Dhar chapter 2 Flashcards
Why do we calibrate?
We want an accurate test result
-To compare results from 1 clinic to another
We need to set a standard
-Across different people we need to know where to start. We need calibration to compare hearing. Also allows all audiologist to give the same results because all of our equipment is calibrated with one standard
Speaker
electrical –> acoustic
We need to know how much electricity were sending and how much were getting in return. Speaker has 2 electrical terminals to drive electricity. Think about ear phones. So you drive xVolts to the speaker will produce ydBSPL. Ex: drive speaker with 2 volts and produces 100dBSPL.
Sensitivity
a critical specification of a speaker. ex.
1volt=50dBSpl
Microphone sensitivity
- How many dBSPL it takes to produce 1volt. Or you could say that the driving unit is backwards. The mic is a speaker going backwords. Sound electricity. This is Volts per Pascal (measure of the pressure)
- dBSPL = 20 x log (Pa/Pr)
instrumentation
instrumentation needed to at least evaluate whether the audiometer meets the reference equivalent threshold sound pressure level (RETSPL), freq, linearity, and distortion. Sound level meter, multimeter, freq counter, oscilloscope, digital spectrum analyzer
What determines if a speaker is good or not?
- Sensitivity
- Dynamic range
- Frequency response
- High fidelity/distortion
Dynamic range
how much can it take before it starts distorting. Basically dynamic range…can it do low frequency and high frequency well? Otherwise classical music sounds flat and sucks
-The frequency its capable of producing. Draw a graph with a straighter line and a curved line. You want the straight because it is good at all frequencies. Narrow band working headphones would be from the dollar store and have a curved line on the graph.
High fidelity
Measure distortion. Measure every frequency and calculate the energy at 1000 HZ compared to energy everywhere else. The good speaker should have a big ratio because the energy at 1000 Hz should overwhelm any other energy elsewhere. If ratio is close to 1 then the speaker sucks
does this speaker still produce xdBSPL when I drive it with xVoltsRMS?
need a sound level meter. then couple speaker to sound level meter. calibrate all headphones using the artificial ear.
Sound level meter
need acoustic calibrator and coupler (2cc/6cc), microphone, and the SLM.
should be type 1 audiometer, mic
condenser microphone
most commonly used type 1 mic. 1/8’, 1/4’, 1/2’,1’. smaller the mic, the higher its upper freq cutoff, less sensitivity.
how to couple speaker with sound level meter
By ‘couple’ he means figure out a way to channel all of the sound into the microphone. To get an accurate measure you don’t want it to be in an undefined environment. You need to have a calibrated sound level meter but a defined coupler (into which ur earphone will fit and not let anything leak and direct all of the sound into the microphone).We need different types of couplers to be able to couple the sound to the sound meter. The coupler also has to be standardized.
Artificial Ear
for AC. Imagine a cup made out of metal. The opening is sized perfectly for the TH49s (big headphones). imagine screwing a sound meter in the bottom. The cup is defined as 6cubic centimeters (cc). the mic in the bottom is the surrogate for the TM. Sound from headphones mic how many dBSPL?A 2cc coupler. To approximate the volume captured between end of earphone to the ‘TM’
• We usually use a flat plate coupler.
• The 6cc coupler is too small for our small earphone to deal with. So we attach a plate to fit the smaller ear phone but use the same “metal cup”/coupler. For BC. We put that bone vibrator on the the plate and the vibrations are recorded with circuitry and it converts vibes to sound and send it on its way
physical volume/unit of sound pressure. dBSPL
If we were to use that, laura would come to the clinic and wed know that laura at 1000 shes 11 dBSPL. But xaviers is different. So we came up with dBHL (hearing level) and they took a bunch of people and asked if they should have any past problems..and they tested at 1000 dbSPL and created a threshold for ER3 heaphones. We average it all and its about 7dbSPL. That make 1000hz with ER3 headphones has a 7dBHL threshold. When bryan comes in at 20dbSPL he knows that hes got bad hearing compared to the population
distortion
If youre playing 1000hz you should only register energy at 1000hz. That’s a measure of the distortion.
