Reading this and other threads on testing prompted some thoughts on the subject of tone testing.
The experts can look away now. Or hopefully correct any of my mistakes.
You should always start a run of sinewave test tones at your
highest test frequency and work
downwards. This should avoid you blasting your equipment at inaudible frequencies. Which could easily happen if you start very low and then turn the level right up just to hear these strangely inaudible tones!
When you hit a room resonance it is the
room that amplifies the signal.
Not the equipment. Which merely "excites" the room resonance. The equipment is
not being put in danger of overload by a true room resonance. Just be sure you haven't got the level too high if it's your
first test tone!
The
test signal should remain at the same level throughout a test run. Which means that you should choose a reasonable level on the SPL meter while still in the clearly audible region. Starting at somewhere around 70-80dBs @ 120Hz seems to work for me. This is impressive enough to be fun without damaging the scenery too much on room resonances.
There is no
correct level. It is a matter of taste. If you start testing at 100dB on the meter then your sub may decide to commit suicide. Rather than play the really deep notes. If the neighbours start banging on the wall then your test level is probably a bit too high.
The danger is if you set your test level in the middle of an unknown (but deep) room response trough. If you discover the response is rising fast as you lower the frequency of your tones. Then you should lower the
test level and start again from the beginning. Though you may simply be approaching a response
peak slightly below your starting frequency. If the level doesn't get out of hand and starts to fall again with lower frequency tones then it
was a peak.
Have your SPL meter propped up where your head is at your normal listening position. With the test microphone pointing towards the speakers or sub being tested. The RadioShack SPL meter has a tripod bush underneath if you have a photo or video tripod handy. It is often easier to read the scale if you set the meter on its side. I find it can be read from much further away when you are running back and forth to the computer. To change to the next tone or write down your SPL reading and test frequency.
Remember to have your meter correction figures handy when you finally plot your column of test figures onto a bit of graph paper or computer spreadsheet.
If you don't have expensive Windows Excel on your computer then don't panic. Download "Open Office". A free (64MB?) download. Not too painful if you have unlimited broadband.
Don't blame your subwoofer if it is your
speakers causing a wobbly bass response. I would always test with the speakers working and normally calibrated first. Long before doing a sub only test. Which is largely of acedemic interest.
Remember that your speakers are part of the sound reproduction system on all material (music or film). You can't completely ignore the speakers in the bass. Even when they are crossed over as high as 120Hz! Though most are crossed over lower than this.
If you're running full range, unfiltered floorstanders like me. Then you'd better have your speakers working! Or your test results are completely worthless!
All speakers and subwoofers go through a whole series of being in an out of phase with each other. Depending entirely on frequency and your room's physical dimensions. The bigger the room the lower the affected frequencies. But these phase effects are always still there.
The sub's phase control often seems inneffective. Unless you are cancelling a phase peak between the sub and your speakers. Then you won't notice a thing as you wind the phase control knob back and forth. (Or switch between 0 and 180)
Adjusting the phase should always be done from the listening position anyway.
Not when you're crouched over the sub!
Boo! You can all wake up again now!
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