Impulse / Phase / Frequency – responses

In order to understand what a transfer function measurement is displaying, you need to understand what the impulse response / phase response / frequency response means.

Abbreviated, IR / PR / FR:


WIKI article – Impulse Response
WIKI article – Phase Response
WIKI article – Frequency Response

In my ignorance, I basically ignored the IR and went straight for the FR (incorrectly thinking of a FFT measurement as a RTA of sorts). Knowing better now, the IR is the very first and most important trace. The IR trace reveals acoustics and secondary reflections that may be caused by poorly aimed speakers, poorly placed measurement mics, etc…

The IR can also provide information about polarity. If the impulse trace shows a peak that goes negative instead of positive, you may have to resolve that issue before doing anything else.

It’s essential to be able to read and understand what an IR trace reveals.

Once you have an acceptable IR (which also provides a time offset between source and response / reference and measurement), then you can move on to the PR and FR traces with confidence that you aren’t wasting your time.

Scott Theater – house PA failure

There must be a better way to tell a house person that their PA has issues than just telling them their PA has issues because doing so wasn’t met with much applause.

During the Dog Days loadin / setup it was discovered that the HR main had an issue. What issue? No lows and no highs but nothing blown???? Strange but very noticeable.

Ultimately we took both the HL and HR speakers down (after labeling them) to attempt to fix the HR speaker and to measure and verify what the issue is. With a working HL speaker that is the same make and model, it will be obvious if there is something wrong.

I have no doubt there is something wrong. I just don’t know what it is yet. Once I measure them both, I’ll post the results and hopefully correct the issue and provide new measurements once they are both fully functional again.

More soon.

Dog Days – Scott Theater

On Thursday we began the load in process for Dog Days (the opera). On Friday the traveling crew arrived. The sound designer discovered that something was wrong with the HR main speaker. No lows and no highs. The box is a Renkus Heinz (????) which has a 15″ woofer and 2″ driver on a horn. I’ll know more once I measure the cabinet against it’s working counterpart but my guess is that a component on the crossover has failed and the phase between the drivers is off now. Both components are working so it’s not a blown speaker / driver. What do you do when you find a malfunctioning house speaker? Take it down and put up a different one of course!

The system consists of the following:

(2) QSC KW122 speakers hung as L / R for mains.
(1) TOA HX5 4 box mini line array for a center cluster graciously left behind by TCU’s Bill Eckenloft.
(2) QSC KW122 speakers hung for side of stage monitors.
(4) Mackie SRM150 hotspot type speakers in the orchestra area.
(2) Mackie SRM350 rear of house speakers for the helicopter

Today we tuned the system.

PHOTOS TO COME

10000 plus visitors

Even if there are only a handful of visitors who are actually interested in audio measurments and the rest are google and microsoft searching for information, audiomeasurement.com has hit a milestone of 10,000 + visitors.

Cheers!

Dallas City Performance Hall – Fulcrum Acoustics

I’m back in a local Dallas venue that has a Fulcrum Acoustic sound system.

The system consists of:

(2) DX1265 for upper main L / R
(2) DX1295 for lower main L / R
(4) US212 subs located in cavities under the stage.
(3) CX5D for under balcony fills
(7) CX5D in the face of the stage for front fills

There is a center cluster that is not a Fulcrum product.

(1) D&B TI10L 5 box line array center cluster

d&b audio – Ti10L webpage

DCPH Stage

DCPH house

This is my 3rd visit to the venue with Texas Ballet Theater. Previously I have known that the system wasn’t optimized but didn’t know enough to know what to do about it. I also didn’t have a relationship with the house crew to start digging into their settings. I have simply measured and Eqed at the board as best I could.

This time, there is enough trust between myself and the house crew to get into the DSP settings and discuss what might be wrong and what might be done to make things better.

What are the issues? The PA appears to have issues of speaker placement and aim. Poor coverage on the sides, uneven response, uneven levels, overlapping coverage, possibly incorrect delay times, speakers that are reflecting off near by surfaces, subs located too close to specific seats, subs mounted in resonant chambers, etc…

If the proof is in the pudding, it simply doesn’t sound very good. The good news is that I think the existing system can be greatly optimized with proper aim & correct processing. Neither of which requires much effort and cost to correct.

Since I haven’t finished gathering the necessary information to go forward with the verification process yet, I’ll hold my final judgement but in the meantime, here are some interesting images…

This is the DSP block that is processing one of the DX1265 cabinets.

