slightly odd audio / PA question about subwoofers

Random audio questions for work – I want to drive a subwoofer array – 4 channels right now.

I want them to be loud enough that it can be felt physically in your tummy at 5-10 foot distance. I want 4 independent channels, driven by a computer line-level output. I’m completely not caring about quality, I just need some booms and hums and shakes. As always I want to do this on the cheap.

So my current plan is to find a big 4 channel power amp or two big 2 channel power amps, and use some cheap car subwoofers on it. I think I might possibly be able to scavenge a power amp or two from somewhere in the office, there is a rack of them which I think haven’t been used for a couple of years, need to check out what the specs of them are.

The alternative is to use something like this, a couple of pairs, but I’m not sure if they’ll be loud enough:
http://www.ebay.co.uk/itm/QTX-QT15SA-ACTIVE-SUBWOOFER-15-SUB-1200W-BUILT-IN-AMP-PAIR-/230925250952?pt=UK_Sound_Vision_Speakers_Monitor&hash=item35c437b188

I don’t think I can use a home theatre sub, as they appear to be designed not to point the sound in a direction, and I want something more directional, more like a PA or car speaker.

A few things I don’t know though:

1)Does that combination sound practical, or is there an obvious better way to do this? Is the better way still going to be better if I happen to have a free power amp.

2)Do I need a specific subwoofer amplifier, or will any power amp that is rated for the output of the subwoofer be okay? I don’t necessarily need low pass filtering, as I have full control over the signal, am I right that sending a too high sound accidentally for some reason won’t break the speakers?
3)What sort of output power do I need, for real boom to the stomach kind of boom.

There’ll be people on here who can answer far better than I can, but when I used to be ‘into’ hi-fi stuff one of the hardest things to achieve was a really deep bass note for little money.

A lot of speakers and sub will give a frequency range they will work over but generally the output drops right off at the lower end, so even though the speaker can produce a certain frequency it will be so weak that you won’t really register it.
I remember speaking to a bloke in a hi-fi shop in Uxbridge about this. I was looking to buy some B&W DM602’s and while I was in there he invited me to listen to a set-up that he had in the demo room. There was a REL sub the size of a large coffee table (I think it weighed 19 stone) that went down to silly frequencies and the output was the same across the frequency range). It was an impressive bit of kit but massive and very expensive. He’d been demoing a home cinema set-up to a potential customer.

With regards to the directionality thing, I think the human ear isn’t very good at detecting the direction bass noises come from, hence why in home theatre type set-ups you can have a sub in one box and stick it anywhere in the room. Same with those satellite/sub speaker boxes you can buy. It’s not a ‘home theatre’ feature, but a human ear/brain feature, I think.

I know it isn’t directional in terms of listening; but physically it still is – essentially I want to mess with phasing effects by using varying delays on the 4 signals.

Essentially, if you know exactly where the person is, where the speakers are, and the characteristics of the speaker array, if you mess with the phase of the different speaker array, you can make it so that the sound level at particular points is the full power of all the speakers, whilst having much low zero power at other points. At least that is the theory, and it seems to look fine when I graph it all up. It’d in theory mean being able to target people and make them feel things at different places on their body. I think it might be more complex in real life, and the effect will inevitably be pretty blunt, but I still think it’d be cool to build and test.

The alternative is to use something like this, a couple of pairs, but I’m not sure if they’ll be loud enough:

I’ve used similar active bass bins and they’ve loudness has never been an issue.

So you want to mess with phasing… I take it there aren’t any walls to interfere with the phasing, or they are sound deadened? I could imagine it being quite difficult.

QTX are not a great brand, that sub only goes to 40Hz. When looking for an amplifier, look at the frequency range, same thing for speakers. It might be better for you to use a car sub speakers/enclosures, they are pretty cheap, just watch out for the rubbish.

I used to use a 300W RMS Sub connected to a 500W RMS amp, it provided a nice punch to the chest. Put it in a room for 30-40 people and it was a lot less. You need a hell of a lot more power for bass than you do mids/tops.

Oh, and I think I need probably 40-80 hz, although not sure if 80hz will still be something that you can feel as opposed to hear.

I don’t think I can use a home theatre sub, as they appear to be designed not to point the sound in a direction, and I want something more directional, more like a PA or car speaker.

The only way you can make a sub directional is by making a huge steered array. That won’t be possible to fit inside any room smaller than an arena. Something like lots of these:

http://www.meyersound.com/products/1100-LFC/

Plus a controller. Your concept is interesting but hasn’t a hope of working.

I’d read the wikipedia article on subwoofers – there’s a LOT on multi-sub setups and the effects that can be achieved with them.

How big is the room?

RE Matching amps and drivers, with subs you want lots and lots of power [2,3,400W/more per driver], OR enormous speakers. The amps and the drivers should be quite closely matched, you defo don’t want LESS power from the amps [check what impedance the drivers are and what the amp can deliver into that impedance], I’d be aiming 120% or more of the max RMS INPUT for the driver, per amplifier driving it.

Car subs seldom work as intended in a room, as the car itself is a small enclosure and this effects the design of the speaker.

Cut a long story short I’d guess you’d be best phoning a few pro sound hire places and hiring a 2-4 15 or 18″ bass cabs with matching amps. At least you know it’ll work then.

