How to find safe limit for Driving Speakers?

Axiom Home Page

I've got the bits for my Outside Build using:
- Behringer X1222USB Mixer : Lots of inputs for Audio Devices, Mics, Instuments... even has a "Voice Cancellation" feature for "instant-karaoke", which will mix then feed the signal out to a
- Behringer B1800D-PRO Active Sub : 18" Sub with built in stereo cross overs that then go out to a
- Behringer iNUKE 6000DSP : This is a 2ch amp capable of driving low ohm loads with lots of power + inbuilt DSP you can program for LF cross overs that will then drive two pairs (in parallel)
- Axiom M3 Outdoor Speakers which I already own and each pair will face a different part of the yard.

I've got it all hooked and and it is working well so far but I need some advice on how to set the max power levels for the Amp so as not to blow the Axioms yet have a max SPL from them.

So far I slowly turned up the power keeping an eye / ear on the Axioms (front covers off) and got a bit scared as I don't know at what point bad stuff permanently happens. At one point I heard some harshness / buzzing from one of the tweakers in a particular passage of music so I then turned it down.

Thanks
Nathan

Advice so far from the the Jriver site (cross post):

Quote:

Do you have an SPL meter? Those axiom speakers (according to their website) have an 88dB 1 watt/ 1 meter sensitivity outdoors, and they claim to be able to accommodate 175 watts. That means if you ran 0dBFS pink noise into the speakers at max rated power, an SPL meter at 1 meter in front of them should register about 110dB. Similarly, if you ran JRiver's pink noise calibration (which is -20dBFS), you'd know you found the max power setting when the meter read 90dB at one meter.

But max rated power is not a good goal (and I wouldn't recommend running them that loud even for a test). Most speakers will experience power compression and/or heat related issues if continuously operated anywhere near their max rated power, and sometimes the max ratings are a little exaggerated, meaning that one can do damage running them up to the max.

I'm pretty risk averse, so if it were I, I would cap the power where the SPL meter shows 100dB at 1 meter with a 0dBFS pink noise signal or 80dB with a -20dBFS pink noise signal (like the one in JRiver). That setting would be very unlikely to test the thermal limits of the speakers if their ratings are correct. But that's a very conservative setting; you might be comfortable running them up louder, but if you set your max above 105dB I'd encourage you to keep an eye on them.

Just remember the golden rule: a 3dB increase is double the power and a 10 dB increase is ten times the power.

Nathan, I'd agree with the calculations shown on the Jriver site, but not necessarily with the recommendations as to max volume. The 175 watt Axiom max spec does equate to about 110dB at the standard 1 meter measuring distance, but the Axiom power specs are typically conservative. Measuring with an SPL meter to get a max on peaks(not continuous)of 110 dB per speaker at one meter(113dB with two simultaneously, 116dB with four)shouldn't cause a problem with the M3s.

Listening out of doors with little room reinforcement(a bit from the ground), the sound level would drop off at nearly the theoretical 6dB per doubling of distance which occurs in anechoic conditions(a typical figure in home listening rooms is on the order of 3dB).

Thanks - I've now dialled them in with the test tones from JRiver's Media Center and reduced the power a bit from when I was doing it by ear/sight alone.

Given you advice on the Axioms being conservative so I've gone with a 30% setting on the Amp which gives me around 90dB @1m (+/- 2db pending on the speaker) using MC's "narrow-band pink noise at -20dBFS" mode when both the channel and main mix are at their max (+10) . Playback on a couple of pop tracks with these settings saw peak levels just under 110dB @1m with the mixer's just touching the clip indicator (which according to the 6000DSP manual is what you want).

It sounded clear and loud and should prevent the teenagers from over driving anything.... though I'm now a bit deaf in the left ear! Grin

I take your point on the lack of reinforcement as it is a big open area but my wife complained she could not hear the radio in the house when I was testing.... however we had some tradies over who gave it a thumbs up (they thought I was testing the gear for some upcoming gig!).

Thanks
Nathan