Multiple reflections of sound impulses from the surfaces of the room, such as the walls, floor, and ceiling, produce reverberation. These reflections continue to ricochet around the room, overlapping and forming a complicated pattern of sound waves.
Below is an image of CATT Acoustic ray-tracing the movement of acoustic waves in a space. CSG Acoustics have used CATT Acoustic for a number of years and it’s a great tool for reverberation time prediction.
When sounds are emitted in a room, they travel in all directions and reach the room’s surfaces. These surfaces reflect the sound waves, with each reflection losing a small amount of energy due to material absorption. The reflected waves continue to reflect back and forth until they lose enough energy to become inaudible. The image below shows the initial reflections within a space and the various second order and third order reflections – again the image is taken from CATT Acoustic.
Typically reverberation time it is defined as the amount of time required for the sound level to diminish by 60 decibels (dB) from its initial level. Reverberation time is affected by the room’s size and shape, the surfaces’ materials, and the quantity of absorption and diffusion present.
The movie below shows numerous gun shots taken in a large warehouse. This RT is extremely long and was subsequently controlled by covering all walls and ceiling surfaces in acoustic absorption.
There are multiple perceptual effects of reverberation on sound. It enhances the overall aural experience by adding richness and depth to music and speech. In some instances, excessive reverberation can reduce the intelligibility of speech because reflections can obscure the clarity of individual sounds. On the other hand, a room with insufficient reverberation can sound uninteresting and ‘dead’.
In settings such as concert halls, theatres, recording studios, classrooms, and offices, reverberation control is crucial. To modify the reverberation characteristics of a room and accomplish the desired acoustic environment, acoustic treatments such as the use of absorptive materials, diffusers, and sound barriers can be applied.
Typical procedures for measuring reverberation time in a hall include the following:
Calibration: Calibrate your sound level metre or microphone in accordance with the instructions provided by the manufacturer. This phase assures accurate measurements by adjusting for equipment sensitivity variations and ensures that your measurement set-up is working correctly.
Set-up: Position the sound source at a predetermined location in the space, typically slightly off centre. Ensure that the sound source is positioned at an appropriate height and distance from all adjacent surfaces. Place the microphone at the measurement position, typically a few metres away from the sound source and at a height that corresponds to the average audience head height / receiver head height.
Background Noise: Prior to beginning the measurement, determine the background noise level in the room. The background noise level is measured and recorded at the same location as the microphone. This measurement will be used to correct the reverberation time measurement.
Impulse/Decay: Generate a test signal with a broad frequency range, usually pink/white noise or a instantaneous source such as a starter pistol. The signal must be sufficiently powerful to elicit a measurable response, but not so noisy as to cause distortion or discomfort.
Below is an image of a colleague kitted out and ready to undertake measurements.
Measurement: Start the test signal and record the decay of sound level over time using the sound level metre or microphone. Reverberation time is determined by the amount of time it takes for the sound to decay by 60 decibels (or another specified level) from its initial peak.
Analysis: Review the recorded data in order to ascertain the reverberation time. This involves determining the time necessary for the sound level to decay by the desired quantity and calculating the reverberation time using the appropriate mathematical formulas or computer software. Below is an image of the 01dB dB Bati Software we use to analyse the RTs.
Background Noise Correction: Subtract the measured background noise level from the recorded decay curve to eliminate the effect of ambient noise on the measurement of reverberation time.
Repeat and Average: To ensure accuracy, perform the measurement at numerous locations within the space and average the results to determine the representative reverberation time.
Accurate measurement of the reverberation time requires careful consideration of room geometry, microphone placement, and sound source characteristics, among other factors. Consult CSG Acoustics or refer to applicable standards and guidelines for specific measurement procedures and best practises.
Visit the previous blog on Why We Love Acoustics here: https://www.csgacoustics.co.uk/why-we-love-acoustics-and-further-reading/
Stay tuned for next week’s blog Airborne Sound!