# Sound waves



## whaleboat (Aug 16, 2010)

As I understand it , the length of a sound wave measured fromm crest to crest is always the same at a given frequency . But amplitude is variable , are there any constants regulating amplitude height ? For instance , if I got a reading of 60 db with a meter using a sound source of 400 hz , and a reading of 60 db using a sound source of 4khz would the amplitude of both sound waves be the same ? Thanks


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## derekleffew (Aug 16, 2010)

Promoting this question to the QotD area.


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## DuckJordan (Aug 16, 2010)

If memory serve's correctly, how we measure amplitude on a scope is the same across the spectrum. whether its 50 hertz or 15k hertz it should as far as amplitude measure accordingly


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## gpforet (Aug 16, 2010)

This is correct, the amplitude is the same regardless of frequency.

However, the percieved volume is a completely different issue. The principles of percieved volume are described in a set of curves call the Fletcher-Munson curves. These charts depict that the human ear is not linear with regards to percieved volume across the auditory spectrum as amplitude increases. In other words, as volume increases, our ears do not perceive these volume changes in a uniform fashion.

It is very important for live sound mixers to understand Fletcher-Munson because the human ear changes in reponse curve and therefore, as the show progesses, and we're driving levels up to maintain an exited state for the audience, we may well be required to make changes to eq to compensate for what these curves describe. While mixing is always done by ear, I've yet to find two sound mixers who mix a show the same way, so, where mixing is an interpreted and personal expression, level changes in program content may require eq changes, and to the mix engineer who understands this, their hands move to the eq, rather than the faders and channel strips, to keep the sound spectrum intact as levels are raised.


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## museav (Aug 16, 2010)

whaleboat said:


> As I understand it , the length of a sound wave measured fromm crest to crest is always the same at a given frequency . But amplitude is variable , are there any constants regulating amplitude height ? For instance , if I got a reading of 60 db with a meter using a sound source of 400 hz , and a reading of 60 db using a sound source of 4khz would the amplitude of both sound waves be the same ? Thanks


Let's see if I can rephrase this. If all else remains constant and with flat meter response (no weighting or filtering), would the peak sound pressure level represented by a 60dBSPL measurement be the same with a 400Hz sine wave source as it is for a 4,000Hz sine wave?

And I'll add, tying this back to a previous QotD, would applying an A or C weighting change the answer and if so, how?


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## whaleboat (Aug 17, 2010)

Let's say the meter is set up 2o feet from a speaker and centered on both the horizontal and vertical planes with an infinate amount of space above and below it . As a sine wave is depicted on paper , with crests and troughs , let's define the crest as a 10db loss and the trough as the same , a 10db loss of SPL from the center line . If the meter read 60db regardless of what frequency was used as a sound source , would the crests and troughs of the wave be at the exact same distance above and below the meter for both 400hz and 4khz ?


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## gpforet (Aug 17, 2010)

SPL measurement tools utilize weighing schemes (A or C) and these are indeed filters which affect the displayed SPL measurement based on spectrum content. An explanation of both Fletcher-Munson and the A-weighed curve are explained here. Here's another link which provides explanation of SPL measurements and the affect of the weight curve on these measurements.


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## gpforet (Aug 17, 2010)

By the way, this discussion is really good stuff, and reveals some indiosyncracies when using SPL meters to measure levels.

A tool like SMAART, using calibrated mics and level, will provide far more insight into what's happening in the room with regards to SPL across the frequency spectrum.


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## yert33 (Aug 19, 2010)

I think the graphical amplitude would be the same for 60 db no matter the frequency as we are talking about a graphical representation of the sound pressure level. A sine wave, right? On a graph where the y coordinate is the amplitude and the x coordinate is frequency. So 60db would be at the same place on the graph. Of course, this is predicated on me understanding the question correctly. Of course, gpforet is correct in pointing out that perceived loudness is another animal, as our hearing is not an SPL meter.

Did I pass?


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## DuckJordan (Aug 19, 2010)

yert33 said:


> I think the graphical amplitude would be the same for 60 db no matter the frequency as we are talking about a graphical representation of the sound pressure level. A sine wave, right? On a graph where the y coordinate is the amplitude and the x coordinate is frequency. So 60db would be at the same place on the graph. Of course, this is predicated on me understanding the question correctly. Of course, gpforet is correct in pointing out that perceived loudness is another animal, as our hearing is not an SPL meter.
> 
> Did I pass?


 
Nope just failed, you had to ask


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## yert33 (Aug 20, 2010)

DuckJordan said:


> Nope just failed, you had to ask


 
Now I'm crying!  Thanks for the brutal honesty, though.....


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## DuckJordan (Aug 20, 2010)

yert33 said:


> Now I'm crying!  Thanks for the brutal honesty, though.....


 
oh don't cry, you were right all the way up until you had to ask if you were. thats where the fail came in.


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