Room Acoustics Testing Methods: A Comprehensive Guide
In the field of Room Acoustics, measurements are made to help determine how sound is created, propagated, absorbed, and heard by an audience in an enclosed space such as a lecture room, auditorium, concert hall, or transportation terminal. Generally, these measurements are made to provide information about the quality of the sound as perceived by the audience. An acoustic test can help diagnose potential sound issues, whether you are creating a home cinema or building a theater. Taking accurate acoustic measurements lets you pinpoint issues correctly.
Noise sources such as music systems, televisions, appliances, traffic, aircraft, and manufacturing are all around us. The field of building acoustics evaluates and designs the materials and methods to isolate us all from these sound sources when we are at work, resting, or enjoying leisure time. Building acoustics are typically analyzed and designed to control noise from room to room or from outside to inside, often involving creating and measuring an artificial sound source and then measuring the resulting noise in receiver locations.
Larson Davis offers the sound source equipment, the sound level meters to record the source and receiver locations, and the software to calculate and report important metrics. This helps in determining appropriate countermeasures for areas of concern.
This article covers the main ways in which you can measure your room’s acoustics, from clap tests to software analysis, testing a room’s acoustics is the first step.
Reverberation Time: A Key Parameter
Reverberation Time is the single most important parameter used to describe Room Acoustics. Whether for performance venues, architectural measurements, or workplace acoustics, reverberation time is a key parameter for characterizing a room. A long reverberation time can make speech less intelligible, and music can become garbled. Conversely, a short reverberation time can muffle speech and make a room sound “thin”.
The 831C-RA firmware option adds reverberation measurement and calculation functionality to the Model 831C-RA sound level meter.
Tools and Equipment for Acoustic Testing
A wide range of sound sources are available for your application in the Series BASA, giving you an easy-to-use Building and Room Acoustics test setup.
Sound Level Meters
Reverberation Time-Enabled Sound Level Meter Model 831C-RA
Sound Sources
Series BASA wide range of sound sources are available for your application.
Methods for Measuring Room Acoustics
1. Simplified Tests
To measure sound acoustics, you can take a simpler test with the tools you have. You do not need any additional tools for these tests.
2. Software Analysis
A software test offers a more technical analysis of the room. To take the test, download the software on your PC and sit in your usual seat with the microphone, holding it at ear level. Play a full audio sweep on the software, then analyze the results.
Key Measurements from Software Analysis:
- Reverberation time (RT60): This measures how long the sound decays by 60 decibels.
- Frequency response: The frequency response shows you which sounds are too loud or too quiet from where you are sitting.
Echoes and reverberation can make sounds unclear, affecting their quality. Uncontrolled bass is a low-frequency issue that can make some sounds overpowering and unbalanced. These sounds accumulate in corners, resulting in inaccurate or unpleasant noise.
3. Frequency Sweep Method with SPL Meter
This method uses a CD containing test tones played at different frequencies such as 60Hz, 65Hz, 70Hz, etc and a SPL meter. You then play each tone and record the number shown on your SPL meter on a spreadsheet or on a sheet of graph paper. Very time consuming even to put together a single frequency response. Cannot understand the energy / time or time / energy / frequency behavior of your room.
4. Integrated Software and Hardware Packages
A recent development are integrated software and hardware acoustical measurement packages such as XTZ Room Analyzer and Dayton Audio Omnimic where you literally get everything you need to measure your room in one box. The software side of these packages is designed to be easy to use, since they will be typically used by beginners and those with no experience of room measurement. The last thing you want to waste time on is figuring out how to use the software to take measurements!
Software for Acoustic Analysis
The REW room acoustics software contains comprehensive help information. If you find REW useful you can make a donation.
The current version is V5.31.3, 25th July 2024. Notarized universal binary for Intel and ARM Macs. Code signature and will be requested if necessary. Mic access, such as the SPL meter.
For Mac users there's FuzzMeasure, which is equally capable and costs $150 from www.fuzzmeasure.com.
Advanced Testing Techniques
Audio equipment, including amps and speakers, even listening rooms are at the very least always tested with a frequency sweep to get the dB vs frequency response curve. Lying buried, unseen below these response curves, are additional response curves that if known would provide additional information about the performance of the audio equipment. The most well-known of these hidden curves is the steady state background noise floor. With amps and speakers another of these “hidden” curves in Harmonic Distortion. To reveal these otherwise hidden distortions additional tests are required.
