Understanding the Decibel (dB) Scale


If you are into setting up a home theater system, you might have heard of the term “decibel” (dB) while adjusting your system or checking out speakers, amplifiers, or sound level meters. So what is a decibel, and how does it relate to the sound of your home theater?

What is a decibel?

In regards to sound, a decibel (dB) is a unit of measurement used to gauge the intensity or loudness of sound. It operates on a logarithmic scale, comparing the sound’s pressure level to a reference level. With every 10dB increase, there is a tenfold rise in sound intensity, leading to a perceived doubling of loudness. For instance, a sound at 30dB is ten times more intense and twice as loud as a sound at 20dB, while a sound at 40dB is a hundred times more intense than the threshold of human hearing at 0dB.

The decibel scale is commonly used to measure sound levels in various contexts, such as home theaters, concert venues, industrial settings, and even environmental noise monitoring. It enables us to assess sound exposure, and to ensure proper audio balance for a better listening experience. However, it is essential to interpret decibel measurements with consideration of the reference level and the specific weighting used (e.g., A-weighting and C-weighting) to accurately understand the impact of sound on human perception.

Decibel Weighting

The human ear can perceive a wide range of sound pressures, from the threshold of hearing (about 0dB) to the threshold of pain (about 120dB). However, the human ear is not equally sensitive to all frequencies of sound. The ear is more sensitive to mid-range frequencies (around 1kHz to 4kHz) than to lower or higher frequencies. This means that a sound with a constant intensity across all frequencies will not sound equally loud at different frequencies. We explain this concept in further detail in our article “Understanding Audio Dynamic Range.”

To account for this frequency-dependent perception of loudness, a weighting filter is usually applied to the sound pressure level measurements. There are three main types:

dB(A): The A-weighting filter is the most commonly used when measuring environmental sound, and it mimics the human ear’s response at moderate sound levels. It attenuates lower and higher frequencies more than mid-range frequencies, which are the ones that humans hear best. The dB(A) unit is used to express sound pressure levels measured with the A-weighting filter.

dB(C): The C-weighting filter is similar to the A-weighting filter, but it attenuates lower and higher frequencies less than the A-weighting filter. It is more suitable for measuring peak sound levels, such as home audio or rock concerts, where the human ear becomes less sensitive to frequency differences. The dB(C) unit is used to express sound pressure levels measured with the C-weighting filter.

dB(Z): The Z-weighting filter is also known as the zero-weighting filter, because it does not apply any frequency correction to the measured sound pressure level. It is equivalent to measuring the sound level in decibels (dB) without any weighting filter. The dB(Z) unit is used to express sound pressure levels measured with the Z-weighting filter.

Graph showing the weighting differences between dB(A), dB(C), and dB(Z).

Each weighting has a specific adjustment used to align measured sound intensity with how humans perceive it. For instance, humans are less sensitive to lower frequencies than to higher ones. So, if a 30Hz tone is played at the same dB(Z) level as a 1000Hz tone and we switch to dB(A) weighting, we will notice that the dB(A) level of the 30Hz tone is much lower than the 1000Hz tone.

What dB Level is Best For Home Theaters?

The optimal dB level for a home theater system depends on personal preference and listening environment. However, some general guidelines can be followed to achieve a balanced and realistic sound reproduction while calibrating or measuring your audio system.

According to the THX standard, which is a set of specifications for high-quality audio-visual systems and is the standard when calibrating home audio, the reference dB level for a home theater system should be 85dB for each speaker when playing pink noise test tones. However, for audio processors that are THX-certified and/or running Audyssey room correction, the tones will be emitted at 75dB, which is corrected internally to correspond to 85dB.

When using a THX-certified amplifier or receiver with the -dB scale on the volume control, 0dB represents the THX reference level, which corresponds to an average SPL level of 85dB with a typical +/- 20db swing in peaks. However, with other volume scales, the numbers on the scale may not directly correlate to the THX reference level or the dB scale. So, when using different volume scales, the numbers might not reflect the same sound intensity as the THX reference level. It is important to keep this in mind when setting up your audio system.

Since reference level is quite loud, and it may not be suitable for most audio-listening situations. It is specifically intended for playing sound at a higher volume in larger rooms like commercial theaters or big auditoriums. However, in smaller rooms like our homes, sound behaves differently due to the room’s acoustics. This causes listening at THX reference level in homes to be uncomfortable for most people. For regular home use, lower volume levels are generally more comfortable and enjoyable.

As an example, I tend to enjoy my content around -15dB at the loudest, which corresponds to an average level of 70dB with peaks around 90dB, a little higher than normal conversation level.

How can you measure the sound pressure levels in your Theater?

The dB level of a home theater system can be measured using a sound pressure level (SPL) meter or a microphone.

When using an SPL meter, it is recommended to set it to C-weighting and choose the slow response mode. This ensures accurate measurements of the sound levels, as C-weighting is best to use for measuring peak sound levels and slow speed ensures that there is an accurate response. Alternatively, you can also download an SPL meter app on your phone, but be aware that your phone’s microphone was not designed to take accurate sound readings. However, it can still be used for a ballpark measurement.

The second method is using a microphone connected to a computer, preferably a calibrated microphone. Then, using software like REW, you can take SPL readings of the system. If using computer software, be aware of the weighting that is being used on the measurements. For example, in REW, frequency sweeps are not weighted, but you can pick the weight if you are using the SPL meter function.

To obtain precise and reliable results, it is recommended to measure the sound from the primary listening position at ear height and any other locations you may be interested in. Placing the meter on a tripod or microphone stand is preferable, and care should be taken to avoid positioning it near reflective or absorbent surfaces, which may interfere with the measurements.

If you are trying to calibrate your home theater, you may want to follow the auto-calibration procedure for your device as it will set all speaker levels for you, which you can then verify with one of the two methods above for accuracy. We offer comprehensive guides to help you through the calibration process of your audio system. Check out our guides below:

To Wrap It Up

Understanding the decibel and its connection to sound is crucial for achieving a well-balanced and captivating home theater experience. The logarithmic scaling of decibels can be confusing for many, but a simple rule to remember is that each 10dB increase corresponds to a perceived doubling in volume.

By using proper calibration techniques and taking individual preferences into account, you too can have high-quality audio within the comfort of your home.

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