directional properties of sound

[robnold]

Hello all, I'm wondering if someone can help me shed some light on a stage monitoring problem I am experiencing.

My monitors always sound great until I bring up the mains and get an abundance of low frequency coming back from the mains. My question is: At what frequency does sound become omnidirectional. I am hoping the answer to this question will help me to e.q. some bass out of the monitors to compensate for the excess bass on stage due to the sound coming back off of the main speakers. I've read through my sound physics book and am not able to find the answer. Any help will be sincerely appreciated. Thanks, Rob Nold

[Scott Gould]

The reason you can't find the answer in your book is that there is no specific answer. Sound does not 'become' omnidirectional at any particular frequency. What does happen, is that your speaker cabinets block or reflect the higher frequencies to give the sound directionality. How much (& below what frequency) bass gets through the back of the cabinets is purely a fuction of the speakers themselves (design/materials/power output). The best way to determine what you need to know is by experimentation - put a CD through your system and use a low shelving filter, varying the cut-off frequency 'til the results are what you want.

Scott

[JeffBarnett]

Scott speaks the truth. Sound is always directional. It travels in a straight line until it is absorbed or bounces off of something, regardless of frequency. You'll find that there is no frequecy at which your speaker cabinets start projecting the sound forward, but rather a range of frequencies over which the cabinet's polar pattern becomes less and less directional. Reputable speaker companies usually publish polar pattern charts at a variesty of frequencies.

[xstatic]

Thank you guys for answering this question reasonably. I am a live sound engineer who runs a lot of larger rigs. I am soooo tired of people telling me it doesn't matter where I place my subs because they are omnidirectional. It is sooooo NOT true. It matters probably more than the tops do where they are placed since my subs are manifold style cabinets and couple very well. Also, you don't have to run them in mono like so many people say. The stereo image can still greatly be affected by sounds as low as 45 hz.

As for Robs question, this is a common problem. I always do my best to set monitor levels with the mains on just for that purpose. Not only do crossover points, power amps and cabinet design have a lot to do with the low end in your system, but the shape/size of the room and volume of your mix come into play as well. The reason I always try and check monitor levels with the mains on is so that the performers can set the levels based around what they will already be hearing coming from the backs of the stacks. This will make a huge difference on the center mix (vocals usually) and the drum mix. If the drummer is hearing the mains already when you start bringing kick drum up in his mix he is less likely to ask for as much which saves your wedges and power amps a little stress. Same is true for bass guitar.

Every system, room, speaker, amp, mic and mixer combination makes things a little different. However, here are what might be some useful starting points. Assuming the use of a decent monitor, make a steep roll off from 50 hz all the way down. Don't just activate the high pass, roll it off also. I highly doubt your floor wedges are efficient below those frequencies, and it will save a lot of stress for your power amp which will reproduce those frequencies for your speakers to receive, but they will probably not audibly output them. Not even many sidefills out there hit that low very well. Also, this will have a smoothing effect on all your other frequencies to about 400 hz due to the harmonic structure of frequencies. For me, on just about every wedge I have mixed on, I also start with a 3db cut at 100, 200, 400, and 630, and often times also around 1.25khz or 1.6khz (depends on where the 15" driver in the wedge crosses over with the horn). I also always try to avoid much cutting between 800 and 4k. This is because cuts here tend to make the performer feel like he has lost volume. Then you have to turn it up, then re EQ, then turn it up etc... You see the problem there I am sure.

I have actually seen monitor engineers check phase with front of house. Making sure stage mics don't sound better out of phase on stage due to the two systems bleeding over. If you have a really nice crossover like a BSS or an XTA, you can also delay your main stacks in time so that they line up with the backs of the monitors. The theory? If bleed from one system is hurting the other and it can't be fixed without the FOH system suffering from over EQ'ing, then use it! Let the bleed back from FOH be a part of the monitor mix. If your monitors are bleeding a lot of low and low mids into the house, time align the two and make it so at least it hits you in phase:) I have even seen a couple of guys delay the mains to line up with the guitar stacks if they are that loud. Good luck, and hope some of my rambling helps:)

[robnold]

Thanks much for taking the time to answer my post. I found all of the information to be of use. I have an additional question, if I may. My front of house speakers and monitors are both mackie srm 450's. my mixer is a 1604 vlz pro. I plan to consult my mackie manual shortly for the polar pattern charts. (thanks for the tip). As for applying e.q., is there a signal routing technique that can enable me to make use of the e.q. built into the channel strips for a monitor send? in other words, use an input channel as a bus for a monitor send. Be aware I like to use as the least amount of gain stages possible. Thanks in advance for anyone willing to help me on this one. Cheers, Rob

[xstatic]

You could route the auxilliary output that you are using for monitors to the line in of a channel and then use the tip part of the insert to output the signal towards your amp, but I highly recommend against this. There are several reasons. First this is definately not proper gain staging, but I have had to do crazy things like this before. Also, if you do route say aux 1 back to channel 16, make sure you do not route aux1 on channel 16 anywhere. Giant feedback loop.... 2nd, Mackie EQ is pretty useless. I don't think I would want it on the output busss and on the channels. You really need Graphic EQ's for monitors. At least 15 band EQ's or even a good 5 or more band parametric can be very useful.

As for the polar patterns, I would be surprised if the cab was more than 90 degrees wide and 45 degrees vertical dipersion. They will only list the driver splays. Not the actual coverage that occurs outside the direct driver radiation. Too much of that is rom dependant. I can tell you that the Mackie 450's are a little bumped up in the 100 to 400 hz range. This is to make the smaller 2 way cabinet sound a little bigger and fuller. You may want to start by EQ'ing in that range on both monitors and mains. That frequency bump is a lot of what makes them sound a little louder and fuller than their predecessor the RCF 500A. The RCF however sounds much more natural, with a smoother high end and a tighter low end. Kind of like thte Mackies were designed to be used by musicians, while the RCF was designed to be used by engineers:D