On the acoustic side of audio, the listening room is the component. Acoustic distortion is created in the space between the speaker and the listener, in the listening room. There are acoustic tests beside FRC. The RTA real time analyzer shows the spectrum of sound at any given moment. The ETF energy time frequency shows the waterfall spectral decay of sound.
The issue of musicality is not that we don’t hear the musical sound as it was originally sounded. It’s when we can’t hear the low-level leading edge of attack transients and the subsequent musical dynamics of the sound as it was originally played. The test for accurate variation in loudness, amplitude modulation, is an acoustic modulation test. This then is also the test for dynamic distortion in playback rooms.
There are two parts to musicality, one is hearing each of the complex tones and the other is hearing the rapidity of their coming and going, essentially the variation in loudness of the tones being heard, dynamic level fluctuation. Music is not a steady state sound. It is a sound that rapidly varies in frequency and loudness. A simplified version of music can be boiled down to a series of tone/chord bursts.
The Hybrid Swept/Multi-Tone Test
The hybrid Swept/Multi-Tone test signal is played into the room. This MATT test, Musical Articulation Test Tones, is symmetric about 780 Hz and is a linear (not log) gated frequency sweep. The resulting sound level vs time chart as shown below is the basis for evaluating Dynamic Distortion caused by passing the test signal through the listening room. The test tones are 2 Hz apart. Test begins at 20 Hz ascends to 780 Hz then reverses back down to 20 Hz. It is comprised of 760 individual tone burst test responses which are seen as closely spaced vertical lines. Each test tone burst lasts 67ms, (1/16th sec) and is separated from the next test tone burst by 67 ms during which the residual noise can be measured. The upper envelope of the tone burst test outlines the FRC of the room. The lower envelope of the bursts outlines how quiet the room gets between each pair of test tone bursts.
The lower envelope of the MATT test outlines in terms of room acoustics what is found in the third, the right-hand graph in the above section, Discussion of Multi-Tone Testing. The noise response curve.
Below is an expanded view of the above gated sweep test. It has the same vertical scale but horizontal frequency scale has been expanded by a factor of x15, showing the first 100 Hz, between 20 and 120 Hz. A series of low frequency tone bursts created this sound level chart at the listening position. This is essentially the room acoustic version of the Multi-tone display for amps.
Acoustic Treatment and its Measurement
In the realm of acoustic treatment, it's crucial to differentiate effective products from incompetent ones. Many acoustic treatment products and materials claim to be actively absorbing, but their actual performance when installed can vary widely.
Understanding Absorption Coefficients
Acoustic panels are often more effective when spaced from a room surface because they work better with an air gap. This allows sound to wrap around the panel, increasing its exposed surface area and making it more effective. However, the standard absorption coefficient no longer applies because coefficients consider only the front surface area.
Corner Placement of Bass Traps
Some absorbers are intended for mounting in corners as bass traps. Placing a bass trap straddling a corner of the same room can yield four times more absorption than placing it flat on a wall. However, bass traps meant for corner placement are not always flat panels. Some are designed for wall-ceiling tri-corners where two walls meet the floor or ceiling.
It's important to note that lab tests may not always accurately reflect real-world performance. Factors such as boundary conditions during testing versus when installed can significantly affect the results. Additionally, different labs may produce varying results due to differences in testing environments. Therefore, corner placement anywhere other than in a corner can be ineffective.
Limitations of Standard Tests
Standard tests often ignore the size of the device, making it difficult to assess low-frequency absorbers reliably. While 125 Hz is within the range of certified frequencies, the reverberation room method may not be reliable at very low frequencies. A more reliable way to assess low-frequency absorbers and compare proposed trap designs involves measuring modal ringing decay times and reducing the peak Qs.
One limitation with the standard tests is that they do not give any numbers. Data is needed to measure absorber performance. By lowering resonant frequencies, the peaks become broader, indicating that more resonance is needed.
Table: Acoustic Treatment Considerations
| Consideration | Description |
|---|---|
| Air Gap | Spacing panels from the wall improves absorption. |
| Corner Placement | Bass traps are most effective in corners. |
| Low Frequencies | Standard tests may not accurately assess low-frequency absorbers. |
| Size of Device | Standard tests often ignore the size of the device |
Ultimately, understanding the limitations of testing standards and considering real-world installation conditions are crucial for selecting effective acoustic treatment products.