Sound pressure, sound intensity, and sound power are three different ways to quantify sound. This FAQ begins by defining each one, looks at various types of measurement techniques that can be used to quantify them, presents a range of international testing standards for measuring sound, looks at instruments used for measuring sound pressure and intensity, and closes with a brief look at the nonlinear characteristics of human hearing.
Sound pressure, sound power, and sound intensity are measured in Pascals (Pa), Watts (W), and Watts per area (W/m2), respectively. Using a suitable reference level, all three can be expressed in decibels (dB) (Table 1).
Relating pressure with intensity
Sound pressure is related to sound intensity. In an acoustic-free field, a space in which no sound reflections occur, sound intensity is equal to the sound pressure multiplied by the sound particle velocity. The speed at which the air molecules vibrate while transmitting a sound is the particle velocity and vector quantity. Sound pressure is a scalar value; combining it with particle velocity results in a vector quantity, sound intensity.
Sound pressure can be measured with a single microphone. Measuring sound intensity is more complex and requires at least two measurement instruments, for example, two microphones. Sound pressure is usually measured relative to the nominal atmospheric pressure, and for many sounds, it’s a variable quantity (Figure 1).
① silence;
② audible sound;
③ atmospheric pressure;
④ sound pressure.
Measurement techniques
If specific requirements are met regarding the type of sound being measured, sound intensity-based sound power can be measured in any sound field. ISO 3740:2000, Acoustics – Determination of sound power levels of noise sources, provides guidelines for the use of specific sound power measurement methods, including:
- Precision method: has to be performed in a laboratory environment with high precision equipment and is called a grade 1 method that gives the most accurate results.
- Engineering method: considers the influence of the acoustic environment and the source type and provides a medium grade of accuracy suitable for engineering analysis. It’s a grade 2 method that gives very accurate results.
- Survey method: requires the least time and less precise equipment and compares sources with similar characteristics. This is a grade 3 method that has limited value in evaluating noise, except on a comparative basis.
Testing standards
There are a range of standards for implementing precision and engineering test methods for sound power, sound intensity, and sound pressure. Some representative standards from the International Standards Organization (ISO), American National Standards Institute (ANSI), American Acoustical Society (ASA), and the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) include:
AHRI 230 – Sound Intensity Testing Procedures for Determining Sound Power of HVAC Equipment, is based on ISO 9614-1 (grade 2) or ISO 9614-2 (grade 2) with certain exceptions and extensions.
ANSI/ASA S1.13-2020 – Measurement of sound pressure levels in the air applies primarily to indoor measurements but may be utilized in outdoor measurements under specified conditions.
ANSI/ASA S12.2-2019 – Criteria for evaluating room noise is an engineering method that employs expanded noise criteria (NC) curves; and a method for evaluating low-frequency fluctuating noise using room noise criterion (RNC) curves.
ISO 3741 (ANSI/ASA S12.51) – Determination of sound power levels and sound energy levels of noise sources using sound pressure, includes precision methods for reverberation test rooms and is designed for testing machines, computers, medical devices, components, or consumer products for regulatory compliance.
ISO 3744 (ANSI/ASA S12.54) – Determination of sound power levels and sound energy levels of noise sources using sound pressure, includes engineering methods for reverberation test rooms and is designed for testing machines, computers, medical devices, components, or consumer products for regulatory compliance.
ISO 9614-1:1993 – Determination of sound power levels of noise sources using sound intensity, employs measurements at discrete points. This engineering grade method applies to sources situated in any environment which is neither so variable in time as to reduce the accuracy of the measurement of sound intensity to an unacceptable degree nor subjects the intensity measurement probe to gas flows of unacceptable speed or unsteadiness.
ISO 9614-2:1996 – Determination of sound power levels using sound intensity is an engineering-grade method for measurement by scanning. The scanning operation can be performed either manually or using a mechanical system.
ISO 9614-3:2002 (ANSI/ASA S12.54) – Determination of sound power levels using sound intensity is a precision method for measurement by scanning. The scanning operation can be performed either manually or using a mechanical system.
