Hi fellow DIYers,
I work in a Hospital. We have one decibel sound level meter using a single measurement microphone monitoring a 4 bed ward. This provides inadequate range to evenly measure all 4 beds.
Rather than buy 3 more expensive sound level meters I'd like to add 3 relatively cheap measurement microphones above each bed and mix them with a circuit. The level meter should simply display the loudest microphone's signal. Which bed produces the noise doesn't matter.
I seek advice on circuit design or whether an affordable off the shelf product exists. The other problem to solve is the microphones have to run via fairly long cable (10-15mtrs) and are thus prone to interference. The microphone included with the sound level meter has a built in preamp to allow cable length of up to 20 meters.
Existing calibrated measurement microphones are available fairly cheaply such as this Dayton Audio IMM-6. Whether this has a built in preamp I dont know:
https://www.daytonaudio.com/product/1117/imm-6-idevice-calibrated-measurement-microphone
So I need to mix (sum) the 4 microphones. I should ideally be able to calibrate each one individually with a 94db reference calibrator such as this:
https://www.amazon.co.uk/Calibrator-Microphone-Professional-Calibration-Measurement/dp/B01FFI2OAU
What kind of simple circuit would you design to 'mix' 4 sound level measurement microphones or are you aware of an existing product.
Any help appreciated.
Thanks
I work in a Hospital. We have one decibel sound level meter using a single measurement microphone monitoring a 4 bed ward. This provides inadequate range to evenly measure all 4 beds.
Rather than buy 3 more expensive sound level meters I'd like to add 3 relatively cheap measurement microphones above each bed and mix them with a circuit. The level meter should simply display the loudest microphone's signal. Which bed produces the noise doesn't matter.
I seek advice on circuit design or whether an affordable off the shelf product exists. The other problem to solve is the microphones have to run via fairly long cable (10-15mtrs) and are thus prone to interference. The microphone included with the sound level meter has a built in preamp to allow cable length of up to 20 meters.
Existing calibrated measurement microphones are available fairly cheaply such as this Dayton Audio IMM-6. Whether this has a built in preamp I dont know:
https://www.daytonaudio.com/product/1117/imm-6-idevice-calibrated-measurement-microphone
So I need to mix (sum) the 4 microphones. I should ideally be able to calibrate each one individually with a 94db reference calibrator such as this:
https://www.amazon.co.uk/Calibrator-Microphone-Professional-Calibration-Measurement/dp/B01FFI2OAU
What kind of simple circuit would you design to 'mix' 4 sound level measurement microphones or are you aware of an existing product.
Any help appreciated.
Thanks
The Dayton mic may not be suited to running 15m. From the connector I expect that the preamp is very simple that requires bias voltage that you would get from a PC. I.e. it is compatible with a headset mic.
A mic like the Behringer ECM8000 would work better over longer distances. It does require 48V phantom power which a standard 4 channel audio mixer could supply. The mic would need balanced XLR cables which can be purchased off-the-shelf. The mixer could also feature audio capture over USB to a PC if your budget stretches.
The output of the mixer would feed the level meter. However, you are not going to (easily) arrange a situation where the loudest mic only reaches the level meter. If the mixer has LED meters for each channel, which pushes the cost up, you can by visual inspection route the loudest mic to the level meter and mute the other channels. No automatic operation.
If you were to go the audio capture route, there is software (eg. Audio meter) that can compare two or more different sources after recording, and measure the loudness levels. 140 hours of capture with this software is claimed. I have no personal experience of these products. You will need to research.
A mic like the Behringer ECM8000 would work better over longer distances. It does require 48V phantom power which a standard 4 channel audio mixer could supply. The mic would need balanced XLR cables which can be purchased off-the-shelf. The mixer could also feature audio capture over USB to a PC if your budget stretches.
