IR Measurements, Part 1: Pre-Smaart Preparation

IR Measurements, Part 1: Pre-Smaart Preparation

The Smaart side of taking an impulse response measurement is a rather simple process, and will be detailed in Part 2 of this guide. That being said, the usefulness of any information you capture will depend on a number of decisions made before you even open Smaart. This decision-making process is a comparatively more complex task, and is detailed in this article.

The overall process for measuring an acoustical response can be summarized as follows:

• Pre-Smaart:
  • Selection of measurement technique/stimulus type
  • Selection of excitation source(s) and position(s).
  • Selection of measurement position(s).
  • Estimation of the reverberation time and background noise.
• Within Smaart:
  • Estimation of the reverberation time and background noise.
    
  • Selection of measurement parameters (measurement length/duration, excitation level)
  • Exciting the system.
  • Recording results

Several key questions will determine each decision made in the preparation stages of IR measurement.

What are We Measuring and Why?

Before even considering booting up Smaart, it is necessary to define your objective clearly. Obviously, we want to measure the acoustical impulse of the system under test (SUT), but what exactly is "the system"?

• Is it a room?
• Is it a sound system?
• Is it a combination of both the sound system and its acoustical environment?

Additionally,

• What do you want to know about the system? 
• What equipment and measurements will be needed to make sure you get the information you need?
• If you want to measure the reverberation time of a room with an installed sound system, are you more interested in the room or the system? 

If you're looking to specifically measure the acoustic properties of the room independent of an installed sound system, it might be ideal to use an omnidirectional loudspeaker specifically designed for acoustical measurement. If your objective is to instead measure the performance of a loudspeaker system installed in a room, early-to-late energy ratios and speech intelligibility metrics might matter more to you than the reverberation time of the room.

Direct vs. Indirect IR Measurement

Smaart has the capacity to measure Impulse Response via both direct measurements and indirect measurements. Additionally, Smaart's indirect IR measurement method can be considered deterministic if it utilizes a pseudorandom noise signal or non-deterministic if it utilizes a random noise signal. 

Direct IR Measurement: Using an Impulsive Stimulus

The most intuitive way to measure the impulse response of a system is to simply use an impulsive stimulus and record what happens. To do so, you don't need a sound system or even a measurement system to perform the measurement, which is certainly convenient. You just need a way to make a short, loud sound and a way to record it. 

The main issue with this approach is twofold:

• It doesn't tell you anything about an installed sound system (if applicable).
• There is a scarcity of effective impulsive stimulus sources.

The ideal impulsive stimulus would be a perfectly instantaneous, perfectly omnidirectional burst of energy with equal energy at all audible frequencies. In the time domain, it would appear as a single vertical spike no more than one sample in width. In the frequency domain, it would produce perfectly flat magnitude and phase traces. Any changes to its "flat" response would be visually apparent, making it a blank canvas against which to view any nonlinearities. In physics, this is known as a Dirac pulse.  Unfortunately, such a stimulus signal does not exist in the physical world. 

When directly measuring the impulse response of an acoustical system, our options are limited to less-than-ideal stimulus sources. Blank pistols and balloon pops have been commonly used, as well as signal cannon, spark gaps, fireworks and even spot welders. 

Problems with these excitation sources include these:

• Their spectral content is not uniform.
• Their envelopes are not instantaneous.
• They aren't as omnidirectional as they may seem.

These factors vary from one measurement to the next, limiting repeatability. Additionally, the unpredictability of these factors creates uncertainty as to which part of the completed measurement is stimulus and which is response. For this reason, indirect measurement systems infer the response of a system using an ideal impulse, and are much more commonly used.

Indirect (Dual Channel) IR Measurement

Indirect impulse response measurements are made using dual-channel measurement techniques that mathematically estimate the response of an SUT by using either continuous or periodic test signals. These measurement systems calculate the frequency-domain transfer function of a system under test (SUT) by comparing the signal going into a system to the output of the system in response to said input.

When creating the transfer function of a stimulus signal and the SUT's response to it, it strongly resembles an ideal impulse. This is roughly what happens when you use a pseudorandom, period-matched excitation signal. Using random signals for this purpose, however, can lead to extra noise, but repeating the measurement several times and averaging the results will serve to lower the noise floor, a simple feat for Smaart.

Selecting a Measurement Signal

Dual Channel IR Measurement Using Period-Matched Signals

Fourier transforms technically only work with signals of infinite length. When using random signals, the DFT gets around this by "pretending" a finite chunk of signal being analyzed is one instance of an infinitely repeating series of chunks that look exactly like it.

