Sound consists of pressure waves with direct sound from the source, but after you here the direct sound, the sound will bounces off any reflective sources. Sound waves will reflect off of the floor, walls, ceiling, and any other reflective surface. Depending on the absorption factor of each surfaces, will determine the amount of that reflection.
Reverberation is the collection of these reflected sounds and reverberation time is the time after the source of the sound has ceased that it takes the sound to fade away. The reduction of this RT60 would be to include soft, absorptive surfaces in the room and remove any smooth reflective surfaces that will cause sound waves to bounce off the surface.
Critical Distance is the distance from the sound source where the direct and reverberant sound energies become equal. This Critical Distance will be different at all frequencies.
- The more reverberant a room, the closer the Critical
Distance is to the sound source.
- The more absorbent a room, the further the Critical Distance is from the sound source.
A good acoustic design will provide this Critical Distance as far as possible from the sound source. This will resultant in the reverberation to be minimal at all frequencies.
Direct sound from the source will diminish at a level as a function of the distance (inverse square law). Reverberation constantly spreads throughout the room. With new incoming sound from the source reverberation keeps building up until the new sound equals the sound absorbed or steady-state. When the reverberant sound becomes 12dB or greater than the direct sound is intelligibility and totally lost.
The simplest way to find ‘Critical Distance’ is to play compressed pop music through the sound system. Begin with one speaker (left or right). Walk back and forth around the room, and you will be surprised how easy it is to identify the critical distance. Repeat the exercise with the other speaker, then both speakers. It’s surprising how accurate our ears are when compared with acoustic measurement microphones.
The Measurement of Reverberation Time
This is done using Reverberation Time 60 or RT60. RT60 is defined as the measure of the time after the sound source ceases that it takes for the sound pressure level to reduce by 60 dB. Why 60 dB? The loudest sound level of music will average typically at 100 dB and reasonable background noise level will be measured at 40 dB which is a difference of 60 dB. This defines the RT60 measurement as the time it takes for the loudest noise in a room to fade to the background level. It is also important to note that reverberation time will be defined by the frequency. This will mean that a test signal should have to be at a level of 100 dB to measure for RT 60.
RT60 Measuring Difficulties
The measured sound would have to be greater than 60 dB above the background noise and that would be rather loud for testing. This would require for the sound level to be greater than 90 dB if not as much as 100 dB or more.
An alternative has been provided by measuring the first 20 or 30 dB of the decay and then calculate for 60 dB. By using a portion of the decay, you can measure the sound pressure level to decay by 20 dB, which is called the T20 measurement, and multiplied the results by 3. Another option provided is to measure the time for the sound pressure level to decay by 30 dB, which is called the T30 measurement, and multiplied the results and multiply by 2. Note that this measurements will begin after the first 5 dB of the decay.
Now the question is, which one of these do I use?
The T20 or T30?
- When your maximum SPL ≥ 35 dB higher than back- ground noise,
use the T20 measurement for reverberation time.
- When your maximum SPL ≥ 45 dB higher than back- ground noise,
use the T30 measurement for reverberation time.
How is RT60 measured?
In order to measure RT60, the room must first be energized with noise. At a minimum, the popping of a balloon or the clapping of the hands to provide the noise. You can also use a dedicated sound source of the existing system that is in place. I typically use the existing system if available and us a frequency sweep of ten seconds from 40 to 18k Hz. I wish to provide every frequency because of the frequency dependency. A 100 or 300 ms burst of pink or white noise would do as well.
A sound level meter with room acoustics options to measure the time for the sound level to decay and reports the result would be one option. This lacks the sophistication that I prefer, but today there is an app for everything for your phone, tablet or laptop as well as software application for your laptop.
Based upon the size of a room, standing waves will occur at certain frequencies. To better explain, because the varying length of frequencies, a reflected wave will have a phase relationship with the direct wave. If the two are less than 45 degrees in phase of each other, they will be summed together providing this one frequency to be predominate and is defined as a standing waves.
At the same time, when the direct wave and the reflected wave are greater than 45 degrees out of phase of each other, they will be subtract from each other providing this one frequency to be dead at that location and is defined as a dead spot.
For this reason, it is recommended as a best practice is to measure at several different positions in the room.
The RT60 required results will depend on the use of the room. The following provides some reference:
- Less than 0.3 s would be considered acoustically dead.
- Less than 1 s would be good for a classrooms for speech articulation.
- Equal to 1 s: would be considered good for speaking: articulation of speech is clear.
- Music doesn’t sound full, rich, or warm at this level and would require artificial effects
- Greater than 2 s would be considered live or echoic room.
- Equal to 1.5 s to 2.5 s would be considered a good compromise if used for both speaking and music.
- Equal to 3.5 s: Better for music, but some loss of articulation. Possible difficulty in understand of speech.
Please note that there can be a considerable difference from an empty room and a fully populated room. Humans provide for a good amount of absorption factor that will average at a .01 factor or .1% across the frequency spectrum.
C50 is related to the clarity of speech and is expressed in dB values. Late reflections are unfavorable for the understanding of speech when merged with direct sound making speech unclear.
C50 measures the ratio of the early sound energy (between 0 and 50 ms) and the late sound energy (that arrives later than 50 ms). Now if the delay does not exceed this 50 milliseconds, the reflections will contribute positively to this clarity. The ratio will provide a ±20 dB range where +20 dB would have no reflective sound and -20 dB be all reflective sound. The goal of a 0 dB or greater will provide the clarity required.
C80 is related to the clarity of music and is expressed in dB values. Late reflections are unfavorable for the understanding of speech when merged with direct sound making music unclear.
C80 measures the ratio of the early sound energy (between 0 and 80 ms) and the late sound energy (that arrives later than 80 ms). Now if the delay does not exceed this 80 milliseconds, the reflections will contribute positively to this clarity. The ratio will provide a ±20 dB range where +20 dB would have no reflective sound and -20 dB be all reflective sound. The goal of a 0 dB or greater will provide the clarity required.
This clarity is influenced by:
- The location and the amount of absorption.
Absorbing wall panel at the rear wall for large or long room will reduce late reflections.
- Reflecting surfaces, close to the speaker or sound source, can increase the speech clarity.
- Preferred room shapes that supports short distance between the speaker and listeners.
- The signal to noise ratio can increase clarity (speech related to the background noise).
- Wall absorbers will eliminate disturbing echoes that can occur between walls.
D50 is the C50 ratio expressed in percent. This is another way to express the C50 clarity aspect.
This is the total percent of the first 50 ms of energy where:
- 0% = -20 dB; no direct sound and 100% reflective sound
- 25% = -10 dB; 25% direct sound and 75% reflective sound
- 50% = 0 dB; 50% direct sound and 50% reflective sound
- 100% = ±20 dB; 100% direct sound and no reflective sound