help needed understanding dbm, db

Randall_Oelerich

Randall_Oelerich

Veteran
I want to use my Zoom H4 audio recorder as a sort of decibel sound meter to measure the noise levels at coffee shops (which I find quite loud at the coffee shop I am a regular at). Its audio meter goes from -40dbm to I think +20dbm or so. Can I use my H4 as a sort of sound meter so I do not have to buy a decibel sound meter off Amazon?

As audio intensity increases, the H4 meter goes from -40dbm to -30dbm to whatever, in other words less negative. So how do I interpret that in terms of decibels for the purpose of labeling a noise level at the coffee shop as a certain number of decibels?

I found this site and graph, somewhat useful:
http://www.phys.unsw.edu.au/jw/dB.html

Help understanding this appreciated, thank you. Keep in mind I am not out to totally understand sound engineering, I just want to know how to use the H4 for measuring decibels. Not sure about its dbm level and how that converts to decibels.
 
Just buy a sound meter. Radio Shack has them for ~$50 if I remember correctly.

Zoom mics aren't for calibration, aren't necessarily flat, don't have fast vs. slow metering, ect... for anything where knowing real numbers makes a difference, Zoom just isn't going to do it.

If you want to use it for scouting locations, still not too helpful as buzzes and narrow band issues aren't taking into account by broadband SPL readings.
 
Not going to hack it. For all the above reasons and because the metering isn't readable with any accuracy and if it were you still have no way to calibrate levels. About all you could do is compare recordings (providing you didn't change any levels) and say shop X is louder than shop Y. You could also average how much different but you would still not know WHAT that level was. Also it's kind of meaningless. The noise level is going to change radically depending on time of day and day of the week. Try it just after they open on a Sun., it will probably be very quiet then.
 
Randall_Oelerich said:
I want to use my Zoom H4 audio recorder as a sort of decibel sound meter to measure the noise levels at coffee shops (which I find quite loud at the coffee shop I am a regular at). Its audio meter goes from -40dbm to I think +20dbm or so. Can I use my H4 as a sort of sound meter so I do not have to buy a decibel sound meter off Amazon?
Click to expand...

There's the mentioned Radio Shack sound meter for about $50.

For 'fun' there is an iPhone app, RTA, that is produced by a company:

http://studiosixdigita.com

The App will give you a spectrum of the ambient sound.

These are not absolutely NBS traceable devices and should only be thought of as 'guides' or 'gauges', but can 'answer' some questions one may have about the sound environment.
(As a note the RTA app does run on the iPad... but the spectrum is 'wyrd'. I think it may be due to some sort of 'estimate' on the various frequencies, and the mic in the iPad is different
than the iPhone...
 
Gohanto said:
Just buy a sound meter. Radio Shack has them for ~$50 if I remember correctly.....
Click to expand...

Picked one up today. Barnes and Noble (like a Borders) coffee shop was 75db, I am at a Caribou Coffee shop now and when they make an expresso with their screaming banshee machine as I call it, noise sparks up to about 80db. Apparently chronic exposure above 85db can cause permanent hearing damage so this is sort of close.
 
OHSA regs state that repeated exposure above 90 may cause damage, so they have limits on how longer you can be exposed to a given dB level.

(A scale, slow response)
dB...................time limit per day
90........................8h
92........................6h
95........................4h
97........................3h
100......................2h
102......................90m
105......................60m
110......................30m
115......................15m

Keep in mind that dB are measured with a logarithmic scale. A change of 3dB is a doubling / halving of the power level, though most people barely register the change. A 10dB change is perceived as twice / half as loud, though it's a tenfold change in power level.

Of course this all depends on the frequency / level we're talking about, as our ears are decidedly non-linear in response.
 
JordanBlock said:
OHSA regs state that repeated exposure above 90 may cause damage, so they have limits on how longer you can be exposed to a given dB level.

(A scale, slow response)
dB...................time limit per day
90........................8h
92........................6h
95........................4h
97........................3h
100......................2h
102......................90m
105......................60m
110......................30m
115......................15m

Keep in mind that dB are measured with a logarithmic scale. A change of 3dB is a doubling / halving of the power level, though most people barely register the change. A 10dB change is perceived as twice / half as loud, though it's a tenfold change in power level.

Of course this all depends on the frequency / level we're talking about, as our ears are decidedly non-linear in response.
Click to expand...


Speaking of odd uses of audio frequencies... I've seen this mentioned in german magazines, but I don't recall it much in my local US press...

