Reverberation time is measured, not judged: a loud impulse, a calibrated microphone and the number of seconds for the level to fall by sixty decibels. Across this hall it is one point one. At one position it is one point six, and the seat next to it is one point one.
I am an acoustic consultant. Most of my work is school halls, because a hall that holds sound is a hall where a child at the back cannot make out a word, and there is a standard with numbers in it that new buildings have to meet.
How the measurement works.
You need an impulse or a burst of noise loud enough to rise well above the background, and then you measure how fast the level falls. The figure is the time for sixty decibels of decay, extrapolated from the part of the curve that is clean, and you report it in octave bands because a room can be live in the bass and dead in the treble.
I use a starting pistol with blanks, which is old fashioned and perfect, and a class one sound level meter on a tripod at ear height.
You measure at several positions because a room is not one number. A hall with a hard back wall will be livelier at the back. Under a balcony it goes dead. That variation is normal and expected and you average it.
Normal variation between two positions three feet apart is a few hundredths of a second.
This hall is a 1960s school hall. Parquet floor, plastered walls, a stage at one end, high windows, two hundred and forty stacking chairs.
It measures one point one seconds at nineteen of the twenty positions I took, within the spread you would expect.
Position fourteen, which is in the ninth row of seating, four seats in from the left, measures one point six.
One point six is not a bad reading. It is a different room.
Half a second of extra reverberation at one point in a hall is not something a building does. Sound does not stay in a cubic metre of air.
I measured the seats either side of it, and in front, and behind. One point one, one point one, one point one, one point zero nine.
What I did next, in 2022, and this is two days of a job I quoted as one.
Repeated it with a different source. Balloon burst instead of the pistol, then a swept sine through a loudspeaker, which is the modern method and which computes the impulse response mathematically. All three give one point six.
Moved the microphone ten centimetres at a time through the position, on a grid. The area that does it is about forty centimetres across. Inside it, one point six. Outside it, one point one. The edge is sharp enough that I could find it blind.
Changed the microphone. Two mics, both calibrated against the same acoustic calibrator before and after, which is the thing you do so a figure can be defended.
Raised the tripod. At ear height, one point six. At a metre and a half, one point six. At two metres, one point two. At the floor, one point one.
So it is a volume of air about the size of a person, sitting in a seat, holding sound.
What is under it and round it.
Parquet on a screed, and the chairs are steel framed and identical and I moved them all out for the second day.
I have the drawings and the hall has nothing below it. No void, no duct, no service run, and the school has never had a basement.
The position is not under a light, not near a pillar, and it is not at a point where any geometry of that room focuses. I modelled it afterwards out of professional stubbornness and the model says one point one everywhere.
What I put in the report.
The average, which is what the standard asks for, and a note that position fourteen was excluded as an outlier, and the figures for all twenty positions in the appendix because I do not hide data.
The school got the recommendation they paid for, which was absorbent panels on the rear wall and the upper side walls, and they had them fitted, and I went back in 2023 to verify.
The hall came down to nought point nine, which is a good result and a quiet hall and worth every penny of it.
Position fourteen is one point four.
It came down by two tenths when the rest of the room came down by two tenths, which means whatever is holding that sound is in the room with us and is listening to the same panels as the rest of the air.
The part that is not in the report.
I did the verification on a Tuesday in the holidays with the site manager in the building and nobody else.
The swept sine method plays a tone that rises for fifteen seconds and you stand still and wait, and what the software then gives you is the impulse response of the room, which is a picture of every reflection arriving at the microphone in order.
At position fourteen there is a reflection at about twelve milliseconds that is not in any of the other nineteen.
Twelve milliseconds is four metres of travel, which is two metres there and two metres back, and there is nothing two metres from that seat in any direction except the floor under it and the air above it.
I have the file. It is a perfectly ordinary looking early reflection, the sort of thing you get off a music stand somebody left out.
There was nothing left out. I moved every chair in that hall myself.