Tag Archive for: free

Polar Plot (MDT-4000 Turntable) Sequence

This sequence measures the polar response of a loudspeaker in both the vertical and horizontal dimensions. It is designed to work with the Portland Tool & Die MDT-4000 turntable, and has all the necessary commands to automatically rotate it via RS-232. The sequence uses a log sweep stimulus with the Time Selective Response algorithm so that the measurements can be run in a non-anechoic environment. Note that the time window needs to be adapted to the user’s measurement space.

The sequence plays the stimulus and measures at 10 degree increments from 0 to 180 degrees. This process is repeated with the speaker positioned horizontally. The two results are mirrored to display full 360 degree polar plots for each axis. A directivity index curve is also calculated for each axis and is displayed at the end of the test.

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Receive Loudness Rating with ITU Real Speech Test Sequence

The purpose of this sequence is to measure the Receive Loudness Rating (RLR) following the ITU-T P.79 standard using a Head and Torso Simulator (HATS). First, real speech from the ITU-T P.501 standard is sent to the Device Under Test (DUT) speaker by an electrical interface. The HATS right ear captures the DUT‟s speaker response. SoundCheck calculates the frequency response and then RLR based on that recording.

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Send Loudness Rating with ITU Real Speech Test Sequence

The purpose of this sequence is to measure the Send Loudness Rating (SLR) following the ITU-T P.79 standard. This sequence can be used with handsets, headsets, and conference call devices. First, real speech from the ITU-T P.501 standard is played out of a mouth simulator. The Device Under Test (DUT) microphone then captures the signal and transmits this back to SoundCheck. SoundCheck calculates the frequency response function in 1/3 octaves and calculates SLR based on that frequency response.

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Loose Particles Sequence

This sequence demonstrates how to use SoundCheck to detect loose particle defects in loudspeakers. Loose particles typically reveal themselves as randomly spaced impulses, so they may not be detected when performing frequency based measurements such as THD, even though they can be clearly heard as undesirable artifacts. The loose particle algorithm, which is an available function in all analysis algorithms, analyzes a time waveform to detect these impulses. The user sets a customized threshold level for detection.

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Bluetooth Headset Test Sequence

The purpose of this sequence is to test a Bluetooth headset using a mixture of analog and digital channels. First, a Multitone stimulus is created with SoundCheck, played back over the Bluetooth headset (at 8 kHz) and recorded by a head and torso simulator’s ear (at 44.1 kHz). Then the same Multitone stimulus is played back through the head and torso’s mouth simulator (at 44.1 kHz) and recorded via the Bluetooth headset (at 8 kHz).

Due to inaccuracies of clock frequency, the Bluetooth device playback sampling rate is often slightly different than it is specified. Therefore, in SoundCheck, the Recorded Time Waveforms are frequency shifted to correct for the inaccurate sampling rate. The exact device playback sampling rate is displayed.

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Headphone Test Sequence

This headphone test sequence measures a stereo headphone. Both left and right earphones are measured simultaneously using a standard 1/12th Octave stepped-sine sweep from 20 Hz to 20 kHz.

The analysis is then performed using Listen’s HarmonicTrak™ algorithm that measures harmonic distortion and fundamental frequency response simultaneously. Then the diffuse-field and free-field corrected Fundamentals are calculated. The diffuse-field correction curve compensates for the overall frequency response from the diffuse-field (sound in every direction) to the eardrum and includes the effects of the head, torso, pinna, ear-canal and ear simulator. The free-field correction curve compensates for the overall frequency response from the free-field (sound at 0 degree incidence to the nose of the Head and Torso Simulator – HATS) to the eardrum.

Further post-processing of the signal compares left and right earphone responses to show the difference curve (magnitude and phase are available). The average sensitivity from 100 to 10 kHz for both left and right earphone is calculated and the total harmonic distortion displayed.

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Amplifier THD+N Sequence

This sequence measures an audio amplifier’s Frequency Response, Gain, THD, THD+Noise, and Self-noise. It accomplishes this by playing a 1/3rd octave sine sweep through the amplifier. A HarmonicTrak™ analysis step calculates the fundamental frequency response curve as well as the distortion plots. The sequence then records and analyzes a spectrum of the amplifier’s self-noise.