The output of the mixer would feed the level meter. However, you are not going to (easily) arrange a situation where the loudest mic only reaches the level meter. If the mixer has LED meters for each channel, which pushes the cost up, you can by visual inspection route the loudest mic to the level meter and mute the other channels. No automatic operation.
If you were to go the audio capture route, there is software (eg. Audio meter) that can compare two or more different sources after recording, and measure the loudness levels. 140 hours of capture with this software is claimed. I have no personal experience of these products. You will need to research.
I doubt if it is allowed (under the rules of the hospital and/or under the terms of the hospitals liability insurance and/or under the laws in your country) to use diy equipment for a monitoring system like this. And it would not surprise me if essential (or maybe even all) hospital equipment needs some sort of certification, which could limit the choice of existing products.
I agree if these devices were used as clinical tools for actual treatment or care or if clinical decisions were made on the basis of the sound level meter's data then yes it's a different ballgame and I wouldnt even consider asking the question. However the sound level device, a simple chinese Db meter serves only to notify staff they're being too loud and it does a fair job of that. It was a donation and there is no budget for more. I am looking to donate my time and some materials to extend this existing meter to all 4 beds.
I suppose you are on point regarding the DIY aspect, yes there may indeed be liability issues there so maybe its best to just look for an off the shelf solution or forget it for the hospital setting.
I'd still like to know how one would do this , just to learn something new. How would one merge the signal from 3x cheap omni condenser microphone elements into one. Average them but pass the peaks through. What kind of opamp circuit would be right for this. Would this in effect be a simple audio mixer / summing mixer.
I suppose you are on point regarding the DIY aspect, yes there may indeed be liability issues there so maybe its best to just look for an off the shelf solution or forget it for the hospital setting.
I'd still like to know how one would do this , just to learn something new. How would one merge the signal from 3x cheap omni condenser microphone elements into one. Average them but pass the peaks through. What kind of opamp circuit would be right for this. Would this in effect be a simple audio mixer / summing mixer.
Yes good point on on using a phantom powered mic like the Behriger. I have one as it goes and it uses a very cheap simple condenser element. There are mods on how to improve these behringer mics to deliver performance in line with higher end scandinavian type Mics used for scientific / certified applications albeit the real deal mics use very high quality and expensive condenser elements. One could probably diy a smaller ecm8000 clone for 1/3rd the cost of the Behringer if time isnt a factor in the cost equation of course.
So what kind of mixer circuit would apply for merging 3 measurement microphones and making them act as one. I guess a simple summing type audio mixer based on a few opamps
By summing the signals you get the "average" of all microphones, including eventual nulls in case of cancellation. If you need to measure the loudest of three mics I suppose you need a preamp+rectifier (including eventual "A" filtering) for every mic and connect them via diodes to measure the rectified dc voltage.
True, but it may be in practice that a loud area is many dB above the others, in which case the linear sum is still useful.
4 input channel audio interface and a Raspberry PI or other cheap single board computer? after all you're not looking for a simple peak reading your looking to measure something like LAeq(1min). For one microphone REW works fine but for multiple microphones the suggested software is expensive stuff like smaart. The application is so simple though perhaps you could write your own software? 15m XLR cable will be fine for a phantom powered measurement mic, the cheapest will be fine for this application.
It doesn´t work that way.
If you need calibrated SPL readings, only an original untouched SPL meter can provide it, so you walk to desired measuring point and measure or if you want 24/7 monitoring of highest peaks you buy 4 and install them where needed.
Ans each of them NEEDS a "memory"highest peak retained" function built-in.
You can NOT add random mikes to a calibrated measurement instrument and still call it a calibrated measurement instrument because it is not any more.
Even less in a Medical environment.
I just shudder at the mere idea.
I DID run my late Father´s 43 bed private Hospital and our Lawyer would have murdered me if I had tried anything remotely similar, on liability concerns.
You simply can´t mix them at signal level, each needs its own EQ/weighing filters, proper amplifier, probably a log converter and display, with peak memory: meaning you need 4 meters, period.
