monnay
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Joined: Wed Sep 16, 2015 7:58 am

Maximum Audio Output

Wed Sep 16, 2015 8:13 am

Hi everyone,

I'm trying to produce a sound with a specific dB SPL value. How can i achieve this?

Thanks

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Burngate
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Re: Maximum Audio Output

Wed Sep 16, 2015 11:38 am

SPL, aka sound pressure level, implies measurement of instantaneous pressure (normally air, but could be any fluid or even a solid)
Since the Pi is electronic, there must be some conversion between current flow and pressure - speaker one way, microphone the other. Neither is 100% efficient.

Maybe a bit more info could be useful?

monnay
Posts: 5
Joined: Wed Sep 16, 2015 7:58 am

Re: Maximum Audio Output

Wed Sep 16, 2015 1:00 pm

Burngate wrote:SPL, aka sound pressure level, implies measurement of instantaneous pressure (normally air, but could be any fluid or even a solid)
Since the Pi is electronic, there must be some conversion between current flow and pressure - speaker one way, microphone the other. Neither is 100% efficient.

Maybe a bit more info could be useful?
Hi Burngate,

Thanks for your reply. I will use a headphone directly without a power amplifier in between. Headphone spec tells me 108 dB SPL, i assumed its maximum is that. But with which voltage level it reaches 108 dB SPL i do not know. I need help on the maximum output voltage of audio jack.

Thanks

klricks
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Re: Maximum Audio Output

Wed Sep 16, 2015 1:40 pm

The RPi produces 'line level' output same as any other audio equipment... TV, PC, etc.
https://en.wikipedia.org/wiki/Line_level
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

ghans
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Re: Maximum Audio Output

Wed Sep 16, 2015 3:34 pm

You cannot drive your headphones satisfactorily from the Raspberry Pis audio output , not directly at least.

ghans
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Burngate
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Re: Maximum Audio Output

Wed Sep 16, 2015 5:28 pm

monnay wrote:Headphone spec tells me 108 dB SPL, i assumed its maximum is that
Probably depends on what the manufacturer thought the customer would want. :(

Headphones can vary in impedance by an order of magnitude, amplifiers can also vary in power out of the headphone socket by a similar amount, and people's ears probably vary by twice that.
Then the efficiency of the transducer could be anywhere from 10% to 90%, better-sounding ones being less efficient
H&S regulations put a limit on sound levels for the general public, so headphone manufacturers will probably design them to approach (but not exceed) that level, for some amount of distortion, when some undefined signal is applied. Putting a larger signal in may destroy your hearing, may clip (and sound horrible) or may destroy the voicecoils. Or may not.
TL:DR there's almost no correlation between what you put in and what you hear.

The Pi can put out about 1v from 100Ω, which isn't very much. If your headphones are efficient and have between 32Ω and 200Ω, and the room's quiet, you may hear something.
Otherwise you'll need an amplifier.
My own amplifier has an output stage driven from a ±25v supply.
That's enough to have blown my speaker coils, but only when I turned it right up - but then the volume control is a five-turn pot. Turn it up to 11? mine goes to 55! :evil:

monnay
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Joined: Wed Sep 16, 2015 7:58 am

Re: Maximum Audio Output

Tue Sep 22, 2015 5:33 am

Thanks everyone for their responses. I will work out something depending on the sound output :)

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