Sound Intensity vs Sound Power Density: Key Differences and Applications

Content Summary

This article provides a detailed introduction to the definitions, units, and key differences between Sound Intensity, Sound Power Density, sound energy flux density, sound intensity level, and sound energy flux density level. Based on the national standard GB/T 2900.86–2009, it explains the applications of these parameters in assessing microphone directivity, environmental noise control, wireless microphone stability, and other areas.

By analyzing their specific impacts in condenser microphones, dynamic microphones, and wireless microphones, the article helps readers better understand how to utilize sound intensity-related parameters to optimize the overall performance of audio systems.

Understanding Sound Intensity, Power Density, and Their Levels in Microphone Systems

In audio engineering and sound system design, “sound intensity,” “sound power density,” and “sound intensity level” are important physical quantities that describe the directionality and spatial distribution of sound energy propagation.

According to the national standard GB/T 2900.86–2009 definitions 801-21-38 and 801-22-06:

“Sound intensity is the ratio of sound power passing through an area perpendicular to a specified direction divided by that area.”

“Sound intensity level is the logarithm of the ratio of the given sound intensity to the reference sound intensity, expressed in Decibels (dB), equal to 10 times the value.”

These concepts not only help us understand how sound propagates in space from a physical perspective but also play a crucial role in microphone selection, sound field analysis, and noise control.

This article will delve into the following five core parameters:

Sound Intensity

Sound Power Density

Sound Energy Flux Density

Sound Intensity Level

Sound-Energy Flux Density Level

We will introduce their definitions, units, and application scenarios one by one, and provide comparative analysis to help you better understand the relationships and differences between these parameters.

Additionally, we will combine content from “Detailed Explanation of Microphone Frequency Response” “Detailed Explanation of Microphone Sensitivity,” “Detailed Explanation of Phantom Power for Microphones,” and “Detailed Explanation of Sound Power and Sound Power Level,” to explore the specific impacts of these parameters on devices such as condenser microphones, dynamic microphones, and wireless microphones.

1. What is sound intensity? Definition, units, and physical meaning

Sound intensity refers to the amount of sound energy passing through a unit area per unit time and has a clear directionality. It is a vector quantity used to describe the flow of sound energy in a specific direction.  

National Standard GB/T 2900.86–2009 Definition 801-21-38 provides the following definition:  

“The ratio of the sound power passing through an area perpendicular to a specified direction divided by that area.”

In other words:  

If a sound source emits sound power in a certain direction;  

Then the sound intensity in that direction is equal to the power divided by the corresponding area;  

For example:  

The sound intensity measured in front of a loudspeaker is relatively high;  

While it significantly decreases on the sides or rear;  

This indicates that sound intensity has a pronounced directional dependency, which is critical for microphone placement and pickup angle selection.  

What are the units of sound intensity?

The international unit of sound intensity is watts per square meter (W/m²). Since sound intensity values are typically small in everyday environments, commonly used units include:

milliwatts per square meter (mW/m²): 1 mW/m² = 1×10⁻³ W/m²

microwatts per square meter (μW/m²): 1 μW/m² = 1×10⁻⁶ W/m²

PicoWatt per square meter (pW/m²): 1 pW/m² = 1×10⁻¹² W/m²

2. What are sound power density and sound energy flux density?

Sound power density (Sound Power Density) refers to the sound power carried per unit area. It is an indicator of the concentration of sound energy in a given area.

Although the terminology differs slightly, in practical applications, sound power density is often considered equivalent to sound energy flux density (Sound Energy Flux Density).

Sound energy flux density (Sound Energy Flux Density) is the amount of sound energy passing through a unit area per unit time. It emphasizes the flow characteristics of sound energy in space, particularly suitable for analyzing non-steady sound fields or complex reverberant environments.

These two parameters collectively reflect the energy distribution state of sound during propagation, aiding in the assessment of Sound Pressure changes in various directions within the microphone's environment.

3. What are sound intensity level (SIL) and sound energy flux density level (SEFL)? How are they calculated?

To facilitate comparison and standardized expression, the logarithmic forms of sound intensity level (SIL) and sound energy flux density level (SEFL) have been introduced.

The basic formula is:  

SIL (dB) = 10 × log(I₁ / I₀)  

Where:  

I₁ is the measured sound intensity;  

I₀ is the reference sound intensity, typically set to 1 pW/m²;  

This formula converts the linearly varying sound intensity into a logarithmic scale expressed in deciBels (dB), allowing us to represent large energy changes with smaller numbers.

