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Obviously, one UL standard that the telephone headset need to be qualified before distributing in US market is Acoustic shock. Check UL item 6.5 for the test requirement.

I see... Thank you for BDJ's website and Johnwiss.. Actually I have tried to find the info. from the website before but I couldn't get the information that I want.. (or maybe I'm not clever enough to understand their 'mathematical explanation') What I want to find out is that, normally the MLS...

I think you mean one-third octave, right? The you can follow Pheon from his one-third octave band table.. I can give you some principle in case you then later hear about one-sixed, one-tenth, etc. octave band.. :-) if one tone has a frequency that is twice another tone then the tones are said...

Can anybody explain the concept of MLS to me? Why this signal has the flat frequency spectrum? Also, How can I calculate the power from this signal?

There are 2 standard for sound level meter (SLM) depend on where are you living. For the European, The IEC Publication 651 is used while in America the standard is ANSI 1.4. They are completely alike except for one important point.. The IEC standard requires sensitivity calibration in a...

If I generate the MLS (Maximum Length Sequence) and feed it into the loudspeaker to measure the frequency response, how can I calculate the input power from this signal?

Try this website.. maybe it's useful for you.. :-) http://www.hvac-toolbox.com/35.html

I do agree with MikeyP. Probably most of the energy of HVAC noise you've measured fall into low frequency range where human ears are less sensitive. The better way to observe the noise character is to use the sound analyser which will give you the sound level per each individual frequency range.