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COMD 3700 Basic audiology Lesson 4 The Measurement of Sound Highlighted information refers to a change between the audio recording (using 10th edition) and the 11th edition of the textbook 1. COMD 3700 for distance education. This is lesson four, a continuation of the measurement of sound. This will cover pages 5068 in your textbook 2. We will begin with a review of psychoacoustics. This was covered in your ComD 3400 course, so I will just quickly review some of the main points. Psychological acoustics or psychoacoustics includes everything that can be perceived by the human ear from a sensory standpoint. This refers to how the sound “feels” to us. Unlike physical acoustics, which are the same with or without the human ear, psychoacoustics requires a human ear. What we measure as frequency, the ear perceives as pitch. Intensity measurements we interpret as loudness, timbre as tonal quality. Frequency, the physical measurement in Hz, or cycles per second, relates to the listeners interpretation of pitch, the psychological measurement of whether a sound is low or high. Raising the frequency of a sound makes the pitch higher. Doubling the frequency raises the pitch one octave, but does not double the pitch. Mels measure pitch. 1000 Mels is the pitch of a 1000 Hz tone at a 40 dB sensation level for normal ears. A higher pitched sound will have more Mels, a lower pitched sound, less. Intensity, measured in dB IL, SPL or HL is a physical measurement. Other words that describe intensity are amplitude and Maximum Sound Pressure. But loudness is psychological. We perceive a small increase in the intensity of a soft sound as a significant increase in loudness. We require a greater change in intensity to make a loud sound louder than to make a soft sound louder. If we make a sound twice as intense, we would expect it to be twice as loud. This is not the case. Most auditory systems follow a power law, sometimes considered a logarithmic concept, because a 10 dB increase doubles the loudness over most of the range of intensities. Remember that when we double the sound pressure of intensity, a 6dB increase occurs. A threefold increase in sound pressure, 10 dB, doubles the 1 loudness. For example, a 1000 Hz pure tone at 80dB SPL is twice as loud as 70 dB SLP, and half as loud as 90 dB SPL. Loudness levels across frequencies are measured in phons. Loudness, up frequency (not across), uses sones for measurement. So to review physical acoustics vs. psychoacoustics, remember that the physically measured loudness of a sound includes frequency, intensity, and duration. These measurements remain constant regardless of whether hearing is normal, or a conductive, mixed or sensorineural hearing loss is present. Although the measurements remain fixed, they are perceived differently by each patient because of individual hearing differences. This perception is psychoacoustics. 3. Audiologists usually make 2 kinds of measurements. One is to test the ability of patients with possible disorders of the auditory system. The other type is to measure the sound pressure levels in the environment. We are going to discuss the equipment used for both types of testing. Before we start into measuring hearing and pure tone audiometry, we need to talk about the equipment we use to measure sound and how to calibrate, or looking at the integrity of, our testing instrumentation. In an advanced class, talking about sound measurement and calibration will be a lengthy portion of the class but we want to talk about it in a brief form here in basic audiology. We will talk about the equipment used in measuring the ability of a patient’s auditory system and measuring sound pressure levels in the environment as well as calibrating the audiometer. 4. We will start with measuring the amount of a patient’s hearing loss. To do this we use an audiometer. I have heard people pronounce it audio-meter. But we refer to this instrument as an audiometer. Audiometers are made by a number of manufacturers, and vary in complexity and control layout. The controls or indicators may be dials, buttons or switches. For the most part, every audiometer has the following: Supra-aural and/or insert earphones and a bone receiver- we will discuss these in greater detail later Hearing Level indicator or attenuator- This controls the volume or intensity of each tone you present. This dial measures the volume, expressed in decibels Hearing Level or dB HL. Frequency Indicator-Tests the following frequencies: 125, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 6000 an 8000 Hz. The sound 2 produced at each frequency is a pure tone. Ear Indicator-to select the ear to be tested, either left or right. Interrupter switch-this controls both when the test tone is on and the duration of the tone. 5. So let’s look at some audiometers. This is a very simplified face on a basic pure tone audiometer. As you can see, we have a frequency dial. That's the Hz dial frequency. On a simple audiometer like this, we can select frequencies from 125 to 500 on up to 8000 Hz. Those are the octaves or frequencies at which we test hearing. On the right side, you see the other large dial. That is the hearing level dial measuring dB HL. It runs from minus ten to 110. So we have our frequency range and intensity range we can select. On top, we have the masking level dial. We're not going to discuss this right now, but masking is the noise that's put in the non-test ear to keep it from hearing the test tone. Down at the bottom, is the tone interrupter. It's been called an interrupter switch from the beginning of time. When these instruments were first produced, the tone was constantly on. It had a spring in it that would pop up. When you pushed down on it, it would interrupt the tone. On most modern audiometers, the tone is off until you depress an interrupter switch and then that causes the tone to be delivered the client. Then you have the power switch with off and on. A simple audiometer like this is designed for pure tone testing only and is used as a screening audiometer. 