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I've tried several Electret Condenser Microphones as input to a generic USB audio sound card: MO064202-4 MO064402-4 MO064402-5

All mics work great at close range (about 1 or 2 inches). However, none of them would pick up any sound at ranges further than that.

I tried two different pre-amp circuits, they actually made the problem worse.

I suspect, I'm using the wrong type of microphone to pick up sound from across the room. What should I search for at DigiKey/Mouser/Arrow?

Thank you!

Additional info: This is for a custom wall mounted all-in-one computer. So a separate boundary microphone is not ideal. The microphone is only used for short spoken communication, whispers would be nice, but normal conversational level is sufficient. Not for music, not for voice recognition.

The pre-amp I tried uses LM358 op-amp chip. Powered with 9V PSU.

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  • \$\begingroup\$ What is your use-case? A USB condenser microphone could be an all-in-one solution (e.g. Blue Yeti), or a boundary microphone designed for conference calls (also comes in USB variants). What preamplifier circuits have you tried? \$\endgroup\$
    – loudnoises
    Jun 20, 2018 at 5:11
  • \$\begingroup\$ What kind of sound are you trying to record? Voise? Music? What is your preamp circuit? How was it powered? How loud is the sound source: whispers? shouts? \$\endgroup\$
    – Henry Crun
    Jun 20, 2018 at 5:12
  • \$\begingroup\$ Digi-Key has lots of directional mic’s made of MEM’s in SMD that will work better so you don’t get as strong echoes. You have a “noise cancelling mic ( short range) \$\endgroup\$
    – Tony Stewart EE75
    Jun 20, 2018 at 6:47
  • \$\begingroup\$ Is the amplifier circuit in the link to the 358? The link doesn't work for me. \$\endgroup\$
    – JRE
    Jun 20, 2018 at 9:10
  • \$\begingroup\$ Pick a microphone with a proper data sheet. \$\endgroup\$
    – Andy aka
    Jun 20, 2018 at 9:17

3 Answers 3

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You need to pay attention to the following items:

  1. High sensitivity - measured in dB, and always negative. You want a value closer to 0. That is to say that -38 is better than -44. Sensitivity is measured relative to a given pressure and voltage. 0dB would mean your microphone is as sensitive as the standard in the definition. See this article from the Arrow site (actually linked on the microphone pages you posted.)

  2. High signal to noise ratio - the microphone (and its internal amplifier) produce noise. You want as little noise as possible, else it will drown the quieter signals from across the room. This is also measured in dB, but positive numbers. You want the highest you can find. 60dB is better than 55dB.

  3. Directionality - if your device is on one side of the room and you want to pick up sounds from the other side of the room, then you will want a cardioid microphone rather than an omnidirectional (make sure it faces the correct direction.) If you need to pick up sounds from all around, then stick with omnidirectional microphones.

  4. Amplification - even the best microphone will have a low level output. You will want a good, low noise amplifier between the microphone and the sound card. You could use gain (multiplication) on the digital data from the sound card, but the sound cards are usually noisy so you would be amplifying the noise as well as the signal - your signal to noise ratio will stay the same (or get worse.) I'd find a pre-amplifier that can take microphone signals to line level and use the line in on the sound card. You said you tried an amplifier and it made it worse - well, too much amplification is bad, and a bad amplifier (poor signal to noise ratio) would hurt things, too.

  5. Automatic gain control - if you are trying to make recordings of sounds from all over the room, then the volume will vary greatly. The further the source is from the microphone, the lower the level. AGC attempts to keep the recorded sound level approximately the same. You can do this in software or as part of the pre-amplifier.

Points 4 and 5 are obviously not part of the microphone, but are parts of the system you are building.

If you use the microphone input of the sound card, then it expects a low level signal. An amplifier intended for line level would produce a signal far above what is expected. You would then hear a lot of noise, and extremely distorted speech.

You can usually change the gain of an amplifier. If the circuit you use provides too much amplification, then you could post the circuit here (new question) and ask how to change its gain.

Or, use the same amplifier on the line in input of the sound card. Some (many?) cheap USB sound cards only have a microphone in, though.

Your microphone is most likely picking up sounds from all around the room - you just can't hear it. Record using Audacity, then use Audacity to increase the volume (try the normalize effect.) I think you will find that your signal is there even without a pre-amplifier, though it may be too close to the noise floor of the sound card to be useful.

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  • \$\begingroup\$ Thank you for taking the time explaining everything in great detail. Very helpful. You are absolutely spot on with item#1. All the mics I purchased are -44dB. I will purchase this -25dB Electret Condenser mic with 70db s/n ratio. I also saw some MEMs microphones with -26dB sensitivity. Could you please recommend a chip to interface these to a normal mic input? Thank you! \$\endgroup\$
    – Hari Wiguna
    Jun 20, 2018 at 18:37
  • \$\begingroup\$ JRE, the -24dB mic arrived and it works wonderfully! It is super sensitive. I was able to hear everything in the room without any pre-amps. Thank you! \$\endgroup\$
    – Hari Wiguna
    Jun 25, 2018 at 14:29
  • \$\begingroup\$ If 24dB did the trick, then that explains why your preamplifier caused problems. 28dB is only 14dB better than the microphone you were originally using. That's a gain of about 5. Preamplifiers are usually setup for much more gain - like 100 or so. That would have been way too much. \$\endgroup\$
    – JRE
    Jun 25, 2018 at 15:53
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When I wanted a microphone to record voice, the USB headset was so noisy that it was unusable. (with a microphone right in front of the mouth)

It required a BC547 transistor pre-amp, extra power supply filtering on the USB power, the USB/ADC chip power, and power filtering on the feed to the microphone capsule. That brought it up to usable, but only after bandpass filtering in software.

So integrated usb audio chips: not very good.

The bigger the microphone the more sensitive it should be.

You probably need a lot of preamp gain to overcome the usb audio chips internal noise, until you can hit the microhones noise floor. You must have a clean power supply to the preamp and the microphone capsule. USB power is very noisy. Sart by using a battery for the preamp and microphone.

Your noise will be much reduced if you can bandpass filter to a narrower bandwidth eg 350Hz-2700Hz if it is speech. Experiment with Audacity.

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Without seeing your schematic we are just guessing. Is the electret mic powered properly? Does the preamp have enough gain or have too much gain?

An LM358 opamp is horrible for audio. It is noisy (rummmble and hisss) and produces crossover distortion. You are using a single microphone but why does our hearing use two ears? So that our brain can do echo cancellation. Try it. Plug up one ear and try to hear speech from the other side of a room and you will hear plenty of echoes unless the room is well padded.

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