Acoustic Analysis

Data Acquisition Systems for Acoustic Measurements and Analysis

Factors to consider when purchasing a DAQ for acoustic measurements:

1. Number of channels: The number of required channels will depend on the complexity of the application and the number of sources to be simultaneously measured.
2. Maximum sampling rate: The sampling rate must be sufficient to accurately capture the highest frequencies of interest.
3. Dynamic range: The dynamic range refers to the range of amplitudes that can be accurately measured by the DAQ system. A wide dynamic range is important to ensure that both weak and strong signals can be accurately measured.
4. Compatibility with software: The DAQ system should be compatible with software that allows for easy data visualization, analysis, and manipulation.
5. Portability: If the DAQ system will be used in multiple locations or in the field, portability is an important consideration.

While all of Crystal Instruments' data acquisition systems are capable of performing acoustic measurements, certain models may better suit specific application requirements than others.

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Octave Filters

The Fractional Octave Filter Analysis applies a bank of real-time octave filters to the input time streams and generates two types of signals at the same time: fractional frequency band signals, i.e., octave spectra, and the RMS time history of each filter band. The output of each real-time filter bank is in fact a 3D waterfall signal that is arranged in the x-axis as logarithmic frequency and z-axis as time. In the frequency direction, frequency weighting is applied. In the time axis, the time-weighting is applied.

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Sound Level Meter

An analog sound level meter measures the sound pressure level. The standard sound level meter is more correctly called an exponentially averaging sound level meter as the AC signal from the microphone is converted to DC by an RMS circuit and thus it must have a time-constant of integration.  This is referred to as time-weighting.

In a sound level meter, the time weighting exponential averaging mode supports the following time constants:

• Slow uses a time constant of 1,000 ms.  Slow averaging is useful for tracking the sound pressure levels of signals with sound pressure levels that vary slowly.
• Fast uses a time constant of 125 ms. Fast averaging is useful for tracking the sound pressure of signals with quickly varying sound pressure levels.
• Impulse uses a time constant of 35 ms if the signal is rising and 1,500 ms if the signal is falling. Impulse averaging is useful for tracking sudden increases in the sound pressure level and recording the increases to provide a record of changes.
• User Defined allows users to specify a time constant suitable for a particular application.

The output of the RMS circuit is linear in voltage and is passed through a logarithmic circuit to generate a readout in decibels (dB). This is 20 times the base 10 logarithm of the ratio of a given root-mean-square sound pressure to the reference sound pressure. Root-mean-square (RMS) sound pressure is obtained with a standard frequency weighting and standard time weighting.

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Sound Power

The sound power emitted by a noise source in an anechoic or hemi-anechoic environment can be determined by measuring the sound pressure levels throughout the surrounding space.

A fully anechoic chamber is constructed with sound-absorbent materials that provide complete insulation in all directions whereas a hemi-anechoic chamber typically contains a hard ground floor that acts as a reflecting plane.

Crystal Instruments has developed software compliant with the ISO 3744:1994 and ISO 3745:2003 standard – Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision methods for anechoic and hemi-anechoic rooms.

Sound pressure can be measured using any of the Crystal Instruments data acquisition systems. However, accurate analysis requires knowledge of the microphone positions. Fortunately, the CoCo systems include dedicated software designed for this purpose, allowing users to define key factors such as microphone geometry, environmental conditions, and test room type.

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Sones & Phons

Two sounds with equal intensity do not necessarily have the same ‘loudness’ to humans. Loudness is a subjective term used to describe the human ear’s perception of the strength of a sound. Sones and phons are used to quantify the loudness of a sound.

• A phon is a logarithmic unit of loudness. The number of phons of a sound is defined as the sound pressure level, in decibels, of a 1000 Hz tone that would be perceived to be equally as loud as the sound being measured.
• If a sound is perceived to be as loud as 40 dB at 1000 Hz then it is said to have a loudness of 40 phons.

To better understand this concept, Equal Loudness Curves provide an excellent visual representation of how tones with different frequencies and intensities can be perceived as equally loud. The plot below displays the set of contours, which indicate the sound pressure levels required for tones at different frequencies to be perceived as equally loud by the human ear.

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Noise Criterion

Noise criteria curves are commonly used to characterize background noise in buildings and spaces. The curves were developed to ensure that the sound levels in indoor spaces are safe, comfortable, and appropriate for their intended use. There are multiple noise curves because the acceptable amount of sound pressure varies depending on the type of room. The table below serves as a guideline for selecting the appropriate noise curve to use when evaluating the sound level of a room.

Noise criteria are calculated by measuring the background noise in a building using a 1/1 octave spectrum ranging from 63 Hz to 8000 Hz. The curves can be superimposed on an octave spectrum in the Dynamic Signal Analyzer or Post Analyzer software. The DSA software allows users to measure and view real-time sound pressure and compare it against the curves.

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Acoustic Control

Crystal Instruments provides hardware solutions that not only serve as data acquisition systems but also function as controllers. A simple control system comprises of three main components:

1. Input - The reference or target signal.
2. Operator (Crystal Instruments Spider system) - The system that acts on the input signal. This can involve processes such as computations, manipulations, or transformations, depending on the application and goal of the control system.
3. Output - The result of the operation performed on the input. The output is then routed back to the input of the control system, creating a feedback loop.

Acoustic control encompasses the process of manipulating audios sources based on a predefined frequency spectrum. Building upon the outline of the control system described above, the Spider system functions as the operator in this context. It drives the audio source which serves as the output. A microphone serves as the input and is used to measure the audio output. The microphone, Spider, and audio source combine to form a closed-loop control system.

The Vibration Controller software provides users with a customizable acoustic testing option to accommodate any specific needs.

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Microphones for Acoustic Acquisition

Users should carefully choose an appropriate microphone to realize the full capabilities of Crystal Instrument’s data acquisition systems. The quality of acoustic signal analysis, specifically in terms of signal-to-noise ratio and frequency response, relies not only on the data acquisition system but also on the microphone. It is important to recognize these factors which can become potential bottlenecks that impact the overall analysis quality.

Some parameters to consider when selecting a microphone for acoustic measurements:

1. Sensitivity – The sensitivity refers to the microphone’s ability to convert sound pressure into voltage. A higher sensitivity is beneficial for accurately capturing low-level sounds.
2. Frequency Response – The microphone should ideally have a flat frequency response across the desired measurement range to avoid distortion or attenuation of specific frequencies.
3. Signal-to-Noise Ratio – A high SNR ensures that a microphone accurately captures the desired sound while minimizing the background noise.
4. Directionality – Omnidirectional microphones capture sound from all directions whereas directional microphones focus on specific angles. The appropriate directionality is based on the desired sound source isolation and application.
5. Environmental Considerations – In certain scenarios, a microphone necessitates the ability to endure environmental factors such as temperature, humidity, dust & debris etc.

The GRS is specifically designed to remotely measure sound pressure in harsh environmental conditions. For this reason, we recommend a G.R.A.S. ½” LEMO free-field microphone since it can withstand a large temperature and humidity range. Additionally, the microphone would be placed into an enclosure for protection against rain, dust, and debris.