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In Reply to: RE: Division of labor in PA system-More posted by Duke on February 07, 2011 at 12:36:45
Hi
I believe the test Ivan had suggested will show the point.
If you play some typical rock music at a decent level and adjust it to suit your taste, then look at it with an RTA, you will most likely find the low frequency end is about 10dB hotter than the mid and the high end rolls off some. If you’re talking about BASS heavy music, then this difference is larger.
Google up “equal loudness curves” and examine the low end vs mid levels. This shows actual SPL vs “how loud it sounds”.
Follow Ups:
Hi Tom,Thanks for joining in.
Yes I understand equal loudness curves.
Is there any merit to my previous post, which theorizes that the RTA readings back in the audience (farfield) are going to be bass-heavy relative to the RTA readings nearfield, due primarily to the directivity of the mains but possibly also due to absorption coefficients in an occupied room?
I look at two-piece (two fullrange mains) PA systems, and don't see evidence that they are designed to have a low end cranked 10 - 20 dB louder than the mids and highs.
As another example, at the link below is a four-piece JBL system: Two mains, two subs. The mains max out at 133 dB each, and the subs max out at 127 dB each, so the subs combine in-phase for 133 dB. Is this system a complete joke, with the subs running out of gas 10 to 20 dB before the mains?
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Edits: 02/07/11 02/07/11 02/07/11 02/07/11
Yes but….
The directivity of the system comes into play just as audibly out of doors.
It is a non-constant directivity that causes the spectral balance to change with distance. Usually the balance is struck at some distance where hopefully most of the people are.
The LF lift is pretty normal though for that spectrum shape with much music.
In a room, one adds the fact that it is the total acoustic power and room absorption that governs the reverberant sound level for any given frequency.
Lastly, keep in mind that the theoretical peak output figure has nothing to do with how loud the system might actually go or how loud it can go and still sound acceptable.
Best,
Tom
Thank you, Tom.
"In a room, one adds the fact that it is the total acoustic power and room absorption that governs the reverberant sound level for any given frequency."
That's pretty much what I was trying to say, though my wording may have been unclear. The application I'm looking at is in-room-only at this stage.
"Lastly, keep in mind that the theoretical peak output figure has nothing to do with how loud the system might actually go or how loud it can go and still sound acceptable."
Ah yes, I'm well aware that the real-world distortion-limited power handling is often far less than the thermal power handlng. I'm doing bass guitar cabs now, and fartout @ well below the rated power handling is a classic example of that.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Yes, room and people absorption will tend to make things sound "bass heavy", due to greater absorption at progressively higher frequencies. This is one reason why bands can get away with putting the subs on the floor but need to have the mids and uppers above the audience's heads.Getting to the directivity issue... Bands often have a hard time getting the system to sound "right" throughout the room. At the risk of giving away one of my sound reinforcement "secrets" ;) , this is often because the mids and uppers are too directional for the venue, and so I always designed systems to have wide dispersion as high up as possible. This improves both the direct sound and the reverberant sound, and makes it much easier to tune the system, and have people coming away saying "wow, that band sounded great!". Along that line, one thing I'd avoid is a 2-way 15" design. Inevitably, the crossover has to be much higher than a 15 should be used - they get too beamy.
Regarding the JBL system you mentioned... It's fairly easy to get a lot of output from the main boxes, which is one reason why you'll often see them capable of more output than the subs. It's not that the system is mismatched, it's just harder and more expensive to get lots of output at lower frequencies (but you already know that!), and so you'll see systems which have similar mids and uppers, but increasingly more "oomph" on the low end, as the price point goes up - effectively - it's beefing up the low end to "catch up" with the capabilities of the mids/highs. But, remember that the waveform peaks (especially snare drum, cymbals, and other fast rise-time sounds) are created by the mid and upper harmonics, so some extra headroom there is often necessary. This is one reason why so many systems which are driven to near their max levels sound so distorted - the peaks are clipped.
In a lot of ways, a really good sound reinforcement system is like a really good home stereo system - only bigger and louder. :)
Edits: 02/08/11
"At the risk of giving away one of my sound reinforcement 'secrets' ;) , this is often because the mids and uppers are too directional for the venue, and so I always designed systems to have wide dispersion as high up as possible. This improves both the direct sound and the reverberant sound, and makes it much easier to tune the system, and have people coming away saying 'wow, that band sounded great!'."
Dangit, that was gonna be MY secret!
I get to design the mains and subs for this system, so a lot of attention is being paid to the off-axis sound. Since this is an indoors-only system, that approach pays dividends in the oft-neglected reverberant field (at the expense of maximum SPL per dollar). Or so I theorize, not having done it on this scale before; must say that your experience along these lines is most encouraging. Thanks!
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
"In a lot of ways, a really good sound reinforcement system is like a really good home stereo system - only bigger and louder. :)"
+10
What makes a system "balanced" is not always the way people actually run it.
Yes you will get all kinds of different RTA readings around the room.
Of course an RTA is the wrong tool to actually be using, I only suggested it because it is easy to get ahold of and for most people to seem to understand/
Go ahead and walk up close to a loudspdeaker system and move the mic above your head and down at your knees and you will get very different readings-in relation of low to mids.
I am going on my experience in doing thousands and thousands of live shows. Maybe those are not typical, but do cross all kinds of genre from rock-country-jazz-rap-punk-ethnic-and much more.
Thanks for your input, and I was not aware of the depth of your experience. I appreciate your taking the time to respond to me here.
What I'm trying to do is take your observation that the bass may be 10 dB louder than the rest of the spectrum when you RTA it out in the farfield (indoors of course), and work backwards from that to figure out what the relative target 1-meter on axis maximum SPL would be from the mains & from the clustered subs in order to end up with a balanced system. I'm not sure it's a straightforward 1:1 correspondence because of the difference in radiation patterns and the different absorption coefficients that different wavelengths encounter in-room.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
I owned my own PA&Lighting rental company for 25yrs doing everything from clubs to festivals to concerts to churches to fairs and so forth.
For the last 12+yrs I have been in the install side of the industry designing sound systems.
In a typical install the SPL across the freq band will be the same at the rear seats as it is at the front seats.
So you figure out how loud it needs to be at the rear seats and the distance involved and go from there.
Indoors you will need less bass cabinets to match the tops than outdoors.
This is because the bass reflections off of the walls will help to reinforce itself. Think of it being "contained" in the building instead of constantly expanding outside.
Outside you generally need at least twice (if not more) bass to match the tops for the same sound "experience".
A lot of this is very scientific and a lot of it is "seat of the pants"/experience type of thing.
Yes and that is why the best sub bass inside I have heard is in concrete and brick buildings
:)
I think the bass coupling to the room/building might be the right word inside verses no coupling effect outside?
"Indoors you will need less bass cabinets to match the tops than outdoors.
"This is because the bass reflections off of the walls will help to reinforce itself. Think of it being "contained" in the building instead of constantly expanding outside.
"Outside you generally need at least twice (if not more) bass to match the tops for the same sound "experience"."
This all totally makes sense to me; producing loud low bass for the back row outdoors has got to be very demanding.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
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