

Consumer Electronics:

In consumer electronics, vibration sensors are utilized in various applications, including smartphones, wearables, and gaming devices. These sensors enhance user experience by providing haptic feedback, allowing devices to communicate with users through vibrations. The demand for advanced consumer electronics with immersive user experiences is driving the integration of vibration sensors in these devices. Furthermore, as manufacturers focus on developing more intuitive and responsive products, the role of vibration sensors is becoming increasingly important, leading to innovation and growth in this segment of the market.

Healthcare:

The healthcare sector is witnessing the increasing use of vibration sensors in medical devices and equipment monitoring. Vibration sensors are utilized in devices such as ultrasound machines, patient monitoring systems, and imaging equipment to ensure performance accuracy and reliability. By monitoring the vibrations of medical equipment, healthcare providers can detect potential malfunctions and ensure that devices are operating within optimal parameters. With the rise of telehealth and remote monitoring solutions, the demand for reliable medical devices equipped with vibration sensors is expected to grow, contributing to improved patient outcomes and operational efficiency in healthcare settings.

By Distribution Channel

Online Stores:

Online stores have emerged as a key distribution channel for vibration sensors, providing customers with convenient access to a wide range of products. The growing trend of e-commerce has made it easier for manufacturers and retailers to reach a global audience. Online platforms offer detailed product information, customer reviews, and competitive pricing, making them an attractive option for end-users looking to purchase vibration sensors. Moreover, the convenience of online shopping allows businesses to streamline their procurement processes and access the latest technological advancements in vibration sensor solutions.

Direct Sales:

Direct sales remain an essential distribution channel for vibration sensors, particularly for companies that offer specialized solutions and customized products. This channel enables manufacturers to engage directly with customers, providing tailored solutions that meet specific industry requirements. Direct sales teams play a crucial role in educating clients about the benefits and applications of vibration sensors, fostering long-term relationships and customer loyalty. Furthermore, direct sales allow for immediate feedback from customers, supporting product development and innovation in the vibration sensor market.

Distributors:

Distributors serve as vital intermediaries in the vibration sensor market, facilitating the movement of products from manufacturers to end-users. Distributors often provide value-added services such as technical support, inventory management, and training, making them crucial partners for manufacturers. The extensive network of distributors helps manufacturers reach diverse markets and customer segments, enhancing their market presence. Additionally, distributors can offer insights into regional market trends and customer preferences, enabling manufacturers to adapt their strategies accordingly.

OEMs:

Original Equipment Manufacturers (OEMs) represent a significant distribution channel for vibration sensors, integrating these sensors into their products and solutions. Many industries, including automotive, aerospace, and industrial machinery, rely on OEMs to deliver high-quality vibration sensors as part of their offering. By partnering with OEMs, vibration sensor manufacturers can access larger customer bases and ensure that their products are included in critical applications. The collaboration with OEMs also fosters innovation, as manufacturers can develop customized solutions tailored to specific market needs.

Others:

This category includes various other distribution channels, such as retail outlets and specialized distributors that cater to niche markets. While not as prominent as the previously mentioned channels, these outlets still play a role in distributing vibration sensors, particularly in local markets where customers may prefer to purchase products in person. The diversity of distribution channels in the vibration sensor market ensures that customers have access to a wide range of products and solutions, facilitating growth and competition among manufacturers.

By Technology

Piezoelectric:

Piezoelectric technology is widely used in vibration sensors due to its ability to convert mechanical energy into electrical signals. This technology is particularly effective for detecting a wide range of vibration frequencies and is commonly used in industrial applications. Piezoelectric sensors are known for their high sensitivity and reliability, making them ideal for monitoring machinery in real-time. The growing emphasis on predictive maintenance and condition monitoring in industrial settings is driving the demand for piezoelectric vibration sensors, as they provide critical data for enhancing operational efficiency and reducing downtime.

