Measuring reverb time without any tools?

J

Jason Lavoie

Junior
Jan 13, 2011
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Ottawa
So I had this thought while operating a scissor lift in a super reverberant gym where the RT was so bad that the intermittent warning beeper on the lift sounded like a continuous tone.

Is it possible to develop a series of carefully created sounds/tones stuttered at certain rates so that an untrained ear could play a CD and determine the ballpark RT? (or at least a pass/fail)
I figure they would get to a certain track where something is indistinguishable from something else and that would tell you about what the RT is.

it wouldn't hold up in court, but it could be a great self-diagnosis tool for a customer to check their space and see if they might need treatment, or if a new customer calls up saying that their system is unintelligible we could have them run through the exercise and see what comes out.
it seems like a lot of people have varying opinions on what constitutes "good" or "bad" acoustics

I know someone smarter than me will either link me to someone who already researched this in the 60s, or will tell me why it just can't be done :)

Jason
 
Re: Measuring reverb time without any tools?

I can't point you to a listening analysis resource, but I will say that the spectral content of the RT60 is equally important as the length of the RT60.
 
Re: Measuring reverb time without any tools?

of course. but I'm looking for a way for the customer to find out for themselves how bad their room is. in most cases if the room hasn't been professionally treated then the RT will be pretty bad at all frequencies.

Jason
 
Re: Measuring reverb time without any tools?

Rather than try to impute the RT60, why not listen for intelligibility with suitably percussive sources, like speech samples with differing degrees of difficulty.

The bottom line is how does the room work, or not. Why not try to test that, at least for informal results? I am speculating, but a range of samples that get harder to follow might serve as a rough quantification of the problem. Multiple raters could help, but all things equal, one person could learn to judge by listening to same samples in a treated or known good room vs. not.

or not...

JR
 
Re: Measuring reverb time without any tools?

Talk to another person. See how far apart you can get before you can't understand the other guy anymore.

Room I worked last week about 20 feet is all I got before I couldn't understand a thing - and the crowd complained they couldn't understand the performers!
 
Re: Measuring reverb time without any tools?

Silas Pradetto said:
I find that to be an awfully general generalization.
Click to expand...

Agreed. My venue isn't treated (yet). RT60 is fine for lows. It's the upper mids that suffer the most with an RT60 approaching 2 seconds. Then there's a good amount of honk at 400-500Hz but the RT60 at those frequencies isn't as bad as 1K to 4K. Once the room fills with warm bodies. Things improve tremendously. But treatment is still need to help with reflections from the direct stage sound off the ceiling. I can hardly wait until the treatment is in...
 
Re: Measuring reverb time without any tools?

Silas Pradetto said:
I find that to be an awfully general generalization.
Click to expand...

Haha, yes. I guess I should qualify that a bit more.. most of my business is in gyms, pools, and arenas. all big boxy rooms that have large hard surfaces all around by nature, and even at full capacity don't get enough people in them to count as absorption..
without the involvement of someone pushing for better acoustics the best we can usually hope for in these spaces is slotted block on the walls.

Jason
 
Re: Measuring reverb time without any tools?

John Roberts said:
Rather than try to impute the RT60, why not listen for intelligibility with suitably percussive sources, like speech samples with differing degrees of difficulty.

The bottom line is how does the room work, or not. Why not try to test that, at least for informal results? I am speculating, but a range of samples that get harder to follow might serve as a rough quantification of the problem. Multiple raters could help, but all things equal, one person could learn to judge by listening to same samples in a treated or known good room vs. not.

or not...

JR
Click to expand...

my issue is that I'm often talking to people in other cities and getting someone's opinion of the acoustics or inteligibility is hit and miss..
if I could devise a quick test that might give me an indication of roughly how the acoustics are then the first trip out can be much more useful
if the RT is off the charts then selling them better speakers aren't going to make much difference right?

Jason
 
Re: Measuring reverb time without any tools?

Unless you are trying to meet numerical criteria that you or others have set, in which case it would seem important to have accurate numbers, then wouldn't it simply be a matter of a subjective "too live", "too dead" or "just right" judgement?

And to add to the spectral content comments, it's also the shape of the decay. A room with a decay that initially drops off quickly but then transitions to a slower decay may sound sound different from a room with the same RT60 but that initially decays slowly and then sharply drops off. That's why RT60 is limited in what it can tell you, especially since few rooms and systems can support a true reveberant field that is at least 60dB, and preferably 72dB or more, above the ambient noise at all frequencies.

FWIW, was told of a case where apparently one party measured the RT60 using calibrated instrumentation while a second party determined it by walking into the room and and having someone clap their hands a couple of times. From what I heard later, the latter was a professor with all sorts of degrees and letters after their name, none of which related to acoustics, and when it went to court that seemed to make their results perceived as being the more reliable and accurate evaluation.
 
Re: Measuring reverb time without any tools?

Somebody got sued over an RT60? Now you got us curious. Depending on details, I might have bet on a hand clapper with good ears who knows acoustics over someone with much less experience but with a calibrated instrument. What was the issue being litigated?
 
