Audio Question of the Day #1

mbenonis

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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 Ω
Input Sensitivity: 0 dBV
Maximum Output Level: +24 dBm
Output Impedance: 10k Ω
Dimensions: 10" W x 0.75" H x 12" D
 

Radman

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

JahJahwarrior

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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
 

JahJahwarrior

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Jul 7, 2004
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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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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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.
 

mbenonis

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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?

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
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.

---

20 to 20K is missing +/- spec
eg: 20 to 20K, +2, -3 db
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.
 

avkid

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

mbenonis

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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.
 

Scooter

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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
 

BNBSound

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Mar 17, 2005
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Alexander, NY
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 Ω
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 Ω
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.
 

Chris15

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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.