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Author Topic: Clarity on the math while Bridging in amps  (Read 6311 times)

Steve Moland

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Clarity on the math while Bridging in amps
« on: December 04, 2012, 10:37:07 AM »

I work in the running road race and triathlon industry where the rock concert mentality of sound doesn't work because of the spread venue of multiple places sound needs to be. It's all outdoors btw. I use more speakers with less volume each to reach everyone without blowing the heads off those close-in

All my speakers are 8 ohm 400 watt rated at the lowest number and higher with some other numbers like Peak. I never run my gear into the clip zone.

I've a situation in a very open area where I do need to push some volume.

I use Mackie 1400i amps which are rated 700 watts to each of the internal amps at 2 ohms and I've run 4 8 ohm speakers from each at times. I've never blown a speaker or over heated the amp even with my long run cables I make myself, and that's after 160 events in 4 years. [Well I did have 1 amp overheat when a runner toss a shirt over the back of the amp and it blocked the cooling.]

The rating says in bridge mode that it's 1400 watts to 4 ohms.  To me that means, It's one big amp and instead of pushing 8 8 ohm speakers [double the 2 times 4 ohm speakers] I'm limited to 2 8 ohm speakers total for the whole amp.  Is that correct?

Since my speakers are rated at 400 watts I'd say that 1400 watts out of my 2 speaker isn't going to gain me anything but perhaps blown speakers.

What are my options for speakers that would let me push the 1400 watts to them and will the perceived sound actually be much louder over open ground?  The speakers need to sound good but what I need is far from the best as to cost.

Steve in NH

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Steve Moland
Announcers/Emcee and sound systems
Endurance Sports Event Announcing & Management
Road Races  -Triathlons
Adventure Races - Open Water Swims

John Roberts {JR}

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Re: Clarity on the math while Bridging in amps
« Reply #1 on: December 04, 2012, 10:58:23 AM »

I work in the running road race and triathlon industry where the rock concert mentality of sound doesn't work because of the spread venue of multiple places sound needs to be. It's all outdoors btw. I use more speakers with less volume each to reach everyone without blowing the heads off those close-in

All my speakers are 8 ohm 400 watt rated at the lowest number and higher with some other numbers like Peak. I never run my gear into the clip zone.

I've a situation in a very open area where I do need to push some volume.

I use Mackie 1400i amps which are rated 700 watts to each of the internal amps at 2 ohms and I've run 4 8 ohm speakers from each at times. I've never blown a speaker or over heated the amp even with my long run cables I make myself, and that's after 160 events in 4 years. [Well I did have 1 amp overheat when a runner toss a shirt over the back of the amp and it blocked the cooling.]

The rating says in bridge mode that it's 1400 watts to 4 ohms.  To me that means, It's one big amp and instead of pushing 8 8 ohm speakers [double the 2 times 4 ohm speakers] I'm limited to 2 8 ohm speakers total for the whole amp.  Is that correct?
When operated in bridge mode, the effective impedance seen by the amplifier drops in half, so running 2x 8 ohm boxes in parallel and  bridged would use up your entire 2 ohm per channel budget.

Note: 2 ohm operation is heavy work for a power amp, so thermal headroom for playing music loudly, outdoors, may behave differently than voice announcements, and low level background music.
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Since my speakers are rated at 400 watts I'd say that 1400 watts out of my 2 speaker isn't going to gain me anything but perhaps blown speakers.
Not always apples and oranges, since music is not continuos sine waves there is a duty cycle factor..  Just like you can run longer/further with walk breaks, than running full speed until you drop. So speakers may not be the weakest link. (400 watts Peak? or 400 watts continuous? )
Quote
What are my options for speakers that would let me push the 1400 watts to them and will the perceived sound actually be much louder over open ground?  The speakers need to sound good but what I need is far from the best as to cost.

Steve in NH

It is harder to get loud outdoors, but race participants should not be expecting disco SPLs. But don't let me tell you about your customers.

JR
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Paul G. OBrien

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Re: Clarity on the math while Bridging in amps
« Reply #2 on: December 04, 2012, 11:21:28 AM »

The old saying... "if it ain't broke.. don't fix it" applies here. It sounds like you already have a system that does the job so what are you trying to accomplish? Do you have new requirements or are you trying to do the same job with less equipment by pushing it harder? That's often a recipe for letting the magic smoke out, like most things audio equipment tends to last much longer running at most a little below it's maximum rated potential. So following that theme if you want speakers that can handle 1400w all day long they really need to be rated for 1.5 to 2x that amount which is how you spell expensive, and given that there's no guarantee they will be appreciably louder at the listening position than your current setup if they are setup significantly further away since sound dissipates at 6db per doubling of distance outdoors from point source speakers.
While your current setup may appear to be working the amplifiers hard those long speaker runs may be doing you a favor and adding a little impedance to the total load, that will result in less power delivered to the speakers but if you're getting the sound levels you need without blowing anything up then mission accomplished.
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Steve Moland

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Re: Clarity on the math while Bridging in amps
« Reply #3 on: December 04, 2012, 11:45:38 AM »


It is harder to get loud outdoors, but race participants should not be expecting disco SPLs. But don't let me tell you about your customers.

