The STA program enables the DSP30 to measure the acoustical propagation time from a loudspeaker to the analyzer’s microphone with a high degree of precision. This data can then be used to determine the necessary delay to align signal arrivals between multiple loudspeakers, as referenced to a point in the room, or may be used as a quick check for the internal propogation delay of the electronics. In the default mode, the ranges for measuring delay are from 1ms (1.13feet) to 999ms (1,129 feet). In the unlikely event that you needed to measure a delay of more than one second, additional ranges are available for delays of up to 10 seconds.
ACOUSTICAL PROPOGATION delays are typically the result of using multiple loudspeakers at very different distances, as referenced to a point in the room. It is not possible to set one delay time, which would result in perfect arrivals at every point in the room. Accordingly, the measurement is made from a location in the room where the problem of mis-aligned arrival time is most serious. Typically, if one individually measures the SPL from both loudspeakers, the point in the room where the arrivals are closest in SPL, is the optimal position to set the delays. Many factors, including seating positions and reflections in the room can effect the contractors decision of where to set the delay position. The above rule, however, provides a good starting point.
HAAS EFFECT is a psychoacoustic response of the human brain, which allows us to fool the mind into believing that sound is coming from a different position than the actual arrival. Most often it is used to move the image towards the person that is talking, and away from the loudspeakers. The brain expects sound to come from a person, not from a loudspeaker, and Haas Effect is one method of achieving this result by utilizing delay.
CALCULATING THE DISTANCE from a loudspeaker to the microphone is very simple. Take the time in milliseconds “ms”, and multiply it by 1.13 to get the distance in feet. Temperature and humidity modify this equation slightly. At a 100 feet the variation rarely exceeds a few inches. Remember, however, that sound can be refracted by changes in temperature, and the acoustical path may be longer than the direct path as measured with a tape measure of laser.
CALCULATING THE DIGITAL DELAY between two loudspeakers will be discussed in detail below. The digital setting typically is the difference between the arrival time from the nearest loudspeakers, and the arrival time from the furthest loudspeaker. The delay is always applied to the nearest loudspeaker, for reasons that become obvious if one considers the alternative.
ELECTRICAL PROPOGATION delays are typically the result of digital components in the signal path. If you wanted to know whether a system currently has a digital delay engaged, this test will quickly identify the amount of delay. Additionally, most digital components have between 1ms and 5ms of delay. If several of these devices are used in the audio chain, the net result can become very significant.
THE TEST PULSE FREQUENCY is based on a 1kHz sine wave. Accordingly to make the measurement the PN3B must be set to the 1kHz position. To make the measurement the Loudspeaker must be capable of producing 1kHz. In the case of subwoofers, this may not be possible. And, in that case place a small mid-high frequency cabinet as close as possible to the sub, and use that to make your measurements.
MULTIPLE LOUDSPEAKERS WITH HAAS EFFECT:
There are cases where simultaneous arrival is not the objective. The most common of these is the use of the Haas Effect.
Typically a person singing or speaking from a podium produces a certain amount of sound directly from their voice. Generally the brain will identify the loudest arrival as the true direction of the sounds origin, and in most sound systems the PA is louder than the voice itself. In determining direction, however, the brain also considers the time of the arrival. The shortest distance between any two points is a straight line, and accordingly sound should arrive first from the true point of origin. Haas effect utilizes this general rule of the brain, by placing the reinforced sound 15-20ms behind the arrival of the natural voice. There is little degradation in intelligibility from this offset in time, and in some instances a dramatic improvement in perceived image can be obtained.
To use the Haas Effect try the following steps.
1. Place a small loudspeaker at the location where the person will be talking. Measure the arrival time from that location as per the steps set forth above.
2. Measure the time of arrival from the Main Cluster.
3. If the cluster arrives either earlier, or less than 20 ms after the location of the natural voice, delay the cluster back into the Haas Zone.
4. Then measure the near loudspeaker, and delay it to arrive with the Main Cluster. If you have never tried this method, you will be amazed by the results. The 20ms is not an exact science, so try a couple of settings in that area, and let your ears judge which sounds most natural.
ELECTRICAL PROPOGATION DELAY - QUICK FIELD CHECK:
Place the microphone within a few inches of a loudspeaker driver that will produce 1kHz. The sound should arrive from the PN3B to the loudspeaker at approximately 1/5th the speed of light. Any delay detected is electrical propogation of the system, and should be taken into account in system design. If the delay is the same on all channels it is not typically a problem, but this is an excellent way to determine if a line has a digital delay already engaged. Follow the setup procedure above for multiple loudspeaker propogation. Then send a test pulse through the system.
The minimum time that can be measured is around 2ms. This is not a laboratory test of components, but does provide a quick method to determine if a digital delay is currently in use within the system. If no delay is being used, the time should be very close to zero. Make sure that the reference for ambient noise is below the zero reference line on the DSP30, and that the pulse is slightly above the reference line. Otherwise a zero reading could indicate merely that no data was received. Remember that digital equalizers, control systems or any other device which converts signal from analog to digital, or digital to analog will typically add around 1 ms of delay. Accordingly most audio systems have at least a small electrical propogation delay.
USING THE STA OPTION AS A YARD STICK:
If you want to know the distance from a cluster to the 23rd pew in a church, the microphone would be placed at the 23rd Pew, and an STA test would be performed. The arrival time is multiplied by 1.13 to obtain the distance in feet. Remember that there can be a variation of approximately 1 foot in the data. To determine the distance to the loudspeaker in meters, multiply the time in seconds by 344m/s, or the time in milliseconds by 0.344m/s. Note that for temperatures and humidity other than 70°F (21°C), and 60% humidity, the speed of sound will be slightly different.
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