John Sayers' Recording Studio Design Forum Archive

Up until 10 minutes ago, this is what I believed to be true (about decibel levels):
a change of 3dB is barely notable by our ears.
double the distance between source and ear=6dB decrease
an increase of 10dB doubles the sound (although this is debated).


I just read in two spots (physicists) that 30 dB is TEN TIMES louder than 20dB. what is the deal with this?


what I was looking for when I read this was (can you answer it for me...I don't want to dig in my books....lazy, I know) : how to add dB levels. For example: The bass amp puts out 80 dB at point A, the guitar 82, and the monitor 88. What is the total dB at that point?

For example #2: The house sound puts out 90 dB at PointB and the stage noise is 105 dB. If PointB is 20 feet away from the stage........
you get the idea.


Thanks guys.

Jeff Z

Ok. I was feeling guilty for not doing enough leg work of my own. I persisted and found the answer to question 2.

SOURCE: http://www.10db.co.uk/html/how_do_sound ... _add_.html
(is copying the text OK to do here?)

If there are two sound sources in a room - for example a radio producing an average sound level of 62.0 dB, and a television producing a sound level of 73.0 dB - then the total sound level is a logarithmic sum ie

Combined sound level = 10*log ( 10^(62/10) + 10^(73/10) ) = 73.3 dB

Note: for two different sounds, the combined level cannot be more than 3 dB above the higher of the two sound levels. However, if the sounds are phase related there can be up to a 6dB increase in SPL.


How does sound change with distance ?

As you get further from a sound, it’s level will decrease and this property of sound is often used in noise control exercises. The change in sound pressure level for a point source in free field can be calculated using the equation:-

attenuation = 20*log(d2/d1)

The practical effect of this is that if you double the distance, the sound pressure level decreases by 6dB.

Jeff,

> a change of 3dB is barely notable by our ears. <

I can easily hear changes of 1 dB and I'm sure you can too.

> double the distance between source and ear=6dB decrease <

This probably depends a lot on the size of the room and where in the room you and the source are located. Room reflections play a big part in perceived (and actual measured) level.

> an increase of 10dB doubles the sound (although this is debated). <

Yes, I think it's often accepted that a 10 dB increase sounds about twice as loud.

> I just read in two spots (physicists) that 30 dB is TEN TIMES louder than 20dB. what is the deal with this? <

Decibels are logarithmic, so adding them is really multiplying by some amount.

> The bass amp puts out 80 dB at point A, the guitar 82, and the monitor 88. What is the total dB at that point? <

How signals combine in level depends partly on how "coherent" they are. As an extreme example, if two sources are identical but of opposite polarity, adding two loud sounds will yield a very soft result.

--Ethan

The decibel, as related to sound levels, was originally used because 1 dB was considered the least amount of change discernible to humans. There are "golden ears" types that claim to hear 1/4 dB - since I wasn't there I'll be kind and say "maybe" :=) I only know that it wasn't ME that claimed that...

As to 6 dB loss per doubling of distance, that only applies to sound in a free field - once you contain it in a "box", all bets are off. From then on, wall surface, mass, flexibility, frequency, etc, all kick in. Due to the infinite combinations of materials, methods, shapes, sizes, dynamic absorption (# of people in the room), etc, the result is unpredictable enough to require measurement tools.

All this is why my personal fave for what DB REALLY stands for, is Drive (you) Batty... Steve

This is more along the lines of frequency/perception/dBs, but note-worthy-

I walked the floor and walked through every row of seats in the building. Was locating the "Blast Paths" of the speakers. I also carried a SPL meter.

The SPL only increased about 3 dB from the rear of the room to right in the path of the horns. But the TYPE of sound was DRASTICALLY different.
No wonder people are complaining!

Another nugget- the board is at the rear of the room and no where near the path of a those higher frequencies. And it is boomy a bit, too. So they have been mixing and EQing something they don't hear. 80% of the house hears what they don't- sound wihtout a high frequency fall off!

QUESTION
Can't move the mix position, so I ?guess? we should put a speaker in front of the board to supplement? Is that a legit idea?

Thanks.

Jeff

Hey steve. I was a previous naysayer but as i have now spent the past 9 years living in a studio, I can hear changes as subtle as .3 to .4 db .2?? Nah not there yet

Bryan

Can't re-fly the speakers for more even coverage either? Just putting a speaker near the board won't necessarily guarantee a similar sound field.

Went to a Moody Blues concert in Portland a few years ago (wife loves 'em, I don't DISLIKE 'em) and the first 1/3 of the concert the sound was PAINFUL !!! Actually saw several dozen people get up and LEAVE after paying for tickets - would have been WITH them, but for the wife... upper mids were taking people's heads off, it was so bad. Then, FINALLY, somebody with the sound company walked the floor (musta seen the mass exodus) and all of a sudden the sound got good (not great) and the rest of the concert was OK.

I'm not into live sound all that much, but from what little I know about it, it sounds like some of that $$$ needs to be spent on a multi-array, time delayed system for more even coverage without "blast areas" -

There's a brand new BBS in town, dedicated to live sound and started/run by a mate of John's from Portland, Ore, known on Home Recording BBS as Sonusman - that might be a good place to post your questions on coverage, etc - here's a link

Damn, thing's already broken, I'll try again later... Steve

Well you got the array/delay idea correct. Coverage is the true name of the game in live sound.

I know of a church with a GREAT sound system with the most even coverage of any facility I have been in. Heritage Christian Center in Denver, Co.

Honestly at mix position, 84db is 84db throughout the room. I walked and metered it. And as far as intelligibility and accuracy, one of the best.

The next best thing is a VDosc Line Array. there are others and I have heard them, and none compare to the V-Dosc System.

Bryan