High Sensitivity HeadPhones, new direction

I have a pair of what looks like "old timie" headphones that i bought maybe 30 years ago from one of those catalog places that catered to "old radio" stuff. The headphones are by Newcomb audio products of Hollywood CA and measure 550 ohms DC. The caps do not screw off - VERY tight so cannot say what the guts look like.

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=A0Thanks, MikeK

Dont even think about reusing enamelled copper wire

NT

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If you want maximum sensitivity, the last thing you want is any damping. Damping reduces output massively. Early moving iron horn speakers had no damping, in band resonance, linearity issues etc, but were still quite listenable, despite the quality. Or would that be horribility.

NT

Tabby wrote in news: snipped-for-privacy@30g2000yql.googlegroups.com:

Only if you are interested at output at specific single frequencies.

What do early moving iorn horn speakers that had no damping, in band resonance and linearity issues have to do with earphones for use in a crystal radio?

snipped-for-privacy@30g2000yql.googlegroups.com:

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i didn't imagine my crystal radio headphones had flat frequency response over the audible spectrum, but i don't think for an application where the headphones are powered solely from an unpowered device where the energy driving the headphones is what is scooped outa da air, i don't think you want any resistive components that don't come naturally with the wound coils or in the crystal diode (or the headphone windings). perhaps there is a transformer for optimal impedance matching and maximum power transfer, and there is some loss in that transformer.

but you don't want any damping put in there. you deal with the resistance you can't avoid (and, perhaps, make use of it to broaden out some frequency response).

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snipped-for-privacy@30g2000yql.googlegroups.com:

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no, I mean for audio speech & music use.

Point is they had the downsides of no damping, yet were good enough for speech & music. Ie it is realistic to use such an approach to maximise sensitivity for speech & music.

NT

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=A0 =A0Thanks, MikeK

Geeze, you've shot my whole approach. I must have about ten different sizes or recycled enamel wire. I haven't just thought about it, I've done it. It's true that some windings are better than others for reuse though.

I have wound a few toroids, and hear that the level of detail does descend down to whether the copper is close to the core, which for the rectilinear cross section forms poses a problem for any heavy wire, but certainly work hardened will be the worst. I've heard of guys annealing the stuff, but I haven't gotten that far with it. Any tips are welcome; often just a few short words go beyond any reference material, particularly when recycling parts. Anyway, the DIY approach will always be more enlightening; partially for the small sidetracks and sensetivities that are learned along the way, and though we may not get as much done the discoveries along the way are worthwhile. The stepper motor wire that I have is on a nice diameter roll, straight out of the motor.

For this project of the large diaphragm headphone I think there is an interesting puzzle of selection of the winding core, and what it's proper material and diameter should be. It is possible that winding, say three, smaller cores could be superior to the single large core, which could then also give some nice experimental feedback if there is variation amongst the three cores. Particularly the ability to (de)tune the diaphragm will come in here in terms of resonances and so forth and placement of coils. This is a ton of dynamics to play with. The old style headphones use two coils off center, on small rectangular iron cores. I've been wanting to build a transducer for some time and one day I'll get to it; probably from a coffee can, and admittedly the first will just use one coil, where max gain is likely at the fundamental frequency of the diaphragm, which then should be at say 600 Hz or so. This would be an ideal CW transducer, and I don't know why they are not on the market; tune it right up!

- Tim

Classical room acoustics (the earcup on head is a room for this discussion) has it that regardless of the volume, the sound level in a room is calculated directly from the sound power inserted and the absorption, volume does not matter (!!). In decibels,

SPL = PWL-1*log(A) + 6 (A in square meters)

SPL = PWL-10*log(A) + 16.3 (A in square feet)

One picowatt is 0dB PWL

Assume the cup and ear to have a surface area of about 30 square inches (0.2 square feet), with an absorption coefficient 50%, A = 0.1, so the 10*log(A) is -10 dB.

This leaves us with 6 dB of SPL. At 1 or 2 kHz, this is ( for a good ear, fellas) about at threshold...

Now, that's 1 picowatt of Sound Power... Considering audio transducer conversion efficiency of 10%, the electrical-audio-frequency power needs to be 10 picowatts of audio ac. Figure a factor of ten for demodulation (in)efficiency of 10% ups this level to 100 picowatts.. The RF power that engendered that, let's say ten times more, or 1,000 picowatts (one microwatt).

Therefore AM RF absorbed or captured by the crystal set and its antenna must be more than one microwatt of RF energy.

I think we can assume that he earcup volume SPL is uniform throughout for frequencies up to about 2 kHz. The ear canal resonance will be as it is for a open ear.

Ange

And there is the where I started! How do I increase that 10% to 20% or 30%? The easy 1 to 3db is to eliminate the impedance transformer, so make the speaker impedance high, with a tapped coil (for matching) on the speaker. (I don't know if it is feasible to make a speaker coil with 500kohm impedance) Or a multi peizo speaker with series/parallel switching arrangement to alter the impedance. All this used in the gallows style diaphragm. Thanks, MikeK

Proper enclosure design.

