Re-Wiring a Microphone to use in an Oscilloscope

Thread Starter

Cool_Runnings

Joined Oct 2, 2017
2
Hi,
Could someone please offer some guidance on how to wire a microphone so that I can plug it into an oscilloscope. I know it can be done because I have seen it being used with an oscilloscope. So the input into the oscilloscope requires two banana plugs. Currently the microphone has a jack on the end. On the other end of the microphone lead is a 3 way female adaptor which the microphone plugs in to. I have seen people re-wire a stero jack to two banana plugs but can this be done for a microphone?

Thanks for any help offered.
 

Reloadron

Joined Jan 15, 2015
5,667
Depends on the microphone. Some microphones generate a voltage out and some don't requiring some external circuitry.
Certain crystals change their electrical properties as they change shape (see How Quartz Watches Work for one example of this phenomenon). By attaching a diaphragm to a crystal, the crystal will create a signal when sound waves hit the diaphragm.So a Crystal Microphone or Piezo Microphone generate their own voltage out. Condenser Microphones require some external power so they would not work directly connected to a scope. There are other types of microphones also so it depends on what you have?

Now if you have a computer with a microphone plugged into it and want to just see wave shapes you could download Audacity and talk into your mike and see what you would see on a scope. Audacity is free software and pretty neat.

Ron
 

MrChips

Joined Oct 2, 2009
22,101
There are two basic types of microphones:

a) a dynamic microphone (which is basically a coil in a magnetic field)
b) a condenser microphone (or electret mic)

An electret mic has a FET (field effect transistor) amplifier and this requires a power source.

Both microphones will produce very low currents and voltages, too low for proper observation on an oscilloscope, You will need to amplify the signal by a factor of 100 to 500.
 

gerty

Joined Aug 30, 2007
1,292
If you just want to see a waveform on your oscope you can connect a simple speaker to the scope, turn the volts down low and there you go..
 

Thread Starter

Cool_Runnings

Joined Oct 2, 2017
2
There are two basic types of microphones:

a) a dynamic microphone (which is basically a coil in a magnetic field)
b) a condenser microphone (or electret mic)

An electret mic has a FET (field effect transistor) amplifier and this requires a power source.

Both microphones will produce very low currents and voltages, too low for proper observation on an oscilloscope, You will need to amplify the signal by a factor of 100 to 500.
Thanks for the input. So I have done some digging around and this is exactly what I want to do with it:


Measure the speed of sound.

What sort of microphone will this guy have and how do you get one that connects into an oscilloscope?
 

Reloadron

Joined Jan 15, 2015
5,667
The video, in my humble opinion. is poorly made. The person doing it does not explain the test equipment used or settings used. He looks to be using an audio frequency generator do drive a speaker. The microphone used appears to be a crystal type as mentioned earlier. He makes no mention of the microphone specifications or if the microphone pickup is amplified. Microphone sensitivity is defined as how much sound pressure level is required to get how much output. Microphone sensitivity is typically measured with a 1 kHz sine wave at a 94 dB sound pressure level (SPL), or 1 pascal (Pa) pressure. The magnitude of the analog or digital output signal from the microphone with that input stimulus is a measure of its sensitivity. This reference point is but one characteristic of the microphone, by no means the whole story of its performance. I suggest you give this a read. So the microphone you use must be sensitive enough to detect the volume from the speaker used at several given distances. The scope is used to measure phase shift of a fundamental frequency on what looks to be Channel 1 which is also the scope trigger and the microphone pick up on channel two allowing the scope to be used to calculate phase shift between the two signals of the same frequency. CALCULATING PHASE SHIFT WITH AN OSCILLOSCOPE is a good read on the subject. Before I forget there are unidirectional and omnidirectional microphones, for this a unidirectional would be the better choice. You may get by just using a cheap piezo sensor with a homebrew cone wrapped around it. As long as you can pickup the speaker signal. As to generating the signal I mentioned audacity in my first post. You can drive a PC speaker using it and it will generate tones. Again, the video leaves quite a bit to be desired in my opinion.

Ron
 

Alec_t

Joined Sep 17, 2013
11,746
He makes no mention of the microphone specifications or if the microphone pickup is amplified.
Hard to tell from that pic, but it looks as though the mic cable is slimmer than the scope input cables: so my guess would be that the mic signal goes via an amplifier.
 

Reloadron

Joined Jan 15, 2015
5,667
Hard to tell from that pic, but it looks as though the mic cable is slimmer than the scope input cables: so my guess would be that the mic signal goes via an amplifier.
I agree. The guy has a pile of stuff and explains next to nothing. The video just leaves out too many important details in my humble opinion. :)

Ron
 

MrChips

Joined Oct 2, 2009
22,101
If your objective is to study the speed of sound in air, then you need to be able to send a burst of a few cycles at a fixed frequency.
The speed of sound in air is about 340m/s which is 34cm/ms.
In order to discern sub-millisecond phase delays you want to transmit acoustic waves at the highest frequency possible.
You would be better to work with ultrasonic transducers which typically operate at 40kHz which is beyond the upper limits of human hearing.
 

Tonyr1084

Joined Sep 24, 2015
5,249
The speed of sound will vary with changes in atmospheric conditions. Air pressure is the first and probably the foremost contributor to change in the speed of sound. Higher air pressure (typical hot dry summer weather) is more dense, thus, the speed of sound will move more quickly, but be attenuated by distance more so than by cold air. Humidity changes the speed to. Sound through water is vastly different from sound through air. Cold dense air will conduct sound a longer distance, which is why you often see the lonesome train whistle from across the valley in the winter time. Lower air pressure (stormy weather) doesn't conduct sound as well. Given more moisture in stormy atmospheric conditions, and the sound is more quickly attenuated again.

