Measure with Spectroid App
On this page we cover “Spectroid”, a real-time audio spectrum analyzer. We use this app in
our Facebook group. You will find a few frequently asked questions and instructions for measuring annoying hums at the end of this page.
But first you have to download the Android app to your smartphone or tablet. A smartphone is more suitable here, since it usually comes along with a better microphone. After installation, the app is already ready for use. To our Facebook group it makes sense that every member uses the same settings to help comparing your results.
You should apply the following setting. You get into this menu by touching the three dots at the top right corner.
Now simply tap on the corresponding word within this menu.
First select „Audio“ by tapping on the word and change the values accordingly:
Audio source – default
Sampling rate – Auto (48000 Hz)
Processing: FFT size 2048 bins (23 Hz/bin)
Window function – Blackman-Harris
Desired transform interval – 50 ms (20Hz)
Now head back by tapping ←Audio to apply those settings.
Moving on to Display:
Spectrum: Frequency axis scale – Linear
Waterfall – ½ screen oder 2/3 screen
Max-hold trace – off
Peak markers – off
Again apply the settings we just made by tapping ←Display
Next is Miscellaneous, where we activate “Subtract DC” (tap on the slider located on the right).
Apply the settings you have just made by tapping (←Miscellaneous; ←Settings) twice.
Spectroid will now show you its first results. However it makes sense to adjust the measuring range.
The numbers located between the lower part (the waterfall) and the upper part (spectrogram) show the frequencies. By default you will see a range between 0 and up to 10.000 hz, which is not really helpful.
Limit this range from 20 to 200 hz
To do this, place two fingers on the display simultaneously and then pull them apart. It is the same gesture you would use to enlarge an image. If the frequencies we want to monitor disappear from the visible range, you can make them visible again by moving both fingers in parallel. That way you will only move the measuring field.
To increase the contrast of the lines within the spectrogram, you can adjust the value range on the y axis as well. To do this, pull your fingers apart vertically.
Now what data can we evaluate with this spectrogram? Below you will find a sample.
In the upper area (spectrogram) you will see a horizontal yellow and red line. The yellow line shows the different frequencies, the red line shows the maximum db value (volume) that was measured. In the lower area (waterfall) you can see a vertical purple stripe. This is a constant hum with a frequency of 49.8 Hz. You can read the frequency and the measured db value in the spectrogram. The many small horizontal stripes are other noises, but they do not show a hum that lingers.
The vertical gradient we are measuring is the timeline. This allows us to determine whether a noise suddenly occurs or stops. In our sample on the right the buzzing sound stops abruptly.
Vertical gradients are sounds / noises that can be heard over a constant period of time.
Horizontal stripes, regardless of their length, show a short tone that extends over several frequencies, just like our voice, which can also produce a variety of frequencies.
Take a Screenshot.
To take a screenshot it might be useful to pause the app. Just tapp on the pause-symbol on the upper right.
Now take your time and take a screenshot as you would normally do on your smartphone / tablet. If you don’t know how to do that, simply google up the modell or brand you are using and add “how to take a screenshot”. You can share this screenshots so that it can be viewed by others.
If you want to reset all the settings we have just made, simply follow the instructions below and everything will be reset back to default.
Click on the image to enlarge it.
Frequently Asked Questions:
F: Why are all the dB-values negative?
A: Spectroid uses dBFS (full scale). 0 db determines the maximum level input level of your microphone. That being said, the values are negative because the sound levels we are measuring are normally far below that level. In other words: -20 dB are louder than – 80 dB. This is shown more clearly through the peaks in the upper graph.
F: Can I zoom in horizontally or vertically?
A: Yes. place two fingers horizontally on the display and pull apart. It’s the same gesture that you would use on your smartphone or tablet to zoom in. That way you can change the graph to your needs.
F: Why is there a vertical black gap within the spectrum and the waterfall?
A: Spectroid makes use of several different FFTs (algorithms) that overlap in frequency to improve results in reading lower frequencies. The disadvantage of this method is that it deviates when responding to an impulse and a minor frequency deviation. On the other hand the results shown are closer to a person’s perception of natural sounds.
F: Anything else I need to consider?
A: It’s best to put your smartphone / tablet into flight mode. That way your measurements will not be adulterated by a call or by receiving a message.
F: What is a spectogram?
A: A spectrogram is a visualization of the spectrum of a signal. It visualizes how a signal is composed of different different frequencies. Typically this is shown in a graph that visualizes data on a time axis.
Note: The app is not available for iOS. If you want to proceed with an Apple product we recommend using SpectrumView.
How to improve your results
There are many ways to track down a hum. Following this guide you will be able to get to the bottom of your issue. We start by measuring the room that is bothering us the most. While taking a measurment it is important to lie your device down flat on a surface. Do not move the device or expose it to anything that might cause vibrations (also: do not move) because that might distort our readings. Follow up by measuring other rooms and take screenshots of your individual results. Name each screenshot accordingly (name of the room or magnitude of the event). When you have completed this, turn off power at your fuse box and repeat. Again, take screenshots and name them. That way it is possible to determine whether the hum is connected to any electrical source located within your apartment or house.
If everything is turned off and no hum is visible in the app you can easily determine what is causing the problem by turning off everything one by one. If the hum still persists after that you might continue. Try to narrow it down by checking while standing by an open window or at other places in front of your house. Again, take screenshots. The closer we get to whatever is causing it, the higher the sound levels in your app will be. Spectroid shows negative values for dB, so f.e. -80 dB is louder than -100 dB. Try to take some measurements during different times of the day and compare them. This might help to determine whether something that is located outside your house is causing the hum.
Because low frequency sounds are all around us, you will need to verify that your measurements are actually matching the sound that is bothering you. By using a tone generator you will be able to determine whether the measurement that you have taken meets the actual sound: https://www.brummton.info/bestimmen-der-brummtonfrequenz-mittels-tongenerator/
Many people encounter a 50 Hz or 100 Hz peak. Our power grid uses 50 hz, thus those frequenies are pervasive. If a 100 Hz peak accompanies a 50 Hz peak, that is usually caused by a fridge. A hum that is caused by a fridge is usually a solid-borne sound that is capable of being transferred through ceilings. Heating systems might also cause a 50 Hz hum, while ventilation systems usually cause peaks between 60 to 80 Hz.
It is highly recommended to take measurements when you are unable to actually hear the hum. That way you will be able to compare everything more easily. If both measurements are near-equal to each other, an internal issue (f.e. a low-frequency tinnitus) might be the cause of your issue. Same applies if you might hear something, but Spectroid is unable to capture an external sound while the hum seems to be present for you.
We have listed some characteristics of a low-frequency tinnitus here: (https://www.brummton.info/tinnitus-merkmale-und-eigenschaften/).
To make sure that your smartphone is technically able to capture low frequency sounds you might try using Spectroid while holding your device close to a subwoofer or any other device that can play low frequency sounds. Most devices that we have tried are able to capture sounds up from 17 Hz. Smartphones are usually far better at capturing sounds than your own hearing would be, it might be helpful to check whether you are actually able to hear the lower frequencies that your smartphone is catching.
Please note: This guide is ment for people who are not familiar with Spectroid or with taking measurements. There are many other factors, like resonances within a room, standing waves, room modes, differrent aspects of structure-borne sounds and infrasounds that we are not going to cover here. We consider this a quick guide that might be useful for someone who wants to get started with Spectroid.