Humans can hear sounds up to about 20kHz for children and 16kHz for adults. Sounds with a higher frequency than this are considered ultrasonic. Many microphones can easily hear ultrasonic frequencies. But what about Metal Marshmallow contact mics, how high can they hear?
Theoretical Limits
The theoretical upper limit of the frequency range of each Metal Marshmallow microphone is listed below.
Model
Upper Limit
MM II White
234 kHz
MM II Red
73 kHz
MM II Black
23 kHz
MM Pro (Lowest Gain)
13,000 kHz (13 MHz)
MM Pro (Highest Gain)
250 kHz
Note that the theoretical upper frequency limit decreases with higher gain. Note also that Metal Marshmallow Pro has an upper frequency limit that is about 13 times higher than Metal Marshmallow II for equivalent gain settings.
Sampling Frequency
In order to record ultrasonic sounds, you will need to use an audio sample rate that is at least twice as high as the highest frequency you are trying to record. For example, using the standard 44.1kHz sample rate, you can't record frequencies much above 22kHz, which is barely ultrasonic. Using a 192kHz sample rate you can record frequencies up to 96 kHz. Note that you will need a recording interface that supports frequencies in the desired range. I personally do not have a recording interface that goes higher than 192kHz, and I have never tested Metal Marshmallow microphones at higher frequencies than this.
Some Data
To illustrate the ultrasonic capabilities of Metal Marshmallow Pro, I did the following. I stuck the microphone onto an ultrasonic dog bark deterrent, as illustrated in the image at the top of this article. This device emits loud ultrasonic tones when its input level surpasses some threshold. I recorded these tones using a 192kHz sample rate. The original recording is in Figure 1 below.
Figure 1: Raw recording (wav file) of an ultrasonic dog bark deterrent recorded with Metal Marshmallow Pro using a sample rate or 192 kHz.
On inspection, it consists of three consecutive approximately sinusoidal tones. The frequencies are about 13.7k kHz, 20.5 kHz, and 27.3 kHz, respectively. In solfeggio this would be do - sol - do. The average spectrum over the whole recording is plotted in orange in Figure 2 below. The noise floor of the mic + audio interface is plotted in blue for reference.
Figure 2: Average spectrum of the dog bark deterrent over the entire recording.
Note that there are peaks at each of the aforementioned frequencies, and integer multiples of those frequencies due to harmonic distortion.
For the sake of illustration, I also pitch shifted the original recording down by 2 octaves and resampled it to a 48 kHz sample rate and re-encoded it as an mp3. The result is in Figure 3;
Figure 3: The same recording as in Figure 1, but pitch-shifted down by 2 octaves for illustration.
It is a nice little melody for the dogs. Notice that each consecutive tone is lower in amplitude than the previous one. This could be because a) the dog bark deterrent plays them at successively lower amplitudes, b) because the higher frequencies transmit through the plastic more poorly, or c) because the mic is not as sensitive in this range. I'm not sure which of those options is the correct one. Notice also that the noise floor increases substantially above 50 kHz. I am again not sure whether this additional noise originates in the microphone or the audio recording interface.
Conclusion
All Metal Marshmallow mics can record ultrasonic sounds to some degree, with Metal Marshmallow Pro having the widest frequency range. Have a blast with it and don't let te dogs get you down.