Capacitive:

Capacitive vibration sensors operate based on changes in capacitance caused by vibrations. These sensors are known for their robustness and durability, making them suitable for harsh industrial environments. Capacitive sensors are often used in applications where high-frequency vibrations need to be monitored accurately. The increasing adoption of automation and smart technologies in various industries is fueling the demand for capacitive vibration sensors, as they offer reliable performance and accurate measurements for critical machinery and equipment.

Strain Gauge:

Strain gauge technology is another important method used in vibration sensors to measure deformation and strain due to vibrations. These sensors are particularly valuable in structural health monitoring, where detecting minute changes in structure can prevent catastrophic failures. Strain gauge sensors are used in various applications, including civil engineering, automotive, and aerospace. The growing focus on infrastructure safety and maintenance, coupled with the increasing regulatory standards, is driving the adoption of strain gauge vibration sensors in monitoring applications.

Optical:

Optical vibration sensors utilize light to measure vibrations, offering high sensitivity and accuracy, particularly for low-frequency applications. These sensors are less susceptible to electromagnetic interference, making them ideal for sensitive environments. Optical sensors find applications in aerospace, automotive, and research fields, where precise vibration measurements are critical. The increasing demand for advanced monitoring solutions and the rise of smart technologies in various sectors are driving the adoption of optical vibration sensors, contributing to the growth of this segment.

Others:

This category encompasses various other vibration sensor technologies, including magnetic and inductive sensors. While these technologies are less commonly used than the aforementioned types, they still play a role in specific applications where unique measurement requirements exist. The diversification of vibration sensor technologies allows for a broader range of applications, ensuring that end-users have access to suitable solutions for their specific needs. The introduction of innovative sensor technologies is expected to further enhance the market's growth, as manufacturers continue to develop advanced products that cater to evolving industry demands.

By Region

The North American region holds a significant share of the global vibration sensor market, driven by the presence of advanced industrial sectors and a strong focus on technological innovation. The region's robust automotive, aerospace, and manufacturing industries are major contributors to the demand for vibration sensors, as companies increasingly adopt predictive maintenance practices to enhance operational efficiency. The market in North America is expected to grow at a CAGR of 6.5% from 2025 to 2035, supported by ongoing investments in smart manufacturing and automation technologies. Moreover, the increasing emphasis on regulatory compliance and safety standards in industrial operations is further propelling the adoption of vibration sensor solutions in this region.

In Europe, the vibration sensor market is also witnessing substantial growth, driven by advancements in industrial automation and the rising demand for condition monitoring solutions. The region's diverse industrial landscape, including manufacturing, energy, and transportation sectors, presents significant opportunities for vibration sensor manufacturers. The European market is expected to grow at a CAGR of 7.0% from 2025 to 2035, fueled by the increasing integration of IoT technologies in industrial applications. Additionally, the growing awareness of workplace safety and the need for predictive maintenance practices further contribute to the rising demand for vibration sensors in Europe. Meanwhile, the Asia-Pacific region is emerging as a rapidly growing market, with significant investments in manufacturing and infrastructure development driving the demand for vibration monitoring solutions.

Opportunities

One of the significant opportunities in the vibration sensor market lies in the increasing adoption of Industry 4.0 practices, which emphasize automation, data exchange, and real-time monitoring in manufacturing processes. As industries transition towards smart manufacturing, the demand for advanced vibration sensors that offer connectivity and data analysis capabilities is expected to surge. Manufacturers are focusing on developing intelligent vibration sensors that can integrate with existing industrial systems, providing valuable insights into equipment health and performance. This technological shift presents a lucrative opportunity for vibration sensor manufacturers to innovate and cater to the evolving needs of the market.

Another promising opportunity for the vibration sensor market is the rising demand for safety and compliance in various industries. Regulatory bodies are increasingly emphasizing the importance of equipment monitoring and safety standards, leading to greater reliance on vibration sensors to ensure operational reliability. Industries such as aerospace, automotive, and energy are investing in advanced monitoring solutions to comply with safety regulations and enhance overall equipment effectiveness. As organizations strive to reduce operational risks and improve equipment performance, the demand for reliable and accurate vibration sensors is set to grow, presenting significant opportunities for manufacturers to expand their market presence.

