1) what measurement software do you personally use? I will like to give it a try
REW 5.11 (
http://www.roomeqwizard.com/)on Win XP, soundcard = EMU0204 on 192/24 samplerate, rig = WM61A on homemade rig + dedicated and compensated pre-amp.
The WM61A has a peak at 16kHz which I the pre-amp removes.
Furthermore a mic that is designed to be used in free air (a measurement mic as well) has a different LF response when mounted on a(n infinite) baffle so must compensate for this as well.
These compensations can also be applied in REW but haven't experimented with this.
The output of my mic amp is thus already compensated 'flat' (at least as far as I know) and outputs line level.
I use a homemade amp for driving the headphones.
It's irrelevant what you use as long as it has enough output power to reach 90dB SPL in each headphone (+ headroom) and doesn't alter phase and FR and has low distortion.
All amps that meet those minimum requirements ALL will give the exact same readings on the same headphone.
As Purrin also explained, REW needs also needs a separate dB meter for calibration (only for the SPL part).
Bought a cheap one from fleabay and already had a (modified) Realistic meter which I calibrated a while back when I had an opportunity to use it alongside expensive mics that were being calibrated.
The cheap fleabay meter is about 0.5 to 1 dB 'off' so cheap meters MAY be accurate enough (you never can know for sure though).
It really doesn't really matter if the actual SPL is 88, 90 or 92dB when it displays 90dB.
You set the SPL meter in the 90dB range and crank up the amp/vol control sliders in the calibration mode till it reads 90dB (using noise generated by REW)
Make sure you seal the headphone (using a CD with some felt on it ?) and poke the mic though the hole in the CD.
ENSURE you have a good seal around the mic and headphone to get a proper SPL reading.
Put that headphone on the rig.
Then you adjust the input of the soundcard so it is around -10dBFS (leaving headroom while maximizing S/N ratio)
Then you tell REW that its input level is 90dB SPL and never touch any of the controls again or note the settings so you can set them in the same position later on.
So I am assuming that I can find from the manufacturer the reference they used and this spec will be listed as a certain SPL at 1kHz (or using noise). And then I will run a measurement trial, I will attempt to match the threshold of the measured distortion to what they say the spec should be using the gain settings?
the dB spec the manufacturer gives is irrelevant, it only says something about its sensitivity at a certain frequency.
4) what program would I use to determine the THD? any recommendations?
I use REW (it can also give distortion in % and or dB) but ARTA (properly set up also gi
ves good results.
Measuring headphones is not as easy as it looks and every one that does is produces different plots, some are somewhat comparable, others are not.
Some use smoothing others do not or use less.
A measurement is just an 'aid' and lies to you.
It's up to you to know if the rig is telling lies or not.
That takes knowledge and experience ... and thus takes time and lots of practice as well as understanding what the plots mean and learnig to correlate aspects to sound.
There will NEVER be a 100% correlation to the sound for many obvious reasons but such a rig can be helpful when modifying headphones or when looking for major flaws.