6. Here, we are looking at a block diagram of this simplified pure tone audiometer. This figure is found on page 56 in your textbook. These are the basic components necessary to accomplish the task of delivering a pure tone stimulus to the client's ear. On the left, you see a pure tone generator. That is the same as the frequency dial. So we need to generate a tone of given frequency. For instance, let's generate a tone at 1000 Hz. That is delivered to an amplifier. Then the amplifier amplifies the tone to the maximum level. So if the audiometer is putting out 110 dB at the maximum level, that's the level at which the amplifier is producing the tones, 110 dB.The attenuator is the same as the hearing level dial. Here's where we attenuate. Attenuate means to subdue or tone down or bring down. Just like if you have a bright color and it's attenuated by adding white to make it a softer color. This is what the dB dial is actually doing. We take a high level amplified stimulus and attenuate it down to 70 dB or 3 30 or 10 dB. When you have the dial, you're not pumping energy in as you turn the dial up. You're allowing less attenuation so you're getting a stronger signal. You send that to the silent switch. You saw the interrupter switch on the previous slide. You see if you push the switch, it will make the connection and deliver the tone. There will be a switch somewhere on the audiometer, other than the basic one we just used, where you can select either a bone conduction or air conduction as the mode of delivery of the stimulus. So if you choose air conduction, you can also choose to have the tone delivered to the right earphone or the left earphone. Or if you choose to have the stimulus delivered to the bone oscillator or vibrator it is just delivered to one place. On the diagram it is the black square. You can deliver tones to the individual by way of air or bone conduction. We talked about these two pathways when we talked about tuning fork tests. 7. In a clinic environment when you are conducting advanced testing, other than basic hearing screenings, then you will use a diagnostic or clinical audiometer for testing pure tones as well as speech testing. A speech audiometer is part of a clinical audiometer. We will discuss testing with speech stimuli in a later lesson. In addition to the equipment we listed earlier, a diagnostic or clinical audiometer will also include: Presentation Indicator-Changes the presentation of the tone from interrupted (normal choice) to pulsed, continuous, or warble tones Function Indicator-changes the audiometer function from microphone to air conduction, to bone conduction or to speech testing Microphone-Allows you to communicate with the patient while the headset occludes the ears Monitor-Important when you use a two-room test suite, a sound booth, or recorded speech tests Masking Control-produces a white or other noise to isolate an ear during certain tests CD Player (or tape player)-used for speech testing VU Meter-Monitors the input level by an averaging voltmeter called a Volume units or VU meter. The meter reads in dB VU, implying an electrical reference in watts. 8. So, let me summarize. This is a diagnostic audiometer, sorry you 4 can’t see the words indicating what the buttons on this audiometer are for. But you can see the 2 dials for frequency and intensity. With any pure tone audiometer, the objective is to deliver a tone at given frequencies or pitches. We use the term frequency more often. The frequencies we use to sample a person's hearing in audiometry, the lowest is 125 Hz. Then we go up in octaves. 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz until the high end at 8000 Hz. With an audiometer, we can select any of these octaves or tones. You can see on the screen of the audiometer that 1000 Hz is chosen. With the attenuator you can take any one of these octaves and adjust the intensity level at that octave. Now we can deliver a 1000 Hz tone at 20 dB, 50 dB, 70 dB, 90 dB, for instance. Then we can switch to a different octave. We can deliver 250 Hz at 0 dB, 15 dB, 25 dB. On this audiometer 0dB is chosen. The pure tone or speech signal is routed through supra-aural headphones, insert earphones, a bone conduction vibrator or in the sound field. We will review those now. 9. Remember when we discussed the pathways of sound and said that most of the time we hear via air conduction? When we are testing to determine type and severity we start the testing of the air conduction pathway using some type of earphone or rather a headset that consists of two earphones on an adjustable band that fits over the head. Earphones are color coded, red