Threats

Despite the positive growth outlook for the vibration sensor market, certain threats could impact its expansion. One of the primary threats is the intense competition among manufacturers, which could lead to price wars and reduced profit margins. As more companies enter the market, the pressure to innovate and offer cost-effective solutions increases, making it challenging for existing players to maintain their market share. Additionally, the rapid pace of technological advancement requires manufacturers to continually invest in research and development to stay ahead of competitors. Failure to adapt to changing market dynamics and customer preferences could result in lost opportunities and diminished market presence.

Another potential threat to the vibration sensor market is the economic fluctuations and uncertainties in global markets. Economic downturns may lead to reduced capital expenditures by industries, affecting the demand for vibration monitoring solutions. Industries may prioritize cost-cutting measures, leading to delays in investments in advanced monitoring technologies. Moreover, geopolitical tensions and trade disputes could disrupt supply chains, impacting the availability and pricing of vibration sensors. Manufacturers must remain vigilant to these external factors and develop strategies to mitigate risks associated with economic uncertainties.

Competitor Outlook

• TE Connectivity
• Honeywell International Inc.
• Analog Devices, Inc.
• National Instruments Corporation
• PCB Piezotronics, Inc.
• Emerson Electric Co.
• Siemens AG
• Murata Manufacturing Co., Ltd.
• Omron Corporation
• Wilcoxon Sensing Technologies
• Vishay Precision Group, Inc.
• Brooks Automation, Inc.
• Meggit PLC
• Kistler Instrumente AG
• Rion Co., Ltd.

The competitive landscape of the vibration sensor market is characterized by a mix of established players and emerging companies that are continuously innovating to capture market share. Key players like TE Connectivity and Honeywell International Inc. are leveraging their expertise and extensive product portfolios to meet the growing demand for advanced vibration monitoring solutions. These companies invest heavily in research and development to enhance their sensor technologies and offer comprehensive solutions that cater to various industries. Moreover, strategic partnerships and collaborations between manufacturers and end-users are becoming increasingly common as companies seek to develop tailored solutions for specific applications.

Additionally, companies such as Analog Devices and National Instruments are focusing on integrating IoT capabilities into their vibration sensors, enabling real-time data analysis and monitoring. The rise of smart manufacturing and Industry 4.0 is driving the demand for intelligent vibration sensors that can seamlessly connect with existing industrial systems. This trend presents opportunities for market players to differentiate themselves through innovative product offerings and enhanced customer experiences. Furthermore, the growing emphasis on predictive maintenance practices is pushing manufacturers to develop advanced vibration sensor technologies that provide valuable insights into equipment health and performance.

Emerging players like Murata Manufacturing and Omron Corporation are also gaining traction in the market by offering cost-effective solutions and leveraging their expertise in sensor technology. These companies are focusing on niche applications where specialized vibration monitoring solutions are required. The competitive landscape is rapidly evolving, and companies must remain agile and adaptive to changing market dynamics to capitalize on emerging trends and opportunities in the vibration sensor market.

• 1 Appendix

  • 1.1 List of Tables
  • 1.2 List of Figures

• 2 Introduction

  • 2.1 Market Definition
  • 2.2 Scope of the Report
  • 2.3 Study Assumptions
  • 2.4 Base Currency & Forecast Periods

• 3 Market Dynamics

  • 3.1 Market Growth Factors
  • 3.2 Economic & Global Events
  • 3.3 Innovation Trends
  • 3.4 Supply Chain Analysis

