Audio apps are extremely common nowadays, and they represent a significant part of our lives. To ensure an excellent user experience, their specifications need to be tested thoroughly and work flawlessly.
In this article, we’ll discuss the theoretical aspects of audio files and audio testing, creating a testing base before starting the actual process of testing audio files.
We’ll start by talking a bit about the audio apps for mobile devices (tablets, phones) on different operating systems (Android, iOS, etc.).
These apps must recognize and run quite a few file formats, ranging from uncompressed audio formats (AIFF & WAV) and lossless audio formats (ALAC, FLAC, WMA) to compressed, lossy audio formats like MP3 and AAC. It depends entirely on what the owner of the app requires from the end product.
All audio files come with their own characteristics. The most important ones are sample rate/frequency, bit rate, and bit depth.
We’ll go over brief definitions for these terms before going any further.
What is an audio file format?
Essentially, it’s a way to encode information in a computer file.
Each audio format has its specifications and limitations regarding the 3 parameters described below:
- Sample rate – how many times per second a sound is sampled. Sampling involves taking snapshots (“reading”) of an audio signal at a fast rate measured in Hertz (Hz). 1 Hz = 1 sample/second, 44100 Hz means 44100 samples per second. The higher the number, the better the audio quality.
- Bit rate – how much data (bits) is transmitted in a given amount of time. 320 kbps (kilobits per second) means that 320000 bits are transmitted in a second.
- Bit depth – the number of bits of information found in each sample. A bit depth of 16 or 24bit means that the device uses 16 or 24 bits per sample.
As apps have to face such a wide range of file types, each audio app must be able to play at least the most common of them (MP3, AAC, WAV, and FLAC).
- MP3 is a file format that contains an audio stream of MPEG-1 Audio, or MPEG-2 Audio encoded data.
It’s a lossy data compression format – during the encoding process, some of the data is discarded or lost. It happens because certain sound components are beyond most humans’ hearing capabilities and due to space-efficiency concerns. A 320 kbps MP3 file can be in excess of 75% smaller than a WAV file.
- AAC (Advanced Audio Coding) was designed to succeed MP3 by having a higher sound quality at the same bit rate.
However, it’s still a lossy data compression format. Although MP3 is an almost universal standard format, AAC is the default audio format for iOS-based devices and others.
- WAV (Waveform Audio File Format) was developed by Microsoft and IBM for raw and typically uncompressed audio.
Its encoding format (linear pulse-code modulation or LPCM) retains all the samples of an audio track. For this reason, WAV-LPCM is used by professional users and audio experts for maximum audio quality.
- FLAC (Free Lossless Audio Codec) is another lossless audio encoding format, but it is an open format with royalty-free licensing.
Finally, audio files can have 1, 2, or more audio channels (mono, stereo, and multi-channeled files). They can be played through devices with one speaker, two speakers, or surround speakers.
However, that doesn’t mean a mono audio file can’t be played through a surround system or a multi-channeled file cannot be played through a single speaker.
It only means that these files are designed specifically, not exclusively, for a certain amount of speakers.
Plus, it also depends on the hardware used to distribute each channel to each speaker (in case files have more than one channel).
Some file formats support more than two channels, but most are designed to output stereo sound.
We know this amount of tech-talk can be a bit overwhelming, but bear with us; they’ll help you understand what comes next.
Testing Audio Apps
First, we have to make sure we have the right test tracks to be played in the app.
The most common sample rates for MP3 files are 44100 Hz (44.1 kHz) and 48000 Hz (48 kHz). The bit rates are usually between 256 and 320 kbps.
It would be best to have files with a constant bit rate as they can be more precisely used. The bit depth for such files is usually 16 or 24 bit, rarely reaching 32bit.
To ensure that most aspects of playback are covered, a tester should use MP3 test tracks that have these specifications:
- 16bit, 44100 Hz, 256 kbps;
- 16bit, 44100 Hz, 320 kbps;
- 16bit, 48000 Hz, 256 kbps;
- 16bit, 48000 Hz, 320 kbps;
- 24bit, 48000 Hz, 320 kbps;
The main purpose of an audio app is to render audio files as sound. Therefore, testing these apps should always be accompanied by a good pair of headphones because the playback shouldn’t work only on internal speakers.
If your device also has Bluetooth capabilities, a BT speaker should also be available to the tester.
The tester should also know exactly the type of devices the owner wants the app to run on. Usually, audio applications are meant for most Android or iOS devices, but some have specific filters or effects for certain tablets or phones.
Follow us for the second part of this article, where we’ll discuss the practical aspects of audio testing:
- How to create audio files.
- How to use them in apps.
- What to look for when running tests while using them.
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