Decibels & Sound - Page 1
Forum section: electronics / general

author: MikeTannel
author_rank: Contributor
timestamp: September 03, 2025, 01:08:40 pm
content: Hi All,So I think I get the basic idea of a decibel. A quick example to make sure: I present 1V at a circuit's input and I get (1/100,000)V at the output. The circuit has attenuated the input by -100db. Likewise, if I got 100,000V out, it will have amplified the input by 100db. Got it. It's the ratio of one value to another.Now let's say I have a jet engine that's rated at 120db. Well that's a million times louder than what? You can't really call a jet engine an amplifier (ha,ha), there's no sound input, it just makes noise when it's running. Or, is a decibel a completely different animal when applied to sound?Can someone explain/describe what's going on?Cheers!Mike

author: madires
author_rank: Super Contributor
timestamp: September 03, 2025, 01:57:43 pm
content: The SPL reference value is 20 µPa for air.

author: themadhippy
author_rank: Super Contributor
timestamp: September 03, 2025, 02:10:43 pm
content: Quotea million times louder than when its not there/running

author: MikeTannel
author_rank: Contributor
timestamp: September 03, 2025, 02:27:19 pm
content: Okay, so we're talking loudness as the amount of pressure on the ear drum ie SPL. Then given that reference value, the jet engine would produce 20Pa of pressure on the ear drum. Yes/No? But why that as the reference value? Is it given for some standard altitude? Also, sound is in a sense like AC, so is this an RMS value that represents a static pressure (constant force against the drum) such that the instantaneous peak pressure would be higher or is this the peak value and the RMS value would be lower? Just curious now that I think about it a little.In any case, assuming I understand correctly about the SPL, you've answered my original question. I wasn't considering what sound (or more accurately loudness) actually was.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 03, 2025, 02:37:45 pm
content: To bring the concept to more electrical terms, I believe the accepted hearing threshold is 10^-16 W/cm^2. So, 100dB would be 1uW/cm^2, and 160dB would be 1W/cm^2.

author: MikeTannel
author_rank: Contributor
timestamp: September 03, 2025, 02:45:32 pm
content: 0.1 Femto Watts! Ha, ha - it'll take more than a Fluke 87V to measure that. Though a few fluke owners might argue.

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2025, 02:51:38 pm
content: The original definition of 0 dB SPL was an estimate of the threshold of human hearing.Sound intensity is measured as the amplitude of the longitudinal pressure variation, so it is expressed in Pa rms.Sound power (in W) is proportional to the square of that pressure value, with properties of the air and geometry factored in.Of course, “dB” by itself is a power ratio (or a corresponding amplitude ratio); adding a suffix (e.g. dBm or dB SPL) specifies the reference for a value.Roughly, undamaged human hearing threshold is slightly above the random Brownian motion of the atmosphere, which avoids annoying noise.

author: madires
author_rank: Super Contributor
timestamp: September 03, 2025, 03:54:22 pm
content: Quote from: MikeTannel on September 03, 2025, 02:27:19 pmOriginally it was believed that the SPL threshold for a human to start hearing an 1 kHz sine wave is 20 µPa. Later it turned out that 20 µPa isn't quite right but was kept as the reference value.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 03, 2025, 04:15:56 pm
content: There was a thread recently here about an article author of which found decibels are confusing. The general consensus of the forum was that decibels are not confusing and therefore, you are just stupid. But you have me on your side on this one. The fact that the exact same "dB" piece of text is used for many totally different measurementsisconfusing. Those with enough domain expertise always deal with the thing on the domain they are familiar with. If you work with audio, it is obvious that dB refers to actual absolute sound pressure - not relative - and how the 0dB reference point is defined. For others who remember dB from different contexts, it's not obvious at all.

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2025, 04:21:35 pm
content: A few more details:“Loudness” as a technical term refers to perception, including the human hearing system and processing.“Intensity” is physical, and refers to the pressure waves hitting the observer or microphone.The power = force x velocity, includes the area hit by the pressure to get the force.The velocity is the speed of sound in the medium (air).Since perception of stimuli by humans is approximately logarithmic, dB are handy for estimating loudness. Often, 1 dB is assumed to be the least perceptible change in intensity. Old rule of thumb for dB as a ratio: if you ask a child to turn down the music, he will reduce it by 1 dB.In audio, the important suffixes are often ignored, but that is not careful usage. Besides the suffix “SPL” that gives the reference level, there are also important suffixes for acoustic purposes that define frequency weighting relevant to dangerous sound levels, such as “dBA” for A-weighting (q.v.).

author: Tation
author_rank: Regular Contributor
timestamp: September 03, 2025, 06:34:23 pm
content: I find interesting that really loud sounds (e. g. 120 dB ~ 20 Pa) are really small compared to the "static" pressure they are super-imposed to (at sea level ~ 101315 Pa).

