Input SPL for 9 octave bands and instantly compute A/C/Z-weighted levels, overall SPL, Zwicker loudness, and NC curve compliance in real time.
Band Level Input
Presets
NC Curve Target
NC Target
NC
Spectrum Display
Results
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Overall SPL (dB)
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LA (dBA)
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LC (dBC)
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Loudness (sone)
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Dominant Band
Octave
NC Compliance:NC-40 Pass
Engineering Note
NC curves (ASHRAE) are the dominant indoor noise standard in North America. ISO 1996 and JIS Z 8731 use A-weighted equivalent level Leq for environmental assessments. When LCC − LCA > 20 dB, low-frequency content dominates and may cause perceptual discomfort even if dBA is within limits.
A-weighting (key bands): 31.5 Hz: −39.4 dB / 125 Hz: −16.1 dB / 500 Hz: −3.2 dB / 1 kHz: 0 dB / 4 kHz: +1.0 dB / 8 kHz: −1.1 dB
Zwicker Loudness: $S = 2^{(P_{phon}-40)/10}$ [sone], where $P_{phon}$ is derived from the A-weighted overall level via equal-loudness contours.
What is Octave Band Noise Analysis?
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What exactly is an "octave band," and why do we break sound into these specific frequency groups instead of just measuring the total loudness?
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Great question! Basically, our ears don't hear all frequencies equally. An octave band is a range of frequencies where the highest frequency is double the lowest. The simulator uses the 9 standard bands from 31.5 Hz to 8 kHz. By measuring noise in each band separately, we can understand its "fingerprint" – like whether it's a rumbling bass or a hissing high-pitch sound. Try entering different levels for the low (31.5 Hz) and high (4 kHz) bands above and see how it changes the weighted results.
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Wait, really? So what's the point of A, C, and Z-weighting? They give me different numbers from the raw octave band levels I typed in.
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Exactly! Each weighting is a filter that mimics how the human ear responds. A-weighting heavily reduces very low and high frequencies, approximating how we hear at quiet levels. C-weighting is flatter, used for louder sounds. Z-weighting is "zero" weighting – no filter at all. In practice, environmental noise regulations often use dBA. For instance, a fan might have strong low-frequency rumble. The raw levels (Z) might be high, but the dBA value could be acceptable. Slide the 63 Hz and 125 Hz bands up high and watch the gap between LZ and LA grow.
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That makes sense. But then what are these NC curves and "loudness in sones"? They seem like another layer of complexity.
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They are, but they're crucial for real design. NC (Noise Criterion) curves are the dominant standard for rating background noise in buildings like offices and hospitals. Each curve defines a maximum allowable level per octave band. The simulator checks your spectrum against the NC Target you select. Loudness in sones (and phons), calculated by the Zwicker method, goes a step further by modeling how our perception of loudness changes with both frequency and sound pressure level. A common case is a spectrum with a prominent tone – it might pass an NC check but still be perceived as annoyingly loud.
Physical Model & Key Equations
The overall Sound Pressure Level (SPL) in decibels is calculated by energetically summing the pressure from all frequency bands. You cannot simply average decibel values.
Where \(L_i\) is the sound pressure level (in dB) for the i-th octave band. This is the formula behind the "Overall SPL (Lin)" result in the simulator.
The A-weighted sound level applies a standardized frequency-dependent correction to each octave band level before summation, simulating human hearing sensitivity.
Where \(A_i\) is the A-weighting correction for the i-th band (e.g., −39.4 dB at 31.5 Hz, −16.1 dB at 125 Hz, −3.2 dB at 500 Hz, 0 dB at 1 kHz). The same principle applies for C-weighting with different correction values. The simulator applies these corrections instantly as you adjust each band slider.
Frequently Asked Questions
A-weighting is a weighting that approximates human hearing and is often used in environmental noise standards. C-weighting evaluates low frequencies almost flat and is suitable for confirming peaks in machinery noise. Z-weighting is the unweighted physical sound pressure level and is used as a reference value for analysis. Please switch according to the application.
Zwicker loudness is an index that models human loudness perception more precisely, with units in sone. While dBA uses a single frequency weighting, Zwicker considers masking effects and critical bands, making it effective for evaluating the 'annoyance' of complex noises.
NC curves indicate acceptable noise criteria for indoor environments. If the fitting result is NC-30, it means that the levels in each octave band do not exceed the NC-30 curve. The lower the number, the quieter the environment; typical guidelines are NC-30 to 40 for offices and NC-25 to 30 for bedrooms.
Use a precision sound level meter or real-time analyzer to measure 9 bands from 31.5 Hz to 8 kHz with 1/1 octave analysis. Position the microphone at the ear height of the evaluation subject (1.2 to 1.5 m above the floor), and ensure sufficient sampling time to avoid the influence of background noise.
Real-World Applications
HVAC System Design: This is the primary use of NC curves. Engineers measure or predict the octave band spectrum of air handling units, fans, and diffusers to ensure the background noise in a conference room or hospital patient ward meets a specified NC rating (e.g., NC-30 for open-plan offices). The simulator lets you model a proposed system's spectrum and check compliance instantly.
Product Noise Labeling & Development: Manufacturers of appliances, power tools, and vehicles use octave band analysis to diagnose noise sources. By identifying which frequency bands dominate, they can target specific components for damping or isolation. A-weighted overall levels are often used for regulatory noise labels.
Environmental Noise Assessment: Community noise from industrial plants, wind farms, or traffic is often evaluated using A-weighted equivalent levels (Leq) over time. However, analyzing the octave band data is essential when low-frequency noise complaints arise, as dBA may underestimate the annoyance, a phenomenon you can explore with the simulator.
Architectural Acoustics: Consultants use this analysis to design wall and floor partitions. The Sound Transmission Class (STC) rating is derived from octave band transmission loss data. Ensuring a balanced spectrum prevents "bass flanking" where low-frequency sound leaks through a structure even with a good STC rating.
Common Misconceptions and Points to Note
First, understand that a lower overall sound pressure level (L_total) does not necessarily mean it's quiet. Low-frequency sounds are less audible to the human ear (lower sensitivity), so even a high overall level can be rated low when measured with A-weighting (dBA). For example, the reason you might feel a sense of pressure or vibration near a transformer without it sounding loud to your ears is precisely this. Conversely, if you focus only on dBA for countermeasures and neglect low-frequency noise, you risk overlooking the root cause of complaints.
Next, there's a tendency to underestimate the impact of moving a single slider. Increasing the level in one frequency band by 10 dB means its energy becomes tenfold. While its effect on the overall level depends on the levels in other bands, consider this: if all bands are at 60 dB and you raise only the 125 Hz band to 70 dB, the overall level appears to increase by only about 1 dB, from approximately 60.4 dB to 61.4 dB. However, this 1 dB change is often enough to push the noise above an NC curve, causing the design to fail the criteria. You cannot afford to dismiss subtle changes.
Finally, be aware of the danger of not considering the source of your measurement data. This simulator is just a calculator; its entire premise relies on the nine input values coming from accurate measurements. In practice, factors like microphone orientation, the presence of a windscreen, or background noise can introduce significant errors, especially in low-frequency ranges. No matter how good your theoretical analysis looks, remember the principle of garbage in, garbage out (GIGO).