How loud is too loud

[litemog]

The panto I'm working on is using the le maitre large glitter cartridges. These are rated at 91db at 5m, but I have no idea how this relates to a safe and permissable volume, particularly as two cartridges will be detonated at the same time. Presumably there is a decibel level that should not be exceeded.

[paulears]

We're using the same - 1230A in our case. We use two, one either side and I can't say the bang from these causes any problems - certainly not loud enough to make the hearing sensitive members of the cast and crew worry. Has somebody voiced concerns? They don't seem any louder than the others we're using?

[litemog]

Its a fairly enclosed performance space so we are only just achieving the 5m clearance to the nearest person. Nobody has complained, but I find them unpleasantly loud, which is what got me to thinking that there must be some kind of limit that shouldn't be exceeded.

[Simon Lewis]

... which is what got me to thinking that there must be some kind of limit that shouldn't be exceeded.

 

Yes - the Control of Noise at Work Regulations gives an exposure limit value of " 87 dB Lep,d (daily exposure) and 140 dB Cpeak (peak noise) which take into account the effect of wearing hearing protection and which must not be exceeded"

[litemog]

so if two of these effects are fired together how many decibels will be generated?

[Jivemaster]

IIRC 91 +91 =94 if fired at the same time. If fired sequentially 91 +91 may only equal 91 !! -Sequentially meaning greater than 25milliseconds between bangs.

[MarkPAman]

My guess would be 94dB :P

[Jivemaster]

Realistically a 91dB bang could be lost in the general noise on stage as the finale builds to its own crescendo, a pit orchestra and some PA will easily be that loud -maybe louder! Just that it sounds loud in isolation.

[Brian]

And any Regulations only apply to 'employees' not the audience.

[Simon Lewis]

The manufacturer's data sheet states "Noise level / dB(A): 101, recorded at 5m indoors". However, this isn't too helpful, given that the sound is a short duration impulse.

The best way to determine employee risk from a noise at work perspective is to measure the C weighted peak noise level.

It will be more than 101dB, but hopefully not more than the three action levels of 135, 137 and 140 dBCpeak.

[tommulliner]

IIRC 91 +91 =94 if fired at the same time.

 

Just for the record, how does that work??

[Biskit]

IIRC 91 +91 =94 if fired at the same time.

 

Just for the record, how does that work??

To keep it (very) short, each time you double the amount of energy being converted to sound (in this case by using two pyros at the same time) the SPL in dB increases by 3. This is because the dB scale is logarithmic rather than linear. This makes it more representative of the way our ears perceive differences in sound level.

 

You can find formulae and far more in-depth explanations online for 'what would happen if I added this sound level to that sound level'... etc but hopefully that explains 91 + 91 = 94!!

 

Ben.

[Andrew Edwards]

dB is a dimensionless unit which expresses sound as a logarithmic ratio to a reference level.

 

It also come in 2 flavours; sound pressure level (the familiar dB SPL) and sound power level (dB SWL - w is watts).

SPL is the effective pressure variation in air (or any other fluid for that matter) and SWL is the force behind it.

 

0dB SPL is the threshold of hearing and not total silence. Think of a mosquito across the room.

 

Pressure is measured in pascals but it involves quite a large range of numbers. The quietest sound a human can hear is about 20 micropascals and a full symphony orchestra is about 2,000,000 micropascals. Logarithms allow us to reduce the range into something more convenient and better suited to sound perception. Log base 10 is used in this case.

 

Using a formula which can be googled if so desired the relationship between pascals and dB SPL can be calculated. If you put dB on the horizontal axis and micropascals on the vertical axis of a graph, you get a rather steep curve rather than a straight line.

 

14dB is about 100 micropascals

34dB is about 10 times more micropascals as 14dB

54dB is about 10 times more micropascals as 34dB

74dB is about 10 times more micropascals as 54dB

 

and so on....

 

A general rule of thumb is a doubling of sound pressure level equates to about 6dB SPL. If you want to know the resultant increase in sound power level then this is half the increase in sound power level (i.e. 3dB SWL).

 

To make matters worse, as it were, the human ear does not perceive sound equally across the frequency spectrum (there is a peak around 2.5-3kHz). The human ear developed to 'hear' frequencies better around those involved with human speech. There are resonances in the ear canal at frequencies that relate to this (one of which is 1kHz). So the scale has to be weighted to mean anything useful.

 

Back to the bangs.

 

If said bangs occurred during a relatively quiet period then some nearest the source may develop a minor and temporary ringing. Completely depends on age, past exposure and/or physical make up.

If the ambient noise level is quite loud then the ear's built in compressor will be operating and this may not occur.

For the curious this is because there are muscles attached to the small bones of the inner ear which reduce the transfer of the movement of the tympanic membrane (eardrum) to the fluid of the cochlea.

 

Edit: Grammar

[Simon Lewis]

put numerically...

 

You can't add dBs directly, so you need to convert back to sound intensity, 'do the maths' then calculate the sound intensity level (which in most cases is numerically the same as sound pressure level).

 

SIL = 10log(I/Io) (where Io = reference level for intensity, 1x10^-12 N/m^2)

hence

I = 10^(SIL/10) x Io

Now we have I, we can double it (since we have two sources at 91dB). Let's call this new intensity In...

 

SIL=10log(In/Io)

 

Another way of seeing it - if the values are the same is:

 

10log(2I/I) = 10log2 = 3dB

 

It may also be worth mentioning that a decibel is 10log of the ratio of two powers. It works for electrical power, just as well as acoustic power.

If we replace one of the power terms with a reference level (for, say, acoustic pressure, intensity or power), we can then refer to the value as as sound pressure level, sound intensity level or sound power level.

[litemog]

The manufacturer's data sheet states "Noise level / dB(A): 101, recorded at 5m indoors". However, this isn't too helpful, given that the sound is a short duration impulse.

The best way to determine employee risk from a noise at work perspective is to measure the C weighted peak noise level.

It will be more than 101dB, but hopefully not more than the three action levels of 135, 137 and 140 dBCpeak.

 

There does appear to be a discrepancy between the data sheet and the cartridge packaging which states 92 at 5m indoors. Measuring it on the Iphone decibel meter app gave a reading of 96db in the wings which was on a par with the cast singing unaccompanied on a vocal warm up. Using a slightly more sophisticated piece of equipment, the sound op measured 104 in the auditorium for two fired together.