Using SPL meters

[toddalin]

FWIW, and you should really have thought about this, I am very protective of my hearing. I do a lot of shows, but mine are quieter than most (Saturday's festival was a searing 88-89dBA at FOH, during our set, dood, and was commented on by dozens that it was wonderful to have such a great set at such a comfortable volume).

I assume that this is measured using a cheap Rat Shack meter and that being the case, you were probably just reporting what you perceive as the "average" reading off the meter.

But this "average" reading actually represents the L50, (i.e., the value that is exceeded 50% of the time). This is not the number that you should be concerned about.

You should be concerned about the Leq (equivelent sound pressure level) that represents the logrithmetic average over the duration). Remember sound propagation is based on a log scale.

If half the time your meter reports 85 dB and half the time is shows 95 dBA, you are not listening at 90 dB, you are listening at 92.4 dB.

In this case, the calculation is Leq = 10 x log ((10^8.5) + (10^9.5)/2) = 92.4 dB

(And all you guys with those cheap Rat Shack meters don't really know what your equivelent sound level is, but it's certainly greater than you have been reporting.)

The Leq is typically 3-4 dBA greater than the L50 and this is based on the measurement of traffic noise. In the case of a live venue, the peaks could/would be considerably higher than would be expected with traffic noise and the Leq would be even higher than this 3-4 dB above the L50. Rat Shack meters don't react fast enough to display these peaks. Your 89 dBA value actually represents an Leq of about 92-93 (or more depending on the peaks) and OSHA says this level is "safe" for a period of ~5.28 - 6.06 hours at a time (or less if your Leq is greater).

[boputnam]

I assume that this is measured using a cheap Rat Shack meter...

Assumption is wrong. But thanks for the schooling.

[toddalin]

Assumption is wrong. But thanks for the schooling.

Regardless of the make, if you are not reading on the Leq scale (a time-weighted logrithmetic average) you are under-evaluating your sound levels. This requires the use of an integrating sound level meter.

[grumpy]

Some of this would be cool info if it was in a "how to interpret measures and indicators for the protection of your hearing" thread.
(It'll get lost in here)

[Doc Mark]

Some of this would be cool info if it was in a "how to interpret measures and indicators for the protection of your hearing" thread.
(It'll get lost in here)

Hey, Grumpy,

This is a fantastic idea!! How about it, Todd, and some of you other engineers and Tech Folk, out there? How about posting a guide on how those of us who will never have much of the quality test gear that you have, can use and interpret information from such low caliber gear as the Rat Shack DB meter? I know this would be very helpful to all us Lower Life Forms!! I'd love to see a "sticky", or whatever we call it here, that would be permanent, so all of us could access such valuable information at any time we need it! Again, great idea, Grumpy! I hope some of our good learned Friends, here at LH, will undertake to do just as you asked. Thanks, and God Bless!

Every Good Wish,
Doc

[Robh3606]

Regardless of the make, if you are not reading on the Leq scale (a time-weighted logrithmetic average) you are under-evaluating your sound levels. This requires the use of an integrating sound level meter.

Hello Todallin

Coundn't it also go the other way as well?? Where you over estimate based on a short passage?? I always tend to bring out a meter when it's the loudest part of the music piece I am listening too. I would think that would be the most common way to do it if your concern was hearing damage due to high average SPL measurements.

Rob

[toddalin]

Hello Todallin

Coundn't it also go the other way as well?? Where you over estimate based on a short passage?? I always tend to bring out a meter when it's the loudest part of the music piece I am listening too. I would think that would be the most common way to do it if your concern was hearing damage due to high average SPL measurements.

Rob

If you just read your "L50" readings during the loudest passages, then you know what level is being exceeded half the time for those loud passages. You still don't know what the peaks are unless your meter stores them. You cannot physically see the peaks on the meter, even if its digital. You can't even see the Lmax (maximum root-mean-square value) unless its stored internally. The display is not updated as fast as the peaks occur, so you only catch an occasional peak, unless the meter has provisions to store this value over the measurement period.

For sound reinforcement, having an idea of the maximum "average" volume may be OK. You're probably not up against OSHA nor are you preparing a million dollar Environmental Impact Report that will be scrutinized by all the agencies as well as the general public.

If your measurement is over the duration of the period (e.g., you want to know the average level of the song), no, the L50 can NEVER be higher than the Leq. In the case of a continuous, unvaring, sine wave, the L50 will equal the Leq, as will the Lmin, Lxx, Leq, and Lmax.

[boputnam]

You should be concerned about the Leq (equivelent sound pressure level) that represents the logrithmetic average over the duration).

OK, but the example you gave was not what I posted. So, for my example, last weekends' festival set was in the range 88 - 89dBA (it was most often sitting on 88.3'ish, although I don't know it's resolution precision):

Leq = 10 x log ((10^8.8) + (10^8.9)/2) = 90.1dB

Correct? A plenty acceptible level and far below any show I attend as audience. FWIW, IIRC FOH was 83-ft back. The system was a line-array, which by-design have minimal dB drop over the field. We took measurements up-front too, and other than the obvious increased contribution from on-stage SPL ( ), things were not so different.

Granted, I'm not facing OSHA inspections like you are, but I do get guys with badges and proprietors looking over my shoulder and it feels about the same! But that is not what motivates me to "keep it down" - it is a heathy respect for our hearing, and the basic awareness that you can usually get by with less. Usually, a lot less.

