Ozarks Technical Community College

HIS 110: Acoustics & Psychoacoustics

Difference Limens & Wrap-Up of

Acoustics/Psychoacoustics in Normal-Hearing

Listeners

We have talked about the frequencies and intensities that the human ear can receive

We’ve also discussed audiometric concepts of determining threshold (softest audible sound intensity at different frequencies) and masking

Now, let’s discuss the just noticeable difference (JND) or Difference Limen for frequency and intensity

Difference Limen (DL) = smallest difference between two sounds that can be perceived by a listener

Difference Limens

The minimum difference in decibel level that can be perceived as sounding “different” by a listener

In very simple terms, our ears can generally hear the difference between two sounds when they are separated by 1-2 dB SPL.Very low intensity sounds require a greater difference

in intensity to be perceived as different (i.e. a sound that is 10-20 dB must be increased by ~2 dB to be perceived as different than the original sound)

High intensity sounds require a smaller difference in intensity to be perceived as different (i.e. a sound that is 40 dB or louder only needs to by 1 dB louder to be perceived as different than the original sound)

Difference Limen for Intensity

This does vary with frequencyAt 125 Hz, we require a DL of 5dB when we

are near threshold to detect a differenceThis is a very good reason for why we test

hearing using 5 dB steps (plus it saves time)

Difference Limen for Intensity

Difference Limen for FrequencyThe minimum difference in frequency that

can be perceived as sounding “different” by a listener

The DL for frequency is generally about 0.5 percent of the frequency of the original toneSo, at 1000 Hz a change of ~5 Hz would be

noticed by the listener

Difference Limen for DurationTemporal refers to timing (tempo)Definition=The shortest gap/interruption of a

signal that can be detected by a listenerGenerally, a gap of 2 to 3 msec can be

detected by a listener (heard as two separate tones)When a gap is less than 2 msec, most listeners

will hear one steady tone

Temporal Discrimination

Review of Acoustic/Psychoacoustic Concepts

A pure tone has almost all its energy located at one frequencyWe use pure tones during audiometric testing

A complex sound is composed of multiple frequenciesMost of the sounds in our day-to-day lives are

complex in nature

Pure Tone vs. Complex Sound

Frequency & WavelengthWavelength=the distance a

soundwave travels during one cycle of vibration

Low frequency sounds have a longer wavelength

High frequency sounds have a shorter wavelength

If the waves at left occurred over 1 second, what would the frequency of each be?3 Hz10 Hz

Image from: blog.leeburrows.com

The distance between the ears is approximately 7 inches

For frequencies of 2000 Hz and higher, the wavelength is less than 7 inches, and therefore the shadow of the head results in a reduction of sound intensity at the ear furthest from the sound source This makes it easier to locate high frequency

sounds due to intensity cuesNote: Our ability to locate low frequency

sounds is primarily due to arrival time differences at the ears

Wavelength and Head Shadow Effect

Speed of SoundIn air, the speed of

sound is approximately:1100 feet/second340 meters/second

The speed of sound varies depending on the medium it is traveling throughSound travels faster

through media that have the highest elasticity (i.e. steel)

Image from: meritsoundwaves.weebly.com

Reflection of sound that occurs when it bounces off of a physical object (i.e. wall) and results in a prolongation of the sound

Reverberation is reduced by increasing the absorption of the environmentUse of acoustic tiles in a classroom or

gymnasiumCarpeting instead of hard-surface flooringDraperies

o An understanding of reverberation will be important when we discuss the hearing-impaired ear

Reverberation

A periodic sound is a sound in which the wave shape repeats itself and is predictableA pure tone is periodic

An aperiodic sound is a sound in which the wave shape does not repeat and is randomNoise is aperiodic

Note: this will be important when you learn about modern hearing aids. HAs are constantly analyzing the frequency spectrum of incoming sound to determine if it is speech (periodic) or noise (aperiodic).

Periodicity vs. Aperiodicity

Fundamental FrequencyThe fundamental

frequency is the frequency in a complex sound that has the lowest frequency and the most energy (100 Hz)

Harmonics are multiples of the fundamental frequency and contain less energy than the fundamental frequency or the preceding harmonic (300, 500, 700, 900 Hz) Image from: beausievers.com

The resonant frequency is the frequency at which a system will be most easily set into vibration It is the point where mass and stiffness are equal and

there is no opposition to the flow of energyIn a tube, with one open end and one closed end:

The longer the tube, the lower the resonant frequencyThe shorter the tube, the higher the resonant

frequencyThe average resonant frequency of the earcanal is

approximately 3000 Hz, but will vary depending on the patient’s earcanal size and shape!

Note: this will be very important in the fitting of hearing aids and is the reason why individual REAL-EAR VERIFICATION is so important

Resonant Frequency