Not sure asking at an Audio Forum is the best path, even less sure the Forum might in any way want to be implied in your liability risk, not even in the most indirect way.
Extra concern: even if you swear it´s only for peak level monitoring purposes, suspect Patients or their Families won´t be pleased, AT ALL, by having microphones near them.
Hint: privacy concerns.
Your apparently "Technically simple" question is opening many cans of worms on other levels.
Thanks for your reply. I guess mere mention of clinical environment throws all of this onto another plane that leads to strong replies so as I mention in my reply to the earlier message above, yes indeed going a DIY route in such a setting is not worth any of the headaches. I'll therefore leave this line of thought for all the valid reasons as far as actually 'doing this' in that setting. You and the other poster are I suppose right on that point.
So forget the clinical environment for all its liability concerns and lets just think of the engineering challenge of how to 'mix' 4 signals, each from a microphone that has been independently calibrated feeding into one sound level meter to essentially give it wider reach in a larger space. If 4 seperate devices is the only way so be it but does everyone agree with this.
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Indeed that seems to be the case so let's leave that line of thought and just think of how to solve the engineering challenge regardless of the environment. There are already a couple of good replies in this thread
In that "neutral" case , I would design and build my own system.
I am a minimalist at heart and to strictly meet your needs, no bells and whistles, I would:
1) forget mixing microphones, or try to match random store bought ones (even if calibrated) with unknown specs one present at current Level Meter.
2) get 4 Electret capsules (one of the few things in Life which are always WAY better than their asking price)
3) for each one:
* load/bias it.
Use, say, a 4k7 resistor feeding its Hot terminal from a 9V source, the other terminal is grounded.
Capsule/resistor junction is where Audio appears.
* amplify that ; a simple Op Amp such as half TL072 can supply up to 200X Gain.
And remember Electrets are quite sensitive and produce a "good" signal on their own.
I would add a Gain trimmer to Preamp, you might easily have more signal/gain than necessary.
I assume trigger level is not whisper quiet but "annoying", say "shop/office" level.
* feed each preamp output into a "perfect diode rectifier", easily made using the other TL072 half.
So now each "head amp" sub module will produce a DC voltage proportional to SPL.
Do we need a Log Converter, a digital display, an exact dB scale?
Not at all, just a simple LED showing a threshold has been surpassed ... or not.
A simple comparator with and adjustable DC reference level is enough ... and adding another adjustment point as/if needed.
4) you can build 4 units, to make it clear "who" is being noisy OR send that DC to a central unit which in turn lights up a signal stating anything from "HUSH!!!!" to "STFU!!!" to anything in between 😉
OR: you can build all of the Electronics inside a single box, including one 9V supply for all and all 4 4k7 resistors, plain unbalanced but of course shielded microphone wire is more than enough to connect "just the raw Electret capsules" many meters away to your monitoring central.
5) to calibrate the system, you first measure what means an "annoying noise/chat level" at real locations (so SPL level Meter is not "useless", just nor needed real time as an alarm system, only as a "calibrator"), then at home/lab you play some pink noise to that same level, through any amp+speaker you have available, and use that noise field to calibrate each electret+detector module.
It can all be done in a day or two, at least on Protoboard.
Final layout - PCB - Case - Supply - display (from 4 Leds to lighting a warning sign to sounding an alarm bell at Director´s office he he) will take more, of course.
Cost is nil .
PS: possible enhancements: add a couple Time delays, so:
* it takes more than, say, 1 to 5 minutes of "loud talking" to trigger it, so a door slamming, a car honking, etc. does not trigger it unnecessarily.
* Also Hush/Warning sign stays ON for, say, 2 minutes or more as a reminder that the "keep quiet" rule has been broken.