for right ear and blue for left ear. Everything in audiology uses these color codes, so you’ll need to remember that. I just remember R is for red & right. You can use headphones that fit over the ear, Supra Aural, or insert receivers that fit inside of the ear. There are definite advantages for using insert earphones when conducting testing and in addition, they are usually more comfortable for the patient. So whenever possible, I recommend using them. 10. A bone receiver on a spring steel headband is usually used to test the bone conduction pathway. It is usually placed on the mastoid process. This is the bone directly behind the pinna of the ear. Bone conduction testing can also be placed on the forehead using a plastic strap that circles the head. 11. There are times when we will test using the sound field of the audiometric suite or sound booth. This is usually done in speech testing. The signal is fed into the booth using loudspeakers, rather 5 than earphones. If pure tones are used for testing in the sound field, then the stimulus of the sound must be changed to a warble, or wavy tone. In this picture you can see the loudspeakers behind the patient being tested. 12. So, we have discussed the equipment usually used to test the auditory system of a person. Now we are going to discuss testing airborne sounds to determine the sound pressure level in decibels. To do this a sound level meter is used. Here are some pictures of various sound-level meters. This testing may be done to determine the level of sound in factories, schools, offices, stores, airports, etc. Recently our students have been using them to measure the intensity in the environment of sporting events. For those of you familiar with USU, you won’t be surprised to hear that the intensity is much louder at the basketball games than the football games! Sound level meters are instruments that will collect the sound energy from the surrounding environment, convert that energy to electricity, then give us a decibel readout as to the intensity of the sound in the surrounding environment. In other words, if we wondered how intense a sound was in a given environment, we'd take our sound level meter into that environment to measure the sound and the intensity, and some of the characteristics of the sound in that environment. Most sound level meters are battery operated and vary in size from half a loaf of French bread down to as small as a mouse you use on your computer. There are basic analog sound-level meters and sophisticated digital sound level meters. 13. A sound-level meter can also be used to calibrate an audiometer. Calibration is necessary to determine if an audiometer is performing properly in terms of its acoustic output, attenuator linearity, frequency accuracy and harmonic distortion. We need to know that if the audiometer shows that it is emitting a 1000 Hz signal at 45 dB, that it is actually doing it. At the USU Hearing Clinic we have all of our equipment professionally calibrated once a year. However, we want to make sure that in between the annual checks, the audiometer is functioning properly. So we perform periodic checks on the equipment. If an office is equipped with a sound-level meter and couplers, then the calibration can be done at any time. We are going to discuss some of the basic information regarding calibration. 6 14. Here is a basic drawing of a very simple sound-level meter. As you saw in the previous slide, most sound-level meters today are digital and much more advanced than this picture. I am using this basic drawing so that you can see the parts that we will be reviewing. Observe the meter at the middle of the diagram. It says dB SPL in the meter. The stylus on the meter is pointing at zero. That would indicate there was some sound energy coming in. If we were in silence, the stylus would drop to the left and we wouldn't measure anything. The fact it's standing at zero indicates we're measuring sound now. On the right, the meter is marked off in decibels steps. 1 dB 2, 4, 5. On the right hand, there's also equal demarcations. Move down to the dial just below the meter, note that it says sensitivity dial. We can adjust the sensitivity of the meter. If you move down, one more dial there, you see the frequency dial. What we can do with most sound level meters is actually take through a filtering process and look at the intensity in the environment of a sound centered close to the meter, there is a lot of sound energy around 1000 Hz, you can select that band with the dial at the bottom of the meter. If we would like to know what the sound intensity is at 500 Hz, we could change the Hz down and get an indication of how much sound energy was in the environment around 500 Hz or 250 Hz or 2000 Hz or whatever. In order to transform the acoustical energy into electricity, we need to have a microphone, which you can see is placed at the top of the sound level meter. More particularly you can see the diaphragm of the microphone sits atop the microphone. On top of that is a 6 cubic centimeter coupler. Why 6 cubic centimeters? Sound reacts to coupler or environmental size in predictable ways. If we introduce an amount of sound energy into a small cavity, we'll measure a relatively large sound in the cavity. If we increase the size of the cavity, the sound pressure level in the cavity drops or lowers. On the averages, when you put an earphone over the normal human ear, you've occluded or covered the ear. The diaphragm in the earphone over the opening of the ear canal causes a cavity to be formed. The walls of the external auditory meatus and the tympanic membrane on one end and the earphone on the other, that closed cavity on average is 6 cc. Thus we use the 6 cc coupler when we do calibration measurements on a sound level meter. 