Fulcrum Acoustics DX1265 super module blocks

HPF

Fulcrum Acoustics  DX1265 HPF

LPF

Fulcrum Acoustics  DX1265 LPF

LF EQ

Fulcrum Acoustics  DX1265 LF PEQ

HF / LF EQ

Fulcrum Acoustics  DX1265 HF LF PEQ

HF / LF FIR (Finite Impulse Response)
WIKI article – FIR / Finite Impulse Response

Fulcrum Acoustics  DX1265 HF LF FIR

This is interesting,

The upper DX1265 Main L / R boxes have a delay between the HF/LF & LF driver of .438 ms:
Fulcrum Acoustics DX1265 LF delay

The lower DX1295 Main L / R boxes have a delay between the HF/LF & LF driver of 1.479 ms:
Fulcrum Acoustics  DX1295 LF delay

Same box, same dual 12″ LF drivers. The DX 1265 has a 60 x 45 horn. The DX1295 has a 90 x 45 horn. What accounts for the 1 ms+ difference in delay times between lower and upper boxes (between the LF and HF / LF drivers) ???

Dog Days – pre loadin mic check

The Fort Worth Opera festival loadin starts tomorrow. Dog Days is one of the operas in the festival but it’s being produced in a different venue. The Dog Days load in starts on Thursday.

WIKI article – Dog Days opera

NY Times – Dog Days

Here is the input list that came with the show:Dog Days original input list

I have some of these mics and some will be substituted for arguably “better” mics. Some will be substituted with arguably “comparable” mics.

This afternoon I setup a mic test rig using a QSC KW122 self powered speaker. I used the speaker because it is relatively flat and being self powered, took about 1 minute to have pink noise coming out of it. I set up a mic stand at roughly 6′ away from the speaker. The speaker’s HF horn was located at roughly 6′ off the ground with no objects between the mic stand and the speaker.

Dog Days mic test rig

Note that the KW122 has a 3 way switch on the back. The choices are “Ext Sub” (reduces low end), “Normal” (flat response), “Deep” (extends low end). After measuring those 3 settings with the first mic, I selected “Normal” for all subsequent measurements.

This trace shows what the switch does:

INSERT TRACE HERE

Then I proceeded to take some reference measurements using my matched pair of Earthworks TC30K omni mics. Then I measured with my newly acquired Audix TR40 pair (non matched). After that I went through the rest of my measurement mics including an Earthworks M30BX (self powered), TC25K omni and then moved on to the mics to be used in the production.

This includes:

AKG 535
AT 4033
Earthworks SR20 / SR69 (same thing but older model)
Earthworks FM500 (flex mic)
Oktava MC012 / MK012 (same thing but newer model)
Sennheiser E604
Shure SM57
Shure SM58 / SM58S (switchable)
Shure Beta 52A

The Oktava bodies can work with different capsules so I compared all (6) cardioid, (4) omni (2) hyper cardioid capsules.

All in all it took about 3 hours to go through the process of documenting the serial numbers of the mics, measure and save the data.

INSERT TRACES HERE

The Fort Worth Opera festival loadin starts tomorrow. Dog Days is one of the operas in the festival but it’s being produced in a different venue. The Dog Days load in starts on Thursday. Maybe by next week I have some time to update this page and add the measurement data…

How to calibrate your SPL meter in Smaart 7

My AMPROBE SM-CAL1 Sound Meter Calibrator just arrived and I thought I would figure out how to setup an accurate SPL meter in Smaart 7. Fortunately, Rational Acoustic’s “Calvert Dayton” has already provided instruction on how to go through the process via a post on the Rational Acoustics forum. Calverts response is #7.

Re: SPL –measurement does not what it should

Instructions as follows:

1. Open the Amplitude Calibration dialog window by clicking CalibrateSound Level Options button in the dialog (Options > SPL/LEQ…).
2. Select the audio device and input channel that you want to calibrate.
3. Turn on your calibrator and wait a moment for the average to stabilize — say 10-15 seconds for a microphone calibration using an SLC, maybe 20 to be on the safe side. 20 seconds would be enough time to build from -138.5 dB (all 0’s in a 24-bit integer) to 0 dBFS (all 1’s), given a 0 dBFS reference tone.
4. Click the Capture button.
5. Type your nominal reference level (e.g., 94, 104 or 114 dB) in the Calibrated Level field — when performing a microphone calibration in the field you generally want to use the highest output level that your sound level calibrator offers.
6. Hit the Enter key to recalc the Calibration Offset.
7. Click the Apply button.
8. If you want to calibrate additional inputs, go back to step 2 and loop through again. Otherwise click the Done button to exit the dialog.

