How can I measure dB output?

[Duo]

Lol... I'm going to look for a project for that in my amateur radio magazines. I've seen them before and they are easy to build, they just have to be calibrated.

[zx3chris]

the worlds loudest cars at DB comps now are in the mid 180s, and the highest i believe is 187.... most are using 10's of thousands of watts (running from 18vdc as per the rules) and tons of subs.

-chris

[Da5id4Vz]

I once checked a couple of the RS SPL meters against some very expensive Bruel and Kjaer (drool) big iron. It was a simple test using the B-K microphone capsule-calibrating unit. Surprises of surprises, the RS meters mic is the same diameter as the Bruel...

Not only were the cheap little RS things reasonably consistent among them selves, they also tracked the lab grade stuff within a dB or two.

I had this CD produced by B-K demonstrating their studio microphones that had a bunch of test signals recorded at the end. The octave and 1/3 octave noise recordings worked great with that cheap little meter for setting up EQ. I lent them to someone a while back and haven’t seen them since...

Of course for next to nothing it was a nice setup, but honestly, its the Bruel Kjaer stuff that I really miss.

Mr. Joe Dirt (R) has a nice looking SPL meter for sale in the trading post: professional sound level meter

I think that if a person were to set out and try and build a descent SPL meter from scratch that there would be two big challenges. The first would be finding a reference source to calibrate against. The second would be getting defining and calibrating the response curves for A, B, and C weighting. These weights correspond to human hearing response (Fletcher-Munson equal loudness curves), inverse hearing response, and something else that I cant think of at the moment. This link explains it better than I can: http://www.me.psu.edu/lamancusa/me458/4_metrics.pdf

If you find a good article that covers this stuff, it would be an interesting reed.

[zx3chris]

here is an intersting scale,
note that the pressure produced after an atomic bomb ranges from 10-15psi... these super db cars are nearing that... and since it in a log scale, you can see a system of 200db (30psi) would destroy nearly anything near it. my car measures in at a bit over 130db, and i can really really feel it (makes your hair jump), its amazing this it is only .01psi.

SPL = 20 Log(p/p ref)
p = pressure in pascals
p ref = reference pressure in pascals = 2 x 10^-5 Pa (pascals)
to convert psi to pascals you need this:
Pa/psi (approximately)
so take your relative PSI value convert it to pascals and dump it back
into the above formula.
2 psi = 13638 Pa
SPL = 20 Log [13638/(2x10^-5)] = 176.7 dB acoustical
More dB SPL versus PSI
dB
acoustical - PSI
50.7 - 0.000001
56.7 - 0.000002
60.2 - 0.000003
62.7 - 0.000004
64.6 - 0.000005
66.2 - 0.000006
67.6 - 0.000007
68.7 - 0.000008
69.7 - 0.000009
70.7 - 0.00001
76.7 - 0.00002
80.2 - 0.00003
82.7 - 0.00004
84.6 - 0.00005
86.2 - 0.00006
87.6 - 0.00007
88.7 - 0.00008
89.7 - 0.00009
90.7 - 0.0001
96.7 - 0.0002
100.2 - 0.0003
102.7 - 0.0004
104.6 - 0.0005
106.2 - 0.0006
107.6 - 0.0007
108.7 - 0.0008
109.7 - 0.0009
110.7 - 0.001
116.7 - 0.002
120.2 - 0.003
122.7 - 0.004
124.6 - 0.005
126.2 - 0.006
127.6 - 0.007
128.7 - 0.008
129.7 - 0.009
130.7 - 0.01
136.7 - 0.02
140.2 - 0.03
142.7 - 0.04
144.6 - 0.05
146.2 - 0.06
147.6 - 0.07
148.7 - 0.08
149.7 - 0.09
150.7 - 0.1
156.7 - 0.2
160.2 - 0.3
162.7 - 0.4
164.6 - 0.5
166.2 - 0.6
167.6 - 0.7
168.7 - 0.8
169.7 - 0.9
170.7 - 1.0
171.5 - 1.1
172.2 - 1.2
172.9 - 1.3
173.6 - 1.4
174.2 - 1.5
174.7 - 1.6
175.3 - 1.7
175.8 - 1.8
176.2 - 1.9
176.7 - 2.0
180.2 - 3.0
182.7 - 4.0
184.6 - 5.0
186.2 - 6.0
187.6 - 7.0
188.7 - 8.0
189.7 - 9.0
190.7 - 10.0
191.5 - 11.0
192.2 - 12.0
192.9 - 13.0
193.6 - 14.0
194.2 - 15.0
194.7 - 16.0
195.3 - 17.0
195.8 - 18.0
196.2 - 19.0
196.7 - 20.0
197.1 - 21.0
197.5 - 22.0
197.9 - 23.0
198.3 - 24.0
198.6 - 25.0
199.0 - 26.0
199.3 - 27.0
199.6 - 28.0
199.9 - 29.0
200.2 - 30.0

-chris

[Duo]

It's really amazing how little an eardrum can detect.

[Da5id4Vz]

Ive heard the analogy that if our vision were as sensitive as our hearing that we would be able to see a candle burning in on the beach in California while standing on the beach in North Carolina. (Of course ignoring that part about the curvature of the planet)

[Duo]

That makes sense. Just think about all the different nuances of thing you can detect with hearing. So many things can be observed with sound.

[Dave Bullet]

How about this. does anybody know how to make a dB meter here? I'd be into doing that.

Assuming you have a PC with a soundcard (not a bad assumption to make, since you are posting on this board Checkout making your own electret mic at Speakerbuilder.net . Parts are only a few dollars.

Ok - it won't be callibrated, but even if a few dB off you should be able to get some rough idea.

Oh yeah - you'll need some software like Speaker Workshop (free from Audua) to measure the response.

Hope that helps,
David.

[AuroraB]

For some years, I worked with 1Joule pulsed lasers, and we used to joke about that you could look at it,- twice...

For the 200dBs,-..

" I will say thiz only onze...."

( I think the euros get the pun...)

[BigDaddyD]

92db efficiency speakers are like 2x as loud with the same power at the same distance as 89db speakers. Not 3db like you would figure.

Actually, this is almost right. To get 3db louder takes twice as much power, doesn't mean you perceive 3db as twice as loud. Actual perception of twice as loud is 9db which would require eight times the power.