• 4 Consumer Behavior

  • 4.1 Market Trends
  • 4.2 Pricing Analysis
  • 4.3 Buyer Insights

• 5 Key Player Profiles

  • 5.1 Meggit PLC
    • 5.1.1 Business Overview
    • 5.1.2 Products & Services
    • 5.1.3 Financials
    • 5.1.4 Recent Developments
    • 5.1.5 SWOT Analysis
  • 5.2 Siemens AG
    • 5.2.1 Business Overview
    • 5.2.2 Products & Services
    • 5.2.3 Financials
    • 5.2.4 Recent Developments
    • 5.2.5 SWOT Analysis
  • 5.3 Rion Co., Ltd.
    • 5.3.1 Business Overview
    • 5.3.2 Products & Services
    • 5.3.3 Financials
    • 5.3.4 Recent Developments
    • 5.3.5 SWOT Analysis
  • 5.4 TE Connectivity
    • 5.4.1 Business Overview
    • 5.4.2 Products & Services
    • 5.4.3 Financials
    • 5.4.4 Recent Developments
    • 5.4.5 SWOT Analysis
  • 5.5 Omron Corporation
    • 5.5.1 Business Overview
    • 5.5.2 Products & Services
    • 5.5.3 Financials
    • 5.5.4 Recent Developments
    • 5.5.5 SWOT Analysis
  • 5.6 Analog Devices, Inc.
    • 5.6.1 Business Overview
    • 5.6.2 Products & Services
    • 5.6.3 Financials
    • 5.6.4 Recent Developments
    • 5.6.5 SWOT Analysis
  • 5.7 Emerson Electric Co.
    • 5.7.1 Business Overview
    • 5.7.2 Products & Services
    • 5.7.3 Financials
    • 5.7.4 Recent Developments
    • 5.7.5 SWOT Analysis
  • 5.8 Kistler Instrumente AG
    • 5.8.1 Business Overview
    • 5.8.2 Products & Services
    • 5.8.3 Financials
    • 5.8.4 Recent Developments
    • 5.8.5 SWOT Analysis
  • 5.9 PCB Piezotronics, Inc.
    • 5.9.1 Business Overview
    • 5.9.2 Products & Services
    • 5.9.3 Financials
    • 5.9.4 Recent Developments
    • 5.9.5 SWOT Analysis
  • 5.10 Brooks Automation, Inc.
    • 5.10.1 Business Overview
    • 5.10.2 Products & Services
    • 5.10.3 Financials
    • 5.10.4 Recent Developments
    • 5.10.5 SWOT Analysis
  • 5.11 Honeywell International Inc.
    • 5.11.1 Business Overview
    • 5.11.2 Products & Services
    • 5.11.3 Financials
    • 5.11.4 Recent Developments
    • 5.11.5 SWOT Analysis
  • 5.12 Vishay Precision Group, Inc.
    • 5.12.1 Business Overview
    • 5.12.2 Products & Services
    • 5.12.3 Financials
    • 5.12.4 Recent Developments
    • 5.12.5 SWOT Analysis
  • 5.13 Wilcoxon Sensing Technologies
    • 5.13.1 Business Overview
    • 5.13.2 Products & Services
    • 5.13.3 Financials
    • 5.13.4 Recent Developments
    • 5.13.5 SWOT Analysis
  • 5.14 Murata Manufacturing Co., Ltd.
    • 5.14.1 Business Overview
    • 5.14.2 Products & Services
    • 5.14.3 Financials
    • 5.14.4 Recent Developments
    • 5.14.5 SWOT Analysis
  • 5.15 National Instruments Corporation
    • 5.15.1 Business Overview
    • 5.15.2 Products & Services
    • 5.15.3 Financials
    • 5.15.4 Recent Developments
    • 5.15.5 SWOT Analysis

