Crash Course

[decz101]

Hey,

Straight to the point.. I've just got a very cheap pio-vsx-c100 with a set of yamaha nsp100 speakers.

I can't find any guides anywhere that tell me how to calculate my output in decebels..

So for starters - when the volume of my amp is at -50dB does that mean 50% of the amps power is being sent to the speakers, and -25dB means 75% etc?

Cheers all,
D.

 

[MDS]

As the numbers approach zero (become less negative) the receiver is putting out more power, but are basically meaningless unless you calibrate the receiver to a known level. The output Sound Pressure Level (SPL) depends on lots of factors such as your speakers and room size so saying my volume is at -50 dB means nothing if you haven't calibrated. That would mean 50 dB below some level - but what level?

Typically you calibrate by picking a number on the volume dial to be your reference volume position and calibrate such that turning the knob to that position yields a specific output SPL. The level most commonly used is Dolby Reference Level and the volume position most commonly used is 0 dB.

You need a Radio Shack SPL meter set to C weighting and Slow response. Set the volume dial to 0 dB. Now play the receiver's internal test tones (which are at -30 dB = 30 dB below 'full scale digital') and adjust the channel levels so that each channel reads 75 dB on the SPL meter. Now all the channels are balanced and at 0 dB on the volume knob, the system is producing 105 dB peaks (75+30=105), which is Dolby Reference Level.

Now if you turn the volume knob to say -20, you are listening at 20 dB below reference level which means the peaks will be 95 dB (20 dB below 105 db). The average level will be much lower because music/movies aren't constantly at peak values.

If you just want to know what the SPL level is at any point on the dial, whether you calibrated or not, set the meter to A weighting, Fast response and just read the meter.

 

[decz101]

Cheers, that was very helpful.

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=33-2055

That seems like a good product with the necessary features.

Woah at 0dB! - that'll be a first for my reciever!

 

[decz101]

Just found that it will go to a maximum of -4dB only, so i guess i would just set the channels so they read 79dB instead?

 

[MDS]

That is the digital SPL meter I have. I like it better than the analog version because it's easier to read the big lcd numbers and it has a few extra features the analog version lacks.

The range of the numbers on the volume scale will vary depending on the values of the channel trims. You want to set the channel trims to 0 before you start the test tones. If they are all set to zero and you select an unused input, you can turn the volume up all the way to see the max value (or see if the manual tells you what the range is). If for example, the range is -80 to + 10 but the channel trims are set to 10, the range will become -80 to 0.

Don't confuse the output SPL, which is the sound pressure level at your ears at the listening position, with the numbers on the dial. You adjust the volume level in combination with the channel trims to reach the output level on the meter. You can use any number near the top of the range for your 'reference' position - 80% of the max is a good rule of thumb.

I know this can be confusing and I have explained it many times before but the goal is simply to get all the channels balanced. You can calibrate to any level you want, although Dolby reference is the standard. 75 dB of pink noise is a bit annoying. Let's say you do want 75 dB at 0 on the volume scale. You can set the receiver volume to -20 and calibrate so the meter reads 55 dB and you will still have reference level of 75 dB at 0 on the volume scale. The only difference is that 55 dB is alot less annoying to your ears.

Make sense?

 

[decz101]

Its all starting to make sense now - ta very much.