This is a document summarising discussions at research symposia held as part of this network. It is a best practice or methodology document discussing sound archaeology or archaeoacoustic field tests. 

Discussion of methodology/best practice for archaeoacoustic study (the acoustic study of archaeological sites).

Work should be based on a scientific methodology, which aims to define and separate out what is known and unknown. Data is of course never neutral, or neutrally connected, but efforts can be made to make data as uncoloured as possible. Pragmatism also means sometimes the issue is what can we do with what we have available. Thus there are different levels of methodological approach, including

  • this is what we know
  • what can we do

It is acknowledged however that in this field a phenomenological or experiential approach can further develop our understanding, but that results may be less certain, and that they may raise as many questions as they answer. Clear, open statement of the level of certainty of results, and of the type of methodology used is therefore essential in this field, where there are a number of appropriate methodological approaches.

It is also important to consider the resolution of the result that is required, in terms of what level of confidence is most appropriate, from the highly probable, which may allow only very simple results to be obtained, to the more speculative, which may provide interesting possibilities that can then be further explored.

What methodology of study and of presentation one chooses has to be tailored to one’s audience to some extent, it may be quite different depending whether the data is aimed at the general public or to a scientific community.

Bibliographic and Ethnographic Research

There have been a number of studies of the acoustics and sound of various sites and structures, and it is important to familiarise oneself with any existing literature when carrying out a new study. There is a bibliography of relevant sources at

http://AMBPnetwork.wordpress.com/bibliography

If there are indications that there may be interesting sonic features at a site, it will be useful to investigate existing archaeological research on a site or study, as there may be information that could be relevant to a sound based study that may have seemed insignificant or been overlooked in other contexts.

It is of key importance to consult the archaeological history of any site that is being studied, in order to determine the extent of restoration and
degradation, and whether there is evidence for now-missing elements
such as timber posts.

It may also be of relevance to investigate collections of finds to see if there is anything that could be of relevance. It is useful in this context to develop good working relationships with archaeologists, curators and others who have worked on or at the site or structure, as they are likely to have relevant information.

It may also be useful to search other sources to see if other relevant information available. There may be traditions, local evidence or sonic culture in other periods that although not directly relevant may provide useful information. There may be evidence available from periodicals, novels, local history societies, newspaper reports that may prove useful. It may also be useful to explore the sonic culture or traditions of the area, exploring the ethnography of the area.

While broader study of the wider area, region or country, and study of different time periods must be dealt with carefully, as there is a risk of inferring links between unrelated traditions or practices, this may be of some use to show what is possible. Some practices may develop from fundamentally human or physiologically driven situations, and comparisons with other ethnographic studies from even substantially different contexts have proved of some use for some studies.

Acoustics

There are a number of types of methodological approach for archaeoacoustic studies. Field work involving acoustic measurements is one approach. Acoustic tests can be carried out using scale models, as long as it is possible to scale results appropriately. Digital modelling of sound is useful but the restrictions of software must be taken into account.

For a field work based methodological approach involving acoustic testing one of a number of levels of study may be appropriate.

Level 1

An initial study may involve a number of simple observations that aim to identify whether there are acoustic or sonic features present that may be of interest. This may include:

  • Providing an impulse such as clapping, and listening for an acoustic response
  • Listening for echoes, and identifying where echoes seem to come from, as well as from where they are most readily generated and most clearly heard
  • Investigating different positions of sound generation such as clapping (source positions) and listening in different positions (receiver positions)
  • Clapperboards, mouse traps, balloons or clave type instruments can be used to give a louder and more repeatable sound source than clapping
  • Listening to the effect on the human voice of the acoustic of the space
  • Investigating subjectively for a number of source and receiver positions terms such as clarity, speech intelligibility, definition or loudness
  • Recording if there are any unusual sounds in the space
  • Describing any natural sounds in the space such as wind or water

Even in such a simple investigation it is important to record certain basic information that can affect sound, including:

  • Temperature
  • Air pressure
  • Windspeed
  • Weather conditions including rain, mist, snow, sun
  • Time of day
  • Position

It is vital to record exactly where any assessments are made, to record positions accurately, using diagrams and maps. A long tape measure is essential for this, and most archaeological projects will have staff involved that are highly skilled at recording and mapping positions. Photos can also be useful for this process. A recording of any acoustic effects, or of sound in the space of whatever quality available, is useful. Video cameras if available can be used to make sound recordings, also handily recording what is happening visually. Even mobile phones and stills cameras are often capable of making basic recordings.

