It is not recommended that ultrasonics be used with earphones because of the easier possibility of listening at too-high volume, and therefore damaging your hearing. You have most likely read this before, but I just had to repeat it.
However, if you go ahead with listening to ultrasonic ("US") subs on earphones, here are my recommendations (based on what I have done).
MP3 player
I suggest you specifically use a dedicated MP3 player which has "clicky" volume buttons. Therefore, the likelihood of accidentally changing volume is negligible. Also, having a manual "Lock" switch is double insurance against accidentally changing the volume (or changing the track). MP3 players are very cheap nowadays, and for the purpose of improving your life in the long-term, it is well worth buying a dedicated MP3 player.
Volume
I suggest that you listen to US subs at an even lower volume than 25%. Instead of calibrating using the masked tracks, use the free "Speaker Test" track to calibrate. Imagine playing that track constantly for hours on end - you realise that you want the volume VERY LOW, not just one or two notches lower than the masked tracks. On my MP3 player, I might have the volume set at "1" or "2", out of "25". At the most, I will have it at notch "3", when I am on a noisy train/bus.
Effectiveness at low volume
Do the US subs work at such low volume? In my experience - Yes.
My theory is that the concentrated range of sound in the US subs means that the volume of the track is much higher than the masked tracks, which have a wider distribution. Therefore the US track needs to be played at a MUCH LOWER volume. Whether that theory is true or not, my experience is that the US subs will work at the lowest volume setting on my MP3 player.
However, if you go ahead with listening to ultrasonic ("US") subs on earphones, here are my recommendations (based on what I have done).
MP3 player
I suggest you specifically use a dedicated MP3 player which has "clicky" volume buttons. Therefore, the likelihood of accidentally changing volume is negligible. Also, having a manual "Lock" switch is double insurance against accidentally changing the volume (or changing the track). MP3 players are very cheap nowadays, and for the purpose of improving your life in the long-term, it is well worth buying a dedicated MP3 player.
Volume
I suggest that you listen to US subs at an even lower volume than 25%. Instead of calibrating using the masked tracks, use the free "Speaker Test" track to calibrate. Imagine playing that track constantly for hours on end - you realise that you want the volume VERY LOW, not just one or two notches lower than the masked tracks. On my MP3 player, I might have the volume set at "1" or "2", out of "25". At the most, I will have it at notch "3", when I am on a noisy train/bus.
Effectiveness at low volume
Do the US subs work at such low volume? In my experience - Yes.
My theory is that the concentrated range of sound in the US subs means that the volume of the track is much higher than the masked tracks, which have a wider distribution. Therefore the US track needs to be played at a MUCH LOWER volume. Whether that theory is true or not, my experience is that the US subs will work at the lowest volume setting on my MP3 player.
