How to reproduce audio output in dB SPL at specific level?
I am designing an experiment in which the reproduced audio output via headphones in dB SPL should be fixed. In my case 70 dBSPL. I am aware that the dB encountered in live.gain~ object is in dBFS. For the hardware settings, the headphones are connected to an external soundcard. I am also aware that I need a sound level meter to check the level in dB SPL. Can someone guide me as to how this procedure should be carried out? In addition, are there any specific objects in MAX/MSP which take into the account the properties of headphones such as sensitivity? Does the audio status in MAX/MSP will be helpful in this procedure?
you need to set up a sound of a given peak, for a example a white noise of 1., then amplify your headphone until you reach 70 db, which you need to measure with a pressure level meter.
i am not sure if white noise or a cosine or a LUF of -12 is the industry standard for getting the usual 70 db monitoring situation, but there is no other way than measuring the headphones output at a digital level set by you. (and then later produce roughly at that peak/power, too.)
of course heardphones are far more difficult to measure than speakers.
Shouldn't the source be pink noise, and SPL read with C-weighting?
yes, rather pink than white noise.
@Roman Thilenus and @Florian1947, thank you very much the suggestion. In order to measure, do I need a coupler? Could you share any link any video or forum where can I get a rough procedure of the same?
There doesn't seem to be a standard for measuring headphone levels, especially for the in-ear-type.
I would 'adapt' the line-up rules suggested by Dolby (see chapter 5.3; - although it is for cinema loudspeakers). I'd put an SPL-meter between the two phones cups and play the attached Dolby Noise at unity gain from DAW/interface.
Good luck!
@Florian1947 thank you very much for the audio file. It seems I need a coupler to the procedure precisely. I also did not play the audio from a DAW instead I directly played from Windows Media Player. Since, I am using MAX/MSP in the experiment, is it a good idea to play pink noise via MAX/MSP?
It makes practical sense to use Max for playback, at unity gain.
For measurement, most efficient solution would probably be an RTA app on a smart phone, like Audio Tools, in combination with a calibrated line-up mic like the i436.
Set level on the soundcard (not in the playback software) to hit 70 dB SPL C. This adjusts for headphone sensitivity.
beside the fact that it is almost impossible to do a proper measurement of earphones the other problem is that the cheaper SPL meters such as the police uses have a limited frequency range on the low end, which might be less than a good pair of studio speakers or headphones.
@Roman
absolutely, the cheap SPL meters tend to be for outdoor use, normally at A-weighting, which limits frequency response both ends.
That is why I suggested C-weighting, which is the flattest of the three standard filters. With a suitable smart phone and an external mic plugged in, you can get metering performance close to a much more expensive discrete SPL-meter. A small mic could be placed directly inside the headphone can.
I haven't tried this myself, but I would expect this way to get results which are at the very least useful, and on a budget.