• 6 Market Segmentation

  • 6.1 Vibration Sensor Market, By Technology
    • 6.1.1 Piezoelectric
    • 6.1.2 Capacitive
    • 6.1.3 Strain Gauge
    • 6.1.4 Optical
    • 6.1.5 Others
  • 6.2 Vibration Sensor Market, By Application
    • 6.2.1 Automotive
    • 6.2.2 Aerospace & Defense
    • 6.2.3 Industrial
    • 6.2.4 Consumer Electronics
    • 6.2.5 Healthcare
  • 6.3 Vibration Sensor Market, By Product Type
    • 6.3.1 Accelerometers
    • 6.3.2 Velocity Sensors
    • 6.3.3 Displacement Sensors
    • 6.3.4 Proximity Probes
    • 6.3.5 Others
  • 6.4 Vibration Sensor Market, By Distribution Channel
    • 6.4.1 Online Stores
    • 6.4.2 Direct Sales
    • 6.4.3 Distributors
    • 6.4.4 OEMs
    • 6.4.5 Others

• 7 Competitive Analysis

  • 7.1 Key Player Comparison
  • 7.2 Market Share Analysis
  • 7.3 Investment Trends
  • 7.4 SWOT Analysis

• 8 Research Methodology

  • 8.1 Analysis Design
  • 8.2 Research Phases
  • 8.3 Study Timeline

• 9 Future Market Outlook

  • 9.1 Growth Forecast
  • 9.2 Market Evolution

• 10 Geographical Overview

  • 10.1 Europe - Market Analysis
    • 10.1.1 By Country
      • 10.1.1.1 UK
      • 10.1.1.2 France
      • 10.1.1.3 Germany
      • 10.1.1.4 Spain
      • 10.1.1.5 Italy
  • 10.2 Asia Pacific - Market Analysis
    • 10.2.1 By Country
      • 10.2.1.1 India
      • 10.2.1.2 China
      • 10.2.1.3 Japan
      • 10.2.1.4 South Korea
  • 10.3 Latin America - Market Analysis
    • 10.3.1 By Country
      • 10.3.1.1 Brazil
      • 10.3.1.2 Argentina
      • 10.3.1.3 Mexico
  • 10.4 North America - Market Analysis
    • 10.4.1 By Country
      • 10.4.1.1 USA
      • 10.4.1.2 Canada
  • 10.5 Vibration Sensor Market by Region
  • 10.6 Middle East & Africa - Market Analysis
    • 10.6.1 By Country
      • 10.6.1.1 Middle East
      • 10.6.1.2 Africa

• 11 Global Economic Factors

  • 11.1 Inflation Impact
  • 11.2 Trade Policies

• 12 Technology & Innovation

  • 12.1 Emerging Technologies
  • 12.2 AI & Digital Trends
  • 12.3 Patent Research

• 13 Investment & Market Growth

  • 13.1 Funding Trends
  • 13.2 Future Market Projections

• 14 Market Overview & Key Insights

  • 14.1 Executive Summary
  • 14.2 Key Trends
  • 14.3 Market Challenges
  • 14.4 Regulatory Landscape

Segments Analyzed in the Report

The global Vibration Sensor market is categorized based on

By Product Type

• Accelerometers
• Velocity Sensors
• Displacement Sensors
• Proximity Probes
• Others

By Application

• Automotive
• Aerospace & Defense
• Industrial
• Consumer Electronics
• Healthcare

By Distribution Channel

• Online Stores
• Direct Sales
• Distributors
• OEMs
• Others

By Technology

• Piezoelectric
• Capacitive
• Strain Gauge
• Optical
• Others

By Region

• North America
• Europe
• Asia Pacific
• Latin America
• Middle East & Africa

Key Players

• TE Connectivity
• Honeywell International Inc.
• Analog Devices, Inc.
• National Instruments Corporation
• PCB Piezotronics, Inc.
• Emerson Electric Co.
• Siemens AG
• Murata Manufacturing Co., Ltd.
• Omron Corporation
• Wilcoxon Sensing Technologies
• Vishay Precision Group, Inc.
• Brooks Automation, Inc.
• Meggit PLC
• Kistler Instrumente AG
• Rion Co., Ltd.