Acoustic Envelope Detector

When you bite into an apple, a sharp, crisp crunch tells you it’s fresh. Without such a characteristic sound the apple would be less appealing. The science of sound emission is called acoustics and its analysis gives you vital information on the quality and acceptability of the food – both actual and perceived.

In response to requests from both university and commercial research departments, Stable Micro Systems developed a method of measuring the acoustic energy release during a physical test and delivered the Acoustic Envelope Detector.

How the Acoustic Envelope Detector works

The Acoustic Envelope Detector is an instrument that measures the acoustic energy released by a sample as it is being deformed by the Texture Analyser (Plus models only) and, in conjunction with Exponent Connect software, converts and displays this in decibels (dB) – the standard measurement of sound. Acoustic emissions, in the audible range up to a frequency of 12.5kHz, are detected close to the sample using a microphone and pre-amplifier from Danish sound and vibration experts Brüel & Kjær which has high sensitivity to the frequencies emitted by brittle products, but has low sensitivity to mechanical noise emitted by the Texture Analyser. This information is converted from an analogue signal (voltage) that represents the total acoustic energy released from the product as it varies with time. This voltage is measured while the TA.XTplusC and TA.HDplusC Texture Analyser simultaneously measures the force, distance and time to provide the 4th dimension in texture analysis.

See tests being performed in Exponent Connect software

In addition, Exponent Connect software can now process audio and synchronise this data as .wav files. The audio files (.wav files) recorded through the Acoustic Envelope Detector are useful for several purposes:

• The audio files greatly aid comprehension when analysing test results. Being able to hear the audio while seeing the audio data and Acoustic Envelope data helps the user to understand the nature of the data they are analysing.

• Keeping the audio files from your tests allows a better comparison between different acoustic environments. One laboratory may have different background noise to another laboratory, and listening to the actual audio may allow you to easily identify this.

• When analysing the test results, listening to the .wav files allows you to immediately hear whether there were unwanted background noises that have polluted your test data.

• When setting up acoustic tests to compare products against previously stored test data, the ability to HEAR the test environment is useful to ensure the environment is acoustically similar.

• When used with the Video Capture System the RAED Module provides the audio to accompany the video footage. The user then has Force, Video, Audio and Envelope data that may be studied together to provide a full understanding of the interaction between forces and noise produced from the breaking of a sample.

Force data (blue curve) and acoustic data (red curve) with .wav file below for a bulk compression test of cornflakes

The measurement of sound

Acoustic data is measured and displayed in Exponent Connect software in real time alongside mechanical measurements (force, distance and time) to identify certain events during a test, e.g. the popping sound when a beverage can is opened or the break of a biscuit as it is snapped. This sound measurement provides another dimension of quantitative data on fractures or audible events that occur when a sample is deformed during testing.

Perhaps the most salient characteristic of crunchy and crispy foods is the sound emitted upon their disintegration/fracture during chewing or in mechanical testing. When a crisp food is broken or crushed they invariably have a jagged stress-strain relationship. In addition, characteristic sounds are produced due to, for example, the brittle fracture of the cell walls of fruits or vegetables or the fracture of individual layers within a matrix product such as a crisp biscuit.

Sensory ‘crunchiness’ and ‘crispness’ are perceptions of not only force-deformation-time events but also, and almost certainly primarily, of their acoustic effects. It has been found, in fact, that the combination of acoustic and mechanical techniques more adequately describes food sensory perception than either technique alone.

The sound emitted from a product becomes its characteristic ‘acoustic signature’. The Acoustic Envelope Detector provides a tool for the quantification of this sound emission whilst measuring the textural properties of the product.

The marketing of sound

Advertisers use onomatopoeia so consumers will remember their products – words for things are created from representations of the sounds these objects make. In the past few years, advertisers have made use of the crispness and crunchiness of foods. Television viewers cannot experience the taste or the smell of a product being advertised. They can only see it and, of course, hear it. Advertising the aural assets of a food acquaints the potential customer with this most important quality attribute of many products. Consumers then associate ‘acoustic signatures’ with various products which enhances their appeal, success and brand loyalty.

