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[Ivan Beaver]

Let's try to clarify a few things. While SPL RMS is not a customary term in day-to-day sound work, it does have a useful meaning. Sound pressure and volume velocity scale as the square root of sound power. The microphone in a sound level meter produces a voltage proportional to sound pressure. To get an estimate of sound power this voltage is generally full-wave rectified and low-pass filtered. (The cutoff frequency of the low-pass determines "fast'" or "slow".) This estimate is accurate for sine waves and while good enough for most purposes it is not a good estimator of sound power for complex waveforms. It was used (and incorporated into standards, I think) because of the expense and difficulty of implementing a true RMS computation in the analog days. For complex waveforms the RMS of the sound pressure is the correct estimator of sound power. This is analogous to the measurement of electrical voltage (sound pressure), current (volume velocity), and electrical power (sound power).

On the loss of sound pressure and power with distance. For a spherical wave in a free field the sound power falls as the inverse square of the distance and the sound pressure falls as the inverse of the distance. This results in the familiar 6dB drop for each doubling, whichever way you figure it. In a room the sound pressure falls more slowly due to reverberation, the difference between the actual and free field sound pressure being referred to as room gain. This is why you need more subs outdoors.

This is not in defense of the OP's admittedly malformed rider. Just trying to keep the discussion from being "simple and wrong".

--Frank

I would "argue" that the voltage that is the result of a specific pressure on the microphone is measured as RMS.  So that is the normal "SPL voltage".  There is no reason to state it.

It starts to get cloudy when you say "peak" SPL.  Is the peak, the peak voltage of the voltage waveform?  OR is it the maximum short duration signal that is received.

I know of one SPL meter, that when you put a know sine wave (like a 1K calibrator), you will read the same SPL on all scales and weighting, EXCEPT the peak scale.  It shows 3 dB louder.

So not only is it reading the peak voltage of the sine wave, but also the maximum short duration pressure reading.


And of course, being "technically correct", the rate of falloff is less inside rooms depends on a number of factors, including the size of the room.

The boundaries of the room will cause reflections (reverberation), which can result in the sound being louder further away, in extreme cases.

A friend of mine did a gig this past weekend in "whisper room", with other whisper rooms attached to the entrances.

We were looking at some waterfall plots he took and often out in time the reflections were several dB louder than the direct sound arrival.

There are a lot of "it depends", when it to trying to predict the rate of rolloff inside, one of which is totally freq dependent.

A simple easy answer is usually wrong.

[Chris Jensen]

Hi Everyone,

Thanks for all the info.

I knew some of you would respond just as you did.  I put the quotes around what they asked for as it was directly pulled from the rider.  That was my first clue that they are not quite sure what they are asking for other than as a few of you pointed out, they just don't want to get screwed.  There was no weighting, bandwidth or FOH location requirements.  Really what I wanted to get here was the basic real world math.

Art, thanks for pointing out I subtracted 40 not 30.  I will edit so that for the rest of time I don't look too bad.  Also as you pointed out Art, FOH is not defined meaning that if closer than it's louder.  I don't intend on being that far, but again for the sake of application beyond this issue, I want to be able to have the right concept for future.  It's not often that I have to worry about contracts and requests, so I don't usually do the work to qualify the system to any given constraints.

I know I didn't give much detail as I was more interested in the math.  The event will be outside.  It is a small 2 stage festival on a college campus that is in its infancy.  I am partnering with the programmers to supply PA and labor.  The rider in question is on stage 2 and getting the QSC K12 and KW181 system.  The other stage gets the ARCSII and SB28s.  Due to how campus works with vendors I want to make sure that the rider is taken into consideration as the band contract is way nicer and simpler than what campus makes the band go through.  I don't want to be in a position where they have entered into a giant contract with campus yet we can't provide their somewhat simple requirements.

Tim, the next step is to talk to someone on their end and I'm sure as soon as we start talking they will be fine.  I just wanted to have my math confirmed so if it comes up I have some confidence.

Thanks All!

[Caleb Dueck]

Hi Everyone,

Thanks for all the info.

I knew some of you would respond just as you did.  I put the quotes around what they asked for as it was directly pulled from the rider.  That was my first clue that they are not quite sure what they are asking for other than as a few of you pointed out, they just don't want to get screwed.  There was no weighting, bandwidth or FOH location requirements.  Really what I wanted to get here was the basic real world math.

Art, thanks for pointing out I subtracted 40 not 30.  I will edit so that for the rest of time I don't look too bad.  Also as you pointed out Art, FOH is not defined meaning that if closer than it's louder.  I don't intend on being that far, but again for the sake of application beyond this issue, I want to be able to have the right concept for future.  It's not often that I have to worry about contracts and requests, so I don't usually do the work to qualify the system to any given constraints.

I know I didn't give much detail as I was more interested in the math.  The event will be outside.  It is a small 2 stage festival on a college campus that is in its infancy.  I am partnering with the programmers to supply PA and labor.  The rider in question is on stage 2 and getting the QSC K12 and KW181 system.  The other stage gets the ARCSII and SB28s.  Due to how campus works with vendors I want to make sure that the rider is taken into consideration as the band contract is way nicer and simpler than what campus makes the band go through.  I don't want to be in a position where they have entered into a giant contract with campus yet we can't provide their somewhat simple requirements.

