Reflected Sounds

What are reflected sounds and how do they impact sound quality?

When we listen to music in a small room what we hear is a combination of the direct sound from the speaker and the multiplicity of reflections from the surfaces of our room. Most listeners actually show preference for high levels of reflections that are spectrally similar to the direct sound. They add to timbral richness and give body and fullness to images within the soundstage.

As Benade (From Instrument to Ear in a Room, Journal of the Audio Engineering Society, 1985) states:

"The auditory system combines the information contained a set of reduplicated sound sequences (authors note - i.e. the direct sound and its reflections) and hears them as if they were a single entity, provided:

a)       that these sequences are reasonably similar in their spectral and temporal patterns

and

b)       that most of them arrive within a time interval of 40ms following the arrival of the first member of the set.

The singly perceived composite entity represents the accumulated information about the acoustical features (tone color, articulation, etc) shared by the set of signals. It is heard as though all the later arrivals were piled upon the first one without any delay – that is, the perceived time of arrival of the entire set is the physical instant at which the earliest member arrived (authors note – this is known as the precedence effect).

The loudness of the perceived sound is augmented above that of the first arrival by the accumulated contributions from the later arrivals.

The apparent position of the source of the composite sound coincides with the position of the source of the first-arriving member of the set, regardless of the physical direction from which the later arrivals may be coming"

What Benade is saying, in simpler terms, is that our brain combines all of direct and reflected sound received at our ear. What we hear is the combination of the direct and reflected sound. In small rooms the contribution of reflections to the sound we hear can be as much as an 60%. What we hear from a sound quality perspective in a small room is therefore determined as much or more by the reflected sound as the direct sound.

A critical point is that spectral distortion in the reflected sounds due to poor speaker off axis performance or materials at reflection points such as thin absorbers that do not operate consistently across the spectrum interfere with our perception of both localization and timbre.

These concepts are explained in much more detail in Toole's excellent book Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms.

How can I identify problematic early reflections using acoustic measurements?

The best measurement to use is the Energy Time Curve (ETC), which shows energy against time.

What is the target for reflected sounds?

The key requirement with reflected sounds is to properly mate the speakers intended response at a specific distance and axis to the response as measured at the listening position. Typically this means keeping the spectrum of direct and reflected sounds as consistent as possible.

A popular approach is simply to analyze the level of reflections on an ETC and compare these to the direct sound, setting a target for the reflections to be 10dB or more less than the direct sound. This analysis is not sufficient since ETCs are spectrally blind (i.e. they contain no information as to the spectral content of the reflected sound) and the auditory system is very discerning in its requirements for spectral balance between the direct and reflected sounds in a room.

Per our white paper on acoustic measurement standards, the ETCs for the left and right speaker should:

• Be visually identical (with only minor deviations) from 0-40ms
• Be down to -10dB by 40ms to prevent breakdown of the precedence effect
• Clearly show a decrease in the amplitude of energy over 0-40ms. The decay pattern may or may not be continuous.
• Show the consecutive peaks of the highest amplitude reflections viewed across the time axis to be relatively smooth in pattern and density.
• The criteria above should be considered in conjunction with the other stated targets for reverberation time and L/R frequency response.

Deviations from the targets above warrant examining one-octave bandpass filtered ETCs at 500Hz, 1kHz, 2kHz and 4KHz.

 Back to the Acoustic Distortion overview.