Measurement of Sound Pressure In Practical Environments
Based on the ANSI S1.13–2005 standard
Your entire body is under pressure. If you’re located in San Francisco today, your body is under approximately 29.9 inHG of atmospheric pressure. This just means the air particles surrounding you are exerting a force against your body by a certain amount.
When the air particles around you translate vibrational energy from a source, into pressure differentials in your ear canals, the perception of sound is created. These sources can be ambient sounds of the room you’re sitting in, or sounds of interests such as another person you’re speaking to. A scientist or engineer may want to measure the pressure level of sound sources to learn a variety of applicable information such as:
How loud a certain point is in or around the home, at a concert, or in a working environment.
Assess if a space meets a background noise specification (ie a movie theatre).
Evaluate hearing-damage risk caused by noise exposure.
Standardize acoustic testing measurements from one laboratory to the next.
Measure the performance of loudspeakers, musical instruments, or other sound sources.
An acoustical environment has a significant impact on sound pressure waves. The practical acoustical environment is always a combination of free-field and diffuse field environments, which means a combination of a theoretical infinitely large room with no reflections, and a room with a homogeneous even sound field with a continuous amount of reflections. Reflective surfaces like chairs and tables in a room will also have an impact on arriving pressure waves at a microphone being used in a real-world audio measurement application.
Source: Siemens paper on sound fields
Indoors vs Outdoors:
An indoor setting is a relatively small room with hard walls might approximate a diffuse field, whereby a very large room with absorptive acoustic foam on the walls plus a measurement microphone placed in the near-field of a sound source may approximate a free-field. If an acoustics engineer were to make several measurements at various locations of the room, a more diffuse environment would maintain a consistent pressure level, where a free-field room would produce a diminishing sound pressure level the further away from the sounds source the microphone becomes.
Diffuse field indoor room. Source: RDacoustic.cz
Outdoor settings are large, flat and open areas such as the roof of a building, or an empty parking lot. If there are obstacles like walls or cars present, or the contamination of uncontrolled sound sources ie a plane flying by, then the outdoor acoustical environment becomes less ideal. The flooring of an outdoor source needs to also be taken into consideration, as a carpet acts as an absorptive material, whereby a smooth cement is reflective.
Outdoor acoustic test setup. Source: GRAS
For the purpose of this article, I will be ignoring laboratory environments such as anechoic chambers, as they are expensive and are difficult to access for the every-day acoustics enthusiast. What is most important to this audience is the method of measurements and the standards used from one measurement to the next.
Instrumentation:
The accuracy of sound pressure depends on selecting the right instruments for the right situation. For example, if the sound signal being measured is the maximum SPL (sound-pressure-level) of a rocket engine, the proper sound meter will require a high amount of dynamic range, otherwise there will be significant distortion and even damage to that sound meter if exposed to a large enough sound pressure.
A conventional sound-level meter is capable of measuring and computing the time-averaged A-weighted sound level in an environment. The Breul & Kjaer 735 sound level meter can be purchased a variety of vendors, including amazon. It is recommended to use an integrated sound level meter like the 735, as it satisfies a variety of standard requirements such as:
System integration of microphone, pre-amplifier, power supply and on-board display for the user to interface with the device.
Frequency weighting and octave-band analyzers to obtain different types of insights of the audio signals being recorded.
Data logging and data-sampling features that convert the audio recordings into bits of data and digital files that can be used for further FFT and post-processing applications.
Wide-band frequency response and signal-to-noise ratio
Each sound level meter will come with manufacturer instructions to setup the recording equipment and controlling the measurement system for optimal results.
Planning and preparation of measurements is required to execute an affective measurement. All aspects leading up to this point will affect the acquisition methods, such as the type of sound and what environment it is in. Depending on the purpose of the measurement, a detailed plan will vary in complexity, however it should always denote:
Type of sound being measured
Instrumentation needed
What frequency analysis will be performed
Sequence of measurements
Microphone position/distance
When the measurement system is setup properly, and all the details above are noted and understood, it is time for calibration.
Calibration
My favorite step that most engineers in our field overlook is calibration of the measurement system. All equipment used in the measurement chain that can affect the sound pressure level values should be referenced to a known pressure level. This basically tells the tester to increase/decrease the gain of the input system to produce a value that is equal to a known real-world value.
In real world applications, acoustic engineers always reference their measurement microphones to 1 pascal (ie 94 dBSPL) at the 1-kilohertz frequency band. This represents the mid-band frequency range whereby the microphone should produce its most consistent and well-behaved response. In order to do that, a manufacturer such as B&K will offer a sound calibrator that is carefully calibrated and stabilized in their factory to produce a 1 pascal, 1kHz signal.
Image source: Bruel and Kjaer website
Measurement Procedure
This article will focus on steady-continuous sound for simplicity. As such, the measurement of a sound of interest in an environment will require a steady continuous sound emanating from the source. The standardized A-weighting of the steady pressure wave from the sound source will produce a small variety of levels at the microphone. The level shall be taken as the average of the maximum and minimum levels indicated on the meter’s display or scale during the measurement time interval. This value is often written as dBA, with the A denoting the decibel weighting.
All pertinent information, observations, and data should be documented at the time of measurement. The acoustical data required for analysis will be used in post-processing exercises in order to make valuable insights about the particular sound of interest. For instance, each measurement of sound-pressure level comes at a single point in space. When multiple measurements are done at different points in the same space, a scientist can characterize how the sound pressure level varies over space from one point to another.
If you need help organizing all this pertinent information in such a way where it becomes time consuming to find, access and analyze, I am extremely interested in speaking with you. I have a tool that will help, so please contact me at josh@thelyceum.io. Otherwise please leave comments on how you already manage your measurement data and analyze it.