JR

Thanks JR, for the other information I did not quote here.

I've observed the issue of loudness outside, though I've noticed that base sound seems to bushwack through people, bushes or whatever when the speakers are placed low. The highs and voice do better over the heads of people so I always get my speakers as high as possible.

What I have noticed at some events where sound companies provide the gear is there is are [my words] speakers that appear to look like large horns either overtly or inside cloth covered boxes. Not the little horns inside my speakers.

My need for louder sound is more for communications with some background music but the clarity of the voice is important. The need is not all day and often won't be used for 80% of the clock time of the event once it starts.

It's also needs to cover areas where wires don't play well. Parking lots, large open park areas where running clubs have setup canopy tents.

Prior to the start of a race you have move hundreds or thousand of people who didn't read the playbook they were sent in an email about pre-event logistic. Its like herding cats, but it's very important to be able to hit a timeline of an on-time start.

I can use wireless nano stations to broadcast a line level signal for miles [if the line of site is clear] to become "line in" on a completely separate sound system. The problem is I cannot leave my gear out unattended. People steal the generators first, then the other stuff the figure has value. Promised security often fails to materialize.

My announcing partner who I work with 20% of the time had his backup $1200 Honda generator stolen from under the stage on Saturday. With about 5 minutes left to go the main power circuit use by everyone popped. We always have our generators large enough to run our gear as backup because we believe we are paid to announce not give justifiable excuses why we couldn't.

I jumped off the stage with the power lead cord in my hand and reached for the generator that wasn't there. Bummer.

So some booming speaker back near me would seem best. These would not be things I use regularly but would geared to the task at the time.


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Steve Moland
Announcers/Emcee and sound systems
Endurance Sports Event Announcing & Management
Road Races  -Triathlons
Adventure Races - Open Water Swims

Robert Weston

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Re: Clarity on the math while Bridging in amps
« Reply #4 on: December 04, 2012, 12:27:43 PM »

Correct - in bridged mode, the amp becomes a single large one-channel amplifier.  The 4 ohm load on the amplifiers can be either 1 x 4 ohm speaker; 2 x 8 ohm speakers; 4 x 16 ohm speakers; you get the idea.  The power rating of 1400 watts would be distributed between the two 8-ohm speakers, (~700 watts each speaker) assuming that level of power is actually reaching the speakers.

You mentioned long cable runs -  how long are these runs?  Depending on the ohms, gauge of cable, and wire distance, a good amount of amplifier wattage can be wasted before it reaches the speakers. 
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Steve Moland

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Re: Clarity on the math while Bridging in amps
« Reply #5 on: December 04, 2012, 01:05:08 PM »


You mentioned long cable runs -  how long are these runs?  Depending on the ohms, gauge of cable, and wire distance, a good amount of amplifier wattage can be wasted before it reaches the speakers.

Longest is 500 and that's early AM in a places with no wind and not much other ambient sound completely open ground with some people scattered so as to leave lots of holes through the bodies. Voice only and then is only one speaker off one of the separate amp output channels, with whatever off the other amp side.

I use 12 gauge wire

There are some mornings where the sound carries almost more than I want. Some people living in the area get upset when the sound starts at 6:30am. I place, angle and move speakers often to keep the peace. Races often have trouble getting permits based on sound. Part of what people pay me for is to think about communicating with the participants and not upset the locals. Tree lines, open fields, open water especially, wood buildings vs brick and concrete and glass all effect sound movement. I love a downtown setting. A little sound carries down a tunnel of buildings ever so great. The worst place on the planet is often a gymnasium. There I promise only noise and am happy if it gets better than that :)

Whenever I do this long run the speaker placements are facing away from each other so there is no sound arrival time difference which people might hear.

200 is common with 100 to 150 being the majority.




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Steve Moland
Announcers/Emcee and sound systems
Endurance Sports Event Announcing & Management
Road Races  -Triathlons
Adventure Races - Open Water Swims

Robert Weston

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Re: Clarity on the math while Bridging in amps
« Reply #6 on: December 04, 2012, 01:55:16 PM »

Longest is 500 and that's early AM in a places with no wind and not much other ambient sound completely open ground with some people scattered so as to leave lots of holes through the bodies. Voice only and then is only one speaker off one of the separate amp output channels, with whatever off the other amp side.

