Pop those balloons with RoomScope 1.3

A common request for RoomScope has been to directly capture room impulse response (IR) data using impulsive noise sources, such as balloons or starter pistols. RoomScope 1.3 now supports both single-channel and dual-channel IR data acquisition. The dual-channel capability has been built-in since version 1.0 of RoomScope, enabling the use of an omnidirectional loudspeaker and various excitation signals including broadband noise and frequency sweeps. The new single-channel IR acquisition tool can capture IR data directly by popping a balloon or firing a starter pistol within an acoustic space of interest. The input signal can be triggered in order to automatically begin data acquisition when the instantaneous input level crosses a user-defined threshold.

It should be noted that single channel analysis doesn’t mean that only one channel of data can be captured at one time. In fact, with an iPad and a multi-channel USB audio interface, it is possible to capture IR data from multiple microphones simultaneously (obviously, the mics and supporting cables, adapters, etc, would also be needed). Single channel analysis simply means that each input channel is treated independently from all other input channels. In contrast, dual-channel analysis means that a single measurement is based on data acquired from two input channels (using auto and cross-spectral analysis).

RoomScope 1.3 is now available for download on the App Store.

RoomScope 1.3 Screenshot

Download RoomScope 1.3

RoomScope and IOScope measure longer IRs and use 64-bit FFTs

RoomScope 1.2 and IOScope 2.3 arrived in the iOS App Store this week. Both apps now support impulse response measurements up to 16 seconds long and use double precision (64-bit) FFTs in their measurements. The maximum measurement length is dependent on the amount of memory available on the device, so 16-second measurements are currently only supported on the iPhone 5, iPad 3, and iPad 4. The previous generation of devices, including the iPod touch 5, supports 10-second measurements, and older devices are still limited to measurement durations of 5 seconds or less.

RoomScope 1.2 also offers the following additions:

  • Center time has been added to the list of calculated acoustic parameters.
  • All parameters are now calculated in low, mid, and high frequency bands, as defined by the ISO 3382 standard.
  • The calculation of the clarity and definition parameters (C and D) compensates for the delay of the whole and 1/3-octave band filters, as described in ISO 3382.
  • Raw IR data can now be excluded or included in CSV, MAT, and TXT file data exports.

RoomScope turns your iPad, iPhone, or iPod touch into a room acoustics measurement and analysis tool. With RoomScope, you can measure a room impulse response and then calculate reverberation time, early decay time, center time, clarity, and definition, as defined in the ISO 3382 standard. RoomScope also allows you to adjust the Schroeder decay curve integration limits with the touch of your finger and plot the calculated room parameters versus whole or 1/3-octave band center frequency.



Download RoomScope 1.2

IOScope brings true dual-channel transfer function and impulse response analysis to iOS. With IOScope, measure loudspeaker impedance, frequency response, and sensitivity. Measure a room impulse response. Tune a large sound reinforcement system, time-align a set of surround sound speakers, or optimize your home stereo. Determine the actual cutoff frequencies of your latest speaker crossover circuit, or teach your students the fundamentals of Fourier analysis of dynamic systems.

Measure frequency response magnitude and phase, coherence, and group delay. Time domain functions enable you to measure impulse response and auto/cross-correlation. IOScope includes a built-in signal generator for producing suitable excitation signals to analyze your system or device under test (DUT). See http://youtube.com/faberast for a video demo of loudspeaker impedance measurement.


Download IOScope 2.3


Measuring Loudspeaker Impedance with IOScope

Today, a new video, Measuring Loudspeaker Impedance with IOScope, was published on this site, as well as on the Faber Acoustical YouTube channel. The video is both a demonstration of IOScope, as well as a simple tutorial on measuring loudspeaker impedance. Although the video is largely self-explanatory, I thought it would be beneficial to include some further explanation and tips for those who are interested. The movie is essentially broken into four chapters and a similar format will be followed here.

What is impedance? How is it measured?

By a generalized version of Ohm’s law, we understand that voltage is equal to the product of electrical current and impedance. This means that electrical impedance is equal to the ratio of voltage and current, or:

Z = V/I

where Z represents impedance, V represents voltage, and I represents current. This means that we can calculate impedance as long as we can measure voltage and current. Voltage is easily measured from the iPhone’s audio inputs, but how do we measure current? By returning to Ohm’s law, we see that if we measure the voltage across a resistor (a known impedance), we can compute the current by I=V/Z. So, as long as we have a known resistance and a means to measure the voltage on both sides of it, we can effectively measure current.