Re: Measuring reverb time without any tools?

I work in a problem room. The reverb time isn't bad at all. The problem is the reverb level and spectrum. My ears tell me (to guess that) that the 'critical distance' is only half way back in a 60' deep room.
 
Re: Measuring reverb time without any tools?

Very simple. ANd the result is a "simple answer". Put pink noise in the system-turn it up to a decent level and then mute the signal. Use a stopwatch (or count) until you can't hear it anymore.

That will give a "will guess" as to the time. You can also hear the freq content that "hangs around". Or you can put in band limited pink noise and do the same thing.

At least it will give you a quick idea.
 
Re: Measuring reverb time without any tools?

Was the floor made with marble? I did a wedding gig once in a beautiful room with horrible acoustics. The floor was completely marble and walls were covered with mirrors. It could be a fantastic venue for a chamber orchestra or choral work. The band was 14 piece and I never had so much anxiety for not being able to fix the sound.

Silas Pradetto said:
Talk to another person. See how far apart you can get before you can't understand the other guy anymore.

Room I worked last week about 20 feet is all I got before I couldn't understand a thing - and the crowd complained they couldn't understand the performers!
Click to expand...
 
Re: Measuring reverb time without any tools?

Brad Weber said:
A room with a decay that initially drops off quickly but then transitions to a slower decay may sound sound different from a room with the same RT60 but that initially decays slowly and then sharply drops off. That's why RT60 is limited in what it can tell you
Click to expand...

Very interesting.. that would imply that some materials in the room only absorb loud sounds and then reflect those same frequencies when they get quieter?

Jason
 
Re: Measuring reverb time without any tools?

Ivan Beaver said:
Very simple. ANd the result is a "simple answer". Put pink noise in the system-turn it up to a decent level and then mute the signal. Use a stopwatch (or count) until you can't hear it anymore.

That will give a "will guess" as to the time. You can also hear the freq content that "hangs around". Or you can put in band limited pink noise and do the same thing.

At least it will give you a quick idea.
Click to expand...

Hmm. you might be onto something here. anyone with a smartphone already has a stopwatch, and a pink noise track with some instructions wouldn't be hard to distribute..
still not very accurate, but it should lead us to the ballpark ranges of ok/maybe ok/needs help for sure

and as a bonus, the customer gets a real demonstration of the decay time and hopefully a realization of why it is bad..

Jason
 
Re: Measuring reverb time without any tools?

Burak Yildirmaz said:
Was the floor made with marble? I did a wedding gig once in a beautiful room with horrible acoustics. The floor was completely marble and walls were covered with mirrors. It could be a fantastic venue for a chamber orchestra or choral work. The band was 14 piece and I never had so much anxiety for not being able to fix the sound.
Click to expand...

No, it was a giant old theater converted to a church, probably held about 1200 people on the floor (no balcony). Just the usual place with about 50 foot ceilings, no acoustic treatment, etc etc. Even full it was a disaster.
 
Re: Measuring reverb time without any tools?

Jason Lavoie said:
Hmm. you might be onto something here. anyone with a smartphone already has a stopwatch, and a pink noise track with some instructions wouldn't be hard to distribute..
still not very accurate, but it should lead us to the ballpark ranges of ok/maybe ok/needs help for sure

and as a bonus, the customer gets a real demonstration of the decay time and hopefully a realization of why it is bad..

Jason
Click to expand...
But it easily get much better.

I you have a simple RTA (on a phone for example) set it for the fastest release time it has.

Put in the noise-let the room "fill up" and then stop it.

Simply watch the meter as it decays-and you can get an idea of what the largest problem freq is (it will be the slowest to decay) and you can get a quick idea of the balance of the decay time over the freq range.

This same technique can be used when yo uare dealing a a very live room and a sound system. Let's say that 500Hz has the longest decay time. Then put a wide dip in the response of the system at 500Hz.

This way you are not energizing the room as much at 500Hz-so it doesn't "stand out" as much.

Yes in the measurable response there will be a dip-but in the audible response-it will sound a lot "less echoy".

It won't fix the problem-but it can help to make is not as bad.

Of course if the decay is nice and even-not much you can do in that case.
 
Re: Measuring reverb time without any tools?

Ivan Beaver said:
Very simple. ANd the result is a "simple answer". Put pink noise in the system-turn it up to a decent level and then mute the signal. Use a stopwatch (or count) until you can't hear it anymore.

That will give a "will guess" as to the time. You can also hear the freq content that "hangs around". Or you can put in band limited pink noise and do the same thing./QUOTE]

Using sine wave is very effective for narrowing down what the frequencies are the have the longer decay using the mute n' count method. I have the Audio Tools app in my iPhone which has a signal generator in it. Very handy.

Greg
Click to expand...
 
Re: Measuring reverb time without any tools?

Jason,

The critical bit is that by using a good measurement tool you can measure below the noise floor, which is critical to getting a real RT60 in many rooms. With your ears, you're just guessing. This type of measurement is easy to do wrong.
 
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