Pure Tone Audiometry

Hey there, students! 👋 Welcome to one of the most fundamental and important lessons in audiology - Pure Tone Audiometry! This lesson will teach you everything you need to know about how hearing professionals measure your ability to hear different sounds. By the end of this lesson, you'll understand how air and bone conduction testing works, why different transducers are used, how thresholds are determined, and when masking techniques are necessary. Think of this as learning the "language" that audiologists use to map out exactly how well your ears are working - it's like creating a detailed blueprint of your hearing! 🎧

Understanding Pure Tone Audiometry

Pure tone audiometry is the gold standard behavioral hearing test used by audiologists worldwide to measure hearing sensitivity. Think of it like taking your ears for a comprehensive check-up! This test evaluates how well your peripheral auditory system (your outer, middle, and inner ear) and central auditory system (your auditory nerve and brain pathways) work together to process sound.

During this test, you'll listen to pure tones - these are single-frequency sounds that don't occur naturally in our environment. Imagine the difference between a piano note (which has multiple frequencies) and a tuning fork (which produces a pure tone). The audiologist presents these tones at different frequencies, typically ranging from 250 Hz (very low pitch, like a deep bass drum) to 8000 Hz (very high pitch, like a bird chirping).

The test measures your hearing threshold - the softest level at which you can detect a sound 50% of the time it's presented. This is measured in decibels hearing level (dB HL), where 0 dB HL represents the average hearing threshold for young, healthy adults. Fun fact: Most people can detect sounds as quiet as -10 dB HL, which is actually better than the "normal" reference point! 📊

Air Conduction Testing

Air conduction testing is like taking the scenic route through your entire hearing system! When you wear headphones or insert earphones during this test, sound waves travel through the air in your ear canal, vibrate your eardrum, move the tiny bones in your middle ear (the malleus, incus, and stapes), and finally stimulate the hair cells in your cochlea.

The transducer selection is crucial here, students. Audiologists typically use three types of transducers for air conduction testing:

Supra-aural headphones (the classic over-ear style) are most commonly used because they provide excellent sound isolation and are comfortable for most patients. They're calibrated to deliver precise sound levels and are ideal for testing frequencies from 250 Hz to 8000 Hz.

Insert earphones look like small foam earplugs with tubes attached. They're particularly useful when testing children, patients with hearing aids, or when you need better ear-specific testing. They also provide better attenuation (sound blocking) between ears, which is important for accurate testing.

Circumaural headphones completely surround the ear and provide the best sound isolation, making them ideal for testing in noisy environments or when testing very quiet thresholds.

Air conduction testing reveals your overall hearing ability - if there's a problem anywhere along the hearing pathway, it will show up in these results. Normal air conduction thresholds are typically between -10 and 25 dB HL across all frequencies. 🎵

Bone Conduction Testing

Now here's where things get really interesting, students! Bone conduction testing bypasses your outer and middle ear entirely by sending vibrations directly through your skull bones to your inner ear. It's like having a secret pathway to your cochlea!

During bone conduction testing, a small vibrator (called a bone oscillator) is placed on your mastoid bone (behind your ear) or sometimes on your forehead. When this device vibrates, it causes your entire skull to vibrate slightly, which directly stimulates the fluid in your cochlea. This means that even if your ear canal is completely blocked or your eardrum is damaged, you can still hear the sound!

The bone conduction transducer is specifically designed to deliver vibrations efficiently through bone. It's calibrated differently than air conduction transducers because bone conduction is naturally less efficient than air conduction - typically about 40-60 dB less sensitive.

By comparing air and bone conduction results, audiologists can determine what type of hearing loss you might have:

• If both air and bone conduction are equally reduced, you likely have a sensorineural hearing loss (inner ear problem)
• If air conduction is worse than bone conduction, you probably have a conductive hearing loss (outer or middle ear problem)
• If you have both types, it's called a mixed hearing loss

This comparison is called the air-bone gap, and it's one of the most important diagnostic tools in audiology! 🦴

Threshold Determination Techniques

Determining your exact hearing threshold is both an art and a science, students! Audiologists use a specific procedure called the Hughson-Westlake method, which is the international standard for threshold determination.