Here is another website that described the method:

Smaart V7 calibration for SPL

Shure Beta 52a

I have a production coming up where the visiting engineer wants a Shure Beta 52a for the kick drum. I could substitute an arguably “better” mic but I happen to like the sound of a Beta52A on kick drum and it around $125 street price, it doesn’t hurt to have an industry standard mic in my collection. One of the things that manufacturers do to “kick drum” specific mics is to PAD them down so they don’t overload an input. There is usually some extra low end involved.

As a reference, there are many kick drum specific mics on the market and then some non kick drum specific mics that have been used for that purpose such as the EV RE20, Sennheiser MD421 and Beyer M88.

Here are some articles on kick drum mics, none of which provide more than opinion about the difference between the mics they compared:

recordinghacks.com – kick drum mic shootout

gearslutz.com – 11 kick drum mic shoot out

soundonsound.com – bass drum microphones

What does it all mean without measurements to see what is what?

INSERT Beta52A measurement data
INSERT D6 measurement data
INSERT RE20 measurement data
INSERT MD421 measurement data
INSERT AKGD112 measurement data
INSERT E901 measurement data
INSERT Earthworks SR25 data
INSERT Earthworks TC30K data (for reference)

audiomeasurements.com Youtube channel coming soon…

Unfortunately, there is a limited amount of useful information currently on Youtube regarding audio measurements in general. Consequently, I’ve geared up to build a Youtube video channel specifically related to audio measurements.

I’ll be using Camtasia screen recording software to capture the measurement process, add titles as well as edit and assemble non screen video and still media together.

Camtasia webpage

If you have an idea for a video that explains something you want to learn regarding audio measurements, contact me.

[sweetcaptcha_contact_form]

Ineffective acoustic treatment 040715

I just came across some old install photos and thought I would share.

A while back I hung a new sound system at “Pirates Voyage” in Myrtle Beach South Carolina.

piratesvoyage.com

We replaced the existing EAW PA with a Meyer Sound rig designed and provided by Pro Sound out of their Orlando Florida office.

In reviewing the photos of the old rig, I was reminded of what appears to be “acoustic treatment” on the backsides of the EAW arrays.

Pirates Voyage old PA cluster

I can only guess that someone was trying to reduce some of the low end / low mid content coming off the back of the cabinets. The thickness of the padding appears to be about 2 inches. I would argue that 2 inches of padding will cause zero reduction of any sound coming off the back of the cabinets as that sound would be omnidirectional anyway and completely ignore the padding. Whatever resonance was the culprit might of been better served by putting the padding on the inside of the cabinets.

If you want to know how a speaker will behave when it’s producing sound, tap on the sides of the speaker (any speaker / any side) with your knuckles. If it is relatively muted and of higher pitch, that suggests a well braced speaker. If you excite the walls of a speaker when you tap on them and it sounds like a drum, it will sound like that same drum at certain frequencies when it produces sound.

A perfect speaker cabinet wouldn’t vibrate at all. Think of a speaker cabinet being made of concrete. There are some home stereo speakers made of concrete but for professional applications, concrete speakers just don’t make any sense. Speaker design is a balancing act. Cost, weight, performance, etc… IF you brace a speaker cabinet correctly you don’t need to use concrete to get a relatively vibration free cabinet.

If you mount a speaker inside a structure that doesn’t vibrate at all, then all the energy of the speaker will be transferred into the air. Instead we settle for some cabinet vibration but you certainly don’t want cabinet resonances where the cabinet literally sings along. Inexpensive molded speakers typically make for hand good drums. The better the speaker, the better the cabinet design and bracing. Some inexpensive speakers are mounted in a wooden box with no bracing. That might as well be a hand drum. Speaker design is more than porting and cabinetry. A well designed cabinet lets the speaker does all the work.

There is a balance between light weight and good sound.

Flown subwoofers 040615

In the sound reinforcement industry, flown subs is a relatively new trend. In the early days of large scale sound reinforcement even the main PA was typically stacked on the ground, stage or located on scaffolding to raise it up.