15. In this slide, we have the same sound level meter as we looked at before and all of its components. Here we've taken an earphone from 7 the audiometer head band and placed it atop the sound level meter and positioned in over the cc coupler in preparation for calibrating the earphone with its accompanying audiometer. We have the earphone atop the sound level meter placed atop the 6 cc sound level coupler, in preparation for calibration. We have also put a 500-gram weight atop the earphone. That pressure of the weight is a standard in the field. It's specified in the calibration standards. If you increased or decreased the amount of weight put on top the earphone, sound level pressure would vary slightly. Therefore, we have to go with the standard and put a 500-gram weight on top. 16. In this diagram, you see the very basic audiometer on the left. We have selected 1000 Hz as our audio output. We've selected 70 dB as the intensity of our output. 70 dB is the suggested output if you're going to stay with standard procedure. 70 dB output is standard for each frequency we test. So we'll leave the dB dial at 70 and change the frequency or Hz dial from 1000 to other frequencies. We'll deliver that frequency at that intensity to the earphone. On this basic SLM we've set the sensitivity dial to 75 dB in anticipation of this 70 dB output from the audiometer coming into the sound level meter. The sound level meter when stimulated by the audiometer reads on the positive side 2.5.So you take the 75 dB that is on the sensitivity dial plus 2.5. You get equals 77.5. We had an output of 70dB from the audiometer and are measuring 77.5 on the SLM. The question is: is this audiometer in proper calibration? The answer is yes. It's in perfect calibration. The sound level meter is measuring in the world of sound pressure level. On the left, the audiometer operates in the world of hearing level. If you think back to the conversion we did from SPL to HL. We learned that at 1000 Hz you add 7.5 to convert dB HL to dB SPL. So, at 1000 Hz a 70 dB HL tone is equal to 77.5 dB SPL. This audiometer with this earphone is in perfect calibration. If we change frequencies and go to five hundred or 2000 Hz, we will have to look on our conversion chart from SPL to HL to see the difference there. We would then have to adjust the sensitivity of the sound level meter in anticipation of that difference, then deliver a 70 dB HL tone to the sound level meter and we should read some level above 70 dB. Remember HL is different from SPL and we have to add some dB to get from one to the other. You would go through the same process we just went through at 1000 Hz for all other frequencies to determine whether or not an audiometer is in calibration. 8 17. So, if we conduct all of the testing and everything is in calibration, then you don’t need to do anything. You can feel confident that what you are recording is accurate. However, what if you find that your audiometer is not meeting the specified guidelines or is not measuring as it should be? If calibration reveals marked differences from specification, then it should be seen for recalibration. But if there are just minor differences, then you can correct for those when testing. We are going to discuss how to do that. When calibrating, there is a certain level of tolerance. In terms of intensity, the standard allows us plus or minus 2.5 dB margin of error. If we come up with 2.5 on either side of our desired level, we move to the next whole 5 dB. For example, if we are testing at 70dB at 1000Hz, we're anticipating 77.5 dB HL, like we did on our previous slide, But what if in fact we measured not 77.5 but 81.0? We would be strong by 3.5. So in our calibration report we'd say to the next person who looked at the audiometer, instead of 70 dB at 1000 we'd have to put a plus 5 there to tell the next person the audiometer was strong by 5 dB. In the same vein, what if we ended up with something less than 77.5 and exactly 2.5, which would be 75 dB? If we measure 74 dB, we would be on the minus side so we'd report at 1000 Hz with the earphone that the audiometer was minus 5 dB and we'd have to make adjustments accordingly. So in calibrating intensity if the difference is plus 2.5 or greater, we have to go up to the next five dB and minus 2.5 dB, we'd have to drop down by five dB. Then we'd have to put that on a correction chart so the next person who did testing with the audiometer understood that the audiometer was not in perfect calibration. It would specify that in fact, we were strong by 5 dB or weak by 5 dB or some other dB level. It could be plus or minus 10 or 15 or whatever. With frequency, our tolerance is somewhere between 1-2% percent, depending on the type of audiometer according to the 1996 ANSI standards. So this would mean if we put the earphone on a frequency counter at 1000 Hz, you could measure between 990 Hz and 1010 Hz and still be in the tolerance level if you were using an