Level 2

A more detailed study can use relatively simple and inexpensive equipment to gather information that can be used for acoustic analysis.

Capturing the impulse response of a space can be of perhaps the most use. This can be achieved by making an impulse sound and recording the resultant sound in the space. Suitable impulse source sounds can include:

  • A balloon
  • A mechanical traditional sprung rat or mouse trap
  • Clapperboards
  • Blank firing gun

As loud and short a sound as possible is required. In order to stop the body from masking acoustic, effects the sound can be made away from the body or above one’s head. Earplugs are useful to protect from the loud sounds made.

When using balloons one should try to use large, strong balloons blown up as far as possible. A sharp pin will make as short an impulse as possible. When making a number of recordings, one should try to blow up the balloons to the same size each time. Although balloons are quite effective, the level of sound produced is variable and difficult to repeat accurately. Good results can be achieved using a large, sprung mouse or rat trap. It makes a loud noise that is accurately repeatable, but care should be taken when triggering it. A wooden clapperboard can easily be made otherwise, by attaching two pieces of wood together with a hinge. These are simple to use, but again the sound is not accurately repeatable. Blank firing guns are available at a range of prices, and even a “toy” gun or cap gun can be quite loud and useful, especially since they will give a fairly reliable and repeatable volume level. Care should be taken if these are used, as even toy gunshots can cause alarm in public spaces.

These source impulse sounds need to be recorded. Numerous portable digital recording devices are available at fairly low prices that are useful for this process. Laptop computers with a good quality soundcard can also be used. Some will have one or two built in microphones, if an external microphone is used it should be of as good quality as available, and have a windshield, which is often made of foam. Wind noise on microphones is a major problem when making field recordings. Umbrellas can be used to shield a microphone from wind. Although they will affect the recording to some extent, this may be preferable to wind ruining any recordings.

Mono recordings made with an omnidirectional microphone are quite acceptable, stereo recordings may add complexity to analysis. Stereo recordings will however provide directional information that could be useful.

It is important to follow all the instructions for the level 1 study, and to carry out those basic tests as well. Temperature, pressure, wind, weather, time, date and position data should be recorded for each recording. It may be possible to use a simple methodology such as this to make more recordings in the available time in a greater variety of positions, than would be possible with more elaborate equipment. One will still have to make decisions about source and receiver positions. This should be considered either in advance by a consideration of plans, or by a subjective assessment of the most interesting acoustic positions or areas of study. In order to create a more rigorous set of data, it may be useful to make a number of measurements at set intervals, so that changes in acoustic can be assessed. For example measurements could be taken at one metre intervals through an entrance. It should be noted that acoustic effects can vary dramatically over short distances, and that being next to a wall or in a space can produce very different results.

Recording levels should be set as high as possible, but some headroom should be left, in other words levels should not be set so high so that there is a risk of distortion. Levels can be set by making a control recording. A control can be made by recording one’s impulse source sound in a space where there are no acoustic effects. Few will have an anechoic recording studio to do this, and so the middle of an open, flat field with a good distance in all directions to any object or road, will provide an appropriate space. A recording should be made of an impulse source sound at a set distance, of perhaps one metre. The recording levels of the recording device should be set so that this sound is a few decibels (perhaps 3db), or at any rate a little lower than distortion levels. With a digital device this will be just below the maximum level, which is usually 0db. With an analogue recording device this should also be 0db, and going above this is usually indicated by orange or red markings, or lights.

Once this recording level is set, one would ideally not change the recording level again. This is so that results of different recordings can be compared. However a recording made when the source is near the receiver will be far louder than where the two are far apart. In order to have a good signal to noise ratio, so that system noise is kept to a minimum, the recorded sound should be as loud as possible in comparison to background noise. If making recordings where source and receiver are always going to be far apart, it may be worth making one’s control recording at a larger distance. If making a variety of recordings at close and further source-receiver distances, one could make close recordings first, and make a second control recording, followed by more distant recordings.