What is the benefit of flying subs?

For one when they’re on the ground those closest to them are getting a lot of low end. For instance, those standing right up against the barricade. If you’re 3 feet away from a sub array, it’s going to be really loud in comparison to those who are 30 feet away or 300 feet away. By flying your subs you equalize the distance at which the audience can be in proximity to your subs. Another benefit is that when your subs are flown with your mains you have the opportunity to time align the subs for a larger portion of the audience. When you fly your main speakers and put your subs on the ground, you’ve got a limited amount of time alignment choices and none of them are going to work for the entire audience.

Frequency & Wavelength 040615

I’ve been looking into the concept that a line array needs to be approximately 12 feet long for directional control down to about 100hz.

As you shorten your array, the directional control of your array starts to climb in frequency.

In order to know what frequency your short line array becomes omnidirectional, you can use this website calculator:

MC2 System Design Group – Wavelength calculator webpage

Here are some lengths of frequencies or “wavelengths”.

Wavelength of 100hz

Wavelength of 250hz

Wavelength of 500hz

Wavelength of 1000hz

If the goal is directional control down to to crossover point where the sub kicks in, obvious 100hz / 12feet / 3.44 meters is pretty much a minimum. What if we can get by with a sub crossover point @ 125hz?

Wavelength of 125hz

So we still need 9 feet / 2.75 meters of line array length.

What if we want our directional control to extend below 100hz? How about 50hz? We need 22.6 feet of line array.

Wavelength of 50hz

25hz equals 45 feet! I don’t think I’ve ever seen a 45 foot long line array but I have seen some that are 20+ feet long.

Wavelength of 25hz

What is the benefit of having directional control down to the sub crossover frequency? I’ll have to do some more research to answer this question fully but I can tell you that your stage sound will benefit from not having a bunch of low end and low mid energy coming off the main PA and loading your stage. The concept of putting sound where the people are means the audience and not the stage. The last thing I want is low end from the PA system bleeding into all my stage mics.

Along these same lines are the use of cardioid subs. I would guess that with a long enough array (at least 12 feet) and cardioid subs, you may have very little PA sound on stage which is an obvious benefit to the band / talking head, etc…

Toby Dean Guynn of TOBY Speakers passes away – 033015

Today is a sad day here at audiomeasurements.com…

I just found out that my friend and long time mentor Toby Dean Guynn passed away on March 30th, 2015.

The first time I met Toby, I walked into Toby Speaker Corp on Montgomery with a very simple question. He walked me over to the nearest chalk board and began to explain. I didn’t get the answer I was looking forward. In fact, I think I left with more questions than answers that day. I didn’t realize it at the time but that first Q&A session would lead to a 20+ year friendship with the man. Toby was one of those rare people who understood what he knew very well but was always willing to state “I don’t know…” when he didn’t know something. He was a man who would ride his bike to the bank to make a deposit. A man who never lost his fascination with “good sound” and the pursuit of “better sound”. A musician, artist, recording engineer, inventor and business man who made a career out of selling a good product for a good price with a good warranty. If there is one person who inspired me to strive for being a better sound engineer, gaining better understanding of acoustics, speaker building, recording techniques and just being well rounded, it was Toby.

He will be greatly missed.

Toby Dean Guynn

TOBY Speakers carries onward…

tobyspeakers.com

For those interested, here is a reprint of an article about Toby that appeared in the local newspaper a while back:

A Short History of Toby Corp of America

Friday On The Green – 030315 event

This evening I returned to the site of Friday On The Green to hear what the show sounded like. The event producer states that this event typically draws between 1500 and 3000 people. Based on the people I saw, I would guess there was at least 3000 people. More people means more territory to cover.


fortworthsouth.org webpage
Friday On The Green pano

Friday On The Green pano

Near the stage the PA sounded relatively good. As I moved further away from the PA the intelligibility dropped off along with the SPL level until what I could mostly hear was low end. This is to be expected because low frequencies can travel much greater distances than high frequencies. Consequently, as you move away from the sound source the high frequencies are reduce due to air but the low frequencies keep right on going. Depending on the expected coverage area, you might add delay speakers to compensate for the SPL and high frequency losses due to distance. You can only push a certain PA so much before you run out of headroom. In the case of the PA system in discussion, the PA is simply not capable of producing enough SPL for the outdoor audience which this time was huge in comparison to previous Friday On The Green events.