audiometer with a 1% frequency tolerance (This is stated incorrectly on the recording). As an example I have made a simple calibration chart that would represent the results of a celebration procedure. This would be located with the audiometer that we calibrated. We've indicated the results at 500, 1000, 2000, or 4000 with the right and left earphone. You'd note that at 500 Hz the right earphone is in 9 perfect calibration. In the right earphone at 1000, the audiometer is strong or producing 5 more dB than it ought to. At 2000 Hz, the audiometer is back in perfect calibration. At 4000 Hz, the audiometer is weak in the right earphone. In the left ear the audiometer is weak by 10 dB at 500 Hz. This is getting a little bit concerning, particularly if it was 15. At 1000 Hz, the audiometer is strong by five dB. At 2000 Hz the audiometer is in perfect calibration. It is soft or weak or producing 5 dB less than it should at 4000 Hz in the left ear. 18. So if you are conducting audiometric testing on an audiometer that has a correction chart or factor that necessitates a correction to be done, how is this recorded? We’ll answer that by learning about what’s called the direction of the correction. What we're talking about here is if the audiometer is not calibrated perfectly at zero, it's strong by 5 dB or weak by 5 dB or another value, how do we adjust the final result on the hearing chart before we report the patient's hearing to someone else? If the audiometer is out by 5 dB, you can't take the dial reading as being true, because it is not in calibration. You can't report the dial reading as it stands and report that level to someone else. You have to make the correction before you place it on the hearing chart. So I created some samples here to help understand this. For this example, we are using 1000 Hz. First, let's look at the upper tier here. Note in example #1, the audiometer is reading 70 dB HL and the sound level meter is reading 77.5 dB SPL at 1000 Hz. So, the correction factor is zero. We anticipate the difference to be 7.5 because 7.5 is the difference between HL and SPL at 1000 Hz. In the next example, #2, we're putting out 70 dB HL from the audiometer but measuring 72.5 dB SPL on the sound level meter. The correction would have to be minus 5 dB. We're weak by 5 from the audiometer. If we move to example #3, the audiometer dial says we're producing 70 dB HL but the sound level meter says that's not true, you're measuring 82.5 dB SPL, so we're strong by 5 dB. On example #2 and #3 these correction factors would be noted. So now we proceed to test a patient using these three audiometers. The lower part of the chart represents the results from this patient. In example #1, when we test the patient we find that the client's threshold at 1000 Hz is 30 dB. We've calibrated the audiometer and know at 1000 Hz, it's in calibration. So when we record the results of the hearing test, we can put it on the audiogram at 30 dB. Everything is calibrated and in order. Let’s move on to example #2. The dial on the audiometer says 10 35 dB but the earphone is actually producing 30 dB. We know based on our calibration that this audiometer is weak by 5 dB. It's out of calibration. We think we're producing 35. But when we calibrate, we measured -5. So we're actually only producing 30 dB. The audiometer says the client's threshold is 35 but the audiometer was soft so it is actually giving the patient a threshold that is less than he should have or it's making him look worse than he ought to. So we take the thirty-five from the audiometer and look at our calibration chart. It says minus five because the audiometer earphone was down by five. We take that five dB from the audiometer reading and place the minus five. That puts the threshold on the audiogram at thirty, that's his correct threshold-30 dB. Okay, now let’s look at number three. Here, the audiometer says the threshold is 25 dB. But the audiometer when calibrated was found to be producing 5 dB more than it should have been. This audiometer is strong by 5 dB at 1000 Hz. Instead of placing 25 on the audiogram, we have to add 5 dB to the patient's threshold before we record it. So then we'll record it as thirty on the audiogram. Can you see that the earphone is producing 30-30-30? We have made the adjustments to audiogram as 30-30-30. Everything is good irrespective of the false readings of the audiometer, which read 30, 35, and 25. Number one is a true reading. But #2 & #3 are not true readings, so we make that adjustment. So in summary, you obtain the client's threshold on the audiometer. Don't think about calibration just look at the threshold. Then go to the correction chart. It will say minus 5 at 1000 on number two. Subtract the 5 and record it on the audiogram. For example number three, take the threshold, add five to it, and record it. You'll record it at thirty, which is what's coming out of the earphone. This is the direction of the correction. If you understand what you're doing, you'll do this correctly. Otherwise, you can get in trouble. Find the threshold and add or subtract the number on the correction chart and put that amount on the audiogram. This is the end of lesson 4. Hopefully what we have covered will prepare you to begin learning about conducting hearing evaluations in the next lessons. To me, this will begin the more exciting part of audiology. 11