In part 1 and part 2 of this series of blog posts, we described the acoustics that explain why audiometric asymmetries are found in some musicians (and rarely found in industrial workers). In the last blog, the physics of wavelength phenomena were discussed, and now we turn our attention to the exciting world of reverberation.
Reverberation is one of those phenomena where too little can be as bad as too much. Examples of this are distortion, potato chips, and water; too much of any of these can be bad for you, but the same can be said for too little. It is a physiological fact that the human body requires potato chips (and also some occasional water). And too little distortion would cause music and speech to be thin and, in most cases, unintelligible.
Having a room that is acoustically “dead” may be useful for some very specific tasks, but it would result in odd sounding speech and odd sounding music. Echoes that contribute to the overall sound of speech and music add a certain level of liveness and naturalness to all sounds.
The topic of reverberation can be thought of as the “opposite of wavelength phenomena,” discussed in part 2 of this series. Long wavelength (or low frequency) sounds do not acoustically “see” a wall, head, or other obstruction, and may go right through them. The last thing that low- frequency sounds would want to do is reflect. The same cannot be said of higher frequency, shorter wavelength sounds.
Higher frequencies do see an obstruction such as a head or a wall and are reflected back like a mirror. Walls are “acoustic mirrors” for higher frequency sounds. And it is this reflection that we call reverberation. There is no such thing as low-frequency reverberation unless the obstruction or wall is very, very thick and dense. Perhaps the only time we would see reverberation (reflections) in the very low-frequency region is in underground car garages- the walls are made of concrete and are a million feet thick.
However, in most real-life environments where walls are 6-8 inches wide and made of normal sheetrock building materials, the sounds that do have enough energy left in them to be noticeable as significant reflections are in the upper frequency regions- the mid and high frequency harmonics of many musical instruments.
In an industrial setting, walls and floors are frequently made of concrete and, like those of an underground car garage, can be quite thick. Echoes off of machinery can be quite significant and will include energy throughout the spectrum (with the exception of the very low-frequency sounds). This reverberation means that a machinery off to the left side will be of a similar sound level when measured at both the left ear and the right ear of a worker.
Musical venues are typically less reverberant. There has been a tremendous amount of research into what is the optimal amount of reverberation for different types of music. As it turns out, the amount of reverberation for one type of music (e.g., liturgical music of the middle ages) may be quite different from that for modern forms of pop music.
One problem concerns the issue of how we actually quantify reverberation. And what about the first reflection? Should we be looking at the second and third (and fourth) reflections of a sound as well? This is seen in the literature as “early” vs. “late” reflections. Early reflections (less than 50 msec) can be constructive and can serve to enhance the sound, whereas later reflections (greater than 80 msec) can significantly degrade the sound quality.
One such measure is called the “Reverberation Time 60” or RT60. This is the amount of time (in seconds) for a sound to fall off in energy by 60 dB. The following figure shows a measure of RT60 and shows the expected falloff of sound energy over time. Some musical venues have RT60 values around 0.9 seconds and others have RT60s that approach 2 seconds.
The following chart from the Journal of the Acoustical Society of America shows some RT60 values for various musical venues. It shows that while RT60 is a commonly used measure, it doesn’t tell us the whole story. Similar RT60 values can be associated with great and lousy concert halls. Industrial RT60s can be on the order of 3 or 4 seconds.
Different quality concert halls but similar Reverberation Times (RT).
|Symphony Hall, Boston||Superior||1.85|
|Davies Hall, San Francisco||Good||1.85|
|Barbican Large Hall, London||Fair to good||1.7|
So what does this have to do with audiometric asymmetries found with some musicians such as drummers, violinists, and viola players? I suppose that by now it’s pretty obvious.
If many musical venues have low reverberation times, then sound that is generated on one side of the head just does not have enough energy left over to be of any potential danger to the other ear even after one reflection, never mind two or three reflections. The RT60s in musical venues are just too short.
Between the acoustic properties of different sized wavelengths (part 2) and reverberation times that are inherently shorter in musical venues, music that emanates from one side of the head will be much lower level by the time it reaches the other ear. Audiometric asymmetries are therefore commonplace with music whereas quite rare with industrial settings.
… and one more thing…. what are the two locations where reverberation does not exist… at least above 100 Hz?…
….1. anechoic chambers and 2. sky diving