AVS Forum banner

Status
Not open for further replies.
1 - 8 of 8 Posts

·
Registered
Joined
·
55 Posts
Discussion Starter · #1 ·
Alright, this is for you audio engineers out there. I'm trying to answer a question my wife asked me about our new receiver.


The new receiver has a volume control scale that uses db. For example, when you turn the volume control the readout will say -30db or -70db, etc.


The old receiver used a 1-100 scale, 100 being loudest.


My wife wanted me to explain what the -30db meant and what scale it was based upon. In other words, -30db means 30 db less, but less than what? What's 0db in this scale equate to? Is there some reference level for 0db that the industry uses for this?


Thanks
 

·
Registered
Joined
·
517 Posts
Quote:
Originally posted by jtorello
-30db means 30 db less, but less than what? What's 0db in this scale equate to?
It is my understanding, that the "0 db" is simply an arbitrary number that represents the maximum volume the receiver can produce with those speakers. Be it 100 or 110db or whatever.


For example, if your receiver was set to "0 db", and you used an SPL meter and found it produced a 108db volume, then at "-30 db", your speakers would be producing approximately 78db of noise.


I say approximately, because the "- db" scale of the receiver is only a guage... because the actual volume depends on the efficiency of your speakers... if your speakers are hard to drive, then "-30 db" may be more or less than 30 decibels off the maximum volume.


My new Rockets do not go anywhere near as loud as my older Infinity speakers did on my Sony DA5ES receiver (but they are a much nicer sound!). I can stay in the room at "0 db" with my Rockets, but could only go up to "-10 db" with my Infinities. Not sure what the actual SPL level is... I may get my meter out and take some measurements! All I do know, is that using the Video Essentials DVD, to tune it to be "reference level", I had to have my Sony DA5ES at "-18 db" for my Rockets. I never tuned the Infinity speakers (because I got the SPL meter recently).
 

·
Registered
Joined
·
1,358 Posts
Glen,


your explanation may be correct for some peices of equipment, I'm not sure. But I can tell you that it's not the case with my onkyo TX-DS676 receiver. I know this because 0db is not the maximum of the scale. the scale on it ranges from -60db to +10db (or +15 can't remember). +3db is definately much louder than 0db, so 0 can't be the maximum volume.


It was always my understanding that 0db was a (THX) reference level of 85db average SPL, with 105db peaks. (Of course using speakers of some nominal imedence and sensitivity.)
 

·
Registered
Joined
·
244 Posts
Quote:
-30db means 30 db less, but less than what?
You have already almost nailed it. It is a ratio, or a means of expressing one things compared to another. The decibel scale is a mathematical means to express large ratios. A negative value means that the quantity is less than some reference. A postive value means higher.


The reference used in your receiver is probably just attenuation from maximum output. That is not to say maximum usable output... the amps will probably reach their limits (clip) long before hitting 0 dB.


When used in audio, dB is a ratio of power. It is calculated as such: 10 * log (Power2/Power1) = Power ratio expressed in units of dB. Note that the term (Power2/Power1) is just a power ratio expressed in absolute terms. For example, if Power2=100 Watts and Power1=10 Watts, then (Power2/Power1), or the Power ratio, is 10. (Power2 is 10 times Power1). The value in decibels (dB) is 10*log(10)=10. Don't be confused that it is also 10. What if Power1=1 Watt? Then the ratio is 100. Calculate it in dB and it is 20 dB. Here is where it can be confusing. Why is it 20 dB? Because each 10 dB difference is 10 times. So, 20 dB is two 10 dB differences, or 10 X 10 = 100 times. Likewise, a ratio of 1000 times is 30 dB. A ratio of 2 is approximately 3 dB. A ratio of 1/1000 is -30 dB.


Once you know that 10 dB means 10 times the power (or 10 dB less means 1/10 the power), you will realise just how little power it takes to make loud sound. If your amp clips at -10 dB on the volume readout, and you normally listen to music at -30 dB, then you are listening at (-30dB - -10dB =) -20dB, or 1/100th of the power capability of your amp. If your amp is rated at 100 Watts, you are listening to your music with only 1 Watt (peak) power going to your speakers.


In audio, there is a term "SPL", or "Sound Pressure Level", which is measured in dB. SPL is the difference from the absolute quietest sound that can be heard by average, young, healthy human ears. That "quietest sound" is then by definition, said to have a 0 dB SPL. Any other sound will have some positive SPL in dB. A whisper is about 40 dB; a conversation about 60 dB, loud music about 80 to 90 dB. Prolonged exposure above 90 dB can cause hearing damage; above 120 dB causes immediate pain. The avaerage bass-head's booming car stereo puts out well over 120 dB, often over 130 dB (over 10 times the pain threshold).


Speaker sensitivity ratings are in dB SPL, and are usually the SPL measured when driving the speaker with a 2.83 Volt signal, at one meter (100 cm or 39.4 inches) away. (2.83 volts because that voltage gives one Watt into a resistance of 8 Ohms). If you are comparing two speaker systems, the sensitivity is important. If speaker1 has a sensitivity of 89 dB and speaker2 has a sensitivity of 92 dB, then you will need to double the power into speaker1 (+3 dB) to make it output as much sound as speaker2. So if speaker2 needs 80 W to reach a certain loudness, then speaker1 needs 160 W to reach the same loudness.


I hope that I haven't confused you too much. Understand the dB can be a great help when choosing speakers, amplifiers, and when setting up and EQ'ing your system.
 

·
Registered
Joined
·
904 Posts
Macboy, great explanation!


One question, you mention the formula dB=10*log(power1/power2) and in your example say 10dB = 10x power difference "10*log(10)" which implies base 10, i thought dB was calc'd using base e (natural log), so the formula should be dB=10*ln(power1/power2) is this more accurate?


thanks again :)
 

·
Registered
Joined
·
354 Posts
What should you set your dbmeter to when calibrating the pre/pro with its built in test tones for "reference level"? And what are the different ref levels?


Setting to a higher dbspl will give a higher output to the amp (and send you sooner into clipping) at any volume setting; which is right: 75, 80 or 85?


Or does it matter as long as they are all the same?
 

·
Registered
Joined
·
904 Posts
ham, what's important is that they're all the same when you listen at your normal volume.

for most home use, 75db is a good level to use for calibration. many people find 85 to be uncomfortable, but it's really just personal preference.
 

·
Registered
Joined
·
244 Posts
Quote:
One question, you mention the formula dB=10*log(power1/power2) and in your example say 10dB = 10x power difference "10*log(10)" which implies base 10, i thought dB was calc'd using base e (natural log), so the formula should be dB=10*ln(power1/power2) is this more accurate?
No, decibels uses log base 10, not natural log (log base 'e^1')
 
1 - 8 of 8 Posts
Status
Not open for further replies.
Top