[Bob Womack]

And the upshot is, don't push your levels. Believe OSHA when they say you can only handle 90 db for 8 hours and don't push the level because your permissible exposure goes down logarithmically for each DB added. If you must use a Radio Snack meter, average UP when figuring what you think you are being exposed to and down when figuring how long you think you can be exposed. Simple terms for shorthand.

And remember, your exposure doesn't stop when the concert is over. There is still street traffic, time under the headphones, TV time, time on the tarmac with planes taxiing, etc.

And engineers, remember that the sound check, sussing lines, exposure to boxes and rigging crashing against each other in load-in and load-out, standing by the generator backstage on summer outdoor gigs, and time operating the liftgate and driving the truck all figure in as well.

Bob

[4313B]

"It is said, 'Go not to the elves for counsel for they will say both no and yes.' "

As far as I'm concerned Galadriel can say whatever she wants...

[Bob Womack]

"It is said, 'Go not to the elves for counsel for they will say both no and yes.' "

As far as I'm concerned Galadriel can say whatever she wants...

Ah, but don't expect her to offer advice lightly...

[toddalin]

OK, but the example you gave was not what I posted. So, for my example, last weekends' festival set was in the range 88 - 89dBA (it was most often sitting on 88.3'ish, although I don't know it's resolution precision):

Leq = 10 x log ((10^8.8) + (10^8.9)/2) = 90.1dB

Correct?

No, your math is off.

10 x log ((10^8.8) + (10^8.9)/2) = 88.5 dB

But again, unless you are reading on the Leq scale, this value essentually represents the L50 and as I noted, the Leq for traffic noise is typically 3-4 dB higher than the L50 value. I would think there would be an even greater variation for music, but would have to monitor a "typical" song and compare the two values to determine by how much. This of course would assume that what was to be measured was not subject to dynamic compression either by choice (e.g., compressor/limiter), or because of the amp speakers running out of headroom/excursion compression (unless your system is subject to this too).

Get something like this and dump the Rat Shack. Hard to go wrong for $100 unless this is like the Behringer products (lots of features + little quality = lots of returns).

http://cgi.ebay.com/SoundTeck-Digita...QQcmdZViewItem

[toddalin]

OK, but the example you gave was not what I posted. So, for my example, last weekends' festival set was in the range 88 - 89dBA (it was most often sitting on 88.3'ish, although I don't know it's resolution precision):

Leq = 10 x log (((10^8.8) + (10^8.9))/2) = 90.1dB

Correct? A plenty acceptible level and far below any show I attend as audience. FWIW, IIRC FOH was 83-ft back. The system was a line-array, which by-design have minimal dB drop over the field. We took measurements up-front too, and other than the obvious increased contribution from on-stage SPL ( ), things were not so different.

Granted, I'm not facing OSHA inspections like you are, but I do get guys with badges and proprietors looking over my shoulder and it feels about the same! But that is not what motivates me to "keep it down" - it is a heathy respect for our hearing, and the basic awareness that you can usually get by with less. Usually, a lot less.

It's the peaks that will get you.

Say a song lasts 5 minutes (300 seconds) and has a logrithmetic average (Leq) of 90 dB over 299 of those seconds. But, the drummer's just kicked over his kit wacking the lead player's Marshalls and creating a 1 second value of 110 dB. What is the average over the 3 minutes.

Well on the Rat Shack meter, you will see the needle jump for a moment, then return to the 90 dB level so would probably conclude that it is still about 90 dB. So what is the logrithmetic average over the 5 minutes?

Leq = 10 x log (((299 x 10^9.0) + (1 x 10 ^11.0))/300) = 91.2 dB

So just 1 second at 110 raised the average sound level by 1.2 dB.

If that 1 second value were at 120 dB.

Leq = 10 x log (((299 x 10^9.0) + (1 x 10 ^12.0))/300) = 96.4 dB

This demonstrates why non-integrating sound level meters are only good for a relative levels.

[Doc Mark]

(snip)

Get something like this and dump the Rat Shack. Hard to go wrong for $100 unless this is like the Behringer products (lots of features + little quality = lots of returns).

http://cgi.ebay.com/SoundTeck-Digita...QQcmdZViewItem

Greetings, All,

I found that little meter last night, and asked Todd about it. He graciously gave me his thoughts about it and told me that it would very probably do everything I'd want it to do, for my uses, which is a perfect recommendation. Thanks, Todd, for that info, and for taking the time to check that item out. Much appreciated, and God Bless!

Every Good Wish,
Doc (who has a new SPL meter in his future, me thinks! )

[boputnam]

No, your math is off.

10 x log ((10^8.8) + (10^8.9)/2) = 88.5 dB

Really...?

Here's my equation:

=10*LOG((10^8.8)+(10^8.9)/2)

It looks as you suggest - I am not seeing the error. Excel insists this equates to 90.1. Where is the error - one of hierarchy of instructions?

Get something like this...

FWIW, ironically, that was the meter at FOH last weekend. It was not mine - it was something the contractors had available.

It's the peaks that will get you...

Agreed. One "tool" I never travel without is the Drawmer 1968 - I hand-carry a 4RU rack of Drawmers on every flight. Only my clothes get checked...

As posted elsewhere I insert the 1968 across the Mix buss. It seamlessly takes -3 to -9dB off the transients, entire Mix (obviously settings dependent). I credit this one device, properly used, with providing the most pleasing experience for our fans. Loud never gets too loud, and the subtle compression is inaudible...