Here is a good website that explains how distance affects sound intensity & the inverse square law.

HyperPhysics – Inverse Square Law Sound webpage

Inverse Square Law for sound

Inverse Square Law plot for sound

Simplified, the sound level (intensity) coming from any PA (with a few exceptions) is going to diminish by 1/2 with each doubling of distance. For example, if you measure 100db at 1 foot, you will measure 94db @ 2 feet, 88db @ 4 feet, 82 @ 8 feet, etc…

This means a few important things.

1. In order for a PA to be loud enough far away, it has to be much much louder up close.

2. If the PA is stacked on the ground or the stage and the audience is able to get any where near the PA, they will be subjected to excessive SPL IF the PA is loud enough further away.

2. Raising up the PA reduces the level discrepancy between being close and being far away.

3. The PA you choose for a given event MUST be capable of a certain SPL output without distortion or else you’ll either be forced into distortion to meet the expected SPL requirements at a distance OR you’ll have to accept a lower SPL level and those farther away will have bad sound.

This event is a perfect example of an undersized PA system. There is no way this PA (in it’s current configuration) can provide even coverage to the audience. Obviously there is different levels of “unqualified” when it comes to PA systems. While this PA might seem “unqualified”, This is probably the best PA the event has had to date. The first time I went to an Friday On The Green event, the entire PA was a few small Mackie SR450 mains and (1) Mackie 1×18″ sub per side. The current QSC PA is like a Rolls Royce in comparison.

Given the size and shape of the audience, Friday On The Green warrants a much larger rig with side fills, front fills and if the PA can’t be raise high enough, delay speakers. Budget limitations are what limit the PA configuration.

Friday On The Green happens all Spring and Summer long so I’m sure there will more chances to measure and discuss this event…

Friday On The Green – 030315 measurements

Today I volunteered to measure and help tune a PA for Fort Worth Weekly’s “Friday On The Green” event this evening.
The self powered QSC PA consists of (4) KLA boxes for mains (2 per side on a pole) & (2) KW152 for cross firing front fills. For low end, (4) KLA18 and (4) KW181 subs were used. PA processing was handled by a dbx Driverack 260.

Here is some information about the QSC speaker components used for this PA.

QSC KLA webpage
QSC KW webpage

Friday On The Green PA

Right off the batt, we know that the main PA is too low given the physical circumstances. The ground where the PA sits is lower than the grass that the audience will be on. Maybe as much as 3 feet lower.

QSC KLA12 and KLA18

Ideally the KLA12 speakers (on the poles) would be raised up more. Enough so that those near the front aren’t getting slammed trying to get the PA loud enough @ FOH position and further back into the crowd.

QSC KLA12s & KLA18 subs

QSC KW152 and KW181 sub

Due to the outdoor nature of the gig, I took one of my hardcore windscreens for a Earthworks TC30K. I’m sure the HF response is affected by using such a heavy duty windscreen but better that than wind disturbances that ruin the entire trace.

Earthworks TC30K with windscreen

I also could barely see my screen so I went to my truck and grabbed my coat for a roof.

SpectraFoo Complete under coat

How does one tackle a PA like this? First things first is to verify that your DSP outputs are all feeding the expected speakers. In this case some repatching was necessary. While the dbx Driverack 260 is a capable device, without a PC and RS232, making adjustments on it is tedious. Fortunately FOH engineer Adrea Edmonson was well versed with the device and able to make the necessary EQ and delay adjustments on the fly from the front panel.

We started by measuring the HR KLA array (2 boxes) near FOH but the wind was pretty strong so I moved the mic more into the nearfield of that zone. Same measurement results but with less wind influence.

Here is what we measured before and after eq was added to the QSC KLA12 house right array:

QSC KLA HR duo with and without eq

Ideally you can setup your measurement rig right next to where you make your DSP adjustments and “see” what you’re doing but I was busy hiding under my coat trying to see my screen and Andrea was trying to see the front display on the dbx Driverack 260. Consequently our filter shapes don’t quite match the measured system response and so we ended up with some holes. No matter what, I’d rather have a few =6 to -3 dips in my system response than some +6 to +9db peaks. Especially in the 250hz to 2khz region.

