Whats the SPL at source (@0 m) when SPL is 90dB@1M?

[wonderfulaudio]

The subject says all.

 

[David McBean]

The source output power is 0.0121 watts.

To calculate the SPL you need to know the source surface area.

 

[JonSnell Electronic]

A meaningless question.

 

[wonderfulaudio]

The source output power is 0.0121 watts.

To calculate the SPL you need to know the source surface area.

Lets assume a 1" fullrange driver, about 5 cm2, for the sake of understanding.

 

[DampedAndDriven]

Don't you have to use some small positive distance? Shooting from memory here, but the equations I've seen for pressure (and hence SPL) as a function of distance would go to infinity at distance from source = exactly 0.

 

[steveu]

For a radiating area much larger than 1m then the answer is 90dB, and increases to infinity as the driver size decreases to zero, point source. But, of course, the limits of air pressure total vacuum put a limit of about 200dBA on any sound.

 

[David McBean]

Lets assume a 1" fullrange driver, about 5 cm2, for the sake of understanding.

SPL = 134 dB

Assumptions:

Velocity of sound = 344 metres per second
Density of air = 1.205 kilograms per cubic metre
Solid radiation angle = 4Pi steradians (free space)

 

[wonderfulaudio]

The question occurred to me while trying to find the SPL as I move closer to the driver. It was baffling since the usual halving of distance will never get us to 0m until infinity, hence I posted the question.

Thanks everyone.

 

[JMFahey]

In theory, SPL should be infinite, and you should have an infinitesimal source, that´s why it´s called a "point" source.

By definition, a point has size/dimension zero.

But of course there is not such a sound source in practice, so you won´t have infinite SPL either.

Personal opinion, others may differ, smallest sound source possible is the voice coil diameter, either moving a rigid dome (this is true in dome tweeters and in a few speakers, such as old JBL, some of which had a rigid aluminum dome glued straight to voice coil, not somewhere on the cone) or driving a small cone slice just around it.

Of course, although not very rigid, a cone behaves quite close to a perfect piston, so actual radiating surface (and diameter) is significantly higher than plain voice coil diameter.

 

[chris661]

When you're close enough to consider the speaker as a plane source, SPL will stop rising with proximity.

Chris

 

[David McBean]

In theory, SPL should be infinite, and you should have an infinitesimal source, that´s why it´s called a "point" source.

"A point source is a small source which alternately injects fluid into a medium and withdraws it." ~ Harry F. Olson - "Acoustical Engineering"

In the example considered earlier the source was assumed to have a surface area of 5 cm2, equivalent to a spherical "point source" of radius 6.3mm (less than 0.37 wavelengths at 20000 hertz). The SPL of 134 dB was calculated at the surface of this source.

 

[David McBean]

The question occurred to me while trying to find the SPL as I move closer to the driver.

SPL2 = SPL1 + 10 * Log10(S1 / S2)

Given:

SPL1 = 90 dB
S1 = 4 * Pi * 1 ^ 2 m2 (surface area of sphere of radius 1 metre)
S2 = 5 cm2 = 5 * 10 ^ -4 m2

Then:

SPL2 = 90 + 10 * Log10((4 * Pi) / (5 * 10 ^ -4)) = 134.00 dB

 

[steveu]

Sound Fields - acoustic glossary - article
"Near Sound Field, that part of a sound field, usually within about two wavelengths of a noise source, where there is no simple relationship between sound level and distance, where the sound pressure does not obey the inverse square law and the particle velocity is not in phase with the sound pressure.

Near Sound Field Definition IEC 801-23-29, sound field near a sound source where instantaneous sound pressure and particle velocity are substantially out of phase"

Web search for "near field effect" and you will find you tube videos and more.