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Please explain headphone "sensitivity"

post #1 of 12
Thread Starter 
In as much detail as possible, please explain what the "sensitivity" measurement means. How is this figure determined, how is it measured, and what does it mean (what are its implications)? How does it relate to impedance (if at all)? Thanks.

post #2 of 12
markl: If you take the term seriously (= not mixing it up with efficiency), sensitivity describes the sound pressure level produced when a certain voltage is applied (dB/mV), whereas efficiency tells you the sound pressure level produced when a certain power is applied (dB/mW). So a headphone with higher sensitivity is especially welcome, if you have an amp with a rather low voltage swing, because it will be able to draw more current out of that amp than an equally efficient but less sensitive headphone at the same position of the volume knob.

Greetings from Munich!

Manfred / lini
post #3 of 12
lini's point about voltage is an excellent one, and probably often overlooked!

Something to keep in mind is that even though nominal impedance is stated as one number, in some headphones impedance can vary greatly depending on the sound frequency being reproduced.

I lament the fact that so many manufacturer's published specifications are seemingly full of errata, omissions, typographical errors, inconsistent units, undefined units, and incomplete data. There's much room for improvement, but I'm not holding my breath.

Anyway, a few references:
The sensitivity rating is usually for 1 milliwatt POWER input to the phones and a corresponding sound pressure level (SPL) output (USUALLY 102 to 106 dB SPL output for moderate to high sensitivity rated phones).

However, connecting different sets of phones with the same 1 milliwatt to SPL output sensitivity rating to the same headphones amplifier output may, or may not give corresponding equal or loud enough SPL operation!

The impedance (the ohms rating of the phones voice coil) determines how much audio 'voltage' needs be applied to get that '1 milliwatt' or more power into the phone's motor circuit.

For example: A low 16 to 45 ohm headphone (low ohms impedance that's typical for Sony headphones) will easily get to operate for many milliwatts for providing close to live loudness on battery powered portable equipment that typically output only .5-1.5 volts RMS volts output.

A 65 to 600 ohms phones set impedance (higher ohms typical of Sennheiser, Beyer, & AKG headphones). will require a higher headphones amplifier output voltage signal to get the same milliwatts into the phones voice coil for providing equal loudness.

Most of these higher impedance phones require connection to a headphone amp circuit that has ability to drive at least 2-6 Volts RMS to get reasonable loudness for phones listening. Not the type of headphone to use on most portable powered deck's with low voltage phones outputs. These high ohms phones really need a an AC powered amplifier's headphone jack or even direct connection to 10 watt or more power amp's speaker outputs for realistic (loud) listening. There are headphone amps available for this purpose and some are battery powered with at least (1 or 2) 9 volt batteries, the minimum supply needed for higher impedance phones to work at reasonable loudness.

Bottom line: Phones sensitivity is only a measure of the ability to play loud enough only if comparing phones with about the same ohms impedance that are being driven by the same headphone amplifier.

Best before buying is to try out the phones on the exact equipment you to use while playing typical sounds of interest.

Why are some headphone louder than others?

Loudness is directly related to the impedance and sensitivity of the headphones. Impedance determines how much power the headphones will draw, while sensitivity indicates how much of the electrical signal delivered to the headphones is converted into sound. Because low impedance headphones draw more power from the amplifier they will sound louder, at the same control settings, than high impedance headphones. Also, headphones of higher sensitivity will sound louder than those of lower sensitivity. Check the manufacturer's spec sheet for impedance and sensitivity information.


RaneNote - Understanding Headphone Power Requirements:

RaneNote - Audio Specifications:

post #4 of 12
Thread Starter 
Excellent! Thanls lini and TravelLite.

post #5 of 12
people seem to really get sensitivity and efficiency confused. even one of travellite's references mixed them up.

just to reiterate what lini said, sensitivity is the spl at a standard voltage, while efficiency is the spl at a standard power. because you measure efficiency at a certain power, you need to know the impedance of the driver to know how loud it will play at a certain voltage. (also we shouldn't forget that efficiency and sensitivity are taken at a specific frequency... usually 1khz from what i've come to understand.) here's a bit on the difference between efficiency and sensitivity.