Many recording devices aimed at a consumer (rather than professional) market feature automatic control of recording levels, designed to avoid distortion. This feature should be switched off if possible, as should be automatic compression and limiting of recordings. Instead close attention must be paid to recording levels to ensure that there is no distortion of the recordings. The highest possible available recording fidelity should be used. Digital recording is preferable to analogue. Digital .wav or .aiff should be used where possible rather than .mp3 files. If .mp3 files are used, then as high a quality possible should be used. 16bit files are acceptable, but 20 or 24 bit files are preferable.

A note should be taken of the height from the ground of the recording as well as the horizontal position of source and receiver positions.

From this recording it should be possible to capture the impulse response of the space, and from this calculate a number of acoustic measurements.

In addition to this, it may be useful to record the ambient sound in the space, so that the natural sonic environment can be explored. Recordings can also be made of various musical instruments and sounds. Source sounds could include:

  • Replica period musical instruments being played
  • Speech
  • Clapping
  • Singing
  • Simple percussive sounds using natural objects, hitting sticks or stones together
  • Other ‘natural’ instruments, such as log drums, animal horn trumpets
  • Domestic objects from the period being used musically, for example combs as rasps; bows as simple harps; antlers as drumsticks; cooking pots as drums.
  • Modern musical instruments that may have period antecedents

Level 3

Specialist acoustic testing equipment can be used in order to get more detailed results. This would include sound generators, sound sources, microphones, recording devices, analytical equipment and integrated systems. Equipment should have a frequency response that is as flat as possible, so that it does not colour the results.

Sound Generators

Laptop computers are often used which have specialist equipment for acoustic testing. They are used to generate a number of signals, each of which has advantages and disadvantages.

Sine Sweep

A sine sweep is a sine wave signal that sweeps from a low frequency to a very high one at a set pace and fixed level. This identifies the acoustic behaviour of each individual frequency in the space, and the results are put together to reconstruct the impulse response of the space, and to calculate a range of acoustic parameters. Its main advantage is that by dealing with one frequency at a time a high signal to noise ratio can be achieved, and thus good results obtained, and extraneous noise on the site excluded. The slower the sound sweeps the better the quality of the result.

Noise

Noise signals include all frequencies simultaneously. Noise is sometimes used in order to investigate how different frequencies interact together. If a number of frequencies interact with one another to create an acoustic effect, a sine sweep may not be able to stimulate this affect as it only generates on frequency at a time.

MLS

Maximum Length Sequences are sequenced periods of noise, which provide a combination of the advantages of using a sine sweep and noise.

Impulses

A very short burst of noise can be played through a loudspeaker or a starter pistol (as used in athletics) or blank firing pistol, can be used to provide a high level, short impulse that will be very similar in level each time. It is a more professional version of the use of a balloon. The pistol has the advantage of recording a real signal made by an acoustic source, not relying on a loudspeaker. Although a sine sweep can theoretically start at 0Hz, in reality loudspeakers often do not produce sound at less than 30Hz, and are often coloured at low frequencies. If a sine sweep is used to reconstruct an impulse response, but the loudspeaker is not able to play for example 25Hz, then this frequency will be missing. This may mean that echoes, very low frequencies or other time-based effects may be missed. If an impulse response is required for use within a convolution reverb to simulate the original acoustics of the space, a real impulse recorded from a starter pistol may give a better result.

Some venues may be unhappy about the use of what they regard as a replica gun.

Programme material

It may be useful to have musical examples of different types to play within the space. Hearing music, speech or other sounds in a space will provide a very different experience than the capturing of an impulse response.

Amplifiers

Source materials generated by laptops and other equipment have to be processed by amplifiers and other audio equipment in order to heard. Amplifiers need to be carefully matched to loudspeakers, to ensure that impedances, wattages and frequency responses are carefully rated. An acoustic system is only as good as the weakest link in the chain, and cheap cables, noisy power supplies, or a poorly matched amplifier and speakers can affect results.

Sound Sources

The various sound signals are played through loudspeakers. Dodecahedron loudspeakers are commonly used in order to radiate sound in all directions equally, omnidirectionally. These speakers often have poor or unbalanced frequency response, in order to radiate in all directions the ability of the speaker to deal with very low or high speakers is sometimes compromised. In some cases it may be preferable to use a high quality directional loudspeaker, which can reproduce very high and low frequencies well, especially if very high and low frequencies are of particular interest.

