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I was asked for advice on buying headphones to protect students who have medical conditions that make them sensitive to noise, such headphones would have to allow them to hear human voices.
Due to the significant differences in hearing issues (including physical damage and sensory issues) it seems unlikely that getting identical headphones for all students will give an ideal result. The person who asked me the question didn’t explain what type of students are being taught. If it’s an adult education class then getting everyone the ideal headset wouldn’t be particularly difficult. If however it’s the special needs class in a high school then students would probably want the most shiny headphones rather than the ones that are a best match to their hearing issues.
Also some combinations of hearing problems and ambient noise can’t be addressed by such headsets. A friend who developed Noise Induced Hearing Loss from shooting tells me that he really can’t stand brass instruments. But the high frequencies from such instruments tend not to be filtered well by noise canceling headphones, so any student who has such a problem would probably need hearing aids that filter out high frequencies – I believe that such hearing aids are available but don’t have any particular knowledge about them.
I did a quick test on my Bose QC-15 noise canceling headphones  which cost me $320US including tax and my cheap Bauhn headphones from Aldi  which cost me $69AU (and which apparently could later be purchased on special for $35AU to clear stock).
I found that when not playing music they seemed to perform about equally well in terms of allowing me to hear people speaking, although I admit that just having a conversation with the nearest people wasn’t the most scientific test. When I was playing music I found that the Bose headset made it significantly more difficult to hear people speak than the Bauhn headset. This is an advantage for the Bose for it’s intended use, and I expect that students who need a headset for medical reasons won’t want to listen to music while studying so it’s never a disadvantage.
In both cases, if the headphones are used for just canceling unwanted noise the speaker shouldn’t need to raise their voice significantly to be heard. In some situations the noise canceling headphones make it easier for someone with good hearing to hear what people are saying, for example a conversation in a car or plane could probably be held at a lower volume if all people involved were wearing suitable noise canceling headphones. If however the students have damaged hearing then I can’t make any prediction as to whether the teacher could speak at a lower volume or whether they would be required to use a higher volume if the students wore such headphones.
The Brookstone on-ear headphones that I tested  seem particularly noteworthy in this regard due to the way they canceled the melody of the store background music and just left the singing. If someone wants to buy headphones for people with physical damage to their ears then the Brookstone product is really worth investigating. If however the target market happens to be people on the Autism Spectrum then they may hate anything that presses on their ears (as I do) in which case the Brookstone product can’t be considered. The Brookstone price of $150US (presumably $160 including tax) was also the best price I saw when shopping in the US – but I presume that I could have found something with a similar quality and price to Bauhn in the US if I looked hard enough.
The big advantage of the Bose for this use is that it blocks a wider range of frequencies than some other noise canceling headsets. They all work really well on regular low frequency noise such as car engine noise (whether a car passenger or a pedestrian) but to stop certain higher frequencies such as those from air conditioning systems the Bose wins hands down. I guess this may depend on what noise is to be blocked, if a class was held in the same room every time and noise canceling headsets were purchased specifically for that class then it would probably make sense to ensure that the acoustic capabilities of the headsets match the unwanted background noise and the hearing issue that each student has.
Here’s an Amazon link: Bose® QuietComfort® 15 Acoustic Noise Cancelling® Headphones
I’ve been reading about Sensory Processing Disorder. I’m sure that some children are doing poorly in the default school system because they either have an undiagnosed case of SPD or who don’t have enough symptoms to get a diagnosis. I think it would make a good experiment to try noise canceling headphones on some of the difficult children, I wouldn’t expect a high success rate – but if it worked in as little as 5% of cases and did no harm to the children who didn’t benefit then it would be worth doing.