Why use one amplifier, when you can use two for double the price?

Of course I'm talking about bi-amping - the method of driving a single two way loudspeaker from two amplifier channels.  Most PA Loudspeakers fall into the the following categories:-

• Passive
• Powered (some manufacturers like Mackie call this Active for some reason)
• Bi-amp / Tri-amp Active

Passive speakers use an internal crossover, and are driven from a single amp channel.  Powered speakers have an amplifier (or sometimes more than one amplifier, and other electronics) built into them.  Bi-amp / Tri-amp active speakers have no crossovers built in, and need a separate amplifier for each driver (woofer / tweeter / horn) inside the speaker.

Back of an envelope - Passive vs Bi-amp design

So what's the big advantage of bi-amp speakers, then?  And is it worth shelling out on double the number of amps to use them?

It's a bit techy but basically passive crossovers sap power from the amplifier. They also have very shallow cross-over slopes of around 12dB per octave. That allows a significant amount of range where both HF and LF drivers are radiating the same sound, which can create comb filtering or other bad effects and phase problems. An active crossover allow for much higher crossover slopes of 24bB per octave or higher, so you're getting less overlap between the drivers.  There is also no loss of power to the internal passive crossover.  You'll gain about 3bB in headroom or output, possibly more in some cases.
When amplifiers are connected directly to the drivers, and not through a passive crossover, they have a greater ability to control the speaker cone, and so reduce the possibility of ringing and other negative phenomenon.
So, maybe it is worth converting your current speakers to bi-amp, and buying more amps and active crossovers?

Alternative to hire - build your own speakers!

I designed and made my first set of loud speakers 23 years ago.  They were vast ugly items that sounded, well, like nothing else I'd ever heard.  But not great. I'd taken a few of the principles I'd read about in books on speaker design, and tried to apply them.  Retrospectively, I should have stuck to a tried and tested design rather than think of something new.

Big, ugly, and loud. Home made speakers.

My second set of speakers where much much better, but still had problems.  I remember I purchased the high frequency drivers, I said to the guy in the shop "Don't I need to use a cross over with these?", and he replied "Naa, don't worry about that, just solder in this capacitor."  No wonder those boxes had such an interesting mid-range 'presence'.
Fast forward to today, I've heard and used a great many different types of loud speaker.  And the time may have come to build another set... after all, there are now a myriad of websites and forums dedicated to loud speaker design and construction.
Of course, cheap loud speaker are much better than they used to be, and you can probably buy a pair for less than the materials required to make one.  So I must think carefully!

The MkII speaker was a huge improvement! 

What is a line array, and why would you want to hire one?

Line array has been a buzz word in the live sound and PA hire industry for a good few years now, ever since the French company L'Accoustics introduced the first concert worthy system in the early 1990's. But what is a line array?
In simple terms, a line array is a set of speakers, designed to create a coherent line source sound, normally for reproduction of live music or speach.  Well, what's so special about that then?
A 'normal' speaker system is typically a point source.  Point sources of sound can be measured to reduce by 6dB for every doubling of distance you move away from them. Therefore if you were 10m away from a speaker and you have a sound pressure of 95dB(A), then you move to 20m away, all things be equal, and in free space outside, you'll have a sound pressure level of 92dB(A). That's because a point source (at least theretically) radiates in all directions.  A vertical line source doesn't and to cut a long story short, the sound level is supposed to fall 3dB for every doubling of distance. It's because sound radiating up and down interferes and cancels.  Yes there is science behind it, but all you need to know is, the sound dispersion pattern of the speakers is more like a slice of cake and less like a football.  And the sound carries further. That's at least part of the theory, but real life factors like absorption of high frequencies by air and the restrictive length of the array it's self arguably limit some of the benefits.  Have a look here for a more detailed discussion on line array theory.
Some argue that point source speakers still sound better, but the bottom line is that the direction of the sound is often better controlled with a line array, and with much less space taken up in the truck.
Take a look at a big festival sound system, and 9 out of 10 times you'll see line arrays in use.

Martin Audio Line array

Bloging on the job

In the PA hire business, there are often time working on a show when one has time to kill... and a few years back a newspaper or magazine was the best remedy.  Now the trend is towards iphones et al.  Yes, I'm talking about Facebook, Myspace, and the blogsphere.

Now its great to keep your mates up to date with the latest Allen & Heath iLive you're using and the jobs your working on.  But posting Facebook updates like "this is the most hateful ***** annoying job I have ever worked on!", surely that crosses the line?

I'm happy to report none of my crew have made such postings.  For quite a while now.  What can I say?  It pays to keep tags on your crews myface pages.

The desk is an Allen & Heath GL2800 by the way

Colour changers, for those that don't like moving head fixtures!

Today I received an SGM Idea 575 Colour changer in the post.  By mistake.  How often does that happen?  So I thought I'd at least take the opertunity to test it before informing the sender.
For those that might not be familiar with this kind of lighting fixture, basically it's a large wash light, capable of switching to any colour instantly.  This is done by the use of CMY colour mixing (that's Cyan, Magenta, and Yellow).  By a combination of any of these, and white of course, almost any colour can be projected.
A lot of these fixture also have a few other features, such as a shutter to enable strobing, an iris for dimming, and a few like this SGM have a beam shaper.
If you aren't a fan of moving fixtures, it's still worth a look at these.  They could be a real time saver in a theatre environment.
I can report that in auto mode this is an amazing light, it lit up the room even with daylight coming in and cycled through it's features. It looks like it has a 'zoom' as well which adds to it's usefulness.
I must admit, I'm reluctant to send it back now!

http://www.sgmtechnologyforlighting.com/prodotti.asp?co_id=1076

SGM 575 Colour changer, received by mistake!!!

If you're thinking of hiring some lights, check this lighting hire page out.

Help, my speakers are blown!

If you've worked with, hired, or used sound systems for long enough, you'll come across this scenario.  In my case, two powered subs came back from a PA hire job not working.  A quick test with a multimeter across the terminals of the drivers and a smell of burning confirmed the worst... basically the voice coils had melted.
This is about the most common reason for a speaker to blow.  In my case, the unsupervised DJ had ran the system into the clip lights all evening.  Even with limiters, this can cause square waves to pass through to your speakers, which is bad for both the voice coils and the speaker cones themselves.
The only remedy in the case of a melted voice coil is to replace the driver.  The cost could be from £20 to £400 depending on the speakers you have.  Of course, more expensive equipment is less likely to sustain this kind of damage, but it can still happen, and it's even more expensive when it does!

Driver from a Yamaha MSR800W sub-woofer

Blown voice coil removed from the driver

Hello world!

Hi, Welcome to the new Sound Truth ltd blog.  I'll try and post occasionally to keep you all up to date with the latest news, developments, equipment, anything PA hire related.

This coming weekend we are supplying two PA systems for the Disney store in Milton Keynes, for the grand opening of their new store.  Should be great!

We'll try and fit a PA System next to the magical castle ;-)