* you might reduce bandwidth to "voice signal range", say 250 to 2500 Hertz, so it focuses better on actual people talking, not ambient noise which surprisingly, is not THAT annoying (we get used to it and filter it out)
I am a minimalist at heart and to strictly meet your needs, no bells and whistles, I would:
1) forget mixing microphones, or try to match random store bought ones (even if calibrated) with unknown specs one present at current Level Meter.
2) get 4 Electret capsules (one of the few things in Life which are always WAY better than their asking price)
3) for each one:
* load/bias it.
Use, say, a 4k7 resistor feeding its Hot terminal from a 9V source, the other terminal is grounded.
Capsule/resistor junction is where Audio appears.
* amplify that ; a simple Op Amp such as half TL072 can supply up to 200X Gain.
And remember Electrets are quite sensitive and produce a "good" signal on their own.
I would add a Gain trimmer to Preamp, you might easily have more signal/gain than necessary.
I assume trigger level is not whisper quiet but "annoying", say "shop/office" level.
* feed each preamp output into a "perfect diode rectifier", easily made using the other TL072 half.
So now each "head amp" sub module will produce a DC voltage proportional to SPL.
Do we need a Log Converter, a digital display, an exact dB scale?
Not at all, just a simple LED showing a threshold has been surpassed ... or not.
A simple comparator with and adjustable DC reference level is enough ... and adding another adjustment point as/if needed.
4) you can build 4 units, to make it clear "who" is being noisy OR send that DC to a central unit which in turn lights up a signal stating anything from "HUSH!!!!" to "STFU!!!" to anything in between 😉
OR: you can build all of the Electronics inside a single box, including one 9V supply for all and all 4 4k7 resistors, plain unbalanced but of course shielded microphone wire is more than enough to connect "just the raw Electret capsules" many meters away to your monitoring central.
5) to calibrate the system, you first measure what means an "annoying noise/chat level" at real locations (so SPL level Meter is not "useless", just nor needed real time as an alarm system, only as a "calibrator"), then at home/lab you play some pink noise to that same level, through any amp+speaker you have available, and use that noise field to calibrate each electret+detector module.
It can all be done in a day or two, at least on Protoboard.
Final layout - PCB - Case - Supply - display (from 4 Leds to lighting a warning sign to sounding an alarm bell at Director´s office he he) will take more, of course.
Cost is nil .
PS: possible enhancements: add a couple Time delays, so:
* it takes more than, say, 1 to 5 minutes of "loud talking" to trigger it, so a door slamming, a car honking, etc. does not trigger it unnecessarily.
* Also Hush/Warning sign stays ON for, say, 2 minutes or more as a reminder that the "keep quiet" rule has been broken.
* you might reduce bandwidth to "voice signal range", say 250 to 2500 Hertz, so it focuses better on actual people talking, not ambient noise which surprisingly, is not THAT annoying (we get used to it and filter it out)
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Have you verified that the level actually IS different from one bed to another?
In a reverberant space, beyond critical distance Dc, level is flat.
Dc is 3-4 feet in a living room. 9-12 feet in a concert hall. A 4-bed hospital ward is inbetween, but very much on the hard side, so I'd expect Dc near 3 feet. If SPL is 100dB at 9 inches from a loud mouth, it will be 95dB @ 18", 91dB (3dB above a 6dB slope) at 36"/3', then leveling to 88dB anywhere in the room except near the source, a wall, or an open door/window. (Or a big fuzz pad, but hospitals don't do those.)
In a reverberant space, beyond critical distance Dc, level is flat.
Dc is 3-4 feet in a living room. 9-12 feet in a concert hall. A 4-bed hospital ward is inbetween, but very much on the hard side, so I'd expect Dc near 3 feet. If SPL is 100dB at 9 inches from a loud mouth, it will be 95dB @ 18", 91dB (3dB above a 6dB slope) at 36"/3', then leveling to 88dB anywhere in the room except near the source, a wall, or an open door/window. (Or a big fuzz pad, but hospitals don't do those.)