The fact that his measurements came up with something different from what Google may say it is tells me that he may be at a lower altitude than the reference point given by Google (which I haven't looked up). He's wearing long sleeves, potentially wooly sweater, so I'm guessing that the room he's in is cold. Relatively speaking. Colder air is more dense, and hence, sound will travel faster through denser material.

I recall watching guys hammer on a concrete walkway down a hallway in a school. You'd hear the hit through the concrete faster than you would that of the sound transmitted through the air. However, the sound through air was louder, meaning it was not being as greatly attenuated as it was via the concrete.

Speed of sound is generally calculated at sea level under a specific temperature and atmospheric pressure, whatever that may be. Move away from any or all of those preset conditions and your measurements will be different every time.
 

Reloadron

Joined Jan 15, 2015
5,667
From our friends at Live Science:
:On Earth, the speed of sound at sea level — assuming an air temperature of 59 degrees Fahrenheit (15 degrees Celsius) — is 761.2 mph (1,225 km/h). Because gas molecules move more slowly at colder temperatures, that slows the speed of sound; sound moves faster through warmer air".

From our friends at Wiki:
"The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium. ... At 20 °C (68 °F), the speed of sound is 343 metres per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn), or a kilometre in 2.91 s or a mile in 4.69 s".

What I don't remember and likely will never need to remember is how much the speed of sound changes with change in temperature or temperature. Would those changes be even noticed over a short distance of several yards or meters? I doubt it would matter.

A little off topic but when I was on the Marine Corps rifle range we took turns working "the butts" which is the target impact area. Each line of fire, working back from the targets has phone box jacks so the guys working and pulling targets in the butts can talk with the guys on the firing line. When the shooters were on the 500 meter line and I was on the phone in the butts I could hear the shot on the phone followed by the bullet crack (traveling still super sonic) above my head as it passed over and through the targets. Typical velocity was about 2650 feet per second (807.72 meters sec) Something else which was cool was early surveyors in the western US would fire a revolver on a hill and a gun watching on another hill would wait, counting, to hear the bang. The revolver, in those days would emit a puff of white smoke visible to start counting and you stop counting when you hear the bang. Close enough. :)

Ron
 

Tonyr1084

Joined Sep 24, 2015
5,249
Higher air pressure (typical hot dry summer weather) is more dense, thus, the speed of sound will move more quickly, but be attenuated by distance more so than by cold air.
Boy did I get that one backwards. Hotter air is less dense. Otherwise, what I said about the speed of sound moving more quickly through dense air still remains accurate.

My understanding of wave propagation is that one molecule bumps the next, transferring its energy. In a more dense medium (such as water or concrete) that propagation would be faster than a less dense medium such as air.

Who am I to question the statement by "Live Science"? Still, there have been errors before. Since Live Science and I disagree, there most certainly is at least one error. Just who is at error, let OTHER science define. If I'm wrong - then I'll own the error and amend my thinking on the subject.
 
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Tonyr1084

Joined Sep 24, 2015
5,249
According to NDE-ED.ORG:

Density

The density of a medium is the second factor that affects the speed of sound. Density describes the mass of a substance per volume. A substance that is more dense per volume has more mass per volume. Usually, larger molecules have more mass. If a material is more dense because its molecules are larger, it will transmit sound slower. Sound waves are made up of kinetic energy. It takes more energy to make large molecules vibrate than it does to make smaller molecules vibrate. Thus, sound will travel at a slower rate in the more dense object if they have the same elastic properties. If sound waves were passed through two materials with approximately the same elastic properties such as aluminum (10 psi) and gold (10.8 psi), sound will travel about twice as fast in the aluminum (0.632cm/microsecond) than in the gold (0.324cm/microsecond). This is because the aluminum has a density of 2.7gram per cubic cm which is less than the density of gold, which is about 19 grams per cubic cm. The elastic properties usually have a larger effect that the density so it is important to both material properties.

Looks like the error was mine after all. I own it.
 
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Tonyr1084

Joined Sep 24, 2015
5,249
Wait a minute!

Air Density and Temperature

Suppose that two volumes of a substance such as air have different densities. We know the more dense substance must have more mass per volume. More molecules are squeezed into the same volume, therefore, the molecules are closer together and their bonds are stronger (think tight springs). Since sound is more easily transmitted between particles with strong bonds (tight springs), sound travels faster through denser air.

However, you may have noticed from the table above
(actually below) that sound travels faster in the warmer 40oC air than in the cooler 20oC air. This doesn't seem right because the cooler air is more dense. However, in gases, an increase in temperature causes the molecules to move faster and this account for the increase in the speed of sound. This will be discussed in more detail on the next page.

OK, I'm SO confused right now. But back to what I originally said: The room just have been colder. Just how much colder it would have to be to affect sound in that short a distance - I haven't a clue. But look at this table, which suggests just the opposite:

Speeds of Sound

Material Speed of Sound
Rubber…….…...60 m/s
Air at 40˚C….….355 m/s
Air at 20˚C..……343 m/s
Lead…………...1210 m/s
Gold…………...3240 m/s
Glass…………..4540 m/s
Copper………..4600 m/s
Aluminum……..6320 m/s


my source: https://www.nde-ed.org/EducationResources/HighSchool/Sound/speedinmaterials.htm
 
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