Negative decibels?

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   Insanely nooby question incoming but google and other sites are only so helpful...
So I'm confused about what a negative decibel measurement at a certain frequency means ? (i.e -3db at 130hz). I understand that the scale is a reference and that negative decibels means it has less than the 0 reference point but am confused about it's application in loudspeakers and was hoping someone could clear it up.
TIA

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mrcheggman Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 01 April 2018 Status: Offline Points: 39	Post Options Post Reply Quote mrcheggman Report Post Thanks(0) Quote Reply Topic: Negative decibels? Posted: 26 September 2018 at 7:43pm
	Insanely nooby question incoming but google and other sites are only so helpful... So I'm confused about what a negative decibel measurement at a certain frequency means ? (i.e -3db at 130hz). I understand that the scale is a reference and that negative decibels means it has less than the 0 reference point but am confused about it's application in loudspeakers and was hoping someone could clear it up. TIA

DMorison Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 14 March 2007 Location: Aberdeen Status: Offline Points: 1649	Post Options Post Reply Quote DMorison Report Post Thanks(0) Quote Reply Posted: 26 September 2018 at 8:08pm
	Well, as you've said, dB's are always an expression of one level relative to another. So, if we take the average volume a speaker produces across its whole frequency range and call that 0dB, then at the low and high frequency ends of the spectrum, there will be a point where it is no longer operating as effectively and the output drops. When it has dropped 3dB, it is producing half the sound power as it did when at its nominal or 0dB level. There used to be a good article on prorec.com about it - it's not there anymore but I found a saved version (in 2 parts) here: https://faculty.mccneb.edu/ccarlson/vaca1010/vaca1010_cd/db%20part%201.pdf and pt2 here: https://faculty.mccneb.edu/ccarlson/VACA1010/VACA1010_CD/dB%20part%202.pdf. HTH, David.

odc04r Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5482	Post Options Post Reply Quote odc04r Report Post Thanks(0) Quote Reply Posted: 26 September 2018 at 10:26pm
	For power: db = 10log(power in/power out) For voltage: db = 20log(voltage in/voltage out) (Voltage^2 ~ Power) so via log(a^n) = n*log(a) you can see how the 2 are equivalent if you substitute P=v^2 in the power expression. Wikipedia has a pretty good page on it. The answer is that you need to understand what a logarithm means to get dB. But you're on the right track, -dB means that a signal is less in amplitude or power then a prefefined reference point. E.g. we have in audio various standards such as dbu, dBV, dbFs all of which have different reference amplitudes for 0dB. 0dB = your signal is same amplitude as the reference point. The quick answer is that using a dB scale lets you describe large ratios of signal:reference with a simpler numeric scale. We use it for convenience primarily.

TMH Music Members Profile Send Private Message Find Members Posts Add to Buddy List Young Croc Joined: 07 April 2011 Location: London Status: Offline Points: 637	Post Options Post Reply Quote TMH Music Report Post Thanks(0) Quote Reply Posted: 29 September 2018 at 12:39pm
	Let’s take your figures as an example. Making a few assumptions first, let’s say the speaker you talked about in your original post is a small hi-fi satellite speaker, with a 2 inch driver. Also assuming it’s published sensitivity is 93 Db/w/m, meaning that applying a signal of (usually) 1000hz at 1 watt of power, to the input, will produce an acoustic output of 93Db, when measured with the microphone 1m away. Now, according to your supplied figures, if the frequency of the input were lowered to 130Hz, but all other variables remain the same, i.e power input and measuring distance, the acoustic output will have dropped by 3Db to 90Db. Lowering the frequency further will reduce acoustic output even further, as the speaker struggles to reproduce the lower frequencies. Hope this helps
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