The best way to get around this inherent assumption of cyclicality is to use a period-matched test signal. A period-matched signal either fits completely within the measurement time window or cycles with periodicity equal to the length of the DFT time constant. These signals can produce deterministic, highly repeatable measurements in a fraction of the time it takes to get comparable results using random signals.

When using period-matched test signals for dual-channel IR measurement:

• No data window is required.
• Delay compensation is not a critical requirement.
• Considerably less averaging is typically required.
• Measurement time constants can be kept to reasonable lengths.
• Small time variances become less of a concern.
• Subjectivity in selecting measurement parameters is reduced.
• The measurement system doesn't necessarily need to be connected to the system under test (SUT). 

When using a known test signal, the measurement system and the SUT can get their stimulus/reference signals from two different sources and the measurement will still work, provided that the two signals can be time-aligned post-process (a simple feat in Smaart). You won't get an accurate propagation delay time without an audio feed from the signal source being used to excite the SUT, but if you don't really need delay times this can be a very handy option.

Logarithmic Sweeps

Logarithmic sweeps are called Pink Sweeps in Smaart. Sweeps can be used either as a circular or aperiodic signal source. If the "Triggered by Impulse Response" option is enabled in Smaart's signal generator, the sweep signal is triggered by starting an IR measurement. When you start the measurement, Smaart will insert a short period of silence, run the sweep, and then insert another period of silence afterwards to let the SUT ring out. If the "Triggered by Impulse Response" option is un-checked, the sweep runs continuously when the generator is on. In this case, you would start the generator before starting the measurement as you would with other test signals.

When using dual-FFT-based IR measurements made with logarithmic sweeps, distortion products in the excitation loudspeaker/SUT show up as "pre-arrivals." Because the DFT is a circular function, these typically end up wrapped around past the beginning of the measurement and pile up near the end of the time record. To remedy this, you may need to make the measurement time window a little larger than you would for a matched noise measurement. This will ensure that these artifacts do not intrude on the reverberant decay slope.

Dual Channel IR Measurement Using Random Stimulus Signals

An excitation signal is considered random if it is not completely contained within or, if continuous, has its periodicity precisely matched to the time constant of a discrete Fourier transform. In Smaart's signal generator, the Random pink noise option or any pseudorandom cycle length with periodicity longer than the FFT size used are effectively random (Periods shorter than the FFT size should never be used because they won't contain energy at all FFT bin frequencies). Music as a test signal is also considered "random," as well as any noise signals with arbitrarily long periodicity from sources outside Smaart.

The only absolute requirements for using a random test signal are thus:

• The measurement system needs an exact copy of the signal going into the SUT.
• The signal must contain enough energy at all frequencies of interest to make a solid measurement. 
• If it is a cyclical periodic signal such as pseudorandom noise, the cycle length must be greater than or equal to the time constant of the FFT size used to make the measurement.

Disadvantages associated with random stimulus signals (as opposed to period-matched signals) include poorer noise rejection and increased measurement time required to obtain comparable results - more averaging is required, meaning you must measure over a longer period. Additionally, it is left up to the operator to decide how much averaging or how long a time window to use and the actual dynamic range of the SUT is ambiguous.

There are three basic things you can do to improve the dynamic range of measurements made using random test signals:

• Delay the reference signal to match the timing of the measurement signal so that the data windows line up. (You should always do this when measuring with random signals.)
  
  
• Evaluate the system over a longer period of time by increasing the DFT size and/or by averaging multiple measurements.
  
  
• Simply measure louder, which also applies to deterministic and direct IR measurements - in that case, you're increasing the signal-to-noise ratio of the measurement by increasing the level of the actual signal, rather than statistically.

Selecting Your Excitation Source Position(s)

Excitation source positions should be places that sound would normally emanate from when the SUT is in use. An omnidirectional sound source of some kind should be placed on the stage, podium, lectern, pulpit or whatever location(s) that would best simulate normal use of the room/system, and at an appropriate height.

Directional Loudspeakers, Early Decay Time (EDT), and Reverberation Time (RT60)

For the specific purposes of reverberation (RT60) and early decay time (EDT) measurement, a potential complicating factor can arise if an installed sound system is to be used to excite the room. IR measurements made with directional loudspeakers typically have higher direct-to-reverberant ratios in the higher octaves than those using other excitation sources. 

• ISO 3382-1 unequivocally states that "the sound source shall be as close to omnidirectional as possible" and provides criteria for assessing the omnidirectionality of a prospective source.

• ISO 3382-2 specifies measurement procedures for three levels of accuracy in reverberation time measurements: Survey (quick and "on-the-fly"), Engineering (decent) and Precision (excellent).

