A number needs a reference.
A decibel (dB) expresses a ratio on a logarithmic scale. Multiplying sound intensity by 10 adds 10 dB. Adding 20 dB means 100 times the intensity. Decibels are not a simple percent, and intensity is different from perceived loudness.
Frequency is the number of pressure cycles per second, measured in hertz (Hz). It relates to pitch. Sound pressure is the size of the pressure variation around the surrounding air pressure, measured in pascals (Pa). This model changes frequency and pressure independently.
dB SPL · Physical sound pressure
Sound pressure level uses a reference pressure of 20 micropascals (20 µPa) in air. The measured pressure is an effective average called root mean square (RMS).
Lp = 20 log10(pRMS / 20 µPa)
0 dB SPL is the reference pressure, not silence. Levels below 0 dB SPL are possible.
dBA · Frequency-weighted sound level
A weighting changes how different frequencies contribute to a measurement. A sound-level meter may report an A-weighted level in dBA. Always retain the weighting, averaging time, and measurement location when reporting a result.
Frequency, duration, and measurement settings matter. A single number does not describe every feature of a sound.
dB HL · Hearing threshold reference
Hearing level uses standardized, frequency-specific reference values for the audiometer and its transducer. A transducer converts an electrical signal into a sound or a vibration.
0 dB HL represents a reference hearing threshold, not no sound. There is no single offset that converts every dB HL value to dB SPL.
dBFS · Digital signal reference
Full scale is the reference for a digital audio signal. dBFS describes the signal relative to that limit, not the sound pressure at a listener's ear.
A device's volume percentage does not establish dB SPL or dB HL. This page does not calibrate a phone, laptop, or headphones.
Change the level. Watch the ratio.
A fictional laboratory compares two pure tones in the same medium and conditions. The reference tone stays at 40 dB SPL. Predict what a 10 dB increase will do, choose +10 dB, then compare the pressure and intensity ratios.
Use the arrow keys for 1 dB steps. The baseline is fixed at 40 dB SPL.
Higher frequency puts more cycles in the same time window. The RMS pressures stay unchanged.
Pressure ratio = 10ΔL / 20 · Intensity ratio = 10ΔL / 10
ΔL means the difference in decibels. For this model, pRMS = 20 µPa × 10L / 20. The curves show instantaneous pressure variation for a pure sine wave, whose peak pressure is √2 times its RMS pressure. Intensity ratios assume the same medium and propagation conditions.
Read a worked comparison
At 40 dB SPL, RMS pressure is 2 mPa. At 50 dB SPL, it is about 6.325 mPa. The increase is 10 dB, so pressure grows by about 3.162 times and intensity by 10 times. These are physical ratios. They do not say that a person experiences 10 times the loudness.
Now move the comparison below 40 dB SPL. At 20 dB SPL, pressure is one tenth of the baseline and intensity is one hundredth. A negative decibel difference means the comparison is lower than the baseline; the physical pressure remains positive.
See the change. Then explore sound.
This model is silent so you can compare the same mathematical values without testing your own hearing. To explore a sound-wave demonstration, open PhET Sound Waves from the University of Colorado. Choose Intro. Keep frequency steady and change amplitude, then keep amplitude steady and change frequency. Watch the wave pattern each time.
PhET also has a Listen Audio control. If your teacher uses it, start with the device volume low and choose a comfortable level. The sound played by a computer is affected by its speakers or headphones. An on-screen level cannot certify actual dB SPL at your ear.
WHO explains its hearWHO screening app, intended for ages 12 and older. It uses spoken numbers in noise. It is a screening check, not a formal hearing test and not this pure-tone audiogram activity. No personal score or app installation is required for the lesson.
Find a threshold.
Then build the pattern.
ECHO is an original fictional adult who reports difficulty following speech. A clinician first asks about the concern and relevant history, inspects the ear with an otoscope, and checks the calibrated equipment and quiet test conditions. The concern provides context; it does not identify the cause.
A · Right ear, air conduction, 1,000 Hz
Air conduction tests the route through the outer ear, middle ear, cochlea, and auditory nerve. In this excerpt, after a miss the next tone is 5 dB higher. After a response, the level drops by 10 dB before ascending again. Follow each fictional response.
Choose “Next observation” to see ECHO's first trial.
| Trial | Level (dB HL) | Response |
|---|---|---|
| No observations yet. | ||
A threshold is the lowest level detected on at least two of three ascending trials. The first response alone does not establish it. These six observations show two ascending detections at 40 dB HL, meeting the minimum repeated-response criterion. They do not make three detections mandatory. A clinician may collect further trials when needed. This is a teaching excerpt, not a complete clinical protocol. Source 3
Before reading the graph, learn the two routes
AC means air conduction. An earphone sends sound through the usual outer- and middle-ear route to the inner ear. BC means bone conduction. A small bone vibrator sends vibration through the skull to the inner ear, bypassing the usual outer- and middle-ear sound transfer.
Why use masking? A skull vibration can reach both inner ears. If the clinician is testing the right ear, the left ear might detect the tone and supply the response. The clinician may therefore send carefully controlled noise into the left ear. This is masking. It makes that other ear less able to answer for the ear being tested.
Right masked BC means a right-ear bone-conduction measurement made with masking used to establish which ear responded. In ECHO's fictional case, assume the masking was effective and the measurements reliable. Too little noise may leave the other ear able to respond; too much may affect the test ear. A professional method is needed. Unmasked BC alone does not tell us which inner ear responded.
Now return to the chart. Find 1,000 Hz across the top and follow its line down. Find 40 dB HL on the left and follow that line across. Their meeting point is the right AC circle: one summary of the lowest level ECHO detected reliably, not a circle for every missed tone.
