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#1 2023-05-14 22:27:25

Jai Ganesh
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Registered: 2005-06-28
Posts: 46,162

Reverberation

Reverberation

Gist

1. prolongation of a sound; resonance.
2. a continuing effect; a repercussion.

Summary

What is Reverberation?

Reverberation is the accumulation of soundwaves in a space. Because reverberated sounds stack up, they can make direct communication difficult because there is so much ambient sound and the direct sound can get lost. When that happens, people tend to speak up to be heard over the din which only causes more reverb, and the cycle continues.

Reverberation vs Echo – What’s the difference?

There is a misconception that echo is a reverberation synonym. They are actually two different sonic phenomena. The main similarity between the two is that both reverberation and echo happen after a sound has been made.

We can see the difference when we define reverb and define echo. An echo is a sound wave that has been reflected or otherwise returned with sufficient magnitude and delay to be detectable as a wave distinct from that which was directly transmitted. In layman’s terms, an echo is a reflected sound wave that is audibly distinct from the source sound.

Reverberation, on the other hand, happens when sound persists as a result of repeated reflection or scattering after the sound source has stopped. So while an echo is a distinct sound, reverberated sounds are difficult to hear clearly because the reflections keep repeating.

Basically, echos often sound very clear because the magnitude of the sound and the distance it travels makes it sound distinct.

Reverberation can muddy the sonic waters of your environment because soundwaves reflecting off multiple surfaces pile on top of each other.

What are the Uses of Reverberation?

That doesn’t mean that all reverberation is bad. When using a microphone, speaking publicly, or performing live, some reverb will help give your sound a richer, warmer feeling. On a recording, simulating reverb using software helps create the sonic illusion that the listener is in an actual space with the musician. Without reverberation, those sounds will seem empty or tinny because they don’t feel as real to us.

In a live setting, too much reverb makes the language hard to understand and too little means the sound from the speaker or performer won’t carry as far. This is why designers of lecture halls, theaters, and concert venues pay particular attention to the reverberative qualities of their venues.

When is the reverb undesirable?

Reverberation becomes undesirable when it is out of control. When sounds from several sources all reverberate in the same space together, speakers must speak at higher volumes to be heard which only contributes to the reverb problem.

Environments that are full of sustained, loud noises can lead to tinnitus or even long-term hearing loss for people that occupy those spaces for too long.

What is reverberation time?

The key to controlling reverb is understanding reverberation time. Reverberation time is the number of seconds it takes for reverberant sound energy to decrease by a defined number of decibels—common measurements of reverb time focus on the time it takes for reverberant energy to drop by 60 decibels. Concert hall designers have been factoring in reverberation time for years. Using a reverb time formula, architects have designed concert halls that will stretch reverb time just enough to augment the music on stage without muddying the sound.

In a room like a normal office where several conversations may be happening at once, a short reverberation time is desirable. Limiting reverb and shortening reverb time is important in video conferencing because a microphone won’t discriminate between the sounds that it picks up.

So you may sound unclear to the people on the other end of your call because your microphone is picking up what you’re saying now and the residual sound waves from what you just said.

How do you calculate reverberation time?

Calculating reverberation time is done by determining the volume of space and the absorptive qualities of the materials therein along with the speed of sound in that space. What this means is that by factoring in the size of a space in three dimensions, the materials that make up that space, how much sound those materials absorb or reflect, and the speed and angles at which sound will be traveling, you can find reverberation time.

Once you know the reverberation time of space, you can make adjustments to the materials in that space to increase or decrease it. A sound level meter can be used to measure the level of reverb or reverberation time in a room or other enclosed space by using an impulse response measurement technique.

How to use a Sound Level Meter:

Generate an impulse sound: You will need to generate a short, sharp sound in the space you want to measure the reverb. A starter pistol, a balloon pop or a loud clap can be used for this purpose.

Capture the impulse response: Use the sound level meter to record the sound of the impulse using the built-in microphone, or by connecting an external microphone to the meter. Make sure to capture the sound as accurately as possible and avoid any external noise or reflections.

Analyze the impulse response: Using acoustics software, or app on your phone, analyze the impulse response to calculate the reverberation time. The software will display a graph showing the decay of the sound in the room over time, and the time it takes for the sound to decay by 60 dB (or some other specified amount) is the reverberation time.

Measuring reverb using a sound level meter can be a useful tool for optimizing the acoustic environment of a recording studio, performance venue, or other space to achieve the desired sound quality. It can also be used to diagnose and troubleshoot acoustic problems in a room, such as excessive echo or resonance.

How to control reverb?

When you are surrounded by hard surfaces and angles, you create an environment that will probably have reverberation issues. Taking steps to control the reverb and echo in your spaces will allow for easier communication because there won’t be so many ambient sound waves bouncing around, masking what you want to hear. With the right acoustical materials, you can achieve the best reverberation time for your space..

In conclusion

Reverberation can make a sound seem warm and welcoming or completely overwhelm the people in a space with an excess of noise.

To make sure you are seeing the benefits of reverberation without the potential drawbacks, talk to an expert today to take the first steps toward a better sonic environment.

Details

Reverberation (also known as reverb), in acoustics, is a persistence of sound after it is produced. Reverberation is created when a sound or signal is reflected. This causes numerous reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air. This is most noticeable when the sound source stops but the reflections continue, their amplitude decreasing, until zero is reached.

