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Author Topic: dj sub system questions  (Read 9954 times)

Mark Cadwallader

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Re: dj sub system questions
« Reply #10 on: July 15, 2015, 12:41:17 am »

Bren, the reason I asked the questions was to try to figure out what results you are trying to get.  The requirements are very different if you are playing jazz indoors to 2000 in a 2000 seat auditorium, versus 2000 in an outdoor football stadium that holds 15,000, versus EDM at any of those places.   Outdoors with no barriers (an empty field) is different yet. I don't know where you live, but 40 acres is 1/8 of a mile by 1/8 of a mile, a pretty standard farmers field.  If a person takes up 9 square feet (3'x3'), over 4800 people fit on one (1) acre. That's how many folks fit on 1/40 of the farmers field.  If you are only filling an acre of space (or less), you need a whole lot less power than to fill a mile deep.

For reference, an acre is 43,560 sq feet.  A US football field (360' x 160') is 57,600 square feet (including end zones), or 48,00 sq feet without end zones.

As you will learn if you hang out here, details matter.  No shame in not knowing, if you are willing to learn. I learn lots of stuff here every day.
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Luke Geis

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Re: dj sub system questions
« Reply #11 on: July 15, 2015, 12:52:52 am »

Until you understand the question you will never know the answer. You are essentially at this point stealing the job away from a more qualified and probably deserving person. I digress though.

The inverse square law is probably one of the more crucial math equations that effect a sound engineer. I never thought I would need to know math to do my job, but alas I am better at math now than I was 20 years ago; simply because it must be used to do the job correctly. Wattage, a speakers sensitivity, atmospheric losses and effects, cable losses, power compression, coupling, comb filtering and even terrain all have a large effect on what the end result is. Each one requires a somewhat different mathematical equation to apply for a result and being close sometimes isn't good enough. A really hot day with lots of tarmac in front of the PA can have huge implications to sound over distance as compared to that of a cold day.

You have what many would consider to be a sizable amount of power that would take care of most any normal gig with ease. What is normal though? That depends on what you know? In this case 2,500 people could be scattered over a very large area, or perhaps condensed into a relatively small area too. The system design approach may be totally different for each scenario? do you need the things to cover huge area? Simply stack the things one atop the other so they are 4 tall and forget about stereo. This will give you a pretty huge 360 deg. coverage area around the subs and help get the energy outwards in all directions except upwards. If you stack them side by side width wise you will still have good power, but you will narrow the coverage in the horizontal plane in relation to the width of the line of subs. The longer the line of subs, the narrower the horizontal coverage will be. If you can't understand what that means and why it does that your already in trouble. You can set the subs up in a multitude of ways to get a number of results in regards to their coverage and potential peak SPL. Unless you know what you need to acquire ( the question ) you can never know how to come to the answer. You can even get cardiod coverage ( directional control ) with what you have.

What you have is capable of about 143db at 3' assuming you get them to full rated power. In all reality you could at best expect to get up to about 140db and even more realistically 137db  for the long term. However no one will tolerate being close to you when your putting 130+db in the immediate area. Not to mention your tops more than likely won't keep up either. There are subs out there that can do 140+db each!!!!! So no, the JBL STX's are not all that great in terms of performance ability. The Peavey SP218 speakers actually are about the same in terms of performance to the STX's. With 8 of the SP218's you would beat STX's by 3db. The big difference between the two is the frequency response. The STX's go lower and probably sound better at performance level.
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Lee Buckalew

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Re: dj sub system questions
« Reply #12 on: July 15, 2015, 06:17:59 am »

ok ive been in sound for quite sometime but i never really do spl or any of that im not tech handy and im very old school i own no digital processors and im just recently changing to jbl.... i dont fully understand what is being asked they way i always go about things is how many people around 2000-2500 requirements of gig and type of music..... never really go by dbs or spl level i just typically know what i need but since ive jsut recently switched to jblstx828s i dont know if 4 of them can do the job i used to use 8 peavey sp218 for around 3000 people.... but im assuming these jbls can do more with less speakers am i correct

Well, let's start learning.
Here is a link to a page (I don't know and therefore cannot endorse GC Audio, just liked the chart) that gives a pretty good overview of relative dB/SPL from various sources.
http://www.gcaudio.com/resources/howtos/loudness.html
LF vs. HF reduction over distance different since HF also experiences absorption in the air and not merely the same power being spread out to cover an increasing area.  Loss over distance, excluding air absorption, is typically said to be 6dB/SPL and it follows a law of physics commonly called The Inverse Square Law for point sources.

Now, on to your question re. your 4 new JBLs compared to your previous Peaveys.  How were the Peavey's powered?
If you just compare cabinet specs the JBLs are rated to get about 2x louder in peak SPL for each speaker cabinet than were your Peaveys and they have more extended LF response by almost half an octave.  The Peaveys are rated as being very slightly more efficient than the JBLs which means they would get louder with the same amplifier power until you reach the maximum for the speakers, at that point the JBLs can keep getting louder since they can handle more power.

