Discussion in 'Sound Science' started by jthew, Dec 4, 2017.

1. I was out with a couple hearing aid users this weekend. Both are high level engineers/scientists with inquisitive minds. They were on board with me. From all of our understandings, the ear has different parts responsible for hearing different frequencies. Areas that are damaged result in reduced hearing at those frequencies. Hearing aids boost the frequencies in the diminished areas. They've never been told that a hearing aid would hurt what they have left. And sure, I could make a mistake, and over-do a frequency range, and as a result, damage my hearing in those frequencies. But so can an audiologist. They would be working off the same hearing test results I would be working off of.

2. My step dad has a bluetooth hearing aid with an embeded microphone, he can take calls with his hearing aid and smartphone (which is awesome). I suppose he can listen to music thru it, and that the output is EQed to his calibrated values?

3. Yes. It should be EQed to compensate for his hearing loss. But my guess is that it is only concerned with frequencies associated with dialogue. Some people have about the same amount of diminished hearing over the whole range of frequencies used for dialogue. Those 'lucky' folks can just use something like TVEars or the Sennheiser hearing impaired models that can output as much as 120dB. Then, they just crank it up.

4. In post #3, I outline my anticipated eq. There was an estimate that the actual measured dB that would make music sound like 80dB to me (with my diminished high frequency hearing) would be maybe 90 to 93dB. In other words, after applying a relative boost at the higher frequencies, but maintaining a normal level from 20 to 2kHz, what my younger ears would interpret as 80dB would actually measure 90-93dB.

So, I need an amp. If I have headphones with a sensitivity of 93dB at 1W, and the headphones are 32ohm, every amp that measures 1W or less at 32 ohms would be out of the question. Correct?

5. I'm hoping you're just giving out random values and your headphone doesn't actually have a sensitivity of 93 dB SPL/W... but yes, if we're talking just in theory, assuming that the power draw for all frequencies is completely linear and the headphone impedance is also 32 ohm, anything less that 1W would clip. But since theories don't perfectly apply to the real world, and the fact that we can't assume the manufacturers specifications are perfect, it would be best to have an amp that outputs more than what is required to reach your required dB levels.

When manufacturers give out their sensitivity rating, it's also usually measured at 1kHz, so don't take their sensitivity rating as the be all end all spec.

6. Which brings up another thing I don't understand: HiFiMan HE-400i from one source: Sensitivity measured with a 1 mW signal between 300 Hz and 3 kHz at the rated 35 ohms impedance, is 93.3 dB.

Just 1 mW? Won't any amp drive these to the volume I need, with enough room to do the relative boost in the high frequencies?

I've also seen this: Sensitivity: 94 dB SPL/V

7. Well not any amp but most will yeah. Don't forget, SPL is logarithmic. So to get +3 dB, you need double the power (approximately). For example, to reach 96.3dB you'd need 2 mW and to reach 99.3 dB you'd need 4mW and so on.

If you have it in volts, just use Ohm's law, P=(V^2)/R, gets you the same result.

Last edited: Dec 11, 2017
8. Digesting more....

I'm not getting this. Is this demonstrating what can go wrong if several eq settings act on the same frequencies?

9. So, how well would this:

https://www.minidsp.com/images/documents/Product Brief - HA-DSP.pdf

Drive these:

For this review I, besides the stock cable, mostly used cables from Forza Audio and Charleston. The HE400i has an efficiency of 93 dB/mW and a nominal impedance of 35Ohm. In reality my HE-400i measures 42.9Ohm on both sides, but at least they are balanced. The latest Hifiman headphones should be easier to drive then ever before but you will notice they’re still not the easiest to drive headphones. The new HE-400i though, can easily be driven by the most popular DAPs.

We also measured impedance, which was almost dead-flat in magnitude (at 43 ohms) and phase through the entire audio band. As expected for an open-back, isolation is almost non-existent, with just a slight attenuation above 2 kHz maxing out at about -8 dB. Sensitivity measured with a 1 mW signal between 300 Hz and 3 kHz at the rated 35 ohms impedance, is 93.3 dB. That's pretty low compared to most other headphones, but OK for planar magnetic. We got plenty of volume from an Apple iPod Touch.

Taking into consideration the EQ I plan to apply (stated earlier in this thread). My guess is that they are a little too lightweight, and that I would want something that pumps out closer to 1W or more at 43ohm?