B · Place frequency and level on the chart
The horizontal axis uses logarithmic frequency: each doubling of Hz has equal spacing. The vertical axis is hearing level in dB HL, increasing downward. Lower points mean that a higher test level was needed for detection. One octave has the same plotted distance as 20 dB.
No points recorded yet. Complete the threshold example above.
O and X represent unmasked air conduction. The bracket [ represents right masked bone conduction; it sits just left of its frequency line and opens toward it. Masking helps establish which ear responds. An unmasked bone-conduction result alone cannot identify the responding cochlea. A no-response result would need a symbol with an attached arrow at the equipment limit, not an ordinary threshold point. Source 4
Legend: AC = air conduction; BC = bone conduction; PTA = pure-tone average; Hz = cycles per second; dB HL = hearing level relative to the test reference. O = right AC without masking; X = left AC without masking; [ = right BC with effective masking in the left ear.
| Frequency (Hz) | Right AC · O | Left AC · X | Right masked BC · [ | Right air-bone gap |
|---|
AC = air conduction. BC = bone conduction. A blank marked “Not supplied” is missing data, not a zero threshold. No left BC values or BC values at 250 or 8,000 Hz are supplied. Lines between air-conduction symbols guide the eye; only marked frequencies were measured. These fictional values illustrate interpretation, not a clinical crossover or masking calculation.
C · Read the evidence before naming the pattern
At 1,000 Hz, the right ear needed 40 dB HL through the earphone but 15 dB HL through the bone vibrator. Subtract bone from air: 40 - 15 = 25 dB. This is the air-bone gap at that frequency. It compares two routes for the same ear. It is not a percentage of hearing. The illustrated walkthrough shows the two points joined by a labeled gap bracket.
- Describe each ear and frequency. Right AC rises from 35 dB HL at 250 and 500 Hz to 55 dB HL at 8,000 Hz. Left AC is 10 to 15 dB HL. On this chart, right thresholds slope downward as frequencies rise.
- Compare the two routes at the same frequency. Bone conduction bypasses outer- and middle-ear sound transmission; the cochlea and nerve still need to function. Subtract right masked BC from right AC: the gaps at 500, 1,000, 2,000, and 4,000 Hz are 25, 25, 30, and 35 dB.
- Calculate an average without hiding the shape. The right three-frequency pure-tone average (PTA) is (35 + 40 + 45) ÷ 3 = 40 dB HL, using 500, 1,000, and 2,000 Hz. The left PTA is 10 dB HL. This average does not include every frequency and is different from an air-bone gap.
- Name the degree convention. In the ASHA/Clark ranges used here, 26 to 40 dB HL is mild and 41 to 55 dB HL is moderate. The right PTA lies at the upper edge of mild, while its 2,000 to 8,000 Hz AC thresholds fall in moderate. Calling the entire ear simply “mild” loses information. The supplied left AC and right masked BC values fall within the normal range of −10 to 15 dB HL.
- State the supported pattern and its limit. Assuming reliable ear-specific masked BC, the right ear shows a conductive pattern: higher AC thresholds with BC in the stated normal range. The audiogram alone cannot establish why that pattern occurred or prescribe treatment. Left AC is within the stated normal range, but no left BC is supplied, so do not assign a left-ear loss type from missing evidence.
Make the claim earn its evidence.
Use your notes or discuss with a partner. This is optional practice with fictional data. There is no new submission, score, or personal information to enter.
- Record: Set the model to 43, 46, 50, and 60 dB SPL. For each, record the difference from 40 dB SPL and both ratios. Which changes approximately double pressure? Which approximately double intensity?
- Explain: Hold level constant and change frequency. Name what changes in the curves and what stays constant. Explain why the audiogram's dB HL axis cannot be replaced with this model's dB SPL readings.
- Review: Why does ECHO's first response at 40 dB HL need confirmation? Point to the observations supporting the recorded threshold.
- Reflect: A report says “ECHO has mild hearing loss caused by an ear infection.” Identify the unsupported cause claim and the detail hidden by the single degree label. Rewrite the report using only the supplied evidence.
Optional new case · FOXTROT
FOXTROT is a separate original fictional case. At 500, 1,000, 2,000, and 4,000 Hz, right AC is 40, 45, 50, and 60 dB HL. Right masked BC is 35, 40, 45, and 55 dB HL. Assume reliable ear-specific results. Calculate each gap and the 500/1,000/2,000 Hz PTA, describe the degree and shape, and compare the pattern with ECHO. Name one thing these numbers cannot tell you.
Start with the table, compare routes at matching frequencies, then use the ASHA explanation of loss types. A pattern describes evidence; it does not establish a cause.
Follow the evidence.
Primary educational and professional references. The clinical guideline descriptions are simplified for learning and do not replace professional training. ECHO and FOXTROT values are original invented teaching inputs, explicitly fictional.
- NIDCD · How Is Sound Measured? Sound level, frequency, and A weighting.
- CDC NIOSH · Understand Noise Exposure Measurement instruments and exposure context. Occupational exposure guidance is not a classroom sound target or a guarantee of safety.
- ASHA · Guidelines for Manual Pure-Tone Threshold Audiometry (2005) Preparation, threshold procedures, and retesting. This model displays one short example, not the complete protocol.
- ASHA · Audiometric Symbols (1990) Symbol conventions and chart proportions.
- ASHA · Degree of Hearing Loss Clark degree ranges used in this lesson.
- ASHA · Types of Hearing Loss Conductive, sensorineural, and mixed patterns.
- UNSW Physics · What Is a Decibel? Logarithmic ratios and the airborne reference pressure.
- NIDCD · Noise-Induced Hearing Loss Hearing protection and the roles of level and duration.