Talk:Acoustic impedance

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Distinction has to be made between:

• the characteristic acoustic impedance Z₀ of a medium, usually air (compare with characteristic impedance in transmission lines).
• the impedance of an acoustic component, like a wave conductor, a resonance chamber, a muffler or an organ pipe.

This seems to differs from the electrical definition of impedance in that wave conductors and organ pipes would be considered transmission lines, not components, and would have a characteristic impedance. — Omegatron 18:26, August 26, 2005 (UTC)

Very well observed, Omegatron: the input impedance of a wave conductor will vary with its termination and a great deal of its impedance is explained by transmission line behavior. Think about it in physical terms: you can in fact "look through" a wave conductor. A sound wave within the conductor will be affected by the reflection at the end of the tube and the measured impedance at the entrance will vary accordingly. A closed-end organ pipe is maybe a exception, since its impedance can be expressed as a "fixed" (but still cyclical) component. The termination finds an electrical analogy in a short circuit. In reality however, the acoutical short circuit in an organ pipe is not ideal. The closed end is not optimal rigid and often vibrates. The sound wave will therefore loose some energy and will not be 1:1 reflected.Witger 06:45, 20 September 2005 (UTC)

I think the analogy is actually closer. Electrical transmission lines and impedances are idealisms anyway. I believe a closed and open organ pipe is equivalent to a short circuit or open circuit terminated transmission line. Of course there is no such thing as a perfect short or perfect open, and some leakage will occur. — Omegatron 02:15, 21 September 2005 (UTC)

Hi Omegatron! An open end organ pipe is terminated by what is called a radiation impedance. The term is well known in RF applications. In acoustics it finds applications in solving noise problems in air conditioning systems. The radiation impedance has been also mathematically derived for an ideal sound source in a flat wall (as far as I remember with very complicated Bessel functions). The radiation impedance depends on the radius of the pipe and its proportion to the wavelength. Also exhaust pipe systems of cars benefit from the radiation impedance: this is (one) of the reasons why very noisy engines have several exhaust outlets (though maybe one preceeding muffler casing) and why the tail pipe is usually the longest length of the muffler system. In the particular case of an exhaust pipe: if the open end would be really a "open circuit" with infinite impedance, then the tail pipe wouldn't add up in the insertion loss of the system.Witger 07:35, 21 September 2005 (UTC)

Very cool. That should probably be added to the article. — Omegatron 13:57, 21 September 2005 (UTC)

Is it safe to say that characteristic impedance doesn't change with frequency? I would think that there are limits to the sounds that can go through air, but maybe that's a different concept? Does air have an acoustic absorption spectrum or something when you get above ultrasound? — Omegatron 03:11, 3 May 2007 (UTC)

-Yes characterisic impedance is usually assumed be a constant under normal acoustic conditions. It is not usually considered to be frequency dependent. Unless the the acoustic variables (pressure, density, etc) become extremely large with respect to the ambient characteristics, one can assume that it is constant. There should be a dedicated page on absorption in Air as it can be very complex. emh203 10:57 7, May 2007 (EST)

There is a table of other materials at [1] in rayles that we could convert and include. — Omegatron 03:34, 3 May 2007 (UTC)

As soon as I get some time :-) , there needs to be some more ellaboration of all the types of impedance used in acoustics. This topic is a large source of confusion. I'll try to get everything with proper citations, etc. What is on the page is mostly correct but there needs to be some discussion about how and when the relationships are valid and where the relationships come from. There also needs some topics added such as Radaition impedance of a source. emh203 10:57 7, May 2007 (EST)

[edit] frequency dependence of characteristic impedance

The characteristic impedance of a fluid is density * sound speed. In a dispersive medium the sound speed varies with frequency and all real media are slightly dispersive. As density is independent of frequency it follows that the characteristic impedance must vary with frequency. Thunderbird2 17:05, 12 July 2007 (UTC)

[edit] Impedance confusion

There seems to an error relating to acoustic impedance here. Taking the Kinsler 'fundamentals of acoustics' book as authority (sect 10.4): Rayl is the unit of specific (and characteristic?) acoustic impedance,z, not acoustic impedance, Z, (the units Pa.s/m in http://en.wikipedia.org/wiki/Rayl support this - that section makes this mistake too though). The unit of acoustic impedance is the acoustic ohm (Pa.s/m^3)

Also, looking at the external link http://www.sengpielaudio.com/RelationshipsOfAcousticQuantities.pdf the term acoustic impedance there actually refers to specific acoustic impedance. Presumably people often use the term 'acoustic impedance' to mean 'specific acoustic impedance', but since both are defined here we should keep them distinct. I'm still fairly new to acoustics (and wikipedia), so would appreciate comment before I amend the article. Pgj98r (talk) 11:34, 26 February 2008 (UTC)