Microphones

Specialist acoustic measurement microphones are used that have a flat frequency response. An omni-directional condenser measurement microphone with a half inch or one inch capsule is standard, and ideally will have a calibrator. More than one microphone can be used to capture a stereo result, or to capture results simultaneously in more than one receiver position. A soundfield microphone can be used to capture a three dimensional signal and the results can be used to generate a range of results. This is particularly useful for exploring spatial information, although there may be more colouration than when using an omni-directional measurement microphone. Recording results from a soundfield microphone and omni-directional microphone together produces a wide range of results and possibilites. The microphones should be used with a stand or holder, in order to avoid handling noise.

Recording Devices

A microphone will need a pre-amplifier in order to boost the signal to an appropriate level. In some cases this is integrated into the microphone, other wise it might be integrated into the recording equipment. A laptop computer is often used as a recording device. In this case a professional soundcard is preferable, as the quality of the analogue to digital conversion process is important. In some cases more easily portable equipment is more appropriate, and there are a number of digital audio recorders available, as described in level 2. Some of these can use external microphones, and a system that allows this is preferable. Geophones can be used in order to record subsonic signals below 20Hz.

Integrated Systems

Some companies offer integrated systems that will generate sound, and record and analyse signals. This includes handheld systems with calibrated microphones. Software is also available that will generate signals such as sine sweeps, record them and then analyse the results, producing various acoustic measurements.

Other equipment

Specialist systems are available to insulate microphones from the wind, such as  full Rycotes. A calibrated volume meter can be used to assess variations in level, for example when using noise as a sound source. Differences in level can be assessed for example along a particular line or path, in order in order to see how the acoustic varies in a space, and to detect and describes standing waves.

Practical considerations need to be taken into account such as how to get electricity to equipment. Generators can be used by generate noise. Externally clad generators are quieter than skeletal models, and as small as possible a generator should be used in order to minimise noise. Sound baffles or careful placement of a generator can be used to minimise noise, as can the use of very long power cables so that the generator can be placed as far as possible from the testing. Batteries can be used but can run out very quickly. Camping batteries can be used and recharged, but will give very restricted time. They are however very quiet and will not interfere acoustically. It is also possible to use uninterruptible power supplies as power sources.

Level 4

Advanced and experimental techniques may sometimes be required for specific situations. Sometimes real instruments or voices can have effects that cannot be simulated through loudspeakers, and the effect of a group of people spread throughout a site is difficult to simulate. Sometimes the use of volunteers and volunteers making sounds may be useful for example. Indoor and outdoor sites may require very different methodologies, largely intact or largely degraded sites will also require different approaches. Standard techniques taken from building acoustics may not always produce the information required, and a customised approach may be needed in some situations in order to achieve the best results.

Analysis of Acoustics

Theoretical

It is possible to carry out acoustic analysis of plans of a site, by calculating Eigen frequencies and considering likely acoustic behaviour in the space.

Digital Acoustic Modelling

Digital Acoustic Modelling programmes like Odeon and Catt Acoustic are able to analyse and reproduce the acoustics of a structure or site. In order to carry out this work, an accurate digital graphical 3D model needs to be created. Care must be taken as these programmes principally use ray-tracing methodologies, which result in low-frequency results being inaccurate below a certain point. It may be impractical to use this approach for extremely large sites.

Wavefront analysis methods are more accurate at lower frequencies. A combination of these approaches will produce the most accurate results. Odeon and Catt Acoustic offer auralisation (recreation) of the acoustics of the space, and also offer the automatic generation and mapping of a number of standard acoustic parameters. They are expensive, and there are other less expensive pieces of software that offer some acoustic analysis. (Insert other examples?)

Some acoustic analysis software is designed for the setting up of loudspeaker arrays for PA system applications, and may not be appropriate or as useful for this kind of study.

Noise Mapping

When investigating larger areas, landscapes or acoustic environments, noise mapping programmes can be used to indicate how sound propagates in the space. This may indicate unusual sonic features, and where sound from various sources could be heard by a receiver or listener. Noise mapping considers in particular what is audible and what is not audible, and maps the contours of audibility across space.

There are a number of pieces of software that can carry out noise mapping including:

(insert examples)

Artistic Approaches

Sound Examples

Replicating the experience of acoustics, sound and music can be useful in order to explore a range of possibilities.

The method of presentation of results is important. Sound examples and multimedia and VR techniques should be used where possible.

