Claire handles the customer service emails for the SleepPhones product here at AcousticSheep. She forwarded the exchange in question to me. (I'm more involved in the technical side of things.)
I think this is the response that might be considered controversial:
"I guess you make a good point about no vibrations at 0 Hz. It's just the typical manufacturer designation f0-20kHz. I just tested a pair of our headphones. The minimum detectable frequency is 20Hz using my MacBook. Perhaps if I used a more powerful player, I can hear the 10Hz."
Amplifiers actually DO have minimum and maximum frequency ratings - but I've got no idea what they might be on a MacBook. I figure this is a moot point here, though, as merely listening really isn't an appropriate way to evaluate speakers at frequencies below the range of human hearing (20Hz). I'm aware that some people can hear tones even down to 17Hz, but unfortunately the track she was using for testing only works in increments of 10Hz. Thus it's not surprising to me that she's not hearing anything below 20Hz.
Anyway, the frequency response range numbers on our website come from a datasheet provided by our speaker manufacturer. They should still be up there - I don't think anyone has taken them down. Personally, though, I don't consider these numbers to be very useful, even though I know they are often all consumers have to go by. Instead, it's the frequency response chart that counts. If the response drops off too quickly after a certain point, I don't expect that tracks which rely on frequencies below that point will be effective. I consider this to be the case even if the "official" cited frequency response number is much lower. Here's an article that explains what I'm talking about (even though the included charts don't go below 20Hz):http://forum.ecoustics.com/bbs/messages/34579/131062.html
On our end, we've very carefully evaluated a great number of speakers looking for the best ones we could find with the size and shape we need. The testing we've done here at AcousticSheep, though, has only covered the range of human hearing (20Hz to 20kHz). This is because we don't have access to an "electronic ear" device which can handle infrasonic frequencies (under 20Hz).
Therefore, I do not feel comfortable recommending our SleepPhone product for applications that depend on frequencies below the accepted range of human hearing. (These would include HoloSync's "Awakening Level 4" and beyond.)
The last thing I want is for our customers to purchase our product for an application that it's not suited for.
I've actually tracked down a chart for our speakers since my last email to uniquesoul. It looks like anything down to 20Hz will be fine. (Which would cover any HoloSync programs up to but not including "Awakening Level 4".) I can't be certain of anything below 20Hz, though, as that's where the chart ends.
Up to now, we've just been quoting the frequency range provided by our speaker manufacturer. I've just requested specific measurements for the range below 20Hz, as determined by an appropriate "electronic ear" device. These should provide a fuller picture. I'll keep everyone posted.
I hope this clears up any confusion. If anyone has any further questions, please feel free to contact us! It's our goal at AcousticSheep and SleepPhones.com to be as accessible as possible. Also, I do want to apologize again to uniquesoul for not answering his questions sooner - I was forwarded an email exchange and genuinely didn't realize there were questions left in there that I was meant to answer.
Thanks for reading this far!
Jason H. Wolfe, CTO