Decibels

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thirdcrank
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Decibels

Postby thirdcrank » 20 Oct 2018, 1:16pm

I'm hoping somebody can tell me if there's any perceptible difference between a sound rated at 70 decibels and on rated at 71.

I appreciate that things like the accuracy of measurement come into it but I'm looking for an "all other things being equal" sort of an answer. Would I notice a significant difference or even a difference?

rjb
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Re: Decibels

Postby rjb » 20 Oct 2018, 1:21pm

Every 10 decibel increase doubles the noise level. It's a logarithmic scale so a 1 dB increase would be pretty hard to detect using your ears. I may be wrong but I always thought a 1 dB increase in noise level was the smallest increment detectable by the average human being. :wink:.
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sjs
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Re: Decibels

Postby sjs » 20 Oct 2018, 1:30pm

It's a logarithmic scale, base 10, so a 10 dB increase represents a factor of 10, and 3 dB a factor close to 2. 1 dB is an increase of around 25%. It might be detectable in a side by side test, otherwise I would think not.

thirdcrank
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Re: Decibels

Postby thirdcrank » 20 Oct 2018, 1:38pm

I sort of half remembered from O-Level days that it was logarithmic which made me assume it was more significant that it appears. One on seventy is an increase of one seventieth, but 25% of 70 seems a lot bigger. Perhaps I've got the wrong end of the stick.

axel_knutt
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Re: Decibels

Postby axel_knutt » 20 Oct 2018, 2:11pm

As a student, I was taught that the smallest change most people can hear whilst present at the time is about 1dB, but if the level changes whilst they are out of the room they won't notice anything under 3dB. From this, it's both level and frequency dependent:

Sound Difference.gif


The Bel is just the base-10 logarithm of a power ratio: log(P2/P1), a decibel is a tenth of a bel: 10log(P2/P1), so 1dB is an increase of about 26%, 3dB is double the power, 10dB is ten times, and 70dB is 10,000,000 times.

dB spl Chart.JPG
dB spl Chart.JPG (62.98 KiB) Viewed 345 times
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sjs
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Re: Decibels

Postby sjs » 20 Oct 2018, 2:15pm

thirdcrank wrote:I sort of half remembered from O-Level days that it was logarithmic which made me assume it was more significant that it appears. One on seventy is an increase of one seventieth, but 25% of 70 seems a lot bigger. Perhaps I've got the wrong end of the stick.


Well perhaps the middle of the stick.
It's not 25% of 70, it's 25% of whatever was originally measured at 70. 70 dB is 10 to the power 7, ie ten million times whatever the 0 dB reference is. 71 dB is 10 to the power 0.1 (25%, near enough) more than that. Sound levels, like light, are deceptive. It takes quite a big percentage difference to be noticeable.

I see I've been beaten to it, response-wise!

thirdcrank
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Re: Decibels

Postby thirdcrank » 20 Oct 2018, 3:41pm

Thanks for the replies, especially the stuff aimed at my basic level. I learned many years ago not to try to appear to know more than I do. I appreciate it's very subjective: one person's pleasure is another's discomfort. I'm a bit wiser now than I was this morning, so long as I've not forgotten anything.

tatanab
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Re: Decibels

Postby tatanab » 20 Oct 2018, 3:52pm

sjs wrote:it's 25% of whatever was originally measured at 70. 70 dB is 10 to the power 7,
Exactly. The Bel or Decibel is not a unit of measurent in itself, it is a logrithmic scale of comparison and so must always have a base unit to relate to. In this case the correct measurement unit is dBA (A for audio I guess). In my background in RF electronics we used units such as dBmV i.e dB related to 1 millivolt, or dBmW being dB related to 1 miliwatt.

Using the term decibel to describe level of sound is just common usage like Hoover for vacuum cleaner etc.

thirdcrank
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Re: Decibels

Postby thirdcrank » 20 Oct 2018, 4:26pm

tatanab wrote:
sjs wrote:it's 25% of whatever was originally measured at 70. 70 dB is 10 to the power 7,
Exactly. The Bel or Decibel is not a unit of measurent in itself, it is a logrithmic scale of comparison and so must always have a base unit to relate to. In this case the correct measurement unit is dBA (A for audio I guess). In my background in RF electronics we used units such as dBmV i.e dB related to 1 millivolt, or dBmW being dB related to 1 miliwatt.