I use 12 gauge wire

There are some mornings where the sound carries almost more than I want. Some people living in the area get upset when the sound starts at 6:30am. I place, angle and move speakers often to keep the peace. Races often have trouble getting permits based on sound. Part of what people pay me for is to think about communicating with the participants and not upset the locals. Tree lines, open fields, open water especially, wood buildings vs brick and concrete and glass all effect sound movement. I love a downtown setting. A little sound carries down a tunnel of buildings ever so great. The worst place on the planet is often a gymnasium. There I promise only noise and am happy if it gets better than that :)

Whenever I do this long run the speaker placements are facing away from each other so there is no sound arrival time difference which people might hear.

200 is common with 100 to 150 being the majority.

The cable distance you are running will reduce the amount of power to the speakers.  The 200 ft run will drop the power by about 1.2db; the 100 ft runs by about 1/2 that.  It's not much but could make a difference with other environmental factors. 

The 700 watts/2 ohm rating is not even putting 200 watts into each of the speakers. 

To get more distance, you may need speakers better suited for "throwing" sound, or utilize a more powerful amplifier.  The 400 watt rating on your speakers...?  Is that "continuous" or "Program"?  If it's continuous, you should be using an amp that can produce that wattage (to each speaker) when the amp is placed into a 2 ohm load (stereo). 
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Robert Weston

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Re: Clarity on the math while Bridging in amps
« Reply #7 on: December 04, 2012, 02:08:20 PM »

Check out the Mackie manual..

http://www.mackie.com/pdf/m1400_m1400i_om.pdf

...page 24.  It shows wire gauge sizes in relation to speaker ohms. 
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Steve Moland

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Re: Clarity on the math while Bridging in amps
« Reply #8 on: December 04, 2012, 02:34:10 PM »

The cable distance you are running will reduce the amount of power to the speakers.  The 200 ft run will drop the power by about 1.2db; the 100 ft runs by about 1/2 that.  It's not much but could make a difference with other environmental factors. 

The 700 watts/2 ohm rating is not even putting 200 watts into each of the speakers. 

To get more distance, you may need speakers better suited for "throwing" sound, or utilize a more powerful amplifier.  The 400 watt rating on your speakers...?  Is that "continuous" or "Program"?  If it's continuous, you should be using an amp that can produce that wattage (to each speaker) when the amp is placed into a 2 ohm load (stereo).

I'm curious about speakers that are suited for "throwing" sound. They were the basis for my comment about speaker that looked like horns in one of my other comments. When I saw them I was a running participant with others on vacation road trips at some national events. I didn't have the time or flexibility to inquire what the gear was.

Doing these events is a compromise of what to use.

The gear has to be able withstand very bad weather. These kinds of events do not get cancelled because of anything  short of a hurricane. 

My speakers of choice is Peavey PR12 and PR 15 because they travel well, survive if the wind blows them over off the stands and rains doesn't bother them.  The PR15 stand mounting hole is slightly off verticle so the top of the speaker leans forward just enough to stop a downward rain from hitting the face. They are also well priced.

A 1200 or 1400 watt amp is just about what I can get away with because I run them off a Honda EU2000i generator/inverter. I've other gear that needs power.  Promised power is often not available or is shared with other contractors and my share is not enough or breakers often trip.

Almost all my new customers are folks who attend event as participants but are event managers elsewhere. Being the voice of the event means if the sound stops it's noticed by everyone and even if the power comes back I can't throw others under the bus.

Less than great volume is only noticed by a few and only in a few places. It's a trade off of odds.

I'm aware of the power drop to the speakers but it's only the off speakers far out that cover it's area badly. Because I use more speakers over an area the sound doesn't have to go as far and isn't absorbed by the bodies as much.

I get to hear the sound levels more than most because I do a lot of my work wearing a wireless headset and I roam the venue. I put my wireless receivers way up in the air using double extended lighting stands. That gives me way more range than spec since there is nothing in the way to block the signal. If I need even more mic range I will put the wireless receiver out 100 or 200 feet from the amp in a more central spot and use shielded XLR cable to bring the signal back to the amp.   
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Steve Moland
Announcers/Emcee and sound systems
Endurance Sports Event Announcing & Management
Road Races  -Triathlons
Adventure Races - Open Water Swims

John Roberts {JR}

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Re: Clarity on the math while Bridging in amps
« Reply #9 on: December 04, 2012, 02:55:56 PM »

While this is too complex to describe in 25 words or less, as you correctly perceive "horns" act like a megaphone to direct the same sound energy into a narrower segment of the full sound field.

There are tradeoffs from doing this, but in large sound reinforcement rows of speakers form arrays that combine in a manner that focuses the sound in one direction more than another.

I suspect this is far more serious than you intend to get. More expensive PA cabinets will provide a target effective angle for the horn on the HF section to advise you of the effective coverage area. Look at the Peavey literature for those cabinets and see if they list HF pattern angle.

This is generally used when arraying speakers together to cover a wider angle.

JR

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Re: Clarity on the math while Bridging in amps
« Reply #9 on: December 04, 2012, 02:55:56 PM »


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