Hi all --

I work in a chemistry lab, and need to wear a half-face silicon/plastic respirator with filter cartridges while giving instructions. The respirator muffles my voice when I talk, so folks cannot understand what I am saying. Light surgical masks (ala blue covid-19 ones) do not muffle the voice much, but also do not provide a proper seal. So, I am looking to make a self-contained, relatively compact voice amplifier which I could clip-on onto the respirator. The mic would be on the inside of the respirator, while the speaker, the battery, and the amp box would be on the outside. The mic leads would run through the respirator shell, sealed with silicon for air tightness. Since human voice is 1-2W, I figured the device can be small enough to fit on the mask. It would be also OK if the battery was on one side, and the speaker in the centre, and the amp on the other side, as long as the mask is self-contained.

I have already tried voice amp systems in which the speaker is clipped onto the belt, and the mic lead is tucked under the respirator seal. The system worked, but the the wires get cumbersome (entangled etc), and the mic lead does not allow for the proper seal to form. The voice speakers I found were too large to be attached to the mask, too.

From what I read on this forum, it appears that having the mic and the speaker in close proximity (i.e. directly on the mask) would be a problem for simple circuits. Are there designs that would be resistant to interference despite the physical proximity? Should it be a digital circuit, not an analog one (like I tried below)?

To experiment with, I got this self-contained A386 circuit from DFRobot with a speaker rated at 0.5W (now I understand it's under-powered, as 0.5W is peak power, i.e. poor quality). I connected it to a 9V battery and a smartphone earphone jack. This level of sophistication also tells you what I know about circuits ( I do have all the tools though). I got the sound out, although the sound quality was poor (like cheap 1970s pocket radios), barely approaching normal human voice by volume, with distortions. My laptop speakers seem much smaller, but deliver much better sound. What characteristics should I look in speakers to get "proper" sound quality?

The circuit is supposed to have connections to ground... but if this is to be wearable, where would the ground be? My face? For now, I left the ground lead intact, and just connected - and +.

The A386 circuit did not specify mic parameters (impedance?) at all, so I tried a few mics lying around, and no sound came out at all. What parameters of the mic should I look for, particularly if I want the voice to sound as "natural" as possible?

Overall, it looks like I will have to build a circuit from scratch, but the A386 one was useful to experiment with.

 

Ground in a circuit schematic usually represents a common (COM) reference node. Think “ground” on an automobile or aircraft.

I would investigate a Bluetooth microphone and a speaker which can be located anywhere away from the mask. Even an FM mic to an FM radio would be a quick solution.

 

hi run,
Welcome to AAC.
I would suggest you remove the LED's, especially D1 across the speaker.
How many people are in the listening group and their distance from each other.?

E

 

hi run,
Welcome to AAC.
I would suggest you remove the LED's, especially D1 across the speaker.
How many people are in the listening group and their distance from each other.?

E

Hi Eric --

I forgot to mention: the circuit is self-contained, all components are on an mass-produced board. Not sure if I can remove the LED:

https://www.dfrobot.com/product-165.html

People can be up to 5-10m (15 ft) away, like in a classroom setting, except that each student has her own chemistry set. A setting where one needs to speak clearly, but no yelling is needed to be heard.

 

Ground in a circuit schematic usually represents a common (COM) reference node. Think “ground” on an automobile or aircraft.

I would investigate a Bluetooth microphone and a speaker which can be located anywhere away from the mask. Even an FM mic to an FM radio would be a quick solution.

When I set up my audio system in the car, ground was the metal frame of the car , or so I found by trial and error.

I believe 3M makes (very expensive) masks with built-in voice amps, similar to those seen in popular culture in Breaking Bad. So I was hoping there was a solution, since they figured out how to do it. The speaker does need to be self contained in the mask, b.c. I need to move around, fix equipment, take the mask on/off constantly to wash my face, etc. When I had the speaker clipped on the belt, separately from the mask, it would often get covered with the lab coat, fall on the floor, buttons would get pressed by butt-bumps during experiments, get disconnected, etc.

 

One advantage with a wireless mic is that you can have multiple speakers strategically located around the lab so that each student can hear you well even at low speaker volume level.

 

I second the Bluetooth idea. There are bluetooth speakers available with high quality sound. And the wearable part would be minimal that way.

Bob

 

One advantage with a wireless mic is that you can have multiple speakers strategically located around the lab so that each student can hear you well even at low speaker volume level.

I agree that this would work, but there is a lot going on: I begin by giving instructions to the class, then once they get on with it, I switch to one-on-one. Plus, there is more than one lab per day (i.e. 4-5 different rooms). Thus, I was wondering if there is a more "native" solution to transmit my voice through the mask (even sound-guide pipes or smth, if the electronics idea does not pan out).

Bluetooth idea: would mounting the speaker on the front of the mask, with the mic inside, still cause interference, like in a simple audio 386 circuit, or would the bluetooth layout be more resistant to feedback loops?

 

hi run,
OK, built unit.
An option would be a BT audio transmitter in the head unit, paired with your laptop and using the laptop to drive a low cost ready built audio amp.