Auralisation is the term used within acoustics for the process of producing a digital recreation of the acoustics of space or structure. This can be useful to allow a phenomenological exploration, to allow individuals to experience sonic features. This may provide insights that are not apparent from examining figures and statistics.

An impulse response either recorded at a site, or reconstructed using digital acoustic modelling, can be added to a range of sound sources to evaluate acoustic behaviour. This can allow one to evaluate the behaviour of a range of acoustic sources, and has been described as experimental sound archaeology.

It is important to consider sound production in a wide-ranging way, considering different agencies and sources, such as

  • Environmental sound, including wind, water, fire, animal noises, birdsong
  • Sound from human activity such as work, play, speech
  • Musical sound, by groups or individuals including
  • Singing and Clapping
  • Musical instruments

Archaeological evidence may be able to suggest sounds that were made from the space, such as the knapping of flint, digging or food processing.

It will be important to consider how people in the space would affect the sound present, a large number may generate a loud sound but absorb acoustic effects, smaller numbers may make less sound but allow more subtle effects to be heard. Again, archaeological evidence may be able to provide information about those likely to be using a site. It is also important to consider how a listener’s position might change their experience of sound. Different positions in a space may have significantly different acoustic properties. The effect of moving from one position to another within, entering or leaving a space, might be highly significant.

Sound sources should be as authentic as possible. Where possible sounds that it is known would have been present in the space from evidence, should be used as sound sources. Otherwise sounds should be used for which it is known it was possible that they could have been present should be used. Sounds that it is known could not have existed in the space should not be used as sound sources where this can be avoided, such as modern musical instruments or musical examples. Simple musical sounds, such as handclaps, hitting of wood or stone or vocal sounds may be useful. A range of sounds can be used as sources to evaluate their acoustic effect.

Single static sound examples can be created using impulse responses produced as already discussed. A digital audio file of any sound can have the impulse response of a space added using a digital acoustic modelling programme such as Odeon or Catt Acoustic, or by using a digital audio workstation (DAW) such as Logic Pro or Cubase. Within a DAW an impulse response can be loaded into a convolution reverberation plug-in, which can be used to add the sound of the space to the source sound. These sounds can then be organised and sequenced in different ways.

Audio recordings from a site can also be structured and mixed together using similar software.

Sound examples can be created that are made up of a number of single or static audio files, to create a more dynamic effect, which may include illustrations of changes of sound, or differences of sound in different positions, contexts or time periods. Audio files can be mixed together to illustrate audio transformations in different situations, such as entering or leaving a site. It may be that such changes are when sonic effects would be most noticeable.

Stereo sound examples can give a greater sense of position and reality. Surround sound or ambisonic files can be used to provide examples that create a truly immersive sound environment.

One may wish to create an audio trail, which a person can listen to when walking through the space. This would be an audio guide that could be specifically about audio effects, or part of providing more general information about the space.

One could also consider creating a sound installation using stereo, surround or ambisonic loudspeakers. This would allow one to illustrate the acoustic environment in a larger scale setting, ideally in the original environment, at the site itself.

Video Examples

The combination of the visual and sound is particularly powerful. This can be as simple as video footage of walking into or through a site with either simple recordings or a constructed soundtrack. Sound is a time-based medium, it does not exist as a static entity, and thus combining moving images and sound works especially well. A soundtrack can be created that includes different sources, contexts, possibilities or ideas, using a ranged of mixed sounds and sound examples, spoken descriptions and narration. ‘Dry’ sounds without the acoustic of the site or structure added, may be contrasted with ‘wet’ sounds that have the reverberation of the space added to them. Still images of the space, including photographs, illustrations and drawings, can be used to illustrate where the sounds were set, the surrounding environment, or to show technical information or data. Onscreen text can explain what is being heard in greater detail, and show further information.

Video imagery shot on site or created using digital modelling or various artistic methods, can be added to sound examples and static images. There are several different possible approaches to making video and sound material. One can illustrate results very effectively in this way, providing walkthroughs, and providing evidence of differences in sound in different contexts. One can present data clearly and in an engaging and experiential manner without compromising detail. Completed digital video files can be embedded within websites, allowing complex text and data to be added accompanying the file, including papers and links to other files, and making all of this easily available and accessible.

