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Headphone & Amp Impedance Questions? Find the answers here!

Discussion in 'Sound Science' started by proton007, Apr 25, 2012.
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  1. MrMateoHead
    Let me add that,
    Headphone A is Sennheiser HD 595
    Headphone B is Hifiman HE-400
    According to Inner Fidelity,
    Senn 595 requires (for 90 Db)
    0.55 Volt RMS
    0.05 mW
    Impedance is 55 Ohm
    Hifiman HE-400 (for 90 Db)
    .129 Volt RMS
    0.33 mW
    Impedance is 51 Ohm
    So, evidently, Hifiman wants 4 time less voltage, and almost 7 times the wattage of the Sennheisers to produce 90 decibels. I don't understand how the Sennheisers require barely a miliwatt to reach 90 decibels. They are rated 112 dB @ 1 Volt RMS. By that figure alone, I would assume 92 dB @0.1 Volt RMS.
    I am trying to get myself straightened out.
  2. stv014
    You can ignore the 51 mA power supply current. It is the maximum voltage of 1 Vrms at 32 Ω that matters, so the power is really 1 * 1 / 32 = 31.25 mW.
    If you took the 1 Ω output impedance from the datasheet of the ALC888, rather than the specifications of your motherboard, or measured it yourself, then note that there is often additional serial resistance (a few tens of Ω) added to the headphone output on the motherboard.

  3. stv014
    It is really only 0.055 Vrms, you missed a '0'. That translates to about 115 dB/V, compared to the specification of 112.
  4. MrMateoHead
    Using formulae provided by Inner fidelity and specs, this is what I come up with:
    To reach 110 dB on the Senn 595s:
    .055 * 10 (0.55 V)
    0.55^2 / 55 = 5.5 mW for 110 dB
    To reach 110 dB on the Hifiman HE-400:
    .129 * 10 (1.29 V)
    1.29^2 / 51 = 33 mW for 110 dB
    The basic formula was found here:
    Assuming all specs and math are correct, either headset should get ear-damaging loud without much wattage at all. My complaint is, I thought getting from 90 dB to 110 dB would require 20 times the voltage, since it is 20 dB, not 10 dB louder. Interestingly, the Hifiman's need about 6 times the wattage to reach the same volume level.
  5. obobskivich
    Assuming current can be delivered ad infinitum, sure. But your voltage # for the 595 is wrong, as per stv014 and InnerFidelity:

    They're more sensitive, it's .055v, not .55v. They will get louder sooner.
  6. MrMateoHead

    Ooops. Corrected above.
  7. obobskivich
    Even 90 dB is insane by the way. :)
  8. MrMateoHead

    110 would be like a fricking concert, so I believe that! I am just spec-drunk at the moment, and irritated that I cannot figure out whether I'll have 6 times the wattage I need to get the Hifimans as loud as I would likely ever stand. As is, I can't usually stand turning volume much past 50% with my current setup.
    I assume the Hifi's are going to suck up a lot of whatever headroom I currently have.
  9. obobskivich
    110 is unrealistically loud - you will have a headache after. 60-70 is realistic.
  10. xnor
    I wouldn't go above 85 dB SPL. The HE-6 requires less than 7 mW (or 0.63 V) to reach that volume. Many headphones only need a tiny fraction of a single milliwatt to reach the same volume.
    obobskivich likes this.
  11. MrMateoHead

    Thanks Xnor. Can you speak to the calculations I made above however, or give any guidance on how to make use of the weird specs given for many headphones and amps? I wouldn't want to listen at 110 dB (or even 100 dB) for more than short bursts. I am usually in a quiet environment where 85 dB or less is probably more than adequete.
    It baffles me to think that the HE-6 would require much of an amp to reach high volumes, yet the big hifiman amp for sale puts out something like 6 watts.
  12. xnor
    If you use the Vrms needed to reach 90 dB SPL from innerfidelity just use
    10^(relativedB / 20) and multiply it with the Vrms needed to reach 90 dB SPL.
    So for 85 dB SPL you have to substract 5 dB SPL: 10^(-5 / 20) * 0.055 = 0.031 Vrms for the HD595 to reach 85 dB SPL.
    You can reverse this: 20*log10(0.031 / 0.055) = -5 dB.
    If you have sensitivity for milliwatts just use 10*log10(power / 0.001) instead, or the reverse: 10^(... / 10).
  13. MrMateoHead

    Ok - the first formula makes sense and I can replicate your number. 85 dB is achieved at 0.031 Vrms for the senns, and 0.0725 for the hifimans.
    Hypothetical, lets Change the Vrms to 1.
    20*Log(10*(1 / 0.33)) = 30 dB. Therefore, 1 Vrms yields max SPL of 112 dB. (Hifiman)
    20*Log(10*(1 / 0.055)) = 45 dB. Therefore, 1 Vrms yields max SPL of 135 dB (Sennheiser)
    Though, both of those numbers are pretty close to radically high. Realistic? Or is it likely that driver limits would prevent such SPL? I am surprised that 1 V could yield that much volume considering amps like the Asgard are rated for up to 7 Vrms (but 1 Vrms at 10% THD).
  14. obobskivich
    Power requirements are often misrepresented to sell more gadgets - you really don't need anywhere the "juice" most traderags cry about. As far as when the driver blows up - the isodynamic should outlast because it will like die due to Tmax not Xmax, while the Sennheiser you'll probably pop the cone or cook the motor off fairly quickly (max input power is probably like 200 mW). But there will be time relativity here - instantaneously they will survive more than continuously. But still - this is way beyond suitable listening levels.
  15. mikeaj
    I couldn't follow what you're trying to calculate, but before we even get to that, people have been using log10( ) to be shorthand for logarithm with base 10.  Somehow a 10 ended up inside the argument of your logarithms.
    Senn HD 595 (0.055 Vrms for 90 dB SPL):
    20*log10(1 / 0.055) = 25.2 dB,
    so 1 Vrms would give 90 dB + 25.2 dB = 115.2 dB SPL
    HiFiMAN HE-400 (0.129 Vrms for 90 dB SPL):
    20*log10(1 / 0.129) = 17.8 dB,
    so 1 Vrms would give 90 dB + 17.8 dB = 107.8 dB SPL
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