# Audio Question of the Day #1



## mbenonis (Jan 27, 2005)

What is wrong with the following (abbreviated) set of specifications for an imaginary piece of audio equipment? [Adapted from the Yamaha Sound Reinforcement Handbook - and don't cheat!]

Frequency Response: 20 Hz to 20 KHz
Output Noise: Better than -115 dB
Input Impedance: 600 &Omega;
Input Sensitivity: 0 dBV
Maximum Output Level: +24 dBm
Output Impedance: 10k &Omega;
Dimensions: 10" W x 0.75" H x 12" D


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## ship (Jan 27, 2005)

Excellent idea for a test! Me, it's Greek to of course.


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## Radman (Jan 27, 2005)

Can there be negative decibels? I don't think so!


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## JahJahwarrior (Jan 27, 2005)

um, what exactly is meant by the "noise: better than -115db" ?

I think you could have negative decibals but I'm not sure if you can have that many of them  I also don't know what the whole output noise is. is that like THD? If it is than why isn't it listed like it is for amps, more like "less than .X% THD at _____ watts" or something? 

and I know this thing can't be an amplifier because it has an output of 10k ohms which is not a normal speaker impedance  I think 600 ohms is around a lo z mic XLR impedance, and the 10k sounds more like a hi-z signal. What is the input signal was higher than 24 db? and I've always wondered, what is the input sensitivity? I know there is aknob to select that on the front of a peavey amp I have, but I don't know what it means. It's possible that 0 db insensitivity doesn't work. I might do a google search. 

and did you mean to type "dbm" on the output level? 
t


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## JahJahwarrior (Jan 27, 2005)

I looked it up and it seems to me the problem might lie with the fact that an input sensitivity of 0db would in effect mute the input. 

but I might be wrong, it might also mean that the input is not affected at all. I looked up what input sensitivity is and I think I understand it better, but I still am left with two options, one which says 0 db is muted, the other says it's just not touching the sound at all.


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## RelativeMischief (Jan 28, 2005)

JahJahwarrior said:


> I still am left with two options, one which says 0 db is muted, the other says it's just not touching the sound at all.



0 dB SPL would be no sound. As in there are no vibrating air molecules, so there is no sound.

However, since this is input to a device, the signal is electrical, meaning that 0dB is a gain difference of absolutely nothing.

I'm going to agree with Chris in that the problem with these specs is the missing dB reference for Output noise.


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## RelativeMischief (Jan 29, 2005)

okay okay I exagerated a bit on the no molecular movement part. I had been up for 46hrs when I posted that, I wasn't thinking straight. However I did make a reference to "dB SPL". The SPL ratio is measured against the threshold of sound, it's quite specific.


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## mbenonis (Jan 30, 2005)

OK guys, let's see how we did here:


JahJahwarrior said:


> um, what exactly is meant by the "noise: better than -115db" ?
> 
> I think you could have negative decibals but I'm not sure if you can have that many of them  I also don't know what the whole output noise is. is that like THD? If it is than why isn't it listed like it is for amps, more like "less than .X% THD at _____ watts" or something?
> 
> ...



I believe some people have already answered some of these questions - see below. And yes, I did mean to type dBm.


Chris Hindle said:


> > Output Noise: Better than -115 dB
> 
> I take dB in this context to mean a change or difference. Missing is the reference. -115 compared to what.
> The assumption might be taken that the noise floor is 115 dB below the clipping level at some stage of this magic box, but without the qualifing or context of the measurement, it is just marketspeak.
> ...



Right and Right.


Anonymous said:


> I think the problem here has to do with the impedance ratings. The input impedance should be alot higher, I think and I am not sure if output impedance is even a real thing.



The impedance ratings are indeed part of the problem. The numbers are not the problem however. And yes, there is a such thing as output impedance - however, perhaps it isn't as clear as it should be...

By the way, it seems nobody has touched the dimensions - those aren't entirely correct either.


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## avkid (Jan 30, 2005)

0.75 inches high? that is ridiculous,diameter of 12 inches,it would look a giant tea saucer!


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## SuperCow (Jan 30, 2005)

I think the 'D' there stood for depth, not diameter.


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## TechnicalDirector3-W (Jan 30, 2005)

no it looks to be .75inches in depth or height, the Diameter looks to be 12 inches which would make sense..


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## mbenonis (Jan 31, 2005)

It's .75 inches high, 19 inches wide, and 12 inches deep. The 10 inches wide part of the original post was a mistake on my part, and is not related to the issue with the dimensions. Neither is the way it was written an issue - it's something less obvious that is wrong with the dimensions.


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## avkid (Jan 31, 2005)

they aren't in metric(ha)


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## Scooter (Jan 31, 2005)

JahJahwarrior said:


> I looked it up and it seems to me the problem might lie with the fact that an input sensitivity of 0db would in effect mute the input.
> 
> but I might be wrong, it might also mean that the input is not affected at all. I looked up what input sensitivity is and I think I understand it better, but I still am left with two options, one which says 0 db is muted, the other says it's just not touching the sound at all.



that is what caught my eye


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## soundman1024 (Feb 1, 2005)

Same here Scooter


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## TomyN (Jan 18, 2007)

Hi, 

what about a DI - Box? (But 0dBV seems to be rather high input level)

Tomy


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## BNBSound (Jan 19, 2007)

Frequency Response: 20 Hz to 20 KHz
*the k shouldn't be capitalised*
Output Noise: Better than -115 dB
*Generally input noise is the spec and it usually pertains to a specific impedance*
Input Impedance: 600 &Omega;
*If it's something other than that Firefox doesn't render the code for the Ohm symbol properly that one seems 
like it's a little low, a more typical "Low Z" input would be more like 1.2k to 2k ohms*
Input Sensitivity: 0 dBV
*This input has no dynamic range whatsoever*
Maximum Output Level: +24 dBm
*dBm is a ratio referenced to 1 milliwatt, the + is extraneous, and converted to an actual value 24dBm is
250mW so this box might be useful for driving your headphones (cans) and not much else*
Output Impedance: 10k &Omega;
*Got me on this one... it's getting late.*
Dimensions: 10" W x 0.75" H x 12" D
*Usually it's Length, Width, Height.*


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## Chris15 (Jan 19, 2007)

Realising that this is an old topic so much of the discussion has come to an end, but as for the dimensions, 0.75" is rather strange... Given the 19" width, one assumes that this mythical piece of equipment is rack mountable. However, 1 RU is 44mm, about 1.75" If my conversion is correct. So we are talking about something that is less than half a rack unit high... That might be a problem.


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