Have you ever experienced not being able to hear something due to the presence of a louder sound? Well, that’s basically the issue we’ll be talking about today. If you are intrigued by this occurrence, and are curious to learn more about it, then the appropriate question you should be asking is – What is Auditory Masking?
The subject of acoustical science is not my cup of tea. However, I will try my very best (like I usually do for my wonderful readers) to explain the fundamental principles of this interesting phenomenon. You’ll be able to understand what causes it and also the various factors that alters its effects. Let us jump right to it!
Masking – Does It Happen?
To put it in simple terms, when the perception of one sound is affected by the presence of another sound, that is called audio masking. In the frequency domain, this occurrence is called “simultaneous masking”, “frequency masking” or “spectral masking”. In the time domain however, it is known as “temporal masking” or “non-simultaneous masking”.
To make this post easier to digest, we’ll break it down into several topics:
- Masking threshold
- Simultaneous masking
- Temporal masking
The two masking thresholds are: unmasked threshold (the quietest level of the signal which can be perceived without a masking signal present) and masked threshold (the quietest level of the signal perceived when combined with a specific masking noise). Hence, the difference between the masked and unmasked thresholds represents the amount of masking.
For example, the sound of a cat scratching a wall is at the level of 10 dB SPL. However, when a vacuum cleaner runs simultaneously (masking), the sound of the scratching has to be at least 26 dB SPL, in order for it to be audible. Hence, the unmasked threshold would be 10 dB SPL, while the masked threshold is 26 dB SPL. The difference between the two would then be 16 dB (amount of masking).
Take note that the amount of masking will vary for different listeners. In the example above, the listener could hear the cat scratching at 26 dB SPL, but another person might not be able to hear the scratching (while the vacuum was running) unless the sound level of the scratching is at 30 dB SPL instead. Thus making the new amount of masking for the second listener to be 20 dB.
This form of masking happens when a sound is made inaudible due to another unwanted sound occurring at the same duration as the original sound. For instance, a sudden boost at 1 kHz will tend to mask out a lower-level tone at 1.1 kHz. Also, if two sine tones (440 and 450 Hz) are played separately, they can be clearly distinguished by the listener, but not when they are played simultaneously.
When there are two sounds (of different frequencies) played at the same time, you can often hear two separate sounds, instead of a combination of both tones. This ability that allows frequencies to be heard separately, is called “frequency resolution” or “frequency selectivity”. But when two tones are perceived as a combination tone, they are said to be in the same critical bandwidth.
You will only be able to identify the frequencies of both sounds if they are far enough apart in the frequency spectrum. But if they are very close to each other (for example, 440 Hz and 450 Hz), then they are considered to be in the same critical bandwidth and will often be perceived as one sound instead of two.
Also keep in mind that the greatest level of masking happens when the masking sound and the original signal are of the same frequency. This phenomenon is called “on-frequency masking”, where the listener will be unable to distinguish between the two sounds and they will be perceived as one sound. In this case, the quieter sound will be masked by the louder one.
This form of masking occurs when the masking sound makes other sounds, which are present either immediately preceding or following the masking sound, to be inaudible. When a sound is obscured immediately preceding the masker, it is called backward masking and if a sound is obscured immediately following the masker, it is called forward masking or post-masking.
Just like simultaneous masking, the effect of temporal masking is dependent on the frequency analysis carried out by the auditory system. For instance, forward masking thresholds for complex harmonic tones (such as a sawtooth wave at a fundamental frequency of 500 Hz) exhibit threshold peaks (high masking levels) for frequency bands centered on the first several harmonics.
Auditory masking is used in various kinds of audiometry, including pure tone audiometry, and the standard hearing test to examine each ear unilaterally and to test speech recognition in the presence of partially masking noise. Furthermore, principles of auditory masking are also taken into consideration when designing data compression algorithms for audio signals (MP3).
We have come to the end of the article. There is definitely much more that can be discussed, but I think this should give you a good overview of the subject.
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