For the Precision method, the requirements for the excitation source are identical to those specified in 3382-1, but 3382-2 goes on to say that "For the survey and engineering measurements, there are no specific requirements for the directivity."

Omnidirectional sources are generally preferred for reverberation time measurement, but it is often possible to obtain usable reverberation time estimates using directional excitation sources. 

This can be done by one of two methods, both of which are supported by Smaart: 

• By using the (default) linear regression method (aka "a least-squares fit line).
• By manually fitting a straight line to either the reverse time integration or directly to the IR. 
  
  

When determining EDT, the impact of directional sources on the highest octave bands tends to be more significant. Because of this, the EDT of the source is harder to distinguish from the EDT of the room itself.

That being said, IR measurements made using an installed sound system may be more representative of its actual use than measurements made by any other means (considering it is already in the space in which it will be used). This is why IR measurements start with the question, "What am I trying to measure, and why?"

In some cases, you might need to make measurements using multiple sources. For example, you could use an omnidirectional source positioned onstage, another using the installed sound system, and even a third using the house paging system (to estimate its intelligibility). In other cases, using an installed sound reinforcement system as your lone excitation source can give you all the info you need. The full range of use cases for IR measurement is vast (and beyond the scope of this guide).

Selection of Measurement Position(s)

You generally want to measure from where listeners will be when the SUT is in service. You may also choose to give special attention to any problematic or reverberant areas in the room. Measuring from a single position will only give you an idea of what the room sounds like at that one point in space. Measuring from several different locations, however, will average out any position-dependent differences, thus revealing the most common characteristics of the system response. The more measurement positions included in this average, the lower the theoretical margin of error. 

ISO 3382-2 specifies the number of measurement positions for each method:

• Survey: a single stimulus source is measured from at least two measurement locations.
  • Margin of error: +/-10% for octave bands. 
• Engineering: at least two stimulus source positions and six independent source-microphone combinations.
  • Margin of error: +/-5% for octave bands (+/-10% in 1/3 octave bands). 
• Precision: 12 independent source-microphone combinations using at least two different stimulus source locations.
  • Margin of error: +/-2.5% for octave bands (+/-5% for 1/3 octave bands).

ISO 3382-2 also specifies that all measurement positions should be at least 1/2 wavelength apart and at least 1/4 wavelength from any reflecting surface (including the floor). For example, if we wanted to measure as low as the 125 Hz octave band, the lower band edge is at ~90 Hz. At 68° F (20°C), the speed of sound in air is 1127.4 f/s (343.6 m/s) and so one wavelength at 90 Hz would be about 12.5 ft (3.8 m). From that, we could conclude that no two mic positions should be less than 6.25 ft (1.9 m) apart and all microphones should be at least 3.13 ft (0.95 m) above the floor and at least that far from any wall or other reflecting surface. For the 63 Hz band you would need to double those distances.

Notably, ISO 3382-2 specifically applies to measurement of reverberation time in rooms. What about acoustical measurements made for other purposes? Two other standards that specify microphone placement are ANSI S1.2, Criteria for Evaluating Room Noise, and SMPTE 202M, the current standard for calibrating cinema sound systems. 

ANSI S1.2 has this to say about measurement positions:

"Sound measurements for rating room noise under this standard shall be made at locations that are near the average normal standing or seated height of human ears in the space: 5'-6" for standing and 4'-0" for seated adults, or 3'-6" standing and 2'-6" for seated children. The microphone shall be no closer than 2' from any sound reflecting surface or 4' from the intersection of two intersecting reflecting surfaces, or 8ft from the intersection of three intersecting reflecting surfaces."

SMPTE 202M recommends that microphones be placed:

"In indoor theaters, at position S [...] and position R [...] should it exist, and at a sufficient number of other positions to reduce the standard deviation of measured position-to-position response to less than 3 dB, which will typically be achieved with four positions. [...] It is recommended that measurements be made at a normal seated ear height between 1.0m and 1.2m (3.3ft and 4.0ft) from the loudspeakers(s)." (Position "S" generally works out to be a little to the left or right of the approximate center of the room on the main floor. Position "R" is for balconies.)

Wile serving different purposes, these different guidelines are generally similar. They are likely quite similar to the positions you would intuitively use for frequency-domain transfer function measurements of a sound system.

Minimum Distance from Sound Sources

Another requirement for room IR measurement positions is that they need to be located far enough away from your excitation source to ensure that the measurement is not dominated by direct sound. ISO 3382-2 provides the following formula for calculating this minimum distance (dmin):

Once you have determined your source and measurement positions and chosen a measurement technique, it's time to boot up your copy of Smaart.

Part 2 of this guide focuses on the Smaart side of things when taking and analyzing IR measurements, and can be viewed here.