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2025, 06:38:37 pm
content: Right. Sensible sound level amplitudes are tiny compared with the static pressure, so air is a very linear medium for acoustics.At huge amplitudes, such as nuclear explosions, you can get non-linear effects.

author: EC8010
author_rank: Frequent Contributor
timestamp: September 05, 2025, 10:20:02 am
content: The dB is a ratio. Only. But you can specify a level by comparing to a reference level, so acoustical engineers commonly reference to 20uPa. And they usually add an "A" weighting filter and call it dB(A). Journalists habitually lose the "A" at the end. It gets hairier when you hit audio (electrical analogue of sound). dBm in audio is referenced to 1mW dissipated in 600 ohm. But (a) measuring power is awkward and (b) nobody has used 600 ohm for decades (except US op-amp manufacturers, who invariably get it wrong). So analogue audio in the *EBU uses dBu, which is referenced to the voltage that would have dissipated 1mW in 600 ohm, but it doesn't care about impedances. Then there's dBV, referenced to 1V. And dBW, referred to 1W; if you're lucky, a resistance might be specified. dBr is referenced to something towards the beginning of the document that you skimmed over and have forgotten. In digital audio, there's dBFS where the reference is the largest sine wave that can be accomodated without clipping and is termed Full Scale. Oscilloscopes use dBm on their FFTs, but the 1mW is dissipated in 50 ohm because RF transmission lines are 50 ohm, making the voltage different to audio dBm.I hope you're all taking notes; there may be a test later...* European Broadcasting Union

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2025, 10:47:21 am
content: Quote from: EC8010 on September 05, 2025, 10:20:02 amYeah. Except both professionals and laymen across wide range of contexts say "dB" and mean some absolute value. Pedantically, they should be saying "dB SPL" or "dBA SPL" or dBm or whatever, but they don't. So we are in the situation of either having to say that dB is super widely misused by alarmingly high number of professionals who are stupid. Or we can accept that the meaning has changed, and is fluffy, uncertain. Sucks either way.

author: TimFox
author_rank: Super Contributor
timestamp: September 05, 2025, 02:27:36 pm
content: Yes. The world is filled with careless usage of technical terms. That is why I try to employ careful usage.

author: themadhippy
author_rank: Super Contributor
timestamp: September 05, 2025, 02:55:01 pm
content: QuoteI likes fluffy uncertainty,you can use often use it to your advantage,for example when the council bod demands we take a background spl before a gig and your only allowed to go so many db above that.However they didnt say you cant take the ambient reading as one of the many planes was passing over on its final approach to the airport

author: radiolistener
author_rank: Super Contributor
timestamp: September 11, 2025, 05:20:43 am
content: Indeed, there is some confusion around dB in the audio field, but it is important to note that the confusion comes from the reference value, not from the logarithmic unit itself. The decibel as a relative unit is extremely convenient and that is why it became so popular. The ambiguity usually arises when the reference is not mentioned or is not clearly stated. This is mostly an audio-measurement issue — in RF field, where the reference is usually well defined (dBm, dBW, dBV, dBc), such confusion is encountered rarely.From my perspective, the most confusing case in audio is dBu. Usually the letter u would normally imply “micro” (so dBu would mean dB relative to one microwatt). In audio, however, dBu has a rather unusual definition: it originates from power into a 600 Ω load, but in practice it is expressed as a voltage reference and is independent of load impedance.As for dB SPL, the reference is the conventional threshold of hearing. As already mentioned, this is not meant to be the exact level where a given individual starts perceiving sound, just some agreed value representing the average threshold of audibility.The situation with sound levels is even more confusing, since the values may also be given with weighting corrections — for example, A-weighting. In this case, the reported levels are adjusted according to frequency, following a standardized curve.Regarding to missed reference level, this kind of confusion is somewhat similar to when people mistakenly write “mHz” instead of “MHz” — which, strictly speaking, would mean millihertz rather than megahertz.

author: Simmed
author_rank: Frequent Contributor
timestamp: September 11, 2025, 07:36:02 am
content: some of you may remember this useful toolhttps://www.analog.com/en/resources/interactive-design-tools/dbconvert.htmlyou can expand outwardstake out a spreadsheet and try playing with this(convert to dBm wrt 600R impedance) -60.8dBu <--- -63 (convert to dBV) < ---0.707(divide 2^0.5 = AC rms)<--- 1.000mV (AC peak starting point) --> x2 = 2mV (AC peak peak) --> into 8R loudspeaker 90dB/W (assume flat) = about 38dB SPL --> 0.00158Pa (convert to pressure)now you can roll it upwards to 160dB SPL changing where the formula will work fromat some point you can peg some references =SM58 microphone is rated at 1.85mV rms output from 94dB SPL inputNT1A mic i think is -121dBV noise floor ?i think R68 EBU reference fullscale level is +18dBu ?180DB SPL into SM58 mic will get us about 20V rms (and some serious injury maybe)spreadsheet povQuote from: Ground_Loop on September 03, 2025, 02:37:45 pmbut here is where i dont quite get the conversion ? how do you get watts from pressure ?

