Soundfield Acoustics
Soundfield Acoustics
- The person
- The stimuli
- The ______
- The sound system
The Person
Field-To-Eardrum Transfer Function
- The change in sound pressure from the ______ to the _______
Your Calculations!
For ear canal of 2.3 cm,
what is the resonant frequency?
4(2.3) = (33528
cm/sec)/(frequency)
frequency = ???
Now determine length of ear canal for Shaw's Data
- Resonant Frequency = 2.6 KHz
Field-To-Eardrum Transfer Function in
Adults
- 1) Shaw's data on FTE TF in adults shows
- at 2.6 KHz there's a pressure gain of
- ___ dB at 0 degree azimuth
- 21 dB at 45 degrees azimuth
- 2) Resonance extends to ____ KHz due to concha
- 3) There's an antiresonance of about 3 dB at ____ KHz
Changes in Vertical and Horizontal Planes
- Vertical - changes in _____ KHz
- Horizontal - changes in 4-5 kHz
Artificial Ear
- Much effort directed at trying to develop this
- Zwislocki came up wih a ______ stimulator
- He took into account ear dimensions, ________, and impedances
Zwislocki Coupler
- Fig 16 - shows match to FTE TF
- Fig 17 - shows match to real ear and _____ response
Hint To Remember
- The lack of agreement between 2 cc coupler and _______ is the
first HINT that we have a complex situation if we want to predict the output of a HA at a
person's eardrum!
Field to Eardrum Transfer Function in
Children
- Kruger (1987) reported that the resonance is ___times higher
- 5.3-7.2 KHz
- Declines to adult values by age ___ (others say ___ years)
Effects of the Head
Head diffraction: Field-to-Head Transfer Function---
+/- 2-5 dB at ____ frequencies
(Madaffari, 1974)
Head Shadow: As sound travels around the head,
frequencies whose wavelengths are _____ than the dimensions of the head are attenuated.
Head Shadow Effects
For the typical head, the reductions begin around
________ Hz with attenuation ncreasing as frequency increases up to about 14 dB at
_______Hz.
Effects For Speech
The average reduction is 6.4 dB as determined by
comparing the SRT in the _____ vs. the ______ condition.
Effects of the Body
- Body Diffraction: Field-to-Body Transfer Function---
- 2-6 dB increase _______Hz
- 5-15 dB decrease ______ Hz (Erber, 1973)
- children experience 1-5 ______(less/more)
Microphone Placement
- Top mounted mic had greater increase (about 2 dB) up to 1 kHz and less of
dip at 2000 Hz re: front mounted mic
- Mic on back can lose up to ____dB at 1500 Hz
The Stimuli
- _____Tones
- FM Tones
- ________
- Speech
There is a Tradeoff Between
- Frequency Specificity and Uniformity of ________
Frequency Modulated Tone
- Most desirable stimulus
- Characterized by:
- frequency deviation-expressed in ___
- modulation ____-expressed in Hz
Frequency Deviation and Modulation
- As the frequency deviation _______ the variation in level decreases
- As the modulation rate increases above 100 Hz variation in level ________
The Room
- Ambient Noise
- _________
- Size
Room Acoustics
- Soundbooths attenuate more in the ____ than in the____ frequencies
- Double-walled booths attenuate ______ dB more than single-walled booths
Near Field
- < .1 m from speaker
- level ______ rapidly with increases in distance
Direct Field
- .1 to .7 m
- level decreases 6 dB per doubling of ________
Reverberant Field
- .8 to 2.0 m
- mean level stays relatively constant except for _____ tones
To Get The Most Uniform Sound: Place the listener in the
reverberant field and use a signal with a _____ bandwidth
The System
- Tape recorder
- Amplifier
- Earphones
- Loudspeakers
Sound Delivery Systems should ideally have:
- flat frequency response
- ______ frequency response
- reliable response
- _____distortion
Calibration
- To make the hearing level dial valid for measurement of thresholds via
earphones or soundfield
Calibration Includes Checking:
- Absolute levels
- _________
- Distortion
For Speech The Reference Values Are:
- Earphones - _____dB SPL
- Soundfield -
16.5 dB SPL at 0
12.5 dB SPL at 45
Dirks, Stream, & Wilson (1972)
Compared speech thresholds via MAP
and MAF and found a difference of…
Correction Factors
CF = OBTAINED - WANTED
EXAMPLE:
CF = OBTAINED -WANTED
= 93 dB SPL - 83 dB SPL
= + 10 dB
PROBLEM
So …. If ….
Then
But
For Tones The Reference Values at 1000 Hz are:
- Soundfield - ____ dB SPL at 0
4.0 dB SPL at 45
Morgan, Dirks, & Bower (1979)
Compared puretone thresholds via MAP
and MAF and found a difference of ….
- -1.2 dB at 0 azimuth
- 3.3 dB at 45 azimuth
at 1000 Hz
PROBLEM
So …. If….An FM
Tone is at ……
Then
But
SOLUTION
- Earphones 70 + 7 = 77 dB SPL
- Soundfield 70 + 8 = 78 dB SPL -0
70 + 4 = 74 dB SPL -45
Reference Values (in dB SPL)
FM Tones (5%)
250 500 750 1000 1500 2000 3000 4000 6000
0 15 11.5 8.0 2.5 2.5
45 20 8.0 4.0 4.0 2.5 4.0 -3.0 -4.5 3.5
Morgan, Dirks, Bower (1979)
Reference Values (in dB SPL)
- Speech
- 0 degrees 16.5
- 45 degrees 12.5
Dirks, Stream, Wilson (1972)