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My understanding of the audio domain is very bad and I apologize for that. In a Wikipedia article, they say in the "History" section that "Those headphones used with early wireless radio had to be more sensitive and were made with more turns of finer wire. Impedance of 1000 to 2000 ohms was common, which suited both crystal sets and triode receivers. Some very sensitive headphones, such as those manufactured by Brandes around 1919, were commonly used for early radio work."

In the "impedance" section, they say that "As the impedance of a pair of headphones increases, more voltage (at a given current) is required to drive it, and the loudness of the headphones for a given voltage decreases".

Aren't these two assertions contradictory? if a headphone with high impedance is highly sensitive, should not a very low voltage be sufficient for hearing something?

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2 Answers 2

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There is no contradiction. You misinterpreted the article. Actually, impedance and sensitivity of a speaker are not directly related to each other. Impedance says how much opposition it provides to the current flow through it, expressed in ohms. Sensitivity says how effectively an electrical signal is converted to sound. It is expressed in dBs/mW. Both these factors together determine the loudness of a speaker, or as you wonder, how well it converts a voltage from amp into an audible sound.

If you have an amp of 8 ohms output impedance, then 8 ohm speaker is needed for maximum power output across it (Impedance matching). Now, the level of sound output depends on this power, and the sensitivity of the speaker. If you have another speaker with a higher impedance 16 ohms and same sensitivity, it will sound less loud for the same input voltage from the amp. Because, the output power of the speaker is lesser and hence the sound too. But here, if the 16 ohm speaker had a better sensitivity, it would have produced a louder sound.

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    \$\begingroup\$ Oh, I understand. They say the speaker had to be very sensitive in one hand, and in the other hand it had to have a very high impedance - and no more. OK. While I think this should be the accepted answer (it answers exactly to the question), I also like the answer of Trevor. \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 9:57
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    \$\begingroup\$ Regarding the assertion "If you have an amp of 8 ohms output impedance, then 8 ohm speaker is needed for maximum power output across it (Impedance matching)", I think you mean "if you have an amp designed for 8Ohm speaker etc." because I have read that the output impedance of the amp should not be above 1/8 the impedance of the speaker. Please, tell me if I am wrong. \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 10:01
  • \$\begingroup\$ Hm ?! I have never read anything like that. The assertion that impedance of speaker and amp should match, comes just from the maximum power transfer theorem. If its less than amp's output impedance, speaker will draw more current from amplifier, which may damage amp. If its more than amp's output impedance, less power is transferred to the speaker. \$\endgroup\$
    – Mitu Raj
    Commented Nov 26, 2017 at 10:08
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    \$\begingroup\$ You can read that in the Wikipedia article cited in the question (section impedance): " Amplifiers are not ideal; they also have some output impedance that limits the amount of power they can provide. In order to ensure an even frequency response, adequate damping factor, and undistorted sound, an amplifier should have an output impedance less than 1/8 that of the headphones it is driving (and ideally, as low as possible)." I've also read that elsewhere. \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 10:46
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    \$\begingroup\$ Thx for having answered me. I will mark your answer as the accepted one. You may want to edit it to include the later remark above. \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 10:54
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Your confusion stems from a bunch of mixed statements from Wikipedia that, when you put them together like you have, seem to contradict each other.

Wikipedia is a wonderful reference site, however, you have to be aware that articles ore often written and edited by numerous people. Each section, and even individual sentences, can be written or edited by entirely different people who will put a slightly different slant or emphasis on certain terms. As such, it is often easy to get into the confusion you find yourself in.

So let me try to give you my interpretation of what it says there.

History

If you look at the this section it tells you how the first headphone were developed before we invented audio amplifiers. The first headphone were just two telephone earpieces hooked together. It goes on to say the early ones were simple moving iron models. Then after some background stuff they mention that they were typically about 75 ohms.

Then they mention that early radio required a much more "sensitive" headphone {They omit to say... because early radio signals were almost not there, and as mentioned, audio amps were still to come}.

The only way to make the headphones more sensitive {In this instance the author is talking about amplify the sound more so you can hear it}, was add more turns to the voice coil. That made them high impedance headphones which was fine for the high voltage, high output impedance, radios they were used with.

As such, here it is not really saying more impedance = more sensitivity by the later definition. At the time, in order to get more volume, they had to endure more impedance.

Sensitivity

Further down the article it goes on to explain what we now call the sensitivity of headphones and speakers.

Sensitivity is a measure of how effectively an earpiece converts an incoming electrical signal into an audible sound.

It is measured in db/V.

This is actually a different definition of sensitivity from the one mentioned in the history section, presumably by a different author or editor. This definition is more quantitative and less qualitative. The headphones used for radio did give out more sound than their predecessors, but had to use very high voltages to do so. As such, technically, the early radio headphones were actually LESS sensitive by this definition.

Impedance

This section is fairly easy to interpret. It is basically saying if you use 32 ohm headphones on an amp that was designed for 8ohms or 4ohms, they will be a lot less sensitive, by the second definition, that is less db per volt of signal... i.e. quieter.

The section also goes on to say, as technology has evolved we are using smaller and smaller voltages with less and less impedance headphones.

As such the sensitivity of headphones, db/V, is far greater today than the early radio models.

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  • \$\begingroup\$ Thank you Trevor for answering me. I don't really understand when you say The only way to make the headphones more sensitive "{In this instance the author is talking about amplify the sound more so you can hear it}, was add more turns to the voice coil." In what sense adding more turn can amplify the sound? For example, I have an old grid dip which also work with headphones. They say the headphone must be at least 4000Ohm impedance. I scoped the signal and found 8 mVpp. How is the 4000Ohm impedance susceptible to amplify this signal? \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 9:10
  • \$\begingroup\$ @MikeTeX the headphones were a voice coil in a magnet, For the fixed current they had available, in order to make the coil move more, you had to add turns to the coil to create a bigger field, and more force to move it. \$\endgroup\$
    – Trevor_G
    Commented Nov 26, 2017 at 9:13
  • \$\begingroup\$ are you saying that the input to the headphones acts more or less like a current source and only the voltage varies? \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 9:15
  • \$\begingroup\$ I have incorrectly expressed my question above. Are you saying that the current drawn by the headphones is independent of the impedance of the headphones? \$\endgroup\$
    – MikeTeX
    Commented Nov 26, 2017 at 9:22
  • \$\begingroup\$ @MikeTeX i'm not saying either of those. I am saying in order to make the voicecoil give more sound for the signal they had available they had to increase the magnetic effect of the coil. \$\endgroup\$
    – Trevor_G
    Commented Nov 26, 2017 at 9:28

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