# How can I work out how loud something is in db? **URL:** \ **Category:** Factual Questions\ **Created:** [September 20, 2004, 7:50pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179 "2004-09-20T19:50:14Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![flapcats](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@flapcats](https://boards.straightdope.com/u/flapcats)\ **Post date:** [September 20, 2004, 7:50pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/1 "2004-09-20T19:50:14Z")
I was listening to this PC hum and wondered if there was any way of working out how loud it is. I’m considering a new PC that is spec’d at 45db (12" from case) but don’t know how to compare that level with the noise I’m currently hearing. Is there any way of using something that makes a known amount of noise to work out how loud something is? I don’t want to buy anything to work this out (since I’m saving for the PC)… ideas anyone? ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [September 20, 2004, 8:36pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/2 "2004-09-20T20:36:01Z")
You need two things, a reference level and a meter. 45 dB is a meaningless measurement unless a reference level is specified. Measurements in decibels tell you the relationship between a measured level and a reference level. When you have two measurements, you can compute their difference in decibels. With sound measurements, the measurement process may be complicated by the need to weight the frequencies according to a frequency dependent curve. ---
**Author:** ![flapcats](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@flapcats](https://boards.straightdope.com/u/flapcats)\ **Post date:** [September 21, 2004, 9:44am UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/3 "2004-09-21T09:44:18Z")
Does that mean there’s no home-growable way of telling? What is 45db eqivalent to? Are there any good acoustics sites that give a broad scale (I’ve seen a couple that go from whisper to aircraft but with only a few steps in-between). I’m guessing 45db is like a loudish whisper or rubbing your hands together slowly at arms length. Yes? ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [September 21, 2004, 10:37am UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/4 "2004-09-21T10:37:41Z")
Here [Digital-Display Sound-Level Meter](http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F007%5F002%5F014%5F000&product%5Fid=33%2D2055) ---
**Author:** ![SteveG1](https://avatars.discourse-cdn.com/v4/letter/s/bb73d2/32.png) [@SteveG1](https://boards.straightdope.com/u/SteveG1)\ **Post date:** [September 21, 2004, 12:03pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/5 "2004-09-21T12:03:05Z")
Just saying a sound is 45 dB s meaningless, unless it is referenced to something. That is why I laguh when anyone says “Rock concerts are bad because they are 1,23456789 dB”. It is a ratio. If talking about voltages, the ratio is dB=20 log V1/V2, where V1 is the first voltage and V2 is the second. If talking about power, the ratio is dB=10 log P1/P1 ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [September 21, 2004, 12:11pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/6 "2004-09-21T12:11:47Z")
See [Sound Power, Intensity and Pressure - An introduction to decibel, sound power, sound intensity and sound pressure](http://www.engineeringtoolbox.com/27_57.html). ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 21, 2004, 3:27pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/7 "2004-09-21T15:27:42Z")
> [@SteveG1](#): > > Just saying a sound is 45 dB s meaningless… No, it isn’t. In acoustics, sound pressure level is always referenced to 10[sup]-12[/sup] W as 0 dB, unless explicitly stated otherwise. By the same token, in electronics and RF work, 0 dB is commonly defined as 1 mW into a 600 ohm load. It’s common practice to append a letter designator to the “dB” to indicate what the reference is; for example, the latter unit is termed dBm (the _m_ stands for _milliwatt_, IIRC). In antenna design work, the terms dBi and dBd are commonly used to compare the performance of a given design with either an (idealized) isotropic radiator or a dipole, respectively. Yes, the dB is a logarithmic expression of a ratio, but there is almost always a standard reference for the measurement being discussed. ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [September 21, 2004, 4:38pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/8 "2004-09-21T16:38:38Z")
> [@Q.E.D.](#): > > Yes, the dB is a logarithmic expression of a ratio, but there is almost always a standard reference for the measurement being discussed. We have standards, **lots** of standards. There’s dBµW, dBm, dBW, dBµV, dBmV, dBV, dBi, dBd, dBc, dBsm. That’s why a measurement without a reference is sloppy at best, useless at worst. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [September 21, 2004, 5:51pm UTC](https://boards.straightdope.com/t/how-can-i-work-out-how-loud-something-is-in-db/265179/9 "2004-09-21T17:51:02Z")
> [@mks57](#): > > That’s why a measurement without a reference is sloppy at best, useless at worst. But there _is_ a reference. For acoustics, the refernce is clear. 0dB is 10^-12W, like **QED** said. **mks57,** the link you provided goes into some detail about this. There’s all sorts of conventions and shorthand notation that we use all the time. And its not just the scientific or technical community. When you see a stop sign, its understood that you stop the vehicle. No need to stop talking, breathing, making that cellular call (although…), etc. Since everyone knows the meaning of the term, it only wastes time re-defining standard terms.