author: EPAIII
author_rank: Super Contributor
timestamp: September 11, 2025, 10:39:06 am
content: The short strokes:1. A deciBel (dB) is one tenth of a Bel (B). Like many units, it is named after a prominent scientist in a related field; in this case, Alexander Graham Bell.2. A Bel is a pure number. It has NO UNITS. Bels are simply a logarithmic way of comparing TWO numbers, usually two power levels. If those two numbers do have units, they must be the same units so that they cancel out when the number of Bels or deciBels are calculated. It does not matter what those units are, they simple must be the same.3. 1 deciBel = 10 log(r) where r is the power ratio.OR1 deciBel = 20 log(r) where r is the Voltage or current ratio.In both of these, log(r) is the logarithm to base 10, not the natural logarithm.4. DeciBels are most often used to compare units of sound's power or of the electrical units (Volts, Amps, Watts, etc.) used to produce or represent sound power in a circuit. When electrical/electronic circuits are the case, deciBels (and rarely Bels) are taken to represent power ratios. So, if the units actually being used are Volts or Amps, then their values are normally squared before taking the ratio or the resulting logarithm is multiplied by 20 instead of 10 after the logarithm is taken.5. Decibels are also often used with reference to some particular reference level. In audio circuitry one such value that is often used is 1 mWatt. BUT THIS IS NOT THE ONLY REFERENCE LEVEL. So you will see the reference level added to the the db value by the addition of one or more letters to designate that reference level. 1 dbm would be a decibel value that is referenced to 1 mWatt. Other reference levels may be 1 Watt, 1 KW, etc. I have worked with all of these and others. Since early audio circuits often had a 600 Ohm impedance, 1 Volt across 600 Ohms (1.667 mW) was also used as a reference level. In most cases, knowing the reference level is absolutely necessary to make any sense of the dB values being discussed.6. In measurements of sound intensity other reference units are used but they are still power ratios. Once again, the unit abbreviation is modified to indicate which reference level is being used. Beyond that, I will leave that discussion to others.https://en.wikipedia.org/wiki/DecibelI at least tried to keep it short. I hope it helps.And back to your original question, to say that a jet engine is producing a sound level of 120 dB makes absolute NO SENSE. In order to properly state it's sound level, the REFERENCE UNITS must be stated.120 dB = 1,000,000,000,000, exactly and precisely. Just a pure number.

author: paulca
author_rank: Super Contributor
timestamp: September 11, 2025, 10:55:05 am
content: What dB is the whistling and ringing you hear at though.For sound I recall the 3/6/9 rule from advice I got for running a PA at a club. 3dB is twice the "power", 6db is twice the damage to hearing and 9dB is twice as loud to the ear.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2025, 02:28:44 pm
content: To Simmed:The power level quoted by GroundLoop is watts/cm2.Pressure (Pa) x area (m2) = force (Nt).Force x velocity (m/s) is power (W).The velocity is the speed of sound in the medium (air).

author: radiolistener
author_rank: Super Contributor
timestamp: September 11, 2025, 04:02:30 pm
content: Quote from: Simmed on September 11, 2025, 07:36:02 amthat calculator once inspired me to create my own version.The original is quite useful, but it’s missing some values and there’s a bit of confusion regarding the reference value.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2025, 04:05:00 pm
content: What are the reference values for S9 hf and vhf?I would think the relevant values would be in V/m E-field at the antenna.

author: radiolistener
author_rank: Super Contributor
timestamp: September 11, 2025, 04:34:39 pm
content: Quote from: TimFox on September 11, 2025, 04:05:00 pmCode:const S9REF_HF = -73; // S9 = -73 dBm @ HFconst S9REF_VHF = -93; // S9 = -93 dBm @ VHFThe conversion from dBm to S-units is defined relative to the reference level, with a slope of 6 dB per S-unit.Quote from: TimFox on September 11, 2025, 04:05:00 pmThe S values represent S-meter readings for a specified input signal power at the receiver input. I don’t think it can be meaningfully expressed in V/m without calibrated antenna, since the corresponding field strength would depend heavily on the antenna type.Your point about expressing it in V/m also makes sense in a practical context — that’s exactly how calibrated measurement antennas are used. With a specific antenna of known gain and impedance, one can relate the feed-point power to the incident field strength in V/m. But since this depends entirely on the chosen antenna model, it’s not part of the standard S-meter definition.

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2025, 05:43:33 pm
content: Quote from: EPAIII on September 11, 2025, 10:39:06 amI was going to say this; 20 x log(A/B) is used for voltage or current, and 10 x log(A/B) is used for power.When the voltage or current is doubled, the change in power is 2 squared or 4 because both the voltage and current are both doubled, so that square comes out and multiplies the 10 to make 20.