Tim, the next step is to talk to someone on their end and I'm sure as soon as we start talking they will be fine.  I just wanted to have my math confirmed so if it comes up I have some confidence.

Thanks All!

The quick skim I took of the thread implies the band isn't into math and SPL, but weeding out cheaper systems.  I don't know about them, but if I were the BE I'd take L'Acoustics and feel shafted with KW anything.   

Sent from my VS980 4G using Tapatalk

[Patrick Cognitore]

The event will be outside. 

Ouch. How far out are you putting FOH? I suggest as close as possible. I can't imagine K12's sounding very good if you're trying to get 110db out of them at a great distance from the stage.

Maybe you need to clarify with the festival programmers how large of an area you'll be expected to cover with sound with each system. I'd imagine an event of this type probably has DB limits, I doubt 110dB at 100' is going to be an acceptable volume, particularly if there is another stage playing nearby.

As someone else mentioned, I would be supremely disappointed to be playing on a stage equipped with k12's when there is L'Acoustic right next door. But I'm sure there are budgetary considerations here too.

[Stephen Kirby]

Outdoors a pair of K12s over single 18s would be okay for folk music in the park for a couple hundred people, just.  Maybe a blues band at nice comfortable old folks volume.  80-85dBC at 100' out.  No way will such a system carry an entire band at 110dBC even 50' out in front.  Good reliable stuff that sounds good at moderate volumes, not your pretender pile-o-cheap boxes.  But not rock and roll either.  Tell the acts on that stage what they're going to get.  Let them decide if they want to complain to the promoter and get someone to pony up for more.

[John L Nobile]

I've gotten riders with SPL requirements. None have had any reference (A, C etc) and only a few have stated at FOH. The most common one I've seen is "capable of 110db". My phone probably does that if I put it up to my ear.

What I've always done is contact the sound tech and tell him what's in the room and the bands that have played on that system. SPL has never come up in conversation.

[Tim McCulloch]

Outdoors a pair of K12s over single 18s would be okay for folk music in the park for a couple hundred people, just.  Maybe a blues band at nice comfortable old folks volume.  80-85dBC at 100' out.  No way will such a system carry an entire band at 110dBC even 50' out in front.  Good reliable stuff that sounds good at moderate volumes, not your pretender pile-o-cheap boxes.  But not rock and roll either.  Tell the acts on that stage what they're going to get.  Let them decide if they want to complain to the promoter and get someone to pony up for more.

Sure you can rock and roll them... just don't expect it to sound good.

I "stuffed" an entire 5 piece band mix through a pair of Mackid Thuds® with companion subs for a songwriter showcase where everything else was 2 acoustic guitars and 1 or 2 singers.  It's where I learned to hate on the Thuds® (before I stuffed that mix in these shitty boxes).  It can be done but it sure won't be pretty.

Again the best bet is to advance the show with performers and see if, perhaps, the PA for the B stage need to be butched up a little/lot.

[Ivan Beaver]

I've gotten riders with SPL requirements. None have had any reference (A, C etc) and only a few have stated at FOH. The most common one I've seen is "capable of 110db". My phone probably does that if I put it up to my ear.

.

What many don't realize is how much the "little numbers" matter.

I just retested this, with my peak reading meter.

At 1M, a single hand clap was around 126dB peak.  A slow was 89dB, A fast was 98dB  (give or take a couple of tenths)

I could snap my fingers at 1meter and get a little over 103dB peak, A slow was 68dB, A fast was 74dB (give or take a couple of tenths)

[Ivan Beaver]

The most common one I've seen is "capable of 110db". My phone probably does that if I put it up to my ear.

I just tried the phone measurement.

With "pop" video playing, on speaker phone, mic close to speaker (in the phone), I got 103 A slow, 106A fast and 124dB peak.

YES, details matter.

[Scott Holtzman]

Ouch. How far out are you putting FOH? I suggest as close as possible. I can't imagine K12's sounding very good if you're trying to get 110db out of them at a great distance from the stage.

Maybe you need to clarify with the festival programmers how large of an area you'll be expected to cover with sound with each system. I'd imagine an event of this type probably has DB limits, I doubt 110dB at 100' is going to be an acceptable volume, particularly if there is another stage playing nearby.

As someone else mentioned, I would be supremely disappointed to be playing on a stage equipped with k12's when there is L'Acoustic right next door. But I'm sure there are budgetary considerations here too.

Numbers aside, except for a small gathering of spectators around an acoustic performance outside K12's are completely inadequate. 

Honestly for any type of amplified music I get a little cringe at folks that only want to rent KW153's and 4 KW181's (still speaking about outside).

On a warm day we stacked 3 aside FBT Muse 210's over two STX828s (on 4kw Powerlights each) and I would call that adequate.

Ditto for a texas swing band on the pool deck of a cruise (probably 150' to the back) with 4 Kara and two SB28's aside.  That got the job done, well.  But if a rock band or possibly even a DJ and a rowdy crowd we would have been wishing for more rig.

Small powered suspension speakers just don't cut it outside. 

Maybe you could rent something for the B stage?  Even a pair of VRX on poles (something cheaply rented) would be no sonic masterpiece but it would enough headroom. 

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