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Room Acoustics

----  Theory  ----  The best rooms  ----  The listening room  ----  Whatever happened  ----

 

The Listening Room

The specifics of the phantom acoustic scene that is rendered by a pair of loudspeakers and perceived by a listener's brain depend upon the radiation characteristics of the loudspeakers, their location in the room, the reflective, diffusive and absorptive properties of the room and the listener's location. The first requirement is for lateral symmetry of the loudspeaker and listener setup with respect to large reflecting surfaces. Secondly, the loudspeakers must be placed at some minimum distance from those large surfaces in order to delay specular reflections by more than 6 ms. This allows the brain to give primary attention to the earlier arriving direct sound from the loudspeakers, if the reflected sound streams have similar timbre and spectral content as the direct sound. This in turn requires loudspeakers with constant, frequency independent radiation characteristics in order to illuminate the room spectrally neutral. The LX521 represents such loudspeaker to a high degree. The typical box monitor is omni-directional at low frequencies and becomes increasingly directional as its baffle and driver dimensions become larger than 1/4th of the radiated wavelength. The power response typically changes by more than 10 dB between low and high frequencies. The power response of the LX521 is 4.8 dB lower at low frequencies and nearly constant up to 7 kHz before it decreases. 

Top and bottom baffle of the LX521 can be rotated independently of each other. The orientation of the bottom baffle determines the degree to which the woofer couples to different room modes. This provides some flexibility in dealing with a problematic room mode by turning the woofer a certain amount. Of much greater significance is the ability to minimize the first side wall reflections L' and R' by orienting the top baffles. The optimum amount of toe-in is a function of loudspeaker distance from the side walls and distance to the listener. In the drawing on the left the right dipole is positioned at 0.5 m from the wall and the left dipole at 1 m. In both cases the plane of zero sound output is pointing at the first reflection points L' and R'. The toe-in axis is at right angle to this plane and points to A or B in front of the listener. 

More toe-in is required for the right dipole that is closer to the side wall. Toe-in has to be increased slightly for either dipole if the listener sits farther back.

The right dipole is too close to the wall to meet the 6 ms delay between reflected signal R' and the direct signal R, but attenuating R' by proper toe-in greatly improves the precision of imaging in a narrow room.

Toe-in also reduces the magnitude of the first reflections L" and R" from the wall behind the dipoles. The level of reverberant sound in the room and the ratio of direct to reverberant sound at the listener are insignificantly affected by toe-in.

You can make a drawing for your room dimensions by using your setup distances and corresponding image sources L', L" and R', R'' to determine the optimum toe-in point. 
A listener would see the top baffle from the side, if mirrors were placed on the wall at L' and R'.

Richard Taylor has analyzed the required room dimensions and speaker setups to obtain a >6 ms delay for the first order reflections. He also determines the toe-in angle for equal strength of side wall and front wall reflections in a small room with a dipole source . I consider it more important to minimize the side wall reflection and to diffuse, not to absorb, the front wall reflection for optimal imaging.

James Heddle contributed a spreadsheet, which calculates the strength and delay of first order room reflections. Enter your speaker's distances from the walls and adjust the toe-in angle to minimize the reflected energy from the closest side wall. Suppression of this reflection widens the sweet spot and this top baffle orientation places the aural scene between the loudspeakers even for far off-center listeners. Compare the calculated dipole reflections to those of a monopole.

A room that is not open or acoustically dead behind the listener is likely to cause problems due to rear wall reflections. If the distance dr between listener and wall is l/4, i.e. when the reflected wave has traveled l/2 relative to the incident wave, then incident and reflected waves will cancel. The frequency response at the listener's location therefore has a notch at frequency f = 340/4*dr . Ideally this notch is at a low 10 Hz, which means that dr = 8.5 m (28 feet), a very long room. A more common listener-to-rear wall distance of 1.5 m (5 feet) would put the notch around 57 Hz, right into the bass range and be very problematic. Thus every attempt should be made to attenuate the rear wall reflection. Use a variable low frequency sinewave signal source to hear the notch frequency range at your listening place. Pink noise is not a good test signal for this. 

The ideal listening room acts like a waveguide with the loudspeakers at some distance from the diffuse (live) end of the room and sound traveling past the listener to the open (dead, absorptive) end of the room (see the drawing below). Sound reflections from the wall behind the listener should be attenuated as much as possible, particularly below 200 Hz., the frequency range where discrete room resonances tend to dominate the bass distribution in the room. The LX521 being a dipole has the advantage of minimally exciting lateral room modes due to the null in its radiation pattern.

A sound processing room is usually designed to be quiet and acoustically dead. It is a work environment and not at all representative of the typical living space where people listen to a stereo recording for enjoyment but also pursue other activities. The processing room has to be dead to ensure a direct to reverberant sound ratio of no less than -6 dB at the work place and to minimize the influence of reflections and reverberation due to the colored illumination of the room by the typical box-type monitor loudspeakers. Close-field monitors at short listening distance relax the reverberation time requirements. They approach headphone listening. Headphones are completely unsuited for judging the spatial rendering of a stereo recording that is intended for loudspeaker playback. Headphones are optimally suited for analyzing tonal artifacts in a recording but completely distort distance perception. Recording engineers often claim that they "can hear through the flaws of their monitors" to the real sound. Then why are there so many technically poor recordings?

An acoustically small dipole radiator in a room with T60 = 750 ms will have the same D/R ratio as an omni-directional radiator in a room with T60 = 250 ms. One room is dead, the other very live, but at the same distance from loudspeakers the room contributions to the sound at the listener's ears are equally subdued compared to the direct sound coming from the loudspeakers. The dipole loudspeaker reaches by a factor 1.73 = sqrt(3) deeper into the room. The two graphs below and the Listening_distance.xls spreadsheet show what this means in actual numbers. Note in particular how steeply the required amount of wall absorption must increase to obtain a 250 ms reverberation time. My preference is for a reverberation time of around 450 ms, which also provides a pleasant environment for talking and reading in addition to critical listening. The wall behind the loudspeakers should be diffusive in order not to lose the rear radiated sound from the LX521. Specular reflection from the side walls must not be attenuated as this reduces high frequency energy in the room. The wall behind the listener should be lossy to attenuate room modes. In addition, cloth wall hangings, rugs, pictures, upholstered chairs, open cabinets, plants and other decorative elements are all that is needed to interface a dipole loudspeaker with the room regardless of whether it is intended for work or pleasure.

Example of a 100 m3 volume room for which EBU specifies a 250 ms reverberation time

Listening distance for -6 dB D/R in the 100 m3 volume room for different reverberation times. Required open window area in % of total room surface area.

See also:  Sound Field Control for Rendering Stereo

 
What you hear is not the air pressure variation in itself 
but what has drawn your attention
in the two streams of superimposed air pressure variations at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?

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Last revised: 01/08/2014   -  1999-2014 LINKWITZ LAB, All Rights Reserved