Loudspeakers How To Prevent Subs from "Walking"

djripcord

Member
Hello!

So I have a problem that irks me everytime I'm out DJ'ing with my EV EKX-18SP's. They are constantly moving away from each other. Rarely am I ever in a venue where I can place my subs on carpet. So i'm usually placed on a linoleum or otherwise slippery floor. I have tried taping or strapping my subs together, but then they just tend to walk together, and adjusting them is a pain then.

What do you do to prevent your subs from walking all over the place?
 

Amiers

Renting to Corporate One Fixture at a Time.
No slip mat for sure will help. Think of the think you put under your cutting board to stop is from shooting out when you are slicing up some food.
 

JD

Well-Known Member
Another vote for the slip mat. Had to set up a bunch of subs to fill in for a speaker stop when the built in system failed on a pipe organ. Had to stay put for 4 months, while handling the 32 foot stop. Four months later, they hadn't moved at all despite the best efforts of two Crown Microtech 1200's. Worked so well that when I finally stacked them in the permanent casement, I used mats as well.
 

Skervald

Active Member
This is just slightly off topic but I need someone smarter than me to answer this. Is there any truth to the story that two low-frequency sound sources placed in close proximity can begin to cancel each other out?
 

JD

Well-Known Member
This is just slightly off topic but I need someone smarter than me to answer this. Is there any truth to the story that two low-frequency sound sources placed in close proximity can begin to cancel each other out?
Yes and no. Since a 50 Hz waveform is 22 feet in length, it would be pretty hard to have a canceling effect unless the cabinets were staggered by 11 feet. Having them 2 feet apart or 6 inches apart is not going to make a difference. The only time a problem would occur is if two cabinets were placed together that were out of phase with each other. In other words, one set of cones were pulling in at the same time as the other were pushing out.
Horn loaded cabinets actually benefit from close placement as doubling the mouth area lowers the cutoff frequency of the horn. Cabinets placed on a floor also benefit from the fact that you get a mirror sound image off the floor.
 

RonHebbard

Well-Known Member
Premium Member
Yes and no. Since a 50 Hz waveform is 22 feet in length, it would be pretty hard to have a canceling effect unless the cabinets were staggered by 11 feet. Having them 2 feet apart or 6 inches apart is not going to make a difference. The only time a problem would occur is if two cabinets were placed together that were out of phase (Polarity) with each other. In other words, one set of cones were pulling in at the same time as the other were pushing out.
Horn loaded cabinets actually benefit from close placement as doubling the mouth area lowers the cutoff frequency of the horn. Cabinets placed on a floor also benefit from the fact that you get a mirror sound image off the floor.
@JD @Ivan Beaver @DSL @MNicolai Would you care to expound upon beating between differing frequencies and the creation of a phantom third note?
Personally, I wouldn't delve into it (As it's outside my realm of expertise) but I'd love to see it mentioned and discussed here by someone who's well versed and knows.
Toodleoo!
Ron Hebbard.
 
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GreyWyvern

Well-Known Member
This is just slightly off topic but I need someone smarter than me to answer this. Is there any truth to the story that two low-frequency sound sources placed in close proximity can begin to cancel each other out?

Yes and no. Since a 50 Hz waveform is 22 feet in length, it would be pretty hard to have a canceling effect unless the cabinets were staggered by 11 feet. Having them 2 feet apart or 6 inches apart is not going to make a difference. The only time a problem would occur is if two cabinets were placed together that were out of phase with each other. In other words, one set of cones were pulling in at the same time as the other were pushing out.
Horn loaded cabinets actually benefit from close placement as doubling the mouth area lowers the cutoff frequency of the horn. Cabinets placed on a floor also benefit from the fact that you get a mirror sound image off the floor.
The church I used to work at originally had the subs hung from the ceiling in the front corners of the room. Things sounded just okay. The low end was there, but it just didn't sound full. I questioned it, but there was no one I could really ask about why it was done that way, and at the time, not really anything else to do. After a couple years, someone from a sound design and installation company started attending. He immediately asked me why the subs were located there. He said that if they were what he thought they were, they are designed to not only be on the ground, but next to each other for reinforcement. Unrelated, but not long after, talk of redesigning/updating the room came up. Long story short, the front half of the stage thrust was cut off and a smaller, "moveable" platform thrust was built to go out in front. It was designed to house the subs under it. The difference was night and day! Even after turning the amp down 50%, I would still have to watch my levels on low frequencies. It did not take much to rattle the walls in the room. IIRC, the resonant frequency of the glass in the windows at the back of the room was a G2, 98Hz. Before, you would have to walk to the back of the room, about 80-90', to really hear the low end and it would increase as you went back. After, there was nice even coverage from front to back. It was amazing. It was at that time I finally got around to putting the subs on an aux. While I thought it was better, only one of my volunteers really got it, so I took them off.
Unfortunately, about a year after the new pastor decided to Force-Into-Resigning-Early me (after I asked for a couple month break to avoid burn out), they decided to update again. They decided to build the front wall on each side of the stage out at an angle for the projection screens. To make more room for seats, they wanted to get rid of the small platform thrust. Since that was what hid the subs, they decided they could put them up into the newly created space behind the screen walls. I went back once to check things out after it was all done and it sounded worse than it did when I first started. The low end was very muffled as the subs were separately enclosed in rooms now. That really is saying something, as about two weeks after I started, the pastor commented on how much better things sounded already and asked me what I did. I told him that I had accidently zeroed out the EQ on the DSP! He told me that was why he hired me, to fix things.
 

