In this article, Richard Honeycutt explains the process of sound masking, how it is used, and ways that it can occur unintentionally.
• A family member sitting near a window air conditioner asks you to repeat what you just said.
• You have trouble understanding what your friend across the table in a restaurant said.
• A hearing-challenged person has more trouble understanding speech in a mall.
• People in different open-office areas can have reasonably confidential conversations.
The answer is acoustical masking. Whenever a given sound is rendered inaudible or unobjectionable by other sound, we have a case of masking. Back in the days of vacuum-tube car radios, each radio incorporated a vibrator that chopped the DC from the car’s electrical system, producing a square-wave AC that could be fed to a transformer to provide high voltage for the tubes.
Vibrators made a buzzing sound that few people would describe as pleasant. Radio engineers had to know how loud the average noise in the car was, in order to know how quiet the vibrator had to be to avoid annoying the driver and passengers. This is an early example in which knowledge of masking was beneficial from an engineering standpoint.
Masking by the Numbers
Quantitatively, the amount of masking is the number of decibels by which a listener’s threshold of audibility is increased by the presence of a masking sound. Let’s say that in a quiet room a person is barely able to hear a pure tone at a level of 60 dB. Then a masking sound is introduced, and the tone’s level must be raised to 62 dB for the person to be able to hear it. The masking effect of the noise in this case is 2 dB.
Masking noise can have any wave shape: pure tones, random noise, music, speech, and even race cars can effectively mask sound. Pure tones are especially difficult to mask, since the human hearing system is particularly good at picking pure tones out of noisy backgrounds. But since pure tones are easy to generate, the masking of one pure tone by another sound was the earliest masking combination studied.
If a pure tone is masked by either another pure tone or a narrow band of noise, the closer to the frequency of the pure tone the center frequency of the masking sound is, the greater the masking. In Figure 1, the masked tone’s frequency is varied. The pure masking tone has a frequency of 400 Hz, and the masking noise is centered at about 410 Hz. For both masking sounds, the level was 80 dBSPL.
Naturally, when the masked tone was centered about 400 Hz, masking was greatest. But also note that when the masking noise is lower in frequency than the masked tone, masking is more effective than if the masking noise is higher in frequency. If a tone is to be heard in the presence of masking noise, the RMS level of the pure tone must slightly exceed the level of the masking noise in a specific range of frequencies known as a critical band.
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