Video can be used to aid in the realism of an audio reconstruction of a site or structure. This may mean adding sound or audio to a drawing or digital graphical model to make it more immersive and sonically interesting. It may mean adding images or video to an audio reconstruction or soundtrack to make it more visually engaging. It may mean creating a visual and audio walkthrough/flythrough of a space to illustrate its changing acoustic properties.

It may mean creating an interactive 3D space, for example a digital 3D model with audio examples or soundtrack that change as one navigates the space.

It may mean taking an experiential approach to the video material rather than trying to be realistic. This may mean using colours, effects, animations and a variety of artistic effects to try to provide an indication of what it may have felt like to be in the space. A ‘realistic’ approach can never be completely successful, as one is not in the real space, and is not living in prehistory, and sometimes it may be more helpful, or provide an interesting alternative, to give an impression that might be less real by intend to illustrate or trigger emotional or psychological responses to the source material.

Installations and Downloads

A mixture of video projection or large video displays of images, and sound projected using loudspeakers, can be used to create installations which allow one to explore the space virtually, and to create an immersive experience. CAVE or RAVE environments are examples of such systems. Stereoscopic displays or projectors can provide 3D visual effects. Ambisonic (3D) sound can be used to create realistic sound environments to interact with these visual projection systems.

Files can otherwise be experienced through digital mobile devices, with PDAs and some mobile phones able to play video and audio files. Files could be uploaded to Youtube, and then viewed and listened to as one walked around the site or structure. Files can be embedded into virtual spaces such as Second Life which can be explored as virtual reality. They can be uploaded into Hewlett Packard’s Mediascapes system, allowing them to attached to the geographical location of the site, and downloaded when onsite. They can be made available via handheld devices hired at visitor centres, or downloaded in advance from appropriate websites.

Music Archaeology

Further information on Music Arcaheology can be found at http://www.musicarchaeology.org/ the website of the International Study Group for Music Archaeology and at http://www.ictmusic.org/ICTM/beta/stg/

The International Study Group for Music Archaeology website. There has been substantial study of music archaeology, although less of prehistory perhaps than of classical antiquity. Rather than list basic information in this document, for more information it is suggested that one visits one of these existing sites.

Archaeology

This kind of study may be useful in order to fully understand the impact of road and other modern noise on a site. The acoustics of a space are as much part of any archaeological site as are stones or the visual, and subtle but important features of the space may need to be considered when considering planning or development. The acoustic ecology of the site may be shown to be important at a site by a study of this kind.

Although acoustic analysis and reconstruction are useful, it is important to also consider how people might respond to a space, and what other relevant archaeological studies can tell us about the wider culture and context of the space that may be relevant to its sonic culture. It is likely that any sound based archaeology, archaeoacoustics or music archaeology, will be best suited to adding further information to the existing knowledge about a space. It will be likely to be a piece of the jigsaw of understanding of the site rather than the jigsaw itself . Thus it is important that information is integrated into a wider archaeological consideration of the site under discussion.

However because archaeology has largely focused on the visual rather than the acoustic, it has been and remains important to carry out such sound based studies in order to demonstrate their worth to the wider archaeological community.

Archaeological information can inform the cultural context in terms of subjects such as the technical limitations of those involved in creating these monuments, what choices were open to these societies, where they had choice and where they were doing only what was possible.

Sound study has much to offer an archaeological study, the definition of a site may for example be defined or changed by sound, as a site may expand to an entire area where a particular sound is audible.

2 Responses to “Methodologies”

  1. Palermo Anderstein Says:

    And lastly, and most importantly, post the impulses that you create in conspicuous locations online so interested parties can download the impulse for further research and experimentation. Hiding them on expensive academic paper mills and other hierarchical areas of the internet are not conducive to learning or gaining support (and therefore possible future funding) for your ongoing research in this great area.

    1. ruperttill Says:

      This is a research networking site, and so does not have the research data of individual researchers on it. Please look on the websites of individual researchers if you want to access their impulse responses. I can only speak about my own research on the acoustics of Stonehenge. We have not published all of our impulse responses for a number of reasons. Partly because we are still conducting our own research on the material, and so as is usual in scientific projects some of the data remains embargoed. Secondly because we are wanting to make the impulses as accurate as possible before releasing final versions, and as I said we are still working on the material. However at least one of our Stonehenge impulse responses is available online.

      I would also point out that some of the impulse responses are available free online, for example at least one of our.

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