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Time Selective Response Sequence

This sequence demonstrates the capabilities of the TSR (Time Selective Response) algorithm in SoundCheck. Using TSR with a log sweep is a very fast and effective method for measuring frequency response and THD of a speaker in a non-anechoic environment. This example plays a quick log sweep from 20 Hz to 20 kHz. An analysis step then uses the TSR algorithm with time windowing to output frequency response, harmonics, THD, and impulse response curves.

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100 Things #17: Simulated Free Field Measurements Without an Anechoic Chamber

With SoundCheck you can make simulated free field measurements in an ordinary room without an anechoic chamber – in a typical lab, office, or even your home. By performing near field and far field measurements, we are able to utilize the measurement strengths of each technique; near field measurements for low frequencies and room reflection immunity, and time-windowed far field measurements for high frequencies. Splice these measurements together, and the result is the free field response of the loudspeaker. All without an anechoic chamber, and all within a single SoundCheck sequence.

Simulated Free Field Measurements Without an Anechoic Chamber

Learn more about free field measurements with SoundCheck

In this online seminar, Steve Temme explains how to use simulated free field measurements to provide accurate free-field measurements across the entire audible frequency range without an anechoic chamber. This seminar talks about setup and equipment considerations, testing in different environments, and analysis of results.

Try simulated free field measurements for yourself with our Splice sequence, available in our sequence library.

Video Script:

Free field loudspeaker measurements are often included in loudspeaker specifications. These measurements are usually made in an anechoic chamber. However, anechoic chambers are not cheap, particularly those large enough to give accurate measurements at low frequencies. This means that many engineers don’t have easy access to a suitable chamber, even more so if they are working from home.

With SoundCheck you can make free field measurements in an ordinary room without an anechoic chamber – in a typical lab, office, or even your home. Here’s how it’s done.

First, measure the near field response of the loudspeaker using a stepped sine sweep or Stweep, and placing the microphone very close to the low frequency driver. If the loudspeaker is ported, you also need to make a measurement at the port or ports. This near field measurement is accurate at low frequencies as it’s unaffected by room reflections, but it does not accurately represent the free field response at high frequencies. 

Next, place the microphone in the far field and measure the time-windowed frequency response using a continuous log sweep with the Time Selective analysis algorithm. The far field, time-windowed measurement is unaffected by room reflections, but it is not accurate at low frequencies as the room size limits the width of the time window and therefore the frequency resolution.

Now, if you examine the two responses you can see an overlap range in the middle where the shapes of the curves align. Using this, you can select a precise frequency to splice the two halves of the measurement together and display a response over the entire frequency range. This is implemented in several post-processing steps which include calculating the impulse response, and automatically correcting for differences in amplitude and phase.

Here you can see the simulated free-field response measured using this method compared to the same speaker measured in an anechoic chamber. In fact, the simulated free field response is actually more accurate than the anechoic chamber measurements because of its small size and 120Hz cut-off frequency

We can also compare our measurements to the manufacturer’s published data. Here we see a tight correlation across the whole frequency range, even at low frequencies – likely because a much larger chamber was used.

If you want to know more, there’s a detailed seminar covering both the theory and practical aspects on our YouTube channel, and a pre-written test sequence for use with SoundCheck can be downloaded from our website for free!

Measuring Hearing Protection Devices to ANSI S3.19-1974 Standard

This sequence is used to measure the NRR, or Noise Reduction Rating, of a hearing protection device to the ANSI S3.19-1974 standard. NRR is a numerical representation of the sound attenuation of a device. The sequence first measures the response spectrum of the unoccluded hearing protector test fixture, then makes a second measurement with the hearing protection DUT affixed. A signal generator virtual instrument generates the pink noise stimulus while an RTA virtual instrument simultaneously records the A and C weighted noise spectrums. The unoccluded and occluded measurements are analyzed with a series of post-processing steps according to the ANSI S3.19-1974 standard. The final display shows the NRR numerical value, RTA spectra of the left and right side of the unoccluded and occluded hearing test fixture, average attenuation level of the DUT, and the standard deviation of the DUT on the test fixture.

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