Sub placement

[Lisa Lane-Collins]

I have a dilemma, there are 2 JBL PRX618S-XLFs at my disposal, for use in a long narrow room. They are currently sitting together, under the stage, to the side, pointing at the wall about a metre and a half away. The theory is that bass wavelengths take a while to 'unfold' and pointing at the wall, they can bounce back and forth along the walls instead of straight out the front door.

This is a vast improvement on the previous location, split, one a side, sitting on the stage (over 12 db volume difference).

And it certainly reduces the amount of bass spilling out onto the street front (they used to be sat together on the ground in front of the stage pointing out to the audience/street/entrance to the venue.....reducing the amount of the bass at the entrance does seem to be a desirable outcome according to the manager).

My dilemma is that I have never, Ever seen subs configured that way and despite the gains in volume and convenience of stashing them where they are, I remain unconvinced that it's the optimal location. What kind of compelling argument can I put forward to the boss, and the audio technically minded super handy man to relocate them? Or, is this actually a totally reasonable, smart way to get max bass bang for buck?

 

[Silas Pradetto]

Re: Sub placement

It is totally reasonable and common to locate subs together along a wall, or in a corner. Every boundary adds 3 more dB to your output. Pointing subs at the wall a few feet away does the same thing - think of where the cones are in relation to the wall. If the subs were against the wall with the cones pointing away, the cone itself would be about 2 feet from the wall. With the subs pointing at the wall, if the cone is about 2 feet away, it's the same thing (opposite polarity though). You ideally want to be as close to the wall as possible to take advantage of the quarter or eighth space gains.

The part about bass "unfolding" is BS.

 

[Max Warasila]

Re: Sub placement

To add another question to this conversation, placing the subs in that way would add plenty of phase misalignment, right?

 

[Bennett Prescott]

Re: Sub placement

With the subs pointing at the wall, if the cone is about 2 feet away, it's the same thing (opposite polarity though).

Hey Silas, the polarity will be the same. The only inverse polarity energy is coming off the back of the cone, where it meets a Helmholtz resonator (the enclosure) and emits from the ports in polarity with the woofer. It may be easier to think of a subwoofer as a point source bolted to the side of a box, it helps visualize the radiation characteristics for me.

You are of course correct that there is no such thing as bass "unfolding" or "sticking to walls", if the former were true headphones wouldn't work.

 

[Silas Pradetto]

Re: Sub placement

To add another question to this conversation, placing the subs in that way would add plenty of phase misalignment, right?

Phase alignment is too relative for an answer to your question to exist.

 

[Ivan Beaver]

Re: Sub placement

To add another question to this conversation, placing the subs in that way would add plenty of phase misalignment, right?

At what freq? At some freq they will add and at others they will cancel. It all depends----------------

There is no "easy general answer" that is correct.

 

[Lisa Lane-Collins]

Re: Sub placement

To clarify, subs are currently off axis by 90 degrees. Tops pointing at the audience, subs at the wall. Silas' explains the gain in volume. I am still wondering about fidelility. I know bass is meant to be omni directional but still feelrg weird having it pointed side of stage.

 

[Silas Pradetto]

Re: Sub placement

To clarify, subs are currently off axis by 90 degrees. Tops pointing at the audience, subs at the wall. Silas' explains the gain in volume. I am still wondering about fidelility. I know bass is meant to be omni directional but still feelrg weird having it pointed side of stage.

Don't listen with your eyes; listen with your ears. Does it sound good? You said it sounded better. Then it is good.

 

[Ivan Beaver]

Re: Sub placement

To clarify, subs are currently off axis by 90 degrees. Tops pointing at the audience, subs at the wall. Silas' explains the gain in volume. I am still wondering about fidelility. I know bass is meant to be omni directional but still feelrg weird having it pointed side of stage.

Why is bass "meant" to be omni? Yes on most cases it is-but directional bass-ie pointed at the people instead of energizing areas where you don't want the sound-is a GOOD thing.

The more sound that is "bouncing around" (it doesn't matter if it is low or high freq) means lower overall quality/clarity.

Pattern control is a GOOD thing

 

[drew gandy]

Re: Sub placement

...it meets a Helmholtz resonator (the enclosure) and emits from the ports in polarity with the woofer....

I'm not sure 'polarity' is the right term for this. I mean, the sound coming from the vent is typically 180 degrees out from the sound coming off the cone a few octaves above. Some might be tempted to consider that 'out of polarity'. It sounds kind of weird but I think 'in phase' might actually be a better phrase since, in a good alignment, the port phase meets up with the cone phase without big dips in response. I know I know, it's all semantics.

 

[Max Warasila]

Re: Sub placement

At what freq? At some freq they will add and at others they will cancel. It all depends----------------

There is no "easy general answer" that is correct.

Yeah, I was just wondering if the phases were even a concern here. Probably best for me not to start another acoustic crossover point discussion. :roll:

 

[Lisa Lane-Collins]

Re: Sub placement

Pattern control is a GOOD thing

Motivated by this concept, I found me a friend and did some experiments. Tried the sub pair out from under the stage pushed against that wall pointing out. That sounded and felt PHENOMENAL....also blocks the fire escape so no go. In that same spot I rotated them 90 degrees so they were point at the wall again. Orientating them that way definitely reduces their ability to fill the room (more than cuts it in half I'd say).

Tried them back in their old spot on the other side of the stage (pointing at the bar about 10 metres away). Actually very lame in that location, the bass drop off is as bad as when they are under the stage pointing at the wall. Finally settled on having them on the other side of the stage (the stage is recessed a bit on either side) pointing out. Seemed like the best compromise between sound, aesthetics and safety to me. Alas, when the pub manager saw them there the next day he was not sold on the location so I'll have to move them back. Sigh.

My friend turned out to to be the right person to bring in. He noticed, and better identified than I had, that the whole rig sounded like it had a 'smiley' EQ curve on it. (It didn't, totally flat). His suggestion was that maybe the bass was masking frequencies higher up. We tried rolling the subs off at 75Hz, made it sound much nicer.

So at least when the subs go back under the stage, I can fix the mids by shelving the lows. Can't do anything about the nauseating bass on stage though. DJs don't like it, they'll have to take it up with the boss. And now I know, best bass response comes with having no obstructions between the speaker and the listener with bonus volume if you are next to a wall (and can confirm JBL do a pretty decent job of stopping the bass from emanating out the back of the box.

Ooh, actually, tangent. If the subs out laying on their side, pushed together, would there be any benefit in making it so the ports of each are 'connected'?

 

[Ivan Beaver]

Re: Sub placement

Ooh, actually, tangent. If the subs out laying on their side, pushed together, would there be any benefit in making it so the ports of each are 'connected'?

It is ALL about freq and wavelength. You will gain nothing by having the ports close together (at the freq they are operating).

But you may have some issues with the woofers (depending on how high they are operated-the actual spacing etc) being further apart.

Since the ports "operate" at a lower freq-they can be further apart and still couple. As you go up in freq-things have to get closer and closer together to couple properly.

As you go lower-they can be further apart and couple just fine. Think in terms of 1/4 wavelength at the lowest freq of interest-in order to get good coupling.

Think of it like this-1000hz (1KHz) has a wavelength of 1.13' so 1/4 of that is .28' or less than 3.5". When drivers are spaced more than that-at that freq-"things" start to happen.

So lets go up an octave-to only 2Khz. Now devices have to be about 1.5" apart (or less_) to couple.

So you can easily see where the loss of quality/detail comes from multiple devices.