ISO 11200:2014 – Noise emitted by machinery and equipment – Guidelines for the use of basic standards for the determination of emission sound pressure levels at a workstation and other specified positions, is the frame standard for the group, ISO 11201, ISO 11202, ISO 11203, ISO 11204 and ISO 11205. It provides guidance for writing noise test codes, providing physical explanations of this noise emission quantity compared to other noise quantities, comparing the different measurement methods offered by the group, and facilitating the selection of the most appropriate method(s) in typical practical situations.
ISO 11201:2010 – Noise emitted by machinery and equipment – Determination of emission sound pressure levels at a workstation and at other specified positions in an essentially free field over a reflecting plane with negligible environmental corrections. The grade of accuracy of the procedure can be qualified as precision or engineering.
ISO 11202:2010 – Noise emitted by machinery and equipment – Determination of emission sound pressure levels at a workstation and at other specified positions applying approximate environmental corrections. The grade of accuracy of the procedure with existing conditions can be qualified as engineering or survey.
ISO 11203:1995 – Noise emitted by machinery and equipment – Determination of emission sound pressure levels at a workstation and other specified positions from the sound power level, specifies two methods for determining the emission sound pressure levels of machinery and equipment at a workstation and at other specified positions nearby, by calculation from the sound power level, and permits the comparison of the sound power of different units of a given family of machinery.
ISO 11204:2010 – Determination of emission sound pressure levels at a workstation and at other specified positions applying accurate environmental corrections. The grade of accuracy of the procedure with existing conditions can be qualified as engineering or survey.
ISO 11205:2003 – Noise emitted by machinery and equipment – Engineering method for determining emission sound pressure levels in situ at the workstation and other specified positions using sound intensity.
Measuring sound pressure and intensity
ANSI S1.1-2013 defines sound pressure measurements in the air made in dB relative to a reference, and 1 Pa will equal a sound pressure level (SPL) of 94 dB. When measuring sounds underwater, a reference level of 1 μPa is used.
Sound pressure is commonly measured using an instrument called a decibel meter, sometimes called a sound pressure level (SPL) meter. It uses a microphone to capture the sound and includes a measurement display. In addition to decibel meters, devices are available that can measure the equivalent continuous sound level (Leq), the RMS value of the sound, and other acoustic parameters.
Decibel meter accuracy can be quantified by ANSI and International Electrotechnical Commission (IEC) guidelines. The ANSI S1.4 standard quantifies decibel meter performance as type 1 or type 2, and the IEC 61672 standard classifies them as class 1 or class 2. A type 1 / class 1 meter is a laboratory-grade instrument, while a type 2 / class 2 instrument is a basic unit for general-purpose applications.
Sound intensity can be measured using handheld probes. Some of these probes are designed to be attached to a smartphone with a sound intensity measurement app. The app can also synchronize with a remote data capture system. The probe can be adjusted to compensate for spacer distance and center displacement and locked into a specific angle (Figure 2).
Hearing is nonlinear
Ears detect changes in sound pressure. An SPL level of 0 dB is defined as the lower limit of audibility, but the upper limit is not clearly defined. It’s often considered to be 194 dB peak or 191 dB SPL. Ears don’t have a flat spectral sensitivity. When plotted as sound pressure versus frequency, the resulting constant loudness curves are highly nonlinear. It takes less sound pressure for an equivalent perceived volume between 3 and 4 kHz compared to higher and lower frequencies (Figure 3).
Summary
Sound intensity, sound pressure, and sound power are all important measurements for quantifying noise levels and audio system performance. All three are usually expressed in dB. Various industry standards have been developed for measuring sound under different conditions and for specific purposes.
References
A guide to measuring sound power, Siemens PLM
Sound Intensity, Wikipedia
Sound Intensity Probe, Norsonic
Sound Power and Sound Pressure Explained, Brüel & Kjær
Sound Power Testing – ISO 3731, ISO3744, North Orbit Acoustic Laboratories
Sound Pressure, Wikipedia
Sound Pressure, Sound Power, and Sound Intensity: What’s the difference?, Siemens
