Range of Hearing and Masking

Range of Hearing

• Human hearing ranges from the hearing threshold to the threshold of pain.
• There's a risk of damage before you reach the threshold of pain.
• Hearing threshold and risk of damage are defined by ISO 389-7.

Masking

• Masking occurs when multiple sounds are encountered simultaneously.
• Masking experiments help to understand the auditory system processes complex soundscapes.
• A tone embedded in white noise gauges a participant's threshold to detect the tone.
• Some participants may press the button as soon as they detect a faint tone, while others may wait until the tone is very clear.
• Differences in thresholds can vary up to 10 dB.
• Forced-choice experiments are used to address the bias in simple masking designs.
• Participants indicate which interval contained the tone when presented with two or three intervals.
• Randomizing the interval with the tone reduces bias and is the standard procedure for masking experiments.

Simultaneous Masking of a Tone by White Noise

• Playing a tone into white noise creates a masking pattern, displaying the tone's threshold level across frequencies in the presence of noise.
• The tone's threshold slightly exceeds the noise spectral density at -10 dB.
• Each 10 dB increment in noise level increases the tone's threshold by roughly 10 dB, showing a linear relationship.
• A slight rise in the curve at higher frequencies results from the widening of auditory filters in that range.
• Wideband noise raises the tone detection threshold across the spectrum from about 100 Hz to 10 kHz.
• White noise is a noise with constant spectral density and constant energy per frequency band.
• The spectral density level "l" is constant and frequency independent.

Narrowband Noise

• Narrowband noise is created by filtering wideband noise to focus on specific frequency ranges.
• Masking is most pronounced within the frequency range of the noise, but can extend slightly beyond those limits.
• The masking pattern shifts when shifting the center frequency of the narrowband noise.
• Masking maintains the specificity to the noise's energy distribution-even in areas where the noise energy is minimal.
• A demonstration for masking with narrowband noise: the narrowband noise is centered at 1,200 Hz, and I will play tones ranging from 900 Hz to 1.8 kHz.
• As the tones sweep through the frequency range of the narrowband noise, you'll notice that they initially become audible, then disappear as they are masked by the noise, and finally reappear once they exit the masked frequency range.
• Since 900 Hz to 1,800 Hz spans roughly one octave, this demonstration shows that even when the noise isn't significantly stronger than the tone, it can effectively mask the tone across an entire octave.

Simultaneous Masking of a Tone by a Tone

• Like narrowband noise, masking by tone is frequency-specific and strongest at the center frequency.
• Viewing the masking pattern of a low-level tone as the masker level increases.
• The masking effect strengthens, peaks near the tone's center frequency, and decays further away.
• Masking becomes more pronounced at higher masker levels and frequencies, increasing more than proportionally.
• Each curve represents a masker level 10 dB higher than the previous one, yet with just a 10 dB increase in the masker level, masking increases by nearly 20 dB at 1,500 Hz.
• Masking grows disproportionately and demonstrates an upward spread.
• Sounds can be masked even at distant frequencies at high masker levels.
• Simultaneous masking of tones by a tone is more complex than with noise because the interaction between different tones can alter audibility.

Simultaneous Masking of a Tone by a Complex Tone

• A complex tone consists of a fundamental frequency and its harmonics.
• The overall masking pattern sums individual masking patterns for each component tone.
• There is more masking in the regions between spectral peaks than you would predict from the individual patterns.
• At low levels and low frequencies the spacing between components is relatively large compared to the critical bands.
• The masking is primarily determined by individual tones, creating distinct peaks and gaps where signals can be heard.
• Many natural sounds, like harmonic complexes in vowels, follow this pattern.
• This analysis focuses on simultaneous masking with long stimuli to achieve steady-state conditions.

