Sabine and reverberation / reverberation

By | April 5, 2021

Every now and then in the search for “How It Really Works” you simply have to start from the beginning and sometimes you also find some fun details right at the beginning.

In Sabine’s Collected Papers On Acoustics, I found some about how the concept of reverberation, or reverberation as we say in Swedish, began to take its slightly more scientific form.

Although there is a lot to say about reverberation, I will only address a small amount of science here and instead a little more curious about how it started.

Only that is enough for two parts, so that the post does not get too long. It will be a little Swedish sometimes, you have to put up with it.

Wallace Clement Sabine and reverberation

In 1895, the young professor Wallace Clement Sabine, who then worked at Harvard but had not worked with sound before, was commissioned to repair the relatively newly built Fogg Lecture Hall.

It had a reverberation time of five and a half seconds and it was basically impossible to hear several words in a row in the building.

The first thing he did was to clarify the concepts.

Even then, there was a discussion about the concepts of sound decay, reverberation, and echo.

And even though science had defined the concept of resonance, it was used in everyday speech when it was called reverberation or echo.

So reverberation was defined as multiple reflections from several surfaces where echo is included as a special case. Unlike echo, the reverberation fills a room and individual reflections are indistinguishable and impossible to locate. Echo is a distinct, often short, sound that is repeated.

In Sabine’s case, he could not hear any distinct reflections or echoes. So it became the reverberation he focused on.

Another difference between reverberation and echo is that in the case of reverberation, we only care about how fast it sounds. When it comes to echo, we are also interested in how long it takes from original sound to first reflection.

This meant that you could then fill a room with sound, then turn off the sound and measure the time it took for it to sound off. To be able to measure this, you must be able to measure the intensity of the sound.

There were several ways that were investigated and later abandoned.

An example was to optically measure a manometric gas flame with a micrometer telescope and they also tried to photograph this.

In the end, it was concluded that the ear with the help of an electric chronograph gave surprising precision and was sensitive enough to determine the time for decay.

Do not ask me about all these tools they tested because I do not know much about them except that I have seen some in the picture.

So instead of measuring how fast it sounded, you then chose how long you could hear residual sound (residual sound).

Scientifically, it was a relationship that was inversely proportional, ie “rate of decay” vs “reverberation time”.

In the case of Fogg Lecture Hall, it was reasoned that there were two factors that influenced: the shape including the volume and the material including furniture.

Since it is difficult and costly to change the shape afterwards, the focus fell on interior design and materials.

What we today call absorption capacity simply.

More on how it went will come next time.

Take care for now!