Also – PAGING chiefgrooveguru

EDIT – prescient chap this CGG 😉

The only way you can make a sub directional is by making a huge steered array. That won’t be possible to fit inside any room smaller than an arena. Something like lots of these:

http://www.meyersound.com/products/1100-LFC/

Plus a controller. Your concept is interesting but hasn’t a hope of working.

Actually now I’ve thought about it, it doesn’t need to be directional, it just needs to come from one place for each speaker. Directionality is actually a bad thing for this design, the ideal would be speakers that radiated roughly equally at a wide range of angles in front of them.

It certainly works with ultrasound transducers, which aren’t really directional either.
(eg. this thing works http://big.cs.bris.ac.uk/projects/ultrahaptics) and certainly if you flip the phase on one of a pair of stereo speakers you can make some weird effects happen.

Obviously with very loud audible sound you’re going to get big horrible noises and need ear protection as well, but that is part of the fun of the idea.

There’s probably some other reason it won’t work (wall reflections, variations in the speed of sound making calibration a nightmare etc.), but I don’t think lack of directionality is it.

Just out of interest – is it a sound you want to project or a physical feeling?

What about this method?

EDIT: – That video is so good it deserves to be on this page

I thought ultrasonics WERE directional

http://en.wikipedia.org/wiki/Directional_sound

and

http://en.wikipedia.org/wiki/Sound_from_ultrasound

The ultrasonic sound stuff is a different mechanism to the ultrasonic haptics thing I linked – it uses an array of transducers to create directional ultrasound, I think in a similar way to what the subwoofer arrays chiefgrooveguru was talking about do.

Those ultrasonic sound / hypersonics / audio spotlight things are really very uncanny, went to a show once where some artist guy had one mounted on a tripod with a sight, and was using it to talk to particular people in the crowd, it was quite quiet, but really disturbing, like hearing a voice in your head.

If you look at vortex cannons, once they go electronic control, they bung on a subwoofer and a big amp to do it, like this one:

It looks like good fun, but feels like it might lack the nuance and variation of something that plays with the phase to move things around.

Actually now I’ve thought about it, it doesn’t need to be directional, it just needs to come from one place for each speaker. Directionality is actually a bad thing for this design, the ideal would be speakers that radiated roughly equally at a wide range of angles in front of them.

Any sub of small enough size will be omnidirectional at these frequencies. The problem is the room boundaries – they become additional delayed sources of lower magnitude. I can’t see how you can remove their effect? The other big issue is harmonic distortion – how will you get clean tones at the required frequency and SPL?

Car subs seldom work as intended in a room, as the car itself is a small enclosure and this effects the design of the speaker.

I’ve used car drivers in home setups several times – just a case of picking the driver carefully.

As you suggest, a lot don’t actually have that much low freq output, they assume the car’s small interior size will boost the output.

Intresting idea

I assume you are sticking to one frequency? As other wise you’ll have multiple wave lengths and the various interference patterns will over lap

http://www.falstad.com/ripple/

As you suggest, a lot don’t actually have that much low freq output, they assume the car’s small interior size will boost the output.

Indeed, a well designed car sub will have a 12dB/octave roll-off which is the complement of the ~12dB/oct ‘cabin gain’, so one cancels the other and you get response as low as you can hear. Sadly most car subs aren’t well designed and many that are are misused (more boom bruv!)

I always thought that <100Hz was pretty much omni-directional? I’m sure I read that when I bought my old sub years ago…

In terms of hearing, but I think he’s talking about something else.

Sadly most car subs aren’t well designed and many that are are misused (more boom bruv!)

oh I don’t know, I think you’re missing the full gamut of design requirements:

1) must make 220bpm hardcore techno sound awesumz loud
2) see #1
3) affordable for cash poor, taste deprived, youthful car drivers
4) easily removable from mum’s runaround when she wants it back for shoping, so unfair bruv
5) secret weed stash hideyhole what the scum police will never find

The problem is the room boundaries – they become additional delayed sources of lower magnitude. I can’t see how you can remove their effect?

I don’t think I can remove them, I was just hoping that if I didn’t put the system near a wall (in a large room) they would be lowered enough in power to mean that the main interference effects dominated – after all it doesn’t have to be completely silent in the non-affected places, just significantly lower power.

The other big issue is harmonic distortion – how will you get clean tones at the required frequency and SPL?

Not sure. I can obviously output decent tones from the computer, but presumably the amplifier and speaker is where it all goes wrong?

From Wikipedia

“Subwoofer output level can be increased by increasing cone surface area or by increasing cone excursion. Since large drivers require undesirably large cabinets, most subwoofer drivers have large excursions. Unfortunately, high excursion, at high power levels, tends to produce more distortion from inherent mechanical and magnetic effects in electro-dynamic drivers (the most common sort). The conflict between assorted goals can never be fully resolved; subwoofer designs are necessarily compromises. Hoffman’s Iron Law (the efficiency of a woofer system is directly proportional to its cabinet volume and to the cube of its cutoff frequency) applies to subwoofers just as to all loudspeakers.”

Here’s[/url] another interesting type of “Subwoofer” – though I feel in your case it is not appropriate. Worth mentioning though!