The application of sound measurement

Whilst much work in this area has focussed on measuring crispness of brittle foods like breakfast cereals, or potato chips or the crunch of a chewing gum tablet coating or fresh apple, potential applications include the quantification of::

• The ‘snap’ of a cracker/biscuit/chocolate bar (whilst measuring its breaking strength)

• The ‘pop’ of a cork (whilst measuring its ease of removal)

• The ‘fizz’ of a bath bomb or disintegrating tablet (whilst measuring its firmness)

• The ‘snap’ of a pencil (whilst measuring its breaking strength)

• The ‘click’ of a switch (whilst measuring its actuation force)

• The ‘crack’ of a Christmas cracker (whilst measuring its pull strength)

• The ‘zip’ of a zipper (whilst measuring its pull strength)

Published articles

We have collated a list of online articles which you may find interesting and useful in understanding the field of texture analysis and acoustic measurement:

Eating with our ears: assessing the importance of the sounds of consumption on our perception and enjoyment of multisensory flavour experiences – According to Charles Spence, sound is the forgotten flavour sense. You can tell a lot about the texture of a food—think crispy, crunchy, and crackly—from the mastication sounds heard while biting and chewing. The latest techniques from the field of cognitive neuroscience are revolutionising our understanding. Read more

Not the Sound of Silence – Sound-profile curves add additional metrics about a product’s crispy-crunchy behaviour that cannot be easily picked up in a force profile. By analysing sound simultaneously with forces, companies can very precisely quantify the behaviours they are designing into their products. Read more

Application of acoustic emission for quality evaluation of fruits and vegetables – Food crushing sound is one of the main factors used for food quality evaluation. Crispness and crunchiness are attributes of high quality product and are usually pointed on the top of a list of consumer preferences. The acoustic emission (AE) method is investigated as a promising tool for food properties evaluation. Read more

Acoustic settings combination as a sensory crispness indicator of dry crispy food – Nine commercial food products were submitted to force and acoustic envelope detector analyses, using two settings each for acoustic data acquisition and acoustic data management. Sensory attributes and instrumental properties' strong correlations indicate that the Texture Analyser in combination with the Acoustic Envelope Detector is a good instrument to mimic human mastication and texture perception, through both force and sound stimuli. This study about dry crisp food may contribute to the literature and food industry, as well to future studies about wet crispy and crunchy foods. Read more

Further examples of the application of the Acoustic Envelope Detector in published work can be found here

Technical information

How the Acoustic Envelope Detector allows choice of acoustic spectrum breadth

It is important to understand that the AED measures the total voltage over the set frequency range and converts this into decibels. This is illustrated in the diagram below.

'1 frequency bin' from selected corner frequency to 12.5kHz

The AED uses a 1 ‘frequency bin’ approach that measures how the total acoustic energy within the set bandwidth varies with time. To focus on a specific frequency band a high pass programmable filter can be adjusted within the TA Settings window to alter the breadth of the acoustic spectrum from the lower end. There are 9 settings range from 1kHz to 10kHz. Note: the factory standard is set to 3kHz to reduce background noise.

What about background noise?

The frequency of sounds emitted from crispy materials spread across a large frequency range, up to and beyond the upper range of human hearing. The background noise of the Texture Analyser and general laboratory is mostly below 3kHz and can therefore be ignored as it does not contribute to the characteristic of crispy/crunchy acoustic events. Detailed research and analysis have enabled the design of a robust system that has low sensitivity to noise emitted by the Texture Analyser and general laboratories but a high sensitivity to the frequencies emitted by such crispy or crunchy products. This is effectively achieved by the use of the following:

• A high pass filter, easily configured by a drop-down menu in the test settings

• A directional microphone, mounted close to the sample

• Furthermore, automatic macros can be run on the data to assess the noise floor and a new data set can be generated without unwanted ‘signal noise’.

Example graph: Blade shear through chocolate ice coating

Technical specification

The reference system (A/RAED) uses a highly sophisticated and sensitive microphone which is calibrated by the user to National Standards. This means that fundamental acoustic data is obtained and therefore results obtained at different times, locations or from different units or systems can be compared directly and accurately. Calibrator included.