Simultaneous Masking: Effect of Tone Duration

• Masking increases by = 10 dB/decade when test tones are shorter than 200 ms, independent of masker level.
• A continuous masker with a brief tone burst shows that the Y-axis represents the level of the tone bursts, while the x-axis shows their duration.
• The level required for the tone to be audible in uniform masking noise depends on its duration.
• The pattern when the masker is at 60 dB is represented by the red curve; the pattern when the masker is at 40 dB is represented by the blue curve.
• Regardless of the noise level, as the duration of the tone decreases below about 200 milliseconds, the tone's energy must increase to remain audible.
• Conditions are essentially steady state beyond 200 milliseconds.
• For durations shorter than 200 milliseconds, the required level increases roughly 10 dB for each tenfold reduction -- going from 200 ms to 20 ms, and from 20 ms to 2 ms.
• When signals are shorter than 200 milliseconds, their levels must be raised to compensate for reduced energy integration.
• The auditory system acts as a long-term integrator over roughly 200 milliseconds when detecting tones in noise

Temporal Masking

• The effect of tone duration introduces temporal masking, which can be viewed as a timing issue between the tone and the masker.
• The probe tone is presented with the masker in simultaneous masking.
• A brief probe tone presented before the masker is known as backward masking ( or pre-masking)
• The probe's level must be increased for it to be heard, even before the masker starts.
• Backward masking lasts for about 20 to 50 milliseconds.
• A short probe tone presented after the masker still experiences elevated thresholds is known as forward masking.
• The tone remains masked for up to 200 milliseconds after the masker has stopped.

Critical Bands

• Critical bands are used to examine simultaneous masking with tones.
• The level of the test tone is the same as the masker, the Fixed tone masker.
• If the masker's frequency is close to the probe, a relatively low masker level is sufficient to mask the probe tone.
• The masker must be increased to about 80 dB to mask the 1 kHz probe when the masker is around 500 Hz.
• The sharply rising tuning curve at higher frequencies, means that a low-frequency probe tone cannot mask a masker at a higher frequency.
• The auditory system functions like a bank of filters where the masker effectively masks the probe when its energy falls within the same filter.
• This is known as energetic masking, and it forms the basis of the critical band concept.
• Critical bands quantify the ear's ability to distinguish between individual frequency tones which varie from 20 to 20,000 Hz.
• The ear functions in ‘bands' similar to bandpass filters, with each band defined by its center frequency and bandwidth.
• The bandwidth is where the response drops by 3 dB, is determined by the lower (Fl) and upper(Fu) frequencies.
• There are 24 critical bands in the human hearing range.
• There are 24 critical bands in the human hearing range; each band a ‘Bark' band.
• Critical bands are a phenomenon created by the cochlea.
• Less space is devoted to high frequencies compared to low frequencies; this unequal allocation allows the ear to detect changes in low frequencies more accurately than in high frequencies.
• Different regions of the basilar membrane respond to specific frequency ranges, and these regions correspond to the critical bands.

Impact of Critical Bands on Tone Detection

• Two simultaneous tones within the same critical band are not separate.
• Instead, the ear perceives them as modulating or beating together.
• The two tones occupy different critical bands when the ear can clearly distinguish them from one another.
• The auditory system is a bank of filters whose outputs help decide whether a probe tone is present by detecting changes in intensity or energy.
• A simultaneous masker means that the auditory system relies on its ability to detect energy changes.
• The minimum energy difference required is known as the "just noticeable difference in intensity".
• Masker's bandwidth affects probe tone detection and increases the overall energy of the noise.
• The tone's level must be increased to be detected for noise with constant spectral density (equal energy per Hz).
• As bandwidth increases, the tone level must be raised in a roughly linear fashion.
• The critical band is defined by additional energy outside that filter which no longer contributes to masking.
• Energy falling outside the filter does not mask the tone beyond a certain point, so the tone's level need not be increased further, even if the overall masker energy continues to rise.

Notched Noise Experiment

• The experiment is one of the most widely used masking paradigms.
• This experiment shows that a tone at a fixed level is presented along with two flanking noises that are separated by a certain frequency gap.
• Tones level must be raised in order to be detected if the noise energy encroaches into the filter corresponding to the probe tone.
• The tone must be louder when the noise separation is narrow; tone levels decrease with wider separation.
• The critical bandwidth is estimated at the two regions intersect.