Using the term decibel to describe level of sound is just common usage like Hoover for vacuum cleaner etc.


One of the points I've tried to make is that I'm a layman here so much of the paragraph beginning "Exactly..." is way over my head. I agree it's common usage to the extent that it seems to be commonly used for official purposes, rather than saying Hoover or what was once Biro and became Bic.

Our council wheelie bin is marked "89 dB" which I assume is part of some official noise rating system. I can see that the information is limited and a test in lab conditions doesn't allow for the way the collection team throw it around, shout to each other or the noise of their lorry at 0700.

FWIW, I raised this query over car tyres. When I keyed in the size as marked on the side of the tyre, I got several options for price, fuel economy etc, and then the noise level. All the options but one were at 71, with one rated 70. I presume, therefore, that this is an official rating scheme, which to some extent allows meaningful comparisons.

My impression from what's been posted is that while every little helps, the reduction in noise level here will not be much. And that's what I was trying to find out.

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Paulatic
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Re: Decibels

Postby Paulatic » 20 Oct 2018, 8:21pm

I’ve an app on my iPhone Decibel X which I find cuts out guessing. Avg DB in this home just now is 55. :D How it helps to know that I’ve no idea. :D but when I put the bin out on Tuesday night I’m tempted to measure it to see how it compares to yours :lol:
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Mick F
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Re: Decibels

Postby Mick F » 21 Oct 2018, 7:04am

The Bell is a log scale as has been stated. It's a huge figure, so they use the decibel. Yes, 3Db is doubling or halving.
It's a measurement of amplification or attenuation in an electronic sense and has nothing directly to do with sound that you hear.

The old way of sound levels were to use the Dbm but these days it's been taken that Db is sound level. It's incorrect to use it that way, but usage is king.
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squeaker
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Re: Decibels

Postby squeaker » 21 Oct 2018, 9:24am

AFAIK (or can remember :roll: )

The relevant bit: most people will notice 3dB change in level, 1dB is pushing it (as already implied above).

Background: the Bel is a logarithmic ratio, originally used by the Bell Telephone Co. to measure losses in transmission lines. Technically, you should also state the reference unit to which the measurement applies, and any other qualifiers such as frequency weighting ('A' is common for sound, but others are available...) or whether the measure is peak, RMS, Leq etc., etc..). So a '70dB noise level' probably should have a footnote somewhere to say that the measurement was A-weighted RMS sound pressure level re 20uPa (ie micro Pascals).

Re tyres, a quick search found this gem which implies that the ratings are pretty meaningless, especially on coarse road surfaces :roll: As for wheely bins, you're on your own there :lol:
"42"

thirdcrank
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Re: Decibels

Postby thirdcrank » 21 Oct 2018, 9:58am

squeaker wrote: ... The relevant bit: most people will notice 3dB change in level, 1dB is pushing it (as already implied above). ... Re tyres, a quick search found this gem which implies that the ratings are pretty meaningless, especially on coarse road surfaces ....


Thanks for these bits which are at my simple level. The linked report which seems admirably clear is just what I was looking for. :D Back to my OP, it seems I'd probably not notice a difference of 1dB and the accuracy of the measuring and labelling system isn't good enough anyway.

I only mentioned the wheelie bins to try to illustrate that decibels were officially used as a measure.

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squeaker
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Re: Decibels

Postby squeaker » 21 Oct 2018, 6:49pm

thirdcrank wrote:Our council wheelie bin is marked "89 dB" which I assume is part of some official noise rating system. I can see that the information is limited and a test in lab conditions doesn't allow for the way the collection team throw it around, shout to each other or the noise of their lorry at 0700.
Some explanation given here - file under 'whatever did the EU do for us?' 8)
"42"

thirdcrank
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Re: Decibels

Postby thirdcrank » 21 Oct 2018, 7:16pm

squeaker

Thanks again :D

Here's a link to a list of fifty trademarks like Hoover that have become generic names

https://digitalsynopsis.com/advertising ... and-names/