• If you ever spot Spam (either in the forums, or received via forum direct message) please use the Report button at the bottom of each post to make sure a Moderator can handle it quickly. Thanks for your help in keeping things running smoothly!

Minus decibel Volume reading on pre amps

  • Thread starter QuestForThe13thNote
  • Start date
Q

QuestForThe13thNote

Guest
Here is a question someone might be able to work out as an academic maths exercise and it's very interesting I think.

my pre amp goes from stating 'min' i.e. No volume, at -80db. When I then click one increment on the volume dial it goes to -79db, hence why I know min is at -80db. The lower the number the louder the volume i.e. -40db is louder than -50db. The display always shows it as minus, then the number and then db (for decibel) after the number.

as I understand it the pre amp signal is attentuated (let's not be too techy but just say suppressed) hence why it's shown in a minus scale, with zero being maximum signal and no volume suppression or attenuation i.e. Max volume. So obviously it doesn't relate to the actual delcibel level as it's a signal db reading, and any db reading from say a soundmeter reading speakers is always positive anyway. But I do understand the signal reading still relates to the db scale which is a logarithmic scale. Ie for every 3 db increase I think the volume doubles. I've been told by Cyrus that it's just maths that determines why it can only get to -80 and not say -90 or -100 or lower.

so the question is can this maths be proven and worked out, and if so can you show me how you'd get there in a calculation. Also does anyone else have pre amps that go down to -80db being minimum, to test this principle/theory/assumption etc. I do find it weird how lots of audiophile manufacturers use this type of minus db scale but I guess once you understand it, it's function seems to make more sense.
 

davedotco

New member
Apr 24, 2013
20
0
0
There is no reason for the volume control of any amplifier or pre-amplifier to follow any logic or reason whatsover. The 'law' of the control is entirely in the gift of the designer as indeed is the way it chooses to display the current setting.

The attenuator style is considered 'techy' so is done for that reason, I would be very surprised if the dB settings are remotely accurate across a range of source components.

Few volume controls are linear, most are simply logarhythmic though digital or switched attenuator can follow any law the designer likes.

Decibels are simply a ratio, they have no absolute value whatsoever, your preamp shows -80dB as being minimum volume, which means that the output is 80 dBs below full output, but we do not know what full output is, neither do we know what the units are, though they will probably be volts.

This kind of volume display may look impressive but it is largely meaningless, what is far more important is that the control is smooth and consistent in nature and that there is sufficient range for volume adjustment to be easy.

Regarding the relationship between volume, sound pressure level and power, this has been explained on this forum in depth, but suffice it to say this...

In a hi-fi situation, increasing the power by 3dB, ie doubling the power, will result in a 3dB increase in measured SPL.

Increasing the power by 10dB, ie 10 times the power, will increase measured SPL by 10dB, which is twice the pressure, simple stuff.

Relating those changes to percieved volume is less easy as volume is subjective, not measured.

On a music signal, doubling the power output will increase the measured spl by 3dB, this a a small but clear increase in volume. To double the volume requires 10 times the power from the amplifier, an increase of 10dB.
 
Q

QuestForThe13thNote

Guest
Thanks for taking time to reply, but I think you've misunderstood the question or are not sure of the answer. I was interested in whether mathematically on a log scale -80db is the lowest you can go and how that could or couldn't be calculated, depending on whether the thinking is right.
 

davedotco

New member
Apr 24, 2013
20
0
0
QuestForThe13thNote said:
Thanks for taking time to reply, but I think you've misunderstood the question or are not sure of the answer. I was interested in whether mathematically on a log scale -80db is the lowest you can go and how that could or couldn't be calculated, depending on whether the thinking is right.
It is a ratio. -80dB relative to what? We don't really know.

Looking at this simplistically, let us assume that the reference is the full voltage of the pre-amp and that this will drive the power amp to full output. From the power of the power amp and the sensitivity of the speakers you can get an idea of the peak output, ie sensitivity of 87dB for 1 watt with approx 26dB of gain from the amplifier gives 113dB maximum.

80dB below full output is therefore 33dB, which is very quiet indeed so a signal at that level would be inaudible in all but the quietest of listening rooms.

Is this what you meant?
 