Loudness Comparison Experiment

• A reference stimulus (often a 1 kHz tone) and a probe stimulus determine loudness.
• Adjusting either stimulus can produce more accurate average results.
• Method of adjustment adjusts the probe until matched to the reference; the process is similar to turning a volume knob.
• An adaptive tracking or magnitude estimation measures contents of the louder sound.

Equal Loudness Contours for Tones

• A loudness of 1 sone is the equivalent to 40 phons (a 1 kHz tone at 40 dB SPL)
• When tones are perceived as equally loud, perform comparisons to measure equal loudness contours.
• Using a 1 kHz tone as our reference, its dB SPL defines the loudness level in phons.
• Equal loudness contours extend over a wide frequency and sound pressure range.
• Threshold hearing (red) follows a 3-phon contour because it is through 3 dB at 1 kHz—the threshold for that tone.
• Contours are closer together and not as steep at 30 Hz compared to 1 kHz with a small change in dB produces a larger increase when loudness is percieved for a 1 kHz tone.

Level Change for Doubling/Halving of Loudness

• Compare loudness magnitudes by assigning the 1 kHz reference tone a value of 100.
• Determine when a sound is doubled or halved in loudness by rating the loudness compared to the reference.
• Roughly a 10 dB increase is needed for doubling loudness above 50-60 dB SPL for a 1kHz signal.
• An increase of less than 3 dB may suffice at lower levels (e.g., around 20 dB SPL).

Spectral Effects of Loudness

• The growth of loudness illustrates how perceived loudness increases as a function of dB SPL for 1 kHz signal.
• The loudness is measured in sones and is defined as a ratio scale where a doubling in sone value means the sound is perceived as twice as loud.
• The 1 kHz signals are at 40 dB SPL (or 40 phons) equals 1 sone.
• Doubles the tone increases to 50 dB SPL with more doubling again with about 60 dB SPL.
• For every 10 bB above 40 phons, the loudness doubles with a smaller lower level which can be around 1-2 dB.
• Because 40 phons is defined as 1 sone, doubling the loudness to 50 phons gives us 2 sones, and further doubling to 60 phons yields 4 sones.

Spectral Effects of Loudness: Tones vs. White Noise

• White noise is generally perceived as being much louder than a tone if they both are at the same sound pressure level.
• This demonstrates how psychoacoustics can differ from simple physical measurements.
• Everyday sounds have a much larger bandwidth.
• The level is compared to a 1 kHz tone that is perceived as equally loud.
• The tone's level must be increased by about 20 dB to match the loudness of the white noise, since it is much higher to percieve.
• This increase corresponds to a rise of 15–20 phons or a perceived loudness increase of three to four times in the mid-level range (40–100 dB SPL).

Spectral Effects of Loudness: White Noise

• Imagine a narrowband noise with a specific overall intensity; if the noise bandwidth increases while keeping the overall intensity constant, the spectral density must decrease.
• Loudness is constant when the bandwidth is up to the critical bandwidth.
• Loudness increases by specific loudness contributions from other critical bands when narrow bandwidth.
• A noise presented alongside a tone will have no impact on perceived loudness until it grows to large bands.
• Loudness can be viewed as the sum of the specific loudness contributions from each critical band.

Partially Masked Loudness

• Signals in the environment that are partially masked are still audible; however, their perceived loudness decreases due to other sounds.
• A 1 kHz tone exhibits normal loudness growth without a masker.
• The threshold shifts upward when the masking noise is prevalent
• For example, a 1 kHz tone at 50 dB SPL can be perceived as being about 2 sones, but if the background noise is at 40 dB, its loudness will decrase to one sone at half level.
• In this instance, loudness nearly matches the unmasked level when the tone is about 20 dB above the masked threshold.