Here's how it works: The audiologist starts by presenting a tone that's clearly audible (usually around 30-40 dB HL). Once you respond by raising your hand or pressing a button, they decrease the intensity by 10 dB and present it again. They continue decreasing by 10 dB until you no longer respond. Then, they increase the intensity by 5 dB and present the tone again. They repeat this "down 10, up 5" pattern until they find the lowest level where you respond to at least 50% of the presentations.

This technique is incredibly reliable - studies show that when performed correctly, threshold measurements are accurate within ±5 dB about 95% of the time! The entire process for both ears typically takes 20-30 minutes, testing 7-8 frequencies per ear.

Some audiologists also use automated audiometry systems that use similar algorithms but are computer-controlled. These systems can be just as accurate as manual testing and are particularly useful for hearing screenings in schools or workplaces. 🎯

Masking Techniques

This is where pure tone audiometry gets really sophisticated, students! Masking is used when we need to make sure we're testing the correct ear. Here's the problem: when sound is presented to one ear at a loud enough level, it can actually cross over through your skull to the other ear. This is called cross-hearing, and it can give us false results!

Masking involves presenting noise (usually narrow-band noise) to the non-test ear while testing the other ear. This "masks" or covers up any sound that might cross over, ensuring that responses are truly coming from the ear being tested.

The decision to mask depends on several factors:

• Air conduction masking is needed when the air conduction threshold in the test ear is 40 dB or more above the bone conduction threshold in the non-test ear
• Bone conduction masking is more complex because bone conduction naturally stimulates both cochleae simultaneously

The masking process uses specific formulas to determine how much noise to use. Too little masking won't be effective, but too much can actually interfere with hearing the test tone - this is called overmasking. The audiologist must find the "plateau" - the range of masking levels where the threshold remains stable.

Interaural attenuation (the amount of sound reduction when crossing from one ear to the other) varies by transducer type:

• Supra-aural headphones: 40-80 dB
• Insert earphones: 55-85 dB
• Bone conduction: 0-15 dB

Modern audiometers have built-in masking calculations to help audiologists determine the appropriate masking levels automatically! 🔇

Conclusion

Pure tone audiometry is truly the foundation of hearing healthcare, students! Through air and bone conduction testing with carefully selected transducers, precise threshold determination techniques, and sophisticated masking procedures, audiologists can create a detailed map of your hearing abilities. This comprehensive assessment allows them to identify the type, degree, and configuration of any hearing loss, which directly guides treatment recommendations. Whether you're getting a routine hearing check or investigating hearing concerns, pure tone audiometry provides the essential baseline information that makes personalized hearing care possible.

Study Notes

• Pure tone audiometry - Behavioral test measuring hearing sensitivity using single-frequency tones from 250-8000 Hz

• Hearing threshold - Softest level where sound is detected 50% of the time, measured in dB HL

• Normal hearing range - -10 to 25 dB HL across all frequencies

• Air conduction pathway - Sound travels through ear canal → eardrum → middle ear bones → cochlea

• Bone conduction pathway - Vibrations travel directly through skull bones to cochlea, bypassing outer/middle ear

• Air-bone gap - Difference between air and bone conduction thresholds; indicates conductive hearing loss

• Transducer types - Supra-aural headphones, insert earphones, circumaural headphones, bone oscillator

• Hughson-Westlake method - Standard threshold procedure: "down 10 dB, up 5 dB" until 50% response rate

• Cross-hearing - Sound presented to one ear crosses over to the other ear through skull transmission

• Masking - Noise presented to non-test ear to prevent cross-hearing and ensure ear-specific results

• Interaural attenuation - Sound reduction when crossing between ears (40-85 dB for headphones, 0-15 dB for bone conduction)

• Masking plateau - Range of masking levels where threshold remains stable, avoiding under- and overmasking