You have essentially upgraded your sound quality in terms of frequency response and kept your SPL capability the same (depending on how you used to power the Peaveys) while taking up less room in your pack and having to now place only 4 subs instead of 8.

If your clients liked/didn't dislike how your system sounded previously then the change should be an improvement to them with the extended LF response.  If you were hoping to have this change increase the coverage capacity for your system, it did not.

Lee
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Lee Buckalew
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Re: dj sub system questions
« Reply #13 on: July 15, 2015, 08:16:18 am »

Two neighboring farmers had a problem with their plow horses breaking through the fence and getting mixed up.  They couldn't tell them apart, so they tried various methods to ID them, first cutting off the mane of one...but it grew back. 

They then tried putting some paint markings on one but it washed off. 

Finally they decided to measure them.  That's when they found out that the white horse was 6" taller than the black horse.

Details are important.
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bren collins

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Re: dj sub system questions
« Reply #14 on: July 15, 2015, 11:30:22 am »

now that i know it would be about a half acre maybe alittle more is where the people will be but its a flat field about 2 acres big..... and thanks guys im starting to learn details matter a little more than i thought. im wanting to get my subs as loud as i can because what i do know is the first 100 foot there will be no one there..... as for my highs im running 8 prx712's and 2 prx 725's.... hope to change these out in future for complete stx system.
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Scott Holtzman

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Re: dj sub system questions
« Reply #15 on: July 15, 2015, 11:56:55 am »

Holy smokes....just how are you assembling all those speakers?  You do realize they are not designed to array

Sent from my XT1030 using Tapatalk

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Dennis Wiggins

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Re: dj sub system questions
« Reply #16 on: July 15, 2015, 12:11:15 pm »

now that i know it would be about a half acre maybe alittle more is where the people will be but its a flat field about 2 acres big..... and thanks guys im starting to learn details matter a little more than i thought. im wanting to get my subs as loud as i can because what i do know is the first 100 foot there will be no one there..... as for my highs im running 8 prx712's and 2 prx 725's.... hope to change these out in future for complete stx system.

A picture, or at least a sketch, of how you will physically setup the speaker cabinets (including placement distances) will help get you some useful advice.

Also, include each amp channel and which cabinet it will be connected to.

-Dennis
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Re: dj sub system questions
« Reply #17 on: July 15, 2015, 12:20:46 pm »

now that i know it would be about a half acre maybe alittle more is where the people will be but its a flat field about 2 acres big..... and thanks guys im starting to learn details matter a little more than i thought. im wanting to get my subs as loud as i can because what i do know is the first 100 foot there will be no one there..... as for my highs im running 8 prx712's and 2 prx 725's.... hope to change these out in future for complete stx system.

Bren...

I hope you'll stick around long enough to learn how these things work.  You may end up taking some flack and fielding very pointed questions and requests for details.  Be patient, don't take it personally and you'll have a chance to share the hard-earned knowledge of a host of real, knowledgeable professionals.

It appears that your situation is such that the sound presence will be fairly diluted over distance no matter what you do with 100' of empty space between you and the crowd.  You will likely be told to use no more than four of your tops arranged 2/side and elevated as high as safely possible, going for clarity rather than sheer volume...which you really won't be getting with by simply adding more cabinets.

What you need to understand is that doubling the number of cabinets yields not a doubling of SPL, but rather a minimal increase in volume while affording you broader coverage.  If you then array your cabinets such that their patterns overlap(all facing front) the destructive interference resulting will tend to negate any gain in SPL you might get and will certainly degrade the QUALITY of the audio.  This will all be compounded by the 100' gap.
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bren collins

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Re: dj sub system questions
« Reply #18 on: July 15, 2015, 12:35:37 pm »

i would post a sketch of the system but i cant figure out how to upload it.... again thanks for all the help im receiving
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Ivan Beaver

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Re: dj sub system questions
« Reply #19 on: July 15, 2015, 01:00:22 pm »

its a outdoor event.... around the 2000 people number, new style dj music such as jason derulo etc... its for a marathon run and its the after party its a dance/glow party theme. im wanting 110 db at 1 m away....
Just to add in the "learning process"

If you want a particular dB SPL (at any distance and any dBSPL), you HAVE to have speakers that are capable of AT LEAST 6-10dB (or more) than the level you will actually read on a meter or "experience"

That is because the meter does not respond to the peaks (even on fast response) so it is more of an average.

The actual peaks (and the maximum output numbers quoted on spec sheets) are much higher.

So don't think that if a speaker says it can produce 130dB at 1M that you will actually be able to measure it.

If you have a special "peak reading" meter, it can capture the peaks, but those are expensive and rare.

So with the 130dB speaker, you might read 120dB.  And remember that a 10dB increase is ALSO a 10 fold increase in power.

A 20dB difference is a factor of 100, and 30dB is a factor of 1000!!!!!!!!!

So not only do you have to have the speakers, but also the amps AND the power source for the amps.

IT gets real big real quick, so be VERY CAREFUL when just "throwing numbers around".

MAKE SURE they are realistic :)
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