Talk:Sound intensity

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Although the definition given here can be found in many books (e.g. Norton and Karczub), the time integration is misleading. Active sound intensity is quantity in the frequency domain. By the definition in this page, active intensity is a number independent of time and frequency such that the intensity 'on 1000Hz' is not defined. The reason that the definition has stuck is that many accousticians 'assume that the singals are sine waves' rather than taking a proper Fourier transform. In practice, sound is a broadband phenomenon so I think we should change to a proper definition.

I would suggest that the discussion is started with the definition of the energy flux: $p(t)\cdot v(t)$, and then generalized to the form which is used in measurements: the cross-spectrum between p and v.

Normally, I conclue my comments on wikipedia with the comment that I'm not an expert and I am reluctant to change anything. Since acoustics is my job, I'd be happy to write this one if anyone agrees the current version is misleading.


WikiProject Physics / Acoustics  (Rated Start-class, Mid-importance)
WikiProject icon This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Start-Class article Start  This article has been rated as Start-Class on the project's quality scale.
 Mid  This article has been rated as Mid-importance on the project's importance scale.
This article is supported by Acoustics Taskforce.
 

People seem to get confused by dB vs linear units. When you express sound intensity in dB (giving "sound intensity level"), the physical quantity being expressed is still intensity. dB is just a convenient way of expressing the relative intensity in a convenient, dimensionless form. It's like a change of units. Instead of expressing intensity in watts per square meter, you could express it in horsepower per square foot. Although that would be a strange choice, it doesn't change the physical quantity being expressed.

Note that sound pressure level also started out as a measure of power, not pressure, since the formula is L_\mathrm{p}=20\, \log_{10}\left(\frac{p_1}{p_0}\right)=10\, \log_{10}\left(\frac{p_1^2}{p_0^2}\right), and p2 is proportional to the power per unit area of the sound wave. If the acoustic impedance were always constant, dB-SPL would be a measure of intensity, even though it is based on the pressure. Of course, the impedance is not always constant, so in practice dB-SPL indicates what the intensity would be if the impedence were fixed.

In practice, it is important to distinguish between dB-SIL, dB-SPL, dB-SWL, etc., because they have different reference levels and because different measurement methods may be implied by the choice of units. Obviously if you have a detector which directly measures pressure, it makes more sense to express the results in dB-SPL than in dB-SIL.--Srleffler 04:27, 28 November 2005 (UTC)


As I've said on some other acoustics pages - it's important to remember that sound intensity is a vector and sound pressure is a scalar. So it is not generally true to say that "If the acoustic impedance were always constant, dB-SPL would be a measure of 'intensity'" because one requires the specification of direction and the other doesn't.—The preceding unsigned comment was added by Richardng (talkcontribs) .


Strength[edit]

Wikipedia policy discourages linking directly to a disambiguation page, unless it's the amorphousness of the term that's under discussion. "Strength" appears to be used in a somewhat-specific sense in the article, but I can't figure out what precisely is meant, and which article I should re-link the word to. Signal strength? Would someone with expertise help, please? Sanguinity 19:23, 24 August 2006 (UTC)

Following the lead set on "Intensity", I'm redirecting "strength" to the wiktionary entry, and will do the same on the other sound-measurement pages (phon, sound pressure). Again, if a particular sense of "strength" is meant, please re-link as appropriate. Sanguinity 18:37, 25 August 2006 (UTC)

Intensity at r = 0?[edit]

I came to this page already familiar with the inverse square relationship between distance and intensity, hoping to find something that could be of use to me in a little simulation I'm running regarding interference. Anyhow, that inverse square relationship makes the intensity shoot off to infinity as you approach zero, which doesn't seem right (I mean, I can stick a buzzing fly in my ear and I certainly won't suffer any hearing loss as a result). Surely there is some way to calculate intensity starting from the source. I think this article could use something that would help explain that. Eccomi (talk) 19:50, 18 May 2008 (UTC)

Been a while since I've done the math, but I'm pretty sure that you take the total power and divide by the total area of the sound source. For instance, a vibrating 12in loudspeaker with a 60W driver would have a surface area of \pi r^2=0.072 m^2. The max intensity would then be 60W divided by this area. I think. Like I said, it's been a while.Gunblader928 (talk) 00:05, 24 January 2013 (UTC)
Where you're going wrong is that you are assuming a point source and that assumption breaks down when the distance from the source becomes comparable to the actual size of the source. -—Kvng 15:48, 26 January 2013 (UTC)