Good Luck! Rich

"Angelo Campanella" wrote in news:ibebte$glo$ snipped-for-privacy@news.eternal-september.org:

No reason to go any further, because your numbers thus far don't comport with reality. One of the more efficient headphones, the Senheiser HDA-280, produces 117dB SPL at the eardrum at 1kHz for 1mW of electrical input, or equivalently, 27dB SPL at the eardrum at 1kHz for 1pW electrical input. So, without going into why, the fact of the matter is that your calculation thus far is in error at least 20dB.

Figure a factor of ten for demodulation (in)efficiency of 10%

"Angelo Campanella" wrote in news:ibebte$glo$ snipped-for-privacy@news.eternal-september.org:

If I understand your analysis correctly, your bottom line is that 10pW of electrical power is required to produce 6dB SPL at 1kHz at the eardrum. If so, there is no reason to go any further, because your numbers thus far don't comport with reality.

One of the more efficient headphones, the Senheiser HDA-280, produces

117dB SPL at the eardrum at 1kHz for 1mW of electrical input, or equivalently, 37dB SPL at the eardrum at 1kHz for an electrical input of 10pW. So, without going into why, the fact of the matter is that your calculation thus far is in error at least 30dB.

Remember, guys, these are all WAG's, just to get an analysis framework up.

We can tweak as we go along.

From one milliwatt to one picowatt is -90 dB . OK. = 27dB for 1 pw.

Not too bad for a beginning system analysis, I say... This relates to the audio sensitivity that can be achieved.

37 dB @ 1 kHz is nicely audible in a quiet room, so that the HDA-280 should be useful to use with a crystal receiver. But what is the input impedance of that headset? if it is 8 ohms or even 50 ohms, there will be a impedance mismatch of as much as 30 dB....

Ange

I think you are missing the fundamental quandary,The major cause of inefficiency of a speaker is poor impedance matching between the speaker cone and the wave you are trying to propagate in the air. It is NOT the mismatch between an amplifier and putting power into the voice coil or diaphragm.

Bill

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An old man would be better off never having been born.

It's not that I'm missing it, you'll note I said; "The >easy< 1 to 3db is to eliminate the impedance transformer". The quandary is that I don't know how to do the mechanical matching. As far as the piezo, I think that is just a matter of picking the right piezo? (assuming a completed bender) Thanks, MikeK

Is this a good assumption? Will the audio output SPL be linear with the power input? Is the answer the same for all transducer types? Thanks, MikeK

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THIs from my previous respoNSE

"The big question here is: Do phones that produce 125 dB SPL when 1 mW is applied to them . . . produce 35 dB SPL when 1 pW is applied ?

1 pW is 90 dB below 1 mW. I believe 35 dB SPL should be very comfortable listening. 1 pW audio might be what a crystal radio would deliver from a weak DX signal.

Much debate has transpired in the past over this issue, without any definite conclusions. Anecdotally speaking: some Sennheiser "120 dB/mw" earbud-type phones failed miserably on weak DX when compared to a good set of sound-powereds."

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"Angelo Campanella" wrote in news:ibfs2k$sbn$ snipped-for-privacy@news.eternal-september.org:

The nominal impedance of the HDA-280 is 37 ohms. Because of the relatively low impedance, it is not a headphone that I would be my first choice for this application. I simply used it as an example of the efficiency that is attainable using current technology in the design of a dynamic headphone. Also, increasing the impedance of the headphone with a transformer is problematic even if the transformer were ideal. The reason is because for every N^2 increase in impedance that the transformer provides, there is a

1/N decrease in overall transformer plus transducer sensitivity when measured in terms of eardrum pressure per volt applied to the transformer. I don't know if there is an N that optimizes overall transformer plus earphone sensitivity for a known earphone impedance and a desired load impedance on the diode demodulator. Perhaps someone else can work out the math.

It is typically very difficult to get broadband matches when there is a fundamental mismatch. That applies to antennas, multilayer thin-film antireflection coatings, and speakers.

Bill

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An old man would be better off never having been born.

Just everything, "Answerman". Oddly nobody here seems to have noticed that the famed "gallows" headphone is nothing but a re-invention of the standard old-time telephone earpiece. And I'd point out those things are QUITE sensitive being used for crystal sets from the beginning. The horns in question were ALSO able to work with crystal sets though usually not very loud. The high sensitivity and high impedance of such transducers made them very suitable. "Ear buds" in spite of the sensitivity and not as good because they are low impedance (require a matching transformer) and they are designed to have wideband flat response. As Tesla demonstrated on many occasions if you really want some action you need resonant devices. Little damping as possible. You are all wandering in the wilderness here.