Re: Real-time sound cyphering algorithm

From: David Eather (eather_at_tpg.com.au)
Date: 05/31/05


Date: Wed, 1 Jun 2005 01:38:56 +1000

Unruh wrote:
> "David Eather" <eather@tpg.com.au> writes:
>
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>
>> Jan Panteltje wrote:
>>> On a sunny day (Tue, 31 May 2005 00:28:50 +1000) it happened "David
>>> Eather" <eather@tpg.com.au> wrote in <429b2367@dnews.tpgi.com.au>:
>>>
>>>> http://en.wikipedia.org/wiki/Sound_unit
>>>>
>>>> Forty dB is a factor of 100.
>>> For voltage only.
>
>> No. Your own entry into the reference stakes:
>
>> http://encyclopedia.laborlawtalk.com/Decibel#Acoustics
>
>> quote:
>> Acoustics
>
>> The decibel unit is often used in acoustics to quantify sound levels
>> relative to some 0 dB reference. *The reference may be defined as a
>> sound pressure level (SPL)*, commonly 20 micropascals (20 Pa). To
>> avoid confusion with other decibel measures, the term dB(SPL) is used
>> for this. *The reference sound pressure* (corresponding to a sound
>> pressure level of 0 dB) can also be defined as the sound pressure at
>> the threshold of human hearing, which is conventionally taken to be
>> *210-5 newton per square metre, 210-5 N/m or 20 micropascals*.
>> That is roughly the sound of a mosquito flying 3 m away. *The ears
>> are only sensitive to sound pressure deviations.*
>
>> end quote:
>
>> As you can see all the references to volume (acoustic intensity) and
>> its perception are
>> measured and defined in terms of pressure. The correct unit is
>> dB(spl)
>
> Actually the definition of dB is in terms of energy rate per square
> meter. the zero reference is 10^-12 watts/m^2. (this is the threshold
> of hearing from some standard good listener.) Every factor of 10
> increase in energy is 10dB (or 1Bel).
> For air, with a velocity of sound of 340m/sec, and density of about
> 1.4Kg/m^3, this corresponds to a pressure of about 2 10^-5 P. Note
> that because the energy rate in a sound wave is proportional to
> pressure^2, this means that every factor of 10 in pressure is a
> factor of 100 in energy or 20dB. But the fundamental definition of dB
> is in terms of energy not amplitude.
>
> Note that in water, with its much higher density and velocity of
> sound, the relation between energy and pressure is very different.
> This has caused immense confusion in the definition of dB in water
> (beause of the difference in reference energy rate per unit area and
> pressure).
>
> Ie, 120dB in water can mean either 1 watt/m^2 or a pressure of 20P
> which corresponds to an energy rate/m^2 of about 3 10^(-4) watts/m^2
> (ie 33dB less).
>
>
>>> But 2 x voltage results is 4 x power in same impedance (speaker).
>>> P = (U * U) / R
>>> http://encyclopedia.laborlawtalk.com/Decibel#Acoustics
>
>> I don't wish to embarrass, but I am tertiary qualified in electrical
>> engineering. Ohms law and derivatives of it are not exactly new to
>> me. I could have defined Pi at the beginning and claimed it gave me
>> some sort of authority. Everyone would then rightly point out its
>> irrelevance to measuring a sound's volume.
>
>> We are talking about recording a sound with a microphone.
>> The microphone is like out ear only sensitive to pressure variations.
>
>> Talking about power when the subject is volume is irrelevant. If I
>> have a speaker that produces 86dB(spl) at one meter with one watt
>> (typical test parameters). What are you going to tell me about the
>> volume at 2 meters? One watt or 80dB(spl). One answer is nonsense,
>> one answer has meaning.
>
> The volume (intensity) of a sound IS power per m^2.

So the above (top) reference,(provided by someone else) that states clearly
that acoustic intensity is a pressure, not a power measurement is wrong?
Wiki says the same thing - it's wrong too? And Britannica?

quote:
>> The decibel unit is often used in acoustics to quantify sound levels
>> relative to some 0 dB reference. *The reference may be defined as a
>> sound pressure level (SPL)*, commonly 20 micropascals (20 Pa). To
>> avoid confusion with other decibel measures, the term dB(SPL) is used
>> for this. *The reference sound pressure* (corresponding to a sound
>> pressure level of 0 dB) can also be defined as the sound pressure
endquote:

>Your example is irrelevant.

No.

>The 1 watt here is the INPUT to the speaker.

Yes.

>Speakers are incredibly inefficient.

Yes.

>In your example, 86dB is about 4 10^{-4}w/m^2.

Yes. 86 dB(spl) at 1 watt input measured at 1 metre is typical result for a
mid-sized domestic speaker.

> Since a sphere of 1m radius has about 12m^2 area this would mean a
> total sound energy of about 5 10^-3 w. (Ie, the efficiency is only
> about .5%

Yes.
Example -
http://www.wharfedale.co.uk/model.php?model_id=17
Larger and 3 db more efficient
http://www.wharfedale.co.uk/model.php?model_id=1

Believe me now?

Drivers used in public venue work, where flat smooth response from 20 - 20k
is not required are more efficient - about 96db(spl) 1watt @.1 metre
wouldn't be a surprise. You further boost the output volume by using horns
or stacks/arrays.

>conversion from input energy to sound, assuming uniform
> angular radiation).
>
>> A conversation with the same error as BO's could go like this (sorry
>> I can't think of the crypto equivalent)
>
>> B1 "Power equals volts multiplied by current"
>
>> B2 "Yeah great. What's the car battery voltage?"
>
>> B1 "Five thousand watts"
>
>>>> OP wrote>
>>>> I need the cyphered sound to be audible, but not understandable
>>> I think this can also be done by using a say 5kHz audio bandwidth
>>> (more then enough for speech), and AM modulating a carrier at
>>> 20kHz. The lower sideband will go to 20 - 5 = 15 kHz.
>>> You will hear some high peaked noises.
>
> Actually due to the non-linearity in the ear you are liable to hear
> the original sound. As cats-whisker radio recievers found, a very
> simply non-linear elemnt ( the diode of the cat's whicsker) allowed
> one to hear the radio broadcast very well.
>
>>> An other alternative is to use a FM carrier, and narrow band FM (so
>>> use only one sideband), and use 10 kHz carrier :-)
>
> again a phase shifter (which loudspeakers are really great at) and a
> non-linear element ( the ear) would again allow the sound to be heard.
>
>
>>> This requires some filtering at the rx side, but any dsp board
>>> should
>
>> I like that idea. I would just suggest that it will not suit the OP
>> in terms of resources and ease of implementation
>
>>> do. FM has the advantage that it is not so sensitve to amplitude
>>> changes.
>>> The possibilities are endless.



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