Voltage calibration

Although audio signals come into the iPhone as voltages, those voltages get converted to digital values. In order to measure voltage correctly within IOScope, we need to perform some type of calibration that will define the relationship between the digital values and the actual voltage reaching the iPhone audio input jack.

As is demonstrated in the video, voltage calibration in IOScope can be easily achieved by measuring the voltage going into the iPhone with an rms voltmeter. Once the rms voltage has been measured with the voltmeter, that measured quantity should be entered into the Ref Input Level text box of IOScope’s calibration screen. All that’s left is to press the Calibrate button and confirm the action–IOScope will automatically compute the audio input device’s voltage sensitivity.

  • The accuracy of the calibration will largely depend on the accuracy of the voltmeter.
  • In the video, IOScope’s built-in signal generator is used to produce a single tone with a frequency of 1 kHz. As long as the tone is turned on in the Excitation tab of IOScope’s main screen, the signal will be output when the calibration screen appears.
  • When entering values into a text box in IOScope, you can enter unit magnitude prefixes, like m for milli, or u for micro. For example, in the video, “618m” is entered into the text box for 618 millivolts. Alternatively, “.618” could have been entered for the same value.
  • When performing a voltage calibration, you need to be sure that the input device or input channel units are set to V (for volts).

Current calibration

When working with a stereo input device, like the Belkin TuneTalk Stereo that was used in the video, IOScope adds a special third channel to the device. That channel is the difference between input channel 2 and input channel 1. Such a channel enables convenient measurement of current by measuring the voltage difference across a resistor. Since the previous voltage calibration was performed at the device level, which means that the calibrated voltage sensitivity applies to all input channels, calibrating the difference channel for accurate current measurement simply requires you to enter the value of the resistor you will be using. In this case, the resistor was measured to have a resistance of 999 ohms. Instead of using the Calibrate button on the channel calibration screen, the known sensitivity of 999 Volts per Amp can be entered directly into the Input Sensitivity text box.

How do we know the sensitivity of the Ch2-Ch1 difference channel should be 999 V/A? Again, Ohm’s law gives us the answer. Since V=IZ (or, in this case V=IR, where R is resistance), if we put 1 amp of current through a 999 ohm resistor, we should read 999 volts, or 999 volts per amp.

  • When performing a current calibration, you need to be sure that the input channel units are set to A (for amps).

Setting up the loudspeaker measurement

As I indicated before, in order to measure the impedance of a loudspeaker, we need to measure the voltage across the loudspeaker’s terminals and the current flowing through them. Measuring the voltage across them is as simple as connecting one of our input channels to the terminals. However, we need a little help from Kirchoff’s current law to understand that if we put a resistor in series with the loudspeaker, the electrical current through the resistor and the loudspeaker will be identical. Since we’re already prepared to measure current across a resistor, we’re good to go.

  • IOScope’s Measurement Configuration screen provides a simple graphic representation of the measurement, which makes it easy to identify which input channel should be which. Since we’re measuring loudspeaker impedance, which is equal to V/I, we set the Y signal to Input Channel 1 and the X signal to Input 2 – Input 1.
  • The proper connections of the signals to the loudspeaker are shown in the video (at 3 minutes).
  • Again, the built-in excitation signal is employed, although this time it is in the form of a logarithmic frequency sweep. The length of the sweep corresponds to the length of the FFT used by IOScope, which allows excellent results to be obtained with little or no averaging (which is also evident in the video).

Making the measurement

Once things are configured, making the measurement just takes a tap of the start button in the toolbar of IOScope’s Frequency tab.

  • A double tap in the vertical axis label region of the display will toggle auto scaling on and off (it’s on by default).
  • Notice that the magnitude measurement is displayed in units of ohm’s (IOScope is smart enough to know that volts divided by amps means ohms).
  • The video also demonstrates saving the frequency domain data into a .mat file, which can be downloaded from IOScope onto a Mac or PC, via a web browser. Data contained in .mat files can be loaded in to MATLAB, Gnu Octave or FreeMat.