JD

Well-Known Member
@JD @Ivan Beaver @DSL @MNicolai Would you care to expound upon beating between differing frequencies and the creation of a phantom third note?
Personally, I wouldn't delve into it (As it's outside my realm of expertise) but I'd love to see it mentioned and discussed here by someone who's well versed and knows.
Toodleoo!
Ron Hebbard.

Sub harmonics? Yes, if two notes are played, one at 40 Hz, the second at 50 Hz (to keep the math simple) a third sub-harmonic of 10 Hz is created. It is the same technique used in tuning an instrument where you try to bring two notes together without a beat pattern. In general, this does not come into play in sound reinforcement, but when I did the 32 foot speaker stop for the organ, I had to make sure there was plenty of headroom in power and in the speakers themselves to handle the sub-harmonics without clipping or mechanical damage. Some of this can be handled by a low-cut filter, but that tends to cause problems on a system that often is called on to produce frequencies below 20 Hz. Keeping in mind that an organists will often land on two petals at a time by accident (or intent!) ;)
 

TimMc

Well-Known Member
Premium Member
This is just slightly off topic but I need someone smarter than me to answer this. Is there any truth to the story that two low-frequency sound sources placed in close proximity can begin to cancel each other out?

Another "yes and no" reply.

Everything in audio is about TIME. Time is a measure of chronology - it's 5 seconds later than when I started typing this sentence; a measure of *distance* because sound waves, generated by time, have a physical wave length; and time is a measure of cyclical events - like frequency

In a general sense, loudspeakers placed less than 1/2 wavelength, using the *acoustic centers* of the loudspeakers, will tend to combine in SPL. The question is "less than 1/2 of WHAT wavelength".

This leads us to the Phase Wheel, and as you travel clockwise around it, at 3 o'clock you're 90 degrees out of phase, at 6 o'clock you're 180 degrees out of phase (full cancellation) and at 9 o'clock you're 270 degrees out of phase, but coming back to 360 degrees, where you'll be in phase but 1 period late.

https://books.google.com/books?id=MOnKiy8qRt4C&pg=PT82&lpg=PT82&dq=phase+wheel+wavelength&source=bl&ots=higSmUiXf_&sig=Dqd_C13_o7l9Y06HVSWJfFn4Whg&hl=en&sa=X&ved=0ahUKEwjt0MKLy5bYAhUN1mMKHVyqAXEQ6AEIPTAG#v=onepage&q=phase wheel wavelength&f=false

Those pages are from Bob McCarthy's "Sound Systems - Design and Optimization" (hint - the gifting holidays are fast approaching); not a simple read but an invaluable resource.

At any rate the above should lead you to this conclusion - most of us have been using them "phase" to describe a DC phenomena called "POLARITY". Polarity is absolute, regardless of frequency (hence the "DC phenomena" moniker) - if you flip a dual banana plug around you change polarity. PHASE is the relationship of 2 wave forms in time, relative to each other. A change in frequeny will, by definition, change the phase relationship if both wave forms do not remain the same. Google for "summation of sine waves" as phase relationships are the heart of summation and you'll find some nice illustrations.

So back to your question about cancellations - what frequencies are your worried about? We can safely eliminate frequencies above 100Hz (for the most part). If you're concerned about 60Hz (the thud part of a kick drum sound), then you'd want to find the wave length of 60Hz, then make sure your subs are less than 1/4 wavelength from any boundary (reflective) surface because once it reflects, the distance is doubled and you're now 1/2 wavelength LATE, which will create a cancellation when the acoustic energy combines. If you separate the subs by 1 wavelength or more, you will get the "subwoofer power alley" down the middle of the room, with radially symmetrical nulls of cancellation and lobes of summation. Note this happens whether the subs are flown or on the ground, they only need to be separated by distance. Further note this occurs in both the horizontal plane and vertical plane and will happen if you have both flown subs and subs on the floor (turn your power alley sideways ;) ).

Probably more info than necessary, but I hope this helps. If we need to continue this topic I suggest a new thread.

Have fun, good luck.

Tim Mc
 

TimMc

Well-Known Member
Premium Member
Hello!

So I have a problem that irks me everytime I'm out DJ'ing with my EV EKX-18SP's. They are constantly moving away from each other. Rarely am I ever in a venue where I can place my subs on carpet. So i'm usually placed on a linoleum or otherwise slippery floor. I have tried taping or strapping my subs together, but then they just tend to walk together, and adjusting them is a pain then.

What do you do to prevent your subs from walking all over the place?

Strap them together and put an area rug 'no slip mat' between them and the floor. That's been the best portable solution I've found. You might need a double layer of the matting if the floor has an irregular surface finish.
 

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