Measuring Sound Levels – Decibels (dB) in Home Theater

What the decibel scale is and why it is important

Two of our most important senses are sight and hearing. With ears, we detect sound changes from the softest whisper to the loudest thunderclap.

How We Hear

We hear the sound (which are waves that move through the air, water, or another compatible medium) when it reached our outer ear, which funnels it through the ear canal to the eardrum

What Determines the Loudness of Sound

Sound loudness or volume is determined by a multitude of factors, which include a combination of the amount of air that reaches the ear from the originator of the sound, and the distance of our ears from the origination point of the sound.

The Decibel Scale

To quantify how loud sounds are, the decibel scale was created.

Our ears detect changes in volume in a non-linear fashion. A decibel is a logarithmic scale of loudness.

A difference of 1 decibel is perceived as a minimum change in volume, a difference of 3 decibels is a moderate change, and a difference of 10 decibels is perceived by the listener as a doubling of volume. Decibels are designated by the letters: dB.

0 dB is the threshold of hearing — Other examples include:

  • Whisper: 15-25 dB
  • Background noise: about 35 dB
  • Normal home or office background: 40-60 dB
  • Normal speaking voice: 65-70 dB
  • Orchestral climax: 105 dB
  • Live Rock music: 120 dB+
  • Pain Threshold: 130 dB
  • Jet aircraft: 140-180 dB
A signal strength meter showing input power and output decibels
RonPeigl / Getty Images

How the Decibel Scale Is Applied

The decibel scale is applied to music and home theater in the following fashion.

For amplifiers, decibels are a measurement of how much power it takes to produce a specific sound output level. However, there is more to it.

For one amplifier or receiver to be twice as loud as another, you need 10 times more wattage output. A Receiver with 100 WPC is capable of twice the volume level of a 10 WPC amp. A receiver with 100 WPC needs to be 1,000 WPC to be twice as loud.

Decibels are also used in relation to the sound output of loudspeakers and subwoofers at specific frequencies, at specific volume levels. A speaker may have the ability to output a frequency range of 20Hz to 20kHz, but at frequencies lower than 80 Hz, the sound output level (volume) may be -3dB less. This is because more power output is required at lower frequencies to produce the same volume level.

The dB scale is applied to the sound level output capability of a specific speaker when fed a tone carried by one watt of power. This means a speaker that can produce 90 dB or higher sound output when fed a one-watt audio signal is considered to have good sensitivity.

However, just because a speaker has good sensitivity doesn't automatically mean it's a "good" speaker. A speaker that needs more power to produce sound most often indicates the amount of power required for the speaker to produced audible sound. Other factors, including frequency response, distortion, power handling, and speaker construction, are also important.

For video projectors, the decibel scale is used to measure how much sound is produced by the cooling fan. If a video projector has a fan noise rating of 20dB or less, that's considered very quiet. Unless you are sitting close, you shouldn't be able to hear the fan — if you do, it shouldn't be distracting.

How to Measure Decibels

With the knowledge of what decibels are and how they factor into music and home theater listening, the question is "How can you measure them?".

For consumers, one way that decibels can be measured is with a portable sound meter.

If you take a sound meter to a movie or a concert, you will find out how loud they really are!

Radio Shack Memories - Digital Sound Meter

Examples of Sound Meters include:

There are also sound meter apps that work with the microphone in many smartphones.

In the home, most home theater receivers have built-in test tone generators, you can use those tones to determine the generated decibel level for each speaker at a given volume level setting. Once you determine the decibel level generated by each speaker using the sound meter, you can adjust your individual speaker volume levels so that the entire speaker system matches. When all your speakers register the same decibel level at a given volume level, your sound listening experience will be balanced.

Measuring Decibels Without a Sound Meter

Many home theater receivers have an Automatic Speaker/Room Correction/Setup system which doesn't require the use of a separate sound meter.

Depending on the brand/model of the receiver, these systems go by different names: Anthem Room Correction (Anthem AV), Audyssey (Denon/Marantz), AccuEQ (Onkyo), Dirac Live (NAD), MCACC (Pioneer), DCAC (Sony), and YPAO (Yamaha).

A provided microphone plugs into the front of the receiver and placed in the primary seating position. When prompted, the receiver sends out test tones to each speaker that is picked up by the microphone and sent back to the receiver.

The receiver then determines how many speakers there are, the distance of each speaker from the listening position, and the size of each speaker (small or large). Using that information, the receiver calculates the "optimum" speaker level relationship (stated in decibels) between the speakers (and subwoofer), along with the best crossover point between the speakers and the subwoofer.

Denon AVR-X2100W Speaker Level Settings Menu