How Dynamic Range, Compression, and Headroom Affect Audio

A lot goes into sound performance on a stereo or home theater system. The volume control is the main control people reach for, but it can only do so much to affect the quality of a listening experience. Dynamic headroom, dynamic range, and dynamic compression are additional factors that can contribute to the overall listening experience.

Dynamic Headroom: Power When You Need It

For room-filling sound, a stereo or home theater receiver needs to put out a certain about of power to your speakers. Because sound levels constantly change throughout musical recordings and movies, the receiver needs to adjust its power output quickly and in a consistent manner.

Dynamic headroom refers to the ability of a stereo, home theater receiver, or amplifier to blast the power to higher levels for short periods of time. This is meant to accommodate musical peaks or extreme sound effects in films. It is especially important in a home theater system, where extreme volume changes occur throughout the course of a film.

Dynamic headroom is measured in decibels (dB). If a receiver or amplifier has the ability to double its continuous power output capability, it should have 3 dB of dynamic headroom. However, doubling the power output does not mean doubling the volume. In order to double the volume from a given point, a receiver or amplifier needs to increase its power output by a factor of 10.

This means that if a receiver or amplifier is outputting 10 watts at a specific point, and a sudden change in the soundtrack requires double the volume for a brief period of time, the amplifier or receiver needs to be able to rapidly output 100 watts.

Dynamic headroom capability is baked into the hardware of a receiver or amplifier, and it cannot be adjusted. Ideally, a home theater receiver will have at least 3 dB or more of dynamic headroom. This can also be expressed by a receiver's peak power output rating. For example, if the peak or dynamic power output rating is double the amount of the stated or measured RMS, Continuous, or FTC power rating, this would be an approximation of 3 dB dynamic headroom.

Dynamic Range: Soft vs. Loud

In audio, dynamic range is the ratio of the loudest undistorted sound produced in relation to the softest sound that is still audible. One dB is the smallest volume difference that a human ear can detect. The difference between a whisper and a loud rock concert (at the same distance from your ear) is about 100 dB.

This means that, using the dB scale, the rock concert is 10 billion times louder than the whisper. For recorded music, a standard CD is capable of reproducing 100 dB of dynamic range, while the LP record tops out at about 70 dB.

When it comes to stereos, home theater receivers, and amplifiers, you want something that can produce the dynamic range of a CD or other source. One problem with source content that has been recorded with a wide dynamic range is that the "distance" between the softest and loudest portions can be irritating.

For example, in poorly mixed music, a vocal may appear to be drowned out by the background instruments, and in movies, the dialog may be too soft to understand, even as the sound effects can be heard down the street.

This is where Dynamic Compression comes in.

Dynamic Compression: Squeezing Dynamic Range

Dynamic compression does not refer to the types of compression formats used in digital audio (such as MP3). Instead, dynamic compression is a tool that allows a listener to change the relationship between the loudest and quietest parts of the soundtrack when playing a CD, DVD, Blu-ray Disc, or another file format.

For example, if explosions or other elements of a soundtrack are too loud and the dialog is too soft, you would want to narrow the dynamic range present in the soundtrack. Doing so makes the sounds of the explosions not quite as loud, yet the dialog sounds louder. This makes the overall sound more even, which is useful when playing a CD, DVD, or Blu-ray Disc at low volume.

On home theater receivers or similar devices, the amount of dynamic compression is adjusted using a setting control that may be labeled dynamic compression, dynamic range, or DRC.

Similar brand-name dynamic compression control systems include DTS TruVolume, Dolby Volume, Zvox Accuvoice, and Audyssey Dynamic Volume. In addition, some dynamic range or compression control options can work across different sources, such as when changing channels on a TV so that all the channels are at the same volume level, or taming those loud commercials within a TV program.

The Bottom Line

Dynamic headroom, dynamic range, and dynamic compression are important factors affecting the range of volume in a listening environment. If adjusting these levels doesn't fix the problems you experience, consider looking into other factors like distortion and room acoustics.