This is the final house right frequency response:

QSC KLA HR duo eqed

Next we added the HR subs to the HR main PA and got this:

QSC KLA HR duo eqed with eqed sub

Between +9 and +24db for sub levels!!! We checked the subs to discover that the amp levels (self powered boxes) were above unity (noon). This brought us back into the +12 category. From there we added a filter at 63hz to bring the peak down a bit. I don’t have a screen shot of that post eq measurement. We were running out of time and the audience was already starting to stake their claim on the grass.
I didn’t even discuss sub alignment but if you look at the trace above, look at the lower trace (phase) and notice that around 100hz (between 63 and 125) the two traces (pink and blue) are close to lining up. With a small amount of delay we could of aligned the phase responses at the crossover point which would make the PA sound better for some listeners.

Lastly we had the front fills to manage. A pair of cross firing QSC KW152 with sister KW181 subs.

Here is the before and after EQ measurements:

QSC KW FF with and without eq

Note what appears to be comb filtering. Also ignoring the dips, note that the KW152 / KW181 combination has excessive low end content between around 50hz and 1200hz. It’s possible that the switches on the KW181 was in the DEEP mode and the KW152 was in DEEP mode. Or one cabinet was and one cabinet wasn’t. Regardless there is as much as +12 of low end being produced by one of the front fill stacks. Now combine both and you will have even more excessive low end. We ran out of time but it would of been good to measure the response of both front fill stacks together and also the whole PA acting as a system. I have little doubt that the system would of benefited from another 30 minutes of measurement and tuning. Next time.

Of special note was the sub configuration for this event.

Onsite was a total of (8) QSC single 18″ self powered subs. (4) KLA18 and (4) KW181. Technically these cabinets should be the same as they have the same specs, same amp and same drivers. The difference being that the KLA18 cabinets can be flown and the KW181 cannot. (3) QSC subs were stacked, one on top of the other, with the highest sub being used as a base to raise up the KLA pair. That makes for a total of (6) subs. The remaining (2) were used to create a full range front fill pair. When you need low end having your subs on the ground added extra intensity without costing extra money or requiring extra power but in this case, the subs were used to raise the main speakers up above the crowd so let’s assume that wasn’t a choice to be made. In the same situation, I think I would try a stack of (4) subs per side to raise the main PA even more and to make sure the all (4) subs per side acted together. I see no reason to have a front fill system that can go down to 40hz when in order to stand where you can hear those front fills you are in the power alley of the main subs arrays.

If there was a way of flying the QSC KLA12 pairs per side instead of mounting them on top of the subs, a mono configuration with (8) subs spaced evenly across the front of the stage might be a better configuration. For more on sub configurations, watch Dave Rat’s video on sub configurations here:



Amprobe SM-CAL1 – sound meter calibrator


sm-cal1

With a sound meter calibrator, you can verify the output level of a measurement mic so that you know that your SPL measurements are accurate. Yesterday I secured an AMP SM-CAL1 with 1/4″ adapter for around $100 on Ebay. Between the 1/2″ and 1/4″ adapters I should be able to maintain all of my measurement mics.


Screen Shot 2015-04-03 at 8.59.32 AM

AMPROBE SM-CAL1 sound meter calibrator webpage

AMPROBE SM-CAL1 sound meter calibrator data sheet PDF

AMPROBE SM-CAL1 sound meter calibrator product manual PDF

QUOTE:

“The Amprobe SM-CAL1 is a sound meter calibrator with two output levels of 94 dB and 114 dB. The calibrator generates these fixed sound level signals for calibration of sound level meters. The unit ships with a ½” adaptor installed to accommodate sound meters and microphones with ½ diameter.

Battery: 9V, 006P or IEC 6F22 or NEDA 1604.
Electrical (sound) standard: ANSI S1.40-1984 and IEC942 1988 Class2.
Output sound level: 94dB and 114dB re 20 uPa under reference.
Accuracy: ± 0.5 dB.
Output frequency: 1 kHz ± 4 %
Two output levels of 94dB and 114dB
Output frequency of 1000Hz
Fits ½” microphones
Easy one handed operation
Low battery indicator
CE, conforms to ANSI S1.40 – 1984 and IEC942 – 1988 class 2
Includes ½” adapter, 9V battery and users manual

This is taken from the user manual (ENGLISH)


Screen Shot 2015-04-03 at 9.11.53 AM

In a future post I will provide details as to how to use one of these devices.