Linearity check
If you go up 5db on the audiometer, the sound level should also go up 5db
how to calibrate speech
What’s the problem? Because speech is a complex wave. There’s no sine wave to calculate RMS for. Typically when youre calibrating the speech output signal youll do it with a 1000hz tone rather than a speech signal. Use 1000hz steady-state tone and say that if you set it at dBspl youll get a reference number of 12.5 db. 1000hz tone at 60dBHL is 72.5dBSPL for speech.
biologic check
is basically listening to see if everythings okay.
- What are you gonna listen for? U should know something about your audiometer. Typically youll do 1000hz at 60 dBSPL. Listen when its working to get a good sound and if its different before clinic starts then theres a problem!
- next you listen for distortions…abrupt on and off, scratchiness, etc. in this process youre checking distortion, checking also for physical problems like loose connections that could cause scratchiness, some people take it 1 step farther they play the sound and wiggle the wires to make sure that its still good to go. Or wiggle where wire goes into the wall.
- Then you can do a linearity check. You should increase the sound and notice an increase in the sound.
Chapter 2 take home messages
- What is done, how its done, but we’ll never do it ourselves.
- Know your equipment. Get used to the sound, wiggle connections to check so u know what to listen for.
- Know your own thresholds at a couple of frequencies. You could always measure your own threshold and see if its close to what it should be.
- -Also is useful if you’re going to test hearing in a non-familiar setting. Helps you determine what correction value to use.
- Reference values.
- -For every earphone, at every frequency, there is a reference threshold (sometimes aka conversion factor, or RETSPL ((reference equivalent threshold sound pressure level)) ). Basically the HL -SPL conversion. Relates physical pressure to a clinical scale.
- -The conversion factor dBHL = 0, Dbspl = 7.5 means that at 40dBHL = 47.5 dBSPL
Octave band filter
if you turn it on, you can ask it to only look for certain frequencies
piston phone
Produces 1 sound at the right level. 250 Hz or 1000 Hz at 2 certain levels. Measures reliability of sound meter. Put it on sound meter and it should be right. Sound meter can go to artificial mastoid, artificial ear, or the ec3 coupler for headphones. helps calibrate the sound level meter
multimeter
allows you to make measurements of voltage, current, and resistance. need a true RMS multimeter for accurate voltage readings. Voltage, current, resistance, can be measured with this. Esentailly has 2 leads (+,-). Most of the time we care about resistance or voltage (battery or output of an audiometer)
frequency counter
counts each “event” or “cycle” only ONCE. you dont want multiple triggers/sec.
oscilloscope
Electrical signal and turns it into a visual displays. So u put in 1000hz tone so you see the sine wave traced. If you set the x axis to be 1sec you should see 1000 sine waves. Voltage as a function of time
spectrum analyzer
can be stand alone or computer-based hardware/software. convert an analog input signal to digital format by use of an analog-to-digital convert. if the conversion is too slow it can make false freq.
basic equipment
voltmeter/multimeter, condenser mic, acoustic calibrator, 6cc coupler/2cc coupler, 500 g-weight, mechanical coupler for BC, SLM
rise-fall time
basic parameter of the audiometer which is checked by taking the output directly from the audiometer and routing it into a digital or storage ossilloscope.
monitoring meter
indicators of signal level and are found on the face of most audiometers. calibrated relative to the input signal thatit monitors and shouldn’t be interpreted absolute values
sound field testing
test room, freq response, method for describing the level of the speech signal, speaker location. place marker on ceiling where head would be, face diaphram towards plane-propogated wave (frontal incidence), or the grazing incidence
cd and tape players
electroacousticall checked every 12 months. maintenance weekly. no standards for tape players used with audiometers.
automatic audiometers
automatic (Besky) audiometers begins with freq, level, cross-talk, and attenuation rate and interruption rate for pulsed signals should be checked
auditory evoked potential instruments
(AEP) check output level, freq, and time.its complicated but needs to be checked.
otoacoustic emission devices
no standards. primary signals should be indicated by a machine.
acoustic immittance devices
use freq counter to start. harmonic distortion shouldn’t be over 5% of fundamental freq.probe shouldn’t be over 90dBspl with 1-type coupler. check with contra/ipsilateral reflex with a insert that may be measures on a standard HA-1 coupler. not checking could lead to variability in measurement –> invalid immittance measurement
