Sound / db Meters - Please Help

[matt09]

I am looking to purchase a meter such as this one http://cgi.ebay.co.uk/VISION-Analog-...QQcmdZViewItem
however I do not understand the A and C weighting frequency response, does it really cut off so early at 32hz? I have looked at many of the radioshack meters and they all seem to say the same but surley that can't be right? I wanted a meter which would give an accurate reading down to around 10hz. Can someone advise me please.

Thanks

[Ron E]

What do you want to do with it?

SPL meters are rarely a good buy - not at all useful for measuring frequency response. For the money it would cost to get one that would measure 10Hz accurately you could buy a "real" measurement mic and a preamp.

About the only thing a SPL meter is really useful for is setting or checking levels in a sound or home theater system...and for that 10Hz don't matter much.

[Cloth Ears]

Ditto what Ron said. In the UK, you're looking at a couple of thousand pounds (you read right) to get a hand held meter that reads that low (accurately). Most SPL meters are for industrial purposes, require rugged construction, close tolerances, and the ability to be used in court for evidence. So there's a big jump between home use ones (cheap) and the better ones. There doesn't seem to be any made for in-between.

UK for Svantek is acsoft.

With regards your question on A and C weighting, it is first necessary to acquire a basic working knowledge of the way we recognize sound. Largely significant is the fact that our ears are not equally sensitive to all frequencies and that these differences are more pronounced at low volume levels than at high ones.

These differences, known as Equal Loudness Contours, represent the SPL required at any given frequency to create the same perceived loudness as a 1 KHz tone. For example, to give the same apparent loudness, a 40 Hz tone would have to be 15 dB higher than a 1 KHz tone at 80 dB, and nearly 40 dB louder than a 1 KHz tone at 30 dB.

Now, "A" weighting is intended for relatively low SPLs (20 to 55 dB) and it's response curve correlates most closely to the ear's frequency response at these low listening levels. "B" is intended for medium level (55 and 85 dB) and the "C" for measuring SPLs of 85 dB+.

You should understand, however, that any weighting network can be used at any SPL. For example, the US government uses the "A" network when measuring noise in industrial environment. In general, the network used by authorities will almost always be determined by the circumstances involved and a clearly defined set of rules and regulations (well, hopefully). Whatever the case may be, correct measurements will require a simple understanding of these weightings. Without it, a weighting network is next to useless.

You should be able to find a lot of info on these weightings (and the specific weighting curves) on the Web.

[matt09]

Thanks for your replys, I wanted one as I am soon to embark on a Sonotube Subwoofer project and would use one to help get a flat response, which was why I wanted one with a decent low frequency extension and measure volumes in general. When I said accurately I just meant as in not within the hard limits of the device, ie <80% accuracy, rather than precision.

I don't that SVS sell Radioshack meters to go with their subs, so their only any good to around 32hz? Or are they designed for calibrating crossover frequencys with their subs which are obviously higher.

I understand the weighting system now, thanks I realise about about equal loudness contours, I didn't know that this was what the weighting was for.

So for what I want, there is no such thing avalable cheaply then?

[Cloth Ears]

I'm looking for a laptop that's been thrown into a dumpster (an early Pentium should be enough I think). A mike, plus a soundcard and some measurement software (on the cheap - or freeware if possible) to top it off.

I wouldn't be using it for anything else, so that's why I'm looking for a laptop on the cheap. But I figure that the soundcard and then the mike will be the biggest expense.

A bit more than the radioshack version, but heaps cheaper (and probably more versatile) than the industrial versions.

[Conrad Hoffman]

We have a nice handheld SPL meter at work. It's actually a third octave analyzer, and flat as a board. Unfortunately, it was $5000, and $1000 of that was just for the little mic cartridge. Out of my price range! Something you should be aware of is the amount of time required for good low frequency measurements. Analyzers with individual channels for each frequency band have the edge there, but usually aren't that desirable for other reasons. I believe our $5K device works that way. Swept analyzers, like my ancient GR sound & vib analyzer are inherently slow, as they mechanically chug through each band, then sit there for a while while the readings settle. An FFT spectrum analyzer like Visual Analyser (free & good) will have to be set for a fairly long update time via samples and rate if you want decent resolution at low frequencies. You'll also find that every footstep in the house or building, plus outside noises, will show up clearly! Finally, when you do LF measurements, resonances will drive you crazy. You have to excite the system with noise to do this right. Visual Analyser (and many others) let you generate any signal you want, including noise, and mic the results in real time. You can make measurements nearly for free today that would have cost tens of thousands of $$ a couple decades ago, if they could be done at all. My measurement system consists of Visual Analyser, an E-mu 0202 USB external sound interface, and various small condenser mics.

[RobWells]

Hi Matt,

There's a comparison of my 50 quid digital spl meter from Farnell against my ECM8000 / trueRTA measurement here:

Room Gain in Real Life

I used warble tones to do the spl meter measurements.(dots)

Rob.

[Ron E]

If all you want is to match a sub to your mains - the cheapo meter will work fine.

If you spend $40-60 more on a ECM-8000 and a UB-802 Mixer, you can measure more than just spot frequencies using Speaker Workshop and progress to designing crossovers or tweeking the speakers you already own. The ECM-8000 also rolls off below 20-30Hz IIRC

Measuring spot frequencies with a SPL meter is a drag. I have some "measurements" of speakers I designed, done with a RS SPL meter, and they look almost nothing like "true" measurements.

[Juergen Knoop]

Originally posted by matt09
Thanks for your replys, I wanted one as I am soon to embark on a Sonotube Subwoofer project and would use one to help get a flat response, which was why I wanted one with a decent low frequency extension and measure volumes in general.
So for what I want, there is no such thing avalable cheaply then?

I could send you the new ATB PC demo via email (app. 2MB).
It is limited to 1kHz and works with most build in soundcards. The soundcard mic input is sufficent. Because of the bandwith limitation the choice of microphone is not very critical. Even cheap electret types should work at low frequencys
Regards
Jürgen