For example:  

If the sound intensities of two sound fields are 1 pW/m² and 10 pW/m², respectively, their sound intensity difference is 10 dB;  

If they are 1 pW/m² and 100 pW/m², respectively, the difference is 20 dB;  

This expression allows us to more easily compare sound energy distributions across different devices or environments.

4. Comparative Analysis of Sound Intensity-Related Parameters

Although these terms are closely related in physical meaning, there are key differences between them.

Sound intensity describes the amount of sound energy flowing per unit area over time and has a clear directionality. It is suitable for analyzing the propagation path of sound in space, particularly playing a decisive role in microphone pickup processes.

Sound power density focuses on the total sound power contained per unit area without considering directional factors. It is more suitable for static modeling and room acoustic predictions.

Sound energy flux density is closely related to sound intensity and also emphasizes the direction of energy flow, commonly used in dynamic sound field measurements and impulse response analysis.

Sound intensity level is the logarithmic expression of sound intensity after standardization, commonly used for equipment specifications, industry standard development, and cross-product performance comparisons.

Sound energy flux density level is the logarithmic expression of sound energy flux density, also used in noise level assessment and acoustic testing.

In general, sound intensity and sound energy flux density have directionality and are suitable for dynamic sound field analysis; sound power density is more suitable for static energy distribution modeling; sound intensity level and sound energy flux density level are standardized logarithmic expressions, facilitating cross-device comparisons and industry standardization.

5. The role of sound intensity-related parameters in microphone systems

Although microphones do not directly measure sound intensity or sound power density, understanding these parameters is crucial for selecting the appropriate microphone type, evaluating the recording environment, and optimizing audio system performance.

Influencing microphone directionality and pickup range

A microphone's directionality determines its sensitivity to sounds from different directions. The directional characteristics of sound intensity directly affect the microphone's pickup performance.

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For example:

Cardioid microphones are most sensitive to sound sources directly in front of them;

They strongly suppress weaker sound intensity from the sides and rear;

In high-sound-intensity areas (such as the center of the stage), microphones can capture clear signals;

In low-sound-intensity areas (such as the edges of the audience seating), additional gain or adjustment of the microphone's position may be required;

Combined with an analysis of microphone frequency response, one can further assess the microphone's performance in specific frequency bands.

The relationship between the interference resistance of wireless microphone systems and sound intensity distribution

In wireless microphone systems, the spatial distribution of sound intensity not only affects pickup quality but may also impact the stability and clarity of RF signals.

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For example:

When input sound intensity is too high, the preamplifier may enter compression mode;

This can cause signal distortion or unstable modulation;

An A-weighted Sound Level Meter can be used to monitor input sound pressure, thereby indirectly controlling the impact of sound intensity;

Additionally, in scenarios where multiple microphones are used simultaneously, properly arranging microphone spacing and directional angles can help reduce crosstalk issues caused by differences in sound intensity.

Dynamic Microphones and Compatibility with High-Intensity Sound Environments

Dynamic microphones, due to their robust construction and strong noise resistance, are widely used in high-intensity sound field scenarios.

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For example:

When recording in front of a guitar amplifier, sound intensity can reach 1 W/m²;

in such cases, dynamic microphones with their high Max SPL and excellent mechanical durability are an ideal choice;

in contrast, ordinary condenser microphones may experience clipping due to their inability to withstand such high input power;

6. Sound Intensity and Environmental Noise Control

In spaces with high audio quality requirements, such as recording studios and conference halls, the concept of sound intensity is also applied to environmental noise control.

For example:  

The sound intensity distribution of equipment such as air conditioning and ventilation systems directly affects indoor background noise;  

By calculating the sound intensity levels of each sound source, the overall Sound Pressure Level can be predicted, and sound-absorbing measures can be implemented;  

Combined with “Detailed Explanation of Microphone Sensitivity,” it can be determined whether the microphone will be affected by low-frequency noise interference;  

Additionally, sound intensity data is also used in architectural acoustics design for reverberation time modeling and standing wave prediction.

7. Sound intensity and related parameters are core indicators for understanding sound propagation

Whether you are an audio engineer, sound engineer, or product designer, understanding the concepts of sound intensity, sound power density, sound intensity level, and sound energy flux density level is crucial for understanding sound propagation paths and optimizing microphone selection and system design.

These concepts not only help us understand the energy distribution of sound from a physical perspective but also guide us in reasonably configuring equipment for different application scenarios. By combining content from “Detailed Explanation of Microphone Frequency Response,” “Detailed Explanation of Microphone Sensitivity,” “Detailed Explanation of Phantom Power for Microphones,” and “Detailed Explanation of Sound Power and Sound Power Level,” we can comprehensively evaluate the overall performance of microphones.

Only by combining theory with practice can we truly achieve high-quality audio capture and transmission.