The idea is that 'youth' have better high frequency audio sensitivity, and so the device emits a high frequency tone, which 'irritates' the youth... and 'moves them along'...

http://www.compoundsecurity.co.uk/
 
Randall_Oelerich said:
Picked one up today. Barnes and Noble (like a Borders) coffee shop was 75db, I am at a Caribou Coffee shop now and when they make an expresso with their screaming banshee machine as I call it, noise sparks up to about 80db. Apparently chronic exposure above 85db can cause permanent hearing damage so this is sort of close.
Click to expand...

I'm incredibly curious why it matters to you what the sound levels are specifically in coffee shops? Are you researching this for a worker health and safety program of some sort or some sort of barrista union issue?
 
I even contacted OSHA, but what is odd is that they will not do anything unless a complaint comes from an employee. I don't have a problem with most coffee shops, just this one in particular. Maybe they have a bad expresso machine or something, it is far far louder than anything at any other coffee shop. Everybody complains about it. So I was just curious at to its db level. Was around 80-85 yesterday at that shop. I am down at a Starbucks this morning, and it is pretty much hovering around 70db. So, 10-15 db level difference, which as I take it is 2-2.5x difference in noise level or power, significant. On a purely entertainment facet, it is almost impossible for me to enjoy soundtracks with my headphones up at the noisy coffee shop given the tortured banshee screaming up there most of the time. :) But I like the free parking, unlimited free refills, free wifi.
 
10dB is a tenfold increase in power. It's generally perceived by our ears as twice as loud.

I've heard that some shops will make their machines hiss louder for whatever reasons, but that's strictly anecdotal.

Even at 85dB it's still below the levels the OHSA lists in their literature.
 
JordanBlock said:
...Even at 85dB it's still below the levels the OHSA lists in their literature.
Click to expand...

True. However, the[FONT=arial,helvetica] EPA sets 70 decibels as a maximum safe noise level in the work place-- putting that one coffee shop far above that safe level. Not meaning to get OCD, I just find the noise extremely loud at the one coffee shop, and that brought the whole db safety issue to my attention, especially because I compose music and I need to keep my hearing intact for that. This morning I measured Starbucks, and even with a long line, busy, grinding machines and expresso machines in play, the db level was 68-72, and it just felt far less loud than the Caribou I normally am a regular at (which pulses at 75-85 db from their screaming banshee machine, maybe it needs repair or replacement, common sense tells me it is not normal as I have to shout sometimes at a Barista two feet in front of me at the counter to place an order).
[/FONT]
 
Change shops. OSHA is near useless except in industrial situations. By OSHA you can just about fire a gun off and be "OK" unless you do it a lot. We did a show that had some sounds that were so loud that it changed your heart beat, literally, and because it was under 5 min... no problem. Feature film mix stages regularly peak at over 100 dB. The mix tech's (the smart ones) wear molded flat response ear protection.
 
Randall_Oelerich said:
Yup. I am down here this morning at the much quieter Starbucks, second day in a row. ***** to get on their wifi, they make it very hard to figure that out.
Click to expand...

As in hacking it, or just kicking your browser to go to the web, and then get shunted off to the authentication page?

Just back from InterOP for my day job, and I couldn't get get WIFI to work at all at the show... I think at one time I saw about 60 access points up enough to be broadcasting their SSID's... couldn't even get onto the the Cisco AP's... the 'industry leader' in Networking...
 
j1clark@ucsd.edu said:
As in hacking it, or just kicking your browser to go to the web, and then get shunted off to the authentication page?..
Click to expand...

Legitimate usage. Pain in the arse to get initial accounts, you have to sign up for both startbucks and register a debit coffee card, then you also have to sign up at att, what a pain. Then second day turns out att was down for their linkage with starbucks. Then next day they were down and I called and got a coupon code and that lasted about ten minutes. Worked okay today though. but it is legit. As long you put a few buck on their coffee card to use as emoney for buying their coffee and stuff, the card pin gets you free wifi 2 hrs a day and also free refills on coffee and ice tea.

I also like that starbucks opens at 5:30am, because here in northern MN in the summer I am up at 4am or 4:30 because the sun rises so damn early, gangs of birds outside my bedroom window doing some gang banging, sort of like a mini-Alaska thing land of the midnight sun, does not get dark until 10:30pm (perfect for summer filmmaking if you need lots of daylight!).
 
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