PA Question, SPL

[Mackovyak]

Alright, from school I know that (considering that we're outside) as the distance from the source doubles, the SPL decreases by 6 dB.

I also understand that it takes a 10 dB decrease in SPL for the perceived volume to drop by 50%.

My question is "How do you know your starting SPL from a certain source?"

Is there an equation, for example,to find the SPL for a 15" / Horn speaker pushing 150 watts RMS at 1 ft from the cabinet?

It sure would clear a lot up.

Justin

[tonefight]

One of the more sound knowledgable guys like lonewolf, Ron or onetooloud will answer this but in the mean time you'll probably need to find the SPL for your set of speakers it should be something like " SPL = 98db 1 watt /1 meter.

[Ron]

Justin,
Speakers should have an efficiency rating that is taken at 1 meter with 1 watt applied to it.

Remember though, that most of the formulas we are taught in school can be very misleading. There can be a big difference between sound power (watts), sound intensity (watts/sq. meter), and sound pressure (newtons/sq. meter).

This link should answer all of your decibel questions, (or just confuse you more).

http://www.prorec.com/prorec/articles.n ... 5400514576

[lonewolf]

Alright, from school I know that (considering that we're outside) as the distance from the source doubles, the SPL decreases by 6 dB.

I also understand that it takes a 10 dB decrease in SPL for the perceived volume to drop by 50%.

My question is "How do you know your starting SPL from a certain source?"

Is there an equation, for example,to find the SPL for a 15" / Horn speaker pushing 150 watts RMS at 1 ft from the cabinet?

It sure would clear a lot up.

Justin

If you look under the Tech Sector, you will see WATTS...WHAATT? covering a lot about this.

Its 6db down every double in distance, unless you have a line array, which is 3db down.

Loudness perception is dependent on frequency. A 3db drop at 3khz alone is perceived as a 12db drop for the human voice. This is why most vocal mics have a 3-4khz presence peak in their response curve. That is described in more detail on the WATTS post.

There are many variations to calculate SPL from a speaker. 1st, you need to look at your speakers' 1w/1m frequency response curve in the frequency regions it will be used. If its flat throughout the region, you can use the 1w/1m EFFICIENCY rating of the speaker to calculate SPL based on the actual power in watts that the amplifier is putting out at the time. If you match the output of subs with the output of tops, you take that value and add 3db to get the total. If you have 2 identical subs right next to each other, you calculate the spl for one of them and add 6db for the pair. This only works at frequencies under 400hz. Take a close look at subs' frequency response curves. The efficiency ratings of subs are usually pure bullshit--they will rate it at 1khz when the sub is meant for use below 200hz. If you look at a sub's curve, draw a vertical line at your lowest desired frequency and another vertical line at the crossover point. Find the SPL midpoint between the lowest spl and the highest spl in that region and use that for the sub's efficiency rating. Generally, you can EQ them flat within these bounds. Once you have the efficiency at 1w, when you double the watts, you get a 3db gain. SPL=efficiency+10(log(actual power to speaker))

Also, it depends if they are flying (full space condition), on the floor (half space), in a corner (quarter space). Each time you "half the space", you add 3db gain from the system. Cabs in a corner are 4 times louder than cabs hanging in the middle of a gym. Assuming you have 2 sides of the pa, once you calculate one side of the PA, you add 3db to get the whole system level.

[Mackovyak]

Ok, so in Portage Boy terms

SPL = efficiency + 10 log (actual power to speakers)

Example

we'll say an amp is pushing exactly 200 watts rms, no variation, no problems with cables, ect...

Speakers: Mackie C300 (the first ones I could find) Sensitivy 98 dB

The SPL at 1 meter would then be 98 + 10log200 ???

[lonewolf]

Ok, so in Portage Boy terms

SPL = efficiency + 10 log (actual power to speakers)

Example

we'll say an amp is pushing exactly 200 watts rms, no variation, no problems with cables, ect...

Speakers: Mackie C300 (the first ones I could find) Sensitivy 98 dB

The SPL at 1 meter would then be 98 + 10log200 ???

Yep, for one speaker, but only if the 1m/1w frequency response curve is flat or close to it. The SPL in this case is 121db.

If your PA has 2 of those speakers running at the same level, you add 3db to the 1st calculation to get the overall level. In this case, 124db total system level.

You have to be more careful with subs, though. For instance, some subs report extremely high sensitivity specs, but if you look at the response charts, they are a lot less efficient below 200hz, where you will use them.

[Mackovyak]

I think I got the jist of it. I don't get into subs much, or even live sound for that matter, but part of apprenticeship in the studio down here requires me to do some work in the field. Very interesting. Thanks so much for the help.