More than likely, those who inquire about "efficiency" are actually concerned with the speaker's sensitivity. It is important to remember that sensitivity and efficiency are two very different things, though they are intuitively related. Most people do not realize that it is entirely possible to produce a speaker which has an extremely low sensitivity figure, yet is highly efficient (at least one electrostatic model, no longer made, fit this description many years ago). People are generally concerned with sensitivity because that tells us how loudly a speaker will play when presented with a standard voltage. It does not, however, take into account the impedance of the speaker, and therefore really tells us nothing about the efficiency, because we don't know how much current is being drawn from the amplifier...

it seems that many manufacters make this mistake a lot too, as i've seen them use the terms interchangably. to make sure whether they are stating efficiency or senstivity check the units. if it's db/mv, then it's sensitivity. if it's db/mw, then it's efficiency.
post #6 of 12
I sometimes make the same mistake as most manufacturers: interchange sensitivity with efficiency. Most of Sennheiser's HD5## series headphones are of medium sensitivity (close to 100dB @ 1 Vrms @ 1kHz) - but their efficiency varies at least somewhat from model to model (for example, the high-end HD580/600 is also of medium efficiency, with a rating of about 94dB @ 1mW @ 1kHz). As for compatibility with portable players, you will want relatively efficient headphones. Unfortunately, many cheap Walkman headphones are of low efficiency (in fact, Sony has consistently overstated the efficiency rating on many of its headphones, giving them higher efficiency figures than what they really are) - so that you'll have to crank up the fscking volume control all the way to maximum just to hear much audio out of those pieces of crap. Couple that with their low impedance, and you'll have a cheap, craptacular pair of headphones that actually drains batteries faster than if you had used a good-sounding, moderately efficient pair of headphones with that same portable player to begin with.
post #7 of 12

getting the impression that for portables ,you want  low impedance and high sensitivity ;but A benefit to a higher sensitivity rating is your drivers will respond very well to a wide range of power output and do not require peak power for good performance. A draw back to high sensitivity is the drivers will not take a lot of abuse (listening at high levels of distortion, etc) . Lower sensitive drivers are more power hungry and force you to give them the full power to receive the full performance. The benefit to low sensitivity is that their simply harder to damage

post #8 of 12

I find it almost unbelievable this is a 10 year old bump....

post #9 of 12
Originally Posted by Darkblade48 View Post

I find it almost unbelievable this is a 10 year old bump....


Imagine how markl will feel if he sees this. Wonder how many posts he had at the time...

post #10 of 12

So is it safe to assume that my Sennheiser HD 598s will be less than 112Dbs when running through my Titanium HD's headphone out, rated at 1Vrms?

post #11 of 12

So, by this measure of sensitivity (sound dB / mW) you double your mW of power, and you double your sound dB.   For example, if your sound dB is 0, and you apply 100 times the power, you still have 0 dB.   Yeah, that makes sense.


How might one measure the dB of sound output by the headphone?

post #12 of 12
Originally Posted by K6YVL View Post

How might one measure the dB of sound output by the headphone?

You can calculate the dB SPL output of the headphone by using it's base sensitivity figure and then adding (or subtracting) based on how much power you give it. You need to know how dB adds based on the power. Here are a couple of rules that will help with that:

1. Doubling input power adds 3 dB.
2. Multiplying the power by 10 adds 10 dB.

Let's do a few examples. We will pretend that the headphones we are talking about produce 102 dB SPL with 1 mW of input power.

1. What's the SPL if we give it 2 mW?
2. What is the dB SPL for 10 mW?
3. How about 20 mW ?
4. ...and what about 4 mW ?

Scroll down for answers....

1. The base for these calculations is 102 dB with 1 mW. 2 mW is double of 1 mW. We know that doubling the power adds 3 dB, so we will get 102 + 3 = 105 dB SPL from 2 mW.
2. 10 mW is 10 times 1 mW, so we know we can add 10 dB. 102 + 10 = 112 dB SPL.
3. 20 mW... hmmm... Now we have to combine some rules. We can get from 1 mW to 20 in two steps. 1 x 10 = 10. 10 x 2 = 20. So we use the 10 x rule first and then the 2 x rule. 1 -> 10 gives us +10 dB. 10 -> 20 gives us +3 dB. So: 102 + 10 + 3 = 115 dB SPL.
4. We can get from 1 mW to 4 mW by doubling twice. So we get to add 3 dB twice. 102 + 3 + 3 = 108 dB SPL.

I hope that helps you understand dB calculations a little better.

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