Q

QuestForThe13thNote

Guest
I think the audible sound pressure level measured in db is different to the electrical signal db level of the pre amp. So the two are different. As in my point above you can't have -50db, as a db reading has to be a positive value to hear anything. So the reading shows db on the scale as it's an electrical signal db reading. I was just really interested in why it's -80db being a min signal db reading on my pre, which Cyrus said is just a mathematical thing based on a log scale. In other words if you have full sound at zero, cut it by half (the signal level not sound pressure level), you are now own to -3db (given I believe halving the signal db level halves the sound on a log scale). If you keep doing this from a signal point of view you get to -80 where there is no sound from a signal level. I guess it must depend on the voltage output of current the pre emits at maximum volume of zero, to whether you just get to -80 or not say -90 etc, or is it just mathematical which cyrus said. I don't profess to be right in my thinking, just trying to get to why it might be -80.
 

davedotco

New member
Apr 24, 2013
20
0
0
QuestForThe13thNote said:
I think the audible sound pressure level measured in db is different to the electrical signal db level of the pre amp. So the two are different. As in my point above you can't have -50db, as a db reading has to be a positive value to hear anything. So the reading shows db on the scale as it's an electrical signal db reading. I was just really interested in why it's -80db being a min signal db reading on my pre, which Cyrus said is just a mathematical thing based on a log scale. In other words if you have full sound at zero, cut it by half (the signal level not sound pressure level), you are now own to -3db (given I believe halving the signal db level halves the sound on a log scale). If you keep doing this from a signal point of view you get to -80 where there is no sound from a signal level. I guess it must depend on the voltage output of current the pre emits at maximum volume of zero, to whether you just get to -80 or not say -90 etc, or is it just mathematical which cyrus said. I don't profess to be right in my thinking, just trying to get to why it might be -80.
Read my posts.

Decibels are a ratio, there has to be a reference point. It is neither positive or negative, nor is it a measure of power or voltage or whatever.

-80dB is just shorthand for saying 80dB below (in the case above) full output. It is just a logarithmic ratio. 10dB is 10x, 20dB is 100x etc.

Most of your post is gibberish, there is no such thing as an 'electrical signal dB reading' that is meaningless, furthermore you say 'if you have full sound at zero, cut it by half', what does that even mean? You keep talking about 'sound', are you refering to measured spl, percieved volume or what? I really don't know.

Start from the begining, ask simple questions one at a time.
 
Q

QuestForThe13thNote

Guest
I think I've made it very clear.

There must be such thing as relating volume in the signal output to the db in the display to even write -40db, as opposed to just a 1-10 volume scale or 1-100 etc. The cyrus displays it as -40db for instance as lots do. I know Cambridge audio do this.

So let's assume full volume is zero, as it is, anything else is attenuated and is minus. But it's still using a db designation which we know to be a log scale and a ratio as you say. So -3db is half as loud at a pressure level than zero, in this sense, so let's apply that to a electrical relative level, Why does it get to -80 and the sound is zero.why does the display show db at all.

What you aren't answering or don't seem sure of is why it's -80 to reach a point there is no sound.
 

davedotco

New member
Apr 24, 2013
20
0
0
QuestForThe13thNote said:
I think I've made it very clear.

There must be such thing as relating volume in the signal output to the db in the display to even write -40db, as opposed to just a 1-10 volume scale or 1-100 etc. The cyrus displays it as -40db for instance as lots do. I know Cambridge audio do this.

So let's assume full volume is zero, as it is, anything else is attenuated and is minus. But it's still using a db designation which we know to be a log scale and a ratio as you say. So -3db is half as loud at a pressure level than zero, in this sense, so let's apply that to a electrical relative level, Why does it get to -80 and the sound is zero.why does the display show db at all.

What you aren't answering or don't seem sure of is why it's -80 to reach a point there is no sound.
Ok, you are good up to the highlighted section, just bear in mind that the way volume is indicated is a style thing as much as anything, -80dB (on the Cyrus) means silence as does 0 on a conventional volume control, there is no difference and the numbers are in no way related to volume.

The highlighted bit is nonsense, as I have explained before. A change in measured spl of -3dB is not half as loud, it is a small change in loudness, but it is half the electrical power which may be where your confusion comes in. For a signal to sound half as loud, you would have to reduce the electrical power to 1/10th of what it was.

Again as explained above, -80dB below full output is very quiet indeed but there is nothing magical about this figure, it could just have easily be -70dB or -90dB, you would never know the difference, the numbers are picked by the designers at Cyrus but mean nothing. It is no different than designating the lowest possible setting on a rotary volume as '0', it is just an indicator.

The fact that there is no sound at all at -80dB is just the designed characteristic of the volume control. In my earlier calculation I pointed out that a signal at -80dB ref full output would be 33dB. This is not silence, it is a measurable level providing your listening room is quiet enough, if it is absolutely silent on your pre-amp then it is because the control is designed to mute the output on it's lowest setting.
 
S

SemiChronic

Guest
I always thought 0db was reference.

Given that bands often talk of playing venues at reference level
 

davedotco

New member
Apr 24, 2013
20
0
0
SemiChronic said:
I always thought 0db was reference.

Given that bands often talk of playing venues at reference level
As I have said earlier, the dB measurement has to be referenced to something. In the case of Quests pre-amp it is probably maximum output but there are so many factors involved. So 0dB below reference level is reference level etc.

In my experince of live bands, the only level that is of interest is the loudest possible, that said my experience of live gigs is somewhat out of date. Last proper live gig I worked was 1985.
 

ASK THE COMMUNITY

TRENDING THREADS

Latest posts