Also called loudness level contours and the Fletcher-Munson curves. The curves are plotted for each 10 dB rise in level with the reference tone being at 1 kHz. The RESPONSE to frequencies over the entire AUDIO range has been charted, originally by Fletcher and Munson in 1933, with later revisions by other authors, as a set of curves showing the SOUND PRESSURE LEVEL s of PURE TONE s that are perceived as being equally loud. The ear is not equally sensitive to all frequencies, particularly in the low and high FREQUENCY ranges. A bit is good…a lot is bad.Equal_Loudness_Contours EQUAL LOUDNESS CONTOURS It’s the difference that matters, and this is the basis behind the psychoacoustic concept of the “missing fundamental.” Extending the lower frequency region to the extreme lefthand side of the piano keyboard in a hearing aid fitting is misguided. This is true regardless of whether the 65 Hz difference is between 65 Hz and 130 Hz or between 1000 Hz and 1065 Hz. Our auditory systems are quite amazing and it turns out that all we need to hear is the difference between two adjacent harmonics. We don’t need to hear the note with a tonic at 65 Hz (two octaves below middle C) in order to “hear” it, for example. Yet, being able to hear the lower frequency sounds of speech or music should not be over-emphasized. This is also the reason I use loudspeakers instead of earphones when listening to and transcribing phonetic details in my linguistic research-I am equal parts musician, audiologist, and linguist. This provides me with a bit more bass sound, especially for the quieter elements of music, than is discernible through earphones. I prefer listening to recorded music and speech through loudspeakers rather than earphones. Above 1500 Hz, the unoccluded resonant characteristics of the ear canal come into play-we are dealing with a half wavelength resonator versus the quarter wavelength resonator with an unoccluded earcanal. In fact, if one were to subtract the MAF 0 phon curve from the MAP 0 phon curve, the resulting spectrum would be that of the vascular noise, at least below 1500 Hz. When our ears are occluded with an earphone, vascular noise of the blood coursing through our arteries becomes audible, masking some of the lower frequency sounds. Listening in the sound-field (MAF curves)-the way we listen to people and to live music-gives slightly better acuity in the mid- and low-frequency region than listening under earphones (Minimum Audible Pressure or MAP). Figure courtesy of We listen to music and speech in two ways: under earphones and in the sound-field (unamplified or amplified through loudspeakers). The lower frequency difference is related to the masking spectrum of vascular noise and the higher frequency difference is related to the different resonant characteristics of the outer ear. This is a comparison between the minimal audible thresholds when using occluding earphones (MAP) and when listening in the sound field (MAF). This of course is an inherent limitation for generalizing the usefulness of the pure-tone audiogram (see the March 2016 article, “ Rants”).įigure 2. And while it’s easy to control, there is very little reality for using a pure tone. Pure tones are used everywhere in hearing assessment, from audiometry to evoked response stimuli. This explains the difference between the 3 dB result at 1000 Hz and the RMS value which would be at 0 dB SPL. Since this is a test of detectability, the peak (at +3 dB) is the stimulus, whereas the RMS or “SPL value” of the pure-tone is 3 dB lower. Pure tones have a crest factor of 3 dB-the difference between the peak and the average (or RMS of the signal). This is directly related to the crest factor of the signal used to obtain the MAF curve-filtered broad band noise, or that of a pure tone. Figure courtesy of However, if we look at MAF curves, there is typically a dotted line at 3 dB SPL and not 0 dB SPL. The peak is at 3 dB and the RMS was at 0 dB SPL. This relates to the crest factor of a puretone being 3 dB. In this case the MAF used puretone stimuli, as evidenced by the 3 dB SPL value at 1000 Hz. Equal loudness contours depend on whether sound is measured under earphones (MAP) or in the sound field (MAF).
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