Consideraciones acerca de medición y ecualización

Floyd Toole:

Almost always a cancellation is a "local" phenomenon, occurring only at one or a few locations. Filling such a dip adds a boost to the direct and all other sounds radiated from the loudspeaker, making it a worse loudspeaker and filling the room with excessive sound at the boosted frequency. All such acoustical interference dips, and peaks, are non-minimum phase phenomena and cannot be corrected by EQ. It is the principal reason why room curves are not definitive descriptors of sound quality.

EDIT: I should add that these non-minimum-phase acoustical interference ripples in room curves are almost always not audible problems. They may look like "comb filtering" but because the direct and delayed sounds come from different directions two ears and a brain perceive them as spaciousness, not coloration. Equalizing them flat/smooth is a mistake - it results in a degradation of the direct sound, which is the most important event.

Bueno y aquí una pregunta directa:

Quote:
Originally Posted by Deboi

Are these negative considerations about Room EQ above the Schroeder frequency also valid for manual EQ (measuring the response in the MLP with an omnidirectional microphone)?



The "negative considerations" about broadband EQ apply to all measurements, but the ones you do yourself may be the most trustworthy. And the MLP is where performance matters most.

The first reason for the "negativity" is well described in my book. It shows that steady-state room curves are highly predictable from anechoic data - it is the off-axis radiation - early reflections - that dominate the curve shape. If there are irregularities in the room curve caused by an otherwise well designed loudspeaker having amplitude response problems, EQ can help. This is a rare event, as most competent designers aim for flattish, smooth on-axis response. If, as is more common, an irregularity is caused by frequency dependent directivity problems, EQ cannot fix it - you need a better loudspeaker, or an acoustically dead room.

The second reason involves discrete reflections that acoustically interfere at the measuring location, generating ripples. If these ripples are smoothed by EQ you have changed the on-axis - direct sound - performance of the loudspeaker which is the most important starting point for good sound. Two ears and a brain react very differently to acoustical interference than an omni mic. Humans distinguish between direct and reflected sounds on the basis of direction of arrival and timing. The mic and analyzer do not.

Iniciado por Deboi

Bueno y aquí una pregunta directa:

Quote:
Originally Posted by Deboi

Are these negative considerations about Room EQ above the Schroeder frequency also valid for manual EQ (measuring the response in the MLP with an omnidirectional microphone)?



The "negative considerations" about broadband EQ apply to all measurements, but the ones you do yourself may be the most trustworthy. And the MLP is where performance matters most.

The first reason for the "negativity" is well described in my book. It shows that steady-state room curves are highly predictable from anechoic data - it is the off-axis radiation - early reflections - that dominate the curve shape. If there are irregularities in the room curve caused by an otherwise well designed loudspeaker having amplitude response problems, EQ can help. This is a rare event, as most competent designers aim for flattish, smooth on-axis response If, as is more common, an irregularity is caused by frequency dependent directivity problems, EQ cannot fix it - you need a better loudspeaker, or an acoustically dead room. .

The second reason involves discrete reflections that acoustically interfere at the measuring location, generating ripples. If these ripples are smoothed by EQ you have changed the on-axis - direct sound - performance of the loudspeaker which is the most important starting point for good sound. Two ears and a brain react very differently to acoustical interference than an omni mic. Humans distinguish between direct and reflected sounds on the basis of direction of arrival and timing. The mic and analyzer do not.

Aquí ya saca un poco la "doble vara de medir" que en ocasiones reluce con ese:

"Si hay irregularidades en la curva de la sala causadas por un altavoz bien diseñado que tiene problemas de respuesta de amplitud, EQ puede ayudar. Este es un evento raro, ya que la mayoría de los diseñadores competentes buscan una respuesta plana y suave en el eje. Si, como es más común, una irregularidad es causada por problemas de directividad dependientes de la frecuencia, el ecualizador no puede solucionarlo: necesita un mejor altavoz o un acústico habitacion muerta . "




... porque es él el primero que sabe perfectamente que:

1- Las cajas no son planas +/-0dB... sino más bien "las buenas" suelen oscilar casi todas dentro de un típico +/-2 o +/-3dB (es decir, hay desviaciones entre máximos y mínimos que todavía oscilan entre 4dB y 6dB en la respuesta anecoica). De 4dB a 6dB de diferente intensidad es audiblemente un abismo (cualquiera que haya utilizado un EQ o haya modificado la forma de curva variando el filtro en un HUM lo sabe perfectamente... él el primero)


2- Por otro lado, los problemas causados por la directividad de la caja según su propio criterio (que por cierto, ya hemos visto en este hilo padecen el "95%" de las cajas comerciales) dice "se necesita un mejor altavoz o una sala acústica"... luego de nuevo está diciendo que para el 95% de las cajas del mercado hay que acondicionar la sala si quieres obtener alta calidad.



La otra opción es comprar uno de esos poquísimos altavoces que cumplen su criterio de directividad (aunque personalmente critico que por muy homogénea que sea esa directividad de la caja, el reflejo de las paredes en salas normales sin tratar sigue sin serlo = sigue habiendo claramente coloración tanto en los primeros ms (que afectan a la imagen y timbre) + también espacial en reflexiones más retardadas (porque el rebote no mantiene esa homogeneidad que las cajas "buenas" envían hacia las paredes en salas normales)




- En lo que sí estoy de acuerdo es en que el oído es al final el que te dice si una EQ es o no correcta (por eso mejor una EQ manual en la que puedas tocar todo poco a poco y no una que te lo haga todo de golpe según su criterio... a no ser que la automática tenga potencia suficiente como para variar en el ajuste pre-EQ la impulsiva en frecuencia según nos la zona que nos interesa sobre la que ir trabajando el convolver). De las pocas EQ automáticas que conozco tienen esa potencia está el gratuito DRCop (pero no se cómo esta el tema en multicanal). No se qué tal el software de ecualización de probablemente los monitores más utilizados y reconocidos de estudio: Genelec (software que por cierto ecualiza en toda la banda)


Un saludete

Iniciado por Deboi

Floyd Toole:

Almost always a cancellation is a "local" phenomenon, occurring only at one or a few locations. Filling such a dip adds a boost to the direct and all other sounds radiated from the loudspeaker, making it a worse loudspeaker and filling the room with excessive sound at the boosted frequency. All such acoustical interference dips, and peaks, are non-minimum phase phenomena and cannot be corrected by EQ. It is the principal reason why room curves are not definitive descriptors of sound quality.

EDIT: I should add that these non-minimum-phase acoustical interference ripples in room curves are almost always not audible problems. They may look like "comb filtering" but because the direct and delayed sounds come from different directions two ears and a brain perceive them as spaciousness, not coloration. Equalizing them flat/smooth is a mistake - it results in a degradation of the direct sound, which is the most important event.

Lo que no dice es que si muestras un RTA de 1/3 o 1/6 octava y desprecias los valles y picos bruscos en frecuencias colindantes al ecualizar, en realidad bien poco vas a corregir que sea de fase no mínima (menos aún, si la sala está tratada = casi todo lo que te llegue en la zona media y alta a 1/3 o 1/6 será prácticamente la respuesta anecoica de la caja)


Un saludete