E
https://www.ebay.co.uk/sch/i.html?_...leDesc=0&_osacat=0&_odkw=BT+audio+transmitter


&
https://www.ebay.co.uk/itm/PC-Compu...051242?hash=item36684c17aa:g:zwEAAOSwn9VfM6oJ

 

A wearable system, with decent voice quality, covering that radius, is going to be large. Specifically, the speaker probably is going to be larger than you want hanging on your belt or neck. I think you should adjust your expectations and think in terms of a small sound reinforcement system: Separate audio power amplifier and speakers, powered from the AC powerline. An interesting option would be a small guitar amplifier. My brother has one that is about a bout 12 x 12 x 6 inches that has amazing fidelity.

ak

 

I would suggest you remove the LED's, especially D1 across the speaker.

Actually, that one is not as bad as it first appears. The circuit is an LM386 powered with 5 V, and the output is AC coupled to a grounded load. In that situation, the amp will barely make +/-2 V output, and a typical small LED has a Vf of around 1.8 V - 2.1 V. Yes, the LED can clip the positive peaks, but that will happen only if the output is within millivolts of clipping anyway. Not the world's greatest piece of design, but not as bad as I first thought.

ak

 

Industry have delved into this problem. Look up FM classroom.

 

OK, I think I was mislead by my thinking that since human vocalization only has 1-2W output, this is the number to look for with the speakers as well. I wonder why the perceived power of the human voice does not seem match the rated power of voice-amp circuits. 2W is not a lot of power at all. E.g. my laptop has two 2.5W and 2.5W speakers and it sounds "good".

I will experiment with bluetooth, and work on integrating the mic into the mask seamlessly.

 

hi run,
OK, built unit.
An option would be a BT audio transmitter in the head unit, paired with your laptop and using the laptop to drive a low cost ready built audio amp.

E
https://www.ebay.co.uk/sch/i.html?_...leDesc=0&_osacat=0&_odkw=BT+audio+transmitter


&
https://www.ebay.co.uk/itm/PC-Compu...051242?hash=item36684c17aa:g:zwEAAOSwn9VfM6oJ

Well, I would need to have a laptop with me in different rooms at all times, need to think about.

I could have a small "controller" box (laptop, raspberry pie) in a fanny pack on my back.

Could also look into self-contained 3M masks with a voice amp.

 

yeah, I am familiar with this system: it came up on google when I started researching the topic. This system won't work because I work in multiple classrooms of various sizes every day, and do one-one-one as well. One-on-one does not work with these solutions, as 80 people would need to listen to every one-on-one conversation emanating from all speakers. Plus the costs of wiring up 10-20 labs.

I will see if I can pair a bluetooth mic and a bluetooth speaker with a raspberry pie, then experiment with placing the speaker on the mask or head-lamp style. The raspberry pie box could be a small running backpack, powered with a 5V battery.

 

I wonder why the perceived power of the human voice does not seem match the rated power of voice-amp circuits.

The efficiency of the speaker. A speaker is an electrical-energy-to-mechanical-energy transducer, and as such has a conversion loss. This can be significant, and varies widely from one device to another.

Way back in the day, when "acoustic suspension" speakers appeared on the home stereo scene, a common review was that they sounded great(ly) but took a ton of power. A common specification for better speakers is dB/W - sound pressure level (SPL) output relative to a reference value - per - watt of electrical energy input. Note that this is almost always the Watt's Law value of the input energy (volts RMS at the speaker terminals squared, divided by the DC resistance), not the complex value (Vrms x Irms).

https://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_level

ak

 

Then I would still suggest a wireless (e.g. Bluetooth) solution to minimize the hardware on the mask.
Put the wireless speaker on a belt around your waist. You can adjust the volume with a control on the speaker.

Here is another idea:

https://www.entandaudiologynews.com...ove-communication-in-theatre-in-the-covid-era

 

If a microphone can hear its output in a speaker then the sounds go around and around making howling and squealing acoustical
feedback. You wrongly called it 'interference".
You need to prevent the mic from hearing the speaker by having a wall or sound barrier between the mic and speaker. Record it and play it back a little later with the mic turned off?

Most mics used today are "electret" type. They need some DC power applied with a resistor from a filtered positive voltage.

The LM386 robot amplifier has a gain of only 26dB which is much too low for a mic. Its input is an RCA jack from a
"line level" (not mic level) signal. The datasheet of the LM386 shows that its gain is 10 times more when a capacitor is added between pin 1 and pin 8. Then if the mic is close to your mouth and you speak fairly loudly the mic and speaker will work if you prevent acoustical feedback.

The datasheet for the LM386 shows that with a 6V supply (four 1.5V battery cells) the output power into an 8 ohm speaker is only
0.25W with low distortion. When powered from a new 9V battery the power is 0.56W. A higher voltage or adding more speakers will overload it and make heat instead of more sound.

Have you heard the tinny sounds made by a tiny speaker? You do not want that.

 

A "throat mic" was mentioned. It produces grunts, groans and vowels, not normal speech.

 

Many thanks for the explanations, they do make sense. Indeed, I was able to only pick the sound from a pre-amped audio line. DSRObot did call the RCA input "mic" in the specs; perhaps they should have mentioned specifically that the mic had to be pre-amplified.

I was indeed thinking to write into a file, and then read from it with a 0.1s delay. But since some of my voice still comes through the mask, it may create strange echo sound effects (not necessarily a bad thing, just awkward).

I agree about the throat mic: youtube videos I've looked at sounded distorted.

I think I will go ahead with MrChips suggestion and get a bluetooth mic, a bluetooth speaker, and a portable microPC with a bt dongle. All of these components can then be recycled for other projects anyway.

May be I can also add a layer of foam to the mask to prevent some of the positive feedback loop.