dBm

[Charles Jenkinson]

I’ve just been refreshing my memory on logs with a view to getting up to speed on working in dB as per this thread: http://billfitzmaurice.info/forum/viewt ... =4&t=20384

So I’ve just bought the Sound Reinforcement Handbook also as a general reference, and I was looking up their log stuff in the online click to read section. Page 412, has the calculation shown below. I am a bit stumped by how 1 watt in dBm is 30dBm, and then 10 watts is also 30dBm. i.e. a power ratio of 1,000 is 30dBm and then a power ratio of 10,000 is also 30dBm...?

http://www.amazon.co.uk/Sound-Reinforce ... 0881889008

[Bill Fitzmaurice]

Looks like a typo to me, first they compare 1w to 1mw, which is 30dB, then out of nowhere the 10w figure shows up.

[MissileCrisis]

Looks like a typo to me, first they compare 1w to 1mw, which is 30dB, then out of nowhere the 10w figure shows up.

+1

[MissileCrisis]

Knowledge I've gained from ee school. Dbv = db relative to 1v. We always reference 2.83, different scales for different needs, this may be similar.

[escapemcp]

Knowledge I've gained from ee school. Dbv = db relative to 1v. We always reference 2.83, different scales for different needs, this may be similar.

DBm is the reference to 1 milliwatt isn't it? EDIT: Thanks wikipedia... it is... just checked to avoid egg on face!

1 watt= 1000 milliwatts = 10³ milliwatts
So 1000 milliwatts = 3 Bm (not DBm - I know Bm isn't a measure! ) = 30 DBm (as it's Deci, so x10)

10W would equal 40 dBm btw (an order of magnitude greater is 10dB) or 10000 milliwatts = 10^4

That's how I do it, anyway... seems to work!

[byacey]

You have to qualify dBm; 0 dBm is .775 volts across a 600 ohm load, equating to 1 milliwatt of power dissipation in the load. This is a standard devised by the telephone company around a 100 years ago.

These days, we don't often run into actual 600 ohm loads in the pro audio industry, but the .775 volt reference level is still used. When it is a reference level without a specified load, it is referred to as 0dBu (unterminated).

Where it can become confusing is the relationship between 0dBu and +4dBu.

On most all modern day equipment, the reference level is shifted to +4dBu, meaning that when the level meters read "0 dBu", the actual real world output voltage level is 4dBu higher, or 1.23 volts.

dBV is also sometimes used, but the reference level at 0dBV is 1 volt RMS.

[byacey]

I know Bm isn't a measure! )

Actually, it is a valid measure. The base unit is the Bel (you probably guessed it - from the Bell Telephone Labs) However, the Bel is a rather large unit for our purposes, so it was scaled down to the deciBel.

The metric system is an amazing thing; I used to try and imagine how electronic formulas could possibly inter-relate and work out in a unified manner using some other arbitrary system of measures, but I just can't comprehend this.

As a paradox, I use metric measures every day in my profession, but I still prefer miles, Fahrenheit, inches, feet and yards for everyday measures.

[ncgrove]

As a paradox, I use metric measures every day in my profession, but I still prefer miles, Fahrenheit, inches, feet and yards for everyday measures.

Fox Butterfield, is that you?

[Chris_Allen]

As a paradox, I use metric measures every day in my profession, but I still prefer miles, Fahrenheit, inches, feet and yards for everyday measures.

Try living in a country where they are all interchanged randomly. Buying planks of wood 150mm x 8ft. Recipes with fl oz and gms. Units like bar per cubic inch. Corrected mass flows of kg/s root K MPa. Distances in miles until you get close, then it's in metres.

[Grant Bunter]

As a paradox, I use metric measures every day in my profession, but I still prefer miles, Fahrenheit, inches, feet and yards for everyday measures.

Try living in a country where they are all interchanged randomly. Buying planks of wood 150mm x 8ft. Recipes with fl oz and gms. Units like bar per cubic inch. Corrected mass flows of kg/s root K MPa. Distances in miles until you get close, then it's in metres.

Chris,
I know you're being serious and there's a serious side to the hypocrisy of doing stuff like that, but hell it made me laugh, so much so my wife asked me what was so funny...

[Bill Fitzmaurice]

Try living in a country where they are all interchanged randomly. Buying planks of wood 150mm x 8ft. Recipes with fl oz and gms. Units like bar per cubic inch. Corrected mass flows of kg/s root K MPa. Distances in miles until you get close, then it's in metres.

A visitor to Quebec noted that both of the faucet handles in his hotel bathroom were marked 'C'. He asked the hotel manager what they stood for. The manager replied "C is for the French word for hot, which is chaud." "But what does the other 'C' stand for?" the visitor asked. "Why, that's for cold. We are bi-lingual!"

[byacey]

That's Canada for you. They always reduce everything to the most complicated solution.

I had to look up Fox Butterfield to see who I was being confused with; he looks to be an interesting character. But no, I'm just Bill Y. from the soon to be frozen north.

[ncgrove]

The Fox Butterfield Fallacy is a running gag on James Taranto's daily article "Best of the Web Today" in the Wall Street Journal. In one of the classic cases, "Butterfield made reference to 'the paradox of a falling crime rate but a rising prison population.' The Butterfield Fallacy consists in misidentifying as a paradox what is in fact a simple cause-and-effect relationship."

http://online.wsj.com/news/articles/SB1 ... 4228137272

[BrentEvans]

Try living in a country where they are all interchanged randomly. Buying planks of wood 150mm x 8ft. Recipes with fl oz and gms. Units like bar per cubic inch. Corrected mass flows of kg/s root K MPa. Distances in miles until you get close, then it's in metres.

The thing that's really funny is that most Customary measurements have now been defined in terms of the metric system. For instance, the official definition of a pound is (roughly) 2.2kg. The kilogram is in turn defined by a specific chunk of metal in France, stored in a vault... whose mass seems to change over time for some reason....

So now there is a push to redefine the kilogram as the mass of a certain number of atoms of a particular element... except that it's really hard to count atoms.... so they had to invent a way to estimate it pretty closely... anyway....

So until then I'll just keep on using whatever unit is convenient to my measurement. It's easier to think in millimeters than fractions of an inch (for me, anyway) but easier to think in inches than centimeters. I have no feel for what a gram or kilogram is, but I tend to think of fluid in liters as opposed to quarts, until you get up to about three of them, when thinking in gallons takes over.

I'm all screwed up, apparently.

[byacey]

Canada is surveyed into square miles, so it makes sense for giving rural directions, and a mile is easier for me to estimate than a kilometer. Same thing with inches, feet yards, fractions of an inch; I grew up building things with these units of measure, so I am able to estimate with some degree of accuracy.