Reverberation is frequency dependent: the length of the decay, or reverberation time, receives special consideration in the architectural design of spaces which need to have specific reverberation times to achieve optimum performance for their intended activity. In comparison to a distinct echo, that is detectable at a minimum of 50 to 100 ms after the previous sound, reverberation is the occurrence of reflections that arrive in a sequence of less than approximately 50 ms. As time passes, the amplitude of the reflections gradually reduces to non-noticeable levels. Reverberation is not limited to indoor spaces as it exists in forests and other outdoor environments where reflection exists.

Reverberation occurs naturally when a person sings, talks, or plays an instrument acoustically in a hall or performance space with sound-reflective surfaces. Reverberation is applied artificially by using reverb effects, which simulate reverb through means including echo chambers, vibrations sent through metal, and digital processing.

Although reverberation can add naturalness to recorded sound by adding a sense of space, it can also reduce speech intelligibility, especially when noise is also present. People with hearing loss, including users of hearing aids, frequently report difficulty in understanding speech in reverberant, noisy situations. Reverberation is also a significant source of mistakes in automatic speech recognition.

Dereverberation is the process of reducing the level of reverberation in a sound or signal.

Reverberation time

Reverberation time is a measure of the time required for the sound to "fade away" in an enclosed area after the source of the sound has stopped.

When it comes to accurately measuring reverberation time with a meter, the term T60  (an abbreviation for reverberation time 60 dB) is used. T60 provides an objective reverberation time measurement. It is defined as the time it takes for the sound pressure level to reduce by 60 dB, measured after the generated test signal is abruptly ended.

Reverberation time is frequently stated as a single value if measured as a wideband signal (20  Hz to 20 kHz). However, being frequency-dependent, it can be more precisely described in terms of frequency bands (one octave, 1/3 octave, 1/6 octave, etc.). Being frequency dependent, the reverberation time measured in narrow bands will differ depending on the frequency band being measured. For precision, it is important to know what ranges of frequencies are being described by a reverberation time measurement.

In the late 19th century, Wallace Clement Sabine started experiments at Harvard University to investigate the impact of absorption on the reverberation time. Using a portable wind chest and organ pipes as a sound source, a stopwatch and his ears, he measured the time from interruption of the source to inaudibility (a difference of roughly 60 dB). He found that the reverberation time is proportional to room dimensions and inversely proportional to the amount of absorption present.

The optimum reverberation time for a space in which music is played depends on the type of music that is to be played in the space. Rooms used for speech typically need a shorter reverberation time so that speech can be understood more clearly. If the reflected sound from one syllable is still heard when the next syllable is spoken, it may be difficult to understand what was said. "Cat", "cab", and "cap" may all sound very similar. If on the other hand the reverberation time is too short, tonal balance and loudness may suffer. Reverberation effects are often used in studios to add depth to sounds. Reverberation changes the perceived spectral structure of a sound but does not alter the pitch.

Basic factors that affect a room's reverberation time include the size and shape of the enclosure as well as the materials used in the construction of the room. Every object placed within the enclosure can also affect this reverberation time, including people and their belongings.

Measurement

Historically, reverberation time could only be measured using a level recorder (a plotting device which graphs the noise level against time on a ribbon of moving paper). A loud noise is produced, and as the sound dies away the trace on the level recorder will show a distinct slope. Analysis of this slope reveals the measured reverberation time. Some modern digital sound level meters can carry out this analysis automatically.

Several methods exist for measuring reverberation time. An impulse can be measured by creating a sufficiently loud noise (which must have a defined cut-off point). Impulse noise sources such as a blank pistol shot or balloon burst may be used to measure the impulse response of a room.

Alternatively, a random noise signal such as pink noise or white noise may be generated through a loudspeaker, and then turned off. This is known as the interrupted method, and the measured result is known as the interrupted response.

A two-port measurement system can also be used to measure noise introduced into a space and compare it to what is subsequently measured in the space. Consider sound reproduced by a loudspeaker into a room. A recording of the sound in the room can be made and compared to what was sent to the loudspeaker. The two signals can be compared mathematically. This two port measurement system utilizes a Fourier transform to mathematically derive the impulse response of the room. From the impulse response, the reverberation time can be calculated. Using a two-port system allows reverberation time to be measured with signals other than loud impulses. Music or recordings of other sounds can be used. This allows measurements to be taken in a room after the audience is present.

Under some restrictions, even simple sound sources like handclaps can be used for measurement of reverberation

Reverberation time is usually stated as a decay time and is measured in seconds. There may or may not be any statement of the frequency band used in the measurement. Decay time is the time it takes the signal to diminish 60 dB below the original sound. It is often difficult to inject enough sound into the room to measure a decay of 60 dB, particularly at lower frequencies. If the decay is linear, it is sufficient to measure a drop of 20 dB and multiply the time by 3, or a drop of 30 dB and multiply the time by 2. These are the so-called T20 and T30 measurement methods.

The RT60 reverberation time measurement is defined in the ISO 3382-1 standard for performance spaces, the ISO 3382-2 standard for ordinary rooms, and the ISO 3382-3 for open-plan offices, as well as the ASTM E2235 standard.

The concept of reverberation time implicitly supposes that the decay rate of the sound is exponential, so that the sound level diminishes regularly, at a rate of so many dB per second. It is not often the case in real rooms, depending on the disposition of reflective, dispersive and absorbing surfaces. Moreover, successive measurement of the sound level often yields very different results, as differences in phase in the exciting sound build up in notably different sound waves. In 1965, Manfred R. Schroeder published "A new method of Measuring Reverberation Time" in the Journal of the Acoustical Society of America. He proposed to measure, not the power of the sound, but the energy, by integrating it. This made it possible to show the variation in the rate of decay and to free acousticians from the necessity of averaging many measurements.

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