Temporal Effects of Loudness

• If the signal is reduced by a factor of 10, then the loudness will be reduced by ≈ 10 phon
• Percieved loudness decreases, and the sound appears softer when the duration is shortened.
• For example, reducing duration from 200 milliseconds to 20 milliseconds reduces energy by a factor of 10, to which the level increases by a factor of 10.
• Similarly, shortening the duration further to 2 milliseconds requires an additional 10 dB increase.
• The auditory system integrates intensity over a roughly 200-millisecond window.
• Reverberation (or reverb) is the sound reflecting off surfaces in an enclosed space, like a balloon popping in a hall.
• A listener will hear called the direct sound first with many reflections that arrive at the listener after the some delay.
• The delay length and lost energy amount is based on the material that is on the walls, floor, and ceiling.
• These early reflections continue to bounce, and start creating reverb as they begin to pile up and overlap

Practical Applications of Loudness

• Sound pressure levels expressed in dB SPL capture the energy in a sound but do not fully reflect how we perceive loudness
• Human hearing is not uniformly sensitive across all frequencies, so engineers designed different frequency weighting filters to adjust.
• The dB(A) filter attenuates low and high frequencies where our ears are less sensitive and emphasizes midrange frequencies.
• Engineers use dB(A) to weight measurements often used for environmental noise, which is often specified to monitor assessments.
• High-level and impulses noise, like industrial environments, concerts, and machinery, are ideally measured with dB(C).
• Originally developed as an intermediate dB(B) largely lost use and has historical importance today.

Loudness Equalization in Sound Quality

• Customer perception of products are based on interior sounds in vehicle manufactueres, especially for vehicle intereior, and expands rapidly.
• Desires to buy recreational vehicles is highly correlated with loudness, without equalization. After loudness, results show different properies affect desire.

Pitch and Timbre

• There is a perceptual correlation between Pitch and frequency where Higher are recieving in pitch.
• Low frequencies are associated low pitches .

Roughness and Fluctuation Strength

• The ear distinguishes different tones when the two tones are in separate critical bands.
• Fluctuation Strength - tones at 4 Hertz apart create hearing of a single tone with a low frequency modulation
• Roighness hearing tones at 70 Hertz apart ( up to 300 Hertz) create hearing perception of rapid modulation.
• The average fluctuation strength, i.e. the hearing sensation related to loudness modulations at low frequencies, was 10.7 mvacil for the environment #A and 70.9 mvacil for the environment #B, confirming that the first environment is more stationary (i.e., with less fluctuations) than the second environment.

Mel Scale of Pitch

• Scale of pitch as function of intensity ,1000 mels=pitch of 1000hz tone presented with multiple scales because they were developed
• There is reasonably good correspondence between pitch in mels,critical band internal, bark, and distance on basilar membrane
• 100 mels coorelates 1 bark

Musical Pitch

• Octave is 2:1 frequency ration
• Frequency from given C will by double that of c octave below
• Different divisions can be made; 7 distinct classes consist scales(do re mi fal sol la ti da); 12 are divided into semitones.

Tones overtones

• Frequency 2:1 will also follow intervals.
• One can scale as for in equal tempermant; equal temperament also divides scale in semitones to create logarithmic functions.

Consonance vs. Dissonance

• Consonance is when tones presented are good; dissonance is on pleasant.

Pitch and Intensity

• The percieved pitch changes as sound percieved above 40db; low frequency tones slightly decrease while high shift more.

Harmonics and Combination tones

• Aural Harmonics are the products distortion that appear in output;
• Distortion products happens in the form of auditory harmonices when Stimulus presented at high enough level it generates harminics at multiples - f0, 2f0,3f0.
• Non linear distortion cause two primary function tones
• difference in tone its primarys exceed 50bs level
• When in presence of noise, masked threshold is quickly above normal loudness ~20db

Pitch of Complex Sounds

• Sound with defitnte picth or harmonic, with picth emerging when whitse is filtere
• With is being filer, the picth is based on the cuttoff ferquency.

Missing fundamental

• The undamental is heard when Harmonics are sent one by after another
• if higher frquency Interupted periodiacally can hear picth

Pitch/Spectral Thery

• The auditory analyzes the spectral patter on harmonic

Theories

• Residue pitch thery; is percieved on temporal pattern
• Higher not resolv;ed create period waveform